In the last 60 years, there has been a tremendous surge in popular belief about life residing on planets outside our solar system. While such speculation used to be frowned upon, it is now commonly greeted with enthusiasm. Life on other planets is now popularly believed to be not just possible but virtually certain. Carl Sagan, among others, has done much to promote this idea and gave it scientific credibility. In particular, the movie Contact, based on his book, has done much to popularize the notion of looking for and communicating with extraterrestrial civilizations. Science fiction movies (e.g., Star Wars), television (e.g., Star Trek), and books have also lent support to such notions and have helped these ideas to cross over from dry academic discussions into popular culture. NASA’s spectacular successes of putting a man on the moon and the robotic exploration of other planets in our solar system have brought many of these ideas right into people’s homes and have made believing in intelligent life on other planets seem more plausible.

This emerging popular cultural belief in the existence of extraterrestrial life can be traced in large part to two important developments in the 1950s, one in astronomy and one in chemistry.[1] For astronomy, it was the resurgence of the nebular hypothesis that suggested that planets would readily form around most stars. In the previous decades, the prevailing model held that planets only formed under special circumstances and would therefore be rare in the universe. But if planets are common as suggested by the nebular hypothesis, that reopened the possibility of life on other planets. For chemistry, it was the Urey-Miller experiment in 1952. By simulating Earth’s presumed early atmosphere in a glass apparatus and sparking it with electricity, they were able to produce noticeable quantities of amino acids and other organics. This suggested that the production of simple biomolecules would naturally be present on most planets thus facilitating the later emergence of life.

In September 1959, Cocconi and Morrison published a ground-breaking paper describing rudimentary ideas on how to detect and communicate with beings on other planets via radio waves using existing technology.[2] Frank Drake led the first official search for extraterrestrial signals in 1960.[3] Dubbed Project Ozma,[4] Drake monitored two nearby stars (Epsilon Eridani and Tau Ceti) for four months but found no extraterrestrial signals. The following year, Frank Drake and J. Peter Pearman organized the first ever SETI (Search for Extra-Terrestrial Intelligence) conference at the Green Bank Observatory in West Virginia.[5] In attendance were ten scientists, including Carl Sagan and Nobel Prize winning chemist Melvin Calvin.

In preparation for the conference, Drake developed his now famous Drake equation to help facilitate discussion about which stars might be the best candidates for investigation. It was a simple equation containing seven terms that when multiplied together provided an estimate for the number of intelligent communicating civilizations in our galaxy right now. (This equation will be discussed in detail later.) Using it, the group came up with a range of possibilities, but their best estimate was 1,000 to 100,000,000 communicating civilizations.[6] In 1965, Drake provided a more cautious estimate that there should be 1,000 to 15,000 intelligent communicating species.[7] If these civilizations were evenly distributed throughout the galaxy, then the nearest one would be located between 775 and 2,000 light-years away. In a book published a year later, Iosef Shklovskii and Carl Sagan provided another highly optimistic estimate—one million intelligent communicating civilizations in our galaxy.[8] That would place the nearest one a mere 200 light-years away from us. If any of these projections were correct, then the chance of detecting an extraterrestrial signal would be very high.

Since Project Ozma, SETI projects have expanded dramatically in both scope and sophistication. Some representative examples include Harvard’s Project META and Project BETA, UC Berkeley’s Project SERENDIP and SETI@Home, and SETI Institute’s Project Phoenix. Starting in 2016, UC Berkeley’s Breakthrough Listen is the most comprehensive SETI initiative to date. More detailed descriptions of SETI’s mission and philosophy can be found elsewhere.[9]^{,[10],[11],[12]}

Of great concern is a growing lack of skepticism toward claims of extraterrestrial life. This is well illustrated by the report of possible remnants of life in Martian meteorite, ALH84001 in 1996.[13]^,[14]^,[15] Before the scientific community could respond, the discovery was hailed by President Clinton and the newspapers were filled with many grand statements about the possible evidence for life on Mars. Some even used the initial report to support their own personal philosophical ideas.[16]^,[17] Enthusiasm over the Mars rock has since quieted down as evidence has accumulated against the original conclusion.[18]^,[19]^,[20] Other examples of skeptical or hyped reporting abound. The Mars rover and other studies indicating that Mars may have had liquid water in the past are being popularized as evidence that Mars may have hosted life in the past. Similarly, the finding of evidence suggesting the possible presence of sub-surface liquid water on Europa, one of Jupiter’s moons, has led to speculation about the possibility of life there.[21] Even the comets, like Hale-Bopp, have been promoted as playing a role in the origin of life by pro-extraterrestrial enthusiasts.[22]^,[23]

This pattern of sensationalized reporting can also be seen in the recent excitement surrounding the unusually strong and irregular dimming around Tabby’s Star (officially KIC 8462852). A group of citizen scientists led by astronomer Tabetha S. Boyajian discovered the unusual results while sifting through data from the Kepler space telescope in September 2015. The group published a paper detailing their findings and proposed seven possible explanations although none were completely satisfactory.[24] The story turned sensational when astronomer Jason Wright suggested that the dimming could have been caused by a “swarm of megastructures.”[25] In that scenario, a truly advanced civilization might have constructed giant orbital solar collectors that could block out light and be responsible for the observed dimmings. Later measurements, however, refuted this by demonstrating that the dimming was caused by fine particles (dust), not a solid object.[26]^,[27]

Yet another recent example was the discovery in November 2017 of the first known interstellar object passing through our solar system. Named 1I/2017 U1 by astronomers but popularly known as ‘Oumuamua (Hawaiian for “scout”), it was a cigar-shaped object: 300-3,000 feet long and approximately 115-548 feet in width and thickness. It became controversial when theoretical physicist Abraham “Avi” Loeb proposed that it was an actual alien spacecraft.[28]^,[29],[30] The ‘Oumuamua ISSI Team, however, disputed Loeb’s claims and showed that the information about the object is consistent with it being a purely natural object.[31] As with the previous examples, the media seldom publicize such reversals of previously reported conclusions.

In addition to all this, there are regular reports of UFO sightings and abduction claims that thrive on the public’s belief in extraterrestrial life. While both SETI researchers and the UFO proponents affirm a belief in the existence of extraterrestrials, they differ significantly over whether we have been visited by these beings. Astronomers point out the enormous difficulties that would be experienced by an advanced civilization trying to travel to Earth. These problems include enormous distances to cross, inability to even approach the speed of light using foreseeable technology, long-term radiation exposure, multi-generational travel difficulties, and stellar hazards.[32] Given these considerations and the lack of real scientific evidence for visitation by alien spacecraft, SETI proponents conclude that we should look for extraterrestrial signals rather than spacecraft. Also, SETI is widely regarded as a scientific endeavor, whereas UFO claims are typically viewed as pseudo-science.

Why is the question of extraterrestrial life so important? Aside from the scientific question itself, which most likely cannot be answered definitively anytime soon, there is an ongoing debate in our culture concerning man and man’s place in the universe. Is man “special” in some way or does life arise spontaneously by purely natural processes? Are our planet, solar system, and galaxy exceptional or are habitable solar systems like our own commonplace in the universe? These are deep questions that were once in the realm of theology and philosophy only, but now scientists are weighing in on the issue. Unfortunately, many of the scientists who are shaping the public debate don’t have a strong theological or philosophical background. To make matters worse, reporters, philosophers, theologians, and other spokesmen for popular culture often lack the necessary scientific background to properly assess the reliability of these scientific claims. For this reason, it is important to go beyond the headlines to understand what the science demonstrates. That will enable one to stand on a solid foundation and not be tossed back and forth by the winds of popular culture.

The Copernican Principle

One thing is certain, many SETI proponents are firmly convinced that we will eventually find advanced alien life. For example, Frank Drake stated, “At this very minute, with almost absolute certainty, radio waves sent forth by other intelligent civilizations are falling on the Earth” (emphasis added).[33] In explaining his initial conviction about the existence of extraterrestrials, Drake wrote, “I could see no reason to think that humankind was the only example of civilization, unique in the universe.”[34] Carl Sagan and soviet astronomer Iosef Shklovskii were no less vehement about the certainty of extraterrestrial life. They wrote, “Given sufficient time and an environment which is not entirely static, the evolution of complex organisms is, in this view, inevitable. The finding of even relatively simple life forms on Mars or other planets in our solar system would tend to confirm this hypothesis” (emphasis added).[35] Sagan and Drake together declared, “There can be little doubt that civilizations more advanced than the earth’s exist elsewhere in the universe” (emphasis added).[36]

Why are Frank Drake, Carl Sagan, Iosef Shklovskii, and many others so convinced that alien civilizations must exist and by implication that habitable planets must be abundant? What is really at the heart of these claims? In an article examining the possibilities of extraterrestrial civilizations, the associate editor of Astronomy magazine, Robert Naeye, explained that the unstated assumption underlying SETI and their optimistic projections is a belief in the Copernican principle.

“On the surface, the most obvious evidence bearing on these questions [about the existence of extraterrestrial life] is the fact that our home world and host star seem so ordinary. Nicholas Copernicus shattered the prevailing notion that the Earth was seated at the center of creation. Succeeding generations of astronomers steadily reinforced the Copernican view as they discovered the true nature of the stars, the remote location of our home world within our Galaxy, and the existence of galaxies far, far beyond our own. So pervasive is this view that in the world of modern science, it is almost heresy to assert any special qualities to our solar system, our planet, and to ourselves. With an estimated 200 billion stars in the Galaxy … scientists and laymen naturally conclude that we could not be alone.”[37] (Emphasis added)

The Copernican principle holds that neither humanity, nor Earth, nor the Solar System occupy a privileged status in the universe—that there is nothing to fundamentally distinguish our Sun and planet from the myriads of others that inhabit our galaxy. We must therefore be cosmically mediocre. Based on this assumption, Carl Sagan concluded:

“Where are we? Who are we? We find that we live on an insignificant planet of a humdrum star lost between two spiral arms in the outskirts of a galaxy which is a member of a sparse cluster of galaxies, tucked away in some forgotten corner of a universe in which there are far more galaxies than people.”[38]

This perspective views the Earth as just a single grain of sand on a giant cosmic beach with nothing significant to distinguish it from any other grain of sand. If the Earth is cosmically average yet has life, then there should be millions of “Earths”—each with its own intelligent life forms.

The Drake Equation

The embodiment of the Copernican principle and SETI thinking is the Drake equation. Starting with the Darwinian paradigm of biological evolution, Drake assumed that life is virtually guaranteed to spontaneously arise if certain basic conditions are present. Considering that there are an estimated 400 billion stars in our galaxy, the Copernican principle essentially guarantees that there will be a multitude of planets with conditions that are friendly toward life. Given the enormous number of places to search, Drake realized that it would be critical to narrow the focus of their search to only those stars that had the greatest chance of supporting life. Drake developed his equation for the first SETI conference in 1961 as an attempt to reduce the number of sites on which SETI astronomers should examine by eliminating those that are the least likely to be habitable.

Figure 1: Frank Drake.

The starting point for this analysis is the recognition that the laws of physics are universal and therefore any conceivable lifeform anywhere must obey the same scientific laws that we experience on Earth. Based on our current understanding of chemistry, there are two almost-universally-agreed-upon prerequisites for life: carbon and liquid water. (Regarding the possibility of other life chemistries, see What about “Weird Life”? A Chemist’s Perspective on page 64.) This need to maintain water in a liquid state eliminates from contention planets that are located either too close or too far from their parent star.

Extending this idea a little farther, Drake developed his equation to express in measurable terms the probability of finding intelligent life on planets elsewhere in the galaxy and making contact with them. Based on his equation, it was estimated that the galaxy should be teeming with life. Estimates ranged from thousands to hundreds of millions of possible extraterrestrial civilizations in our galaxy alone—thus lending legitimacy to SETI investigations. While these original estimates may have been wildly optimistic, many still believe that advanced extraterrestrial civilizations are abundant.

Figure 2: The Drake Equation (1961).

Frank Drake’s original equation contained just 7 factors (see Figure 2), but some more modern versions sometimes include an eighth term, f_s, to reflect current thinking. For the purposes of this paper, we will utilize the following amended version of the Drake equation that expresses the number of extraterrestrial civilizations (N) in our galaxy that we could potentially contact as:

where:

N is the estimated number of intelligent communicating civilizations in our galaxy.

R_* is the rate of star formation in our galaxy (in stars/year).

f_s is the fraction of stars that are suitable for hosting habitable planets.

f_p is the fraction of suitable stars with planetary systems.

n_e is the number of planets, per solar system, with an environment suitable for life.

f_l is the fraction of suitable planets on which life appears.

f_i is the fraction of life-bearing planets on which intelligent life emerges.

f_c is the fraction of civilizations capable of interstellar communication.

L is the lifetime of communicating civilizations (in years).

Each of these symbols represents a factor that affects the predicted number of intelligent communication civilizations. Conceptually, the Drake equation starts with every star being an equally valid candidate. Each factor in the equation is then applied in turn to filter out those deemed unlikely to support advanced alien civilizations. For example, f_s eliminates from contention those stars that fail to provide an environment suitable for planets capable of hosting advanced civilizations. The next factor, f_p, then eliminates suitable stars that do not have planets needed to support life and so forth. Once all the factors have been applied, the stars remaining are those assumed to contain advanced civilizations that we could potentially contact.

None of the above factors are known with certainty. In each case, scientists hope to establish a reasonable order-of-magnitude estimate. The left most factor (R_*) is the most well defined and the remaining factors become increasingly uncertain as one progresses to the right. The biological, sociological, and technological factors (f_l_,f_i_,f_c_, and L) are very speculative because we have only one example—Earth life—as a basis for our understanding.

Let us examine each factor in the Drake equation according to how it was understood in the 1960s when SETI was just beginning. (See The Drake Equation: Estimating the Prevalence of Extraterrestrial Life Through the Ages for a thorough discussion of each of these terms and their estimated values.)[39]

R_* represents the rate of star formation in our galaxy (in stars/year).

Of all the variables in the Drake equation, only R_* can be stated with a high degree of certainty. Back in the 1960s, it was estimated to be around 1-5 stars/year although more contemporary estimates place this at 2-16 stars/year. While this number may seem small, the cumulative number of stars formed over the lifetime of our galaxy is staggering. Astronomers estimate that our galaxy alone contains roughly 400 billion stars. This number is so large that it boggles the mind. Some would argue that this number alone virtually guarantees the existence of life elsewhere in our galaxy since even highly improbable events, such as someone correctly picking all the numbers in a lottery, will be probable if there are enough attempts. As we will see later, there are many factors that must be weighed before we can make any reliable conclusions.

Frank Drake originally intended this factor to be the rate of stars formed 4.5 billion years ago, the same age as our own Sun. That would have restricted consideration to hypothetical civilizations whose potential level of development would be comparable to our own to simplify considerations. Star formation was more common earlier in the galaxy’s history.

f_s is the fraction of stars that are suitable for hosting habitable planets.

This factor represents the fraction of stars that are suitable suns for planetary systems. (It was not present in the original 1961 Drake equation but was added later to reflect the growing understanding of the importance of the star type to habitability.) Dead or dying stars (e.g., black holes, neutron stars, and white dwarfs) are obviously poor candidates for hosting planets with advanced life. Other stars may be rejected because they are too cool (such as red M type stars) and others because they are too short-lived (such as massive blue stars, types O and B). Based on these criteria, only about 10-20% of stars would be classified as suitable for life.

f_p is the fraction of suitable stars with planetary systems.

Only suitable stars that contain planetary systems need to be considered, because life presumably requires a planet on which to develop. Observations of the Orion nebula by the Hubble telescope have been able to see stars with proto-planetary disks that will eventually condense into planets. Based on this and other evidence available in 1961, it was estimated that about half of all stars might contain planetary systems.

n_e is the number of planets, per solar system, with an environment suitable for life.

In the 1960s, it was generally agreed that a planet must satisfy at least two essential properties to have a chance at supporting living organisms. First, a planet must be able to support liquid water because that is a key requirement for life. While water is abundant throughout the galaxy, surface liquid water is scarce. We know that planets that are too close to the sun (e.g., Mercury and Venus) are too hot to maintain liquid water while planets too far from the sun (e.g., Mars) are so cold that any water would be frozen. Between these two extremes is the circumstellar habitable zone (a.k.a. the “Goldilocks” zone) where the planet is at just the right distance from the star to maintain liquid water. Second, a planet must be the right size. Very small planets (e.g., Mercury) are unable to retain a significant atmosphere while massive planets (e.g., Jupiter, Saturn, Neptune, and Uranus) will have such a thick atmosphere that it will make life impossible there. For our solar system, typically only the Earth is viewed as satisfying both criteria (n_e ≈ 1); however, at that time some would have also included Mars and Venus (n_e ≈ 3). A few scientists gave an even more optimistic estimate by including the some of the moons of Jupiter and Saturns as possible life sites (n_e ≈ 5).

f_l is the fraction of suitable planets on which life appears.

SETI enthusiasts believe that given a planet that is kept at the right temperature for liquid water and contains simple organic molecules, life will spontaneously arise. The discovery of organic molecules in space is taken as evidence that such molecules are sufficiently abundant, and hence is taken as evidence for a large value of f_l. Since life appeared suddenly and early on Earth under very harsh conditions, then perhaps life can form elsewhere under similarly harsh conditions. The value of f_l is truly unknown, but SETI proponents generally consider it to be rather high—50% or higher.

f_i is the fraction of life-bearing planets on which intelligent life emerges.

If a planet other than our own contains life, will it develop beings with sufficient intelligence to communicate with us? This is a difficult question because we only have Earth-life to serve as a model. Some believe that intelligence is a natural consequence of evolution and so f_i could be as high as 20-100%. In support of this conclusion, some point out that chimpanzees, dolphins, and some bird species have high levels of intelligence. The value of f_i is unknown and is highly speculative.

f_c is the fraction of civilizations capable of interstellar communication.

The term “communicating” here refers to the ability to send and receive signals from other solar systems. Almost certainly, interstellar communication would be in the form of electromagnetic radiation. A civilization with advanced electronics (or similar technology) would likely emit detectable signals (such as radio and TV broadcasts)—even if they were not intentionally trying to broadcast a message. Cocconi and Morrison2 proposed radio waves as the best place to search for extraterrestrial signals, but later SETI astronomers expanded their searches to include the infrared and even optical portions of the electromagnetic spectrum. The value of f_c is unknown but is believed by SETI proponents to be high (perhaps 20-50%).

L is the lifetime of communicating civilizations (in years).

Even if intelligent beings with the ability to communicate with us were to evolve elsewhere in the galaxy, there may only be a brief window of opportunity for us to contact them. They might simply stop broadcasting strong signals or develop technologies that do not utilize radio waves. A more somber possibility is that advanced civilizations might destroy themselves before we could make contact. Some note that here on Earth, the development of nuclear weapons occurred concurrently with the ability to send and receive extraterrestrial signals. A nuclear war could wipe out a civilization or drive it back to the stone ages. Severe environmental pollution and large asteroidal collisions are additional dangers that could devastate an advanced civilization to the point that they were no longer able to communicate. As such, pessimists might argue for a small value of L, perhaps only a few hundred years. However, Carl Sagan, Frank Drake, and most SETI proponents take the optimistic view that most advanced beings will manage to avoid destroying themselves and consequently, L might be thousands or even hundreds of millions of years. Some even take this one step farther, by suggesting communicating civilizations that do survive might be able to guide less advanced civilizations and help them avoid self-destruction.[40] In the absence of any real information about other beings, the best we can do is set a minimum value for L of about 60 years, the length of time that we have had the technology to communicate with extraterrestrials.

Estimates of the Number of Communicating Civilizations

The first SETI conference in 1961 provided the earliest attempt to assign values to the Drake equation. Two additional early estimates come from Frank Drake’s 1965 paper and a book by Iosef Shklovskii and Carl Sagan that was published in 1966. Even 35 years later, their optimistic appraisal has diminished little as is illustrated below by the comparison to estimates taken from two web sites in 1996.

Table 1: Various estimates based on the Drake Equation

	SETI Conference6 (1961)	Frank Drake7 (1965)	Carl Sagan8 (1966)	Active Mind[41] (1996)	SEDS[42] (1996)
R_*	1	1	10	10	20
f_s					10%
f_p	20-50%	50%	100%	20%	50%
n_e	1-5	2-3	1	3-5	1
f_l	100%	100%	100%	50%	50%
f_i	100%	100%	10%	20%	100%
f_c	10-20%	100%	10%	10-20%	50%
L	1,000-100,000,000	1,000-10,000	10,000,000	10,000	> 60
N	20-50,000,000	1,000-15,000	1,000,000	600-2,000	> 15

This is just a small sampling of possible solutions of the Drake equation. These represent the most optimistic possibilities. Other sources take a more cautious or pessimistic approach with some holding N to be essentially zero suggesting that we are alone in the galaxy. This is not surprising given that if even one factor is zero then N will be zero. Of course, the values whether high or low reflect the biases and presuppositions of the estimator.

Detecting Other Civilizations

If intelligent communicating civilizations are as prevalent as suggested by the Drake equation, then how might we find them? There are approximately 400 billion stars in our galaxy that could be targeted in searches. And what frequencies should we scan for signals? There is an enormous range of choices to consider because we have no way of knowing at what frequencies an alien civilization might broadcast. And what is the best strategy to employ to maximize our chance of detecting alien signals?

SETI researchers have given these questions a great deal of thought over the last six decades. Moreover, their technological sophistication has grown by leaps and bounds since the days of Project Ozma. Here are some specific ways that SETI’s capabilities have expanded:[43]

Technological improvements. SETI now has access to more telescopes than ever before and ones with greater detection sensitivities. Detection protocols have likewise dramatically improved to better eliminate spurious signals.
Multi-frequency searches. Because we cannot know at which frequency aliens might be broadcasting at, it is important to simultaneously scan a range of frequencies. As an example, Project META (1985-1995) could simultaneously monitor 8.4 million individual frequencies while Project BETA (1995-1999) expanded that to a billion frequencies.
Wide-sky and targeted searches. Wide-sky searches scan a large portion of the sky allowing millions of potential stars to be examined in a short period of time. Targeted searches focus on specific candidates of interest for a longer period of time thus improving the chances of detection. A mix of both strategies should be utilized because each is useful but under different circumstances.
Exploring infrared and visible light. At its inception, SETI only considered searching for radio wave communications, but today some are supplementing that strategy by looking in the infrared and visible portions of the spectrum.
Looking for technosignatures and biosignatures. Traditionally, SETI has been focused on detecting intentional broadcasts. An alternative approach is to look for indirect evidences—ones that are a byproduct of other activities, such as technosignatures and biosignatures (as discussed below).

Even with all this, there are many ways that SETI could still fail to find genuine extraterrestrial signals (they are too far away from us, their signal is too weak, we did not look at the correct frequency, etc.). SETI is still in its infancy and much work remains to be done.

The Kardashev Scale

One major challenge facing those searching for signals from advanced extraterrestrials is the notion that some civilizations could be far older and more advanced than our own. We know that there are many stars resembling our sun but were birthed billions of years earlier. That means that if intelligent life were to develop on a planet orbiting one of these stars, it would have an enormous head start in its development. Such a civilization could be millions or even billions of years more advanced than us in their technology. Given this almost unfathomable range of possible technological sophistication, how do we account for this in our search strategies?

To begin, we need to understand that our ability to detect a particular signal depends mainly on three things: (1) the sensitivity of the detector; (2) the broadcasting power of the sender; and (3) the distance between us and them. Because we know the detection sensitivities of our receiving telescopes, astronomers can determine for a given broadcast strength how far away the sender could be and still be detected. The big question then is what can we say about the strength of a hypothetical signal? Given that unknown alien civilizations could possess a wide range of possible technological levels, how do we relate that to their potential broadcasting power?

In 1964, Soviet astronomer Nikolai Kardashev came up with an elegant way to model this. Known as the Kardashev scale, he proposed three different levels of advanced technology and estimated how much power each could utilize in broadcasting.[44] This scale was later generalized by Carl Sagan.[45] Others have proposed extensions to apply it to even more technologically advanced civilizations. Let us consider Kardashev’s three types of technological civilizations (see Figure 3):

Kardashev type I civilization (planetary civilization). Can use the energy in broadcasting equivalent to all the energy from its parent star reaching the planet’s surface (10¹⁶ watts).
Kardashev type II civilization (stellar civilization). Can utilize energy at the scale of its planetary system (10²⁶ watts).
Kardashev type III civilization (galactic civilization). Can control energy at the scale of its entire host galaxy (10³⁶ watts).

To put this in perspective, the Arecibo telescope (representing current Earth-level technology) was capable of broadcasting 10¹⁴ watts prior to its recent demise. On the Kardashev scale, that ranks us as only a type 0.7 civilization. It will likely take 100-200 years before we can develop into a type I civilization and a few thousand beyond that to become a type II.

Figure 3: Kardashev Type I, II, and III Civilizations.

Detecting Technosignatures

While SETI has traditionally focused on detecting alien broadcasts, an alternative strategy is to look for their technosignatures, namely any measurable property indicative of technological activity. This is sometimes referred to as artifact SETI.[46]^,[47] One possible technosignature is large scale astroengineering. In 1960, Freeman Dyson pointed out that extremely advanced civilizations will need to gather more energy from the sun than falls on the surface of their planet.[48]^,[49] One way to do that is to build giant orbital solar collectors around their star. In the same way, the aliens might construct numerous orbital space habitats to expand their population. As more and more of these megastructures are constructed, they will eventually completely enshroud the star forming a Dyson swarm.[50]

These megastructures could potentially be detected in one of two ways. First, we can look for transit dimmings. Satellites, such as the Kepler space telescope (now defunct) and the Transiting Exoplanet Survey Satellite (TESS), can monitor the brightness of a large group of stars over a long period of time looking for brief drops in brightness that could represent the presence of an associated planet. These satellites are capable of detecting components of a Dyson swarm as they pass in front of their star (as was alleged for Tabby’s star). Second, a nearly complete Dyson swarm would block out most of the visible light from the star. After all the collected energy has been used, the resulting waste heat would likely be radiated as infrared light. This would result in infrared anomalies that could be detected as Dyson had proposed.

Extending the Dyson swarm idea even further, consider a civilization that has reached type II status having encompassed their parent star in a Dyson swarm. To harness even more energy and allow the population to grow, they would need to expand to other nearby stars and begin building Dyson swarms around them as well. Continuing this process, would result in a readily noticeable empty patch with few stars being visible but shining brightly in the infrared instead. These cosmic voids would be readily noticeable using standard optical telescopes, but none have been detected so far. The absence of these features—known as the Dyson dilemma—argues against the existence of type III and highly advanced type II civilizations, in our galaxy and all the nearby galaxies.

One additional technosignature to consider is industrial pollution in the planet’s atmosphere. For example, the industrial revolution (starting in the 1800s) placed a lot of soot in the air from burning coal. Of course, this would be extremely difficult to detect.

Alternatively, if a civilization wanted to be detectable over interstellar distances, they could inject about 100,000 tons of short-lived radioactive species into the star.[51] This marker would be readily identifiable in the spectra of the star. Alternatively, creating an enormous solar collector or sun shield in a non-circular shape would produce unusual transit dimming patterns that would be detectable from hundreds of light-years away.

Detecting Biosignatures

Another search strategy is to look for biosignatures, that is, any detectable substance that would indicate past or present life. In this study, we are only considering biosignature gases, i.e., gases produced by living organisms that accumulate in the atmosphere and can potentially be detected remotely by space telescopes. As will be described later in this paper, we do have the technology to obtain the spectra of light passing through the atmosphere of an exoplanet to determine which gases are present. (Unfortunately, our current capacity to do this is extremely limited for both technological and astronomical reasons.)

Living organisms operating over long periods of time can produce measurable changes in the composition of the atmosphere. For example, Earth’s atmosphere is currently about 20% oxygen (O₂) as a result of photosynthetic activity. If an alien civilization analyzed our atmosphere, the presence of such a large amount of oxygen would strongly suggest the presence of life here. Unlike the other detection strategies, this could potentially suggest the presence of even simple microorganisms billions of years before the advent of technology. Conversely, an absence of biosignature gases can potentially be used to rule out the presence of life on a planet.

While looking for biosignature gases offers an exciting new avenue of study, it does have some serious limitations. Aside from the extreme difficulties in obtaining the atmospheric spectra, the presence of specific gases rarely constitutes incontrovertible proof of life on a planet because many gases can be produced by non-biological processes. For example, oxygen can be generated from the breakdown of water by ultraviolet light. Methane (CH₄) is another potential biosignature gas, but it too can be produced by abiotic processes. Let us consider two examples in our own solar system:

Methane on Mars. A small amount of methane was discovered in the Martian atmosphere in 2003. Since methane should break down quickly on Mars, there must be a source to regularly replenish it. One possibility is that it is being generated by methanogenic organisms living deep below the surface, however, there are possible non-biological sources of the methane.
Phosphine on Venus. In September 2020, researchers using the Atacama Large Millimeter Array (ALMA) telescope reported the presence of phosphine gas (PH₃) in the upper atmosphere of Venus. This could be explained as the byproduct of anerobic bacteria residing in the cooler upper atmosphere because there are few abiotic sources for the gas. However, others have vigorously challenged this interpretation.

Both claims are fiercely contested with no clear way to resolve the debate at this time.

Could an Advanced Civilization Detect Us?

While SETI is focused on detecting signals from possible alien civilizations, what about the reverse? Could advanced aliens detect the presence of life on Earth from their remote haven hundreds of light-years away? This thought experiment is instructive because we already know what could be found, so it can serve as a reference point.

In 1966, Shklovskii and Sagan considered the hypothetical situation of a Martian trying to detect intelligent life on Earth using the scientific instruments available at the time.[52] This imaginative exercise illustrates some of the difficulties of trying to detect life at interplanetary distances. And if it is difficult at such close range, how hard would it be to try to identify potential intelligent beings on planets thousands of light-years away? The good news is that while the challenges are immense, much progress has been made. There are several ways that extraterrestrials could determine our presence here on Earth:

Radio leakage. TV broadcasts starting in the 1940s leaked radio waves into outer space. These weak signals would be extremely difficult to detect with existing technology, but a more advanced civilization might be able to pick it up. Regardless, these signals have only reached stars up to 80 light-years away. At half that range, a potential civilization could have received the signal and sent a reply that we could detect now.
Planetary habitability. Earth resides in the circumstellar habitable zone of a yellow star. An alien civilization might recognize this as a prime location for further investigation. Of course, we cannot be certain how hypothetical beings would view our circumstances because it may not correspond to their own concept of planetary habitability. Regardless, Earth’s favorable properties would have been identifiable for billions of years and could be noticed from nearly anywhere in the galaxy using technology just barely more advanced than our own.
Biosignatures. The presence of large quantities of oxygen and water along with significant amounts of methane in Earth’s atmosphere would provide strong evidence for life here. These biosignatures would have been detectable for at least the last half billion years using only foreseeable technology.
Technosignatures. Aside from radio leakage, there are few identifiable technosignatures and all have a recent origin. For example, the International Space Station (ISS) is way too small to be noticeable from interstellar distances. Pollution in the atmosphere might be measurable starting a few centuries ago, but the technological difficulties of detecting it make this very unlikely.
Intentional broadcasts. Some deliberate broadcasts have been sent with the intent of communicating with possible alien civilizations. The most notable example is a message designed by Frank Drake that was broadcast in 1974 using the Arecibo telescope.[53] This signal was very brief and directed toward M13 (a globular star cluster), so the likelihood of it being detected is extremely small. CETI (Communication with Extraterrestrial Intelligence) is a branch of SETI that focuses on composing and deciphering interstellar messages that theoretically could be understood by another technological civilization.

Of all these, planetary habitability and biosignatures are the most significant, because they are potentially detectable from nearly anywhere within the galaxy and at any time over the last few billion years.

Consider that in just the 60 years since SETI started, we have made amazing progress in being able to scan the heavens for signals and have already found thousands of extra-solar planets. Therefore, it is reasonable to estimate that it would take us no more than about another 100 years for us to have a reasonably complete catalogue of planets in our galaxy and their possible biosignatures. And the difference in technological development between civilizations would almost certainly be measured on a cosmic scale—tens of thousands or even millions of years, rather than hundreds. We can, therefore, safely conclude that advanced aliens would have the time and capability of finding solid evidence for life on Earth.

The discovery of biosignatures on Earth alone would not constitute proof of life, nor of our technological capacity, however, it would certainly merit a more concerted investigation to look for more decisive evidence. Let us consider how this might influence their search strategies. Prior to discovering a planet with an undisputed biosignature, a broad, general approach would be utilized, similar to our current SETI strategy. But if persuasive evidence for life were found somewhere it would shift to a much more focused effort:

Targeted search. With such a specific target, a tightly focused search would be merited. Long and detailed investigations would be made at different frequencies and using different instruments to gather as much evidence as possible about the planet.
Targeted broadcast. A very tightly focused beam could be used for contact, that would make it much easier for the signal to be detected on Earth.
Probes and spacecraft. If the evidence for life becomes truly compelling, then it would make sense to go to the extraordinary cost and effort of sending robotic probes and eventually manned spacecraft. This step would be even more imperative if interstellar communication could not be established.

This thought experiment has two important implications. First, it helps us consider how our search strategy might change if we were to find strong evidence for life on another planet. Second, it leads to what I call the observation paradox: if we can find them, they will have already found us.

This paradox was first expressed by Ben Zuckerman in 2002.[54] His thinking was inspired by the then proposed Terrestrial Planet Finder (TPF) space telescope. The TPF was later cancelled in 2011, but the more recently launched James Webb Space Telescope (JWST) fills the same role. His argument rests on three simple postulates:

Space telescopes. Soon after the development of technology, all civilizations will build space telescopes capable of measuring the atmospheric biosignatures of Earth-like worlds at distances of hundreds of light-years.
Curiosity. Intelligent life is curious about other life forms, whether or not that other life is technological. Once having used space telescopes to discover a nearby living planet, most if not all technological civilizations will want to make contact.
Visitation. If the other planet lacks interstellar communication capability, then the aliens would send probes or even spacecraft to visit them rather than passively waiting for the other civilization to develop sufficient technology.

According to this line of reasoning, if there is an advanced civilization close enough for us to detect their home planet, then the aliens should have already learned of our presence and sent probes to make contact. The absence of robotic messengers in our solar system represents a paradox implying that there are no nearby civilizations. Interesting, but few find it persuasive.

A weaker version of this argument concludes that any extraterrestrials with technology even a century beyond our own would almost certainly know that we are here. They would be sending focused broadcasts toward us thereby greatly increasing our chances of observing them. Consequently, a failure to detect alien signals would strengthen the case against nearby extraterrestrial civilizations.

SETI in the 21^st Century: 60 Years of Searching

Given the bold estimates in the 1960s suggesting that there might be ten thousand to a hundred million communicating civilizations in our galaxy, the expectation of detecting one of them seemed very high and should have occurred relatively quickly. For example, Frank Drake optimistically predicted that we would detect alien signals by the year 2000.[55] Nevertheless, those performing the early searches had little idea of what to expect because no one had attempted this endeavor before. Now that SETI projects have been actively searching for more than 60 years, just what have they found so far?

Frank Drake’s Project Ozma in 1960 was a simple affair. He just pointed a radio telescope at two nearby stars and listened for any unexpected signals. Much to his great surprise, he soon detected a strong signal and was even able to redetect it weeks later. Of course, after some sleuthing, he learned that he was picking up military aircraft broadcasting on an unauthorized frequency.[56] This amusing anecdote is a good reminder that one of the greatest challenges faced by the various SETI programs is the need to distinguish between a potentially genuine signal and a spurious detection caused by the ever-increasing number of manmade radio sources.

Unknown natural radio sources represent another potential source of false alarms. For example, signal CTA-102 was a strong radio signal detected in the 1960s. (CTA designates it as coming from a California Institute of Technology radio survey.) In 1964, Russian astronomer Nikolai Kardashev suggested that it was coming from an alien supercivilization.[57] Later work, however, determined that the strong radio source was coming from a previously unknown natural class of objects known as a quasars.[58] A second false alarm came in November 1967 when Jocelyn Bell Burnell discovered a regular signal resembling an extraterrestrial beacon that was later determined to be coming from a radio pulsar, PSR B1919+21. (Bell and her PhD supervisor Antony Hewish jokingly nicknamed the signal “little green men 1” or LGM-1.) A third example is the Lorimer Burst found by Duncan Lorimer and his student in 2007 while looking through pulsar survey data. Astronomers now recognize it as the first example of a fast radio burst.

Figure 4: Locations of the five strongestcandidate signals from project META.

Over the course of the last six decades, various SETI projects have processed copious quantities of data from multiple telescopes. The biggest challenge is sorting through all that information to pluck out signals of interest. This is comparable to trying to find the proverbial needle in the haystack. First, from the raw data one must filter out simple electronic “noise” and instrumental effects. Next, one must remove from consideration spurious detections caused by either terrestrial radio signals or natural sources. A true extraterrestrial signal should (a) be stronger than background noise; (b) occupy a narrow frequency band; and (c) display a frequency doppler shift indicating a non-terrestrial origin. A fourth filter used by advanced searches is (d) to quicky follow up potential candidates using a second telescope to rule out local causes. Of course, even satisfying all these criteria does not guarantee that a candidate signal must be coming from extraterrestrial beings. Therefore, (e) the final and most crucial test is attempting to redetect the signal sometime later. Without redetection, a signal no matter how mysterious cannot be considered a genuine alien signal.

Let us, consider the 1993 initial release of results from Project META, the first continuous high-resolution SETI study. Accumulating 60 trillion observations over the course of five years, META found 37 strong and narrow spectral features that passed all the initial detection criteria.[59] After eliminating weak signals (those just above the minimum acceptance threshold), they were left with 11 strong candidates that lacked any obvious manmade or natural explanation. Notably, the five strongest candidates all lie near the plane of the Milky Way galaxy (see Figure 4).[60] All 37 candidates were turned over to other groups for follow up observations. To date, none of these signals have been redetected and so cannot be deemed to be the products of extraterrestrial technology.

Since that time, however, the various SETI endeavors have greatly increased in scope, sensitivity, and sophistication. Fortunately, authentication protocols have improved considerably over these decades. For example, Project Phoenix scanned 800 stars within 200 light-years of Earth without any residual mysterious or unexplained signals. This null result is considered good news by SETI proponents because it means that their sophisticated authentication protocols are now sufficiently robust to recognize and exclude terrestrial interference.[61] That means that if a future signal passes these rigorous criteria, then it will deserve serious consideration.

Figure 5: Print out of the “Wow!” signal.

Of all data collected over the last six decades, the 72-second “Wow!” signal remains the best candidate for an alien radio signal. The signal received its name when astronomer Jerry Ehman noticed it while analyzing the detector printout and wrote “Wow!” beside it (see Figure 5).

This signal was detected in 1977 by Ohio State University’s Big Ear radio telescope. It was a strong (30 times background) unmodulated narrowband (< 10 kHz) signal that lasted for the full 72-second window before the Earth’s rotation moved it out of the detection zone. No one has been able to redetect it despite multiple attempts.[62] A number of hypotheses have been advanced to explain the signal, but none have gained widespread acceptance.

Since then, the only serious candidate signal has been Breakthrough Listen Candidate 1 (BLC1) reported on Dec 18, 2020. It was detected by the 210-foot Parkes radio telescope coming from the direction of Proxima Centauri. This was particularly exciting because Proxima Centauri is our nearest stellar companion only 4.24 light-years away and is known to have a potentially habitable planet, Proxima Centauri b. Astronomers, however, quickly downplayed the event proposing that it was probably a humanly created radio source.[63]

The search for biosignatures and technosignatures is still in its infancy, so it is not surprising that no examples have been confirmed so far. The most ambitious attempt to date to detect infrared technosignatures is a targeted search of 144 exoplanetary systems and a broad scan of 3 million stars near the galactic center but it failed to detect any technosignatures.[64]

Looking even farther out, Penn State’s project Ĝ (G-HAT or Glimpsing Heat from Alien Technologies) has been searching for advanced type III civilizations in other galaxies by looking for their waste heat in the mid-infrared. Using the catalog of galaxies provided by the Wide-field Infrared Survey Explorer (WISE) survey, they focused on 100,000 galaxies looking for excess infrared and found 93 candidates.[65] Further analysis by a different group, however, strongly suggested that they are the result of natural processes.[66] That study concluded that “Kardashev Type-III civilizations are either very rare or do not exist in the local Universe.”

This issue was revisited by a joint study of astronomers at the National Astronomical Observatories of China and the Leiden Observatory in the Netherlands. In August 2021, the group reported an analysis of 16,367 galaxies and found that 21 had enhanced mid-infrared values with 4 being particularly strong.[67] Of the 21 candidates, 19 were sufficiently well-characterized to be ruled out as being the result of a type III civilization leaving the remaining two for follow up study. Contrary to some popular claims, there is no evidence that these two galaxies contain type III civilizations.

The Current State of SETI Progress

As of 2022, there have been no confirmed detections of alien civilizations. Just what should we make of this null result? On one hand, SETI has been operating long enough to conclude that this silence is significant. On the other hand, the search has not been going on nearly long enough, nor has it been performed with sufficient sensitivity, and therefore it would be premature to conclude that there cannot be other communicating civilizations out there. There are just so many places to look and a great many frequencies that we would need to monitor. We have not even come close to meaningfully exploring all these possibilities. So, how can we quantify how much have SETI searches managed to explore so far?

Figure 6: Extent of SETI searches as of the year 2000.

To answer that question, Andrew J. LePage generated a graphical representation of the extent of all SETI searches as of the year 2000 (see Figure 6).[68] The horizontal axis represents the sender’s distance from Earth in light-years on a logarithmic scale. The extent of the Milky Way galaxy and our local group of galaxies are marked for reference. On the vertical axis is the effective isotropic radiated power (the effective broadcasting strength) in watts; also shown logarithmically. Clearly marked are the broadcasting power of Earth-level (10¹⁴ watts), type I (10¹⁶ watts), and type II (10²⁷ watts) civilizations, using a variant of the Kardashev scale. Type III (10³⁸ watts) is not shown because it would be off the scale of this diagram. Color is used to represent the percentage of star systems that have been searched so far. Red represents areas that have been carefully searched while black represents those cases that are undetectable using current technology. The remaining colors represent partially searched regions.

We can see that the region that has been thoroughly searched is quite extensive, yet much more remains completely unexplored. Based on these results, LePage concludes that we can effectively rule out supercivilizations (type II or III) being present anywhere in our galaxy. However, the results do leave room for one hundred type I civilizations and perhaps as many as a million civilizations with a technology equal to our own. Clearly, there remains much to be done.

Figure 7: Extent of partial searches for Earth-level and type I civilizations.

Another way to understand these results is to visualize the spatial extent of the search relative to our Milky Way galaxy (see Figure 7).[69] The yellow circle represents 4,000 light-years, the distance for which we have made at least partial searches for civilizations with Earth-level technology but have detected nothing. The red line is the same for type I civilizations and extends out to 40,000 light-years.

In reflecting on these results, it should be emphasized that the above summary was based on the data available as of the year 2000. More than two decades have passed since then with no confirmed detections. That means that the constraint on possible extraterrestrial civilizations is even tighter than presented above. (Unfortunately, a more up-to-date version of the above analysis is not available.)

Fermi’s Paradox: “Where Is Everybody?”

In the summer of 1950, Nobel physicist Enrico Fermi came up with a simple yet remarkable challenge to the belief that the universe is full of advanced beings. On the way to lunch at Los Alamos National Labs, Fermi, Edward Teller, Herbert York, and

Figure 8: Enrico Fermi.

Emil Konopinski were talking about the possibility of flying saucers, faster-than-light travel, and extraterrestrial beings. During the lunch-time conversation with his three friends, Fermi interjected the question, “Where is everybody?”[70] Everyone immediately understood that the “everybody” referred to their conversation about extraterrestrial beings.

Fermi had reasoned that if extraterrestrial civilizations were as abundant as was assumed in his day, then many of them would have technology far beyond our own and would have spread out from their original solar system by colonizing other planets. Eventually the distant colonies would feel the need to spread out to even more distant solar systems. Given some reasonable assumptions, this would lead to an exponential expansion that would allow them to colonize the entire galaxy within a few million years—a cosmically brief span of time. So, where is everybody?

To state the Fermi paradox more formally, it is the apparent contradiction between the lack of evidence and high probability estimates for the existence of extraterrestrial civilizations. To understand this, let us unpack the logic behind it. Consider:

There are billions of stars in our galaxy like the Sun, and many of these stars are billions of years older than our solar system.
Many of these stars are expected to have Earth-like planets, and if the Earth is typical, some may have developed intelligent life.
Some of these civilizations may have developed interstellar travel, a step the Earth is investigating now.
Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.
Yet, we have no clear evidence of visitation, nor have we detected any extraterrestrial signals.

This is a paradox. There must be a flaw somewhere in this well-reasoned chain of logic. But where? The fourth proposition deserves closer analysis because there are no known examples on which to base our analysis. Just how realistic is it for a single advanced civilization to spread across an entire galaxy? And how quickly might this be accomplished? If galaxy-wide colonization is simply too difficult, then that is one possible way to explain the absence of extraterrestrials on Earth.

The distance between stars is enormous. Consider that the nearest star to Earth is Proxima Centauri some 4.24 light-years away. Currently, the fastest spacecraft created by mankind is the Voyager 1 space probe launched in 1977 and travels at 17.26 km/second (or 38,610 mph) away from the Sun. At that pace, it would take a spacecraft almost 73,000 years to make the journey to Proxima Centauri.[71] If future spacecraft were limited to that speed, then interstellar travel would be so extraordinarily difficult for humans that it would be effectively impossible.

Future technology should enable us to travel much faster, although our current understanding of physics fundamentally limits travel to near the speed of light. Reaching that speed would reduce the travel time to Proxima Centauri to a reasonable 4.24 years (although the travelers would experience a much shorter timespan due to time dilation). Unfortunately, getting a spacecraft capable of carrying human beings up to the speed of light is infeasible for many reasons. Yet, using only known physics and foreseeable technology, it should be possible to deploy spacecraft capable of reaching 10-20% the speed of light.[72] At that speed, the trip to Proxima Centauri would only take a few decades. With such technology, traveling to other stars would be extremely challenging yet manageable.

Figure 9: Intergalactic Colonization.

Now let us consider the scenario where an advanced civilization sends colonists to a few nearby star systems. After these colonies establish themselves, they can send out secondary colonies. Those colonies eventually send out colonies of their own and so on. This process can be continued indefinitely leading to an exponential growth with the outermost edge of colonization moving outward at enormous speed (see Figure 9).

Using this scenario, we can construct a plausible model for intergalactic colonization time relying on only three parameters.[73]

An average of 10 light-years between colonizable planets.
Ships that can travel at 10% the speed of light.
A period of 400 years between starting a colony and sending out a new colony of its own.

Based on these values, a single civilization could colonize our entire Milky Way galaxy in about 5 million years. On a galactic scale, this is a very brief amount of time (only 0.05% the age of the galaxy)!

Because the time it takes for a single civilization to spread across the galaxy is critical to the Fermi paradox, we need to consider factors that might speed up or slow down this process. Using different values for the three parameters leads to different colonization times, but even the most pessimistic choices place it at no more than 50 million years.[74] And there are several additional factors to consider that could greatly speed up this colonization process:

Faster-than-light travel? Although strictly theoretical, some believe that we will eventually be able to travel faster than the speed of light. The two most popularly suggested possibilities are wormholes and Alcubierre’s warp drive.[75] If this type of technology should prove feasible, the time it would take for aliens to arrive here would be dramatically reduced. Of course, these possibilities are extremely speculative and are only included for the sake of discussion.
Improving technology. The above model assumes that technology would be static over the course of millions of years, yet that is clearly unrealistic. Interstellar travel should certainly become cheaper, faster, and more reliable over time greatly accelerating a civilization’s ability to spread out across the galaxy.
Robotic probes. Robots can be expected to always precede biological beings to a given destination. They are cheaper and much better suited for the rigors of interstellar travel. For example, mankind has only made it as far as the moon, but our probes have visited all the planets in our solar system and five (Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and New Horizons) have already left the solar system.

One ambitious possibility is Breakthrough Starshot, an initiative to send a fleet of a thousand postage-stamp-sized probes to Proxima Centauri. Using solar sails powered by a strong ground-based laser, they could reach a speed of 15-20% the speed of light and arrive at Proxima Centauri in just 20 to 30 years.

Self-replicating (Bracewell-von Neumann) probes. Taking the robotic probe idea one step farther, a complex probe could be sent to a neighboring star system where it could mine materials to produce copies of itself that would be sent to more solar systems.[76]^,[77] They would spread exponentially as with interstellar colonization only on a vastly shorter timescale.

Each of these options has the potential to greatly reduce the time it would take for extraterrestrial aliens or their robotic probes to reach Earth. That serves to strengthen the case that advanced alien civilizations are rare at best. But even the pessimistic estimate of 50 million years for interstellar colonization does little to weaken the implications of Fermi’s paradox.

Addressing the Fermi Paradox

Based on the Drake Equation, many expected our galaxy would be brimming over with advanced alien civilizations, yet we have not detected any of their signals despite extensive attempts. Fermi’s paradox took this one step farther by arguing that we should have seen advanced aliens as well as heard from them. Yet we have not. How can we explain this apparent rarity of technological civilizations?

One early response comes from Michel Hart.[78] Although not addressing the Fermi paradox directly, he starts with “Fact A” that there are no intelligent beings from outer space on Earth now. But if advanced extraterrestrials exist, then how might we reconcile that with “Fact A”? He divides the possible solutions into four groups:

Physical explanations. All explanations which claim that extraterrestrial visitors have never arrived because of some physical, astronomical, biological, or engineering difficulty make interstellar space travel infeasible.
Sociological explanations. Explanations based on the view that extraterrestrials have not arrived on Earth because they have chosen not to. This category is also intended to include any explanation based on their supposed lack of interest, motivation, or organization, as well as political explanations.
Temporal explanations. Explanations based on the possibility that advanced civilizations have arisen so recently that, although capable and willing to visit, they have not had time to reach us yet.
Perhaps they have come. Those explanations which take the view that the Earth has been visited by extraterrestrials, though we do not observe them here at present.

Of these four, Hart finds both the physical and temporal explanations unconvincing for the reasons discussed in the previous section. Sociological explanations require us to make questionable assumptions about how potential alien societies might behave. Moreover, he finds these explanations inadequate because they are unlikely to apply to all alien civilizations. Lastly, he finds no support for the notion that aliens have either come in the past but left no traces or that they are currently visiting (i.e., the UFO hypothesis). Having considered and rejected all four sets of explanations, he concludes that we are likely alone in the galaxy, and that SETI is likely a waste of time and money.

A much more thoughtful and balanced presentation of proposed resolutions to Fermi’s paradox is found in Stephen Webb’s book, Where is Everybody?[79] He discusses 50 different solutions to the paradox that people have proposed and divides them in three distinct groups. These are listed along with some representative examples.

They are here. These solutions suggest that aliens may have already visited Earth—but either we haven’t noticed them, or they are cleverly keeping themselves hidden.
Zoo scenario
Interdict scenario
They exist but have not communicated. Perhaps advanced alien civilizations exist but have decided that traveling beyond their own star system is either too difficult or simply undesirable.
The stars are far away
They have not had time to reach us
We have not listened long enough
They are calling but we do not recognize the signal
They stay at home
Everyone is listening, no one is transmitting
They do not exist. The alternative conclusion is that either advanced life never arises or else it is wiped out before it can communicate.
Continuously habitable zones are narrow
A planetary system is a dangerous place
Jupiters are rare
The moon is unique
Life’s genesis is rare

To conclude his work, Webb presents the 50^th solution a synthesis of all the previous 49 solutions. In it, he recognizes that the arguments against advanced life are not mutually exclusive and can be applied simultaneously. While individually they may not be persuasive, the cumulative effect of all of them is very substantial.

While Fermi’s paradox has drawn much criticism, it has never been adequately answered—even after 70 years.

The Great Silence

What are we to make of all this? How are we to summarize the last 60 years of searching?

There are no confirmed signals from extraterrestrials and only a handful of unexplained candidates.
The Fermi paradox: if extraterrestrial civilizations are abundant in our galaxy, we should have direct evidence of them (or their robotic probes) having visited Earth yet we have none.

Let us briefly consider two additional lines of evidence.

We have found absolutely no evidence that Earth or our solar system has ever been visited. Our asteroid belt would be a prime target for extraterrestrials to come and mine large quantities of valuable metals (such as nickel and iron), yet it remains untouched; and our solar system appears to be in totally pristine condition.
UFO investigators have not confirmed even one extraterrestrial visit.[80]^,[81],[82]

Taken together, this is known as the Great Silence. By all appearances, we are alone or extraterrestrial civilizations are exceedingly rare.

So, does that mean that we can truly close the door on the possibility of advanced alien life? Not at all. While SETI searches have been extensive, there remains so much more to explore. The Fermi paradox is a paradox, not a disproof of extraterrestrials. There are multiple possible solutions under which alien civilizations could exist yet remain undetected.

All four lines of evidence presented above represent an argument from silence. This is a type of negative argument, namely the lack of evidence for the existence of extraterrestrial civilizations. The problem with negative arguments is that they are rarely decisive by themselves. To quote a common adage: “Absence of evidence is not necessarily evidence of absence.”

To build a stronger case, we need to consider possible direct (positive) evidence that can be used to establish the commonness or rarity of extraterrestrial civilizations. Much has transpired since that first SETI conference in 1961 and the most important insight comes from the discovery of extra-solar planets starting in 1992. In the subsequent 30 years, our knowledge of these alien worlds has already profoundly reshaped our understanding of stars and planets, including our own.

Extra-solar Planets

Since the late nineteenth century, astronomers have been actively trying to detect planets orbiting other stars. These are known as extra-solar planets or exoplanets. Early efforts were unable to overcome monumental technological challenges, so no exoplanets had been found at the time of the first SETI conference in 1961. The values used in the Drake equation for the astronomical terms, therefore, relied heavily on the Copernican principle. That is, they assumed that our solar system was typical and therefore that most solar systems would resemble our own. The discovery of over 5,000 exoplanets allows us to finally test this idea.

The first confirmed exoplanets were reported in 1992 when Aleksander Wolszczan and Dale Frail found a pair of Earth-sized planets orbiting pulsar PSR B1257+12.[83] This discovery was an important breakthrough yet generated little fanfare because the associated planets were clearly uninhabitable. Three years later, a pair of astronomers finally discovered a planet orbiting a sun-like star. On October 6, 1995, Michal Mayor and Didier Queloz reported a Jupiter-like planet orbiting a sun-like star, 51 Pegasi.[84] Together these two discoveries inaugurated a new era in astronomy. With the subsequent development of new techniques and the advent of space-based telescopes, the rate of exoplanet detection greatly increased. Currently, there are 5,108 confirmed planets in 3,779 planetary systems (as of July 2022).[85] This clearly demonstrates that most stars will have one or more planets associated with them. Given that there are about 400 billion stars, then there should be at least as many planets out there.

Exoplanet Nomenclature

The planets in our solar system have relatively simple and easy to remember names. In contrast, exoplanets are given complex names that are designed for use by astronomers, rather than for common people. We can glean some useful information from these designations by breaking them down into three distinct components. For illustration purposes, we will use Gliese 667 Cc as a representative example:

Star system name: Catalog name (various naming systems)

► “Gliese 667” represents the 667^th star system in the Gliese catalogue

Star identifier: If more than one star is present, they are identified using capital letters

► “C” means exoplanet orbits the third star in this trinary system

Exoplanet identifier: Exoplanets are identified with lower case letters starting with b

► “c” means the second exoplanet orbiting star C

While this naming system may seem unwieldy, its flexibility allows for easy inclusion of an ever-increasing number of confirmed exoplanets.

Exoplanet Detection

While spotting stars is easy because there are so bright, the task of detecting exoplanets is exceedingly difficult for three major reasons. First, an exoplanet is approximately a billion times dimmer than its host star for visible light and so it gets lost in the glare. Even for infrared light, exoplanets are still about a million times dimmer. Second, planets are very small relative to their star. For perspective, Earth’s cross-sectional area is about 10,000 times smaller than the Sun’s. Third, exoplanets are located far away. Even the nearest exoplanet to us, Proxima Centauri b, is 4.24 light-years away. Taken together, detecting exoplanets is comparable to observing a birthday candle next to a searchlight in Boston using a telescope located in Washington DC.[86]

Given these challenges, most methods for detecting exoplanets are indirect, that is, they operate by measuring small changes in the star rather than looking for the planet itself. Several techniques have been developed but only the four most important methods will be described here:

Radial velocity method. A star with a planet will move in its own tiny orbit about their mutual center of mass. This motion can be detected because the star will be periodically moving toward and then away from the observer. This motion causes the star’s light to correspondingly alternate between being slightly blue-shifted and red-shifted. The radial velocity method measures this miniscule frequency shift to determine the planet’s mass and orbital period. However, the measured shift depends on how the solar system is tilted with respect to the observer and therefore only provides the minimum mass, not the actual mass, of the planet.
Transit method. A planetary transit is when a planet passes in front of its parent star blocking out a small portion of its light. In the transit method, astronomers monitor the brightness of a large group of stars for a long period of time looking for brief dimmings caused by a transiting planet. It takes three consecutive transits to confirm the presence of an exoplanet and determine its orbital period. The size of the planet can be determined from the degree of dimming. The primary limitation of this method is that it only applies to star systems that are oriented such that we observe them almost perfectly edge on, otherwise we cannot observe their transits.
Gravitational microlensing. According to general relativity, a star (or any massive object) will cause light passing nearby to bend. If a star passes precisely between the observer and a distant light source (e.g., a galaxy), it will bend the light toward the observer like a lens. When this happens, more light than usual from the background object will reach the observer causing it to appear to brighten for a period of days or weeks. If there are any planets orbiting the lensing star, then there will be additional very brief brightenings. From this, astronomers can work out the masses of the planets but little else. Because gravitational microlensing only occurs when a lens star is precisely aligned in front of the background object, this method of detecting exoplanets is not generally applicable.
Direct imaging. The most ambitious strategy is to observe planets directly. The main challenge is that stars dramatically outshine their planets making them hard to see. To get around this, astronomers use either a coronagraph or an interferometer to selectively remove or cancel out the light from the star without blocking light from the nearby planets. Currently, direct imaging is extremely restricted due to the technological challenges.

Of the four, the transit and radial velocity methods have been the most successful. Both work best for large Jupiter-like planets located very close to low-mass stars. Unfortunately, this may skew statistics on exoplanets because small rocky planets and planets located far from their stars will be underrepresented. Gravitational lensing can detect even small planets but is limited because the necessary alignment is rare. Direct imaging is a powerful technique but is currently restricted to nearby stars having large planets orbiting far from their stars.

While ground based telescopes have played an important role in detecting and confirming exoplanets, space-based telescopes have taken center stage over the last decade. Currently, the most important example is the Kepler space telescope (2009-2018). During its decade in space, it was responsible for detecting 2,662 planets or about 70% of all confirmed exoplanets at that time. Using the transit method, it monitored the brightness of 150,000 stars looking for brief dips in brightness. Astronomers then analyzed the data to identify candidates of interest for follow up and verification by ground-based telescopes. Kepler successor, the Transiting Exoplanet Survey Satellite (TESS) (2018-present) will survey a different group of 200,000 stars to search for transiting exoplanets.

Exoplanet Properties

Now that we can detect extra-solar planets, what can we potentially learn from them and what does this tell us about their habitability? Currently, we can only measure a few planetary properties:

Orbital radius—the distance the planet orbits from its host star. This is of critical importance because astronomers can use it to determine how much light the planet receives and therefore estimate its surface temperature. Astronomers consider a planet to be in the circumstellar habitable zone if its orbital radius falls within a specified range.
Orbital period—the time it takes to complete one full orbit. The orbital period and radius are closely linked, so knowing one means knowing the other.
Orbital eccentricity—the measure of orbital non-circularity. For planets with highly eccentric orbits, the distance between it and its host star will vary throughout its orbit and therefore the heat it receives from the star will correspondingly change. This would dramatically impact the planet’s climate. Planets in our solar system have nearly circular (low eccentricity) orbits and therefore maintain a near constant distance from the Sun allowing for stable temperatures.
Mass. The mass of the planet tells us a great deal about its composition enabling us to distinguish between small rocky planets and large gas giants.
Planetary radius or size. Like mass, the size of the planet is important for categorizing the general type of planet.
Density—the planet’s mass divided by its volume. This property can be determined only if both the mass and size are known. Based on the density, we can infer the relative composition of the planet, namely whether it is primarily gas, water, rock, or iron.
Atmospheric composition—the amounts of different compounds that make up the atmosphere. If a planet transits in front of its star, then potentially we can determine which molecules are present in the atmosphere by the light they absorb. This is difficult to do in practice but yields the greatest information about possible habitability. More importantly, this allows us to check for potential biosignature gases that could indicate the presence of life.

Not all these properties can be determined for every exoplanet. For example, the mass is generally only obtained through the radial velocity method while the size is typically measured by the transit method. (For the special case of solar systems with multiple planets, the transit method can be used to determine both the size and mass.) Based on the information available, astronomers use models to extrapolate additional details. Because habitability is such a complex phenomenon and because we possess so little information, any claims of an exoplanet being habitable should be considered to be extremely speculative.

Planetary Classification by Size

To help facilitate analysis of newly discovered exoplanets, it is important to have a meaningful classification scheme. Naturally, our own solar system serves as a reference because it is the only system for which we have intimate knowledge. The planets of our solar system naturally fall into three main groups:

Earths (Mercury, Venus, Earth, and Mars). Terrestrial (rocky) planets composed primarily of silicate rocks or metals. They have a solid surface and can support a thin atmosphere. Potentially, they can have liquid surface water and even life.
Jupiters (Jupiter and Saturn). Massive gas giant planets composed primarily of hydrogen and helium. These planets are extremely hostile to life having no well-defined surface.
Neptunes (Uranus and Neptune). Giant ice planets. They are like smaller version of Jupiter but in addition to hydrogen and helium, they contain a higher proportion of “ices” such as frozen water, ammonia, and methane. They are inhospitable to life.

Of the three, only small rocky Earth-like planets are considered to be serious candidates for hosting advanced life. Some, however, suggest that the large moons of Jupiter and Saturn could also support life in subsurface oceans as will be discussed later.

Using these three categories as a guide, astronomers have developed a tentative classification scheme for exoplanets. Because most exoplanets have been discovered by the transit method, planetary radius is used for classification instead of mass. This demarcation is tentative and not universally agreed upon. (R_Å equals the radius of the Earth.)

Sub-Earth (< 0.8 R_Å). Too small to have a significant atmosphere; therefore they are unlikely to be habitable.
Earth-sized (0.8-1.25 R_Å). Although difficult to detect with current technology, these offer the best chance of possessing a thin atmosphere and perhaps liquid oceans.
Super-Earth (1.25-2 R_Å). Little is known about this type of planet because there are no analogues in our solar system on which to base a conclusion. Their similarity to Earths implies that they may be habitable.
Mini-Neptune (2-4 R_Å). Mini-Neptunes have thick hydrogen–helium atmospheres, probably with deep layers of ice, rock or liquid oceans (made of water, ammonia, a mixture of both, or heavier volatiles). Unlikely to be habitable.
Neptune (4-6 R_Å). Ice giant planets possessing massively thick atmospheres. Very hostile to life.
Jupiter (6-15 R_Å). Massive gas giant planet with no well-defined surface. No possibility of life but their large moons might support simple life under massive ice sheets.
Super-Jupiter (15-25 R_Å). Larger versions of Jupiter. Even more inhospitable.

Of these, only Earths and Super-Earths are considered likely candidates for hosting civilizations.

Planetary Classification by Temperature

The second way to categorize planets is by their predicted surface temperature. This has obvious implications for possible life. If life requires liquid water as is generally agreed upon, then that requires a host planet that maintains a stable surface temperature between the melting and boiling points of water. Unfortunately, it is generally not possible to measure the surface temperature of exoplanets, so we must infer it from other information.

Figure 10: Circumstellar Habitable Zone.

As everyone knows, stars are extremely hot. Any planet orbiting very close to one will naturally be scorching hot because of the heat it receives from the star. If we were to move the planet farther out, it would receive progressively less starlight and therefore would correspondingly be colder. Planets located in the outer region of our solar system are mind-numbingly cold. Between these two extremes there is an intermediate range where a planet could remain warm enough to allow liquid surface water. This is sometimes referred to as the Goldilocks zone because, like the famous porridge, it is not too hot, not too cold, but just right (see Figure 10). Astronomers, however, refer to this as the circumstellar habitable zone, the liquid water habitable zone, or simply the habitable zone.[87]^,[88]

Based on these considerations, astronomers broadly categorize exoplanets into three temperatures categories:

Hot. Temperature too high to support liquid water.
Warm. Moderate temperatures where water could potentially be maintained in a liquid state.
Cold. Any surface water would be frozen.

The problem with this classification scheme is that it is not well defined because the surface temperature depends on multiple planetary properties. Since those details are generally not available, they must be extrapolated from models.

Habitable Zone vs Habitable

The use of the term habitable zone is regrettably confusing. Many popular reports often conflate it with a planet necessarily being habitable. Habitability conjures images of an Edenic planet with warm temperatures and large oceans like we find on Earth. Actuality, a planet being in the habitable zone simply means that water could potentially exist in a stable liquid state on the surface of a rocky planet. Or to put it another way, it means we cannot immediately rule out the possibility of habitability based on its position relative to the star. To avoid confusion, it has been suggested that this region should be renamed the temperate zone.[89] Unfortunately, the term habitable zone is too well established, so we will continue to use it here.

When SETI was starting, a planet only needed to satisfy two key criteria to be considered a habitable Earth-like planet:

Planetary size. Generally, only Earths and Super-Earths qualify as rocky planets with a surface that could potentially support liquid water and possess a significant atmosphere. Sub-Earths have too little mass to maintain a significant atmosphere. For Neptunes and Jupiters, the atmosphere will be exceedingly thick and there will be no clear planetary surface.
Orbital radius. To remain warm, a planet must reside within its star’s circumstellar habitable zone. If it was either closer to or farther from the star, the planet would be too hot or too cold to maintain liquid surface water.

However, satisfying both of these criteria is not nearly enough to ensure either moderate surface temperatures or liquid surface water.

The parent star type and the orbital radius determine the planet’s heat energy budget, that is, the amount of heat received by the planet. But there are a host of other factors that determine how much of this heat will be retained and how much will be lost. The resulting surface temperature, therefore, depends on several planetary properties, such as:[90]

Albedo. A planet’s albedo is the fraction of incident light that is reflected back into space by the planet’s surface. Its value depends on the surface. Areas covered by snow or ice reflect light increasing the albedo while darker land masses lower it.
Atmosphere. The thickness and composition of a planet’s atmosphere plays a strong role in determining the planet’s temperature. Known as the greenhouse effect, an atmosphere can trap heat thereby raising the surface temperature. For example, the average surface temperature on Earth is 15 °C (59 °F) but if the Earth had lacked an atmosphere, it would likely be a frigid -18 °C (-0.4 °F).[91]
Carbonate-silicate cycle. The habitable zone would be very narrow except that the Earth has a feedback cycle that acts as a planetary thermostat to keep us in the correct temperature range.[92] Carbon dioxide (CO₂) in our atmosphere is a major greenhouse gas. It gets removed from the atmosphere by reacting with water to become carbonic acid (H₂CO₃) that can then react with silicate rocks to form carbonates. Eventually, plate tectonics and volcanic activity return the sequestered carbon dioxide back to the atmosphere and exposes fresh silicate rock to continue the cycle. The efficiency of this carbonate-silicate cycle depends in part on the planetary temperature. Higher temperature increases the rate of carbon dioxide sequestration leading to a cooling effect. Correspondingly, lower temperatures leave more carbon dioxide in the atmosphere resulting in an enhanced greenhouse effect. Acting over very long periods of time, this mechanism helps counteract temperature changes.
Water. Having an inventory of water and maintaining it for billions of years is obviously a necessary pre-condition for having liquid surface water for a long enough period of time to allow for the development of advanced life. However, water also plays a key role in the albedo, the atmospheric greenhouse effect, and the carbonate-silicate cycle.
Magnetic field. A strong magnetic field can protect a planet from highly charged particles emitted by its star by deflecting them toward the poles. (On Earth, this is the origin of the aurora or polar lights.) Stars generate these charged particles in the form of a continuous solar wind as well as brief intense bursts in the form of solar flares and coronal mass ejections. In the absence of a magnetic field, these charged particles can gradually strip away the planet’s atmosphere. The magnetic field also protects planetary lifeforms from harmful radiation.

These planetary properties are generally not known, so astronomers must rely on complex models and approximations to fill in the gaps. Because of this, the boundaries defining the habitable zone are not universally agreed upon.

To further complicate matters, a star’s brightness will change over its lifetime. In this context, the changing luminosity means that the habitable zone will shift outward over time. For a planet to maintain liquid water over the course of billions of years, it must reside in the habitable zone the entire time. The range of distances where this will be satisfied is the circumstellar continuously habitable zone which is narrower than the standard habitable zone.[93]^,[94] Just because a planet is located in the habitable zone now, it may not be in the continuously habitable zone for the entire time needed for advanced life.

For perspective, 4 billion years ago our own sun was 30% dimmer. Working backward in time, this should have resulted in the Earth at that time being 20 °C (36 °F) colder than it is now.[95] If that were true, Earth would have been freezing ‑5 °C (23 °F), yet we have geological evidence for liquid water being present implying warm temperatures. This is known as the faint young sun paradox. Although the details are uncertain, the consensus is that the young Earth had a higher concentration of various greenhouse gases in the past to keep the planet sufficiently warm. Over the course of those four billion years, the extra greenhouse gases must have been carefully removed to maintain a steady temperature over billions of years.

In summary, a planet being located in the habitable zone does not guarantee either surface liquid water or habitability. The habitable zone is primarily intended as a useful rule to help astronomers identify exoplanets worthy of closer inspection. Moreover, exoplanet measurements only provide a quick snapshot while planets and their host stars change dynamically over billions of years.

Habitable Zone: A Tale of Two Planets

An excellent illustration of the habitable zone can be observed in our own solar system. Earth’s two nearest planetary companions, Venus and Mars, share a great deal in common with our own planet. Both are rocky planets located in the inner solar system. Venus is slightly smaller than Earth and Mars’ mass is just 11% of Earth’s mass. (See Table 2 for a comparison.) Based on this quick look, one might expect that these two planets would be Earth’s nearly identical siblings.

Table 2: Comparison of Venus, Earth, and Mars

	Venus	Earth	Mars
Mass (M_Å)	0.82	1.00	0.11
Size (R_Å)	0.95	1.00	0.53
Orbital radius (AU)	0.72-0.73	0.98-1.02	1.38-1.66

After the Copernican revolution, Earth was no longer deemed special having been demoted to being just another planet. This gave new impetus to the plurality of worlds concept. By the seventeenth century, many individuals boldly promoted claims of intelligent beings residing on other bodies in our solar system, including the sun, moon, planets, and even comets.[96] By the mid-nineteenth century, developments in astronomy began to contradict this notion, eliminating as uninhabitable all these bodies, except Venus and Mars.

Starting around 1860, the belief in intelligent beings on Mars was strong enough that several individuals proposed means of communicating with them (a pre-modern version of SETI).[97] To signal our presence, some advocated carving figures large enough to be seen from Mars. Others proposed building large mirrors to reflect sunlight toward Mars instead. However, none of these ideas proved to be practical enough to implement.

Interest in Martian life reached a fever pitch when Giovani Schiaparelli announced the discovery of “canals” on Mars in 1877. Percival Lowell expanded on this idea in a series of books between 1895 and 1908 by suggesting that these were artificial constructs. In science fiction, Mars was frequently portrayed as an old dying world (with the canals needed to bring water in from the poles), while Venus was depicted as a more primordial planet consisting mainly of jungle and swamp. Even as late as the first SETI conference in 1961, both planets were considered potentially habitable.

This belief that Mars and Venus might be inhabited was ultimately squelched when probes sent to our nearest neighbors in the 1960s and 70s proved them to be spectacularly hostile to life. Far from being a primitive Earth, Venus was revealed to be a hellish nightmare:

Water: none
Average surface temperature: 464 °C (867 °F)—hot enough to melt lead
Atmospheric pressure: very high (92 times Earth’s surface pressure)
Atmospheric composition: 96% carbon dioxide with significant quantities of sulfuric acid (i.e., battery acid)
Magnetic field: none

Likewise, the Mariner 4 flyby (1964) and subsequent Viking landers (1975) revealed Mars to be a frozen wasteland:

Water: no surface liquid water (although it may have hosted some in the distant past)
Average surface temperature: ‑63 °C (‑82 °F)
Atmospheric pressure: very low (0.628% of Earth’s surface pressure)
Atmospheric composition: 96% carbon dioxide
Magnetic field: none

So, despite their similarities to Earth, both worlds are dead. So, what turned Earth’s twin sisters into ugly stepsisters?

In the case of Venus, it is located about 30% closer to the sun than the Earth and so receives correspondingly more heat. This led to temperatures rising above the boiling point of water, preventing rain, and therefore shutting down the carbonate-silicate thermostatic regulation leading to a runaway greenhouse effect.[98] As temperatures soared, carbon was baked out of the soil and converted to carbon dioxide further amplifying the greenhouse effect. The temperatures continued to rise until it reached its current scorching temperatures and any water it once possessed was lost into space.

In contrast, Mars is 50% farther from the sun than the Earth, so its climate moved in the opposite direction. Given its smaller size, Mars’s core cooled rapidly eliminating plate tectonics and its early magnetic field.[99] This allowed its atmosphere to be slowly stripped away (by the solar wind) leaving only a miniscule atmosphere today. This combined with its greater distance from the sun led to runaway freezing.

Thankfully for us, Earth is nestled comfortably in the circumstellar continuously habitable zone and so avoided the dire fate of its two neighbors. But we should not take this for granted because Earth’s habitability was far from guaranteed. For example, if Earth’s atmospheric concentration of carbon dioxide had been ten times higher, Earth would likely have ended up like Venus despite our privileged location.[100] Other changes to the atmospheric composition or to Earth’s geology would likewise have dramatically altered our planet’s history.

The Menagerie: The Diversity of Exoplanet Configurations

Having moved from knowing only one solar system (our own), to more than 5,000 has dramatically changed our understanding of what is normal for planetary systems. To begin, let us broadly consider the diversity of exoplanet configurations that have been found so far. For example, finding two exoplanets orbiting a pulsar in 1992 was completely unexpected. A pulsar is formed when its parent star goes supernova. That explosion should have destroyed any planets that were present, so how do we to account for this pair? Because pulsar planets are certainly dead worlds, we will not consider them here in the context of SETI, but they do represent an unexpected extreme case.

The exoplanet discovered orbiting 51 Pegasi in 1995 was an even bigger surprise. It had half the mass of Jupiter but was orbiting very close to its star—seven times closer to its star than Mercury is to our Sun. At that distance, it completes one orbit in just 4.2 days. Consequently, 51 Pegasi b is baked with super high temperatures (1,200 °C or 2,200 °F)! Many similar examples were subsequently found, so hot Jupiters represent a significant class of planets, not a rare exception. These are far more extreme than anything in our solar system, so it begs the question, “Who ordered that!?”[101]

The discovery of numerous hot Jupiters provoked a strong reaction from the astronomical community because they did not fit the existing planet formation models that had been formulated based on our own solar system.[102] Massive gas giants should only form far from their star where temperatures are low enough to freeze water. Consequently, these planets must have formed far out and then drifted inward toward their stars. So, what caused them to migrate? What kept them from falling into their parent stars? Why didn’t our Jupiter end with the same fate? Fortunately, planetary formation models have been revised to greatly improve our understanding of these and other recent discoveries.[103]

Over the last 30 years, astronomers have found an extraordinarily diverse selection of planetary configurations. Let us begin by describing some of the more exotic examples discovered so far with the name of a representative example in parentheses:[104],[105]

Planets around pulsars (PSR B1257+12 b). Planet orbiting a rapidly rotating neutron star. The origin and nature of these planets are not well understood but are certainly uninhabitable.
Hot Jupiters (51 Pegasi b). These large gas giant planets orbit very close to their stars, subjecting them to very high temperatures making them extremely hostile to life.
Super-Earths (Kepler 186 f). Rocky planets intermediate in size between Earths and Neptunes that have no analog in our solar system but could potentially be habitable.
Mini-Neptunes (Kepler 138 d). These planets are smaller, denser versions of Neptune consisting of large rocky cores surrounded by a thick blanket of gas. Unlikely to be habitable.
Multi-star worlds (Kepler 16 b). Sometimes referred to as “Tatooine[106] planets,” they belong to star systems containing more than one star.
Water worlds/ocean planets (Gliese 1214 b). A type of terrestrial planet containing excessive amount of water on its surface or as a subsurface ocean.
Styrofoam planets (Wasp-17 b). These are hot Saturns puffed up with a radius closer to that of Jupiter. This means they have a surprisingly low density, like that of Styrofoam.
Diamond planets (55 Cancri e). Super-Earths that are exceptionally carbon rich and oxygen poor. That could allow for significant quantities of diamond in the mantle.
Hot Earths/lava planets (CoRoT-7 b). Earth-size worlds orbiting so close to their stars that their surface is mostly or entirely covered by molten lava. When such a planet rotates, “snowflakes” made of solid rock fall from the sky.
Rogue planets (PSO J318.5-12). Planets wandering freely and unattached to a star. It is possible that most planets in our galaxy are of this type.
Distant planets (Fomalhaut b). Gas giants orbiting at extreme distances from their stars. In the case of Fomalhaut, the planet is orbiting about 4 times farther out than Neptune is in our solar system.
Chthonian planets/gas giant cores (HD 209458 b). Gas giants that orbit so close to their parent star that their atmospheres have been stripped away, leaving behind rocky cores.
Tidally locked exoplanets (Gliese 581 c). For planets located close to their stars, the tidal force will slow the planets’ rotation so that one side is always facing its star. When this happens, one face will get scorched by the star’s heat while the opposite side freezes. In extreme cases, they may suffer atmospheric collapse with gas boiling off the hot side and then freezing on the cold side. A thick enough atmosphere can prevent this by transferring heat from the hot side to the cold.
Exoplanets with helium atmospheres (Gliese 436 b). Gas giants that lose their hydrogen but retain helium in their atmospheres.
Exoplanets with highly eccentric orbits (HD 80606 b). Planets with decidedly non-circular orbits. These curious cases spend much of their orbit far from the star but then briefly dive inward and whip around their star before moving out again. This strange behavior is likely due to the presence of a nearby star having altered its orbit in the past.
Exoplanets with retrograde orbits (Wasp 17 b). Planets orbiting in a direction opposite of the star’s rotation.
Exoplanets with strongly inclined orbits (ν Andromedae A c and d). In most solar systems the planets all orbit in the same orbital plane. The presence of a nearby star can scramble orbits so that planets orbit in different planes at a large angle compared to each other.

All these planets break the Copernican mold, and most are likely to be uninhabitable. The main exception to that rule are super-Earths although some consider water worlds, rogue planets, and multi-star worlds as additional possibilities. Clearly, our solar system is not as typical as once thought. Still, the above examples only represent the more extreme cases.

Exoplanet Statistics

The study of exoplanets is a new and emerging field—only 30 years old. It has come a long way in that short period of time with more than 5,000 exoplanets found so far. These discoveries provide a unique new window into the universe with insights into how our solar system compares to these others.

Now that we have a large sample of exoplanets, we can draw some preliminary conclusions. However, it should be cautioned that our knowledge of exoplanets is still in its infancy, so we must be careful not to overinterpret the results. Current detection methods favor larger and closer in planets, so some planetary configurations will be underrepresented, which could skew conclusions. We also must be careful in comparing the results of one study with another because the amount of data available to a study depends significantly on what was available at the time. Moreover, this field has not been around long enough for categorization criteria to be fully standardized, so statistics based on planetary size may vary depending on the particular study’s definitions. Despite this, there is much we can learn even at this early stage.

On March 21, 2022, a NASA press release [107] announced a major milestone: there are now 5,000 officially confirmed exoplanets. This was followed by a popular-level summary by Ethan Siegel.[108] Based on the latest data, astronomers expect that at least 80% of all stars will host planets. They also estimate that there should be about 4 to 20 planets per solar system. (The upper limit is very uncertain due to the difficulties in detecting planets far from their stars.) Extrapolating this over the entire galaxy, means there should be trillions of planets out there.

The press release organizes confirmed exoplanets into four main groups according to their size along with their relative abundances shown in parentheses:

Terrestrial (4%): Small, rocky planets. Around the size of our home planet, or a little smaller.
Super-Earth (31%): Planets in this size range between Earth and Neptune don’t exist in our solar system. Super-Earths, a reference to larger size, might be rocky worlds like Earth, while mini-Neptunes are likely shrouded by puffy atmospheres.
Neptune-like (35%): Similar in size to Neptune and Uranus. They can be ice giants or much warmer. “Warm” Neptunes are more rare.
Gas giant (30%): The size of Saturn or Jupiter (the largest planets in our solar system), or many times bigger. They can be hotter than some stars.

Of these four groups, the super-Earth/mini-Neptune category deserves special attention and will be discussed more later.

Figure 11: Exoplanet populations by size and orbital period.

Going one step farther, we can differentiate within these groups by plotting the planet size versus the orbital period. This allows us to visualize differences between planets within the same category. The graph provided by Siegal is fairly plain, so we will instead use a graph produced in a different NASA report dated August 6, 2017 (see Figure 11).[109]

The horizontal axis shows the orbital period in days on a logarithmic scale. Along the vertical axis is the planet’s size relative to Earth’s size, also on a logarithmic scale. For reference, horizontal lines mark the sizes of Earth, Neptune, and Jupiter respectively. The methods used to detect the planets are denoted by color as defined in the legend.

These planets can be broadly organized into five separate populations based on size and expected temperature. Earths and super-Earths are together denoted rocky planets except for a notable subset orbiting close to their stars that are deemed lava worlds due to their extreme temperatures. In the middle are the Neptune-sized planets grouped together as ocean worlds and ice giants. For Jupiter-sized planets, they are divided into two distinct sub-groups. Those orbiting close to their stars are the infamous hot Jupiters while those orbiting farther out are classified as cold gas giants.

Before moving on, Figure 11 provides one more important detail. The lower right is labeled the frontier representing the combination of sizes and orbital periods that are very difficult to detect with current technology. The planets of our solar system fall into this poorly searched frontier region, so potential solar system analogues are not well represented among known exoplanets.

Returning to the NASA study, Ethan Siegel addresses the question, “Is our solar system ‘typical’ in some fashion?” To that he answers, “Yes and no.” We are likely typical in the sense of the number of planets in our solar system, although we cannot be certain because smaller farther out planets are currently difficult to detect. He then points out four specific ways our solar system is atypical:

Planetary arrangement. One uncommon feature of our solar system is that we have rocky terrestrial planets in the inner solar system (with Earth in the circumstellar habitable zone) and large gas giants in the outer regions. In contrast, many solar systems have hot Jupiters located close to their star where they would disrupt smaller terrestrial planets.
No super-Earths/mini-Neptunes. These planets make up 31% of all planets discovered so far, yet there are no examples in our solar system.
Our bright sun. Our sun is a yellow (G type) star; however, most stars are smaller dimmer red (M type) and orange (K type) stars. Taking this into account, we find that our sun is brighter than 95% of stars in the galaxy. Star type has implications for how closely a planet must orbit to be in the circumstellar habitable zone.
Bachelor star. Our solar system contains just one star, while 50% of all stars belong to star systems containing two or more stars.

Based on these facts, we can conclude that our solar system is not typical. This runs contrary to the Copernican principle that presumes that most solar systems should resemble our own. However, this does not mean that we are truly unique, nor does it prove that other solar systems could not be habitable. Given the vast number of exoplanets, at least some can be expected to resemble our own.

Super-Earths versus mini-Neptunes

Prior to the discovery of exoplanets in the 1990s, astronomers only knew three types of planets: small rocky terrestrial planets, massive gas giants, and ice giants. Earth represents the largest of the terrestrial planets and Neptune is the smallest of the ice giants. As astronomers characterized more and more exoplanets, they were surprised to find that many of them fell within this size gap. These newly discovered planets were further divided into two distinct groups: super-Earths and mini-Neptunes depending on which planet they were expected to resemble the most.

Super-Earths. Worlds that are larger than Earth, but still Earth-like, with rocky surfaces, thin atmospheres, and the potential to—with the right conditions—have water existing in the liquid state on those surfaces.
Mini-Neptunes. Worlds that are no longer like Earth, possessing large, volatile gas envelopes surrounding the world on all sides. If you have a thick atmosphere rich in volatiles—things like ammonia, methane, various ices, and raw hydrogen and helium—the pressure and temperature gradients are so severe that by the time you get to the surface, the biological and chemical processes that we know of can no longer occur.

This distinction is of critical importance because super-Earths have the potential to be habitable whereas mini-Neptunes almost certainly do not. Therefore, if super-Earths represent a significant fraction of this population, that will greatly increase the number of places we might look at for life. Even better, super-Earths have two properties that may make them more likely to be habitable than Earth-sized planets.[110] First, their larger mass would help prevent their atmosphere and water from being lost to space. Second, it would increase the planet’s ability to sustain plate tectonics and therefore its carbonate-silicate cycle.

When exoplanets were first being discovered, most were characterized by either their mass (from the radial velocity method) or their size (from the transit method), but not both. In the absence of clear data, astronomers somewhat arbitrarily used two Earth radii (2 R_Å) as the line of demarcation between super-Earths and mini-Neptunes.

Eventually, astronomers were able to measure both the size and mass of many of these planets. From these two values, we can calculate the planet’s density enabling us to directly differentiate between the two types of planets. To be Earth-like, they should have a density close to that of rock while Neptune-like planets would be characterized by having a lower density. Using data from 2016, the boundary between the two types was determined to be about 1.2-1.3 R_Å.[111] This effectively eliminates the entire category of super-Earths—planets that can be either Earths or mini-Neptunes with nothing in between. That means that our Earth is just about as “super” as a rocky planet can get!

One small caveat. Astronomers have found a few cases of rocky planets up to 1.6 R_Å. However, a majority of these are extremely hot (i.e., they orbit very close to their star). In that case, these likely represent mini-Neptunes that have had their thick gaseous shell stripped off leaving a rocky core. These truly qualify as super-sized Earths; however, it is debatable whether they could be habitable.

Percentage of Planets in the Habitable Zone

One particularly useful statistic is the number of planets projected to be in the habitable zone of their star. This involves knowing both the planetary size and its estimated surface temperature based on the orbital radius. One good source for this comes from Planet Habitability Laboratory (PHL) last revised July 2, 2018.[112] One small challenge in using the PHL study is to relate their size categories to the one used in this paper. These are translated as follows:

PHL categories	This paper
Miniterrans and subterrans	Sub-Earths
Terrans	Earths
Superterrans	Super-Earths and mini-Neptunes
Neptunians	Neptunes
Jovians	Jupiters and super-Jupiters

The PHL study examined 3,877 confirmed planets for which the necessary data was available. They classified the planets into these five categories and further identified them as hot, warm, or cold. Unfortunately, the precise criteria for defining the temperatures could not be found. Here are the resulting statistics in tabular form and then in graphical form:

Table 3: Statistics on exoplanets by size and temperature.

	*Subterrans (2%)**	Terrans (18.5%)	Superterrans (25.6%)	Neptunians (20.9%)	Jovians (32.9%)
Hot	60	614	971	777	827
Warm	1	22	32	39	139
Cold	1	8	10	25	301

* Miniterrans have been merged into the subterrans category.

Figure 12: Number of confirmed planets by size and temperature

If we consider only warm Terrans and Superterrans as being potentially habitable, then there would only be 54, corresponding to 1.4% of the exoplanets in the study. However, if we reject Superterrans as being mini-Neptunes and therefore unlikely to be habitable, then this number drops to 0.57%. Of course, we need to be careful because the result depends on the precise criteria used to define these categories.

This conclusion is an important result! Based on just two criteria, we can rule out a large percentage of exoplanets. Clearly, our planet is nowhere near as common as SETI assumed in the 1960s. Yet, there are likely trillions of planets out there. That means we need to consider what other factors may be important in determining habitability.

Reassessing the Drake Equation

The discovery of extra-solar planets as well as the development of newer models of solar system formation have shattered the assumption that our solar system is the cosmic norm. But how does this impact the chances of finding intelligent life? How should this affect how we understand our own planet and solar system?

To help put this in the proper perspective, let us revisit the Drake equation and consider what impact the latest astronomical discoveries have on how we evaluate the relevant terms. To begin, let us reorganize the terms into two distinct sets:

where:

R_astro is the rate of formation of habitable planets.

f_bio is the fraction of habitable planets with intelligent communicating civilizations.

f_bio contains those elements of the Drake equation governed by biological, sociological, and technological considerations. Its value is very controversial because we currently have only one known example of life and extrapolating from a single case is always uncertain. Unfortunately, a discussion of these terms is beyond the scope of this study but is addressed by other authors.[113]^,[114]

That leaves R_astro, which encompasses all the astronomical factors. Of its four components, the first, R_*, is well understood and uncontroversial. Similarly, f_p is certain to be high given the plethora of known exoplanets. That leaves just two terms, the fraction of “suitable” stars, f_s, and the number of “suitable” planets, n_e.

These two terms are central to our understanding of the commonness or rarity of habitable planets. The problem is that both are heavily dependent upon the notion of what is “suitable?” This is the very crux of the debate. For SETI proponents holding to the Copernican principle, suitability must necessarily involve very loose criteria, so that most stars and planets will qualify. That in turn implies that habitable planets are abundant, and that Earth is mediocre. Others challenge this presupposition arguing that suitability involves far more demanding criteria. In that event, habitable planets will be relatively rare, and both the Copernican principle and SETI assumptions are fundamentally flawed.

Let us marshal the latest in astronomical research to try to understand just what it takes for stars and planets to be suitable for life.

Stellar Habitability: Just what makes a star “suitable”?

In the pioneering days of SETI, scant attention was paid to the stars hosting the planets being considered. Obviously, dead stars (e.g., pulsars and black holes) and ones that have left the main sequence (e.g., supergiants, red giants, and white dwarfs) can easily be dismissed as non-starters, but what about ordinary stars? Stars with different masses will have considerably different temperatures, however, there will always be a range of distances from it that a planet could occupy and receive the necessary amount of heat to potentially allow liquid water. At first glance, it seems that most stars should be adequate for the job, but let’s take a closer look.

Unlike exoplanets, stars are well characterized because they are easy to observe and much simpler to model. Here are some of their more important properties:

Star type. Stars can be organized into seven different stellar types based on their spectral characteristics. The star type tells us about the star’s most vital attributes, such as its mass, temperature, and lifespan.
Single vs multi-star system. Our sun is a bachelor star, but half of all stars have one or more stellar companions.
Metallicity. This is the quantity of “metals”—elements other than hydrogen and helium—present in the star. These metals are formed in the furnaces of earlier stars that eventually go supernova blasting these elements out into space where they can be incorporated into new stars. Metallicity varies among stars because it depends significantly on the local environments out of which the stars formed.
Age of star. Very young and very old stars are far less stable leading to large fluctuations in luminosity (brightness). These fluctuations would result in severe climate change, such as runaway freezing or runaway heating on its planets. While that may not pose a problem for simple life, it would be exceptionally problematic for any kind of advanced life. Middle-aged stars like our own offer the greatest stability. This is especially critical for advanced civilizations. The star’s age also determines how much time potential life on one of its planets would have had to develop.

Let us evaluate how each of these affect the suitability of the star to host habitable planets.

Stellar Habitability: Star type

Astronomers divide main sequence stars into seven types depending on their mass. A quick summary of stellar classification:

Table 4: Stellar Classification

Class	Color	*Mass (M_)**	Temperature (K)	Lifetime*	Fraction (%)
O	Blue	³ 16	³ 30,000	£ 10 Myear	~0.00003
B	Light blue	2.1-16	10,000–30,000	10-1,500 Myear	0.13
A	White	1.4-2.1	7,500-10,000	1.5-4 Gyear	0.6
F	Yellow-white	1.04-1.4	6,000-7,500	4-9 Gyear	3
G	Yellow	0.8-1.04	5,200-6,000	9-17 Gyear	7.6
K	Orange	0.45-0.8	3,700-5,200	17-70 Gyear	12.1
M	Red	0.08-0.45	2,400-3,700	70-5,500 Gyear	76.45

*Gyear = billion years; Myear = million years.

The star types listed in Table 4 are organized with the most massive star types at the top and the least massive at the bottom. Each type is designated by both a color and a letter. For example, the top row describes blue (O type) stars. The bottom row shows red dwarfs or M type stars. Our sun, in contrast, is a middle-of-the-road yellow (G type) star.

A star’s mass affects the efficiency of its stellar fusion, with more massive stars burning hotter and correspondingly burning out faster. Consequently, blue stars have extremely short lifespans of no more than 10 million years. Traveling down the table, each subsequent star type is less massive, cooler, and longer lived. At the very bottom are the red dwarf stars with the lightest of them weighing in at a paltry 8% the mass of our sun—just massive enough to ignite stellar fusion. Because of their low masses, red dwarf stars are relatively cool and therefore burn very slowly with lifespans ranging up to a staggering 5.5 trillion years! (In contrast, our sun is expected to shine for about 10 billion years total.) The longevity of red dwarf stars helps explain why they are so plentiful, making up 76% of all stars.

When we consider which stars might support planets with advanced life, we can largely rule out stars more massive than the sun (e.g., types O, A, B, and F). One major reason for this is their short life spans. It took 4.5 billion years before humanity appeared on the scene—about halfway through our sun’s 10-billion-year lifespan. That suggests that even if life were to arise around one of these massive stars, most likely it would not have enough time to develop an advanced civilization. A second issue is that massive stars burn hotter and therefore a higher percentage of their light consists of high energy ultraviolet light is much higher. That could pose problems for life unless the planet has an atmosphere good at blocking it.[115]

At the other end of the weight scale are red dwarf (M type) stars. There is a great deal of interest in their potential habitability because of their great abundance and long lifespans. Adding to their popularity, current exoplanet detection methods work best for finding planets around these small stars and therefore they account for a large portion of confirmed exoplanets.

This current notoriety is a sharp contrast to the 1960s when red dwarf stars were largely dismissed as uninhabitable. Because they are so cool and dim, planets must orbit very close to the star to be in the circumstellar habitable zone. The problem is that at that short distance, they are subjected to three major hazards:[116]

Extreme stellar activity. All stars emit X-ray and ultraviolet radiation—referred to as stellar activity—that can hammer away at a planet’s atmosphere potentially stripping it away unless protected by a strong planetary magnetic field. This is especially problematic in the case of red dwarf stars because their planets’ close orbits result in greater exposure. Even worse, the initial phase of extreme stellar activity of a red dwarf star usually lasts for 2-3 billion years, whereas yellow stars like our own sun remain active for only a half billion years. This combination of higher levels of stellar activity, longer duration, and close in orbits could strip away an Earth-like atmosphere in less than 1 million years.[117]
Severe flaring. Young red dwarf stars are prone to frequent intense flaring—brief but intense bursts of stellar radiation—that could sterilize the surface of their planets. For the first 2-3 billion years of their life, these stars may flare up several times a day showering its planets with 100 to 10,000 times normal levels of ultraviolet light. In contrast, the flaring stage for a yellow star like our own Sun only lasts for the first half-billion years.
Tidal locking. At close range, tidal forces become strong enough that they will eventually slow the planet’s rotation until it becomes tidally locked with one side always facing the star. When this happens, one side of the planet is flooded with never-ending-light while the opposite side is locked in eternal darkness, such that the former side will bake while the latter freezes. In extreme cases, this could lead to atmospheric collapse with gases boiling off the hot side and then condensing on the cold side. A thick enough atmosphere can help mitigate this problem by transferring heat from the hot side to the cold. Regardless, this temperature difference would make conditions hostile to life.

Clearly, red dwarf systems are not simply smaller versions of our own solar system. Instead, they present conditions that would at best make it extremely difficult for life.

Despite this bleak assessment, not everyone is so quick to dismiss planets around red dwarf stars. Ed Guinan leads the “Living with a Red Dwarf” Program in efforts to help rehabilitate our understanding of these cool stars.[118] His main hope is that if a planet can maintain a strong magnetic field for the first few billion years, then it could keep its atmosphere from being stripped away during the early highly active phase. After that time, the star will remain relatively calm for the next 100 billion years or more. (In contrast, our sun has slowly brightened over just a few billion years causing the circumstellar habitable zone to shift outward.) It is this long quiescent phase that offers the best hope for possible life around red dwarf stars. If the planet can somehow retain its early atmosphere (or perhaps have evolved a new one after the star calms down), then such a planet might be a pleasant place for life. The remaining difficulty would be dealing with the effects of tidal locking.[119] In that case, the best chance for life would be in a narrow ring around the planet representing a temperature half-way between the hot and cold extremes. Or perhaps a bit more toward the backside to avoid radiation and flaring coming from the star.

Just how likely is it that these planets will have a strong magnetic field lasting for a few billion years? One problem is that tidal locking reduces the planet’s rotation and that correspondingly reduces the churning of the planet’s core that is needed to sustain a strong magnetic field. That could lead to a weak magnetic field or one that shuts off completely. A super-Earth would likely have a better chance for sustaining a magnetic field and its larger mass would help retain an atmosphere. Currently, we know too little about red dwarf star systems to completely write them off, yet there is little room for optimism.

Taking all of this into account, we are left with orange (K type) and yellow (G type) stars as being the best candidates for supporting life. These populations represent about 20% of stars.

Stellar Habitability: Single vs multi-star system

Roughly half of all stars belong to multi-star systems (multiple stars gravitationally bound to each other). The idea of a planet having multiple suns is a very romantic idea (like the planet Tatooine in the movie Star Wars), but can such planets really host life? Could we even expect to find planets in such a star system? Early evidence seemed to rule out this possibility, but in 1996 astronomers discovered 16 Cygnus Bb, the first exoplanet in a binary star system. Since then, more than two dozen such cases have been confirmed so far.

Of all possible arrangements, there are two main configurations that allow planets to form with stable orbits in multi-star systems (with distances given in astronomical units (au) that corresponds to the average Earth-Sun distance or 93 million miles):[120]

Circumstellar planet (S-type). When the stars are far apart (more than 100 au), planets may form around one of the stars without being disrupted by the other (seeFigure 13a). These systems are comparable to a planet orbiting a single star.
Circumbinary planet (P-type). If the stars orbit very closely (within 1 au), a planet can simultaneously orbit both stars (see Figure 13b).

For distances intermediate between these two cases, sibling stars can disrupt each other’s ability to form planets.

a) b) Figure 13: Two configurations for exoplanets around binary stars: a) circumstellar; b) circumbinary.

Currently, we know very little about multi-star systems. One study, however, estimates for binary systems, 40-50% could support circumstellar (S-type) planets and 10% could have circumbinary (P-type) planets that might be capable of supporting habitable terrestrial planets within stable orbital ranges.[121] However, we should be cautious about these numbers because they are based on a theoretical model rather than direct observations.

Are planets in binary systems potentially habitable? This is a difficult question to answer, but several serious issues have been raised.[122] For the circumbinary case, the shape of the habitable zone is no longer circular and depends on the nature and motion of both stars. For sun-like star or smaller (types G, K, and M), the planet would have to orbit closely enough that its orbit can become unstable due to the gravity of the stellar pair. This could lead to the planet being ejected or falling into one of the stars. Curiously, 8 of the 10 examples found as of 2015 orbit just outside this region of instability and so have remained safe.

In the case of Kepler 16b, a Saturn-sized planet orbiting a pair of stars of moderately different masses (69% and 20% the mass of our sun respectively), this situation leads to a lopsided habitable zone. The planet passes into and out of the zone causing temperature fluctuations of 15 °C (59 °F) four or five times during its year. In contrast, Kepler 453b orbits a star like our sun and a much smaller red dwarf star (20% the mass of our sun). This leads to a more even habitable zone allowing the planet to stay within it for its full orbit.

The circumstellar case has a lot going for it because the habitable zone would function nearly the same as for the single star case because the sibling star’s radiation would have little effect on it. The problem, however, is the second star will exert a gravitational influence that could tug the planet into a more elongated eccentric orbit. If this process continues long enough, the planet will regularly move out of the habitable zone causing severe climatic swings.

In sum, we cannot completely rule out the possibility of habitable planets in these systems, but they would face some considerable challenges.

Stellar Habitability: Metallicity

A star’s metallicity—the quantity of elements heavier than helium—has a strong bearing on the number and size of planets that can form in orbit around it. Included among these elements are those needed for living creatures, such as carbon, nitrogen, and oxygen. Since the big bang produced only hydrogen and helium, “metals” can only be produced later in the history of the universe from the nuclear furnaces of stars. When massive stars run out of fuel, they collapse triggering a massive explosion called a supernova. These events scatter large quantities of metals into the interstellar medium making them available to be included in new stars and their planets. As such, the concentration of metals in a forming solar system is critically dependent upon the quantity of metal-enriched ashes from previous stars. Thus, the metallicity of a star varies significantly based on its location within the galaxy as well as when in the universe’s history it was born.

Our own sun consists of about 2% metals by mass and serves as a benchmark when reporting on other stars, so it is referred to as solar metallicity. The average metallicity in our solar neighborhood is about two-thirds of this value. As nearby examples, Tau Ceti has a metallicity of about 25% of our sun’s while Alpha Centauri has twice the solar metallicity.[123]

Since planets are built up almost entirely of metals, it is not surprising that a star’s metallicity will affect planet formation. If the metallicity is too low, then either no planets will form, or they will be small like Mercury or Mars and unable to retain a significant atmosphere. One estimate is that stars with a metallicity less than one-third of our sun’s will have a significantly reduced probability of having habitable planets.[124] Analysis of extrasolar planets reveals a clear correlation between stars with high metallicity and the presence of hot Jupiters that would endanger small terrestrial planets.[125] Thus, metallicity exhibits a Goldilocks zone, where habitability requires neither too little nor too much concentration of metals. Of course, we need to be cautious because the exact relationship between metals and planet formation is still unknown.

Stellar Habitability: Example of Proxima Centauri b

To help understand the importance of stellar habitability, let us look to our nearest neighbor, the Alpha Centauri star system just over 4 light-years away. It is a trinary system consisting of three stars (see Figure 14):

Star A—a yellow G type star, slightly more massive than our sun.
Star B—an orange K type star, slightly less massive than our sun.
Star C—a red dwarf M type star, 12% the mass of our sun.

Stars A and B form a closely associated pair orbiting each other every 80 years with a separation distance that varies between 11.2 and 35.6 au. The third star, C, is located a distant 0.21 light-years (13,000 au) away from the other two. Because of this wide separation, it has historically been called Proxima Centauri even though it is still considered to be a part of the Alpha Centauri system.

Figure 14: Diagram of the Alpha Centauri star system.

There was great excitement in 2016 when astronomers announced the discovery of an Earth-sized planet (³ 1.07 M_Å) orbiting in the habitable zone of Proxima Centauri. Known as Proxima Centauri b, it is located at 0.05 au and orbits its star every 11.2 days. It has been hailed as one of the best candidates for being habitable. (As of July 2022, it is listed as the sixth most potentially habitable exoplanet.) Even more exciting for astronomers, it is located very close to us allowing for easier study. This makes it a great candidate for sending interstellar probes, such as proposed for Breakthrough Starshot.

As exciting as all this is, the potential habitability of Proxima Centauri b is extremely doubtful.[126] This planet faces the three major threats common to all planets of red dwarf stars: (a) extreme stellar activity; (b) severe flaring; and (c) tidal locking. Astronomers have detected regular superflaring coming from the star.[127] Computer models suggest that the planet will be subject to severe atmosphere loss.[128] While some still hold out hope, the challenges to potential habitability are formidable. This is a powerful reminder that even though Proxima Centauri b is an Earth-sized planet in its star’s habitable zone, it is highly unlikely to be habitable.

Planetary Habitability: What makes a planet “suitable” for life?

The factor n_e in the Drake equation attempts to quantify the number of planets in a solar system with an environment suitable for life. Again, we encounter the notion of being “suitable.” In the minds of SETI proponents, this was trivial—only two criteria needed to be satisfied:

Planetary size. Only Earths and possibly super-Earths have the potential to support liquid surface water. As discussed earlier, newer evidence shows that super-Earths are likely to be mini-Neptunes and therefore can be eliminated as candidates.
Orbital radius. To remain warm, a planet must reside within the star’s circumstellar habitable zone.

While these properties are obviously critical, they are certainly insufficient to guarantee habitability as previously explained in Habitable Zone vs Habitable. A quick review of the five key properties that were discussed there:

Albedo. A planet’s albedo is the fraction of incident light that is reflected back into space by the planet’s surface.
Atmosphere. An atmosphere is crucial for a planet to retain sufficient heat via the greenhouse effect. To avoid problems, the atmosphere must not be too thick or too thin and may need to carefully change over time to reflect changes in its star’s brightness.
Carbonate-silicate cycle. Carbon dioxide can be removed from the atmosphere by reacting with silicate rock and is later returned via plate tectonics and volcanic activity. Because this process is temperature dependent, it serves as a critical thermostat that helps maintain a planet at a moderate temperature.
Water. Water plays an essential role in the albedo, the atmospheric greenhouse effect, plate tectonics, and the carbonate-silicate cycle.
Magnetic field. A strong magnetic field is needed to protect a planet’s atmosphere from being stripped away by stellar activity. It is also critical for protecting any lifeforms from cosmic rays.

Unfortunately, these additional factors are not covered in the Drake equation and are therefore largely ignored by SETI. One reason for this is that these additional factors involve geology or atmospheric science that are currently difficult or impossible to directly measure forcing astronomers to rely on complex models. In contrast, the criteria defining the habitable zone are amenable to astronomical measurement using current technology making them experimentally quantifiable. While these extra factors may not be as well-defined as the astronomical ones, they are no less important when trying to calculate the likely number of advanced intelligent civilizations using the Drake equation.

Including all seven planetary factors gives us a much more meaningful assessment of planetary habitability, yet even that list is far from complete. Some additional properties:

Magnesium/silicon (Mg/Si) ratio. This ratio determines the composition and properties of a planet’s crust and therefore determines a planet’s ability to support plate tectonics and volcanic activity.[129] Earth has equal parts magnesium and silicon (Mg/Si ≈ 1) and therefore our crust is primarily composed of pyroxenes and olivine. In contrast, 55 Cancri e is magnesium poor (Mg/Si = 0.87), so its crust is mainly feldspar, a much more viscous material. The opposite extreme is the magnesium rich Tau Ceti system (Mg/Si = 1.78) resulting in a crust consisting of olivine and ferropericlase. The resulting crust would be much thicker and difficult to fracture. These crustal differences will impact the planet’s ability to sustain plate tectonics for its carbonate-silicate cycle although the full implications for habitability are not completely understood.
Orbital eccentricity. For planets with highly eccentric orbits, the distance between it and its host star will vary considerably throughout its orbit and therefore the heat it receives from the star will correspondingly vary substantially during each orbit. For large eccentricities, the planet would regularly pass outside the circumstellar habitable zone making it uninhabitable. For less extreme values, the planet would still experience dramatic swings in temperatures resulting in extreme climate changes. A weak eccentricity could be challenging for life but would not necessarily be fatal.
Long-lasting radioisotopes in the core. Certain long-lasting radioisotopes, particularly uranium-238 and thorium-232, are needed to provide a steady supply of heat to maintain a molten core.[130] This is necessary to maintain a planetary magnetic field and to sustain plate tectonics.

A more comprehensive list of parameters is given in Fine-Tuning for Life on Earth on page 69. It should be emphasized that not all the listed properties are necessarily critical enough to prevent microscopic life yet would certainly be disastrous for advanced life.

One thing is clear, if we take all these habitability requirements into account, then n_e will be drastically smaller than has been traditionally assumed. Consequently, estimates for the number of advanced communicating civilizations predicted by the Drake equation will be far lower than typically predicted by SETI proponents.

Solar System Habitability

So far, we have only expanded upon the terms already present in the Drake equation, but let us now consider some additional factors. Presented here are properties of the whole solar system (other than the star and planet itself) that directly impact planetary habitability. In other words, other bodies in the solar system can have a surprisingly strong influence on the habitability of a given planet. There are at least three major solar system properties that could critically affect habitability: (a) large moons; (b) planetary migration; and (c) Jupiters. Let us explore each of these in detail.

Solar System Habitability: Large Moons

It may seem surprising that our Moon is more than just a beautiful light to fill the night sky, but it is also critical to maintaining life on this planet. What makes it so unusual is its large size and close orbit. Although it is only the fifth largest moon in our Solar System, its mass relative to its parent planet is 50 times larger than for any of these other moons. The problem is that it is too large to have formed alongside the Earth from the same protoplanetary material, nor could it have formed elsewhere and been captured by Earth’s gravity.

So, how did we get our exceptional Moon? The prevailing model is the giant-impact hypothesis where a Mars-sized proto-planet named Theia crashed into Earth about 100 million years after the formation of the solar system. The resulting collision threw up a debris cloud that eventually coalesced to form the Moon. These circumstances are likely to be extremely rare and every detail of the collision needs to be just right to benefit later life.[131] For example, the impact had to occur at the “just right” angle because head-on collision would have destroyed the Earth while a glancing blow would not have given rise to the Moon.

In addition to giving us the Moon, this impact event transformed the Earth in multiple critical ways that allowed for the later emergence of life. Here are five specific examples:[132]

Removed Earth’s original thick atmosphere. Earth’s original thick Venus-like atmosphere was removed, allowing for the emergence of a much thinner life-supporting atmosphere.
Increased Earth’s mass. Since much of Theia’s mass was incorporated into Earth, this boosted Earth’s surface gravity allowing it to retain large amounts of water for billions of years.
Increased iron in the core. The collision event supplemented the amount of iron in Earth’s core. This was critical for establishing a molten iron core needed for a strong and enduring magnetic field. Our magnetic field is needed to prevent the sputtering away of Earth’s atmosphere and protect surface life from deadly cosmic rays and solar X-rays.
Increased long-lasting radioisotopes in the core. These radioisotopes release the heat needed to keep the core molten and to sustain our magnetic field. This heat is also necessary to drive Earth’s tectonic activity and volcanism that are required for the carbonate-silicate cycle.
Supplemented early Earth’s magnetic field. Prior to about 3.5 billion years ago, the moon had a significant magnetic field. This magnetic field together with being much closer to Earth at that time, reinforced Earth’s magnetic field to help protect Earth’s atmosphere from being stripped away by our young Sun’s radiation.[133] By the time the moon lost most of its magnetic field, its contribution was no longer necessary because the Sun’s activity had settled down to manageable levels.

In addition, the Moon plays an ongoing role in sustaining Earth’s habitability:

Stabilizes Earth’s tilt-angle. A planet’s tilt-angle (or obliquity) is the angle its rotation axis makes with a line perpendicular to the orbital plane. The Earth’s tilt is 23.5° giving us our four seasons. Our Moon stabilizes this tilt-angle so that it varies by no more than about 2.4° (changing from 22.1° to 24.5° and back) over the course of tens of thousands of years (see Figure 15a).[134] In contrast, Mars has only two small moons (Phobos and Deimos) so its tilt can change by as much as 60° (see Figure 15b). If Earth had been subject to such large variations in the tilt-angle, it would have resulted in extreme climate variations that would have been disastrous for advanced life.
Slowing Earth’s rotation. Our moon has gradually slowed Earth’s rotation rate to a life-sustaining level. Around 4.5 billion years ago, Earth rotated once every 2 hours but today it has slowed to once every 24 hours.

All of this in addition to our moon’s daily role in producing tides

Our exceptional moon benefits us in yet one more way—it allows perfect solar eclipses. Its size and distance from Earth are just right to block the sun’s bright photosphere while allowing people to observe its chromosphere. Of the 64 biggest moons in our Solar System, only Saturn’s moon, Prometheus, is a near match in this regard.[135] While observing total solar eclipses is not necessary for life, it is valuable for scientific discovery. Through observing them, astronomers have (a) determined the nature of the Sun’s atmosphere; (b) discovered the element Helium; (c) confirmed General Relativity’s prediction that gravity would bend light; and (d) revealed the timing of Earth’s rotation rate in the past.[136]

a) b) Figure 15: Maximum variation in tilt-angle for a) Earth and b) Mars.

Solar System Habitability: Migrating Planets

In the 1960s, planet formation models based solely on our own solar system were very straightforward and appealingly simple. Small terrestrial planets form close to the star and gas giants form farther out. This simple picture was shattered by the discovery of hot Jupiters starting in 1995 leading to a profound rethink. Planets can migrate from where they initially form, and this poses a direct threat to potentially habitable planets. If Jupiters drift into the inner solar system, simulation shows that smaller planets are either forced into the star or are ejected from the planetary system altogether. Stars with hot Jupiters are unlikely to possess viable planets.[137]

What about our solar system? Why didn’t Jupiter drift into the inner solar system as frequently happened elsewhere? Our current understanding is that both Jupiter and Saturn did move inward but later reversed course. A detailed description of this motion is given by the Grand Tack hypothesis named in reference to a sailboat tacking into the wind.[138]^,[139] In this model, Jupiter formed first at around 3.5 au but there was enough nearby gas to cause it to drift inward. Once Saturn reached full size, it too began its journey inward only faster because of its lower mass. When Jupiter reached about 1.5 au, the two planets were close enough together that they entered a 2:3 mean motion resonance. This had the effect of halting their inward migration and causing them to move outward again until Jupiter reached its current location of 5.2 au. Clearly this intricate dance requires some very particular circumstances.[140]

One of the main motivations for developing the Grand Tack model was to resolve the Mars problem. Previous planetary formation models consistently predicted Mars to be 5-10 times more massive than Earth, whereas Mars is actually one tenth the mass of Earth. The Grand Tack resolves this because Jupiter’s inward migration removes much of the original asteroidal material that would have gone into building up Mars. This model also explains key details about the composition of the asteroid belt.

Solar System Habitability: Jupiters

Another big surprise is that habitable planets require a suite of Jupiters. In our solar system, Jupiter and Saturn contain most of the mass excluding the sun. Specifically, Jupiter is 318 times and Saturn is 95 times more massive than the Earth. Because of their larger masses, their gravity plays an important role in sculpting the solar system. Gas giants can impact the habitability of terrestrial planets in two major ways:[141]^,[142]

Clearing the early solar system. In the early solar system, Jupiters act to clear out a wide swath of comets and asteroids near their path by either absorbing or scattering them outward. Planetary migration can expand the region cleared as illustrated in our solar system by the Grand Tack model. This activity removed most objects large enough to create mass extinctions if they later struck Earth.
Cosmic shield. Our Jupiter has an ongoing role of protecting Earth by either deflecting or absorbing (as was demonstrated by the Shoemaker-Levy collision with Jupiter in 1994) comets and asteroids that wander about the solar system.

According to one study, the combined action of Jupiter and Saturn in clearing the solar system greatly reduced how frequently Earth received extinction-level asteroid strikes.[143]^,[144] Historically, we know that the Earth was hit by major strikes roughly every 100 million years, such as the one that wiped out the dinosaurs. Without our gas giants, this would likely have occurred 1,000 times more frequently, or about every 100,000 years. Under such bombardment, it is difficult to imagine an advanced civilization being able to arise.

It is not enough to simply have gas giant planets—they must have the right orbital properties:141^,142

Right size and location. If the gas giant planets are either too small or too distant, their ability to act as a shield would be inadequate. On the other hand, if they are too large or too close, then their gravitational pull will disrupt the orbit of the smaller planet. In that case, they would eject, destroy, or prevent from forming any small rocky planets that could support life.
Near circular orbit. Jupiter has a near circular orbit (eccentricity = 0.048), yet most cold Jupiters (those orbiting at least 1 au out) have an average eccentricity of 0.25.[145] One specific example is Epsilon Eridani b, a Jupiter-mass planet with highly elliptical orbit (eccentricity = 0.6). Because of their large masses, Jupiter-like planets with highly eccentric orbits would disrupt the orbit of smaller inner planets.
Not a hot Jupiter. The most dangerous scenario for life is when a Jupiter migrates close to its parent star becoming a hot Jupiter. Small rocky planets in the inner solar system would likely be destroyed or ejected from the system.

The discovery of extra-solar planets has confirmed that while Jupiters are common, “just right” Jupiters are neither guaranteed nor even likely.

Galactic Habitable Zone

Yet another class of factors affecting a planet’s chance of being habitable involves its star’s stellar neighborhood and position within the galaxy. These factors are routinely ignored by SETI and therefore little research has been done into identifying which galactic regions would be suitable for planetary habitability. One reason for the lack of interest is that the vast majority of exoplanets discovered so far are located nearby due to the technical challenges of finding them. That means that most confirmed exoplanets share the benefits of our star’s favorable galactic location.

Since most stars in our galaxy are located outside this “safe” region, they will be subjected to additional dangers and problems not experienced near us. Failure to include these considerations in the Drake equation results in erroneously high galaxy-wide estimates for advanced intelligent civilizations. A simple illustration of this kind of sampling bias would be to determine the average household incomes here in American and then apply it to all nations around the globe. Any analysis based on that would be seriously misleading because average incomes vary dramatically around the world.

There are a great number of galactic hazards than can threaten habitability, such as supernovae, gamma-ray bursts, stray black holes, magnetars, and active galactic nuclei.[146] Of these, supernovae represent the most common threat. A supernova is a tremendous explosion that occurs when a massive star runs out of fuel. The resulting explosion showers everything around it with high-energy particles and deadly gamma-ray radiation and for a brief period of time, it can shine brighter than 100 billion suns! A supernova located within 30 light-years would likely destroy most surface life on a planet. However, supernovae are a major source of the metals needed to make planets and life. Therefore, having lots of supernovae very early in history is just as essential as avoiding nearby supernovae once advanced life develops.

Gamma-ray bursts represent a second major galactic hazard. While far less common than supernovae, they pose a far greater threat. These come in two different types. “Long” gamma-ray bursts are hypernovas resulting from an ultramassive star running out of fuel and collapsing into a black hole. For tens of milliseconds to tens of minutes, it can radiate brighter than 10 billion billion suns (i.e., 100 million times that of a type Ia supernova).[147] In contrast, “short” gamma-ray bursts are the result of neutron star mergers. For both types, this blast of energy is highly directional damaging solar systems in its beam up to 1,000 (or even 10,000) light-years away.

Because gamma-ray bursts are very brief, only one hemisphere will take the brunt of the initial blast of high-energy radiation largely sparing the other side. But the real danger comes from the secondary effects in the atmosphere leading to longer and more wide-spread damage. Some of these effects include (1) depletion of the ozone layer permitting larger amounts of ultraviolent radiation to reach the surface; (2) formation of nitrous oxides and acid rain; and (3) global cooling.[148] Very distant gamma-ray bursts may have been responsible for some large-scale extinction events on Earth.[149]^,¹⁴⁸

Early attempts to assess habitability within our galaxy date back to the 1980s. But it was not until 2001 that Gonzalez, Brownlee, and Ward defined the galactic habitable zone as the galactic equivalent of the circumstellar habitable zone.[150] The two main defining characteristics of this region are:

“Just Right” Metallicity. As explained in Stellar Habitability: Metallicity, metals represent the elements inherited from previous generations of stars that are essential to building planets. Too few metals results in either no planets or sub-Earths that are not massive enough to hold onto an atmosphere. Too many metals result in hot Jupiters whose migration inward can disrupt small rocky planets.
Safe Cosmic Neighborhood. The galaxy is a dangerous place. There are multiple threats that can inhibit or even extinguish potential life on a planet. (Not included here are lesser dangers, such as large asteroid impacts, that may result in mass-extinctions but not near sterilizations.) To have an advanced civilization requires the planet remain relatively safe for several billion years.

Later work refined the galactic habitable zone and established it to be an annulus (ring-shaped zone) extending 7,000-9,000 parsecs (23,000-29,000 light-years) from the galactic center.[151] (Our sun is located near the middle of this zone, 8,300 parsecs or 27,000 light-years from the galactic center.) According to this analysis, only about 10% of stars in the Milky Way reside in this habitable zone. It should be noted that this boundary is not sharply defined, so habitable planets could be present outside this region.

To define the galactic habitable zone, we need to understand how metallicity and galactic hazards vary as a function of location within the galaxy. For that, it is useful to broadly divide the Milky Way galaxy into four distinct regions based on the location, age, and composition of its stars. These regions can be visualized by looking at a diagrammatic cross-section of our galaxy (see Figure 16). The inner-most region is the central bulge containing a supermassive black hole (Sagittarius A*) at the galactic center and a high concentration of stars. Extending outward along the galactic plane is the pancake-shaped collection of stars known as the thin disk. This is where our sun resides along with 96% of our nearest neighbors. Surrounding the thin

Figure 16: Cross-section of the Milky Way galaxy

disk is another distinct batch of stars known as the thick disk. These three regions in turn are surrounded by a loosely spherical collection of stars known as the halo. In addition, our galaxy contains at least 150 globular clusters each consisting of a collection of densely packed stars. They are embedded in the halo like blueberries in a muffin. In the diagram, GNP and GSP are the galactic north and south poles respectively.

Each of these galactic regions has qualities that have strong implications for the habitability of their stars.[152]^,[153],[154]

Central bulge. Contains most of the stars in our galaxy. As a rule, these stars are significantly older than our sun and very metal rich. They are more closely packed than in other regions and so have a high probability of orbital disruption caused by stellar neighbors that come too close.
Thin disk. The stars of the thin disk are relatively young compared to other regions with ages varying from very recent to 10 billion years old. In the same way, the metallicity varies from as high as three times solar metallicity to a mere 1% with an average value of 2/3^rds solar metallicity.
Thick disk. Comprising about 10% of our galaxy’s stars, thick disk stars are older than those of the thin disk. Correspondingly, they are more metal-poor, being about 25% solar metallicity on average.
Halo. The halo consists of about 1% of galactic stars but are spread over a very large area resulting in a very sparse population (except for the globular clusters). These stars are even older than the thick disk (about twice our sun’s age) and extremely metal poor (about 1% of solar metallicity).
Globular clusters. These represent very dense groups of stars that can have 100,000 stars packed into a space only tens to hundreds of light-years across. The largest globular cluster in the Milky Way is Omega Centauri consisting of 10 million stars within a radius of 150 light-years. For that case, a study has shown that the habitable zone of any of the stars will likely be violated by another star passing through it about once every million years.[155] That would be devastating for any planet that might otherwise be habitable. There is little chance for either habitable planets or life in globular clusters because of the extreme overcrowding and because these stars are very old.[156]

So, what can we conclude from all this? It is generally agreed that stars in the thick disk and halo are unlikely to be habitable. Their low metallicity means that planet formation would be limited at best. Globular clusters are also ruled out because of the high density of stars leading to frequent disruptions of their planets by neighboring stars. In the big picture, that is not a major issue because together these three groups only make up around 10% of stars in the galaxy. The thin disk with its ongoing star formation is well-suited for habitability and therefore largely defines the galactic habitable zone. Of course, given the wide range of ages and metallicities at least some of these stars will not be life friendly.

But what about the central galactic bulge? Because it has the greatest number of stars, its inclusion or exclusion in the galactic habitable zone will have a dramatic effect on the overall estimate of galactic habitability. Four major factors limit the habitability inside the galactic bulge: (a) radiation from the supermassive black hole at the galactic center; (b) frequent nearby hazards (e.g., supernovae and gamma ray bursts); (c) high stellar densities resulting in frequent close encounters with other stars; and (d) very high metallicities. Given the large number of stars present, could a significant portion be habitable despite these dangers? We cannot be certain at this point, but our current understanding places the bulge region outside the galactic habitable zone.

Galactic Habitability

Before moving on, there is yet one more habitability question to consider. Are all galaxies equally suitable for sustaining life-supporting solar systems? This is a far more difficult question than the corresponding one for individual stars and planets. Little work has been done to address this question because the scope of the Drake equation is generally limited to just our galaxy and in that case, the suitability of other galaxies is largely irrelevant. There are an estimated 100-200 billion galaxies in the observable universe. Just how do they compare to our own Milky Way galaxy in terms of habitability?

While we have a lot of information about other galaxies, it is difficult to fully assess how their features correlate with the potential habitability of their solar systems. As with the galactic habitable zone, galactic habitability is largely determined by two factors: (a) the existence of the right amount of heavy elements to form Earth-like planets and (b) a low occurrence of nearby catastrophic events that could wipe out life. Based on these criteria, we can identify some galactic situations where habitability would be severely limited (although not necessarily completely ruled out):[157]^,[158]

Early Universe. The most distant known galaxies are too young to have enough metals for the formation of Earth-sized planets. In addition, there would be a high prevalence of hazards, including energetic quasar-like radiation and frequent supernova explosions.
Elliptical galaxies. Stars are too metal-poor. Solar-mass stars have evolved into giants that are too hot for life on inner planets.
Irregular galaxies. Large irregular galaxies possess active galactic nuclei that spew out deadly radiation. Small irregular galaxies lack the necessary metals.
Small galaxies. Most stars in these galaxies are too metal-poor.

In addition to the nature of the galaxy, we must consider the neighborhood in which the galaxy resides:

Dense galactic clusters. A typical galactic cluster contains 10,000 closely packed galaxies and most clusters include giant and supergiant galaxies. The resulting dangers include gravitational interactions leading to distorted galactic structures and blasts of radiation coming from neighboring galaxies.
Rogue galaxies. Occasionally a galaxy is ejected from its local group. This avoids the dangers of dense clusters but results in an absence of nearby dwarf galaxies to absorb. Galaxies must periodically cannibalize these captured dwarf galaxies to have enough fresh material to maintain robust star formation for long periods of time.

Clearly, much work remains to be done, but this is enough to demonstrate that the Copernican principle fails even at the galactic level. Similar analysis applies to superclusters and even super-superclusters of galaxies.[159]

But what of our galaxy? Does it exhibit any exceptional qualities beneficial for life? Several details stand out:158

Stable spiral structure. The Milky Way’s spiral arms are exceptionally symmetrical and evenly spaced with respect to each other and they are approximately the same size. To maintain this spiral structure for billions of years, there must be ongoing star formation. For that, our galaxy must regularly absorb a small nearby dwarf galaxy about every half-billion to one billion years.
Galactic safety. The Milky Way galaxy has not experienced any significant collisions or merger events over the past 10 billion years. In contrast, our nearest neighbor, the Andromeda galaxy, has not been so fortunate displaying a huge warp in its arms as a result.
Sparse galactic neighborhood. The Milky Way resides in a small, sparse galactic cluster known as the Local Group located at the outer edge of the Virgo Supercluster of galaxies. The Local Group consists of only two medium-sized galaxies (the Milky Way and Andromeda) and at least 50 dwarf galaxies.

These are just a few of the critical properties that set our galaxy apart from most others.

Alternative Worlds

Our analysis so far has followed the traditional view that only the surfaces of terrestrial exoplanets orbiting sun-like stars are viable abodes for advanced extraterrestrials. Thinking outside the box, what if advanced life could arise in other locations? This question is important because it has the potential to significantly increase the number of places where intelligent life might arise. The two most popular alternative life sites are moons orbiting gas giant planets and rogue planets. But just how realistic are these scenarios?

Alternative Worlds: Moons of Gas Giant Planets

Of all the places in our solar system (other than Earth), the best prospect for harboring life are the moons of Jupiter and Saturn. In their favor, moons of gas giant planets generally have lots of water because they form alongside their host far enough from the sun where frozen water was prevalent in the early solar system. Given the enormous numbers of Jupiter-like exoplanets that have been discovered so far and the likelihood that most of them will host a plethora of moons, that could greatly expand the number of places that life could exist in the galaxy. This is an intriguing possibility even though we cannot currently detect exomoons and only about two dozen candidates are known (as of July 2022).

Let us begin by considering the moons of Jupiter and Saturn that will serve as our role models for exomoons. These two planets together have more than 100 moons, but only the six largest will be considered here. Four belong to Jupiter (Io, Europa, Ganymede, and Callisto) and two to Saturn (Enceladus and Titan). Of interest, these have some remarkable planet-like properties:[160]

Io. Jupiter’s moons receive 3% the sunlight we do, yet Io has temperatures ranging from 1500 °C (2700 °F) to ‑130 °C (‑200 °F). These high temperatures are the result of tidal heating caused by its gravitational interactions with Jupiter. Consequently, Io is also the most volcanically active place in the solar system.
Europa. Europa has 10 km thick ice shell that is extremely smooth. This is the result of the surface ice undergoing the equivalent of plate tectonics. This makes it the only body in the solar system other than Earth (and possibly Mercury) to have ongoing plate tectonics. Beneath the icy exterior is a subsurface ocean (10-100 km deep) kept warm by tidal heating due to its interactions with Jupiter. Europa has one more interesting feature—an induced magnetic field. This is a result of Jupiter’s strong magnetic field interacting with salt ions in Europa’s subsurface ocean.
Ganymede. The largest of all the moons in our solar system. It is slightly larger than the planet Mercury (although less massive because it is 50% water). Like Europa it has an icy shell with a large subsurface ocean. Interestingly, it generates its own magnetic field as well as having an induced magnetic field.
Callisto. Located farther out than the previous three and experiencing no tidal heating, it experiences a frigid average temperature of ‑139 °C (‑218 °F). Despite this, it still maintains a subsurface ocean with a small induced magnetic field.
Enceladus. Saturn’s moons receive only 1% of the light from the sun that Earth does. Like Jupiter’s moons, Enceladus has an icy surface but with a global subsurface ocean. As proof of its water, ice volcanoes can be observed shooting jets of water into space.
Titan. This is the second largest of the moons and is also the most interesting. Titan is the only moon to have a sizable atmosphere (95% nitrogen and 5% methane). At a frigid ‑180 °C (‑290 °F), any surface water would be frozen, but methane remains a liquid. As a result, Titan is the only planet or moon outside Earth with liquid seas on its surface (albeit composed of methane rather than water). This liquid methane can evaporate and rain down again in a cycle analogous to Earth’s water cycle. It also has significant subsurface water oceans.

Future NASA missions are planned that will attempt to probe some of these moons for evidence of life.

There are two main scenarios to consider for life on exomoons: surface and subsurface. For the case of surface life on moons in our solar system, only Titan has an atmosphere and surface liquid and so could hypothetically support non-traditional biochemistry. All six moons listed above have subsurface oceans that could hypothetically support life. If a moon’s core is not completely frozen over, then there could be hydrothermal vents on the exposed mantle that might support life as they do at the bottom of Earth’s oceans. A second possibility is if the ice shell is thin enough, cracks could allow high-energy molecules on the surface to reach the deep ocean.[161] Either of these underwater scenarios might allow for simple life, but an advanced civilization would be highly unlikely because the watery environment would prevent the development of fire, metal processing, and other technologies.

A better and more general scenario for surface life is for a gas giant planet to migrate with its moons inward into its star’s habitable zone. These moons would receive more heat allowing for surface water oceans. Migration is not an uncommon scenario with five times more Jupiters in the circumstellar habitable zone than Earth-sized planets[162] Of course, potential habitability for moons is more complicated than for planets because one must consider interactions with both the planet and the star.

While intriguing, this scenario faces several major problems.[163] First, large moons are generally not massive enough to retain a significant atmosphere. (Titan does, but this may be due to its cold temperature.) In general, for a moon to be massive enough, it must belong to a gas giant with at least 20 times the mass of Jupiter. The problem with such a massive planet is that it would wreak havoc on the other planets and moons of the solar system. Second, most Jupiters that have migrated inward have highly eccentric orbits subjecting their moons to extreme temperature variations. Third, if the moon is situated too close to its planet, it will suffer tidal locking. But if the moon avoids this by being located farther out, its distance from the sun will vary greatly resulting in severe temperature swings. Fourth, moons lack many critical features for habitability, such as a carbonate-silicate cycle to regulate temperature.

Alternative Worlds: Rogue Planets

During the chaotic early stages of planet formation, one or more forming planets may get ejected from a solar system. It has been theorized that at least one planet was ejected from our Solar System.[164] Astronomers estimate that there may be as many of these free-floating rogue planets as there are regular planets. Unfortunately, they are extremely hard to detect, but some have been found using gravitational lensing (with 10 Jupiter-sized planets found as of 2011).[165] Terrestrial rogue planets are expected to be more common than Jupiter-sized ones.

One of the big challenges for rogue planets is retaining enough heat to maintain liquid water in the absence of a nearby star. Without a star, the remaining sources of heat are (a) internal heat from the planet’s formation, (b) radioactive elements in the core, and (c) tidal interactions between the planet and its moons. For large moons of a rogue Jupiter, tidal heating could provide heat along the lines described in the previous section. But what about a solitary terrestrial planet? One possibility is that if it is ejected early enough to retain its primordial thick hydrogen atmosphere (10-100 times thicker than our atmosphere) that could act as a blanket helping it retain some of its initial heat. (This hydrogen is normally stripped off by the solar wind in the early stages, so planets ejected at a later stage would not have this.)

For a planet identical to Earth but starless, the situation is rather grim.[166] The surface temperatures would be close to ‑235 °C (‑391 °F) with only the radioactive elements in the core to keep it warm. Nevertheless, a small subsurface ocean might be possible. The prospects for habitability in this scenario could be improved in four ways:

Increase water. Once the top 15 km (9 miles) is frozen any remaining water beneath it could be liquid.
Increase mass. Large planets would have more radiometric elements to heat the core. This scenario works best for planets weighing at least 3.5 Earth masses.
Adjust the atmosphere. If the atmosphere had a greater concentration of carbon dioxide from volcanism, that could help keep the surface warm.
Moon. The presence of a large moon in an eccentric orbit could heat both the planet and itself through tidal heating.

While these scenarios offer some hope for life, it would likely only be relatively simple life.

The Failure of the Copernican Principle

Historically, the Copernican principle grew out of a series of astronomical developments. First, the heliocentric model of Nicholas Copernicus displaced Earth from the center of the universe and treated it as just another planet. Second, William Herschel proved that the Solar System was moving with respect to the center of the Milky Way galaxy. Third, Edwin Hubble showed that our galaxy was just one of many galaxies in the universe. Taken together, it demonstrates that the Earth does not reside at a privileged location in the universe. The Copernican principle is assumed in many cosmological models.

But how does this relate to the question of extraterrestrial life? Formally, the Copernican principle just means that we do not occupy a special location in space and therefore the universe is more or less the same in all directions. This has generally been found to be true and useful in cosmology. However, a common generalization based on this is the philosophical notion that Earth is just one of myriads of planets that exist in the universe with nothing to fundamentally distinguish it as being in some way special. In other words, most planets are generally viewed as being similar with the individual differences being relatively unimportant. The same idea also applies on the level of solar systems and galaxies. This perspective is essentially the mediocrity principle. If Earth is cosmically average, then whatever we experience here should be true in many other places. There is life on Earth, therefore there should be life on many other planets as well. But is this true?

On the face of it, it seems reasonable. Superficially, there is little to fundamentally distinguish our Sun from the billions of others. And we have already discovered thousands of exoplanets, many of which are small rocky planets like Earth. Yet this notion of mediocrity is an unproven assumption, not an astronomical fact. The Copernican (mediocrity) principle fails if habitability and the origin of advanced life depend on the particular details of planets and solar systems. In that case, extrapolations based on superficial similarity would lead to wrong conclusions.

The last 60 years have done much to shatter the myth of mediocrity. Even a quick survey of confirmed exoplanets shows the extreme diversity of possible planetary configurations. Clearly, our solar system is not as common as once thought. Moreover, the notion that a planet being in the circumstellar habitable zone is sufficient for habitability as embodied by the Drake equation has already been demonstrated to be false. On top of all this, decades of astronomical research have consistently shown that the Copernican principle fails at the level of the solar system and galaxy. A quick summary:

Planetary habitability. In addition to a planet’s size and location, planetary habitability depends on having the right atmosphere, plate tectonics, magnetic field, and lots of water to name a few examples.
Stellar habitability. Largely ignored in the 1960s, astronomers now recognize that a star’s type and whether it is a single star play an enormous role in potential habitability of its associated planets. Even properties, such as age and metallicity need to be considered.
Solar system habitability. To even have a chance of developing advanced life, a planet needs to remain relatively safe for billions of years. To that end, large moons, planetary migration, and the presence of Jupiters all play a critical role.
Galactic zone habitability. It may seem surprising, but even where a star is located inside the Milky Way plays an important role in allowing for habitable solar systems like our own.
Galactic habitability. Curiously, not all galaxies are equally capable of generating potentially habitable star systems. Going further, the density and location of clusters and superclusters of galaxies have important ramifications for possible life.

Clearly, Carl Sagan was wrong when he dismissed Earth as “an insignificant planet of a humdrum star.”38

Strictly speaking, the Copernican principle only maintains that we do not live in a privileged location. More succinctly, we do not reside at the center of either our solar system, galaxy, or universe as had been assumed in the past. Yet, astronomy has also demonstrated that spatial centrality is undesirable. Instead, specialness is now recognized to be a careful balance—a Goldilocks zone of avoiding too much and too little and being just right. In this sense, we are truly in a special location within our solar system, galaxy, and universe. We are indeed special in every way that matters.

Criticisms of the Drake Equation

The Drake equation is perhaps one of the most recognizable equations in science after E = mc². Although it was primarily intended to facilitate discussion at the first SETI conference in 1961, it has cast a long and enduring shadow over the debate about extraterrestrial life. Most discussions of the Drake equation focus on how to estimate the individual components, but what about the equation itself? Does it need to be revised?

To refresh our memories, let us recall the equation as presented earlier.

The computed value, N, is the estimated number of advanced intelligent civilizations in our galaxy that we could potentially contact. This version of the equation includes the f_s term that was not present in the original 1961 formula.

Before criticizing the Drake equation, let us begin by considering what it gets right. First, it is really simple. This allows it to be useful to even non-specialists. Second, it serves as a great way to breakdown the big challenge of estimating the number of alien civilizations into much simpler pieces for easier analysis. SETI Institute’s Jill Tarter explains, “It’s a great way to organize our ignorance.”[167] Third, it is flexible, allowing others to modify it to include new information not available in 1961. Fourth, it still inspires today. Not bad for a last-minute addition to the conference.

While the Drake equation has proven itself to be very useful, it does have several major flaws:

Overemphasis on the lifetime term, L. One of the great flaws is the use of R_* and L.[168] This is because L is the most arbitrary and least-well constrained factor that was free to vary by a factor of 100 million (8 orders of magnitude)! In contrast, the remaining factors were generally assumed to vary by less than a factor of 10. This ensured that the choice of L dwarfed all other contributions in importance. A second issue is that using the values assumed at the first SETI conference led to the conclusion that N ≈ L.[169] This is deeply problematic because it effectively downplays all the astronomical (R_*, f_s, f_p, and n_e) and biological (f_l, f_i, and f_c) terms making the conclusion almost entirely dependent upon the lifetime factor, L. Being so unconstrained, one is effectively free to choose L and therefore, N, according to one’s worldview assumptions.
Useless for estimating N. The Drake equation is not a proper equation and is deeply misleading when used to calculate the number of highly developed civilizations with the results generally having little bearing on reality. One manifestation of this problem is that different suggested input values in the scientific literature vary the value of N by a factor of 100 billion (11 orders of magnitude).[170] The resulting claims for pessimists (N £ 1) suggest that we are alone in the galaxy and for the optimists (N > 1 million) would place the closest civilization only a few hundred lightyears away. This great variability makes the Drake equation one of the most useless equations in history because the results generally reflect the assumptions of the proponents, more than the science. Despite this glaring problem, the Drake equation is still widely treated by published scientists as a reliable guide for establishing the existence of advanced intelligent civilizations.
Failure of the Copernican principle. The discovery of extra-solar planets was a partial vindication of SETI and the Drake equation. We have found that there might be a mind-boggling one trillion or more planets in our galaxy, many of which are small, rocky worlds. The sheer number seems to weigh heavily in favor of finding alien civilizations. A closer look, however, reveals that most of these solar systems bear little resemblance to our own in regard to possible habitability and this conclusion extends to the solar systems and galaxies. These discoveries clearly run contrary to the Copernican principle that is the foundation of the Drake equation.
Habitability requirements used are unrealistically simple. Back in 1961, it was generally assumed that only two things were needed to be true to make a planet habitable—a planet of the right size and located in the star’s habitable zone. As discussed, in Habitable Zone vs Habitable, these criteria are insufficient for permitting even simple lifeforms, much less for advanced life. As presented in this paper, many more properties of the star, planet, solar system, and even galaxy must be just right to have any chance for hosting an advanced civilization.
Grand extrapolation fallacy. Much of our understanding of exoplanets that goes into evaluating the Drake equation is based on stars in our immediate vicinity. The problem is that these cases are not representative of the entire galaxy. This situation leads to a form of sampling bias—applying results from a non-representative sample to the entire set. To compensate for this bias, one needs to include additional habitability requirements relevant to other parts of the galaxy, such as the galactic habitable zone. Failure to do so means that the resulting value of N will be misleadingly high.
Emphasized astronomically measurable properties. Frank Drake and SETI follow a quiet unwritten rule—only assume phenomena you know exist.[171] This conservative strategy is designed to prevent excessive speculation. Thus, Drake focused on terms that could be measured using foreseeable technology making them more quantifiable. He also focused on astronomical factors governing habitability while ignoring geology, atmospheric science, and galactic dangers that would be difficult to determine. In some cases, this conservative approach led to a potential underestimation (such as avoiding speculation about life on Jupiterian moons), but overall, this dramatically overestimates potential habitability.
Needs to be updated with modern discoveries. Since 1961, astronomers have discovered many more factors relevant to the calculation of N that need to be incorporated into the Drake equation if it is to have any real meaning. One way to do this would be to include additional terms beyond the 7 or 8 typically used. As an example of this approach, Rare Earth augmented the Drake equation with 5 additional terms.[172] However, this approach is unpopular because it makes the equation unwieldy. The second avenue is to expand the existing terms to include additional considerations. This too is undesirable because it makes the modified terms ambiguous and hard to quantify. Sadly, very few sources employ either strategy choosing instead to simply ignore these discoveries leading to an unrealistically high value of N.

Clearly, much work needs to be done to improve or extend the Drake equation if it is to produce meaningful estimates.

The main strength of the Drake equation is that it has been helpful in developing useful criteria for prioritizing the list of possible candidates to investigate by removing from contention any planets that are clearly inhospitable. In this context, the equation is deliberately conservative in using fewer and looser criteria to help avoid prejudicing the search by eliminating candidates unnecessarily. In this case, incorporating too few criteria at worst simply wastes time by studying stars with no real potential for life.

It is when one tries to estimate the number of advanced intelligent civilizations that the Drake equation proves to be deficient. The reason for this is that the Drake equation starts with the presupposition that all stars and planets are equally viable for hosting life and then using specific known factors to eliminate those deemed uninhabitable. This habitable by default strategy means that being conservative on including factors leads to overestimating rather than underestimating the number of alien civilizations. One can inflate the value of N by simply ignoring known or suspected factors related to habitability.

In summary, the central error of the Drake equation is assuming that any planet whose habitability has not been decisively ruled out must therefore be fully habitable. Therefore, unless more rigorous criteria are used in the Drake equation, then N should really be understood as the number of places we should consider searching based on a particular choice of habitability criteria—rather than the number of detectable civilizations. Of course, including more constraints and using more realistic values will drive down the value of N dramatically. On this basis, some argue that N should be very small and therefore it is likely we really are alone in the galaxy.

The Design Inference

Astronomers now recognize that our sun, planet, and even our galaxy had to have a lot of “just right” qualities without which we would not be here. This quality of being “just right” or “fine-tuned” is sometimes referred to as “design” because of the comparison with humanly designed systems. For example, the gears in a mechanical watch must be precisely the right size to connect with other gears. In addition to this, the gear sizes must also be precisely chosen for the watch to correctly measure time. Even small changes in the components of the watch would cause the watch to fail or keep the wrong time. We explain the correct functioning of a watch by recognizing that the watch was designed by an intelligent being (the watchmaker). If our solar system and planet have the property of being “fine-tuned” like a watch, then we may infer that it too must have a designer. This is known as the design inference and stands in direct opposition to the Copernican principle.

Figure 17: Number of Design Evidences.

During the inception of SETI and the Drake equation in the 1960s, astronomers assumed that a solar system only had to get two things right (i.e., the planet’s size and distance from its star) for there to be a suitably Earth-like habitat for life. By 1970 when SETI was just a decade old, the number of design parameters (things that must be “just right”) jumped from just 2 to 8. Each new design parameter reduces the predicted number of possible life sites in our universe. By 1980 this number rose to 23, by 1990 it was up to 32 parameters, by 1995 it had reach 41, in 2001 it reached 128, and in 2002 it was 153.[173] Currently, there are at least 402 design parameters as of 2008 and this list of parameters continues to grow with no apparent end in sight.[174] (See graph in Figure 17.) Given just the 153 design parameters from 2002, we can conservatively estimate that even given the entire observable universe with 100 billion galaxies each containing 100 billion stars and planets, the probability of having even one solar system form by chance that has the necessary properties for life is less than 1 chance in 10¹⁷²! (See Probabilities for Life on Earth on page 79.) Clearly then, the Earth cannot simply be a fortuitous accident, but is the product of design by a caring Designer.

For the most part, SETI proponents have simply chosen to ignore this growing list of requirements for habitability. Except for the addition of the term, f_s, the Drake Equation has largely remained the same, effectively refusing to recognize the advances and discoveries of the last 60 years. Similarly, estimates of the number of advanced civilizations have remained almost constant, although modern estimates are a bit more conservative. (See Estimates of the Number of Communicating Civilizations on page 9.) Sadly, outside of astronomy most of these design parameters are not well known, and when they are presented, typically only a few are mentioned. The popular press has done little to challenge popular SETI notions or educate the public about this growing body of evidence for design.

Rare Earth Hypothesis

After decades of mounting evidence, some scientists are beginning to reevaluate their expectations for life elsewhere in the universe. Associate editor of Astronomy magazine, Robert Naeye, provides a cautionary view of extraterrestrial civilizations.

“Recent studies in a variety of fields suggest that life must pass through a series of bottlenecks on the road to intelligence. On Earth, a long sequence of improbable events transpired in just the right way to bring forth our existence, as if we had won a million-dollar lottery a million times in a row. Contrary to the prevailing belief, maybe we are special. Maybe humanity stands alone on a fertile island in the largely sterile waters of the galactic ocean.”[175]

In support of his statement, he discusses 11 different bottlenecks that must be successfully passed on the long and arduous road to becoming an advanced intelligent civilization:[176]

All Earth life is based on liquid water. The same will probably be true on other worlds.
Life needs a good sun, a single star that radiates enough energy to warm a planet. But if a star is too massive, it quickly exhausts its nuclear fuel.
A special planet with all the right conditions is necessary to support life.
A security blanket of air is needed to moderate surface temperatures and provide sustenance for life.
A magnetic field creates the aurora, but also protects a planet from deadly radiation.
Volcanism and shifting crustal plates regulate atmospheric gases and the climate.
Life must arise before it can evolve toward intelligence. Was this a one-in-a-million event?
Our guardian angel Jupiter deflects comets and asteroids that would otherwise smash into Earth, triggering mass extinctions.
Our rock of stability, the Moon, keeps Earth’s spin axis from flip-flopping chaotically.
To develop technology, life needs to evolve high intelligence and manual dexterity, an unlikely combination.
Nuclear Armageddon, pollution, asteroid impacts, and other disasters can extinguish a civilization. What is our fate?

Based on this, Robert Naeye concludes that we are likely to be alone in the galaxy. He states:

“But until [we discover alien life], it seems more prudent to conclude that we are alone in a vast cosmic ocean, that in one important sense, we ourselves are special in that we go against the Copernican grain.”[177]

It should be mentioned that Naeye comes to this conclusion based on existing evidence—even though he sincerely hopes that SETI will eventually prove him wrong by detecting life.[178] If only eleven factors are enough to convince him that we are likely alone in the galaxy, we can only wonder what conclusions he would make if he had considered all 402 design parameters!

Another example of strong doubt comes from physicist, cosmologist, and astrobiologist Paul Davies who has been involved with SETI for three decades and chairs SETI Post-Detection Taskgroup. In his book, The Eerie Silence, he focuses on new ways that we might look for extraterrestrial life and what would likely happen if we did find proof of their existence, yet at the end he offers an unusually bleak assessment:

“I can assign some sort of probability for aliens to exist, based on sifting all the facts, weighted in turn by the relative importance I attach to the various arguments. When all that is put together, my answer is that we are probably the only intelligent beings in the observable universe, and I would not be surprised if the solar system contains the only life in the observable universe. I arrive at this dismal conclusion because I see so many contingent features involved in the origin and evolution of life, and because I have yet to see a convincing theoretical argument for a universal principle of increasing complexity…”[179]

This is his conclusion as a scientist despite having a life-long fascination with life out there and being uneasy with the philosophical implications of humanity being cosmically alone.

Paleontologist Peter Ward and astronomer Donald Brownlee are also among a growing number of scientists who are challenging the prevailing view that advanced life is common in the universe. Based on their own compilation of requirements for habitable planets, they conclude that intelligent life is exceedingly rare in the universe, even though they start from the assumption that microscopic life can arise spontaneously.[180] In the introduction to their book Rare Earth, they state:

“In this book, we will argue that not only intelligent life, but even the simplest of animal life, is exceedingly rare in our galaxy and in the Universe. We are not saying that life is rare—only that animal life is… We combine these two predictions of the commonness of simple life and the rarity of complex life into what we will call the Rare Earth Hypothesis.”[181] (Emphasis original)

They conclude their book by presenting an expanded version of Drake’s equation that includes 5 new factors based on the latest astronomical discoveries. From there, they conclude:

“Many new factors will be known, and the list of variables involved will undoubtedly be amended. But it is our contention that any strong signal can be perceived when only sparse data is available. To us, the signal is so strong even at this time, it appears that Earth indeed may be extraordinarily rare.”[182] (Emphasis added)

It then appears that the Copernican revolution has come full circle. So, while we are clearly not at the geometrical center of the universe, evidence for design is showing that we are indeed very special.

The Evidence is In: A Cumulative Case Approach

While there is currently no definitive answer to the question of whether there is intelligent life out there, the amount of evidence on which to base our conclusion has increased dramatically since the 1960s. No single argument presented in this paper provides a decisive conclusion, however, there are multiple lines of evidence to consider:

Failure to detect any signals. After 60 years of searching, we have not found even a single evidence for an extraterrestrial civilization. While this does not completely eliminate the possibility of advanced beings, it certainly does put strong upper limits on the frequency of their existence and level of technology.
Fermi paradox. The complete lack of any credible evidence that extraterrestrials (or their robotic probes) have ever visited our solar system argues that advanced extraterrestrial civilizations are rare or non-existent.
Discovery of extra-solar planets. Is Earth as commonplace as was assumed by SETI based on the Copernican principle? In the past, this was difficult to evaluate because we had little to serve as a basis of comparison. This situation has changed with the discovery of more than 5,000 exoplanets. Contrary to expectations, we are finding a wide range of planetary configurations undreamt of in the 1960s and most have little chance for supporting life. But even those solar systems that more closely follow traditional expectations offer little hope of finding truly habitable planets. This is perhaps the strongest experimental evidence against SETI claims.
Habitable zone vs habitable. In the 1960s, it was widely held that planetary habitability only required that a planet be Earth-sized and located in its star’s circumstellar habitable zone. The discovery of thousands of exoplanets and better theoretical modelling have refuted this simplistic view. At a minimum, habitability also depends on the nature of the atmosphere, its geology, presence of a strong magnetic field, amount of water present, etc. When considering all these factors, very few exoplanets have a chance at being habitable.
Failure of alternative worlds. Alternative worlds—those other than the traditional warm Earth-like planets—offer a potential game changer because their presence could greatly expand the number of places where life could arise and therefore increase the number of possible extraterrestrial civilizations. One of the more exciting prospects has been super-Earths. They were unknown in the 1960s, but now astronomers have found large numbers of them. Even better, some have suggested that super-Earths have properties that may make them even more likely to be habitable than Earths. Recent research, however, has challenged this by showing that most super-Earths are actually mini-Neptunes and therefore will be very inhospitable. Large moons of Jupiters and rogue planets offer a staggering number of places to look for life, yet the chances of any of them being habitable to anything more than microbes is very small.
Failure of the Copernican principle. The beating heart of SETI is the Copernican principle under which most stars, planets, and galaxies are assumed to be adequate for hosting life. New discoveries continue to undermine this belief revealing an ever-growing list of features that need to fall within a narrow range of values to even have a chance at supporting life.

Taken together, these present a strong cumulative case that intelligent civilizations other than our own are rare at best.

“Why Are We Here?”

Under the SETI paradigm (based on the Copernican principle), our existence is virtually guaranteed without God’s help simply by virtue of the billions of galaxies, each containing billions of possible stars and planets. However, given the enormous and growing body of evidence for design, we can safely rule out the possibility of finding other habitable planets teeming with intelligent life. (See Probabilities for Life on Earth on page 79.) Clearly the Earth cannot simply be a fortuitous accident and thus, we are very likely alone in the universe.

University of Washington astronomer Guillermo Gonzalez states in his article, “Nobody Here But Us Earthlings,” that:

“My answer to the question [‘Are we alone?’] almost always catches people off guard: Very likely yes, we are alone. When one looks at the astronomical data with an open mind, it becomes quite obvious why we have not found any evidence of extraterrestrial life.”[183]

At the end of his article, he concludes:

“We should not be asking: ‘Are we alone?’ We should be asking instead: ‘Why are we here?’”[184]

The question of our existence was for a long time the domain of theology alone, but now science is adding some new details to the story. Modern astronomy is testifying to the fact that our galaxy, our solar system, and our planet reflect the careful craftsmanship of a Designer. Long ago, King David looked up at the heavens and concluded, “The heavens declare the glory of God, the skies proclaim the work of his hands.”[185] If only he could have seen the universe as we see it today.

Acknowledgements

A big thank you to those who helped review this work: Ken Klos, Christopher Miller, Doug Hohulin, Andrew Overholt, Todd Keith and, John Link.

Figure Credits

Figure 1 Wikipedia commons.

Figure 2 Jenny Cheng/Business Insider

Figure 3 Wikipedia commons (with some colors changed for clarity).

Figure 4 Scanned from reference 60 (with red squares changed to light blue for clarity).

Figure 5 Wikipedia commons.

Figure 6 Scanned from reference 68 (with dashed lines added).

Figure 7 Scanned from reference 69.

Figure 8 Wikipedia commons.

Figure 9 Scanned from reference 73.

Figure 10 Wikipedia commons.

Figure 11 NASA/Ames Research Center/Natalie Batalha/Wendy Stenzel.

Figure 12 John Millam.

Figure 13 John Millam.

Figure 14 John Millam.

Figure 15 John Millam.

Figure 16 Wikipedia commons.

Figure 17 John Millam.

What about “Weird Life”? A Chemist’s Perspective

Our understanding of life is ultimately based on Earth life. If life were to spontaneously arise on another planet, it almost certainly would not look anything like us. Sorry to disappoint you Star Trek fans, but aliens certainly wouldn’t look humanoid with two arms, two legs, and a head as is seen in virtually every sci-fi movie, book, and TV series. Likewise, such alien life forms probably wouldn’t even be based on the familiar molecules that make up earth life—DNA, RNA, and proteins (although such life would still require molecules to perform similar functions.) If alien life is not likely to resemble earth life, then perhaps we can take this idea one step farther and consider alternative life chemistries. Of particular interest is the possibility that life might not be carbon-based. Or perhaps it might rely on a liquid other than water. This idea of “weird life” (non-carbon/non-water-based life forms) has largely been the realm of science fiction, although some astrobiologists are seriously studying these possibilities.[186] If life could indeed arise without the need for liquid water or carbon, then perhaps such life might be able to thrive in environments radically different from our own. This possibility could greatly increase the number of possible extraterrestrial life sites and correspondingly increase the odds of SETI discovering alien civilizations. Could there be a grain of scientific fact in this science fiction?

While some still hold out hopes for alternative life chemistries, chemists have largely ruled out this possibility based on decades of research. But how can we be so certain about hypothetical life forms that we can only imagine? We do know that all conceivable life, even “weird life,” must be able to perform certain basic functions to live. First, life must be able to take in matter and energy from its environment and process it for food, motion, and reproduction. Likewise, life must be able to replicate itself and so requires the ability to store its own blueprints. Using our modern understanding of chemistry, we can rule out systems that do not have adequate versatility to support these essential functions.

All conceivable forms of biological life require complex chemical reactions, and a liquid state is the ideal environment for such reactions. Liquids can dissolve both gases and solids, thus allowing all three phases to participate in reactions and sustain a wide variety of chemical environments. In a gaseous environment, molecules lie too far apart, limiting the ability to store information in complex molecules needed for life. In a solid state, molecules are locked in place, greatly slowing the ability to synthesize compounds, transfer information, and reproduce. Even if life were to somehow arise based on a solid rather than a liquid matrix, its evolution would likely be so slow that it wouldn’t develop advanced life before the death of its parent star.[187] (For perspective, advanced life on Earth appeared approximately halfway through our Sun’s stable lifetime. So even a slow-down by a factor of two in life’s ability to adapt and evolve would mean that advanced life would have arrived too late, and solid-based life forms would likely be slower than liquid-based life by many, many orders of magnitude.)

Of course, not all liquids are created equal. Water (H₂O) stands alone in its ability to serve as a host environment for life chemistries.187 The water molecule’s unusual bent shape, with its three atoms forming a 104.5° angle, combined with its high degree of polarity,[188] gives water a large permanent dipole moment. This gives water the ability to dissolve more substances and in greater quantities than any other liquid, so it is known as the universal solvent. Solubility is critical for the transport of nutrients and waste material. In addition, solubility allows all the molecules for life chemistry to be available in solution to quickly carry out needed reactions. Water isn’t simply passive but is an active participant in many different reactions. Moreover, its high dielectric constant and large dipole moment helps it to stabilize reactive intermediates facilitating certain reactions. Neutral water can also be easily split into two ions (H⁺ and OH^–), which is critical in many energy-producing reactions, such as photosynthesis. These are only a few of water’s chemical and physical properties that are vital for life.[189] When water is compared to other related substances, we see that water ranks highest or second highest in at least ten of these important properties.[190]^,[191]

Only ammonia (NH₃) comes close to rivaling water. Yet it is 1/5^th as polar and cannot match water’s versatility. Also, ammonia is much more reactive and volatile than water and so would not be as abundant or ubiquitous as liquid water. Other suggested replacements for water include methane (CH₄), hydrogen sulfide (H₂S), carbon dioxide (CO₂), hydrogen fluoride (HF) and sulfur dioxide (SO₂). All of these are less than one tenth as polar as water and do not share many of water’s anomalous properties.189

Just as life requires water, it also requires carbon. In 1961, physicist Robert Dicke argued, “It is well known that carbon is required to make physicists.”[192] His statement still stands today because no other element even comes close to matching the versatility of carbon. The field of organic chemistry focuses exclusively on carbon compounds and their chemistry; yet it is far richer and more diverse than the chemistry of all other elements combined (the field of inorganic chemistry). Carbon easily forms strong double and triple bonds as well as being able to form up to four single bonds. This gives carbon the widest array of possibilities for forming chemical compounds. Most important, however, is carbon’s ability to form indefinitely long chains. Simple molecular units can be strung together in innumerable combinations to form very complex molecules. It is these long, complex molecules that provide the capacity to store large amounts of complex biological information.

Silicon is by far the most commonly suggested alternative to carbon and is a popular favorite in science fiction works. Its greatest value to SETI, however, is in its great abundance, being available in far greater quantity than carbon. Silicon, for example, makes up 28% of the Earth’s crust (second only to oxygen), whereas carbon represents only a trace element (less than 1%). While silicon shares carbon’s ability to form four single bonds, it lacks carbon’s ability to make strong multiple bonds, greatly limiting its possible uses.[193] Another major problem is that silicon-hydrogen bonds are far more reactive than the carbon-hydrogen bond (due to silicon’s lower electronegativity). Consequently, silicon is unable to form stable long hydrogen-terminated chains that are critical for complex biomolecules.

Boron is the only other significant challenger to carbon. It has the opposite problem of silicon, having the ability to form strong multiple bonds, but generally forms no more than 3 (rather than 4) bonds. While the chemical flexibility of boron has not been completely explored, it suffers from being far less abundant than carbon. The net result is that wherever boron is found, carbon will also be found, so even if boron-based life were to arise, it would likely be beaten out by carbon-based life.

SETI-Related Questions

Am I opposed to looking for extraterrestrials?

No. While I don’t believe that SETI will detect signals from alien beings, I’m never opposed to genuine research. My opposition is to the propagation of SETI philosophy in the guise of science. The public is regularly hammered in newspapers and on television with the “where there is water there is life” myth, so the possible presence of liquid water on Mars or Jupiter’s moon Europa is given as evidence for extraterrestrial life. Articles sympathetic to the SETI belief in extraterrestrials are given frequent and unchallenged support, while those that challenge these assumptions often receive little attention or are trivialized.

I do support genuine research into the question of extraterrestrial life for many reasons. First, such research has been extremely helpful in demonstrating just how special our solar system is. Second, it has led to many important technological developments, such as improvements in signal processing, better telescope designs, mapping of interstellar radio sources, just to name a few examples.

Is Christianity opposed to the idea of life on other planets?

No. The central focus of this paper is on what the scientific evidence can teach about habitable planets and the possibility of life by purely natural processes without God’s intervention. God certainly could create life elsewhere in the universe. In fact, some Christians argue that God’s creative nature suggests that He would create life elsewhere, while other Christians disagree. Ultimately, the Bible is unclear on this issue, leaving room for debate and discussion among Christians.[194]

If we are alone in the universe, then is the vastness of the universe just a waste of space?

Many have argued that the discovery of intelligent life elsewhere in the universe would be evidence against the existence of God. If, in fact, the opposite is true and the universe is a vast, barren desert with no advanced civilizations other than our own, what then should we conclude? Ironically, some have interpreted this possibility as also being evidence against God. For example, the movie Contact based on the book by Carl Sagan, depicts the main character expressing the thought that if there are no other intelligent beings in the universe, then the vast universe is just a “waste of space.” Physicist Stephen Hawking likewise makes a similar charge, declaring that it is “very hard to believe” that God would make so many useless stars if His intent was just to make a home for man.[195] Or to paraphrase J.B.S. Haldane, if God created the universe, He must have an inordinate fondness for empty parking space.

If God had created the universe for mankind, wouldn’t God only need to create one planet in one solar system? Astrophysics is now shedding light on this dilemma and revealing that an enormous universe is in fact a necessary part of God’s design—not a waste. There are two critical reasons why the universe must be the size that it is:[196]

Mass density fine-tuning. The mass density of the universe is a sensitive catalyst for nuclear fusion. Therefore, if the mass density were any less, the universe would contain only hydrogen and helium. If the mass density were any greater, the universe would only contain elements heavier than iron. The carbon, oxygen, and nitrogen necessary for life are only possible in a universe with a hundred billion trillion observable stars.
Expansion of the universe. More mass would increase the gravity thus slowing the rate of expansion so that all the matter in the universe would accumulate into black holes and neutron stars. Correspondingly, less mass would cause the universe to expand too rapidly, and matter would not have a chance to form stars and planets. Consequently, the universe needs to be exactly as massive as it is to allow for our existence.

This reveals that God loved mankind so much that He was willing to create a hundred billion trillion stars just for our benefit.[197]

What is the anthropic principle?

The term anthropic principle was coined in 1974 by British mathematician Brandon Carter in the wake of the growing body of evidence that our universe was not an accident, but was in fact finely tuned to allow for our existence.[198] In his own words, the anthropic principle expresses the notion that “although our situation is not necessarily central it is necessarily privileged to some extent.”[199] As we have already examined in the main body of this paper, our planet, sun, and galaxy exhibit a high degree of fine-tuning. And this fine-tuning extends to the very laws and constants of physics (see Fine-Tuning for Life in the Universe on page 83).

“In 1961, astronomers acknowledged just two characteristics of the universe as ‘fine-tuned’ to make physical life possible. The more obvious one was the ratio of the gravitational force constant to the electromagnetic force constant. It cannot differ from its value by any more than one part in 10⁴⁰ (one part in ten thousand trillion trillion trillion) without eliminating the possibility for life. Today, the number of known cosmic characteristics recognized as being fine-tuned for life—any conceivable kind of physical life—stands at thirty-eight. Of these, the most sensitive is the space energy density (the self-stretching property of the universe). Its value cannot vary by more than one part in 10¹²⁰ and still allow for the kinds of stars and planets physical life requires… An account of scientific evidence in support of the anthropic principle fills several books. The authors’ religious beliefs run the gamut from agnosticism to deism to theism, but virtually every research astronomer alive today agrees that the universe manifests exquisite fine-tuning for life.”[200]

While the anthropic principle is not directly relevant to the question of SETI (since other stars and planets all share the same laws of physics), it is relevant to the design argument and directly contradicts the Copernican principle.

What if life is found on Mars?

For several decades now, there have been both implicit and explicit expectations about possible implications of finding life (or its remains) on Mars. The logic goes like this: If life is found on Mars, then this is proof positive that life can arise easily and naturally, and hence life must be common in the universe. Iosef Shklovskii and Carl Sagan popularized this idea as far back as 1966.35 In other words, the public has been conditioned to accept that evidence for life on Mars would nullify all the evidence presented in this paper—and hence that we should greet such claims with enthusiasm, not skepticism. This was clearly demonstrated with the report of the remnants of life found on a Mars rock in 1996, as described near the beginning of this paper. Credit should be given to those bold scientists who were willing to challenge popular belief and demonstrate that the “fossils” were not the remnants of life.

In the debate over the reported Mars rock finding, one point was completely ignored by both sides: we will find the remnants of life on Mars! This may seem strange considering the thesis of this paper, but it in fact does not contradict it. Just as rocks from Mars can be transported to Earth, so Earth rocks can be transported to Mars (known as interplanetary panspermia). Also, the solar wind can blow hearty microorganisms to Mars and the rest of the solar system. In other words, we have plenty of reasons to believe that terrestrial microorganisms will contaminate other planets in our solar system. Since all our current probes and tests are not capable of distinguishing between terrestrial and Martian life, we must guard against the assumption that finding the remains of life on Mars equals extraterrestrial life evolving on Mars. This argument was in print at least 8 years before the Mars rock debacle![201]^,[202]

Fine-Tuning for Life on Earth

Remarkable fine-tuning must exist before there is any possibility for life to arise on any planet in any planetary system. Earth exists in the only known planetary system able to demonstrate all the characteristics required for life to survive. The following essential characteristics for life show Earth’s matchless fine-tuning within its parameters. (Reprinted from Lights in the Sky & Little Green Men, Appendix A, pp. 171-184. A list of scientific references supporting these design parameters is included at the end of the book.)

1. Galaxy cluster type.

If Earth’s galaxy cluster were too rich, galaxy collisions and mergers would disrupt the solar orbit.
If Earth’s galaxy cluster were too sparse, there would be insufficient infusion of gas into the Milky Way to sustain star formation there for a long enough period of time.

2. Galaxy size.

If the Milky Way were too large, infusion of gas and stars would disrupt the sun’s orbit and ignite too many galactic eruptions.
If the Milky Way were too small, there would be insufficient infusion of gas to sustain star formation for a long enough period of time.

3. Galaxy type.

If the Milky Way were too elliptical, star formation would have ceased before sufficient heavy elements had built up for life chemistry.
If the Milky Way were too irregular, radiation exposure on occasion would be too severe and heavy elements for life chemistry would not be available.

4. Galaxy mass and distribution.

If too much of the Milky Way’s mass resided in the central bulge, the Earth would be exposed to too much radiation.
If too much of the Milky Way’s mass resided in the spiral arms, the Earth would be destabilized by gravity and radiation from adjacent spiral arms.

5. Galaxy location.

If the Milky Way were located too close to a rich galaxy cluster, the Earth would be gravitationally disrupted.
If the Milky Way were located too close to a very large galaxy (or galaxies), the Earth would be gravitationally disrupted.

6. Supernovae eruptions.

If supernovae had occurred too close, life on Earth would be exterminated by radiation.
If supernovae had occurred too far away, there would not be enough heavy element ashes for the formation of rocky planets like Earth.
If supernovae had occurred too infrequently, there would not be enough heavy elements ashes for the formation of rocky planets.
If supernovae had occurred too frequently, life on Earth would be exterminated.
If supernovae had occurred too soon, there would not have been enough heavy element ashes for the formation of rocky planets.
If supernovae had occurred too late, life on Earth would be exterminated by radiation.

7. White dwarf binaries.

If there were too few white dwarf binaries, there would be insufficient fluorine for life chemistry.
If there were too many white dwarf binaries, planetary orbits would be disrupted by stellar density and life on Earth would be exterminated.
If white dwarf binaries had appeared too soon, there would not be enough heavy elements for efficient fluorine production.
If white dwarf binaries had appeared too late, fluorine would be made too late for incorporation in Earth’s protoplanet.

8. Proximity of solar nebula to a supernovae eruption.

If the solar nebula were farther away, the Earth would have absorbed insufficient heavy elements for life.
If the solar nebula were closer, the nebula would be blown apart.

9. Timing of solar nebula formation relative to supernovae eruption.

If the solar nebula had formed earlier, the nebula would have been blown apart.
If the solar nebula had formed later, the nebula would not have absorbed enough heavy elements.

10. Number of stars in parent star birth aggregate.

If there were too few stars in the parent star birth aggregate, there would have been insufficient input of certain heavy elements into the solar nebula.
If there were too many stars in the parent star birth aggregate, planetary orbits would be too radically disturbed.

11. Star formation history in planet star vicinity.

If there had been too much star formation going on in the vicinity of the sun, planetary orbits would be too radically disturbed.

12. Birth date of the star-planetary system.

If the system had been born too early, the quantity of heavy elements would have been too low for large, rocky planets like Earth to form.
If the system had been born too late, the sun would not yet have reached its stable burning phase. Furthermore, the ratio of potassium-40, uranium-235, uranium-238, and thorium-232 to iron would be too low for long-lived plate tectonics to be sustained on Earth.

13. Planet star distance from center of galaxy.

If the sun were too far from the center of the galaxy, the quantity of heavy elements would have been insufficient to make rocky planets like Earth. In addition, there would be the wrong abundances of silicon, sulfur, and magnesium relative to iron for appropriate planet core characteristics.
If the sun were too close to the center of the galaxy, galactic radiation would be too great and stellar density would disturb planetary orbits. Again, there would be the wrong abundances of silicon, sulfur, and magnesium relative to iron for appropriate planet core characteristics.

14. Parent star distance from closest spiral arm.

If the distance were too great, the quantity of heavy elements would be too small for rocky planets to form.
If the distance were too small, the solar system would experience gravitational disturbances and radiation exposure.

15. Z-axis height of star’s orbit.

If the z-axis height were too high, exposure to harmful radiation from the galactic core would be too great.

16. Number of stars in the planetary system.

If there were multiple stars in the solar system, tidal interactions would disrupt Earth’s orbit.
If there were no stars in the system, Earth would have insufficient heat to support life.

17. Parent star age.

If the sun were older, its luminosity would change too quickly.
If the sun were younger, its luminosity would change too quickly.

18. Parent star mass.

If the sun’s mass were greater, its luminosity would change too quickly and it would burn too rapidly.
If the sun’s mass were smaller, the range of planet distances that would make life possible would be too narrow. In addition, tidal forces would disrupt Earth’s rotation period. Also, ultraviolet radiation would be inadequate for plants to make sugars and oxygen.

19. Parent star metallicity.

If the sun’s metallicity were too small, there would be insufficient heavy elements for life chemistry.
If the sun’s metallicity were too large, life would be poisoned by heavy-element concentrations. Furthermore, radioactivity would be too intense for life.

20. Parent star color.

If the sun were redder, photosynthetic response would be insufficient.
If the sun were bluer, photosynthetic response would be insufficient.

21. Galactic tides.

If galactic tides were too weak, the comet ejection rate from the giant planet region would be too low.
If galactic tides were too strong, the comet ejection rate from the giant planet region would be too high.

22. H₃⁺ production.

If H₃⁺ production had been too small, simple molecules essential to planet formation and life chemistry would not form.
If H₃⁺ production had been too large, planets would form at the wrong time and place for life.

23. Flux of cosmic ray protons.

If the proton flux had been too small, there would be inadequate cloud formation in Earth’s troposphere.
If the proton flux had been too large, there would be too much cloud formation in Earth’s troposphere.

24. Solar wind.

If the solar wind were too weak, too many cosmic ray protons would reach Earth’s troposphere, causing too much cloud formation.
If the solar wind were too strong, too few cosmic ray protons would reach Earth’s troposphere, causing too little cloud formation.

25. Parent star luminosity relative to speciation.

If the sun’s luminosity had increased too soon, a runaway greenhouse effect would develop on Earth.
If the sun’s luminosity had increased too late, runaway glaciation would develop on Earth.

26. Surface gravity (escape velocity).

If surface gravity were stronger, Earth’s atmosphere would retain too much ammonia and methane.
If surface gravity were weaker, Earth’s atmosphere would lose too much water.

27. Distance from parent star.

If Earth’s distance from the sun were greater, Earth would be too cool for a stable water cycle.
If Earth’s distance from the sun were lesser, Earth would be too warm for a stable water cycle.

28. Inclination of orbit.

If Earth’s orbital inclination were too great, temperature differences would be too extreme.

29. Orbital eccentricity.

If Earth’s orbital eccentricity were too great, seasonal temperature differences would be too extreme.

30. Axial tilt.

If Earth’s axial tilt were greater, surface temperature differences would be too great.
If Earth’s axial tilt were lesser, surface temperature differences would be too great.

31. Rate of change of axial tilt.

If Earth’s rate of change of axial tilt were greater, climatic changes and surface temperature differences would be too extreme.

32. Rotation period.

If Earth’s rotation period were longer, diurnal temperature differences would be too great.
If Earth’s rotation period were briefer, atmospheric wind velocities would be too great.

33. Rate of change in rotation period.

If the rate of change in Earth’s rotation period were more rapid, the surface temperature range necessary for life would not be sustained.
If the rate of change in Earth’s rotation period were less rapid, the surface temperature range necessary for life would not be sustained.

34. Planet age.

If the Earth were too young, it would rotate too rapidly.
If the Earth were too old, it would rotate too slowly.

35. Magnetic field.

If the Earth’s magnetic field were stronger, electromagnetic storms would be too severe. Also, too few cosmic ray protons would reach Earth’s troposphere, and this would inhibit adequate cloud formation.
If the Earth’s magnetic field were too weak, the ozone shield would be inadequately protected from hard stellar and solar radiation.

36. Thickness of crust.

If the Earth’s crust were thicker, too much oxygen would be transferred from the atmosphere to the crust.
If the Earth’s crust were thinner, volcanic and tectonic activity would be too great.

37. Albedo (ratio of reflected light to amount falling on surface).

If the Earth’s albedo were greater, runaway glaciation would develop.
If the Earth’s albedo were smaller, a runaway greenhouse effect would develop.

38. Asteroidal and cometary collision rate.

If this rate were greater, too many species would become extinct.
If this rate were lesser, the Earth’s crust would be too depleted of materials essential for life.

39. Mass of body colliding with primordial Earth.

If the body were smaller, Earth’s atmosphere would have been too thick and the moon would have been too small.
If the body were greater, Earth’s orbit and form would have been too greatly disrupted.

40. Timing of body colliding with primordial Earth.

If the collision had occurred earlier, Earth’s atmosphere would be too thick and the moon would be too small.
If the collision had occurred later, the Earth’s atmosphere would be too thin and thus the sun would be too luminous for advanced life.

41. Location of body colliding with primordial Earth.

If the body had just grazed the Earth, there would have been insufficient debris to form a large moon. Furthermore, the collision would have been inadequate to annihilate Earth’s primordial atmosphere. Also, there would have been inadequate transfer of heavy elements to Earth.
If the body had collided too close to dead center, damage from the collision would have destroyed necessary conditions for (future) life.

42. Oxygen-to-nitrogen ratio in atmosphere.

If this ratio were larger, advanced life functions would proceed too quickly.
If this ratio were smaller, advanced life functions would proceed too slowly.

43. Carbon dioxide level in atmosphere.

If the level were greater, a runaway greenhouse effect would develop.
If the level were lesser, planets would be unable to maintain efficient photosynthesis.

44. Water vapor level in atmosphere.

If the Earth’s water vapor level were greater, a runaway greenhouse effect would develop.
If the Earth’s water vapor level were smaller, rainfall would be too meager for advanced life on land.

45. Atmospheric electric discharge rate.

If the discharge rate were greater, too much fire destruction would occur.
If the discharge rate were smaller, too little nitrogen would be fixed in the atmosphere.

46. Ozone level in atmosphere.

If the ozone level were greater, surface temperatures would be too low and there would be too little ultraviolet radiation for plant survival.
If the ozone level were too high, surface temperatures would be too high and there would be too much ultraviolet radiation for plant survival.

47. Oxygen quantity in atmosphere.

If the oxygen quantity were greater, planets and hydrocarbons would burn up too easily.
If the oxygen quantity were lesser, advanced animals would have too little oxygen to breathe.

48. Ration of ⁴⁰K, ²³⁵U, ²³⁸U, ²³²Th to iron.

If this ratio were too low, there would be inadequate levels of plate tectonic and volcanic activity.
If this ratio were too high, the levels of radiation, earthquakes, and volcanoes would be too high for advanced life.

49. Rate of interior heat loss.

If the rate were too low, there would be inadequate energy to drive the required levels of plate tectonic and volcanic activity.
If the rate were too high, plate tectonic and volcanic activity would shut down too quickly.

50. Seismic activity.

If seismic activity were greater, too many life forms would be destroyed.
If seismic activity were lesser, nutrients on the ocean floors from river runoff would not be recycled to continents through tectonics. Furthermore, not enough carbon dioxide would be released from carbonates.

51. Volcanic activity.

If volcanic activity were lower, insufficient amounts of carbon dioxide and water vapor would be returned to the atmosphere. Also, soil mineralization would become too degraded for life.
If volcanic activity were higher, advanced life would be destroyed.

52. Rate of decline in tectonic activity.

If the rate were slower, advanced life could never survive on Earth.
If the rater were faster, advanced life could never survive on Earth.

53. Rate of decline in volcanic activity.

If the rate were slower, advanced life could never survive on Earth.
If the rater were faster, advanced life could never survive on Earth.

54. Timing of birth of continent formation.

If the formation had begun too early, the silicate-carbonate cycle would have been destabilized.
If the formation had begun too late, the silicate-carbonate cycle would have been destabilized.

55. Oceans-to-continents ratio.

If the ratio were greater, diversity and complexity of life forms would be limited and the silicate-carbonate cycle would be destabilized.
If the ratio were smaller, diversity and complexity of life forms would be limited and the silicate-carbonate cycle would be destabilized.

56. Rate of change in oceans-to-continents ratio.

If the rate was slower, advanced life would lack the needed landmass area.
If the rate was faster, advanced life would be destroyed by the radical changes.

57. Global distribution of continents.

If the continents were located too much in the southern hemisphere, seasonal differences would be too severe for advanced life.

58. Frequency and extent of ice ages.

If these were smaller, insufficient fertile, wide, and well-watered valleys would have been produced for diverse and advanced life.
If these were greater, Earth would experience runaway freezing.

59. Soil mineralization.

If the Earth’s soil were too nutrient-poor, the diversity and complexity of life forms would be limited.
If the Earth’s soil were too nutrient-rich, the diversity and complexity of life forms would be limited.

60. Gravitational interactions with a moon.

If the gravitational interaction were greater, tidal effects on the oceans, atmosphere, and rotational period would be too severe.
If the gravitational interaction were lesser, orbital obliquity changes would cause climatic instabilities. Movement of nutrients and life from the oceans to the continents and vice versa would be insufficient. Also the magnetic field would be too weak.

61. Jupiter distance.

If the distance from Earth to Jupiter were greater, too many asteroid and comet collisions would occur on Earth.
If the distance from Earth to Jupiter were lesser, Earth’s orbit would be unstable.

62. Jupiter mass.

If Jupiter’s mass were greater, Earth’s orbit would be unstable.
If Jupiter’s mass were lesser, too many asteroid and comet collisions would occur on Earth.

63. Drift in major planet distances.

If the planet drift were greater, Earth’s orbit would be unstable.
If the planet drift were lesser, too many asteroid and comet collisions would occur on Earth.

64. Major planet eccentricities.

If the eccentricities of the major planets in this solar system were greater, Earth would be pulled out of the life support zone.

65. Major planet orbital instabilities.

If the orbital instabilities were greater, Earth would be pulled out of the life support zone.

66. Atmospheric pressure.

If atmospheric pressure on Earth were too slight, liquid water would evaporate too easily and condense too infrequently. Additionally, weather and climate variation would be too extreme and lungs could not function.
If atmospheric pressure on Earth were too great, liquid water would not evaporate easily enough for land life. Also, insufficient sunlight and ultraviolet life would reach the planet’s surface. There would be insufficient climate and weather variation. And lungs would not function.

67. Atmospheric transparency.

If atmospheric transparency were lesser, an insufficient range of wavelengths of solar radiation would reach Earth’s surface.
If atmospheric transparency were greater, too broad a range of wavelengths of solar radiation would reach Earth’s surface.

68. Magnitude and duration of the sun spot cycle.

If the magnitude of the cycle were lesser or the duration briefer, there would be insufficient variation in climate and weather.
If the magnitude of the cycle were greater or the duration longer, variation in climate and weather would be too great.

69. Continental relief.

If the relief were smaller, there would be insufficient variation in climate and weather.
If the relief were greater, variation in climate and weather would be too great.

70. Chlorine quantity in atmosphere.

If there were less chlorine, erosion rates, the acidity of rivers, lakes, and soils, and certain metabolic rates would be all be insufficient for most life forms.
If there were more chlorine, erosion rates, the acidity of rivers, lakes, and soils, and certain metabolic rates would be too high for most life forms.

71. Iron quantities in oceans and soils.

If there were less iron, the quantity and diversity of life would be too limited to support advanced life. And if the quantity were very small, no life would be possible.
If there were more iron, iron poisoning of at least advanced land life would result.

72. Troposphere ozone quantities.

If there were less tropospheric ozone, insufficient cleansing of biochemical smog would result.
If there were more tropospheric ozone, the respiratory failure of advanced animals, reduced crop yields, and the destruction of ozone-sensitive species would result.

73. Stratosphere ozone quantity.

If there were less stratospheric ozone, too much ultraviolet radiation would reach the Earth’s surface, causing skin cancers and reducing plant growth.
If there were more stratospheric ozone, too little ultraviolet radiation would reach the Earth’s surface, causing reduced plant growth and insufficient vitamin production for animals.

74. Mesospheric ozone quantity.

If there were less mesospheric ozone, circulation and chemistry of mesospheric gases would be so disturbed as to upset relative abundances of life-essential gases in the lower atmosphere.
If there were more mesospheric ozone, circulation and chemistry of mesospheric gases would be so disturbed as to upset relative abundances of life-essential gases in the lower atmosphere.

75. Quantity and extent of forest and grass fires.

If these were lesser, growth inhibitors in the soils would accumulate. Soil nitrification would be insufficient. Also, there would be insufficient charcoal production for adequate soil water retention and absorption of certain growth inhibitors.
If these were greater, too many plant and animal life forms would be destroyed.

76. Quantity of soil sulfur.

If there were less sulfur in the soil, plants would become deficient of certain proteins and die.
If there were more sulfur in the soil, plants would die from sulfur toxins. The acidity of water and soil would become too great for life. Also, nitrogen cycles would be disturbed.

77. Biomass-to-comet-infall ratio.

If this ratio were smaller, greenhouse gases would accumulate, triggering runaway surface temperature increase.
If this ratio were larger, greenhouse gases would decline, triggering a runaway freeze.

Probabilities for Life on Earth

This appendix presents an estimate for the probability of attaining the necessary parameters for life support on a planet. It includes 153 known parameters. (Reprinted from Lights in the Sky & Little Green Men, Appendix B, pp. 185-189. A list of scientific references supporting these design parameters is included at the end of the book.)

Parameter	Probability that feature will fall in the required range for physical life
Local abundance and distribution of dark matter		.1
Galaxy cluster size		.1
Galaxy cluster location		.1
Galaxy size		.1
Galaxy type		.1
Galaxy mass distribution		.2
Galaxy location		.1
Variability of local dwarf galaxy absorption rate		.1
Star location relative to galactic center		.2
Star distance from corotation circle of galaxy		.005
Star distance from closest spiral arm		.1
Z-axis extremes of star orbit		.02
Proximity of solar nebulae to a type I supernovae eruption		.01
Timing of solar nebula formation relative to type I supernova eruption		.01
Proximity of solar nebulae to a type II supernovae eruption		.01
Timing of solar nebula formation relative to type II supernova eruption		.01
Flux of cosmic ray protons		.1
Variability of cosmic ray proton flux		.1
Number of stars in birthing cluster		.01
Star formation history in parent star vicinity		.1
Birth date of the star-planetary system		.01
Number of stars in the system		.7
Number and timing of close encounters by nearby stars		.01
Proximity of close stellar encounters		.1
Masses of close stellar companion		.1
Star age		.4
Star metallicity		.05
Ratio of ⁴⁰K, ²³⁵U, ²³⁸U, ²³²Th to iron in star-planetary system		.02
Star orbital eccentricity		.1
Star mass		.001
Star luminosity change relative to speciation types and rates		.00001
Star color		.4
Star magnetic field		.1
Star magnetic field variability		.1
Stellar wind strength and variability		.1
Short period variation in parent star diameter		.3
Star’s carbon-to-oxygen ratio		.01
Star’s space velocity relative to Local Standard of Rest		.05
Star’s short-term luminosity variability		.05
Star’s long-term luminosity variability		.05
Amplitude and duration of star spot cycle		.1
Number and timing of solar system encounters with interstellar gas clouds		.1
Galactic tidal forces on planetary system		.2
H₃⁺ production		.1
Supernovae rates and locations		.01
White dwarf binary types, rates, and locations		.01
Structure of comet cloud surrounding planetary system		.3
Planetary distance from star		.001
Inclination of planetary orbit		.5
Axis tilt of planet		.3
Rate of change of axial tilt		.01
Period and size of axial tilt variation		.1
Planetary rotation period		.1
Rate of change in planetary rotation period		.05
Planetary rotation period		.2
Planetary orbit eccentricity		.3
Rate of change of planetary orbital eccentricity		.1
Rate of change of planetary inclination		.5
Period and size of eccentricity variation		.1
Period and size of inclination variation		.1
Number of moons		.2
Mass and distance of moon		.01
Surface gravity (escape velocity)		.001
Tidal force from sun and moon		.1
Magnetic field		.01
Rate of change and character of change in magnetic field		.1
Albedo (planet reflectivity)		.1
Density		.1
Reducing strength of planet’s primordial mantle		.3
Thickness of crust		.01
Timing of birth of continent formation		.1
Oceans-to-continents ratio		.2
Rate of change in oceans-to-continents ratio		.1
Global distribution of continents		.3
Frequency, timing, and extent of ice ages		.1
Frequency, timing, and extent of global snowball events		.1
Asteroidal and cometary collision rate		.1
Rate of change in asteroidal and cometary collision rate		.1
Mass of body colliding with primordial Earth		.002
Timing of body colliding with primordial Earth		.05
Location of body’s collision with primordial Earth		.05
Position and mass of Jupiter relative to Earth		.01
Major planet eccentricities		.1
Major planet orbital instabilities		.05
Drift and rate of drift in major planet distances		.05
Number and distribution of planets		.01
Distance of gas giant planets from mean motion resonances		.02
Atmospheric transparency		.01
Atmospheric pressure		.01
Atmospheric viscosity		.1
Atmospheric electric discharge rate		.01
Atmospheric temperature gradient		.01
Carbon dioxide level in atmosphere		.01
Rate of change in carbon dioxide level in atmosphere		.1
Rate of change in water vapor level in atmosphere		.01
Rate of change in methane level in early atmosphere		.01
Oxygen quantity in atmosphere		.01
Nitrogen quantity in atmosphere		.01
Chlorine quantity in atmosphere		.1
Carbon monoxide quantity in atmosphere		.1
Cobalt quantity in crust		.1
Arsenic quantity in crust		.1
Copper quantity in crust		.1
Boron quantity in crust		.1
Fluorine quantity in crust		.1
Iodine quantity in crust		.1
Manganese quantity in crust		.1
Nickel quantity in crust		.1
Phosphorus quantity in crust		.1
Tin quantity in crust		.1
Zinc quantity in crust		.1
Molybdenum quantity in crust		.05
Vanadium quantity in crust		.1
Chromium quantity in crust		.1
Selenium quantity in crust		.1
Iron quantity in oceans		.1
Tropospheric ozone quantity		.01
Stratospheric ozone quantity		.01
Mesospheric ozone quantity		.01
Water vapor level in atmosphere		.01
Oxygen-to-nitrogen ratio in atmosphere		.1
Quantity of greenhouse gases in atmosphere		.01
Rate of change in greenhouse gases in atmosphere		.01
Quantity of forest and grass fires		.01
Quantity of sea salt aerosols		.1
Soil mineralization		.1
Quantity of anaerobic bacteria in the oceans		.01
Quantity of aerobic bacteria in the oceans		.01
Quantity, variety, and timing of sulfate-reducing bacteria		.001
Quantity of decomposer bacteria in soil		.01
Quantity of mycorrhizal fungi in soil		.01
Quantity of nitrifying microbes in soil		.01
Quantity and timing of vascular plants introductions		.001
Quantity, timing, and placement of carbonate-producing animals		.00001
Quantity, timing, and placement of methanogens		.00001
Quantity of soil sulfur		.1
Rate of interior heat loss for planet		.01
Quantity of sulfur in the planet’s core		.1
Quantity of silicon in the planet’s core		.1
Quantity of water at subduction zones in the crust		.01
Quantity of high-pressure ice in subducting crustal slabs		.1
Hydration rate of subducted minerals		.1
Tectonic activity		.05
Rate of decline in tectonic activity		.1
Volcanic activity		.1
Rate of change of volcanic activity		.1
Continental relief		.1
Viscosity at Earth core boundaries		.01
Viscosity of lithosphere		.2
Biomass-to-comet infall ratio		.01
Regularity of cometary infall		.1
Number, intensity, and location of hurricanes		.02

Dependency factors estimate		10³⁰
Longevity requirements		10¹³

The probability of a planet anywhere in the universe fitting within all 153 parameters is approximately 10^-194. The maximum possible number of planets in the universe is estimated to be 10²². Thus, less than 1 chance in 10¹⁷² (100 thousand trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion trillion) exists that even one such planet would occur anywhere in the universe.

Fine-Tuning for Life in the Universe

For life to be possible in the universe, several characteristics must take on specific values, and these are listed below. In the case of several of these characteristics, and given the intricacy of their relationship, the indication of fine-tuning seems incontrovertible. (Reprinted from Lights in the Sky & Little Green Men, Appendix C, pp. 191-192. A list of scientific references supporting these design parameters is included at the end of the book.)

1. Strong nuclear force constant.

2. Weak nuclear force constant.

3. Gravitational force constant.

4. Electromagnetic force constant.

5. Ratio of electromagnetic force constant to gravitational force constant.

6. Ratio of proton to electron mass.

7. Ratio of number of protons to number of electrons.

8. Expansion rate of the universe.

9. Mass density of the universe.

10. Baryon (proton and neutron) density of the universe.

11. Space energy density of the universe.

12. Entropy level of the universe.

13. Velocity of light.

14. Age of the universe.

15. Uniformity of radiation.

16. Homogeneity of the universe.

17. Average distance between galaxies.

18. Average distance between stars.

19. Average size and distribution of galaxy clusters.

20. Fine structure constant.

21. Decay rate of protons.

22. Ground state energy level for helium-4.

23. Carbon-12 to oxygen-16 nuclear energy level ratio.

24. Decay rate for beryllium-8.

25. Ratio of neutron mass to proton mass.

26. Initial excess of nucleons over antinucleons.

27. Polarity of the water molecule.

28. Epoch for supernova eruptions.

29. Frequency of supernovae eruptions.

30. Epoch of white dwarf binaries.

31. Density of white dwarf binaries.

32. Ratio of exotic matter to ordinary matter.

33. Number of effective dimensions in the early universe.

34. Number of effective dimensions in the present universe.

35. Mass of the neutrino.

36. Magnitude of big bang ripples.

37. Size of the relativistic dilation factor.

38. Magnitude of the Heisenberg uncertainty principle.

[*] Ph.D. in Theoretical Chemistry from Rice University. Full permission is given to reproduce or distribute this document, or to rearrange/reformat it for other media, as long as credit is given, and no words are added or deleted from the text.

[1] W. Sullivan, We Are Not Alone: The Search for Intelligent Life on Other Worlds, McGraw-Hill Book Company, New York, NY 1964, pp. 33-43,88-103.

[2] G. Cocconi and P. Morrison, “Searching for Interstellar Communication,” Nature 184, pp. 844-846 (1959).

[3] F. D. Drake, “Project Ozma,” Physics Today, 14, 4 (1961), pp. 40-46.

[4] The name Ozma was taken from L Frank Baum’s book, Ozma of Oz.

[5] F. Drake and D. Sobel, Is Anyone Out There? The Scientific Search for Extraterrestrial Intelligence, Delta Publishing, New York, NY 1992, pp. 45-64.

[6] F. Drake and D. Sobel, pp. 59-62.

[7] F. Drake, “The Radio Search for Intelligent Extraterrestrial Life,” Current Aspects of Exobiology, edited by G. Mamikunian and M. Briggs, Pergamon Press (1965), pp. 323-345.

[8] I. S. Shklovskii and C. Sagan, Intelligent Life in the Universe, Holden-Day, San Francisco, 1966, pp. 408-413.

[9] J. Davis, “Is There Anybody out There?” The Planetary Society, Oct 25, 2017, https://www.planetary.org/articles/20171025-seti-anybody-out-there (as of July 2022).

[10] C. Sagan, “The Search for Extraterrestrial Life,” Scientific American, 271, October 1994, pp. 93-99.

[11] C. Sagan and F. Drake, “The Search for Extraterrestrial Intelligence,” Scientific American, 232, May 1975, pp. 80-89.

[12] Seth Shostak, “Listening for Life,” Astronomy, October 1992, pp. 26-33. Seth Shostak, “When E.T. Calls Us,” Astronomy, September 1997, pp. 37-51.

[13] D. McKay, E. Gibson Jr., K. Thomas-Keprta, H. Vali, C. Romanek, S. Clemett, X. Chillier, C. Maechling, R. Zare, “Search for Past Life on Mars: Possible Relic of Biogenic Activity in Martian Meteorite ALH84001,” Science, 273 (1996), p. 924-930.

[14] J. Hartsfield and D. Salsbury, “Meteorite Yields Evidence of Primitive Life on Early Mars,” NASA News, World Wide Web (Aug. 7, 1996), p. 1, https://www2.jpl.nasa.gov/snc/nasa1.html (as of July 2022).

[15] J. Shreeve, “Find of the Century?” Discover, Jan. 1997, pp. 40-41.

[16] T. Monmaney, “Impact of Mars meteorite may be ‘cosmological blow’ for mankind,” Houston Chronicle, Sept. 2, 1996, p. 11A.

[17] A. Albee, “Mars find could be rock of ages,” The Arizona Republic, Aug. 18, 1996, p. H1-H2.

[18] E. Wilson, “Meteorite worms: Martian nanofossils or lab gunk?” C&EN, Dec. 8, 1997, p. 8.

[19] A. Chaikin, “Life on Mars: The Great Debate,” Popular Science, July 1997, pp. 60-65.

[20] E. Wilson, “Cold Water Thrown on Martian Life Hypothesis,” C&EN, Jan. 7, 1998, pp. 8-9.

[21] R. Cowen, “Biology on Europa,” Science News, 151 (1997), p. 210.

[22] D. Cruikshank, “Comet Hale-Bopp: Stardust Memories,” Science, 275 (1997), pp. 1895-1896.

[23] New York Times News Service, “Comets Scattering Seeds of Life? Hale-Bopp Findings Boost Theory,” April 1, 1997.

[24] T. S. Boyajian, et al, “Planet Hunters X. KIC 8462852 – Where’s the Flux?”, Monthly Notices of the Royal Astronomical Society 457 (4): 3988–4004 (September 15, 2015), arXiv:1509.03622.

[25] R. Andersen, “The Most Mysterious Star in Our Galaxy,” The Atlantic (13 October 2015), https://www.theatlantic.com/science/archive/2015/10/the-most-interesting-star-in-our-galaxy/410023/ (as of July 2022).

[26] T. S. Boyajian, et al, “The First Post-Kepler Brightness Dips of KIC 8462852,” The Astrophysical Journal, 853 (1), L8 (January 3, 2018), arXiv:1801.00732.

[27] N. Drake, “Mystery of ‘Alien Megastructure’ Star Has Been Cracked,” National Geographic, (3 January 2018).

[28] S. Bialy and A. Loeb, “Could Solar Radiation Pressure Explain ‘Oumuamua’s Peculiar Acceleration?” Astrophysics Journal Letters 868, L1 (2018), https://iopscience.iop.org/article/10.3847/2041-8213/aaeda8 (as of February 26, 2022).

[29] A. Loeb, “Six Strange Facts About ‘Oumuamua,” ArXiv e-prints (2018), arXiv:1811.08832.

[30] A. Siraj and A. Loeb, “‘Oumuamua’s Geometry Could Be More Extreme than Previously Inferred,” Research Notes of the American Astronomical Society 3, 15 (2019), https://iopscience.iop.org/article/10.3847/2515-5172/aafe7c (as of July 2022).

[31] The ‘Oumuamua ISSI Team “The natural history of ‘Oumuamua,” Nature Astronomy 3, 594–602 (2019), https://authors.library.caltech.edu/97422/2/1907.01910.pdf (as of July 2022).

[32] H. Ross, K. Samples, and M. Clark, Lights in the Sky & Little Green Men, NavPress, Colorado Springs, CO 2002, p. 55-64.

[33] T. Ferris, “Seeking an end to cosmic loneliness,” The New York Times Magazine, October 23, 1977, p. 97.

[34] F. Drake, Is Anyone Out There? 1992.

[35] I. S. Shklovskii and C. Sagan, p. 411.

[36] C. Sagan and F. Drake, p. 80.

[37] R. Naeye, “OK, Where Are They?” Astronomy, July 1996, p. 38.

[38] C. Sagan, Cosmos, Ballantine Books, New York, NY 2013, p. 205. Originally published in 1980.

[39] D. A. Vakoch and M. F. Dowd (eds.), The Drake Equation: Estimating the Prevalence of Extraterrestrial Life Through the Ages, Cambridge, UK: Cambridge University Press 2015.

[40] I. S. Shklovskii and C. Sagan, p. 413.

[41] Values taken from http://www.activemind.com/Mysterious/Topics/SETI/drake_equation.html (site visited in 1996 and 2011 but is no longer available). This site organized the Drake equation in a slightly different way, so the numbers were adjusted to fit the format used here.

[42] Values taken from http://www.seds.org/~rme/drake.html (site visited in 1996 and 2011 but is no longer available). SEDS stands for Students for the Exploration and Development of Space.

[43] J. T. Wright, “Strategies and advice for the Search for Extraterrestrial Intelligence,” Acta Astronautica 188 (2021), pp. 203-214, arXiv:2107.07283.

[44] N. S. Kardashev, “Transmission of Information by Extraterrestrial Civilizations,” Soviet Astronomy 8, 2 (September-October 1964), pp. 217-221.

[45] C. Sagan, J. Agel (eds.), Cosmic Connection: An Extraterrestrial Perspective, Cambridge Press, New York, NY (2000), pp. 233-234.

[46] J. T. Wright, K. M. S. Cartier, M. Zhao, D. Jontof-Hutter, and E. B. Ford, “The Ĝ Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures,” Astrophysical Journal, 816(1), [17], January 1, 2016.

[47] J. T. Wright, “Exoplanets and SETI,” Handbook of Exoplanets, Springer International Publishing, pp. 3405-341, July 12, 2017, arXiv:1707.02175.

[48] F. J. Dyson, “Search for Artificial Stellar Sources of Infrared Radiation,” Science 131 (3 June 1960): 1667–68.

[49] F. J. Dyson, “The Search for Extraterrestrial Technology,” Perspectives in Modern Physics, Essays in Honor of Hans A. Bethe, R. E. Marshak, Editor, New York: John Wiley and Sons, 1966, pp. 641-655.

[50] Instead of a Dyson swarm, many science fiction books and movies present a Dyson sphere, a rigid spherical shell of material surrounding the star. This model derives from a misunderstanding of Dyson’s paper and is implausible for many technical reasons.

[51] I. S. Shklovskii and C. Sagan, pp. 406-407.

[52] I. S. Shklovskii and C. Sagan, pp. 248-257.

[53] F. Drake and D. Sobel, pp. 180-185.

[54] B. Zuckerman, “Why SETI will Fail,” arXiv:1912.08386. This article originally appeared in the September/October 2002 issue of Mercury magazine (published by the Astronomical Society of the Pacific).

[55] F. Drake and D. Sobel, p. xiii.

[56] F. Drake and D. Sobel, pp. 36-41.

[57] N. S. Kardashev, p. 217.

[58] F. Drake and D. Sobel, pp. 102-104.

[59] P. Horowitz and C. Sagan, “Five Years of Project META: An All-Sky Narrow-band Radio Search for Extraterrestrial Signals,” Astrophysical Journal, 415 (1993), pp. 218-235.

[60] C. Sagan, “The Search for Signals from Space,” Parade Magazine, Houston Chronicle, September 19, 1993, p. 5.

[61] C. F. Chyba, “Life beyond Mars,” Nature, 382 (August 15, 1996), pp. 576-577.

[62] R. H. Grey, “A Search of the ‘WOW’ Locale for intermittent Radio Signals,” Icarus, 112, 1994, pp. 485-489.

[63] C. Q. Choi, “Mysterious radio signal from Proxima Centauri was definitely not aliens,” Space.com (October 26, 2021), https://www.space.com/proxima-centauri-radio-signal-not-aliens-breakthrough-listen (as of July 2022).

[64] C. D. Tremblay, D. C. Price, and S. J. Tingay, “A search for technosignatures toward the Galactic Centre at 150 MHz,” Publications of the Astronomical Society of Australia, 39, E008, arXiv:2202.03324.

[65] R. L. Griffith, J. T. Wright, J. Maldonado, M. S Povich, S. Sigurdsson, and B. Mullan, “The ĝ infrared search for extraterrestrial civilizations with large energy supplies. III. the reddest extended sources in WISE,” The Astrophysical Journal Supplement Series, 217(2):25, 2015.

[66] M. A. Garrett, “The application of the Mid-IR radio correlation to the Ĝ sample and the search for advanced extraterrestrial civilizations,” Astronomy & Astrophysics, August 12, 2015, pp. 1-6.

[67] H Chen, M A Garrett, “Searching for Kardashev Type III civilizations from high q-value sources in the LoTSS-DR1,” Monthly Notices of the Royal Astronomical Society, Volume 507, Issue 3, November 2021, Pages 3761–3770.

[68] A. J. LePage, “Where They Could Hide,” Scientific American, July 2000, pp. 40-41.

[69] I. Crawford, “Where Are They?” Scientific American, July 2000, pp. 38-39.

[70] E. M Jones, “Where is everybody? an account of fermi’s question.” Technical Report LA-10311-MS, Los Alamos National Laboratory (1985), https://www.osti.gov/servlets/purl/5746675 (as of July 2022). Reprinted in Physics Today, August 1985, pp. 11-13.

[71] S. Webb, If the Universe is Teeming with Aliens … Where is Everybody? Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life, Copernicus, New York, 2002, pp. 62-63.

[72] B. Zuckerman and M. H. Hart (eds.), Extraterrestrials: Where Are They?, 2nd Ed., Cambridge University Press, Cambridge (1995), pp. 45-85.

[73] I. Crawford, pp. 41-42.

[74] I. Crawford, p. 41.

[75] S. Webb, pp. 67-70.

[76] S. Webb, pp. 79-84.

[77] F. J. Tipler, “Extraterrestrial Intelligent Beings Do Not Exist,” Physics Today, April 1981, pp. 9, 70-71; and associated commentary in Physics Today, March 1982, pp. 26-38.

[78] M. H. Hart, “An Explanation for the Absence of Extraterrestrials on Earth,” Quarterly Journal of the Royal Astronomical Society, 16, (1973), pp. 128-135.

[79] S. Webb.

[80] S. Webb, pp. 29-32.

[81] H. Ross, K. Samples, and M. Clark.

[82] B. Zuckerman and M. H. Hart (eds.), pp. 20-28.

[83] A. Wolszczan, D. A. Frail, “A planetary system around the millisecond pulsar PSR1257 + 12,” Nature 355, issue 6356 (1992), pp. 145–147.

[84] M. Mayor and D. Queloz, “A Jupiter-Mass Companion to a Solar-Type Star,” Nature 378, issue 6555 (November 23, 1995): 355–59.

[85] Exoplanet TEAM, The Extrasolar Planets Encyclopaedia, The Catalog, http://exoplanet.eu/catalog/ (as of July 2022).

[86] M. E. Summers and J. Trefil, Diamonds Worlds, Super-Earths, Pulsar Planets, and the New Search for Life Beyond Our Solar System, Smithsonian Books, Washington DC, 2017, p. 42.

[87] S. Huang, “Occurrence of life in the universe,” American Scientist 47, 3, pp. 397–402 (1959).

[88] J. F. Kasting, D. P. Whitmore, and R. T. Reynolds, “Habitable zone around main sequence stars,” Icarus 101 (1993), pp. 108-128.

[89] E. Tasker, The Planet Factory: Exoplanets and the Search for a Second Earth, Bloomsbury Publishing Plc, New York, NY 2017, p. 229.

[90] E. Tasker, pp. 228-234.

[91] E. Tasker, p. 229.

[92] J. F. Kasting, D. P. Whitmore, and R. T. Reynolds, pp. 109-110.

[93] G. Gonzalez and J. Richards, The Privileged Planet: How Our Place in the Cosmos is Designed for Discovery, Regnery Publishing, Inc., Washington DC 2004, pp. 127-132.

[94] S. Webb, pp. 158-160.

[95] E. Tasker, p. 234.

[96] D. A. Vakoch and M. F. Dowd (eds.), pp. 53-68, 91-110, 163-179.

[97] D. A. Vakoch and M. F. Dowd (eds.), pp. 207-224.

[98] E. Tasker, p. 232.

[99] E. Tasker, p. 230-238.

[100] E. Tasker, p. 233.

[101] “Who ordered that?” was the exclamation of mock horror by Columbia University physicist I. I. Rabi upon receiving news of the discovery of the muon, a new and totally unpredicted subatomic particle. It seems a very appropriate response to the discovery of hot Jupiters.

[102] S. Flamsteed, “Impossible Planets,” Discover, Sept. 1997, pp. 78-83.

[103] E. Tasker, pp. 27-80.

[104] M. E. Summers and J. Trefil, pp. 6-7, 79-138.

[105] E. Tasker, pp. 60, 83-234.

[106] Tatooine is the fictional desert planet that is the home to Luke Skywalker in the Star Wars films. It is notable for having twin suns and therefore the planet would be orbiting two different stars.

[107] Pat Brennan, “Cosmic Milestone: NASA Confirms 5,000 Exoplanets,” Jet Propulsion Laboratory, March 21, 2022, https://www.jpl.nasa.gov/news/cosmic-milestone-nasa-confirms-5000-exoplanets (as of July 2022).

[108] Ethan Siegal, “Here’s what the first 5000 exoplanets have taught us,” Starts with a Bang, Big Think, March 24, 2022, https://bigthink.com/starts-with-a-bang/first-5000-exoplanets (as of July 2022).

[109] Rick Chen, “Exoplanet Populations,” NASA, Ames Research Center, Jun 19, 2017, https://www.nasa.gov/image-feature/ames/kepler/exoplanet-populations (as of July 2022).

[110] D. Sasselov, Life of Super-Earths: How the Hunt for Alien Worlds and Artificial Cells Will Revolutionize Life on Our Planet, Basic Books, New York, NY 2012, pp. 123-124.

[111] E. Siegel, “It’s Time To Retire The Super-Earth, The Most Unsupported Idea In Exoplanets,” Forbes, Starts with a Bang, https://www.forbes.com/sites/startswithabang/2021/06/30/its-time-to-retire-the-super-earth-the-most-unsupported-idea-in-exoplanets (as of July 2022).

[112] Planetary Habitability Laboratory, “HEC: Exoplanet Statistics,” PHL @ UPR Arecibo, July 2, 2018, https://phl.upr.edu/projects/habitable-exoplanets-catalog/hec-exoplanet-statistics (as of July 2022).

[113] P. Ward and D. Brownlee, Rare Earth: Why Complex Life is Uncommon in the Universe, Copernicus, NY 2000, pp. 55-242.

[114] F. Rana and H. Ross, Origins of Life, NavPress, Colorado Springs, CO 2004.

[115] D. A. Vakoch and M. F. Dowd (eds.), p. 120.

[116] Michael Schirber, “Can Life Thrive Around a Red Dwarf Star?” Astrobiology Magazine, April 09, 2009. https://www.space.com/6560-life-thrive-red-dwarf-star.html (as of July 2022).

[117] C. P. Johnstone, M. L. Khodachenko, T. Lüftinger, K. G. Kislyakova, H. Lammer, and M. Güdel, “Extreme hydrodynamic losses of Earth-like atmospheres in the habitable zones of very active stars,” Astronomy & Astrophysics 624, L10 (2019), arXiv:1904.01063.

[118] E. F. Guinan and S. G. Engle, “The ‘Living with a Red Dwarf’ Program,” (January 2009), arXiv:0901.1860.

[119] E. Tasker, pp. 260-265.

[120] E. Tasker, pp. 165-189.

[121] E. V. Quintana, J. J. Lissauer (2010), “Terrestrial Planet Formation in Binary Star Systems,” N. Haghighipour, (eds.), Planets in Binary Star Systems, Astrophysics and Space Science Library, vol 366, Springer, Dordrecht, pp. 265–283, arXiv:0705.3444.

[122] E. Tasker, pp. 265-269.

[123] B. Zuckerman and M. H. Hart (eds.), p. 188.

[124] B. Zuckerman and M. H. Hart (eds.), p. 185.

[125] C. H. Lineweaver, Y. Fenner, B. K. Gibson, “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way,” Science 303 (5654), June 2, 2004, pp. 59–62, arXiv:astro-ph/0401024.

[126] E. Tasker, pp. 249-251.

[127] W. S. Howard, M. A. Tilley, H. Corbett, A. Youngblood, R. O. P. Loyd, J. K. Ratzloff, N. M. Law, O. Fors, D. del Ser, E. L. Shkolnik, C. Ziegler E. E. Goeke, A. D. Pietraallo, and J. Haislip, “The First Naked-eye Superflare Detected from Proxima Centauri.” The Astrophysical Journal, 860 (2): L30, 25 June 2018, arXiv:1804.02001.

[128] C. Dong, M. Lingam, Y. Ma, and O. Cohen, “Is Proxima Centauri b Habitable? A Study of Atmospheric Loss,” Astrophysical Journal Letters, 837 (2017) L26, arXiv:1702.04089.

[129] E. Tasker, pp. 148-150.

[130] H. Ross, Improbable Planet: How Earth Became Humanities Home, Baker Books, Grand Rapids, MI 2016, pp. 26-27.

[131] H. Ross, pp. 48-56.

[132] H. Ross, pp. 56-58.

[133] H. Ross, Designed to the Core, RTB Press, Covina, CA 2022, pp. 178-181.

[134] G. Gonzalez and J. Richards, pp. 4-6.

[135] G. Gonzalez and J. Richards, pp. 7-11.

[136] G. Gonzalez and J. Richards, pp. 10-19.

[137] S. Webb, p. 161.

[138] K. J. Walsh, A. Morbidelli, S. N. Raymond, D. P. O’Brien, A. M. Mandell, “A low mass for Mars from Jupiter’s early gas-driven migration,” 2011, Nature, 475, 206, arXiv:1201.5177.

[139] S. N. Raymond and A. Morbidelli, “The Grand Tack model: a critical review,” Complex Planetary Systems, Proceedings IAU Symposium, No. 310, 2014, arXiv:1409.6340.

[140] H. Ross, Improbable Planet , pp. 45-48.

[141] H. Ross, pp. 44-45.

[142] S. Webb, pp. 160-163.

[143] G. W. Wetherill, “How Special is Jupiter?” Nature 373 (1995), p. 470.

[144] The editors, “Our Friend Jove,” Discover, July 1993, p. 15.

[145] E. Tasker, p. 203.

[146] S. Webb, pp. 166-172.

[147] J. van Paradijs, “From Gamma-Ray Bursts to Supernovae,” Science, 286 (1999), p. 693-695.

[148] A. L. Melott, B. S. Lieberman, C. M. Laird, L. D. Martin, M. V. Medvedev, B. C. Thomas, J. K. Cannizzo, N. Gehrels, and C.H. Jackman, “Did a gamma-ray burst initiate the late Ordovician mass extinction?” International Journal of Astrobiology 3 (1) (2004), pp. 55–61n, arXiv:astro-ph/0309415.

[149] J. Scalo and J. C. Wheeler, “Astrophysical and Astrobiological implications of Gamma-Ray Burst Properties,” Astrophysical Journal, 566 (2002), pp. 723-737.

[150] G. Guillermo, D. Brownlee, and P. Ward (2001), “The Galactic Habitable Zone I: Galactic Chemical Evolution”. Icarus 152 (1): 185.

[151] C. H. Lineweaver, Y. Fenner, B. K. Gibson. “The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way,” Science 303 (5654): (2004) pp. 59–62.

[152] B. Zuckerman and M. H. Hart (eds.), pp. 184-191.

[153] S. Webb, pp. 153-155.

[154] P. Ward and D. Brownlee, pp. 27-29.

[155] S. R. Kane and S. J. Deveny, “Habitability in the Omega Centauri Cluster,” The Astrophysical Journal, Volume 864, Issue 2, (2018), arXiv:1808.00053.

[156] P. Ward and D. Brownlee, pp. 25-27.

[157] P. Ward and D. Brownlee, pp. xxv-xxvi.

[158] H. Ross, Why the Universe is the Way it is, pp. 68-72. Hugh Ross, Improbable Planet, pp. 28-35.

[159] H. Ross, Designed to the Core, pp. 31-69.

[160] E. Tasker, pp. 275-289.

[161] G. Mone, “Frozen. Irradiated. Desolate. Alive?”, Discover, November 2012, p. 30-37.

[162] E. Tasker, p. 291.

[163] H. Ross, Improbable Planet, pp. 231-233.

[164] E. Tasker, p. 211.

[165] E. Tasker, p. 217.

[166] E. Tasker, pp. 219-224.

[167] N. Drake, “Why alien hunters have spent 60 years finding new solutions for the Drake Equation,” National Geographic, (November 30, 2021).

[168] One possible alternative formulation uses N_* (the total number of stars in the galaxy) and f_L (the fraction of advanced civilizations still broadcasting) in place of R_* and L. (For example, see C. Sagan, Cosmos, pp. 315-319.) Curiously, Drake’s choice of using R_* and L reflected his deeply held assumption that advanced civilizations would be common. In that case, the main challenge would be to determine how many are broadcasting detectable signals during the narrow time window when we are listening. Drake likened this situation to a Christmas tree with individual lights (representing currently communicating civilizations) blinking on and then off again. (D. A. Vakoch and M. F. Dowd (eds.), pp. 9-13.) But, of course, this presumed and abundance of advanced civilizations and prejudiced the equation to support this conclusion.

[169] F. Drake and D. Sobel, pp. 61-62.

[170] A. Sandberg, E. Drexler, and T. Ord, Dissolving the Fermi Paradox (2018), arXiv:1806.02404.

[171] D. A. Vakoch and M. F. Dowd (eds.), p. 11.

[172] P. Ward and D. Brownlee, pp. 270-275

[173] Design statistics were collected by astronomer Dr. H. Ross. In H. Ross, The Creator and the Cosmos, NavPress, CO, 1993, pp. 123-135, he lists 33 design parameters with explanations and estimates. In the second edition of the book (1995, pp. 132-144) this was expanded to include 41 design parameters. By the third edition (2001, pp. 175-199), this list had increased to 128. H. Ross, K. Samples, and M. Clark, Lights in the Sky & Little Green Men, NavPress, Colorado Springs, CO 2002, pp. 171-189, gives the current listing of 153 design parameters. These design parameters are reproduced in the appendices of this paper.

[174] H. Ross, Why the Universe is the Way it is, Appendix C, p. 213 with link https://storage.googleapis.com/reasons-prod/files/compendium/compendium_part2.pdf (as of July 2022).

[175] R. Naeye, p. 38.

[176] R. Naeye, pp. 38-43.

[177] R. Naeye, p. 43.

[178] R. Naeye, p. 43.

[179] P. Davies, The Eerie Silence: Renewing Our Search for Alien Intelligence, Houghton Mifflin Harcourt Publishing Company, New York, NY 2010, p. 207.

[180] This paper does not address the question of how easy or difficult the origin of simple life is. Because of this, the conclusion of Ward and Brownlee is particularly important because it establishes that a belief that the Earth and sun are indeed special is not dependent upon a rejection of abiotic evolution or a belief in biblical creation.

[181] P. Ward and D. Brownlee, p. xiv.

[182] P. Ward and D. Brownlee, p. 275.

[183] G. Gonzalez, “Nobody Here but Us Earthlings,” The Wall Street Journal, July 16, 1997, p. A22. This article along with three contrasting viewpoints and a follow-up response by G. Gonzalez can be found in Cosmic Pursuit, Spring 1999, 16-19, 63.

[184] G. Gonzalez, p. A22.

[185] Psalms 19:1, The Holy Bible, New International Version.

[186] The Limits of Organic Life in Planetary Systems Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council, http://www.nap.edu/catalog/11919.html (as of July 2022).

[187] R. Naeye, p. 38-39.

[188] Polarity refers to an unequal distribution of electrons in which some portions of a molecule have a partial positive charge and other parts have a partial negative charge. For water, the oxygen atom has a stronger pull on the electrons than the hydrogen atoms do, so the oxygen has a partial negative charge, and the two hydrogens have a partial positive charge.

[189] R. A. Breuer, The Anthropic Principle: Man as the Focal Point of Nature, Birkhäuser, Boston, 1991, pp. 209-218.

[190] R. A. Horne, Marine Chemistry: The Structure of Water and the Chemistry of the Hydrosphere, Wiley, 1969. Horne lists ten “anomalous” chemical properties of water, their importance in biology, and comparisons to other solvents. The 10 properties are water’s heat capacity, heat of fusion, heat of vaporization, thermal expansion, surface tension, dissolving power, dielectric constant, electrolytic dissociation, transparency, and conduction of heat

[191] F. H. Stillinger, “Water Revisited,” Science 299 (1990), p. 451-457. Stillinger reports 8 “notable” properties of water and comments, “Some of these attributes are shared with other substances; perhaps we will eventually discover that they all are. Nevertheless, it is striking that so many eccentricities should occur together in one substance.”

[192] R. H. Dicke, “Dirac’s Cosmology and Mach’s Principle,” Nature 192, 1961, p. 440.

[193] J. J. Petkowski, William Bains, and Sara Seager, “On the Potential of Silicon as a Building Block for Life,” Life 2020, 10(6), 84, https://www.mdpi.com/2075-1729/10/6/84 (as of July 2022).

[194] David Wilkinson, Alone in the Universe? InterVarsity Press, Downers Grove, IL 1997, pp. 115-134.

[195] S. Hawking, A Brief History of Time, Bantam Books, New York 1988, pp. 126-127.

[196] H. Ross, Why the Universe is the Way it is, pp. 27-41.

[197] H. Ross, “The Haste to Conclude Waste,” Facts & Faith, Vol. 11, No. 3 (1997), p. 1-3.

[198] B. Carter, “Large Number Coincidences and the Anthropic Principle in Cosmology,” Proceedings of the International Astronomical Union Symposium, No. 63: Confrontation of Cosmological Theories with Observational Data, ed. M. S. Longair (Dordrecht-Holland/Boston, U.S.A.: D. Reidel, 1974), 291-298.

[199] B. Carter, “The Anthropic Principle and its Implications for Biological Evolution,” Proc. Royal Soc. Discussion Meetings on the Constants of Physics, edited by W. H. McRea and M. J. Rees, R. Soc., London, 1983 and in Philos. Trans. R. Soc. London, A310, p. 347-363.

[200] H. Ross, “A Precise Plan for Humanity,” Facts For Faith (Q1 2002), pp. 25-31.

[201] H. Ross, “Life on Mars as Proof of Evolution?” Facts & Faith, Vol. 2, No. 3, 1998, pp. 1-2; H. Ross, “Life on Mars Revisited,” Facts & Faith, Vol. 3, No. 2, 1989, p. 2; H. Ross, “Ahead to Mars, or Back to Babel?” Facts & Faith, Vol. 5, No. 2, 1991, p. 1-4.

[202] H. Ross, The Creator and the Cosmos, NavPress, CO, 1993, pp. 144-146.

The Search for Extraterrestrial Intelligence: A 21st Century Perspective

Exoplanet Properties