Fig. 21-0: The first discovered solar system other than our own with more than three planets, discovered using the Kepler satellite. All the discovered planets are larger than Earth. (Courtesy of NASA, Tim Pyle)
One of the most profound questions about the universe is whether life is an emergent property, of which Earth is one example, or whether life is an exception and Earth may be unique. From one point of view, the operation of physical law does not generally lead to uniqueness. It strains common sense to think of our planet as the single inhabited planet given the hundreds of billions of stars in our galaxy alone, particularly if emergence of life may be one of the natural consequences of planetary evolution. An alternative view, however, is that there are so many hurdles that Earth overcame to reach its present state of hosting technological civilization that we may be unique. In the absence of data, we simply do not yet know, but we can question intelligently.
For any solar system there is a “habitable zone” around the star that permits extended planetary evolution with liquid water. Venus may have been within such a habitable zone in its early history, but if life began there it was destroyed by a runaway greenhouse. Venus is too close to the sun and too hot. Mars may also have had early life, since it has water and it conceivably has a small, deep biosphere today. If life did begin there, planetary evolution was halted at the bacterial stage. Mars also lacked sufficient planetary size to maintain a large enough atmosphere and sufficient greenhouse warming. Mars is too small and too cold. Jupiter’s moon Europa has a liquid ocean that conceivably may harbor life, but the lack of solar energy necessarily limits its planetary potential for advanced life. Alternative forms of life with a solvent other than water might make colder planets habitable—but this is pure speculation.
New facts about other solar systems promise to bring much greater clarity. Clever astronomical methods are leading to exponential growth in exoplanet discovery. Because of the specifics of how planets are found, most newly discovered planets tend to be large and close to their stars, showing that there is a great diversity of styles of solar systems in the galaxy. With improved data from satellite missions, discovery of Earthlike planets in the habitable zones around other stars is beginning. Investigation of the spectra of their atmospheres will eventually lead to estimates for other life. If we find life in one other place, the statistics are such that life is overwhelmingly abundant in the universe.
The existence of other technological civilizations, however, is another matter. A critical unknown is the fraction of a planetary lifetime that a technological civilization exists. Does such a civilization self-destruct in a few hundred years or last for millions of years? For such a civilization to last, the species driving the technology must sustain and foster planetary habitability rather than ravage planetary resources. In the absence of that understanding, a planet will regress to an earlier stage of planetary development. On the more hopeful side, intelligent life could foster planetary evolution leading to further stages of development that we cannot imagine.
Only if technological civilizations persist for millions of years can the galaxy have a significant number of solar systems with technologically capable life. In this case, since we have had this capability for only a century, all such civilizations would be immeasurably advanced relative to our own. Their survival also would have required a comprehension of and attention to planetary sustainability that is presently lacking on Earth. Only to the extent these challenges have been met successfully elsewhere is our universe inhabited by intelligent life.
We have been using the term a habitable planet as if the discussion were generic, but of course most of our knowledge is specific to Earth and its planetary experience. In Chapter 17 the possibility was raised that important aspects of the Earth experience are likely to be general phenomena in the Universe. It is then natural to address the popular questions of whether there are other habitable planets in the universe and whether interplanetary communication with other capable civilizations might be possible. Of course, absent such communication in a fully verified and public way, we are inevitably in the realm of speculation. Our views must be so influenced by our own planetary experience that we are incapable of considering the question broadly enough. Nonetheless, there are facts emerging about characteristics of planets around other stars that are highly pertinent, and it is possible by considering the question statistically to at least show where some of the most important questions and sources of uncertainty remain.
Earth’s evolution (Table 21-1) involved formation of a rocky planet during accretion from the solar nebula, differentiation into discrete layers, formation of an atmosphere and ocean, development of a stable climate through feedback mechanisms involving subaerial crust and ocean, the origin of life, evolution of photosynthesis, development of an oxygenated atmosphere and eukaryotic cells, a rise of oxygen to ~20% of the atmosphere, the development of multicellular organisms, and recently the evolution of human beings followed by development of civilization and technology. Each of these events is a quantum step in planetary evolution, where the planet has a different capability of function and very different qualities. The entire development has also taken a long time—more than 4 billion years of a stable planetary environment. We can imagine that a planet could be arrested at any of these stages of planetary evolution. Lack of heavy elements from insufficient nucleosynthesis would make rocky planets impossible in some solar systems. Catastrophic events in the solar nebula or interactions with a neighboring star could also inhibit planet creation, or early destruction could be caused by a nearby supernova. An early atmosphere could be lost or the planet could have an insufficient volatile budget to create one. Climate feedbacks could fail. Life could fail to initiate or to undergo the closely coupled coevolution with the planetary environment that permits the development of multicellular life, with its access to the great energy potential of a planetary fuel cell. And it seems evident that a technological civilization could either fail to begin or self-destruct by war, climate change, disease, or using up in a short period of time the resources and energy necessary for technology. Given this trail of hurdles, what is the likelihood of other habitable planets in the universe?
Table 21-1
Stages in planetary evolution
Stage 1 |
Sufficient metallicity to form rocky planet, good galactic environment |
Stage 2 |
Appropriate volatile budget, tectonic circulation permitting exchange between surface and interior |
Stage 3 |
Ocean and stable climate feedback |
Stage 4 |
Origin of life |
Stage 5 |
Photosynthesis and oxygen production |
Stage 6 |
Survival of oxygen catastrophe and development of multicellular life |
Stage 7 |
Intelligent life |
Stage 8 |
Technological civilization with access to external energy |
In our solar system Venus and Mars are our closest neighbors, and the fact that no life has been discovered on either of them, and that there is no evidence of life on any other object in our solar system, shows that habitability over billions of years leading to technological civilizations requires particular circumstances. Venus and Mars provide our two closest examples of failure of biological planetary evolution, and we can learn much from them as we consider the possibility of life elsewhere in our galaxy. As we learned in Chapter 8, Venus is similar in size to Earth, and with its extensive atmosphere, clearly had a sufficient volatile budget and sufficient internal heat to generate global volcanism. The lack of impacts on the planet’s surface shows that some of this volcanism is recent. Since Venus is 20% closer to the sun, through its history it has experienced almost twice the influx of solar energy. It appears that in the odds of planetary evolution, Venus was too hot.
In its planetary characteristics, Mars differs more substantially than Venus from Earth. Its greater distance leads to about half as much solar illumination. In the early solar system with the fainter sun, Mars would have received correspondingly less solar energy. Mars is also much smaller than Earth and Venus. At about half the radius it is only about one-eighth of the mass. The smaller mass makes retention of early volatiles more difficult and atmospheric escape much easier. Evidence from the surface geomorphology of Mars provides strong evidence for periodic periods of abundant water on the surface, and the mineralogy of the Martian soils as determined by the Mars rover expeditions has shown minerals whose formation requires water. There may still be substantial water buried beneath the surface. Mars also has CO2, and with its cold temperature it has solid CO2 ice caps that vary with the seasons. The present atmosphere on Mars, however, has a pressure <1% that of Earth, and it does not have the disequilibrium composition that must have been present on Earth over much of its history. No evidence of life on Mars has been found, though it is conceivable that a small biosphere occurs in the subsurface. Even if Mars has life, its planetary evolution must have been thwarted at an early stage. Mars has the combination of being too cold and too small. (A planet of several Earth masses, for example, might have had a robust enough atmosphere to overcome the greater distance from the sun with a larger greenhouse effect.)
While Mars and Venus are the two most obvious candidates for life in our solar system, and evidence for ancient life on Mars may at some point appear, these are not the only environments where life may be present. The discovery of life at deep-sea hydrothermal vents revealed that life could be supported by planetary heat in the absence of sunlight in a liquid environment. One of the Moons of Jupiter, Europa, has such a planetary environment, and is the only other body of the solar system where the data suggest that both rock and liquid water may be present in abundance. Because of tidal heating from Jupiter, the moon is not cold enough for the ocean to completely freeze, and therefore we know that the temperature environment is also one where life is able to exist on Earth. Since life requires an external energy source, however, the most likely environment for life on Europa would be at deep-sea vents, rather than life distributed throughout the ocean. This makes detection of such life extremely challenging. Furthermore, the available energy is very limited in comparison to the sun-fed photosynthesis that has permitted Earth evolution, and therefore the likelihood of advanced life on Europa is very slim. It would also have been restricted to the very early stages of planetary evolution. If our solar system is an exemplary model, one planet per star had the opportunity for long-term coupled biological-planetary evolution.
These considerations lead to questions of how common planets are around other stars, and how other solar systems may compare with our own in their physical characteristics.
Until very recently there was no hope of directly observing distant planets around other stars because planets are so small and dim. New techniques, however, have led to spectacular growth in planet finding, with the numbers of discovered planets still increasing exponentially (Fig. 21-1). For such a fast-moving field, this book will be outdated as of its publication date. Fortunately, a website exists that gives up-to-date reports of planet-finding—http://exoplanet.eu—and the reader is encouraged to visit that and other sites to follow the rapid progress of this exciting new area of science.
Fig. 21-1: The number of new planets discovered by year since the first discoveries in the mid-1990s. More than 1,000 new candidates have recently been discovered by the Kepler satellite. (Data from http://exoplanet.eu)
Fig. 21-2: Illustration of the Doppler method of planet detection. The planet exerts a gravitational plug on the star so that its relative velocity changes slightly if the planet is on the near side or the far side. Notice that very small changes in the star’s velocity can be measured, making this a very sensitive technique. For example, 10m/sec is only 36 km per hour. (Modified from Marcy, Butler and Vogt, The Astrophysical Journal 536 (2000): L43–L46)
The first method to detect planets made use of the Doppler shift of stellar spectra, which varies slightly owing to the gravitational influence of the planet on the star. When the planet is farther away than the star, it exerts a slight gravitational pull away from the Earth, and when it is closer than the star it exhibits a slight pull toward the Earth. These changes lead to slight variations in relative velocity of the star and small changes in the exact position of the lines in the star’s visible spectrum (Fig. 21-2). Since the influence of gravity varies as the square of the distance and linearly with mass, this method is most effective for large planets close to their stars; the first detected planets were hot Jupiters, massive planets orbiting their stars in tighter orbits even than Mercury in our own solar system.
The second method, called the transit method, requires a specific geometrical relationship where the planet crosses in front of its star as viewed from Earth. When the planet “transits” the star, it shadows a small proportion of the star’s light, leading to a slight reduction in the luminosity of the star. By observing the changes in luminosity of individual stars, planets can be “seen,” as illustrated in Figure 21-3. Over an extended period of observation, the repeat time of the reduction in luminosity gives the period of the planet’s rotation around its star. The star’s mass and size can be estimated from its luminosity (big stars burn brighter). Then the period gives the radius of the orbit, from which the reduction in luminosity gives the size of the planet relative to the star, from which the radius of the planet can be estimated.
Fig. 21-3: Illustration of the transit method of detection of planets around other stars. As the planet passes in front of the star, a portion of the star is shaded leading to a slight decrease in brightness. The duration of the transit and its periodicity give information about the distance of the planet from the star. The reduction in luminosity then gives information on the planet’s size.
The transit method requires that the planet’s orbit and the star intersect along the line of sight, so only solar systems that are viewed from the side rather than the top or bottom are susceptible to this method. Simple geometry also shows that the larger the planet the greater will be the change in luminosity, and the more rapid the orbit the more likely the signal will be seen. To detect Jupiter going around our sun, for example, would require capturing the short time interval once every thirty-four years when Jupiter occulted the sun. Closer planets have the additional advantage of a broader angle of sight where transits occur. For example, imagine the (absurd) case of a planet rotating around the surface of its star. All orbits but one, whose plane was perfectly vertical to the line of sight, would transit the star. For this reason, the viewing angle for transit of Mercury is about ninety times wider than the necessary angle for a transit of Neptune. In general the probability of a transit scales with the radius of the star divided by the distance of the planet form the star. To see Earth transiting the Sun from another star, the angle would need to be within 0.05° of horizontal to the ecliptic of the solar system. For Neptune it would have to be within 0.02°! For all these reasons, large planets in tight orbits are more likely to be detected, and the first discovered planets were both large and close to their star, a condition that does not exist in our solar system.
Fig. 21-4: Discoveries of planets around other stars (exoplanets) as of early 2010. Note the log scale, showing the vast differences in size and orbital radius around their stars. Letters give the mass and distance from the sun for planets of our own solar system. Prior to Kepler (see text) the transit method was applicable primarily for large planets very close to their stars. Note that many of the discovered planets are a hundred times or more larger than Earth, and about twenty times closer to their star. (Modified after S. Seager and D. Deming, Annu. Rev. Astr. Astrophys. 48 (2010):631–72, with data from http://exoplanet.eu)
If both transit and Doppler shift measurements can be obtained, then the planet’s density is constrained, because the change in luminosity reveals the planet’s size, and the gravitational pull reveals the planet’s mass. Through these clever methods astronomers have been able to extract an amazing amount of information from a small amount of data.
Because of the difficulties of planet finding, discovered planets have a relationship between mass and distance from their star that differs from planets in our own solar system. Figure 21-4 shows the first-discovered planets in this context. While many planets the size of our outer planets have been found, almost all of them are far closer to their stars than the outer planets of our solar system. Small planets at greater distances become progressively more difficult to find.
These discoveries have revealed much about solar systems, showing that the characteristics of our solar system are not ubiquitous. Bode’s Law does not apply generally to planetary systems, and large, low-density planets can occur very close to their star. The contrast between inner and outer planets that characterizes our solar system is only one class of solar system, not the general rule. Much more work is required to know the distribution of types of solar system.
Astronomers have also developed a clever method to gain some information about planetary atmospheres. When the planet is in front of the star, the spectrum of light has contributions from both the star and the planet. When the planet is not occulting the star, the spectrum is that of the star alone. By subtracting one from the other, it is possible to infer the planet’s influence on the spectrum and to say something about the planetary atmosphere.
Atmospheric composition is one of the critical differences between planets with and without life. Figure 21-5 compares the atmospheric compositions of Mars and Venus with Earth. On Mars and Venus, CO2 is the dominant gas. Water and O2 (and therefore O3) are absent. In our solar system, the planetary composition is responsive to abundant life on a planetary surface. As we saw in Figure 9-7, O3 has a distinctive absorption in the infrared spectrum. O3 also is not present in an atmosphere unless there is substantial O2. The spectrum of a planetary atmosphere thus contains indications of planetary life as we know it. With greater refinement, this is the method that would be most likely to detect evidence of life on other planets. High enough O2 to produce a significant ozone layer and low CO2 have been characteristic of Earth’s atmosphere for many hundreds of millions of years. Detection of a similar atmosphere for a distant planet would be taken as strong evidence for life elsewhere.
Fig. 21-5: Illustration of the relative compositions of the atmospheres of Earth, Mars, and Venus. Notice that Venus and Mars are very poor in oxygen and H2O, and have similar ratios of CO2/N2. Life and the carbon cycle impart a distinct set of gases to Earth’s atmosphere, which are detectable in the absorption spectrum of the atmosphere. Spectra of planetary atmospheres are the most likely detection method of life on planets around other stars.
The planet-finding satellite named Kepler was launched to discover the frequency of Earthlike planets that would be in the habitable zone of stars similar to the sun. The habitable zone of distant stars depends on the blackbody temperature for planets in orbit around the star. The blackbody radiation of the star gives the temperatures of the stellar surface. Then calculation of how temperature varies with distance from the star is straighforward, as was discussed in Chapter 9 for our solar system. Stars smaller than the sun would have habitable zones much closer to the star, and those larger and hotter than the sun would have habitable zones more distant from the star. Smaller, cooler stars will have habitable planets in tighter radii, and these planets will be the first to be observed because of the observational advantages for planets that are large relative to their star and in tight orbits.
Kepler makes use of the transit method, using an extremely sensitive light detection system to be able to detect minute changes in stellar luminosity. Measurements are made continuously by monitoring more than 150,000 stars in one small part of space, only 1/400 of the visible sky. To find planets as distant from their star as Earth is from the sun will require many years of observations in order to determine the period of the planet’s orbit. As of this writing, less than one year of data from Kepler are available and results are still restricted to planets close to their stars. These results are nonetheless remarkable, and show that the new Kepler data will over time greatly expand our understanding of solar systems and the number of planets around other stars that might be suitable for life.
Fig. 21-6: Distribution of planets discovered prior to Kepler. The vertical axis shows mass relative to the mass of Jupiter. While Kepler has also discovered many new large planets, the Kepler data also extend to lower masses, as indicated by the shaded box, and will likely lead to the discovery of planets with characteristics similar to Earth in the near future. (Modified from http://exoplanet.eu)
Kepler has found more than a thousand planet candidates, greatly expanding the number and range of new planets and solar systems. The sensitivity also permits discovery of small planets. Initial results extend to lower limits the detected planetary masses, though the planet periods are as yet limited by the time of observation. Over time, the gray box in Figure 21-6 will gradually extend to longer periods, likely permitting Earthlike planets to be discovered by 2015.
Kepler also made the discovery of a multiplanet solar system where six planets were discovered rotating about a single star. The planets are much closer to their star than is the case for our solar system (see frontispiece), and all of them are larger than Earth. Kepler has also discovered candidate planets whose predicted blackbody temperatures would put them within the habitable zone of their respective stars. Major new discoveries are in store over the next decade, showing that our planet and solar system are one small part of a vast and diverse community of planets and solar systems in our galaxy.
With this prelude of data, we can now turn to more speculative considerations of how many other planets in our galaxy may contain life, and how many might have intelligent life that has developed a technological civilization capable of communicating with our own. A convenient way to explore these questions is to use a statistical method of multiplicative probabilities that is known as the Drake equation. The equation can be written as:
N is the number of planets in our galaxy with technological civilization and therefore the possibility of communicating with us. The number can be estimated by combining a series of numbers and probabilities. Ns is the total number of stars in the galaxy. This is the only number in the equation that is currently knowable; there are about 400 billion stars (4 × 1011) in the Milky Way. Fs is the fraction of such stars that are suitable for life. Terrestrial experience suggests that life takes a long time to evolve. Large stars with lifetimes of less than a few hundred million years will not have inhabited planets around them. As an extreme example, the largest stars that become supernovae in a million years would have no time for planets and life to develop, and the explosion of the star would destroy any planets in the surroundings.
There are two others aspects to the number of suitable stellar host stars. One is that the nebula that gives rise to the star must have enough C, O, Si, Mg, and Fe to be able to make rocky planets with the necessary ingredients for life. This requires being close enough to the galactic center that there has been a high enough frequency of supernovae to generate sufficient masses of the heavy elements. On the other hand, the center of the galaxy has such a density of stars that the field of galactic radiation and frequency of supernovae would be too intense for life as we know it. So in the overall geography of the galaxy, stars need to be not only the right size, they have to exist in a galactic “habitable zone” (Fig. 21-7), at intermediate locations within the overall galactic structure. Of course, it is conceivable that life might evolve to take advantage of the high energy of the inner galaxy, but life in that case would be rather different from what we know, and to be conservative in the estimate of inhabited planets, a “habitable zone” restriction seems sensible. These various considerations lead to values of Fs of 0.01 to 0.1.
Fig. 21-7: Illustration of the “habitable zone” principle that applies to our solar system and may apply on the galactic scale as well. The interior of the galaxy has too intense a radiation field for life as we know it. The outer realms of the galaxy have not had sufficient supernovae to generate the necessary heavy-element budget for rocky planets and life. (Modified after Lineweaver et al., Science 303 (2004), no. 5654: 59–62)
The rest of the terms in the Drake equation are Np—the number of planets around such stars that have suitable energy balance for life; fL—the fraction of those planets on which life arises; fI—the fraction of those planets on which intelligent life evolves; fTech—the fraction of those which develop a technological civilization; and Ttech/Tp, the fraction of a planet’s lifetime during which such technological civilization exists. The game is then to pick numbers for all these terms and come up with the number of communicable civilizations currently present in our galaxy.
Dealing with probabilities is a dicey business, particularly in the complete absence of constraints! For example, if one takes a probabilistic approach to the origin of life, and sets it up like the Drake equation, one could rapidly arrive at a near zero probability. For instance, what is the chance that you make organic molecular precursors in the right locations and proportions, what are the chances of having those combine into polymers, what is the chance of those molecules making up the polymers becoming chiral, what is the chance these would be incorporated into a cellular container, what is the chance of developing replication leading to evolution by natural selection, and so on? We do not know any of these probabilities, and cogent arguments could be made for very small probabilities for many of them, leading to Earth being the unique planet with life.
In the same way that we could argue that life on any particular planet is highly improbable, we can make the case for small enough values of the other terms in the Drake equation that N <<1. We can call this the pessimistic scenario. For example, one could argue one star in a hundred has an appropriate size and galactic environment, and one of each hundred of these stars has a solar system with a planet in a stable orbit in the habitable zone with an appropriate volatile budget. The odds of life beginning are one in ten thousand, and the odds of planetary evolution leading to intelligent life are one in a thousand (e.g., most planets would be like Mars, Venus, or Europa). Even if we give a probability of 1.0 to the development of a technological civilization, and give the duration of the civilization the 10-million-year average lifetime of a species (TTech/Tp = 107/4.5 × 109), the result is that N = 10–2 × 10–2 × 0–4 × 10–3 × ~2 × 10–3 = 2 × 10–14. Multiplying that number by the 4 × 1011 stars in the galaxy leads to N = 0.08. There should not be any technological civilizations in the galaxy. Earth is improbable and unique.
The perspective of planetary evolution could provide another set of probabilities that could be additional multiplicative terms to the Drake equation. For example, what is the probability of a planet having just the right volatile budget? What is the probability of the planet having a large moon that provides tides and orbital stability? What is the probability plate tectonics will develop and climate stability will emerge? What is the probability that photosynthesis will evolve? What is the probability that mechanisms will be found to cope with the O2 poison and convert it to more advanced energy production? What is the probability of endosymbiosis leading to larger and more complex cells? What is the chance these cells will develop partnerships to evolve multicellular life? What is the chance that life will survive the inevitable catastrophes from planet and solar system? And so on. When asked to deal with any of these probabilities, they could always be argued to be far less than 1, and the greater the number of terms included in the chain of probabilities, the smaller the total probability.
What this approach misses is that highly improbable events can often have a 100% certain result given enough time and opportunity. Death is one obvious example. On a more positive note, if the odds of a favorable event are one in a million, but there are millions of chances, then the event will always occur eventually. Then, if that improbable event either has finality or the possibility of replication and amplification, very improbable events ultimately become dominant phenomena. The key ingredient is time, and planets have lots of time. This line of reasoning might apply to the various steps in the origin of life, and also to the various stages of planetary evolution. Yes, O2 might be a toxic poison for hundreds of millions of years. But one mutation of protection provides evolutionary advantage, and use of O2 as energy source provides more advantage. O2 respiring organisms gradually and inevitably come to dominate ecosystems. If planetary evolution ultimately conforms to external forcing functions of energy dissipation and increasing relationship, such phenomena may be universal characteristics, however diverse the manifestations may be in detail.
If we adopt this alternative attitude, then life may be a common result of the dissipation of energy in a solar system on planets with suitable environmental conditions, and planetary evolution toward technological life may be energetically preferred. This could provide an optimistic scenario. Perhaps one star in ten could provide the conditions for life, one solar system in ten has the right planet, and one-tenth of planets succeed. Once life begins, intelligent life and technological civilization ultimately develop one time in ten, and persists once it arises for the remaining one half of the planet’s life. Then N = 2 × 107, 20 million technological civilizations exist in the galaxy, and we have just joined them. And we really do not know which of these scenarios is more likely, because in the absence of data from multiple other solar systems, our reasoning is uninformed.
So we may be alone in our galaxy. Or there may be millions of planets with intelligent life. Of course, these estimates apply only to the Milky Way galaxy. With more than 100 billion galaxies estimated to exist in the universe, the total number of civilizations might be proportionally more. However, given that the nearest neighbor galaxy (Andromeda) is 2 million light years away from us, and the speed of light is our current understanding on the maximum speed of interstellar communication of all kinds, civilizations in neighboring galaxies are not accessible to one another in any way that we can fathom.
We will ultimately be rescued from our guesses and biases by facts. The key fact will be evidence for life on one other planet (or moon) in the galaxy. Because we can only investigate a trivial number of other planets relative to the trillion or so planets that may exist in our galaxy alone, if we find life anywhere else, it means life is overwhelmingly abundant. Even if we investigate 10,000 planets and find only one with life, the implication will be that there are millions of such planets. The most likely evidence will come from the planetary atmospheres of planets in the habitable zones of a nearby star. On the day of that discovery, should it occur, we will know that we are one of a vast community of planetary systems for which biological life is a central aspect.
Earth has gone through a series of stages in its planetary evolution, with long periods of relatively constant conditions, punctuated by periods of abrupt change that lead to a new condition. The latest such change is the appearance of intelligent life and modern civilization. This change is phenomenally sudden from a planetary perspective. The earliest known civilizations were less than 10,000 years ago, and technological civilization capable of long-distance communication has been present for about 100 years. To put that in a planetary perspective, that is 0.000002 percent of Earth’s history. Most mass extinctions that dramatically punctuate the geological record have occupied hundreds of thousands to millions of years, thousands of times as long. From a planetary perspective, human beings are a sudden and recent phenomenon. Relative to the life of a human being, it would be as if such an epochal and transformative event in your personal history happened just as you read this sentence.
So, if someone else were looking for life on planets like Earth, they would find billions of years of unicellular life, a few hundred million years of multicellular life, and a few hundred years of intelligent civilization capable of interplanetary communication. We do not know how long such intelligent civilizations last. We can easily imagine the extinction of our own technological civilization in a short time period from war, famine, disease, exhaustion of resources, or destruction of biodiversity, and there would be great difficulty resuscitating it because we would have used all the readily available energy sources generated by different stages of planetary evolution and over 400 million years of photosynthesis and fossil fuel production.
This gives us an important perspective about searching for life on other planets. Using Earth’s history as a guide, the odds of finding a technological civilization on Earth in any particularly year is currently 100 years divided by 4.5 billion (about two-millionths of a percent. The odds of finding a high oxygen atmosphere would be 600:4,500 (13.3%), and the possibility of finding some form of life no matter how primitive would be 3,500:4,500 (85%). These are big differences. Therefore, perhaps the most important term in the Drake equation is the length of time that an intelligent civilization would last. If it lasts only a thousand years, then even the optimistic assessment of 20 million communicable civilizations, based on a 4.5-billion-year duration of a civilized planet, drops to 4 such civilizations in the galaxy. Since the diameter of the galaxy is 100,000 light years, no communication is ever possible, and once again we are alone.
One consequence of this reasoning is the far greater likelihood of finding a planet with microbial life than one with intelligent life. The number for the pessimistic scenario increases to 4,000, and for the optimistic scenario to 400 million. Slightly smaller numbers might apply to the likelihood of finding oxygenated atmospheres elsewhere.
A second and more intriguing consequence is that if intelligent life exists elsewhere in the galaxy, then civilizations must last for a long time, such as the average species length of 10 million years, let alone the likely billions of years left to Earth’s history. If we consider the technological advances on Earth that have occurred in a hundred years, then beings on planets with millions of years of civilization would be unimaginably more advanced than we are. We cannot possibly imagine an advanced civilization with thousands of years, let alone millions of years, of exponential growth in knowledge and technological development.
This factor leads also to a perspective on what is known as the Fermi paradox—if there is intelligent life throughout the galaxy, then where is everybody? Why don’t we at least see evidence of their radio transmissions? The timescale of planetary evolution throws light on this question. It is not clear there would be any point to a million year civilization contacting us in the obvious ways we consider. Presumably they would understand the mysteries of dark energy and dark matter and have a direct knowledge of the universe that makes our understanding more primitive than that of a universal cave man. A perspective very different from our usual one might be necessary. Communication among advanced planetary civilizations may be a rather different phenomenon than anything we can yet imagine.
Our speculations about life elsewhere in the universe are inevitably biased by our experience of life, planet and solar system, which until very recently were our sole data points. We are also limited by our understanding of the nature of matter and the universe. Based on these biases, leading to phrases such as “life as we know it,” life would be restricted to a solar system in a star’s habitable zone, where liquid water could persist throughout a planet’s history, and to stars within a galactic habitable zone in a similar middle region of galactic geography. The likelihood of life elsewhere in the universe is at the moment partly a philosophical question, since it requires assigning probabilities to many parameters for which we have no reliable data. The latest discoveries of planets in other solar systems hold the promise of providing the necessary data for evaluation of life in the galaxy. Discovery of life in any single other location than Earth would imply that life is pervasive.
The question of other technological civilizations is a more refined question that puts our own planetary behavior in context. Earth as a natural system has gone through billions of years of evolution developing to the present situation of a technological civilization that has appeared in the blink of a planetary eye. For technological civilizations to persist they would need to correspond with the planet as a natural system, following the principles outlined in Chapter 1. To be long lived, a natural system must be sustainable, making use of feedbacks and cycles to preserve resources and living within the limits of the gifts of energy provided by the sun and planet. Natural systems are also in relationship to larger and smaller scales. Human civilization would need to understand its relationship to the planetary scale, to the ecosystems of which we are an important part, and to the smaller scales of other living organisms. This is the challenge of human civilization, to become a part of a natural system to permit and perhaps even to participate in further planetary evolution. Only to the extent that other planetary civilizations have met this challenge are they abundant in the universe. Only if we are able to meet this challenge ourselves will we become a member of this galactic community.
James Kasting. 2010. How to Find a Habitable Planet. Princeton, NJ: Princeton University Press.
Sara Seager and Drake Deming. 2010. Exoplanet atmospheres. Annu. Rev. Astron. Astrophys. 48:631–72.
William J. Borucki, et al. 2010. Kepler planet detection mission: introduction and first results. Science 327:977–80.
Jack J. Lissauer, et al. 2011. A closely packed system of low-mass, low-density planets transiting Kepler-11. Nature 470:53–57.
