Origins: Fourteen Billion Years of Cosmic Evolution (19 page)

Circle it as we may, we cannot avoid the problem, evident from the orbits of the planets around 55 Cancri, of explaining why and how many exosolar planets, with masses much like Jupiter’s, orbit their stars at stunningly small distances. No planet with a Jupiter-like mass can form, experts will tell you, much closer to a Sun-like star than three to four times the Earth-Sun distance. If we assume that exosolar planets obey this dictum, they must have somehow moved to much smaller distances after they had formed. This conclusion, if valid, raises at least three burning questions:

1. What made these planets move into smaller orbits after they had formed?

2. What stopped them from moving all the way into their stars and perishing?

3. Why did this occur in many other planetary systems, but not in our solar system?

These questions have answers, supplied by fertile minds once they had been properly stimulated by the discovery of exosolar planets. We may summarize the scenario now favored by experts as follows:

1. “Planetary migration” occurred because significant amounts of material left over from the formation process continued to orbit the star within the orbits of the new-formed giant planets. This material gets systematically flung by the big planet’s gravity to outer orbits, which in turn forces the big planet to creep inward.

2. When the planets had approached much closer to their stars than their points of origin, the tidal forces from the star locked the planet into place. These forces, comparable to the tidal forces from the Sun and Moon that raise tides in Earth’s oceans, forced the planets’ rotational periods to equal their orbital periods, as happened to the Moon from Earth’s tidal forces. They also prevented any further approach of the planet to the star, for reasons that require sufficient involvement with celestial mechanics to merit passing over here.

3. Presumably the luck of the draw determined which planetary systems formed with large amounts of debris, capable of inducing planetary migration, and which, like our own, had relatively little debris, so the planets remained at the distances at which they had formed. In the case of the planets around 55 Cancri, it is possible that all three migrated significantly inward, with the outermost planet having formed at several times its current distance from the star. Or it may be that the details of how much debris lived inside the planet’s orbit, and how much outside, caused significant migration of the two inner planets, while the third has remained in its original path.

Some work remains to be done, to put things politely, before astrophysicists can proclaim they have explained how planetary systems form around stars. Meanwhile, those who hunt for exosolar planets continue to pursue their dream of finding Earth’s twin, a planet similar to Earth in its size, mass, and orbital distance from its parent star. When and if they find such a planet, they hope to examine it—even from a distance of dozens of light-years—with sufficient precision to determine whether the planet possesses an atmosphere and oceans similar to Earth’s, and perhaps whether life exists upon that planet like our own.

In pursuit of this dream, astrophysicists know that they need instruments orbiting above our atmosphere, whose blurring effects prevent us from making extremely precise measurements. One experiment, NASA’s
Kepler
mission, aims at observing hundreds of thousands of nearby stars, seeking the tiny diminution in starlight (about one hundredth of 1 percent) caused by the motion of an Earthsized planet across our line of sight to a star. This approach can succeed only for the small fraction of situations in which our view lies almost exactly along the planet’s orbital plane, but for those cases, the interval between planetary transits equals the planet’s orbital period, which in turn specifies the planet-star distance, and the amount of starlight diminution reveals the size of the planet.

However, if we hope to find out more than the planet’s bare physical characteristics, we must study the planet by direct imaging and analysis of the spectrum of the light that the planet reflects into space. NASA and ESA, the European Space Agency, have programs under way to achieve this goal within two decades. To see another Earth-like planet, even as a pale blue dot close to a far brighter star, could inspire another generation of poets, physicists, and politicians. To analyze the planet’s reflected light, and thus to determine whether or not the planet’s atmosphere contains oxygen (a likely indication of life) or oxygen plus methane (an almost completely definitive mark of life), would mark the sort of accomplishment that the bards once sang, elevating mere mortals into heroes for the ages, leaving us face to face (as F. Scott Fitzgerald wrote in
The Great Gatsby
) with something commensurate with man’s capacity to wonder. For those who dream of finding life elsewhere in the universe, our final section awaits.

Part V

The Origin
of Life

CHAPTER 14

Life in the Universe

O
ur survey of origins brings us, as we knew it would, to the most intimate and arguably the greatest mystery of all: the origin of life, and in particular of forms of life with which we may someday communicate. For centuries, humans have wondered how we might find other intelligent beings in the cosmos, and with whom we might enjoy at least a modest conversation before we pass into history. The crucial clues for resolving this puzzle may appear in the cosmic blueprint of our own beginnings, which includes Earth’s origin within the Sun’s family of planets, the origin of the stars that provide energy for life, the origin of structure in the universe, and the origin and evolution of the universe itself.

If we could only read this blueprint in detail, it could direct us from the largest to the smallest astronomical situations, from the unbounded cosmos to individual locations where different types of life flourish and evolve. If we could compare the diverse forms of life that arose under various circumstances, we could perceive the rules of life’s beginnings, both in general terms and in particular cosmic situations. Today, we know of only one form of life: life on Earth, all of which shares a common origin and uses DNA molecules as the fundamental means of reproducing itself. This fact deprives us of multiple examples of life, relegating to the future a general survey of life in the cosmos, unachievable until the day we begin to discover forms of life beyond our planet.

Things could be worse. We do know a great deal about life’s history on our planet, and must build on this knowledge to derive basic principles about life throughout the universe. To the extent that we can rely on these principles, they will tell us when and where the universe provides, or has provided, the basic requirements for life. In all our attempts to imagine life elsewhere, we must resist falling into the trap of anthropomorphic thinking, our natural tendency to imagine that extraterrestrial forms of life must be much like our own. This entirely human attitude, which arises from our evolutionary and personal experiences here on Earth, restricts our imagination when we attempt to conceive how different life on other worlds may be. Only biologists familiar with the amazing variety and appearance of different forms of life on Earth can confidently extrapolate what extraterrestrial creatures might look like. Their strangeness almost certainly lies beyond the imaginative powers of ordinary humans.

Some day—perhaps next year, perhaps during the coming century, perhaps long after that—we shall either discover life beyond Earth or acquire sufficient data to conclude, as some scientists now suggest, that life on our planet represents a unique phenomenon within our Milky Way galaxy. For now, our lack of information on this subject allows us to consider an enormously broad range of possibilities: We may find life on several objects in the solar system, which would imply that life probably exists within billions of similar planetary systems in our galaxy. Or we may find that Earth alone has life within our solar system, leaving the question of life around other stars open for the time being. Or we may eventually discover that life exists nowhere around other stars, no matter how far and wide we look. In the search for life in the universe, just as in other spheres of activity, optimism feeds on positive results, while pessimistic views grow stronger from negative outcomes. The most recent information that bears upon the chances for life beyond Earth—the discovery that planets are moving in orbit around many of the Sun’s neighboring stars—points toward the optimistic conclusion that life may prove relatively abundant in the Milky Way. Nevertheless, great issues remain to be resolved before this conclusion can gain a firmer footing. If, for example, planets are indeed abundant, but almost none of these planets provide the proper conditions for life, then the pessimistic view of extraterrestrial life seems likely to prove correct.

Scientists who contemplate
the possibilities of extraterrestrial life often invoke the Drake equation, after Frank Drake, the American astronomer who created it during the early 1960s. The Drake equation provides a useful concept rather than a rigorous statement of how the physical universe works. The equation usefully organizes our knowledge and ignorance by separating the number that we dearly seek to estimate—the number of places where intelligent life now exists in our galaxy—into a set of terms, each of which describes a necessary condition for intelligent life. These terms include (1) the number of stars in the Milky Way that survive sufficiently long for intelligent life to evolve on planets around them; (2) the average number of planets around each of these stars; (3) the fraction of these planets with conditions suitable for life; (4) the probability that life actually arises on these suitable planets; and (5) the chance that life on such a planet evolves to produce an intelligent civilization, by which astronomers typically mean a form of life capable of communicating with ourselves. When we multiply these five terms, we obtain the number of planets in the Milky Way that possess an intelligent civilization at some point in their history. To make the Drake equation yield the number that we seek—the number of intelligent civilizations that exist at any representative time, such as the present—we must multiply this product by a sixth and final term, the ratio of the average lifetime of an intelligent civilization to the total lifetime of the Milky Way galaxy (about 10 billion years).

Each of the Drake equation’s six terms requires astronomical, biological, or sociological knowledge. We now have good estimates of the equation’s first two terms, and seem likely to obtain a useful estimate of the third before long. On the other hand, terms four and five—the probability that life arises on a suitable planet, and the probability that this life evolves to produce an intelligent civilization—require that we discover and examine various forms of life throughout the galaxy. For now, anyone can argue almost as well as experts can about the value of these terms. What is the probability, for example, that if a planet does have conditions suitable for life, then life will actually begin on that planet? A scientific approach to this question cries out for the study of several planets suitable for life for a few billion years to see how many do produce life. Any attempt to determine the average lifetime of a civilization in the Milky Way likewise requires several billion years of observation, once we have located a sufficiently large number of civilizations to provide a representative sample.

Isn’t this a hopeless task? A full solution of the Drake equation indeed lies immensely far in the future—unless we encounter other civilizations that have already solved it, perhaps using us as a data point. But the equation nevertheless provides useful insights for what it takes to estimate how many civilizations exist in our galaxy now. The six terms in the Drake equation all resemble one another mathematically in their effect on the total outcome: each of them exerts a direct, multiplying effect on the equation’s answer. If, for instance, you assume that one in three planets suitable for life actually produces life, but later explorations reveal that this ratio actually equals 1 in 30, you will have overestimated the number of civilizations by a factor of 10, assuming that your estimates for the other terms prove correct.

Judging by what we now know, the first three terms in the Drake equation imply that billions of potential sites for life exist in the Milky Way. (We restrict ourselves to the Milky Way out of modesty, plus our awareness that civilizations in other galaxies will have a much more difficult time in establishing contact with us, or we with them.) If you like, you can engage in soul-searching arguments with your friends, family, and colleagues about the value of the remaining three terms, and decide on numbers that will provide your own estimate for the total number of technologically proficient civilizations in our galaxy. If you believe, for example, that most planets suitable for life do produce life, and that most planets with life do evolve intelligent civilizations, you will conclude that billions of planets in the Milky Way produce an intelligent civilization at some point in their time line. If, on the other hand, you conclude that only one suitable planet in a thousand does produce life, and only one life-bearing planet in a thousand evolves intelligent life, you will have only thousands, not billions, of planets with an intelligent civilization. Does this enormous range of answers—potentially even wider than the examples given here—imply that the Drake equation presents wild and unbridled speculation rather than science? Not at all. This result simply testifies to the Herculean labor that scientists, along with everyone else, face in attempting to answer an extremely complex question on the basis of highly limited knowledge.

The difficulty that we face in estimating the values of the last three terms in the Drake equation highlights the treacherous step that we take whenever we make a sweeping generalization from a single example—or from none at all. We are hard pressed, for example, to estimate the average lifetime of a civilization in the Milky Way when we do not even know how long our own will last. Must we abandon all faith in our estimates of these numbers? This would emphasize our ignorance while depriving us of the joy of speculation. If, in the absence of data or dogma, we seek to speculate conservatively, the safest course (though one that might eventually prove to be erroneous) rests on the notion that we are not special. Astrophysicists call this assumption the “Copernican principle” after Nicolaus Copernicus, who, in the mid-1500s, placed the Sun in the middle of our solar system, where it turned out to belong. Until then, despite a third-century
B.C.
proposal for a Sun-centered universe by the Greek philosopher Aristarchus, the Earth-centered cosmos had dominated popular opinion during most of the past two millennia. Codified by the teachings of Aristotle and Ptolemy, and by the preachings of the Roman Catholic Church, this dogma led most Europeans to accept Earth as the center of all creation. This must have appeared both self-evident from a look at the heavens and the natural result of God’s plan for the planet. Even today, enormous segments of Earth’s human population—quite likely a significant majority—continue to draw this conclusion from the fact that Earth seemingly remains immobile while the sky turns around us.

Although we have no guarantee that the Copernican principle can guide us correctly in all scientific investigations, it provides a useful counterweight to our natural tendency to think of ourselves as special. Even more significant is that the principle has an excellent track record so far, leaving us humbled at every turn: Earth does not occupy the center of the solar system, nor does the solar system occupy the center of the Milky Way galaxy, nor the Milky Way galaxy the center of the universe. And in case you believe that the edge is a special place, we are not at the edge of anything, either. A wise contemporary attitude therefore assumes that life on Earth likewise follows the Copernican principle. If so, how can life on Earth, its origins, and its components and structure provide clues about life elsewhere in the universe?

In attempting to answer this question, we must digest an enormous array of biological information. For every cosmic data point, gleaned by long observations of objects at enormous distances from us, we know thousands of biological facts. The diversity of life leaves us all, but especially biologists, awestruck on a daily basis. On this single planet Earth, there co-exist (among countless other life forms), algae, beetles, sponges, jellyfish, snakes, condors, and giant sequoias. Imagine these seven living organisms lined up next to each other in order of size. If you didn’t know better, you would be challenged to believe that they all came from the same universe, much less the same planet. Try describing a snake to somebody who has never seen one: “You gotta believe me. I just saw this animal on planet Earth that (1) stalks its prey with infrared detectors, (2) swallows whole live animals up to five times bigger than its head, (3) has no arms or legs or any other appendage, yet (4) can slide along level ground almost as fast as you can walk!”

In contrast to the amazing variety of life on Earth, the constricted vision and creativity of Hollywood writers who imagine other forms of life is shameful. Of course, the writers probably blame a public that favors familiar spooks and invaders over truly alien ones. But with a few notable exceptions, such as the life forms in
The Blob
(1958) and in Stanley Kubrick’s
2001: A Space Odyssey
(1968), Hollywood aliens all look remarkably humanoid. No matter how ugly (or cute) they may be, nearly all of them have two eyes, a nose, a mouth, two ears, a head, a neck, shoulders, arms, hands, fingers, a torso, two legs, two feet—and they can walk. From an anatomical view, these creatures are practically indistinguishable from humans, yet they are supposed to live on other planets, the products of independent lines of evolution. A clearer violation of the Copernican principle can hardly be found.

Astrobiology—the study of the possibilities for extraterrestrial life—ranks among the most speculative of sciences, but astrobiologists can already assert with confidence that life elsewhere in the universe, intelligent or otherwise, will surely look at least as exotic as some of Earth’s own life forms, and quite probably more so. When we assess the chances of life elsewhere in the universe, we must attempt to shake from our brains the notions that Hollywood has implanted. Not an easy task, but essential if we hope to reach a scientific rather than an emotional estimate of our chances of finding creatures with whom we may someday have a quiet conversation.