The history of science since Copernicus has been one of growing recognition that human beings are not at the center of the universe. The sheer number of galaxies, stars, and planets, which we are discovering at an increasing rate, seems to make a more radical argument: that we might also not be alone in the cosmos. As a matter of simple mathematics, the idea that no other planet in the universe but Earth supports life would seem implausible. Astronomer Jeffrey Bennett, whose most recent book is Beyond UFOs: The Search for Extraterrestrial Life and Its Astonishing Implications for Our Future (he is also the author of a series of science books for children, including Max Goes to Jupiter), has thought seriously about these matters and offers 15 questions on the implications for our lives and of our venturing farther and farther beyond the reaches of our galaxy.
1. First we learned that Earth orbits the Sun, then that the Sun lies in the outskirts of our galaxy, then that our galaxy is just one of billions of similar galaxies in the universe. This trend suggests we will next find that we are no more central to the biological universe than to the physical one. Yet some have advanced seemingly reasonable arguments (the “Rare Earth” hypotheses) suggesting that life, and especially intelligent life, should be a rarity. Are these arguments sound, or are they simply the last whimpers of the ancient geocentric belief? [For an essay arguing that science helps prove man is the center of the universe, see “Putting Man Before Descartes.”]
2. What constitutes life, and will we recognize it on another world if we stumble across it? Are there biological laws that make all life similar—such as having a DNA-based genome and cell components built from proteins, carbohydrates, and lipids—or can life have many different chemistries?
3. As my colleague Nick Schneider puts it, Venus, Earth, and Mars have all been “sneezing on each other”—via the exchange of rocks blasted into orbit by impacts—for billions of years. That means that life from Earth has almost certainly migrated to Mars; if it did so at a time when Mars was habitable, it could have survived. How would finding life on Mars that is clearly evolved from Earth be different from finding indigenous life? What would it tell us about the likelihood of extraterrestrial life if we fail to find it on Mars at all?
4. Is the search for life in our solar system, especially on Mars, better carried out with robotic missions or by humans?
5. Does Jupiter’s moon Europa really have a giant subsurface ocean—and, if so, what’s swimming in it?
6. The most distant place upon which we have landed a spacecraft is Titan, Saturn’s moon. Firing a spaceship from Earth and hitting a target the size of Titan is equivalent to shooting a gun and hitting a dime from 2,500 miles away. Can technological achievements like this help people to realize what we are capable of when we put our human potential to work for discovery rather than conflict?
7. We’re reaching a point where our telescopes will soon be able to obtain spectra of planets around other stars. Can we identify an unambiguous spectral signature that would prove an extrasolar planet hosts life?
8. Recent discoveries of extrasolar planets have shown that many solar systems are laid out quite differently from ours. Is this a selection effect, meaning that solar systems like ours are actually quite common, or is our solar system unusual?
9. The more than 300 extrasolar planets discovered to date are all significantly larger than Earth, but this is presumably due to limitations of current technology. The upcoming Kepler Mission should discover dozens of Earth-sized planets—if they exist. Will it? And what do we conclude if it does not? [For an essay on the subject of what to call extrasolar planets, see here.]
10. A central part of the argument that life should be common throughout the universe is that it arose during a geologically brief time on Earth, suggesting that its origin is “easy” under the right circumstances. But it took nearly 4 billion years for life on Earth to evolve into intelligent forms. Does the lag time between microscopic life and intelligent life suggest that the latter will be rare, or does it suggest that intelligence is inevitable on planets that survive for billions of years?
11. Given the number of stars in our galaxy, even a probability of only one “civilization” forming out of one million opportunities leads to 100,000 possible civilizations among the 100 billion or so stars in the Milky Way. If we assume that these civilizations have arisen at random intervals during the past 5 billion years, we find that the galaxy gives birth to a civilization about once every 50,000 years. Although these numbers are based on a guess about the probability, it seems clear that we should expect other civilizations—if they exist—to be at least thousands to tens of thousands of years ahead of us technologically. Is it reasonable that they would use methods of communication that we could hope to receive? Or, to paraphrase Arthur C. Clarke, would they be so technologically advanced that to us their technology would be indistinguishable from magic?
12. Polls indicate that the percentage of the public that believes we are being visited by aliens in UFOs is larger than the percentage that accepts the theory of evolution. Given that extraterrestrial life is possible only if life evolves in similar ways on other worlds, can we use the public interest in ufos as a tool for building understanding of evolution?
13. A vocal minority of both scientists and the general public does not accept that global warming is being caused by human emissions of carbon diox- ide and other greenhouse gases. And while other scientists do an admirable job of explaining the science behind it, few of them ever mention the planetary elephant in the room—Venus, where there is no scientific doubt that a huge concentration of carbon dioxide is responsible for its 900 degrees Fahrenheit temperature. Could a greater emphasis on comparative planetology eliminate the remaining resistance to combating one of the greatest challenges in human history?
14. Although we cannot rule out the possibility that we are alone in the universe, the numbers make it seem almost inevitable that civilizations will turn out to be common. And if we take the hopeful viewpoint that some or most of those civilizations survive for the long term, then vast numbers of civilizations must be out there right now. If we further assume that other civilizations arose through evolutionary pressures similar to those that affected us, then we would expect that just as we would probably colonize the galaxy if we had the chance, they would already have done it. Do these ideas point to the conclusion that a galactic civilization already exists? Or is that just wild science-fiction speculation?
15. No matter how we look at it, we face the reality that we have been placed at the turning point of history: If we destroy ourselves, then all the efforts of our predecessors will have been for naught, and there will be no future for our children and grandchildren. But if we find a way to overcome our present crises, it will mean that we have developed social systems enabling us to live peacefully with ourselves and our technology, creating a society in which our descendants will be poised to begin their journey to the stars. To all of us alive today, the generation here now: Will we prove that there is intelligent life on Earth?
