It used to be that we were pretty smug about how special we humans were - we were at the center of the universe with everything else circling us, and we were also lords of creation, far removed from all the animal kingdom. We are still special, no doubt, but just how special is harder to assess. That doesn't keep astrobiologists from trying. Astrobiology is the study of life in space, and has been criticized for being all about stuff we don't even know exists. Life seems to crowd into just about every niche in our own world, but elsewhere, it's just too hard to say right now, but it is not too early to ask good questions and think about how to get answers. In _The Living Cosmos: Our Search for Life in the Universe_ (Random House), Chris Impey, a professor of astronomy, has laid out the history of our understanding of cosmology, and has summarized the intelligent speculation that scientists have brought on a peculiar realm of inquiry. His book is clearly written, covering even difficult cosmology in jocular, vivid analogies.
Guesswork about extraterrestrial life has continued for centuries, but we do now have evidence from different fields that the existence of extraterrestrial life may not simply be speculation. For one thing, we know that there are indeed planets orbiting other suns. Impey spends a chapter describing just how astronomers have spotted these planets. There is looking outside the solar system, and then there is looking at our own planet with a view to seeing what life on it is really like. We take for granted the plants that get energy from the sun, and the animals like ourselves that get energy from those plants, but only recently has it become clear that even on Earth, things don't always have to be that way. "Extremophiles" is the term we use for microbes that can live in extreme environments, though as Impey points out, that's an anthropocentric term, because these "extreme" environments are normal for those microbes, and our own environment would be extreme to them; and anyway, given the original microbes that started life billions of years ago, we ourselves are descendants of microbes that thrived in inferno-type heat. Extreme living situations we know from our own Earth can be just the place for certain microbes to live, but can it be (as in our case) that on other worlds they can be a foundation for evolution to build more complicated animals? And if those more complicated animals have evolved intelligence, Where Are They? This is the great question posed by Enrico Fermi. If there are plenty of other worlds which have evolved intelligence, and plenty of time during which the intelligent beings could have visited us or sent us a message, what does it mean that this has yet to happen? Impey proceeds from considering the Fermi paradox to an overview of the famous Drake Equation, which calculates the possibility of intelligent life elsewhere, and would give an accurate percentage chance, if we just knew all the variables. We are learning some of them, but there are still too many mysteries. No one knows.
Nonetheless, we cannot help thinking about life out there, and hunting for it, especially via the SETI project looking for radio signals from extraterrestrial broadcasters. Buckminster Fuller wrote, "Sometimes I think we're alone. Sometimes I think we're not. In either case, the thought is staggering." The ancient Greeks speculated about life in other parts of the universe, and so do we, only we are doing it though our culture's scientific lenses. "Is there a God?" used to be the great philosophical question, to which almost everyone has an answer and no one can prove that the answer is correct. "Is there life elsewhere?" has become in some ways more pertinent to our times, because at least to that profound question there might someday be a scientific answer. In the continuing silence, however, we have only our questions, questions which, as Impey reminds us, tell us more about ourselves than about any other life forms out there.