The search for life elsewhere in the universe is among the most awesome of scientific endeavors. For the first time in the 4.5 billion year history of our planet, creatures from the Earth are now plumbing the depths of space in search of our neighbors. It is a daunting and frustrating job: the vast distances between our solar system and others, the sheer number of stars that must be surveyed, and the complicated set of circumstances required just to allow for the possibility of life on another world, make the work exceedingly difficult. Even as we have become accustomed to the idea of extraterrestrials through the science fiction of our time, and have made the most breathtaking discoveries about the cosmos, there are those who deride the search for life as mere fantasy, a waste of taxpayer dollars. Meanwhile, there are some enthusiasts who persist in the delusion that intelligent extraterrestrials are probably close-by; hiding on the Moon, perhaps, or on Mars. Such 19th-century thinking may be responsible for some disillusionment when it comes to genuine searches for extraterrestrial life, slow and painstaking as they are. In light of the fact that interstellar travel will almost certainly not happen in our lifetime, there is a kind of exploration defeatism. If the aliens are not reachable in our time, so the thinking goes, is it even worth trying to contact them?
When we truly grasp the profound void that is the ocean of space, we begin to understand the Herculean task that is the search for extraterrestrial life. The universe is far grander in scale than anyone before the 20th century ever imagined. We now know that even our fastest spacecraft, traveling at several kilometers per second, would take many thousands of years to reach the nearest star, just a few light years away. The technology to reach the stars in any reasonable amount of time is unlikely to be within our grasp for at least another century, so our best bet for the moment is long range reconnaissance. We have made some remarkable strides in this vein over the last few decades, but we still have much work to do.
Less than two decades ago we had not yet discovered even a single planet around another star, so the search for life outside our solar system seemed, in a sense, to be putting the cart before the horse. But today, through several elegant techniques, we have now cataloged hundreds of exoplanets. Exquisitely sensitive equipment has been employed to detect the minute wobbling of stars, tugged by the gravity of their planetary companions; and the Kepler spacecraft, staring at one patch of sky, has discovered almost 1800 planetary candidates by observing tiny, periodic dips in starlight intensity -- the telltale sign of transiting planets. From tens, hundreds and even thousands of light-years away, we have discovered the faint signature of alien worlds... gas giants whipping around their parent stars in a few days, and enormous rocky planets, a few of which might just have the right characteristics to support life as we know it. The results are tantalizing, the planets as exotic as they are diverse. Some scientists suggest the Milky Way alone could be home to at least 50 billion planets, and something on the order of 500 million habitable planets.
The discovery of life elsewhere in the Universe would be a defining moment in the history of our planet, a truly revolutionary breakthrough. Never again would we wonder whether we are alone; we would wonder, instead, how many others are out there. The discovery of even a microorganism on a planet or moon in our solar system, living or fossilized, would fundamentally de-provincialize biology, giving us invaluable insight into the origin of life, still one of the most compelling and contentious questions in modern science.
The signature of life in another star system, meanwhile, would prove at last that we are not alone in the galaxy, that there must be countless inhabited worlds in our universe. We might find life by detecting a planet's biogenic atmosphere, though it would be anyone's guess as to what might be there... recall that only in the last century have humans left traces of technology that could be detected from far away, meaning that biological signatures in the atmosphere of an exoplanet -- molecular oxygen, for instance -- could be alien algae, or little green men living on the cusp of their own industrial revolution.
A radio signal, of course, would be an unambiguous sign of advanced civilization. After a few billion years of isolated development, our groping in the darkness would finally begin to subside. In time we would come to know our brothers and sisters in the galaxy, and begin to bridge the great gulf between us. The vast distances between the stars preclude the likelihood of two-way communications, but we could learn much from one-way transmissions. There's no telling how much more advanced our neighbors might be. They could be hundreds or thousands of years ahead of us; indeed, there may be some civilizations out there that are so advanced that we are not even capable of detecting their means of communication -- as though we are an isolated tribe in the jungle with no telephone and someone is trying to send us an e-mail. What magnificent achievements must have been made by these advanced beings of another star, we can only guess. What mysteries of the universe have they solved? How did they survive technological adolescence? Have they found a purpose for existence? The lessons we could learn from an extraterrestrial civilization are incalculable.
But so far, there is only silence. Our galaxy is so immense -- 100,000 light years across, comprised of hundreds of billion of stars -- that there is a good explanation for having not yet found our neighbors; we are listening for the whispers of a needle in a haystack. But could there be no one out there to find? There is always the possibility, however remote, that we are the first of our kind, the first intelligent beings sending our voice out into space. But if this turns out to be true, it is also a profoundly important discovery. Instead of joining the multitude of galactic civilizations, we will have found that we are true cosmic pioneers, far more alone than we ever imagined. Where once we thought we were only carrying the torch of life on Earth, we would now be carrying the torch of life in the galaxy. A more serious responsibility is hard to imagine.
Either way, the question is clearly worth answering, and it can be answered at a reasonable cost. The excitement surrounding exoplanet discoveries, the crowd-sourcing of data analysis, and the grassroots funding for the Allen Telescope Array (a collection of radio telescopes built expressly for the purpose of searching for extraterrestrial life), all suggest that more people are taking SETI seriously. We may still have years ahead of us before we find, or don't find, what we're looking for. But things are looking up.