For decades now, and with ever-increasing sophistication, we have been searching the universe for signs that we are not alone. Rovers are probing the surface of Mars. Scientists are planning missions to other promising spots such as Saturn鈥檚 moon Enceladus and Jupiter鈥檚 moon Europa, both of which may harbour liquid water. The James Webb Space Telescope, scheduled for launch in 2018, will let astronomers sample the atmospheres of exoplanets for chemical signatures of life. And radio astronomers have long been listening for transmissions from intelligent extraterrestrials.
So far, we have found nothing. What if it stays that way? What if, by mid-century, we have visited every life-friendly place in the solar system, eavesdropped on radio transmissions from a hundred million stars and peered at millions of exoplanets without finding the slightest trace of life? When should we give up and decide we鈥檙e alone?
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Never, say those involved in the search. 鈥淚 can鈥檛 even imagine when it鈥檚 time to give up,鈥� says Mary Voytek, director of NASA鈥檚 astrobiology programme. 鈥淲ith all the other planets around all the other stars, it鈥檚 impossible to imagine that life wouldn鈥檛 have arisen somewhere else.鈥� There are, after all, roughly 100 billion galaxies in the known universe, each with roughly 100 billion stars. Such numbers make even highly improbable events likely to happen somewhere 鈥� and the origin of life may not even be all that improbable (see 鈥�The world in 2076: Human-made life forms walk the earth鈥�).
A failure to find alien life would make the searchers doubt their methods and assumptions, not the existence of their target. 鈥淵ou have to think really hard about how you define life. It鈥檚 not like in Star Trek where you have a tricorder and get a nice big 鈥渂ing!鈥� sound when you point it at something alive,鈥� says Jon Willis, an astronomer at the University of Victoria, Canada, and author of a on the search for alien life. We can look for water and oxygen in the atmospheres of exoplanets, for example, but there鈥檚 no guarantee that alien life needs those molecules. We can listen for radio signals from an extraterrestrial civilisation, but who鈥檚 to say they don鈥檛 have a better method of communicating that we鈥檙e unaware of?
And even if life exists on an exoplanet, we鈥檙e unlikely to know it unless it鈥檚 abundant enough to modify the entire atmosphere. If today鈥檚 searches meet with silence, researchers will just head back to their drawing boards to design better, more sensitive ways to look next time.
Of course, they might be wrong. Maybe we really are utterly alone, an astonishing and highly improbable one-off. If so, scientists might have to rethink their ideas about the conditions required for the origin of life, and about what makes Earth unique. And with that comes a great burden, says Willis. 鈥淚f the universe really is empty, and we鈥檙e the first, there鈥檚 a huge responsibility on us not to kill ourselves.鈥�
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This article appeared in print under the headline 鈥淲hat if鈥� We don鈥檛 discover alien life?鈥�
