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’s moon Enceladus and Jupiter’s 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’re alone?
Never, say those involved in the search. “I can’t even imagine when it’s time to give up,” says Mary Voytek, director of NASA’s astrobiology programme. “With all the other planets around all the other stars, it’s impossible to imagine that life wouldn’t have arisen somewhere else.” 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 – and the origin of life may not even be all that improbable (see ” The world in 2076: Human-made life forms walk the earth “).
A failure to find alien life would make the searchers doubt their methods and assumptions, not the existence of their target. “You have …