Astronomical advances mean that over the next decades we may discover if other life existsby Martin Rees / December 15, 2010 / Leave a comment
Out of this world: an artist’s impression of an exoplanet based on data from Nasa
Are we alone in the cosmos? Evidence for any extraterrestrial organisms, even mere bugs or bacteria, would be of huge scientific importance. But what fuels popular imagination is the prospect of advanced life—the “aliens” familiar from science fiction. Speculations about other worlds are nothing new. Great astronomers such as Johannes Kepler in the 17th century and William Herschel a century later imagined that the Moon and planets might be inhabited. In the late 19th century, Jules Verne and HG Wells further popularised such ideas. But the space age damped such speculations. The prime target, Mars, was revealed as a frigid desert with a thin atmosphere. There could be vestigial life there, but nobody expects a complex biosphere anywhere in the solar system.
Yet prospects are brighter if we widen our gaze. We know that many other stars have a retinue of planets circling them, just as the Earth, Mars and Jupiter circle around our own star, the Sun. These “extra-solar” planets, called exoplanets, have mainly been inferred indirectly, by detecting the wobble that their gravitational pull induces in their parent star. Several hundred nearby stars are already known to have planets. One star has at least five. But this evidence pertains mainly to “giant” planets the size of Saturn or Jupiter. Earthlike planets—hundreds of times smaller than Jupiter—cannot be detected this way.
But there’s another technique. A star would appear to dim slightly if it had a planet that transited in front of it, blocking out a fraction of the light reaching us. (A classic phenomenon in our solar system is a transit of Venus, when Venus appears as a dark spot that moves across the face of the Sun.) The Kepler spacecraft, launched last year by Nasa, carries a telescope that has been pointing at the same patch of sky for 18 months. It monitors the brightness of 100,000 stars—and does this every half hour with a precision better than 0.01 per cent. The signature of an orbiting planet would be a slight diminution in a star’s brightness, lasting for a few hours, which repeated regularly. An announcement is expected in February that Kepler has enough data to tell how common such planets are. We’ll be especially interested in planets like ours, orbiting other Sun-like stars at a distance such that water, if present,…