Thousands of planets – but is there life out there?

On October 6, 1995, Michel Mayor and Didier Quéloz of the Geneva Observatory announced the discovery of the first planet orbiting a star other than our sun. Twenty years on, the catalogue of extrasolar planets, or exoplanets, is still growing; around 2,000 have been detected to date. No traces of life have yet been found, but what exactly are we looking for: little green men, or bacteria? Some have compared the breakthrough by the two Swiss scientists to Columbus' discovery of America; the only difference is that no one is likely to set foot on one of these exotic worlds in the near future. In any case, most of them are more akin to Dante's hell than any kind of Promised Land. The confirmation that the universe is teeming with planets (and not just stars, where all life is impossible) opened up a phenomenally rich new field of scientific research. The resources that observatories and space agencies are devoting to exoplanetary discovery show clearly that this is now ...