Not content with seeking earthly equivalents of extraterrestrial oceans, he is looking for analogies with the atmospheres of other planets as well.

ECONOMIST: Planetary pretenders

In the second decade of this century, Esa plans to launch the Darwin flotilla - a fleet of four or five spacecraft that will hunt for Earth-like planets around other stars and analyse their atmospheres for signs of extra-terrestrial life.

BBC: Corot will be launched just after Christmas (Image: Cnes)

However, just like the other planets, we haven't seen or tested the atmospheres of any of these planets, so whether they're habitable remains theoretical.

CNN: 3 new planets could host life

They have used similar observations of more distant stars to find "exoplanets" (planets orbiting stars other than the sun) and in a few rare cases, to measure characteristics of the exoplanet atmospheres.

CNN: A transit of Venus, an age of wondrous science