In 1995, when Michel Mayor of the University of Geneva detected the first exoplanet (a planet that orbits a star other than the sun) he started a race that has gained pace ever since. Some 360 such planets have now been detected, but none is exactly equivalent to the Earth.

The closest so far is Gliese 581c, which was discovered in 2007 by Dr Mayor’s colleague, Stéphane Udry. It is both rocky and orbits its parent star at a distance where liquid water could reasonably be expected to exist. However, since its parent star is a red dwarf—a far smaller and fainter object than the sun—that orbit is, in fact, much smaller that the Earth’s around the sun. That, in turn, suggests Gliese 581c is likely to be tidally locked to its orbital period, so that one side of the planet always faces the star and the other never does. Having half a planet in permanent daylight and the other half in permanent darkness does not sound like a good recipe for life.

As astronomers heard this week at the International Astronomical Union meeting in Rio, two new missions — a French one launched in December 2006 and an American one launched on March 6th — are in the process of trying to add to the list. Dr Mayor told the meeting that the French mission, CoRoT, has now found 80 exoplanets. It does so by watching for small diminutions in the amount of light from a star as the planet in question passes in front of it, a phenomenon known technically as a transit. The details of all but seven of these transiting planets are still unpublished, but Dr Mayor gave the meeting a preview.

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