Published online 3 February 2009 | Nature | doi:10.1038/news.2009.78
Corrected online: 3 February 2009

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Tiniest exoplanet found

Satellite spots a planet less than twice the width of Earth.

Exooplanet intransit across a starThe exoplanet has a density close to that of Earth's.CoRoT

A European satellite has spied the smallest and fastest-orbiting extrasolar planet to date, bringing astronomers closer to finding a habitable planet outside our Solar System.

The Convection, Rotation and Planetary Transits (CoRoT) mission, a French Space Agency (CNES) satellite that scans for exoplanets â€" planets outside our Solar System â€" has spied a rocky planet whose radius is slightly less than twice that of Earth. At 5â€"10 Earth masses, the planet is not the lowest-mass extrasolar planet ever found, but the mass and radius measurements suggest a density similar to that of earth.

"It's much more Earthlike than previously found planets," says Suzanne Aigrain, a researcher at the University of Exeter who is part of the CoRoT team. The results were announced today at a CoRoT symposium in Paris.

Astronomers have used a variety of space and ground-based techniques to observe about 330 extrasolar planets. The majority have been 'gas giants' similar in size to Jupiter, but a handful have been rocky planets that bear a passing resemblance to Earth. The lightest of these have been found using microlensing, which measures changes in the light of a background star, amplified by the mass of a foreground star and its planet.

CoRoT, by contrast, measures changes in a single star's apparent brightness caused by a planet. The method allows astronomers to get a rough estimate of the planet's girth, and in some cases, it has even allowed them to glean information about its atmosphere. Because the effect depends in part on the planet's proximity to its parent star, this technique is particularly good at spotting close-in planets.

Speedy orbit

With an orbital period of just 20 hours, the new planet, known as CoRot-Exo-7b has the shortest orbit yet seen in an extrasolar planet (see video). This short time period means that the planet is extremely close to its parent star â€" and thus uninhabitable to life as we know it. "If you are orbiting at 20 hours around a star, it is hell on the planet," says Jean-Philippe Beaulieu, an astronomer at the Institute of Astrophysics in Paris.

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Follow-up measurements of the planet's mass show it to have an Earth-like density and suggest a bizarre environment. "It's likely that there is a solid surface somewhere," says Aigrain. But the extreme surface temperatures of around 1000°C could mean that the planet is host to vast lava fields and boiling oceans. It also may be 'tidally locked' to its parent star, leaving one face bathed in constant, searing sunlight while the other is shrouded in continuous night. "It would be a very odd place to set foot on," she says.

Beaulieu believes that the find brings astronomers ever-closer to discovering habitable extrasolar planets. Such planets would need to be further from their star to support life as we know it, but they could be closer in mass to CoRoT-Exo-7b than Earth. "Being focused on a one Earth-mass planet as the Holy Grail is, I would say, geocentric," he says. 

Corrected:

The original article incorrectly stated that CoRoT is led by the European Space Agency.
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