A planet-hunting space telescope has detected the most Earth-like world ever found around another star. The terrestrial-type planet is the smallest yet discovered outside our own solar system.

It is less than twice the size of Earth and orbits a star resembling our own Sun in the constellation of Monoceros, the Unicorn.

Astronomers believe the planet, dubbed COROT-Exo-7b and 457 light-years away, has a rocky surface which may be covered with lava or water vapour.

It was found by the European Space Agency’s COROT satellite, which was launched in December 2006 to seek out new worlds.

Most of the 330 planets discovered so far in the galaxy are gas giants resembling out own giant Jupiter. This one is more like Earth, Mars or Venus and is smaller than a previous discovery revealed last year. Another world, Gliese 581c, detected two years ago, was said to be Earth-like because it lies in the habitable “Goldilocks zone” around a star.

The new find, which orbits its star once every 20 hours, lies very close to its parent star, and has a high temperature, between 1,000 and 1,500°C. Astronomers detected the new planet as it passed in front of the star, dimming its light.

The density of the planet is still under investigation: it may be rocky like Earth and covered in liquid lava. It may also belong to a class of planets that are thought to be made up of water and rock in almost equal amounts. Given the high temperatures measured, the planet would be a very hot and humid place.

Daniel Rouan, of Paris Observatory, said: “COROT was designed precisely in the hope of discovering some of these objects.

ESA’s Malcolm Fridlund said: “For the first time, we have unambiguously detected a planet that is ‘rocky’ in the same sense as our own Earth. We now have to understand this object further to put it into context, and continue our search for smaller, more Earth-like objects with COROT.”

Details of the discovery will be published in the journal Astronomy and Astrophysics. NASA have said that they expect 60 per cent of stars like the Sun to have rocky planets in orbit.

Picture: An artist’s impression of the planet passing in front of the Sun-like star. The curved line represents the fade in the star’s light as the planet passes in front of it. (Credit: CNES).

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