By Paul Rincon
Science reporter, BBC News
When planets transit their star, they block out light - like Mercury above
The smallest planet yet found outside the Solar System has been detected by a French space telescope.
The rocky world is less than twice the size of Earth.
Only a handful of planets have so far been found with a mass comparable to Earth, Venus, Mars or Mercury.
The discovery was made by Corot, an orbiting observatory with a 27cm-diameter telescope to search for planets orbiting other stars.
About 330 of these "exoplanets" have been discovered so far. But most of them have been gas giants similar to Jupiter or Neptune.
"For the first time, we have unambiguously detected a planet that is 'rocky' in the same sense as our own Earth," said Malcolm Fridlund, Corot project scientist from the European Space Agency (Esa).
"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," he added.
The new find, Corot-Exo-7b, orbits its Sun-like star once every 20 hours. Although very close to Earth in terms of width, its mass is several times that of our own planet.
Because the planet is so close to its parent star, its temperature is between 1,000 and 1,500C - far too hot to support life.
Rock v gas
The vast majority of exoplanets have been discovered using the radial velocity method.
This looks for spectral signs that a star is wobbling due to gravitational tugs from an orbiting planet.
But the method favours the detection of large planets orbiting close to its parent star.
Astronomers detected the new planet as it crossed the face of the distant "sun", dimming the star's light as it passed in front. This is known as the transit method.
Ian Roxburgh, professor of astronomy at Queen Mary, University of London, said the transit method still favoured the detection of big planets, because they blocked out more light from the parent star.
But he added that if you had a small star - as this one is - then a moderate-sized planet would block out enough light to be detected by telescopes.
Professor Roxburgh told BBC News there appeared to be another planet orbiting the same "sun" - a Neptune-sized gas giant.
Advanced science instruments aboard future spacecraft, such as the proposed European Plato mission, could find many more Earth-mass planets orbiting Sun-like stars.
The Corot mission is led by the French space agency (CNES), with contributions from Esa, Austria, Belgium, Germany, Spain and Brazil.
Its main objectives are to search for exoplanets and to study the interiors of stars.