An illustration from France's space agency, the Centre National d'Etudes Spatiales, of a planet transiting a star. (CNES)

A European space telescope has found a unique planet 390 light years away with a diameter less than twice that of the Earth, making it the slimmest planet yet detected outside our solar system.

The planet also lies very close to its parent star — about 2.5 million kilometres — and so completes an orbit in a speedy 20 hours. By comparison, the closest planet to our sun, Mercury, orbits at an average distance of 58 million kilometres and completes a circuit of the sun once every 88 days.

European astronomers, who announced the finding Tuesday at a symposium in Paris, also believe the planet may be the fastest yet discovered.

Because of its close proximity to its sun-like star, the planet is also extremely hot, with temperatures estimated at between 1,000 and 1,500 C.

Astronomers found the planet using the COROT space telescope in large part because its close proximity made it easier to detect when it passed in front of — or transited — its star, dimming the star's light.

COROT had to look far away to spot the change: the star the planet orbits lies 3,690 trillion kilometres distant, or about 24.6 million times the average distance from the Earth to the sun.

The density of the planet, named COROT-Exo-7b, is still under investigation, so it is too early yet to say what its mass is relative to the current record-holder for least massive planet: a small planet discovered in 2008 orbiting a spinning neutron star.

Because of its temperature, however, the researchers speculate the newly discovered planet may be rocky like Earth and covered in liquid lava, or it could belong to a class of planets thought to be made of rock and water in equal amounts.

'Very important step'

"This discovery is a very important step on the road to understanding the formation and evolution of our planet," Malcolm Fridlund, the European Space Agency's COROT project scientist, said in a statement.

"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," he said.

The discovery has been submitted for publication in the journal Astronomy and Astrophysics.

COROT-Exo-7b is one of the few terrestrial planets — small rocky planets like the Earth, Mars, Venus and Mercury — discovered outside our solar system.

About 330 planets outside our solar system — called exoplanets — have been discovered thus far. Most of these have been huge gas giants that are easier to detect when they pass in front of a star or through an alternative method called gravitational lensing, whereby the presence of the planet is inferred based on the way a star's light bends in response to the planet's gravitational influence. A select few very large exoplanets have also been seen directly.

COROT, launched in 2006 from the Baikonur Cosmodrome in Kazakhstan, was specifically designed to detect tiny changes in brightness from nearby stars as a result of transiting planets.

NASA's Kepler telescope, set to launch on March 5, is designed to use similar methods of detection, but with a more focused mission — limiting its search to transiting Earth-like planets in habitable zones of space, where the temperatures on the planet might support liquid water and where it might be possible to sustain life.