An orbital French observatory has spotted a distant world beyond the Solar System -- similar in size to Earth but boilingly hot -- that could be dubbed a "sauna planet," astronomers reported on Tuesday.

The exceptional find is just twice that size of Earth and may also be a rocky planet, they said at a symposium in Paris.

But even the most ardent sci-fi fan would admit that it would not be a home from home.

CoRoT-Exo-7B is located so close to its star that its surface is scorched, to between 1,000 and 1,500 degrees Celsius (1,800-2,700 degrees Fahrenheit).

So-called exoplanets were first detected in 1995, and the tally of them now reaches 337.

Almost all of them, though, are gas giants, similar to Jupiter, rather than planets with a rocky surface.

The new object was spotted by a team using France's exoplanet hunter, the CoRoT satellite, launched in December 2006.

They detected it thanks to a tiny dimming in light that occurred every time the planet passed in front of an orange star 400 light years away.

CoRoT-Exo-7B follows an ultra-fast track that means its "year" -- the time to complete an orbit -- is just 20 hours.

The observational method yields the approximate diameter of the planet, but not its mass, which has to be calculated using ground-based telescopes.

"It could be a rocky object like Earth and covered with liquid lava," the discoverers, led by Daniel Rouan of the Paris Observatory, added in a press release issued at the two-day exoplanet symposium.

"It could also belong to a predicted class of planet consisting of half water and half rock. In that case, it would be a 'sauna planet', considering its extremely hig temperature."

An exoplanet that could harbour life -- or at least, life as we know it -- would have to inhabit the "Goldilocks zone", experts say.

This means the planet would have to be orbit close enough to its star to enable water to exist in liquid form and have an atmosphere.

Too close an orbit could cause these precious assets to be stripped away by the blast of solar radiation, and too far could make the planet a permanent iceball.