The first planet that could support life as we know it outside our solar system has been discovered.

A team of European astronomers say they have detected a rocky world, possibly only 50 per cent larger than the Earth, circling a small red star called Gliese 581, 20.5 light years away in the constellation Libra.

What makes the discovery so important is that the planet orbits in what astronomers call the "Goldilocks zone" - where makes it neither too hot, nor too cold for life.

Astronomers have found more than 200 planets circling other stars, but, until now, all have been unsuitable for life because they are either massive gas balls, resembling Jupiter, that circle scorchingly close to their parent, stars, or have eccentric orbits that take them out into the bitterly cold depths of space.

The newly found "super-Earth," about five times more massive than our planet, is 14 times closer to its star than the Earth is from the sun. As a result, its year lasts only 13 Earth days. It has been named , named 581 c.

However, because the star is only a third the mass of our sun, it is also much cooler.

Astronomers estimate that the world's surface temperature would therefore be between 0 and 40 degrees.

"Water would thus be liquid," one of the discoverers, Stephane Udry, from Switzerland's Geneva Observatory, said.

"Models predict that the planet should be either rocky - like our Earth - or covered with oceans."

Another team member, Xavier Delfosse, from France's Grenoble University, said: "Because of its temperature and relative proximity, this planet will probably be a very important target of future space missions dedicated to the search for extraterrestrial life. On the treasure map of the universe, one would be tempted to mark this planet with an X."

Gliese 581, one of the 100 closest stars to our solar system, is a red dwarf, one of the most common types of stars in the universe. It is already known to have a planet about the same size as Neptune, the fourth biggest world in our solar system.

The astronomers say they also have strong evidence that the star is circled by a second rocky planet, about eight times as massive as the Earth.

That such a common type of star could have three planets, including two rocky "terrestrial" planets, has boosted speculation that many other red dwarfs might also have their own Earth-like worlds

"Red dwarfs are ideal targets for the search for low-mass planets where water could be liquid," another astronomer, Xavier Bonfils, from Lisbon University, said.

The new "super Earth" was not directly observed by the astronomers who discovered it with the European Southern Observatory in Chile.

Instead, they tracked the red star's wobble, which revealed it is being tugged from side to side as it is circled by the unseen planet.

Chris Tinney, an Australian planet hunter whose team at the Anglo-Australian Observatory has found 30 planets, said today he was still to "check the homework" of the European team.

"These guys have done excellent work in the past," he said.

"But what's a little bit more up in the air is whether it really is Earth- like."

Dr Tinney said science was not advanced enough to be sure that all planets of similar mass had rocky surfaces and could not be worlds of ice and gas, as is Neptune.

"But if it does indeed have a rocky surface it could indeed have liquid water and it could indeed be habitable for life."

Dr Tinney said living on the planet could be a gloomy experience. The "sun" hanging in the planet's sky would "be very dark red. There would not be a lot of light, but a lot of heat".