An Earth-like planet that may be capable of supporting extraterrestrial life has been discovered orbiting a distant star.

Gliese 581c may be 1.5 times the diameter of our own planet but in galactic terms it is only slightly larger. It is the only small planet yet to be found in another solar system that orbits in the “Goldilocks zone” around a star where the conditions for life are just right.

It is by far the best candidate yet identified for the existence of living organisms elsewhere in the Universe. It could have liquid water on the surface, which is thought to be a prerequisite for life, and it is several billion years old, allowing sufficient time for evolution to have progressed.

“On the treasure map of the Universe, one would be tempted to mark this planet with an X,” said Xavier Delfosse, of Grenoble University in France, who is a member of the discovery team.

However, although the planet’s parent star is among the 100 closest to the Sun, humankind’s chances of visiting it, let alone escaping there in the event of environmental disaster on Earth, is vanishingly small. Even with the fastest manned spacecraft to be built, it would take astronauts 554,000 years to get there.

More than 225 “exoplanets” beyond our own solar system were already known to science before the latest discovery, but the overwhelming majority of these are gas giants, mostly several times larger than Jupiter or Saturn. Such large planets are not thought to be capable of sustaining life and though any rocky moons they might have could be suitable, these are far too small and dim to be detected from Earth.

A small number of smaller rocky worlds have been found, most notably OGLE-2005- BLG-390Lb, which is five times the mass of Earth. But until now all have been either too close or too far from their parent stars to sustain life.

OGLE-2005-BLG-390Lb, for example, has an average surface temperature of about -220C (minus 364F).

Gliese 581c, however, is different. It is also about five times the mass of Earth, but it is in the “habitable zone”, where any water is likely to be in liquid form. Though it is 14 times closer to Gliese 581, its parent star, than the Earth is to the Sun, that star is a red dwarf — much smaller and cooler than the Sun. That makes it a prime candidate for life.

“We have estimated that the mean temperature of this super-Earth lies between 0C and 40C, and water would thus be liquid,” said Stéphane Udry, of the Geneva Observatory in Switzerland, who led the team.

“Moreover, its radius should be only 1.5 times the Earth’s radius, and models predict that the planet should be either rocky — like our Earth — or covered with oceans.”

The nature of the parent star enhances the possibility of life. Gliese 581 is not especially active, meaning that the planet would not be bombarded with so much radiation that life could not emerge, and it is several billion years old. Life on Earth emerged about four billion years ago.

The planet, details of which are published in Astronomy and Astrophysics, was discovered at the European Southern Observatoryin La Silla, Chile.

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