Two planets previously discovered around Gliese 581 had a very small chance of being habitable because they were right on the edge of the habitable zone. One of them is shown in this artist's concept. (European Space Observatory)

Astronomers believe they have found the first Earth-sized planet outside our solar system that is likely to support liquid water and therefore life.

Planet "g," which orbits a red dwarf star called Gliese 581, is right in the middle of the star's "habitable zone," reported a team led by Steve Vogt of the University of California Santa Cruz and Paul Butler of the Carnegie Institution for Science in Washington, D.C.

Two of the planets previously discovered around that star were right on the edge of the zone around the star that has the possibility of supporting life, giving them only a small chance of being habitable.

The evidence suggests that Planet "g" is a rocky planet with a diameter about 1.2 to 1.4 times larger than Earth's, they said in a paper posted online at arXiv.org, a pre-print archiving service. It will be published in the Astrophysical Journal.

The planet's mass means its gravity would be the same as, or slightly higher than, Earth's, and a person could easily walk upright, Vogt said in a release. That is also enough gravity to hold onto an atmosphere.

The planet is "tidally locked" to the star that it orbits every 37 days, which means one side is always facing the star and one side faces away in perpetual darkness (similar to the way the Earth always faces the same side of the moon). That means that even though the average surface temperature is likely between -31 C and -12 C, one side is extremely hot, and the other side is always freezing cold.

The "most habitable" zone would be right between the light and dark sides.

The characteristics provide a "very compelling case for a potentially habitable planet."

Habitable planets likely common

Given that very few stars have been monitored so far by planet hunters, the researchers believe the discovery of such a planet so soon has wider implications.

"If these are rare, we shouldn't have found one so quickly and so nearby," Vogt said in a statement.

"The number of systems with potentially habitable planets is probably on the order of 10 or 20 per cent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that's a large number. There could be tens of billions of these systems in our galaxy."

Gliese 581 about 20 light years away from Earth in the constellation Libra.

Researchers found the planet by analyzing 11 years of observations with the high resolution eschelle spectrometer on the Keck Telescope at the W.M. Keck Observatory in Hawaii. The spectrometer, which was designed by Vogt, measures the star's radial velocity — that is, its movement toward or away from the Earth. The gravitational pull of orbiting planets can cause a star to wobble.

Because that force depends on the planets' orbits and masses, those characteristics can be teased out from the radial velocity measurements.