Astronomers in Europe have discovered the most Earth-like planet to date outside our solar system, one they say could potentially hold liquid water, a necessary ingredient to support life.

Scientists with the European Southern Observatory say the planet, discovered orbiting a red dwarf star 20.5 light years away, is the smallest of those found outside our solar system.

An artist's impression of the planetary system around the red dwarf Gliese 581.
(Courtesy: ESO)

The planet has a diameter 50 per cent larger than Earth's and a mass five times as much as our planet. It orbits the red dwarf star Gliese 581 in 13 days at a distance 14 times closer than Earth's orbit to the sun.

But because red dwarf stars are smaller and produce less heat than our sun, the planet's close proximity puts it potentially within the temperature range that might support life, the astronomers said in a paper submitted as a letter to the editor in an upcoming issue of Astronomy and Astrophysics.

"We have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius, and water would thus be liquid," explained Stephane Udry, who is from the Geneva Observatory in Switzerland and the lead author of the paper reporting the result.

"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," he said in the statement.

It's the second planet the astronomers have detected around the red dwarf star, the ESO said. The first one, with a mass of 15 Earth-masses, or about the size of Neptune, orbits the star every 5.4 days and was discovered two years ago. The scientists also found less conclusive evidence of a third planet about the mass of eight Earths orbiting the star.

The planets were discovered, but not actually seen, by studying the variations in the velocity of the star as a result of the gravitational pull from unseen planets. Measuring these velocity variations allowed the scientists to deduce the orbits, distance and minimum mass of the planets.

Water plays key role

Victoria Hipkin, a scientist with the Canadian Space Agency, said the discovery is an impressive one, and added that the planet's potential to hold liquid water is important.

"Liquid water is important to anyone involved in the search for life," she told CBC News Online.

Water plays a key role in creating an environment supportable to life, she said. It is an excellent solvent for other compounds and also possesses the unique quality of being less dense as a solid than as a liquid, which allows ice to float atop the liquid water and potentially act as a protective shield for liquid water when a planet goes through a freezing cycle.

The newly discovered planet could also give more powerful telescopes a good place to start looking, she said. NASA's Terrestrial Planet Finder is expected to be able to see at far greater distances when it goes into operation, though the project currently has no launch date.

Hipkin said finding planets like Earth will provide a unique perspective of our own planet.

"One nice thing about finding an Earth-like planet is it's like looking both forward and backward in history," she said.

"We could see a planet as ours was, providing a look at our early evolutionary history, or we could see a planet older than ours that might provide clues as to where we're heading."