Gliese 581g was discovered orbiting a nearby star at a distance that places it squarely in the "habitable zone" where liquid could exist on its surface. Of around 500 planets that astronomers have found outside Earth's solar system, this is the first to be considered habitable.
The planet is a similar size to Earth and its mass indicates that it is probably rocky with a definite surface and has enough gravity to hold an atmosphere, according to Prof Steven Vogt, who led the team that discovered it.
It is as yet unknown whether water does exist on the planet or what kind of atmosphere it has. But because conditions are ideal for liquid, which is always a precursor for life on Earth, Prof Vogt believes that life will undoubtedly have begun there.
"Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say, my own personal feeling is that the chances of life on this planet are 100 percent," he said during a press briefing. "I have almost no doubt about it."
The findings are based on 11 years of observations by ground-based telescopes at the W. M. Keck Observatory in Hawaii.
The close proximity of Gliese 581g and the fact that it was found relatively early in the astronomers' search suggests that there may be billions of other habitable planets like Earth in the universe.
Prof Vogt estimates that as many as one in five to 10 stars in the universe have planets that are Earth-sized and in the habitable zone.
With an estimated 200 billion stars in the galaxy, that means that around 40 billion planets could have the potential for life, he said.
The new findings by Prof Vogt and Paul Butler, of the Carnegie Institution in Washington, will be published in the Astrophysical Journal.
The paper reports the discovery of two new planets around the nearby red dwarf star Gliese 581.
The most interesting of the two new planets is Gliese 581g, which has a mass three to four times that of Earth and an orbital period of just under 37 days.
Gliese 581g is located 20 light years away from Earth in the constellation Libra.
The planet is tidally locked to the star, meaning that one side is always facing the star and basking in perpetual daylight, while the side facing away from the star is in perpetual darkness.
The researchers estimate that the average surface temperature of the planet is between -24 and 10 degrees Fahrenheit (-31 to -12 degrees Celsius).
Actual temperatures would range from blazing hot on the side facing the star to freezing cold on the dark side.
The surface gravity would be about the same or slightly higher than Earth's, so that a person could easily walk upright on the planet, Prof Vogt said.
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