Astronomers believe they may have found a habitable planet at last, in the widely-studied Gliese 581 system 20 light years away in the constellation of Libra.
The Gliese 581 system has already yielded evidence of a number of exoplanets. This one, though, is in the 'Goldilocks zone' where the temperature could be bearable, liquid water could exist on the surface and an atmosphere be sustained.
The team that discovered it, led by astronomers at the University of California, Santa Cruz, and the Carnegie Institution of Washington, says it's the most Earth-like exoplanet yet discovered.
"Our findings offer a very compelling case for a potentially habitable planet," said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz. "The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common."
Gliese 581g is one of two newwly-discovered planets orbiting the red dwarf star, bringing the total number found in the system to six, the most yet discovered in a planetary system other than our own.
Gliese 581g has a mass three to four times that of the Earth and an orbital period of just under 37 days. Its mass indicates that it's probably a rocky planet, and that it has enough gravity to hold on to an atmosphere.
It isn't the first planet in the system to raise hopes of habitability. Another, 581d, lies on the outer edge of the habitable zone, and there has been speculation that a strong greenhouse effect, if it existed, would make it capable of sustaining life.
"We had planets on both sides of the habitable zone - one too hot and one too cold - and now we have one in the middle that's just right," Vogt said.
The planet is tidally locked to the star, meaning that one side is always in daylight, while the side facing away from the star is in perpetual darkness. The most habitable zone on the planet's surface would be the 'terminator' line between shadow and light.
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.
"Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude," Vogt said.
If Gliese 581g has a rocky composition similar to the Earth's, its diameter would be about 1.2 to 1.4 times that of the Earth. The surface gravity would be about the same or slightly higher than Earth's.
The new findings are based on 11 years of observations of Gliese 581 using the HIRES spectrometer. This allows precise measurements of a star's radial velocity, which can reveal the presence of planets.
The discovery comes just a couple of weeks after predictions from scientists that the first habitable exoplanet would likely be discovered in the first half of next year.
"If these are rare, we shouldn't have found one so quickly and so nearby," Vogt said. "The number of systems with potentially habitable planets is probably on the order of 10 or 20 percent, 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."