Astronomers find what could be first habitable planet outside solar system

It might be a place that only a lichen or pond scum could love, but astronomers said Wednesday that they had found a distant planet capable of harboring water on its surface, thus potentially making it a home for plant or animal life.

Nobody from Earth will be visiting soon: The planet, which goes by the bumpy name of Gliese 581g, is orbiting a star about 20 light-years away in the constellation Libra.

But if the finding is confirmed by other astronomers, the planet, which has three to four times the mass of Earth, would be the most Earthlike planet discovered and the first to meet the criteria for being potentially habitable.

"It's been a long haul," said Dr. Steven Vogt of the University of California, Santa Cruz, who, along with Dr. R. Paul Butler of the Carnegie Institution of Washington, led the team that made the discovery. "This is the first exoplanet that has the right conditions for water to exist on its surface."

In a recent report for the National Academy of Science, astronomers declared the finding of such planets one of the major goals of this decade. NASA's Kepler satellite — which was launched in March 2009 as a way to detect Earthlike bodies — is expected to harvest dozens or hundreds.

Gliese 581g (whose first name is pronounced GLEE-za) circles a dim, red star known as Gliese 581 once every 37 days, at a distance of about 14 million miles. That is smack in the middle of the so-called Goldilocks zone, where the heat from the star is neither too cold nor too hot for water to exist in liquid form on its surface.

"This is really the first Goldilocks planet," Butler said.

Other astronomers hailed the news as another harbinger that the search for "living planets," as Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics calls them, is on the right track.

"I'm getting goose bumps," said Caleb Scharf of Columbia University.

But they expressed caution about this particular planet, noting uncertainties about its density, composition and atmosphere, and the need for another generation of giant telescopes and spacecraft to find out anything more about it. Other Goldilocks planets have come and gone in recent years.

The discovery was announced at a news conference in Washington, D.C., and the findings have been posted on the National Science Foundation's website and will be published in The Astrophysical Journal.

The authors said the relative ease by which the planet was found — in 11 years — led them to believe that such planets must be common.

"Either we have just been incredibly lucky in this early detection, or we are truly on the threshold of a second Age of Discovery," they wrote.

Pressed about the possibility of life on Gliese 581g, Vogt protested that he was an astronomer, not a biologist. He then relented, saying that, speaking strictly personally, he believed that "the chances of life on this planet are almost 100 percent."

Asked the same question, Butler squirmed and said, "I like data." After a pause, he added: "And what the data say is that the planet is the right distance from the star to have water and the right mass to hold an atmosphere. What is needed simply to find lots and lots of these things is lots and lots of telescope time."

The latest results from Gliese 581 were harvested from observations by two often-competing teams, using telescopes in Chile and Hawaii to measure the slight gravitational tugs the star gets as its planets swing by.

Gliese 581 is a longtime favorite of planet hunters and now is known to have six planets in its retinue. It is a dwarf star about one-third the mass of the sun and only about one-hundredth as bright, allowing planets to huddle closer to the campfire.

"It hauntingly reminds us of our own solar system," Butler said.

Two of Gliese's planets have had their moment in the limelight as possible Goldilocks planets. One, known as Gliese 581c, circles on the inner edge of the habitable zone and, thus, was believed to be habitable three years ago. But further analysis suggested the greenhouse effect would turn it into a stifling hell. Another planet, on the outer edge of the Goldilocks zone, probably is too cold.

"One is on the hot side, the other is on cold side," and the new planet is right in between, Vogt said. "It's book-ended."

He and his colleagues estimated the average temperature on the surface of Gliese 581g to be between 10 and minus 24 degrees Fahrenheit, about the same as a summer day in Antarctica.

But that means little, he said, because the planet, like all others in that system, keeps the same face to the star all the time. So the temperature could vary wildly from the dayside to the nightside of the planet, meaning an organism could perhaps find a comfortable zone to live in.