Distant planet found that may be able to support life

CHICAGO — European astronomers say they have found the first Earth-sized planet beyond this solar system with temperatures mild enough to allow liquid water — a crucial step toward answering whether our cradle of life is unique in the universe.

The planet circles the star Gliese 581, which at 20 light years away is among the 100 stars closest to Earth. Dubbed Gliese 581c, the planet orbits very close to its star — closer than Mercury is to our sun. But astronomers with the European Southern Observatory say the star is dim enough that average temperatures on the planet would fall in the range of an ordinary Chicago spring day.

If the planet has water — a big unknown — its size and climate could make it habitable, experts said. The planet appears to be about 50 percent larger than Earth and has five times more mass, making it one of the smallest far-off planets ever detected.

The conditions look promising enough that officials with the California-based SETI Institute, which looks for signs of radio communication from alien civilizations, said they hope to give the planet a fresh look this summer. Previous radio observations of Gliese 581 in the 1990s turned up nothing unusual.

But the finding is a milestone in any case because it suggests that Earth-like planets may be common throughout the universe, astronomers said. Our galaxy alone could be home to 100 million habitable planets, if such worlds are as easy to spot as the new study indicates.

"This is a marvelous discovery," said astronomer Geoff Marcy, a principal investigator for the California and Carnegie Planet Search. Marcy's group had been racing the Europeans to find the first potentially Earth-like planet.

Next year NASA plans to launch the Kepler probe, designed to find even more Earth-sized planets outside our solar system. That probe will survey thousands of stars in hopes of catching sight of planets that cross in front of them.

As with nearly all such planetary discoveries, astronomers could not observe Gliese 581c directly because it is invisible in the glare of its sun. Instead, the European group led by Stephane Udry and Michael Mayor calculated the planet's presence from its tiny gravitational tug on the star.

The research group released its results Tuesday night and has submitted the work for publication in the journal Astronomy and Astrophysics.

The hunt for far-flung planets has made dizzying progress since 1995, when Mayor and his Swiss colleagues discovered a large extrasolar planet circling a star called 51 Pegasi.