Young Writers Society

(04-24) 16:59 PDT SAN FRANCISCO -- A rocky planet not much larger than Earth has been detected orbiting a star close to our own neighborhood in the Milky Way, and the European astronomers who found it say it lies within the star's "habitable zone," where life could exist - possibly in oceans of water.

The object is the smallest of all the 200 or more so-called "exoplanets" whose discovery around far-off stars in the past dozen years has sparked a burst of excitement worldwide among astronomers and astrobiologists.

Most of those planets are huge, hot, and appear to be primarily gas giants like our own Jupiter, but water has been widely detected in these distant solar systems, and many astronomers agree that rocky planets like this one might well hold abundant liquid water as the abodesof alien life.

The announcement Thursday was greeted with enthusiasm by many American astronomers.

"This appears to be the very first detection of a whole new habitable world, with liquid water and the possibility of life. It's a huge milestone in astronomy," said Alan P. Boss, an astronomer withthe Carnegie Institution in Washington said in a phone call to The Chronicle from Sweden, where he is attending a meeting.

The lead author of the discovery report, Stephane Udry of the Geneva Observatory in Switzerland, said the planet's sun is named Gliese 581, one of the galaxy's extremely common "red dwarfs." It lies in the constellation Libra, the Scales, about 20.5 light years away from Earth -- a relatively close neighbor compared to other "exoplanets" that have been detected thousands of light years away.

Udry's group estimated the planet's average temperature at between 32 and 104 degrees Fahrenheit. It orbits Gliese 581 every 13 days only about 6.5 million miles out, which is 14 times closer to its sun than Earth is from ours. But the planet is well within the star's "habitable zone" because Gliese 581 is much smaller and colder than our sun.

"This planet will most probably be a very important target of future space missions dedicated to the search for extraterrestrial life," Xavier Delfosse of Grenoble University in France, another member of the discovery team, said in the announcement. "On the treasure map of the Universe, one would be tempted to mark this planet with an X."

Jill Tarter, the famed seeker of signals from extra-terrestrial civilizations at the SETI Institute in Mountain View, told The Chronicle on Thursday that her colleagues have already been observing the new-found planet's parent star closely through radio telescopes in America and Australia.

"It's a tantalizing discovery," Tarter said in an e-mail message from Chicago. "Water will be the key, and that's going to be a real observational challenge. ... We may be witnessing the start of a cosmic real estate boom."

The new planet's detection was announced from the European Southern Observatory's headquarters in Garching, Germany, whose team leaders used the world's most sensitive spectrograph mounted at the observatory's main telescope at La Silla, Chile.

From Doppler measurements of the tiny wobble in the star's motion caused by the tug of the planet's gravity, the astronomers calculated the object must be only five times more massive than our own Earth and its diameter only half again more than Earth's.

The European astronomers discovered a much larger Neptune-size planet about 15 times more massive then Earth two years ago, circlingextremely close to Gliese 581 in a 5.4-day orbit. In their announcement Wednesday, they said they have also recently found a third new one they call a "super-earth" with a mass 8 times Earth's and an 84-day orbit.

Among the team's leading members are Michel Mayor and Didier Queloz, the Swiss leaders of the group that discovered the first known "exoplanet" in 1995 -- a gas giant circling a star named 52 Pegasi. That discovery was immediately confirmed by a Bay Area team led by Geoffrey Marcy of San Francisco State University and now at UC Berkeley. Of the 205 "exosolar" planets recorded, Marcy's team has found 135.

But Marcy was more reserved than other astronomers in his reaction to the European announcement, although he did call it "a marvelous discovery."

"I know that star well, as both the Swiss and we have been observing it for a long time," he said in an e-mail. "There is no evidence of water, so their guess is just that. This new planet could be a rock, a small Neptune-like planet, or even mostly gaseous."

David Spergel, chairman of Princeton astrophysics department, noted in a telephone interview that the new planet, if it is indeed rocky, must be "tidally locked" to its sun the way our moon is to the Earth -- which means that it must always show its same face to its sun, and that while one side would be too fiercely hot to support life, the other side would be too cold.

But at the terminator -- the margin between the hottest and coldest parts of the planet -- Spergel agreed that liquid water could well exist, hurricane force winds would blow, and although the planet would be "radically different" from Earth, life might well exist in that difficult environment. "This is a big, impressive step," Spergel said.

The European team has submitted its full report for publication in the international weekly journal Astronomy and Astrophysics.