WASHINGTON - In a significant advance in the search for extraterrestrial life, European astronomers have discovered what they say may be the first habitable planet orbiting a nearby star.

They described their find as an Earthlike, probably rocky planet small enough and warm enough that it might have water in liquid form on the surface, a necessary condition for life as we know it.

At 11/2 times the diameter of Earth, the new planet would be the smallest of more than 200 such bodies detected so far outside the solar system. It weighs about five times as much as Earth, apparently the lowest mass of any other known planet outside the system.

The planet - named GL-581 c - is one of three orbiting a star called Gliese 581, a "red dwarf" much smaller and cooler than the sun. Red dwarfs are the most common type of stars in this part of the universe.

Gliese 581 is nearly 21 light-years (120 trillion miles) from Earth in the constellation Libra (the Scales).

Scientists' estimation that surface conditions on 581 c suggest the possibility of liquid water "makes it even more fascinating and arguably the first habitable planet," said Alan Boss, an astronomer with the Carnegie Institution of Washington, who wasn't part of the discovery team.

In a paper to be published in the journal Astronomy and Astrophysics, astronomer Stephane Udry of the Geneva Observatory in Switzerland wrote that his team had estimated the planet's average temperature at between 0 and 40 degrees Celsius (32 and 104 degrees Fahrenheit), "and water would thus be liquid."

NASA scientists cautioned that determining the size, weight, and other characteristics of distant planets is an uncertain art.

"It might be the smallest planet around a normal star, but they cannot be sure," said Jack Lissauer, a planetary scientist at Ames Research Center in Mountain View, Calif.

And, of course, habitable - the term favored by astronomers yesterday - isn't the same as inhabited. Scientists believe liquid water is essential for life, but its presence doesn't mean that anything is alive there.

Nevertheless, Xavier Delfosse, a colleague of Udry's from Grenoble University in France, said the new planet "will most probably be a very important target of future space missions dedicated to the search for extraterrestrial life. On the treasure map of the universe, one would be tempted to mark this planet with an X."

NASA plans to launch such a mission next year, named Kepler, to scour the skies for Earthlike planets.

"Kepler will monitor 100,000 stars for four years with enough precision to find Earth-size planets in the habitable zone," said William Borucki, a space scientist at Ames. Astronomers define a "habitable zone" as one that's not too hot or too cold - and not too close to its star to permit life.

In a sad coincidence, Princeton University yesterday announced the death of astrophysicist Bohdan Paczynski, whose team last year discovered the first extra-solar planet that was thought to be rocky, like Earth, although it is not considered to be habitable. All previously discovered planets were gaseous giants.

The European team that announced its findings yesterday discovered a planet 15 times heavier than Earth, about the size of Neptune, orbiting the same star two years ago.

The European Southern Observatory's 3.6-meter (140-inch) HARPS telescope at La Silla, Chile, discovered the new planet. Udry and Michael Mayor, a Swiss astronomer who discovered the first extra-solar planet 12 years ago, led the team.

Read more about the search for Earthlike planets via http://go.philly.com/science