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 rocky planet that's small enough and warm enough that it might have liquid water, a necessary condition for life, on its surface.
With an estimated radius only 50 percent larger than Earth's, the new planet would be the smallest of about 200 such bodies that have been 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.
NASA scientists, however, cautioned that determining the size and weight 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 NASA's Ames Research Center in Mountain View, Calif.
The new object is one of three planets in a mini-solar system orbiting a star called Gliese 581, a "red dwarf" that's much smaller and cooler than the sun. Red dwarfs are the most common type of stars in the universe.
"The fact that it (the new planet) could have 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.
"We have estimated that the mean temperature of this super-Earth lies between 0 and 40 degrees Celsius (32 and 104 degrees Fahrenheit), and water would thus be liquid," Stephane Udry, an astronomer at the Geneva Observatory in Switzerland, wrote in a paper to be published Wednesday in the journal Astronomy and Astrophysics.
Xavier Delfosse, a colleague of Udry's from Grenoble University in France, said the 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 NASA's Ames Research Center. Astronomers define a "habitable zone" as one that's not too hot or too cold, and not too near its star to permit life.
"Habitable," of course, isn't the same as "inhabited." Scientists say liquid water is essential for life, but its presence doesn't mean that anything is alive there.
Gliese 581 is about 20 light-years (120 trillion miles) from Earth in the constellation Libra (the Scales), in the southern hemisphere. The European team discovered a planet that's 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 extrasolar planet 12 years ago, led the team.
The high-precision telescope detected a very slight wobble in the star's motion. The wobble is caused by the gravitational tug of the planet as it whirls around it every 13 days. The star's velocity varied by only about 5 miles per hour, a brisk walk, but it was enough, if followed long enough, to reveal the presence of the planet.
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