Steamy water planet discovered

Astronomers said Wednesday that they had discovered a planet composed mostly of water.

You would not want to live there. In addition to the heat — 400 degrees Fahrenheit on the ocean surface — the planet is probably cloaked in a crushingly dank and dark fog of superheated steam and other gases. But its discovery has encouraged a growing feeling among astronomers that they are on the verge of a breakthrough and getting closer to finding a planet something could live on.

“This probably is not habitable, but it didn’t miss the habitable zone by that much,” said David Charbonneau of the Harvard-Smithsonian Center for Astrophysics, who led the team that discovered the new planet and will reports its findings today in the journal Nature.

Only 2.7 times the size of Earth and 6.6 times as massive, the new planet takes 38 hours to circle a dim red star, GJ 1214, in the constellation Ophiuchus — about 40 light-years from here. It is one of the lightest and smallest so-called extrasolar planets yet found, part of a growing class of planets that are less than 10 times the mass of the Earth, often called “super-Earths.”

The planet, only 1.3 million miles from its home star, is distinguished by its relative coolness. It bakes rather than roasts, a consequence of the dimness of GJ 1214, which puts out one three-hundredth our sun’s energy. Charbonneau and his colleagues had set out to search for planets around such stars, noting that they are more numerous and that it is easier to discern planets around them.

“There is no question,” Charbonneau wrote in an e-mail, “that small stars provide us with the fastest track to looking for life outside the solar system.”

His planet-hunting equipment is a bank of eight telescopes called MEarth, pronounced “mirth,” on Mount Hopkins in Arizona. They are only 16 inches in diameter, no bigger than those that grace the backyard of many amateur astronomers. They monitor the light of 2,000 nearby stars, looking for the regular blips caused when a planet passes by, or transits.

Charbonneau said the weight of the new planet’s presumptive atmosphere kept the water liquid rather than just boiling into space. If other such planets have this type of atmosphere, he and his colleagues write, none of them is likely to harbor life.

« National AP

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