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Call it Sauna World.

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

You would not want to live there. In addition to the heat — about 200 degrees Celsius 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 on Thursday in the journal Nature.

Geoffrey W. Marcy, a planet hunter from the University of California, Berkeley, wrote in an accompanying article in Nature that the new work provided “the most watertight evidence so far for a planet that is something like our own Earth, outside our solar system.”

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 that are less than 10 times the mass of the Earth.

Charbonneau’s announcement capped a week in which the list of known planets, including these “super-Earths,” grew significantly.

An international team of astronomers using telescopes in Australia and Hawaii reported in one paper that they had found three planets, including a super-Earth, orbiting 61 Virginis, a star in the constellation Virgo that is almost a clone of the Sun. In a separate paper, they reported finding a planet somewhat larger than Jupiter at the star 23 Librae. It was the first time, they said, that a super-Earth had been found belonging to a star like the Sun; the other home stars have been dwarfs.

And in yet another paper, a subset of the same group reported finding a super-Earth and probably two bigger planets circling HD 1461, a star in Cetus.

Paul Butler of the Carnegie Institution of Washington, who was involved in all three papers, said astronomers thought that from one-third to one-half of all Sun-like stars harbored such super-Earths orbiting at scorching distances much closer than Mercury is to the Sun.

In the 15 years since the first extrasolar planet was found, more than 400 have been detected. The field is getting more intense as dedicated planet-hunting instruments like the Kepler satellite from the National Aeronautics and Space Administration, due to report a new batch next month, get into the game.

Alan P. Boss, a planetary theorist at the Carnegie Institution of Washington, said of the planet hunters, “Give them a couple more years and they’re going to knock your socks off.”

Charbonneau’s 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 the Sun’s energy. He 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 message, “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 was in Washington later that day preparing for a State Department dinner when he got a group e-mail message that began: “We have a winner. Congrats Zach!”



The New York Times