Sultry World Found Circling the Star GJ 1214 Call it Steam World. Astronomers said Wednesday that they had discovered a planet composed mostly of water. You would not want to live there. Besides the heat - 400 degrees Fahrenheit on the ocean surface - the planet is probably cloaked in a 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 that 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 of planets that are less than 10 times the mass of Earth. Dr. 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. 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 a separate paper, they reported finding a planet somewhat larger than Jupiter at the star 23 Librae. 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 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 of such planets 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." Dr. Charbonneau's planet, only 1.3 million miles from its home star, is distinguished by its relative coolness - a consequence of the dimness of GJ 1214, which puts out one three-hundredth of 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. His planet-hunting equipment is a bank of eight telescopes called MEarth, pronounced "mirth," on Mount Hopkins in Arizona. Each telescope is 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. In May, Zachory Berta, a first-year graduate student of Dr. Charbonneau, called the group's attention to blips in the Ophiuchus star that seemed to be happening every 1.6 days. If he was right, Mr. Berta said, the next transit would occur at 6 a.m. on May 13. Dr. 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! From the drop in starlight, the astronomers could calculate the diameter of the Ophiuchus planet, known now as GJ 1214b. Then they used a sensitive spectrograph on a 3.6-meter telescope in Chile to measure its gravitational tug on the star, thus deriving the planet's mass. Using those two numbers, Dr. Charbonneau and his colleagues could calculate the density of the planet, about one-third that of Earth. "What we probably have here is a water world," Dr. Charbonneau said. Dr. Charbonneau said the weight of the new planet's presumptive atmosphere kept the water liquid rather than just boiling into space. He acknowledged that a different recipe, with more rock and a very puffy atmosphere, would also fit the data. That is unlikely, he and other planet experts say, but the steam-world theory may be soon tested. The new planet is close enough to be studied directly by telescopes on or near Earth. Indeed, Dr. Charbonneau said his team had already applied for observing time on the Hubble Space Telescope. "Our own TV signals," he said, "have already passed this star."