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As the ranks of these planets grow, astronomers are beginning to plan the next step in the quest to end cosmic loneliness, gauging which hold the greatest promise for life and what tools will be needed to learn about them.
On Monday, another group of astronomers said they had managed to weigh a set of small planets and found that their densities and compositions almost exactly matched those of Earth. Both groups announced their findings at a meeting of the American Astronomical Society in Seattle.
Reviewing the history of exoplanets (those that orbit a star that is not our sun), Debra Fischer, a Yale astronomer, recalled that the first planet found orbiting another sunlike star, a Jupiterlike giant, was discovered 20 years ago. She termed the progress in the last two decades "incredibly moving."
So far, Kepler has discovered 4,175 potential planets, and 1,004 of them have been confirmed as real, according to Michele Johnson, a spokeswoman at NASA's Ames Research Center, which operates Kepler. Most of them, however, including the new ones announced Tuesday, are hundreds of light-years away, too far for detailed study.
"We can count as many as we like," said Sara Seager, a planet theorist at the Massachusetts Institute of Technology who was not involved in the new work, "but until we can observe the atmospheres and assess their greenhouse gas power, we don't really know what the surface temperatures are like."