NASA's Kepler spacecraft is starting to put the pieces together in its search for virtual Earth twins in other planetary systems. Kepler, which launched in 2009, is on the lookout for planets that are about the size of Earth and have temperate surface conditions. One half of that formula was realized on December 5 when mission scientists announced the discovery of a planet in the so-called habitable zone, called Kepler 22 b, a few times larger than Earth. Now Kepler has located its first two Earth-size worlds, and although neither are plausibly hospitable to life, it seems only a matter of time before the mission scores its ultimate goal.
The two new worlds orbit a sunlike star 950 light-years away called Kepler 20. One has dimensions almost identical to our own planet; the other is just 87 percent Earth's diameter. The planets, which by convention have been assigned the names Kepler 20 f and Kepler 20 e, respectively, are the smallest exoplanets for which diameters are known. Francois Fressin and Guillermo Torres of the Harvard–Smithsonian Center for Astrophysics and their colleagues announced the discoveries in a paper published online December 20 in Nature. (Scientific American is part of Nature Publishing Group.)
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