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Gains in the quest for Earth-size planets

Discovery of smallest 'extrasolar' planet yet is next step in search for bodies with Earth-like conditions.

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The scene could have been lifted from an Isaac Asimov novel: a planet a little bigger than Earth orbiting a star halfway to the center of the Milky Way.

But it's for real - among the latest in a string of discoveries by astronomers searching for alien solar systems and clues to how they form. Astronomers studying the puzzling variety of newly discovered planets ultimately hope to learn how many Earth-like planets are out there, some perhaps hosting life.

Wednesday, a team of astronomers announced that it has discovered a planet some 21,500 light years away. Its mass falls between Earth's and Neptune's - the smallest "extrasolar" planet yet found. It orbits its parent star - a tiny speck with only 22 percent of the sun's mass - at roughly twice the distance that Earth orbits the sun.

The discovery is significant because the new solar system is so far away, signifying the greatly expanded volume of space astronomers can search. It is also notable for the team's technique, which is naturally attuned to spotting smaller planets, researchers say.

While the newfound planet is too distant for detailed study, it reveals enough information to become a useful part of the planetary census astronomers are conducting. In the past 12 years, astronomers have bagged at least 170 planets, according to records at the Paris Observatory.

With so many planetary confirmations, "it's kind of tempting at this stage for people to go, 'Ho-hum, they've found another planet, big deal,' " says Stephen Kane of the University of Florida, a member of the team of astronomers from observatories worldwide who made the discovery. "But we're starting to cross new thresholds" that allow researchers to spot smaller planets at greater distances from their host stars.

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