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Discovery of smallest planet yet a 'milestone' in search for another Earth

The Kepler space telescope has found a planet smaller than Mercury orbiting a distant star. The discovery suggests Kepler has the precision to find a planet more like Earth.


An artist's illustration compares the planets in the Kepler-37 system to the moon and planets in the solar system.


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For nearly 20 years, astronomers have filled the pantheon of planets beyond the solar system with objects ranging from behemoths several times Jupiter's mass to small orbs somewhat less hefty than Earth.

Now, a team reports finding the smallest planet yet – a true pipsqueak orbiting a sun-like star some 215 light-years away near the constellation Cygnus. The planet has a mass at least 1 percent of Earth's mass, probably more. And it's about 30 percent Earth's size. This is slightly larger than the moon, which is 27 percent the size of Earth.

The planet, one of three in the system, orbits its host star once every 13.4 days.

Finding the smallest planet yet is more than an "isn't it cute" moment, suggests Thomas Barclay, an astronomer at NASA's Ames Research Center at Moffett Field, Calif.

"This is a milestone" on the path to finding Earth-like planets, says Dr. Barclay, who is the lead author of a report of the find, which appears in Thursday's issue of the journal Nature.

The observations come courtesy of NASA's Kepler mission, a sensitive space telescope that observes more than 150,000 stars simultaneously in and near the head of Cygnus around the clock. Kepler's ultimate goal is to find Earth-like planets orbiting sun-like stars at Earth-like distances.


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