Illustration courtesy T. Pyle, Caltech/NASA

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The newfound star system, Kepler-47, is seen in an artist's illustration.

Illustration courtesy T. Pyle, Caltech/NASA

Double Planets Found Orbiting Twin Stars

Double planets have been spotted circling double stars for the first time, astronomers say.

The Kepler Space Telescope has notched another milestone discovery: double planets circling double stars in the constellation Cygnus, some 5,000 light-years away. It's the first time multiple planets have been found in a binary star system.

"It's much more difficult to form planets around a pair of stars," said San Diego State University's William Welsh, who presented the discovery Wednesday at the International Astronomical Union meeting in Beijing.

Shifting positions and gravitational forces create a chaotic environment that may disrupt the process of planet formation. Only four other planets have been identified as belonging to binary stars. (See star pictures.)

The new system, named Kepler-47, has one sunlike star and a companion about a third its size. The two planets—one three times bigger than Earth and the other slightly larger than Uranus—orbit around both stars in what is likely a delicate dance.

Single Stars a Cosmic Rarity

The new study, published this week in the journal Science, found that Kepler-47's larger, outer planet lies in the much scrutinized "habitable zone," where temperatures are just right for the presence of liquid water on a rocky surface. Its orbit is an unusually long 303 days, similar to Earth's.

Although the planet is likely a gas giant and not suited to life, its presence proves that circumbinary planets—meaning they go around both stars—can exist in habitable zones, said Welsh.

The majority of stars in the Milky Way are, in fact, binaries. "Single stars like the sun are the exception," said study leader Jerome Orosz, an astronomer at San Diego State University.

"The fact you can have planets at binary stars is important because you've just opened up a whole new landscape to look at."

Binary stars are a tough study for scientists searching for planets because they are prone to giving false positives.

That is, they can mimic the characteristics of planetary transits—the crossing of planets in front of a star that causes a dip in the star's brightness.

But Kepler's ability to detect very slight changes in brightness eliminates much of the problem. (Also see "NASA's Kepler Finds Two Earth-Size Planets Around Sunlike Star.")

In just under a year, the Kepler mission has discovered all six planets now known to revolve around a pair of stars. The telescope is aimed at a field in the Milky Way containing about 4.5 million stars. This spring NASA extended its planet-hunting mission by four years.

Richard A. Lovett contributed reporting to this story.