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Unexplained / Space exploration / Universe / Between the Stars / 

Between the Stars

A planet orbits the more massive star of the Gamma Cephei binary

On Earth, children have two parents. In space, though, it's more common for a planet to be born and grow up with just one "parent": the star it orbits. But now astronomers have found a planet whose parent star is coupled with a close stellar partner. The discovery could signal that there are even more planets in the galaxy than astronomers anticipated.

The planet orbits one of the two stars in Gamma Cephei - a tight binary star about 45 light-years from Earth. The third-magnitude star is the third-brightest in the constellation Cepheus. "This is the brightest star in the sky to have a planet," co-discoverer William Cochran told reporters Wednesday at the American Astronomical Society's Division of Planetary Science meeting in Birmingham, Alabama.

The Gamma Cephei stars are never more than 30 astronomical units (AU) from each other - about the same distance as that between Neptune and our sun - and they can come as close as 12 AU. Powerful telescopes can't even resolve the two stars.

"There are other planets in binary star systems," Cochran noted, "but never in a binary system where the stars are this close." In other binary systems with planets, the stars are so far apart that they could be considered independent, he explained.

Cochran is part a team that detected the planet around Gamma Cephei's more massive member using the 2.7-meter Harlan J. Smith Telescope at McDonald Observatory in Texas. Since 1988, the McDonald Observatory Planet Search project has measured wobbles in the spectrum of this star, which are superimposed on the velocity variations caused by the star orbiting the binary system's center of mass.

The planet-related wobbles were actually first seen by a Canadian group who studied Gamma Cephei from 1982 to 1992 with the Canada-France-Hawaii Telescope on Mauna Kea. However, that team discounted the variations as being intrinsic to the primary star, Gamma Cephei A. "They couldn't really tell what was going on," Cochran said. But now, both teams realize the phenomenon must be due to a planet which orbits Gamma Cephei A once every 2.5 years.

"We think this is a planet because the variation has been nice and steady for eight complete cycles," Cochran said. "The star itself would not be varying that nicely for eight cycles over 20 years : and we see no variations that we can attribute to the star itself. The only logical thing that's left is a planet."

The exoplanet is at least 1.76 times as massive as Jupiter and orbits about 2 AU from Gamma Cephei A (which is a bit farther than Mars is from the sun). This star is a subgiant about 1.6 times as massive as the sun and probably about 3 billion years old. The secondary is a redder, M-class star that is only a few tenths of the sun's mass. The binary's orbital period is at least 71 years.

The discovery says a lot about our galaxy's exoplanet population. "Most of the stars in the sky are in binary star systems," Cochran points out. "Our sun is unusual in this aspect." If many binary systems have extrasolar planets, then "nature is very efficient at forming planets," he added. There could be a lot more alien worlds than astronomers once expected.

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