A planet larger than Jupiter has been discovered in a two-star system in which the stars are closer together than any pair previously found to harbor a planet.
Astronomers said the finding was important because most stars are not alone in space but instead are part of one of these so-called binary systems.
The planet is estimated to be 1.76 times as massive as Jupiter. It orbits the primary star of the binary system (called Gamma Cephei) a little farther out than Mars is from our Sun. The star is about 1.6 times as massive as our Sun.
The second, relatively small star is about the same distance from the primary star as Uranus is from the Sun.
The discovery was led by Artie Hatzes of Thueringer Landessternwarte Tautenburg and Bill Cochran from the University of Texas, Austin and the McDonald Observatory.
In previous discoveries of planets orbiting stars in binary systems, the stars were a hundred times farther apart than those of Gamma Cephei, Cochran said. "The stars were far enough apart to be essentially acting totally independently," he said.
Theorists have wondered to what extent the gravitational mechanics of tight binary star systems could support planets.
Observations that led to the discovery date back 20 years and involve studying variations in the light output from the star.
Researchers monitor minor shifts in the light's wavelengths that indicate a star is wobbling due to the tug of an orbiting planet. The method, akin to listening to the wail of an ambulance change as it moves toward and then away from you, is the only one used so far to find planets outside our solar system. About 100 have been detected, but none has ever been photographed.
"We think this is a planet because the variation has been nice and steady for eight complete cycles," Cochran said. The planet orbits the star every 2.5 years.. "The star itself would not be varying that nicely for eight cycles over 20 years. Our observing techniques include several good indicators of stellar variability, and we see no variations that we can attribute to the star itself. The only logical thing thats left is a planet."
The discovery is being presented at the American Astronomical Societys Division of Planetary Sciences meeting in Birmingham, Ala.
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