Astronomers with the McDonald Observatory Planet Search project have discovered the first planet orbiting a star in a close-in binary star system. The discovery has implications for the number of possible planets in our galaxy, because unlike the Sun, most stars are in binary systems. The team announced their finding in a news conference at the American Astronomical Society's Division of Planetary Sciences meeting.

Artist's conception of the planet and its view of the two stars that make up the Gamma Cephei system. The planet orbits the bright yellow star on the right every 2.5 years. Credit: Tim Jones/McDonald Observatory

Artie Hatzes (Thueringer Landessternwarte Tautenburg), Bill Cochran (UT-Austin McDonald Observatory), and colleagues found that the planet orbits the larger star of the binary system Gamma Cephei, about 45 light-years away in the constellation Cepheus. The primary star is 1.59 times as massive as the Sun. The planet is 1.76 times as massive as Jupiter. It orbits the star at about 2 Astronomical Units (A.U.), a little further than Mars's distance from the Sun. (An A.U. is the distance from Earth to the Sun.) The second, relatively small star is only 25 to 30 A.U. from the primary star -- about Uranus' distance from the Sun.

Astronomers have found planets orbiting stars in binary systems before, but the stars in those binary systems 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.

The planet orbits 2 AU from the larger star in the Gamma Cephei system, while the secondary star is a mere 28-30 AU distant. Orbits drawn to scale; star and planet sizes NOT to scale. Credit: McDonald Observatory

Cochran's team began observing Gamma Cephei with the 2.7-meter Harlan J. Smith Telescope at McDonald Observatory in 1988. Prior to that, a Canadian team of astronomers used the Canada-France-Hawaii Telescope (CFHT) to study Gamma Cephei. Together, the observations total 20 years.

In the past, some astronomers thought that the 2.5-year variation in light output from the binary star could be caused by physical processes in the stars.

"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. 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 that's left is a planet."

A third-magnitude star, Gamma Cephei can be seen with the unaided eye. But even powerful telescopes cannot split the light from the system into two individual pinpoints.

The McDonald Observatory Planet Search began in 1987. The team uses the 2.7-meter Harlan J. Smith Telescope to monitor about 180 nearby Sun-like stars for Jupiter-sized planets. In addition to Gamma Cephei, the team has found planets orbiting the stars 16 Cygni B and Epsilon Eridani. The program is supported by grants from the National Science Foundation and NASA.