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. [larger view] (Credit: Tim Jones/McDonald Observatory) |
Austin, TX- 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 will announce
their finding this week in a news conference at the American Astronomical
Society’s Division of Planetary Sciences meeting in Birmingham,
Ala.
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’ 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.
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.
The planet orbits 2 AU from the larger star in |
"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.
For more details on this discovery, please visit:
http://www.as.utexas.edu/planet/gamcep.html
