09 October 2002

McDonald Observatory Planet Search finds first planet orbiting
close-in binary star

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.

"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

IMAGE CAPTIONS:

[Image 1:

(124KB)]
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)

[Image 2:

(115KB)]
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)

[Image 3: (.eps file, 98KB)
http://mcdonaldobservatory.org/images/news/releases/2002/d_Ceph.eps]
Around 9:00 p.m. CDT tonight, Gamma Cephei will be visible
to the unaided eye in the northern sky, just above and to
the east of Polaris. (Tim Jones/McDonald Observatory)