A
team of astronomers from the University of California, San Diego and two other
institutions has made the first discovery of a planet orbiting a giant star, a
find of special interest to astronomers because it provides insight into the
fate of planets during the late life cycles of stars.
The
discovery was announced today at a meeting of the American Astronomical
Society in Washington by Sabine Frink, David S. Mitchell and Andreas
Quirrenbach from UCSD’s Center for Astrophysics and Space Science, together
with Debra A. Fischer and Geoffrey W. Marcy of the University of California at
Berkeley, and Paul Butler of the Carnegie Institution of Washington.
The
result is of special interest because it provides insight into the fate of
planets during the late life cycles of stars. What makes this discovery
remarkable is that the host star, iota Draconis, is not a sun-like star, but
an old star that has already burned the hydrogen fuel in its core. Such
“giant stars” grow much bigger toward the end of their lives and iota
Draconis has expanded to a radius that is 13 times the radius of the sun.
“Until
now, it was not known if planets existed around giant stars,” says Frink, a
postdoctoral researcher at UCSD. “This provides the first evidence that
planets at earthlike distances can survive the evolution of their host star
into a giant.”
The
giant star iota Draconis is located at a distance of 100 light years from
Earth in the constellation of Draco and is currently visible with the unaided
eye in the morning sky, just east of the Big Dipper.
Like
all of the extrasolar planets that have been discovered orbiting sun-like
stars, the discovery was made with the Doppler technique, where the
gravitational pull of the planet causes a wobble in the measured velocity of
the host star.
The
planet completes one full orbit around iota Draconis in 1.5 years and the
shape of its orbit is elliptical rather than circular. The derived mass of the
planet is 8.7 times the mass of Jupiter. Because the Doppler technique
determines the minimum mass, the astronomers say, it is possible that the true
mass of this companion is a brown dwarf, a “failed star” that lacks enough
mass to start nuclear fusion. Brown dwarfs are physically similar to giant
planets, but may form in a different way. Even if this companion is a brown
dwarf, the researchers say its detection around an evolved star represents a
first.
Planet
hunters note that it is more difficult to detect the signature of a planet
orbiting a giant star rather than a dwarf, because giant stars often pulsate.
Those pulsations can produce patterns in the radial velocities similar to
planetary companions, so it is more difficult to interpret the origin of the
observed signal. However, in the case of iota Draconis, the relatively high
eccentricity distinguishes orbital motion from pulsation as the cause of the
velocity variations.
The
scientists say that the sun will eventually undergo a similar fate to iota
Draconis. Several billion years from now, when the sun evolves into a giant
star, the Earth will receive about 60 times more radiation than it does today
and the temperature will rise to several hundred degrees centigrade.
“The
oceans will evaporate, and the water vapor will escape the Earth’s atmosphere
because of the high temperature,” notes Andreas Quirrenbach, a professor of
physics at UCSD.
“Observing
the fate of this companion to a dying star is a reminder of the ultimate fate
of our own Earth,” adds Fischer, a research astronomer at U.C. Berkeley.
The
team’s observations, which were financed by the National Aeronautics and
Space Administration, were carried out with the 0.6 m (24 inch) Coudé
Auxiliary Telescope at the University of California’s Lick Observatory. The
astronomers say that follow-up work will be needed to determine the
exact nature of the companion to iota Draconis. NASA’s Space Interferometry
Mission, scheduled for a launch in 2009, will be able to observe this star and
determine the total mass of the object, helping astronomers to eventually
determine whether it is a massive planet or a brown dwarf.
