FIRST DISCOVERY ANNOUNCED OF A PLANET
ORBITING A GIANT STAR
January 8, 2002
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A team of astronomers is announcing today the first discovery of a planet orbiting
a giant star. The report is presented by Dr. Sabine Frink, David S. Mitchell and
Dr. Andreas Quirrenbach from the University of California, San Diego, Dr. Debra A.
Fischer and Dr. Geoffrey W. Marcy from the University of California at
Berkeley, and Dr. Paul Butler of the Carnegie Institution of Washington to the
American Astronomical Society meeting in Washington, DC. 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 sunlike star, but an old star that has already burned the hydrogen fuel in it's
core. Such 'giant stars' get much bigger towards the end of their lifes, 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", said Dr.
Sabine Frink, a post-graduate researcher at UC San Diego. "This provides the first
evidence that planets at earthlike distances can survive the evolution of their
host star into a giant."
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Artist's concept of the iota Draconis system. The top panel
shows the giant star iota Draconis in orange and the giant
planet resembling Jupiter in the foreground. For comparison,
the bottom panel shows the Sun-Earth system to scale.
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Like all of the extrasolar planets that have been discovered orbiting sunlike stars,
this 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
the orbit is elliptical rather than circular. The derived mass of the planet is 8.7
times the mass of Jupiter.
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Observed radial velocities of iota Draconis (dots) along
with the fitted orbital solution (solid line). The long rise
and steep fall are a result of the high eccentricity of the
orbit. The orbital period is 1.5 years.
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Because the Doppler technique determines the minimum
mass, it is quite possible that the true mass of this companion is in the brown
dwarf regime. Brown dwarfs are 'failed stars' that do not possess enough mass to
start nuclear fusion. They are physically similar to giant planets, but may form
in a different way. Even if this companion is a brown dwarf, it's detection around
an evolved star represents a first.
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Illustration of the orbit of the substellar companion to iota
Draconis (solid line) as seen in the plane of the sky. Iota
Draconis itself is indicated by the purple dot. The shape of
the real orbit in the plane of the sky might look somewhat
different from this illustration because not all orbital
parameters are well determined. 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.
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Our Sun will 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 Celsius. "The oceans will evaporate, and the water vapor will
escape the Earth's atmosphere because of the high temperature", notes Dr.
Andreas Quirrenbach, a professor at UC San Diego. "Observing the fate of this
companion to a dying star is a reminder of the ultimate fate of our own Earth",
adds Dr. Debra Fischer, a research astronomer at UC Berkeley.
The observations were carried out with the 0.6 m (24 inch) Coudé Auxiliary
Telescope at the University of California's Lick Observatory. Follow-up work will
be needed to determine the exact nature of the companion to iota Draconis. NASA's
Space Interferometry Mission (SIM), scheduled to launch in 2009, will be able to
observe this star and determine the total mass of the object, helping us to
understand whether this is a massive planet or a brown dwarf.
The giant star iota Draconis is located at a distance of 100 light years in the
constellation of Draco and is currently visible with the unaided eye as a third
magnitude star in the morning sky, just east of the Big Dipper.
This work was supported by NASA.
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