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.