SANTA CRUZ, Calif., June 14 -- A team of astronomers discovered the smallest planet ever detected beyond our solar system -- a major milestone in the search for Earthlike planets. About seven and a half times as massive as Earth, with less than twice the radius, it may be the first rocky planet ever found orbiting a normal star not much different from our Sun.
All of the nearly 150 other extrasolar planets discovered to date around normal stars have been larger than Uranus, an ice giant about 15 times the mass of the Earth.
The new planet orbits the star Gliese 876, just 15 light-years away and located in the constellation Aquarius. This star also harbors two larger, Jupiter-sized planets. The smaller planet whips around the star in a mere two days, and is so close to the star's surface that its temperature probably tops 200 to 400 degrees Celsius (400 to 750 degrees Fahrenheit)--oven-like temperatures far too hot for life as we know it.
Nevertheless, the ability to detect the tiny wobble that the planet induces in the star gives astronomers confidence they will be able to detect even smaller rocky planets in orbits more hospitable to life. Though the team has no proof that this new planet is rocky, its low mass precludes it from retaining gas, like Jupiter. And because it is so close to the star, it is unlikely to be a cold ice giant like Neptune. Three other purported rocky planets have been reported, but they orbit a pulsar, the flashing corpse of an exploded star.
"We keep pushing the limits of what we can detect, and we're getting closer and closer to finding Earths," said team member Steven Vogt, a professor of astronomy and astrophysics at the University of California, Santa Cruz.
Recent improvements to the Keck Telescope's high-resolution spectrometer (HIRES) provided crucial new data. Vogt, who designed and built HIRES, worked with the technical staff in the UC Observatories/Lick Observatory Laboratories at UC Santa Cruz to upgrade the spectrometer's CCD (charge coupled device) detectors last August.
"This planet will be historic," said team leader Geoffrey Marcy, professor of astronomy at UC Berkeley. "Over 2000 years ago, the Greek philosophers Aristotle and Epicurus argued about whether there were other Earthlike planets. Now, for the first time, we have evidence for a rocky planet around a normal star."
Marcy and three other members of the team -- Paul Butler of the Carnegie Institution of Washington, UC Santa Cruz postdoctoral researcher Eugenio J. Rivera and theoretical astronomer Jack Lissauer of NASA/Ames Research Center -- presented their findings at a press conference Monday at the National Science Foundation in Arlington, Va. The research, conducted at the Keck Observatory in Hawaii, was supported by NSF, the National Aeronautics and Space Administration and the University of California.
"It is the higher precision data from the upgraded HIRES that gives us confidence in this result," Butler said.
A paper detailing the results has been submitted to the Astrophysical Journal. Coauthors are Vogt and Gregory Laughlin of UC Santa Cruz, Debra Fischer of San Francisco State University and Timothy M. Brown of the National Center for Atmospheric Research in Boulder, Colo.
For more information, visit: www.ucsc.edu