The planet sleuths from the University of California at Berkeley, NASA and other institutions discovered the planetary pair locked in what appears to be "resonant" orbits, moving in synch around the star with orbital periods of 60 and 30 days. Because of the 2-to-1 ratio, the inner planet goes around the star twice for each orbit of the outer one. They gravitationally tug on each other to maintain this synchronicity.

"The resonance between the two orbiting planets is among the most exciting planet detection discoveries to date," said Dr. Jack Lissauer, a NASA Ames Research Center scientist based in the heart of California's Silicon Valley. A "resonance" is similar to the harmonic vibration produced by plucking two notes on a stringed instrument. This gravitational pas de deux between the two planets is common among moons and asteroids, but not planets. The axes of the two newly detected planets' elliptical orbits also appear to be nearly perfectly aligned.

Lissauer and State University of New York at Stony Brook graduate student Eugenio Rivera used a numerical model to demonstrate the stability of the nearly twin orbits around the star known as Gliese 876, a dim red dwarf 15 light years from Earth in the constellation Aquarius. "Questions about planetary migration and gravitational influence are still very much unsolved," Lissauer said.

"This discovery is significant for several reasons," said Lissauer. "This is the first extra-solar planetary system to show a strong resonance. It also is the smallest star known to have any orbiting planets, much less two," he said.

The two gravitationally linked planets have masses of at least 0.5 and 1.8 times the mass of Jupiter, he said. The inner planetary companion was not recognized at first because the orbital resonance allowed the pair of planets to masquerade as a single planet with an elongated orbit.

The two orbiting planets are located relatively close to each other, within 0.08 Astronomical Units (the distance between the Earth and the sun) of each other, less than one-third the distance from the Earth to its nearest neighbor, Venus. In our solar system, the only known resonances between a pair of planets is Pluto, which orbits the sun twice for every three times Neptune circles the sun.

Besides Lissauer, the planet-hunting team that discovered the system includes Drs. Geoff Marcy and Debra Fischer of the University of California at Berkeley; Dr. Paul Butler of the Carnegie Institution of Washington; and Dr. Steve Vogt of the University of California at Santa Cruz.

Though significant and unusual, the discovery will require more modeling before researchers can determine what the resonance they discovered actually means.

The team based both sets of its conclusions on 6 years of precise Doppler measurements and observations made at the Keck I telescope atop Mauna Kea in Hawaii and the Lick Observatory telescope in California. The research is part of a multi-year project to look for planets among 1,100 stars within 300 light years of Earth. The project is supported by NASA, the National Science Foundation and Sun Microsystems.

More information about these discoveries is available at http://www.exoplanets.org.

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