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The newly discovered planets probably closely resemble Jupiter (shown here). Although three of the planets are more massive than Jupiter, their stronger gravity would compress them, so they should be no bigger than Jupiter.

"As of a month ago, we thought we understood the lay of the land in planetary systems," said Geoffrey Marcy, an astronomer at the University of California, Berkeley, and leader of the world's leading team of planet hunters, during a Tuesday press conference. "We also thought we understood the lay of the land with regard to planetary orbits....So we were a bit cocky."

But Marcy said his team was humbled by the discoveries of a pair of two-planet systems. One system's planets follow "resonant" orbits, while the second system contains the largest planet-like object discovered to date. "Both planetary systems are entirely unique, and I think a little frightening," he said.

Astronomers have discovered about 50 planets orbiting stars other than our own Sun, but the new discoveries are among only a handful of systems in which more than one planet has been found.

Home of the Whopper
One of the new systems is around HD 168433, a star similar to our own Sun, and contains two planets or other objects. One is at least seven times as massive as Jupiter, the largest planet in our own solar system, and orbits the star at about three-tenths of the distance from Earth to the Sun. The second is at least 17 times as massive as Jupiter and is about three times farther from HD 168433 than Earth is from the Sun.

"This is the whopper," said team member Paul Butler, a staff scientist at the Carnegie Institution of Washington. "This is literally off our scale....It's twice as massive as anything else we've ever seen."
The Marcy-Butler team has discovered almost 40 planets orbiting stars other than the Sun. Most are as massive as Jupiter or heavier, but none approaches the heft of the object in the HD 168433 system. Current theories say that any object about a dozen times as massive as Jupiter or heavier should be a brown dwarf -- a not-quite-star that can shine briefly by "fusing" the nuclei of deuterium atoms (a form of hydrogen) in its core.

But because of its close proximity to the star, and its presence in a system that contains a known planet, Butler said it's too early to classify their discovery as a brown dwarf. Instead, it may be some previously known "missing link" between planets and brown dwarfs. "I prefer to just say it's a mystery system and hope our friends in the theoretical community will consider it a gift," Butler said.

Planetary Reverb
The second pair of planets orbits Gliese 876, a faint, cool star known as a red dwarf. It's one of our closest stellar neighbors, at a distance of 15 light-years. The team had already reported the discovery of one planet in the system, but continued observations revealed a second planet.

One planet is about one-half as massive as Jupiter, while the other is 1.8 times Jupiter's mass. What makes the system especially interesting, though, is the planets' orbits: The innermost star orbits Gliese 876 once every 30 days, while the other orbits every 60 days. This relationship is called a resonance, and it suggests that the two planets act as mutual "shepherds," with their gravitational pull keeping their orbital periods in 2-to-1 harmony.

Team member Debra Fischer of UC-Berkeley called this orbital relationship a true "music of the spheres," and compared it to two harmonizing notes played on a stringed musical instrument.
Similar harmonies are found in our own solar system. The planet Pluto completes two orbits around the Sun for every three orbits of Neptune, for example. -- — Damond Benningfield