Astronomers discover bizarre planet-like objects

By Robert Sanders, Public Affairs

17 January 2001 | A team of planet-hunting astronomers that includes Berkeley scientists have discovered two multi-planet systems that are among the most bizarre planetary systems found to date.

One system calls into the question the very meaning of the term "planet," said Geoffrey Marcy, professor of astronomy in the College of Letters & Science. The second system has two planets eternally locked in sync, with orbital periods of 60 and 30 days. Both systems were known previously to contain one planet, but now appear to have two planets each orbiting a central star.

One of the systems is around the sun-like star HD 168443, which is 123 light years away in the constellation Serpens. It is orbited by both a massive planet and another bulkier object at least 17 times more massive than Jupiter, the largest planet orbiting the Sun. Whether this bulky companion is a planet, a dim failed star called a brown dwarf or some as-yet unidentified astronomical object is unclear, said co-discovered Paul Butler of the Carnegie Institution. Astronomers are debating the origin and nature of this bulky beast, and no one is even sure what to call it.

"This massive planetary object defies our expectations for the largest planets,'' Butler said. "But it's orbiting right there next to another planet. We never expected nature would make such gargantuan planets, and indeed maybe they aren't planets at all.''

In a paper submitted for publication in the Astrophysical Journal, Butler and Marcy propose that planets can be no larger than the mass at which deuterium, or heavy hydrogen, would start to burn in the object's core: about 13 times the mass of Jupiter. The newly discovered object breaks that barrier, yet calling it a brown dwarf is too simplistic, Marcy said.

"How did this form so close in orbit around the star, and in close association with another planet?" asks Steve Vogt of UC Santa Cruz, another scientist who helped discover the planetary system. "This is going to bug astronomers."

The two planets orbiting HD168443 have masses of at least seven and 17 times that of Jupiter. They orbit in eccentric orbits, as do most planets around other stars, residing 0.3 and 3 A.U. from the star. (One A.U. or astronomical unit is the distance from the Earth to the Sun: 93 million miles.) The larger planet wasn't detected until now because the astronomers needed several years of data in order to see the wobble created by its 4.7-year orbit.

Around a second star the team discovered a pair of planets locked in resonant orbits, moving in synchrony around their star with orbital periods of 60 and 30 days. Because of this 2-to-1 ratio, the inner planet goes around twice for each orbit of the outer one. They gravitationally shepherd one another to maintain this synchrony, their elliptical orbits nested and aligned like Russian Matrioshka dolls, said Debra Fischer, a post-doctoral researcher at Berkeley.

"These two resonant planets seem to be humming in harmony," she noted. "They are like two harmonic notes on a stringed instrument with commensurate frequencies.'' These two resonant planets have masses of at least 0.5 and 1.8 the mass of Jupiter, and orbit Gliese 876, a dim red dwarf star only 15 light years from Earth in the constellation of Aquarius.

The team had enough data on the wobble of Gliese 876 two years ago to detect the second planet, but the resonant orbits confounded them. In early December, Marcy sat down to figure out why the data did not fit perfectly with the model of a single planet orbiting the star. He decided to model a system with two planets, and after much trial and error found that two synchronous orbits with periods in the ratio of 2 to 1 fit the data best. This is the first known 2-to-1 nested multi-planet system.

"We were fooled," Vogt said. "The synchrony allowed one planet - the smaller, inner planet - to hide in the wobble of the other." "Maybe we have found the music of the planetary spheres,'' Fischer. "Now we are straining to hear the lyrics, as these planets are surely telling us something of their birth.''

The research is part of a multi-year project to look for planets among 1,100 stars within 300 light-years of Earth.


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