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Outcasts of the Milky Way

29 May 2007 (All day)
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Ron Miller

Starless.
Young planetary systems may have seeded the galaxy with rogue giant planets.

HONOLULU, HAWAII--Our galaxy may teem with rogue giant planets that were ejected from young solar systems, according to a new theoretical study presented here at the 210th meeting of the American Astronomical Society. The number of homeless planets drifting through interstellar space may actually exceed the number of planets orbiting stars.

The rogue giants are part of a possible solution to a long-standing mystery. Most giant planets in other solar systems are in elliptical orbits, unlike the almost circular paths of Jupiter, Saturn, Uranus, and Neptune. No one knows why. Now, theoretical astrophysicists Mario Jurić and Scott Tremaine of the Institute for Advanced Study in Princeton, New Jersey, suggest a possible explanation.

Jurić and Tremaine simulated the evolution of young planetary systems containing a few to a few dozen massive planets. Over tens of millions of years, systems in which the giants have a random and chaotic collection of orbits should lose those planets. The reason? Gravitational attraction between neighboring planets further distorts the orbits and eventually flings the objects away. Eventually, only two or three giants should remain, each in an elongated orbit--just as astronomers have observed in recent years.

All other planets in the simulations--well over half of the total--end up as free-floating orphans. By these results, our own solar system, with four giants in near-circular orbits, is an exception. Jurić says the new theory is easily testable: "Our model predicts that systems with a giant planet in an elongated orbit are unlikely to harbor more than one or two additional giant planets," he says. Moreover, if there are more giants in such a system, their orbits should be tilted with respect to each other. It will be much harder to find the ejected planets, which, without a sun to bask in, are cold and dark.

Theoretical astrophysicist Alan Boss of the Carnegie Institution of Washington, D.C., is skeptical about Jurić and Tremaine's assumption that most planetary systems are characterized by orbital chaos during their early days. "I think that's very unlikely," he says. According to other simulations, he notes, young planetary systems are much more stable. Another force that might have elongated the orbits is the gravitational tug of the remains of the disk from which they form, although it's unclear how that would work.

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