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How to explain a mini-planet's odd orbit?

Discovery of 'Buffy,' an object on the Kuiper Belt's fringe, prompts a rethinking of solar system's formation.

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Could the outer solar system harbor planetary samples nabbed from a passing star?

That's a question some astronomers are asking as they try to explain "Buffy," a recently discovered, frigid mini-planet 300 to 600 miles across. It orbits the sun just beyond the edge of the Kuiper Belt, a broad swath of icy objects that extends far beyond Neptune.

Because of its odd orbital features, when it comes to testing ideas about how the solar system formed, this new object may become known as Buffy the Theory Slayer.

The object "is a challenge to theories of the evolution of the solar system," acknowledges Lynne Allen, a postdoctoral fellow at the University of British Columbia in Vancouver who discovered Buffy. "It points out that a lot more went on than we think" as planets, particularly the gas giants, formed and wandered to their current locations around the sun.

Dr. Allen is part of a team using the Canada-France-Hawaii Telescope on Hawaii's Mauna Kea to conduct a survey of the Kuiper Belt.

The team announced the discovery last week, a year after Allen spotted the object as she was processing the survey data. During the interim, astronomers at the Kitt Peak National Observatories outside Tucson, Ariz., and the Mt. Palomar Observatory near San Diego conducted additional observations that helped pin down Buffy's orbital characteristics.

Buffy orbits the sun once every 440 years

Buffy orbits the sun once every 440 years at a distance ranging from 52 to 62 astronomical units from the sun (one AU is 93 million miles).

Buffy's arresting traits begin with its nearly circular orbit. Those of its nearest neighbors are more elliptical. More puzzling, Buffy's orbit is severely tilted compared with those of planets, comets, and Kuiper Belt objects - some 47 degrees off kilter from the rest of the solar system. It's a feature that defies all but the most convoluted explanations for how the solar system achieved its current structure, some astronomers say.

Enter Scott Kenyon and Benjamin Bromley, two researchers who model solar-system evolution on high-powered computers. Dr. Kenyon, a senior scientist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., explains that the sun probably was born as part of a cluster of stars whose combined gravity bound members only loosely to the group.

A sun drops planetary material?
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