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

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 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.

Thus, over the 4 billion years since the sun formed, the clutch of suns would have scattered. One escapee could have dropped off samples of its planetary building blocks - perhaps Buffy - in the outer reaches of our solar system as it headed off on its own spin about the galaxy.

Kenyon and Dr. Bromley, a planetary scientist at the University of Utah, were running computer simulations to see if an encounter with another cluster member early in the sun's history might have given Sedna, another object beyond the Kuiper Belt, its highly inclined, extremely elliptical orbit.

The two calculated that if another star passed close enough to the sun and with the right trajectory, there was at least a 50 percent chance that it could have tugged Sedna out of the Kuiper Belt and into its current orbit.

But as the two reviewed their calculations, they also found that there was a 10 percent chance that the two stars actually could have swapped material from the extended disks of planetary building blocks that surrounded each of them. Depending on conditions, the sun could have captured up to one-third of the objects orbiting the passing star at distances of from 60 to 80 AU.

In a paper the two published last year in the journal Nature, they concluded that finding objects beyond the Kuiper Belt with orbital tilts larger than 40 degrees could confirm the existence of "extrasolar planets" in our solar system's own backyard.

Now, with Buffy representing the third highly inclined orbital oddball discovered over the past few years, "maybe this idea we had isn't completely out to lunch," Kenyon says.

To be sure, no theory at the moment, including his and Bromley's, "makes enough predictions for finding the distinctive features that would point to one" as the correct explanation, he says.

But he adds that the stellar-flyby is the most straightforward explanation to date. He and his colleague would like to run the model again to see if it yields estimates of the number of these objects astronomers could expect to find.