Distant Object Could Hold Secrets to Earth's Past

The Kuiper Belt, however, ends abruptly 4.7 billion miles from Earth. Sedna, whose closest point of approach to Earth is 7 billion miles, cannot be a Kuiper Belt object and has no obvious relationship to anything else ever seen.

"Sedna is awesome," said planetary scientist Harold F. Levison of the Southwest Research Institute, expressing the view of many astronomers. "I started working in the Kuiper Belt before it was discovered, and every time I turn around, something sensational happens."

Brown suggested that Sedna might be a migrant from the Oort cloud, a spherical shell of icy bodies surrounding the solar system and extending out toward the nearest star. The Oort cloud, however, is supposed to begin beyond Sedna -- about five-sixths of a light year from the sun. Sedna, Brown said, could be the first sighting of something from a possible "inner" Oort cloud.

Such an explanation would account for Sedna's inclined orbit because Oort cloud comets, the principal evidence that the Oort cloud exists, come into the solar system from all angles, probably after passing stars jog them from their icy habitat.

"There's no disagreement that Sedna's orbit had to be disturbed," Stern said, and gravitational pull from a passing star is a likely way this could have happened. There is, however, much disagreement on where Sedna was in the first place and how it got there.

Last year Levison and Alessandro Morbidelli of France's Cote d'Azur Observatory examined various theories for Sedna's formation and concluded that it most likely came from part of the sun's dust disk that was "scattered" and flung outward in different directions by repeated gravity boosts from the giant outer planets. This view in part echoes Brown's inner Oort cloud hypothesis.

In a separately researched paper, Scott J. Kenyon of the Harvard-Smithsonian Center for Astrophysics and Benjamin C. Bromley of the University of Utah suggested that Sedna could have formed near Neptune in the scattered disk or beyond the Kuiper Belt, as Stern suggests, then had its orbit radically altered by a close brush with a passing star.

"It rips out the outer part of the solar disk, making the edge" of the Kuiper Belt, Kenyon said in a telephone interview. "We would like a messiness to the encounter to explain these messy orbits." A sufficiently messy encounter, he said, could cause the two passing solar systems to exchange material -- including Sedna.

Some theories require evidence for something that has not yet been seen in the vastness beyond the Kuiper Belt, either Brown's inner Oort cloud or Stern's extended disk, and there is only one way to determine whether these things exist: "We must find more objects," Levison said.