IMPLICATIONS OF SEDNA DISCOVERY IN 2004

December 2, 2004
Sun Might Have Exchanged Hangers-On With Rival Star

By DENNIS OVERBYE

The Sun may have captured thousands or even millions of asteroids from another planetary system during an encounter more than four billion years ago, astronomers are reporting today.

Such an interstellar ballet would explain many mysteries of the outer solar system - including the strange behavior of the recently discovered Sedna, the system's most distant known object, which occupies a strange elongated orbit far beyond Pluto.

The astronomers' calculations, from supercomputer simulations, suggest that gravity from the star at the center of the other planetary system could have kicked Sedna out of a more conventional orbit. In the process, the Sun and the other star would have swapped their outer entourages. Indeed, the astronomers estimate that there is a 10 percent probability that Sedna itself was one of those strangers.

If the alien asteroids could be found and studied, these bodies could provide testimony to the conditions under which the Sun and the solar system formed, a time otherwise lost in the mists. Astronomers say the Sun was born 4.5 billion years ago as part of a dense cluster of more than 1,000 stars that has long since disappeared. The star that nearly collided with our solar system so long ago could be on the other side of the galaxy by now.

"I don't think anyone has considered that extrasolar planets would be in our own solar system," said Scott J. Kenyon of the Harvard-Smithsonian Center for Astrophysics. He is the co-author, with Benjamin C. Bromley of the University of Utah, of a paper being published today in Nature.

Their paper is the latest in a series of efforts to consider an intruding star as a way of explaining the weird properties of Sedna.

"Sedna surprised the hell out of everybody," said Harold F. Levison of the Southwestern Research Institute in Boulder, Colo., who has proposed a slightly different intruder scenario, with Alessandro Morbidelli of the Observatory of the Côte d'Azur in France.

The strange planetoid was discovered last spring. About 600 to 1,000 miles in diameter, Sedna travels in an elongated oval of an orbit far outside the main planets of the solar system, taking roughly 10,500 years to complete an orbit. By comparison, distant Pluto takes about 248 years to complete a trip around the Sun. At its closest approach, Sedna, named after the Inuit goddess who dwells in the frigid depths of the seas, will still be about seven billion miles from the Sun, 75 times farther than the Earth, which circles at a comfortable 93 million miles - one astronomical unit, in cosmic lingo.

Before Sedna's discovery, astronomers knew of no objects beyond 50 astronomical units from the Sun, where a disk of icy worlds known as the Kuiper Belt, left over from the formation of Neptune and Uranus, abruptly falls off. (They have deduced the existence of a vast halo of icy objects from which comets occasionally descend, known as the Oort cloud, extending much farther out, but these objects cannot be seen.)

Out there, Sedna is far beyond the gravitational influence of the other planets, so how did it get so far out? Both Dr. Kenyon's group and Dr. Levison's agree that a kick from a passing star is the best and perhaps the only way to do it, but they disagree on the details.

Dr. Kenyon and Dr. Bromley conclude that a star and its disk of planetary building materials passing from 160 astronomical units or so from the Sun could not only lift Sedna into its present location, but also truncate the Kuiper Belt, thus explaining its abrupt edge.

Dr. Levison said his and Dr. Morbidelli's work favored a more distant and gentle kick from a distance of about 800 astronomical units. He pointed out that Dr. Kenyon's scenario predicted a plethora of objects in Sedna-like orbits but slightly closer and thus easier to see that are nonetheless not seen.

Dr. Kenyon acknowledged that this was a potential problem.

Either encounter would also leave alien planetoids in our solar system (and some of ours in the alien system) orbiting at a steep angle to the plane in which the planets go around. And so the next step is to search for such objects.

Sedna itself has only a moderately inclined orbit , the astronomers say. A more likely candidate for an extra-solar origin is another icy wanderer, known as 2000 CR105, about half the size of Sedna, discovered out beyond Neptune in 2000. Its orbit is inclined 20 degrees to the planets.

The detection of objects with inclinations of 40 degrees or more, the authors write in Nature, "would clinch the case for extrasolar objects in the solar system."

Alan P. Boss, a planetary expert at the Carnegie Institution of Washington, called the idea that Sedna could have been a captured object formed around a different star "intriguing even if it is a low-probability event."

Dr. Bromley said the study of such objects could help scientists understand how stars and solar systems form and whether the forces that shaped Sedna and the solar system are common to other systems. "We are a long way off from having the technology to observe Sedna-like objects around other stars," he wrote in an e-mail message, "but a captured planet would be within our reach."

Dr. Levison said these objects could be the few relics left with information about the star cluster that gave birth to the Sun.