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Odd Visitor From the Oort Cloud

29 September 1997 8:00 pm
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A 10-kilometer object that looks like an asteroid may have come from the Oort cloud, a spherical shell of frozen bodies far beyond the orbit of Pluto that is thought to be the exclusive lair of comets. If correct, the finding, which is based on computer simulations to be published in the 20 October Astrophysical Journal, has implications for the tumultuous birth of the solar system. It could be the first evidence that material was flung out into the Oort cloud not just from the icy region of Uranus and Neptune, but also from rocky areas much closer to the sun.

The curious object, called 1996 PW, was detected last year by an automated search telescope in Hawaii. It is traveling through the outer solar system in an elongated elliptical orbit typical of a comet, and yet it resembles a rocky body and lacks the gaseous emissions or dusty coma of comets. To find out where the object might have originated, Paul Weissman of the Jet Propulsion Laboratory in Pasadena, California and Harold Levison of the Southwest Research Institute in Boulder, Colorado, ran computer simulations of how 1996 PW's orbit might have evolved.

Given the large size of 1996 PW's orbit and its high, approximately 30-degree inclination to the plane of the solar system, the object most likely evolved from a much larger original orbit that extended to the Oort cloud. The two researchers argue that the object is an asteroid, not a comet that has lost much of its water from flying past the sun too many times, because their simulations suggest that it has only orbited the sun 27 times, far short of the 800 trips of known inert comets. From estimates of how much rocky material might have been thrown out of the early solar nebula, the pair guesses that about 1% of Oort cloud's inhabitants are rocky asteroids.

"There is a lot of merit to what they are saying--that the object is unlikely to have the sort of orbit it does unless it had been in the Oort cloud at some point," says Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "But, I'm less convinced that it is not an inert comet stripped of its ice or covered up by dust."

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