A 10-kilometer object that looks like an asteroid may have come from the Oort cloud, a spherical shell of frozen bodies far beyond Pluto's orbit where only comets are thought to lurk.

The finding, based on computer simulations to be published in the 20 October Astrophysical Journal, could have implications for the tumultuous birth of the solar system--providing 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 object in question, called 1996 PW, is traveling through the outer solar system in an elongated elliptical orbit typical of comets, which are basically dirty snowballs. But it lacks a comet's gaseous emissions or dusty coma. To learn more, Paul Weissman of the Jet Propulsion Laboratory in Pasadena, California--the object's discoverer--and Harold Levison of the Southwest Research Institute in Boulder, Colorado, ran simulations of how 1996 PW's orbit might have evolved. They deduced that it used to be a much larger orbit that extended to the Oort cloud. They also concluded that the object has only orbited the sun 27 times, far short of the 800 trips of known inert comets, which suggests that it's an asteroid and not a comet that has lost water from many solar flybys. From estimates of how much rocky material might have been thrown out of the early solar nebula, the pair goes on to guess that about 1% of Oort cloud's inhabitants are rocky asteroids.

Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, says he's not completely convinced that the object is not an inert comet. But he agrees that it is "unlikely to have the sort of orbit it does unless it had been in the Oort cloud at some point."