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Far out. 2000 CR105 has a very elongated orbit that lies well outside Neptune's orbit.

Signs of a Hidden Planet?

A giant comet in the distant reaches of our solar system has an extremely large, elongated orbit that can't be explained by the gravitational pull from the giant planets in their current positions. The finding is the first solid evidence that a big planet once roamed the farthest outskirts of the solar system; maybe Neptune, before it settled down, the researchers say, or a mysterious, Mars-sized planet that could still be lurking there.

The supercomet, 2000 CR105, was first spotted in February 2000 and is some 400 kilometers wide. It has a highly elliptical orbit well beyond Neptune, the outermost giant planet in the solar system. There are hundreds of other icy "Trans-Neptunian Objects" (TNOs) which are believed to have been flung into their eccentric orbits early in the solar system's history by the gravitational pull of a giant planet--most likely Neptune.

New observations by Brett Gladman of the Observatoire de la Côte d'Azur, France, and his colleagues now reveal that 2000 CR105's orbit is much larger and more distant than had been assumed: Even when it's closest to the sun, at 6.6 billion kilometers, it lies far outside Neptune's orbit. (It's farthest point is 58.2 billion kilometers from the sun.) The giant planets couldn't have put it there, Gladman and his colleagues argue, at least not from their current orbits.

Instead, Neptune may once have had a much more eccentric orbit itself, from which it could have tugged the comet to its current orbit, the researchers say in a paper submitted for publication in Icarus. Or it was pulled there by a massive population of planetary "embryos" that may have inhabited the early solar system but were later expelled. The most exciting possibility is that a mid-sized planet at some 10 billion kilometers from the sun caused 2000 CR105's orbit. And because such a planet would not be very vulnerable to orbit disruptions, it could still be there, the team says.

"This is the first conclusive evidence that somewhere out there, there once was something big," says Hal Levison of the Southwest Research Institute in Boulder, Colorado. "It's a very important result." However, Levison doesn't think there's another Mars waiting to be discovered, because it's unclear how it would have formed. Gladman suspects there must be many more TNOs in orbits like 2000 CR105; "finding more would give us a better idea of how they got there," Levison adds.

Related sites

The paper by Gladman and colleagues

The Kuiper Belt (in which most TNOs reside)

An electronic newsletter about the Kuiper Belt and TNOs