A comet's odd orbit hints at hidden planet. (Science News of the week).

Far beyond the solar system's nine known planets, a body as massive as Mars may once have been part of our planetary system--and it might still be there.

Although the proposed planet would lie too far away to be seen from Earth, its gravitational tug could account for the oddball orbit of a large comet spotted in the outer solar system a year ago.

Known as 2000 C[R.sub.105], the comet moves about the sun in a much more elongated pathway than originally thought, astronomers now find. Observations over the past year by Brett Gladman of the Observatoire de la Cote d'Azur in Nice, France, and his colleagues show that the comet's orbit takes it further than 200 astronomical units (AU) from the sun and as close as 44 AU. One AU equals the Earth-sun distance of about 150 million kilometers.

Such an oblong orbit is usually a sign that an object has come under the gravitational influence of a massive body. But 2000 C[R.sub.105], which may be an escapee from the distant reservoir of comets known as the Kuiper belt, never gets anywhere near any of the solar system's familiar team of nine planets. Even at its closest approach to the sun, the approximately 400-km-wide ball of ice comes no closer than 14 AU to Neptune, the nearest known candidate for a significant gravitational interaction.

The astronomers concede that feeble and random pushes from Neptune could have slowly nudged 2000 C[R.sub.105] into its current orbit. However, preliminary analysis suggests this scenario isn't likely, note Gladman, Matthew Holman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and their collaborators.

In an article the researchers recently posted on the Internet (http://arXiv.org/ abs/astro-ph/0103435), they suggest that a massive body lurking among the tiny, frozen residents of the Kuiper belt could have been the culprit.

That object could have been Neptune itself. According to one theory, Neptune and Uranus first formed between the orbits of Jupiter and Saturn and were then flung out to greater distances from the sun.

If that kick propelled Neptune into the Kuiper belt before the planet settled into its current nearly circular orbit, its gravity could have caused the orbits of several objects like 2000 C[R.sub.105] to stretch into elongated trajectories.

Alternatively, the comet's orbit could be the handiwork of an as-yet-unseen planet whose mass lies somewhere between that of Earth's moon and Mars, the researchers say. It's likely that such an object would have coalesced in the outer solar system from the same debris that formed Neptune, Uranus, and the cores of Jupiter and Saturn, Holman notes.

There's only a 1 percent chance that a planet could have survived in the Kuiper belt or its surroundings over the 4.5-billion-year age of the solar system, says Holman. If the planet found a secluded nook of the belt, however, it could remain intact today.

If the proposed planet is as massive as Mars, it would have to lie some 200 AU from the sun--about 7 times Neptune's distance--Holman calculates. Were it closer, observers would have spotted it.

A planet lurking in the Kuiper belt now or in the past might also explain why many members of the belt have orbits that angle away from the plane in which the nine known planets orbit the sun.

"Undoubtedly, something [massive] knocked the hell out of the belt," says Harold F. Levison of the Southwest Research Institute in Boulder, Colo. "The question is whether it's there now."

The stability of the orbit of 2000 C[R.sub.105] suggests that any planet that influenced the comet's path has long since departed. If astronomers find a family of objects similar to 2000 C[R.sub.105], the nature of their orbits could indicate whether the hidden planet is in fact still there, Levison says.