Another planet for the solar system?

Two new studies hint--but by no means prove--that the solar system could harbor a 10th planet. The object, roughly the mass of Jupiter, would lie far beyond Pluto in a distant reservoir of comets known as the Oort cloud.

In his study, John B. Murray of the Open University in Milton Keynes, England, analyzed the orbits of comets in the inner solar system that are believed to have emigrated from the Oort cloud. He finds that a subset originates from a region in the cloud shaped like an arc stretching across the sky.

One explanation for the unusual pattern, according to Murray, is that it marks the wake of a massive body moving through the outer part of the Oort cloud. As the object travels, its gravity would kick comets from the arc-shaped region. To provide the required kick, the object needs to be at least as massive as Jupiter, Murray calculates. He argues, however, that it must be less than 10 times Jupiter's weight. Otherwise, it would be so bright that astronomers would have already detected it.

The object probably orbits the sun at a distance 32,000 times farther away than the Earth does, Murray reports in the Oct. 11 MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY. Computer models show that it couldn't have resided in the cloud since the beginning of the solar system but must have been captured in its current orbit later on, he notes.

Analyzing the same data, another team also finds evidence of clustering within the cloud that may signify the presence of an unseen planet. The researchers report that the object orbits the sun at an average distance 25,000 times as far as Earth does and has a mass between 1.5 and 6.0 times that of Jupiter.

John J. Matese, P.G. Whitman, and Daniel P. Whitmire of the University of Southwestern Louisiana in Lafayette presented their findings last month at a meeting of the American Astronomical Society in Padua, Italy, and will publish them in an upcoming ICARUS.

Harold F. Levison of the Southwest Research Institute in Boulder, Colo., is skeptical of both studies. He notes that the two teams assume, as planetary scientists often do, that comets are distributed uniformly throughout the Oort cloud. In fact, new simulations by Levison and his colleagues suggest that comets in the cloud naturally form clusters. If more extensive simulations bear that out, an unseen planet need not be invoked to explain the clustering, he says.