10th Planet

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More signs that solar system has tenth planet

By Charles Arthur, Technology Editor

12 December 2002

The solar system may have a tenth planet lurking beyond the orbit of Pluto, calculations by astronomers in Britain and Argentina indicate. "Planet X" could lie 60 times further from the Sun than the Earth, roughly 600 million miles out. But nobody would have spotted it directly because, if it exists, it orbits in a direction that astronomers rarely study.

The new planet, thought to be the same size as Earth, would lie on the inside edge of the Kuiper Belt, a distant region of the solar system principally composed of small pieces of rock and interstellar leftovers from the creation of the solar system. Yet it could have the seeds of life, because astronomers have detected ice and complex molecules on the surface of some of the rocks there.

One of the scientists who developed the idea, Dr Mario Melita, of the astronomy unit at Queen Mary, University of London, said yesterday: "The Belt has a sharp edge to it, which isn't well understood. But people have noticed gaps in planetary rings and when they've looked there, they've found orbiting moons that have swept up the matter by their gravity."

Dr Melita, and Dr Adrian Brunini, of the University of La Plata in Argentina, suggested the sharp edge of the Kuiper Belt was caused by a similar sweeping, and that had, over time, created a planet-sized object, whose orbit of the sun would be almost circular, but angled by 20 degrees to that of the inner planets.

"We could observe it with telescopes, but there's a lot of space to cover," Dr Melita said. "And we don't normally look in that direction relative to Earth."

The idea has had qualified approval from other astronomers. "There's something funny going on out there [in the Kuiper Belt]," Marc Bule of the prestigious Lowell Observatory in Arizona told New Scientist magazine. Alan Stern of the Southwestern Research Institute in Boulder, Colorado, said the area was "a region of planetary formation, with 100,000 objects that are 'miniature planets'."

Modern astronomers reckon that Pluto, discovered in 1930, actually originated in the Belt. Its orbit sometimes passes within that of Neptune, the next most outlying planet. In October, a team at the California Institute of Technology discovered a Kuiper Belt object half the size of Pluto, which they named Quaoar.

Dr Stern told New Scientist the discovery was probably only the first of many. "There are more likely 900 planets in the solar system than nine," he said. "And all but eight are in the Kuiper Belt."

But the existence of a planet is not yet confirmed. Other hypotheses could explain the Belt's shape, Dr Melita said, such as a small star about one-tenth as large as the Sun, passing near when the solar system formed about six billion years ago. "That could have created something like this," he said.


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