Small Planet Discovered in Solar System's Farthest Reaches

NEW YORK (June 5, 1997 07:25 a.m. EDT) -- Astronomers have found an icy miniplanet that orbits the sun well beyond Pluto, providing evidence that the solar system extends much farther than was once thought.

The little planet is about 300 miles across, which gives it a surface area comparable to Texas. It is the brightest solar system object to be found beyond Neptune since the discovery of Pluto's moon Charon in 1978.

At its most distant, it wanders three times farther from the sun than Pluto, tracing a looping, oblong path into an astronomical terra incognita.

"It's the first object in a sort of no man's land, an area we never thought we could get a glimpse of with our current technology," said Jane Luu, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

And it's probably not alone. Theoretical calculations suggest that there are millions of small, icy solar system objects well beyond the outermost planets.

Astronomers consider their new discovery an extension of the Kuiper belt, a collection of small, icy bodies that circle the sun beyond the orbit of Neptune. About 40 Kuiper belt objects have been discovered since 1992.

Before then, the only known Kuiper belt objects were the planet Pluto, discovered in 1930, and Charon.

Luu discovered the new object, known as 1996TL66, with colleagues from Harvard, the University of Hawaii and the University of Arizona, as well as an amateur astronomer based in Cloudcroft, N.M. They describe the find in today's issue of the journal Nature.

"I wouldn't call this a major planet," said Brian Marsden, a Harvard astronomer and contributor to the Nature paper. "But then I tend not to call Pluto a major planet."

In fact, 1996TL66 is considered too minor to be named for a Roman god, like the other planets.

Astronomers surmise that it is composed of the same material as other outer solar system objects -- water, carbon dioxide, methane and other materials -- all frozen solid.

The astronomers found 1996TL66 with a University of Hawaii telescope as the object passed among the outer planets last October. They and others followed it for several months with telescopes in Hawaii, Arizona and New Mexico.

The object's motion over the last few months shows that it follows a lopsided orbit unlike that of any other Kuiper belt object. It swings through the neighborhood of the outermost planets every 800 years, then loops far out into space before making its next pass.

Astronomers have never seen such a thing.

"It just reminds us that we really don't know what the outer solar system holds," Luu said.

Some researchers had an inkling that the object would be out there, however. Hal Levison, a scientist at the Southwest Research Institute in Boulder, Colo., has been telling his colleagues for the past year that objects such as 1996TL66 ought to exist.

The research behind that prediction, conducted with Martin Duncan of Queen's University in Canada, is scheduled for publication in a future issue of the journal Science.

"Though we knew that we had an interesting scientific result, it just never occurred to me that anybody would find one," Levison said. "It's very satisfying when someone confirms your theories."

In his Science paper, Levison describes how Uranus and Neptune probably generated the Kuiper Belt during the formation of the solar system more than 4 billion years ago. According to that theory, Neptune kicked a small percentage of the Kuiper belt objects into oblong orbits such as the one followed by 1996TL66.

Luu and her colleagues found the object at the very beginning of a systematic search for objects at the edge of the solar system. Because it was so easy to find, the researchers calculate that there are hundreds, and perhaps more than 1,000, objects similar to 1996TL66.

"Unless we are improbably lucky, it is merely the first detected of a larger population of similar bodies," the astronomers wrote.

By MATT CRENSON, The Associated Press