Scientists debate planet puzzle

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LOS ANGELES (AP) -- Our solar system is undergoing an identity crisis.

For decades, it has consisted of nine planets, even as scientists debated whether Pluto really belonged. Then the discovery of an object larger and farther away than Pluto threatened to throw this slice of the cosmos into chaos.

Should this newly found icy rock known as "2003 UB313" become the 10th planet? Should Pluto be demoted? And what exactly is a planet, anyway?

Ancient cultures regularly revised their answer to the last question, and present-day scientists aren't much better off: There still is no universal definition of "planet."

That all could soon change, and with it science textbooks around this planet.

At a 12-day conference beginning today, scientists will conduct a galactic census of sorts. Among the options at the meeting of the International Astronomical Union in the Czech Republic capital of Prague: Subtract Pluto or christen one more planet, and possibly dozens more.

"It's time we have a definition," said Alan Stern, who heads the Colorado-based space science division of the Southwest Research Institute of San Antonio. "It's embarrassing to the public that we as astronomers don't have one."

The debate intensified last summer when astronomer Michael Brown of the California Institute of Technology announced the discovery of a celestial object larger than Pluto. Like Pluto, it is a member of the Kuiper Belt, a mysterious disk-shaped zone beyond Neptune containing thousands of comets and planetary objects. Mr. Brown nicknamed his find "Xena"; pending a formal name, it remains 2003 UB313.

The Hubble Space Telescope measured the bright, rocky object at about 1,490 miles in diameter, roughly 70 miles larger than Pluto. At 9 billion miles from the sun, it is the farthest known object in the solar system.

The discovery stoked the planet debate that had been simmering since Pluto was spotted in 1930.

Some argue that if Pluto kept its crown, Xena should be the 10th planet by default -- it is, after all, bigger. Purists maintain that there are only eight traditional planets, and insist Pluto and Xena are poseurs.

"Life would be simpler if we went back to eight planets," said Brian Marsden, director of the astronomical union's Minor Planet Center in Cambridge, Mass.

The trick for astronomers meeting in Prague is to set criteria that make sense scientifically. Should planets be grouped by location, size or another marker? If planets are defined by their size, should they be bigger than Pluto or another arbitrary size? The latter could expand the solar system to 23, 39 or even 53 planets.

It's not an academic exercise; the public may not be open to a flood of new planets. But scientists agree any definition should be flexible enough to accommodate new discoveries.

"Science progresses," said Alan Boss, an astrophysicist at the Carnegie Institution in Washington. "Science is not something that's engraved on a steel tablet never to be changed."