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PRAGUE, Czech Republic (Aug. 24) - Leading astronomers approved historic new planet guidelines Thursday -- downsizing Earth's neighborhood from nine principal heavenly bodies to eight by demoting distant Pluto.

After a tumultuous week of clashing over the essence of the cosmos, the International Astronomical Union stripped Pluto of the planetary status it has held since its discovery in 1930. The new definition of what is -- and isn't -- a planet fills a centuries-old black hole for scientists who have labored since Copernicus without one.

Fans of Pluto, which was discovered in 1930, are expected to go into orbit with the International Astronomical Union's decision to downgrade it. But under pressure from opponents, the organization has backed off its original plan to retain Pluto's status and bring three other objects into the cosmic club.

If the 2,500 astronomers from 75 nations meeting in Prague agree, Earth's neighborhood will officially shrink to eight planets from the traditional nine.

"There would be only eight planets, plus the dwarf planets," said Japanese astronomer Junichi Watanabe, a member of the IAU's planet definition committee.

"Some say, 'No, Pluto is a nice planet"' and should remain one, Watanabe said. "But this is a natural way to draw a line."

Resolutions being considered by the group, the official arbiter of heavenly bodies, would define a planet as "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit."

Anything less would be either a dwarf planet, as in Pluto's case, or a "small solar system body," which would cover many asteroids, comets or other natural satellites.

Ahead of the vote, a group of astronomers led by Brian Marsden, director of the astronomical union's Minor Planet Center in Cambridge, Massachusetts, were pressing for an amendment that would substitute "planetino" for "dwarf planet."

It was unclear how Pluto's possible demotion could affect the mission of NASA's New Horizons spacecraft, which earlier this year began a 9 1/2-year journey to the oddball object to unearth more of its secrets.

Astronomers want to draw a sharp distinction between the eight "classical planets" - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune - and Pluto, which is smaller than Earth's moon, no larger than many objects in its area and has an eccentric orbit.

Joining it as dwarfs would be its largest moon, Charon; the asteroid Ceres, which was a planet in the 1800s before it got demoted; and an object known as 2003 UB313 recently discovered by Michael Brown of the California Institute of Technology in Pasadena, Calif. Nicknamed Xena, it is a slightly larger Kuiper Belt object.

Just a week ago, all three objects were poised to become planets under an initial draft definition that would have created a new class of planetary objects to be dubbed "plutons."

But that idea left many astronomers cold, triggering days of spirited and sometimes combative debate that led to the latest proposal to dump Pluto.

Many believe there's simply no scientific justification to grant full planet status to most of what's floating in the vast sea of rocks that reside in the Kuiper Belt - a mysterious, disc-shaped zone beyond Neptune containing thousands of comets and planetary objects.

Forget the term "pluton" - it's already history, replaced by "plutonium object."

The IAU backed off after getting dozens of objecting e-mails from scientists, including geologists who pointed out - somewhat embarrassingly to astronomers - that "pluton" is already a prominent term in volcano science for deep igneous rock formations.

"What were they thinking?" said Allen F. Glazner, a geologist at the University of North Carolina. "It would be like botanists trying to distinguish between trees and shrubs and coming up with the term 'animal."'

Suddenly, the future looks dim for much-maligned Pluto, named for the God of the underworld.

Its underdog status has inspired scores of tributes, including one by New York folk singer Christine Lavin that laments: "I guess if Pluto showed up at a planet convention, the bouncer at the door might have to ban it."

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