PRAGUE, Czech Republic – It's a cosmic clash, a space squabble, a mutiny in the Milky Way.
Leading astronomers are bitterly divided over new galactic guidelines that for the first time would define what is and isn't a planet. The debate all but dooms a proposal being put to a vote Thursday to expand the solar system to 12 planets from the traditional nine.
Caught yet again in the crossfire is puny Pluto, scorned by many as a poser that could be demoted as a dwarf — slightly shrinking Earth's neighborhood instead.
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Opponents "smell blood, and I think they're going to get it," Alan Boss, an astrophysicist at the Carnegie Institution in Washington, D.C., said on the eve of a vote by members of the International Astronomical Union.
Leaders of the group, the official arbiter of heavenly bodies, caused a sensation last week by proposing that Pluto's largest moon and two other objects officially be designated as planets. They suggested that Pluto and the three newcomers be the first of a new class of planet dubbed "plutons."
The rationale was their initial draft definition of a planet: any object larger than nearly 500 miles in diameter that orbits the sun, has a mass roughly one-12,000th that of Earth and has enough self-gravity to pull itself into a round shape.
But for many of the 2,500 astronomers from 75 countries meeting in Prague, the universe hasn't been the same since.
After days of spirited and sometimes combative debate, renegade scientists have won some key concessions.
A planet, they insist, must be the dominant object in its area. That would draw a sharp distinction between the eight "classical planets" — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune — and Pluto, which would be known as a "dwarf planet."
Pluto's largest moon, Charon, the asteroid Ceres, which was a planet in the 1800s before it got demoted, and a recently discovered object known as 2003 UB313 and nicknamed Xena also would be dwarfs.
The precise wording of the definition remained a work in progress Wednesday. However, if astronomers agree that a planet must have "orbital dominance" in its own neighborhood, the new guidelines would eliminate Pluto and the trio of tentative candidates as proper planets.
"It's a kind of compromise: There would be only eight planets, plus the dwarf planets," said Japanese astronomer Junichi Watanabe, a member of the IAU's planet definition committee.
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.
"It's impossible to draw the line between the new dwarf planets and large asteroids," said Mark Bailey, director of Britain's Armagh Observatory.
And forget the term "pluton" — it's already history.
Under pressure from a growing faction of astronomers, the planet definers have been tossing around other options: plutoids, plutonids, plutonoids, plutians, or Tombaugh objects or planets in honor of Clyde Tombaugh, the American who discovered Pluto in 1930.
Among the scientists who torpedoed "pluton" were geologists, who pointed out — somewhat embarrassingly to astronomers — that it's already a prominent term in volcano science for deep igneous rock formations.
"What were they thinking? The reaction in the geologic community was rolling of eyes," 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.'"
Harvard's Owen Gingerich, who chairs the planet definition panel, conceded: "We perhaps stumbled."
After the panel got dozens of objecting e-mails, "we backed off," he added.
Suddenly, the future looks dim for much-maligned Pluto, which is smaller than Earth's moon.
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."
"Some say, 'No, Pluto is a nice planet'" and should remain one, Watanabe said.
"It's an important object that has played an important role," he said. "But this is a natural way to draw a line."