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Correction to This Article
A graphic accompanying an article about Pluto and a new definition of planets on Aug. 16 incorrectly characterized the orbits of the eight larger planets in the solar system. Their orbits are nearly circular but are actually ellipses.

Pluto's New Place in Space Could Be as a 'Pluton'

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By Rob Stein
Washington Post Staff Writer
Wednesday, August 16, 2006

Hoping to end the agonizing over whether Pluto is really a planet, an international committee of astronomers has come up with a new definition that would save the tiny body's place in the sun's family.

Under the long-awaited proposal, Pluto would remain in the pantheon of planets by becoming the prototype of a new subcategory of small, outer solar system objects dubbed "plutons" -- planets, but distinct from the eight larger "classical" planets closer to the sun.

The changes would require astronomy textbooks to be rewritten and every schoolchild to be taught a new vision of the solar system, because three other orbs would get promoted to planet status, as well -- expanding the total from the traditional nine to 12.

"Everybody's been wanting to know: 'Is Pluto a planet?' " said Richard P. Binzel of the Massachusetts Institute of Technology, who served on the seven-member committee assembled by the International Astronomical Union (IAU) to settle the explosive issue. "The answer is: 'Yes, Virginia, Pluto is a planet.' "

The proposal to resolve the dispute is being officially unveiled today at the IAU's general assembly in Prague. It will be hotly debated until Aug. 24, when about 1,000 astronomers will vote on it. Some astronomers expressed misgivings about the new definition, but it generally drew initial praise, and several predicted it will be ratified.

"I think it's a good compromise," said Larry W. Esposito of the University of Colorado, who had opposed maintaining Pluto and similar bodies as planets. "They're really too small and don't amount to much. But it would be too difficult to demote Pluto. This way, we don't have to scratch it off the list."

The status of Pluto, the smallest of the nine planets, has been called into question by the discovery in recent years of other objects of similar size and distance from the sun. But suggestions that Pluto be demoted prompted heated debate and angry denunciations.

In an attempt to settle the issue, the IAU assembled a 19-member committee, which deadlocked after two years of intensive debate. That led to creation of the smaller committee, which met in Paris June 10 and July 1 to find a way out of the thicket.

Under the new definition, a planet would be defined as any body massive enough to be round that is not a star but is orbiting one.

"These are the most fundamental physical parameters that apply not only in our solar system, but everywhere in the universe," Binzel said. "That's what's so appealing about the definition -- it can be applied universally."

The eight "classical" planets would be Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Ceres, an object located between Mars and Jupiter that has long been considered an asteroid, would be considered a planet.

"One might call it a 'dwarf planet,' but that's not an official term," Binzel said.


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