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.

Pluto, another object discovered orbiting it in 1978 called Charon, and a body discovered in 2003 that is slightly farther from the sun -- temporarily named UB313-- would be plutons. A pluton would be any planet beyond Neptune.

"Currently, we know of three, but there are other objects that are close in size to Pluto that will have to go through an evaluation process to determine whether they will be considered plutons," Binzel said. "We fully expect there are even more discoveries to come that are likely to be in this class of plutons."

Binzel and other committee members stressed that categorizing Pluto as a pluton was in no way meant to downgrade its longtime status as the ninth planet.

"We might be demoting it from the list of eight classical planets, but we're promoting it by making it the head of its own special class," said Owen Gingerich of Harvard University, who chaired the panel.

Other astronomers praised the committee for developing a relatively simple but scientifically valid definition to categorize the new objects being discovered, thanks to advances in technology and more powerful telescopes.

"I think the group brilliantly came up with a simple scientific sieve that can be used to decide whether something should be considered a planet," said S. Alan Stern of the Southwest Research Institute in Boulder, Colo., who served on the committee that deadlocked. "I'm very pleased."

But Mike Brown of the California Institute of Technology in Pasadena said he is disappointed, even though the proposal would characterize the object he discovered, UB313, as a planet.

"My first reaction was, 'Wow, this would mean this thing I found is a planet,' which is pretty exciting," Brown said. "Then I started looking at the details, and I don't think they got it quite right."

Brown questioned the caveats in the details of the proposal, which, for example, would make Pluto a planet but not Earth's moon.

"I find the definition oddly inconsistent. It makes no sense to me," Brown said. "They are sort of trying to have things both ways: They want to have a scientific definition, but they also don't want to offend cultural sensibilities."

Brown questioned why a committee would need to vote on what gets planet status.

"That's weird," Brown said. "That's not science."

But the proposal came as welcome news to Neil deGrasse Tyson, director of the Hayden Planetarium in New York. Tyson was inundated with angry and anguished e-mail and letters from schoolchildren when he opened an exhibit that displayed Pluto apart from other planets.

"This is about the only way you could define planethood in a way that would include Pluto. So I find it a little suspicious," Tyson said. "But I'm happy to finally have an unambiguous definition, so I don't have to worry about it after this."