Plan Would Add Planets to Solar System

The International Astronomical Union (IAU) President Ronald Ekers makes his inauguration speech during the opening ceremony of the 26th General Assembly of the IAU in Prague, on Tuesday, Aug. 15, 2006. The main aim of the 12-day conference is to agree on the definition of planets. The supreme astronomical conference, which is held in Prague after 39 years, is to be attended by 2,500 delegates from 75 countries. (AP Photo/CTK, Michal Krumphanzl) (Michal Krumphanzl - AP)

But the IAU said Pluto meets its proposed new definition of a planet: any round object larger than 800 kilometers (nearly 500 miles) in diameter that orbits the sun and has a mass roughly one-12,000th that of Earth. Moons and asteroids will make the grade if they meet those basic tests.

Roundness is key, experts said, because it indicates an object has enough self-gravity to pull itself into a spherical shape. Yet Earth's moon wouldn't qualify because the two bodies' common center of gravity lies below the surface of the Earth.

"People were probably wondering: If they take away Pluto, is Rhode Island next?" Binzel quipped. "There are as many opinions about Pluto as there are astronomers. But Pluto has gravity on its side. By the physics of our proposed definition, Pluto makes it by a long shot."

IAU President Ronald D. Ekers said the draft definition, two years in the making, was an attempt to reach a cosmic consensus and end decades of quarreling. "We don't want an American version, a European version and a Japanese version" of what constitutes a planet, he said.

Neil deGrasse Tyson, director of the Hayden Planetarium at New York's American Museum of Natural History _ miscast as a "Pluto-hater," he contends, merely because Pluto was excluded from a solar system exhibit _ said the new guidelines would clear up the fuzzier aspects of the Milky Way.

"For the first time since ancient Greece, we have an unambiguous definition," he said. "Now, when an object is debated as a possible planet, the answer can be swift and clear."

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AP Science Writers Alicia Chang in Los Angeles and Seth Borenstein in Washington contributed to this story.

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On the Net:

International Astronomical Union, http://www.iau.org