PRAGUE, Czech Republic -- Earthlings, beware: Your neighborhood is about to get much more crowded.
The International Astronomical Union today proposed enlarging the world's understanding of the solar system to encompass 12 planets instead of the traditional nine, changing what billions of schoolchildren have been taught for generations.
But experts said that by the end of 2007, there could be dozens more.
"There's a whole list of candidates knocking at the door," said Owen Gingerich, who chairs the planetary definition committee of the IAU, the arbiter of what is -- and isn't -- a planet.
"Don't get stuck on this number 12, because that won't last the whole of next year," he told reporters in Prague, where 2,500 of the world's leading astronomers are considering a new definition for planets. "By the end of next year, there's going to be more."
The IAU says it has a "watchlist" of about a dozen potential candidates for planethood. Most would be known as "plutons" under a proposed new definition that would distinguish between eight classical planets -- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune -- and Pluto, along with objects like it in the far reaches of the solar system beyond Neptune.
Under the proposal, which will be voted on next week, Pluto would remain a planet but would be known as a pluton. Joining it on the list of planets would be three other plutons: Pluto's largest moon, Charon; 2003 UB313, or Xena, the farthest-known object in the solar system; and the asteroid Ceres, which was a planet in the 1800s before it was demoted.
Critics, who include 2003 UB313 discoverer Michael Brown of the California Institute of Technology, contend the new classification would be ungainly and admit objects to the planetary club that don't belong because of their small size and mass or other factors.
"This a step backward," Alan Boss, an astrophysicist at the Carnegie Institution in Washington, D.C., said Wednesday. "It's a demotion of the word 'planet.'"
Boss, who believes there are only eight true planets, said the term "planet" would lose its luster under the proposed definition because anything round of a certain size could join the club -- and that could have serious repercussions.
"Folks aren't going to have to worry about learning the planets anymore because it's a list that doesn't mean anything," he said, adding, "We're not going to have a NASA that wants to visit every planet in the solar system because we're not going to be able to do that anymore."
Defending the proposed resolution, Richard Binzel of the Massachusetts Institute of Technology said the IAU had taken pains to consider all points of view and forge a "reasonable and workable" compromise based on sound science.
"There could be hundreds of plutons. But that's fine, because we have this new way of classifying them," said Binzel, a professor of planetary science. "We have to let the solar system be the solar system."
Its proposal: A planet is any round object larger than 800 kilometers (nearly 500 miles) in diameter that orbits the sun and has a mass at least about 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 would not qualify because the two bodies' common center of gravity lies below the surface of the Earth. Pluto and Charon would because their center of gravity lies in the space between them.
Astronomers also were being asked to get rid of the term "minor planets," which long has been used to collectively describe asteroids, comets and other non-planetary objects. Instead, those would become collectively known as "small solar system bodies."
Opponents of Pluto, which was named a planet in 1930, still might spoil for a fight. Earth's moon is larger; so is 2003 UB313 (Xena), about 70 miles (113 kilometers) wider.
IAU President Ronald D. Ekers said the proposed definition -- a benchmark that has eluded astronomers for centuries -- would help settle the endless bickering among astronomers about the significance of their finds.
"We realized we were moving into a minefield on this," Ekers said Wednesday. "But as more objects are discovered, the lack of a complete definition of a planet has become increasingly confusing."
First Published: August 16, 2006, 4:00 a.m.