Experts want solar system enlarged

The International Astronomical Union (IAU) has proposed enlarging the world's understanding of the solar system to encompass 12 planets instead of the traditional nine.

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 the 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.

"Science wouldn't be science if you didn't have a diversion of opinions," conceded IAU President Ronald Ekers.

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.