Pluto - A dwarf

Astronomers gave Pluto the Mickey Mouse treatment Thursday, classifying the world a "dwarf" rather than a full-fledged planet.
Like its cartoon counterpart, the celestial body became a sidekick when the International Astronomical Union meeting in Prague took a hand vote and decided to downsize the solar system to eight planets.

"Pluto is still Pluto; it is still the same scientifically interesting object at the edge of the solar system," says astronomer Richard Binzel of the Massachusetts Institute of Technology, a member of the IAU's planet definition committee. "Science has advanced to the point where we realize there are lots of Plutos out there."

Last year's announcement that a world larger than Pluto had been detected — named UB313 and nicknamed "Xena" by discovery team head Mike Brown of the California Institute of Technology — put pressure on the IAU to redefine planets, says planetary scientist Will Grundy of the Lowell Observatory in Flagstaff, Ariz. Over the last decade, some science museums and astronomers had removed tiny Pluto, which is smaller than Earth's moon, from the list of planets.

The IAU's new definition of a planet is that it must:

• Orbit the Sun

• Be big enough for its own gravity to compact it into a ball

• Have "cleared the neighborhood around its orbit," meaning it is not surrounded by objects of similar size and characteristics.

The last condition was added to a draft planet definition proposed by the committee last week that would have added three more planets to the solar system immediately: UB313, Pluto's moon Charon, and the asteroid Ceres, along with dozens more in the Kuiper comet belt in the next decade.

Instead, the orbit-clearing requirement means Pluto, Ceres and UB313 become "dwarf planets" under a second resolution also adopted. None have cleared their orbital belts of similar-sized objects. The new definition passed with 95% of the votes, says the IAU's Lars Lindberg Christensen.

"I'm of course disappointed that Xena will not be the 10th planet, but I definitely support the IAU in this difficult and courageous decision," says Brown. Others were less enthusiastic. "I think this is utter nonsense. How can we ever say whether something has cleared out its orbit?" says astronomer Sanjay Limaye of the University of Wisconsin in Madison.

Meanwhile, textbook and encyclopedia writers, including publication of the 2007 World Book Encyclopedia, have been awaiting the new definition. "I suspect kids are going to be more interested in Pluto now than before," says editor in chief Paul Kobasa.