Pluto Not a Planet, Astronomers Rule

Mason Inman
for National Geographic News

August 24, 2006

Pluto has been voted off the island.




The distant icy ball is no longer a true planet, according to a new definition of the term voted on by scientists today.

"Whoa! Pluto's dead," said astronomer Mike Brown, of the California Institute of Technology in Pasadena, as he watched a Web cast of the vote. "There are finally, officially, eight planets in the solar system."

In a move sure to generate controversy even as it forces textbooks to be rewritten, Pluto will now be dubbed a "dwarf planet."

But it's no longer part of an exclusive club, since there would be more than 40 of these dwarfs, including the large asteroid Ceres and 2003 UB313, nicknamed Xena—a distant object slightly larger than Pluto discovered by Brown last year.

Researchers voted on the definition at a meeting of the International Astronomical Union in Prague, in the Czech Republic. This group decides on the official names of all celestial bodies.

The tough decision comes after a two-year search for a scientific definition of the word "planet." The term never had an official meaning before.

In the new definition, to qualify as a full-fledged planet, an object orbiting the sun has to be large enough that it has become round due to the force of its own gravity, and it also has to dominate the neighborhood around its orbit.

Pluto would be demoted, because it does not dominate its neighborhood. Charon, its large "moon," is about half the size of Pluto and orbits along with it. All the true planets are far larger than their moons.

A previous proposal, unveiled last week, defined a planet only as an object large enough to have become rounded due to the force of its own gravity.

That proposal would have kept Pluto as a planet in a subcategory called "plutons." Charon and Xena would also have become plutons. The asteroid Ceres would have become a new planet.