Pluto to Get Partners? New Definition of "Planet" Proposed

Mason Inman
for National Geographic News
August 16, 2006
Astronomers proposed a new definition of "planet" today.

The proposal would add at least three more planets to the nine we're familiar with, instantly outdating textbooks.

But it could add as many as 50—and that's a problem, some researchers say.

The term "planet" has never had an official definition.

Many scientists have long argued that it was a mistake to call Pluto a planet, since it is much smaller than the other eight generally accepted planets.

Recent discoveries have intensified the debate over Pluto's status and brought the issue to the forefront of the astronomy community.

Now the International Astronomical Union is meeting in Prague, in the Czech Republic, to debate how to best define the term based on physical properties.

Astronomers vote on the proposal next week.

Planets and Plutons

The IAU proposal says that a planet is an object large enough to have become rounded due to the force of its own gravity.

But it's not that simple. What counts as a planet also depends on what it's orbiting around.

A planet has to orbit a star, so rounded objects floating freely through space won't make the cut.

But if an object is orbiting another, much larger object that's not a star, it wouldn't count as a planet either.

So Earth's moon, if it was orbiting the sun by itself, would qualify as a planet. But according to the proposed definition, since the moon circles Earth, it is actually a satellite.

The proposed definition also introduces two classes of planets.

The accepted eight planets—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune—would simply be called planets.

But Pluto, its large moon Charon, and more distant objects would be called "plutons."

Plutons are all much smaller than the rest of the planets; they also have orbits that are tilted compared to the rest of the planets.

Problematic Pluto

No one is debating the status of the eight biggest planets. They are all large bodies that have become almost perfectly round under the influence of their own gravity.

But the solar system also contains many other objects—such as asteroids—that are small and somewhat irregularly shaped.

(See a virtual solar system.)

Ceres, for example, was the first asteroid discovered in the asteroid belt between Mars and Jupiter. For decades after its discovery in 1801, it was called a planet.

But after astronomers discovered many more of these bodies, and the number of supposed planets mounted, they demoted them to asteroids.

Pluto's large moon Charon is only a bit smaller than Pluto itself, and the two bodies orbit the sun together.

And earlier this year, researchers announced the discovery of a "tenth planet"—called 2003 UB313 and nicknamed Xena—beyond Pluto's orbit.

Under the proposed definition of "planet," all these bodies—Ceres, Charon, 2003 UB313, and perhaps many more—would be granted planethood.

"I think it's good they're grappling with getting a good definition [of the term planet]," said planetary scientist Peter Thomas of Cornell University.

Thomas and colleagues recently announced studies showing that Ceres is smooth and round. Thomas says there are a few other asteroids in the belt that might get upgraded to planetlike status as researchers gather more data.

Flawed Approach?

Despite the scientific basis of the proposed definition, it is stirring up controversy.

Astronomer Mike Brown of CalTech, who led the group that discovered the "tenth planet," opposes the new definition—even though it would make his discovery officially a planet.

Brown calls the proposed definition "leave no ice ball behind," an approach that's flawed, he said, because it will include far too many objects—53 and counting, he figures.

"I'd be sad to miss the chance to have discovered the tenth planet," Brown wrote in a statement. "But I'd get over it."

Astrophysicist Alan Boss of the Carnegie Institution in Washington, D.C., is also critical of the proposed definition.

"It doesn't have the elegance I was hoping for," Boss said. "It looks like it was written by a committee of lawyers rather than scientists."

Boss adds that this is somewhat unavoidable, because of the wide variety of objects in our solar system. But he objects to the broad scope of the definition—and thinks many others will as well.

If there are 50 or more planets, then "being a planet isn't worth what it used to be," Boss said. "Then what's the big deal about being a planet?"

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