What is a planet anyway?

In early times, "planet" simply meant anything in the sky that moved in a predictable way, said Mike Brown, an astronomer at the California Institute of Technology. Things got complicated with the discovery of asteroids (which move predictably but are much smaller than things we call planets). For a while asteroids were called planets, but that changed.

Today, not all scientists agree on a "planet" definition, said astronomer Michael F. A'Hearn of the University of Maryland. Some say it is an object that forms out of the gas and dust left from the formation of a star. Others think more factors are important, such as its size and whether it orbits around a star.

How long have we known about Pluto?

Pluto was discovered in 1930 almost by accident. Mathematical calculations predicted a planet beyond Neptune. The calculations were wrong, but an astronomer at the Percival Lowell Observatory in Arizona didn't know that and kept looking. When he found it, Pluto was thought to be the size of Earth. Today we know it is smaller than Earth's moon.

Why are some astronomers changing their minds about calling Pluto a planet?

Pluto is different from the other planets. Four -- Mercury, Venus, Earth and Mars -- are rocky. The others -- Jupiter, Saturn, Uranus and Neptune -- are made of gases. But Pluto is mostly ice. It also has an odd orbit. It rotates in the opposite direction from most of the other planets, and there are periods of time -- the last being 1979-1999 -- when it is closer to the sun than Neptune.

Pluto is the only planet in the Kuiper Belt, a ring of chunks of ice and rock beyond the orbit of Neptune. Those objects were discovered in the 1990s. This fall, Brown discovered another icy Kuiper belt object: 2002 LM60, called Quaoar (pronounced kwa-whar). It's about half the size of Pluto and the largest one found since Pluto. Brown says this shows that Pluto is just like the other non-planets in the Kuiper Belt.

So is Pluto a planet or isn't it, and what does it matter?

That depends on which astronomer you ask. Brown says that if Pluto were discovered today, it wouldn't be called a planet. A'Hearn thinks Pluto should be classified as both a planet and as a Kuiper Belt object because it will allow researchers to look at it in different ways.

Both said it shouldn't really matter what Pluto is labeled. It's like the question of whether Australia is the smallest continent or the largest island, Brown said. "Australia is what it is whether we call it a continent or an island. This fact doesn't stop people from getting very passionate about Australia's status, though!"

How did Pluto get its name?

The planets are all named after gods from Greek and Roman mythology. Pluto is the shadowy Roman god of the underworld, and the planet has that name because it receives very little sunlight. It is also believed that the first two letters stand for the initials of Percival Lowell, an astronomer who believed another planet existed beyond Neptune but died before he found it.

The name really had nothing to do with Mickey Mouse's dog, Pluto?

It so happens that in 1930, the same year Pluto was discovered, Mickey Mouse's dog made his first appearance in a Disney cartoon, but it wasn't for another year that it was named Pluto, in a cartoon called "The Moose Hunt." A coincidence?