So what's a planet?

Here's what the New York Times is reporting today:

A ball of ice and dust discovered last year in the outskirts of the solar system is 30 percent wider than Pluto, a team of German astronomers is reporting today.

The finding definitively makes the icy ball — temporarily labeled 2003 UB313 and nicknamed Xena — the largest known object to be discovered orbiting Earth's sun since Neptune was identified in 1846, and adds to the debate over what should be considered a planet.

The diameter of 2003 UB313, the astronomers found, is 1,860 miles, give or take 250 miles. Like Pluto, which has a diameter of 1,400 miles, 2003 UB313 is a member of the Kuiper Belt, a ring of icy debris that orbits beyond Neptune.

You would think that would qualify this as a planet, but not so. Seems no one really knows what a "planet" is - it's a word in search of a strict scientific defnition.

The similarities between 2003 UB313 and Pluto extend beyond size and reflectivity. Like Pluto, 2003 UB313 also has a moon and has methane ice on its surface.

The discovery of 2003 UB313 has intensified a debate over the definition of a planet, which has swirled about Pluto since the late 1990's. Pluto is by far the smallest of the nine planets, and 2003 UB313, while larger than Pluto, is still smaller than Earth's moon.

According to one definition, a planet has to gravitationally dominate its surroundings. That would exclude both Pluto and 2003 UB313.

Some astronomers contend that any object in the solar system large enough that gravity has shaped it into a sphere should be called a planet. But that would also add quite a few asteroids and other Kuiper Belt objects. Another possibility is to arbitrarily call anything larger than Pluto a planet.

The debate has kept astronomers from bestowing a formal, more memorable name for 2003 UB313. If planet status is bestowed, 2003 UB313 will probably be named after a Roman or Greek deity, like the other planets except Earth.

"There is still a stalemate on the meaning of 'planet,' " said Brian G. Marsden, director of the Minor Planet Center of the International Astronomical Union, "and no progress on providing a name for 2003 UB313."