Update 8/24: The news is official. Neither Xena nor Pluto are planets.
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When Xena (UB313) was found in July 2005 by Caltech’s Mike Brown and colleagues, it changed the way astronomers thought about what planets are. Next week, the International Astronomical Union will clarify the definition of a planet and either upgrade Xena to planet status or downgrade Pluto to just another object in the Kuiper Belt. It’s also possible that the asteroid Ceres and Pluto’s moon Charon will become planets.

According to Brown’s op-ed piece in today’s NY Times

If you were to look unemotionally at the hundreds of thousands of bodies orbiting the sun, only eight (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) would clearly distinguish themselves by their large sizes.

The remaining objects, which are significantly smaller, are mostly either rocky bodies in the asteroid belt between Mars and Jupiter or icy bodies in the Kuiper Belt in the distant regions beyond Neptune. Of the more than 1,000 known objects in the Kuiper Belt, 2003 UB313 and Pluto are the largest and second largest.

Since Pluto was discovered back in 1930, many kids have made models of the solar system with 9 planets. It’s become a cultural phenomenon and many people don’t want to see it downgraded.

Brown is hoping that the committee keeps Pluto and upgrades Xena. I’m rooting for it too.