Update 8/25/06: A group of Pluto supporters have decided to sue the IAU for violating the Administrative Procedure Act when it decided to demote Pluto to dwarf planet status. I can’t imagine this will make it very far in the court system… but what do I know?
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Today, astronomers voted to demote Pluto of its planet status after 76 years of equal status with the Big 8… can you imagine how many books and model solar system toys now have to be reproduced?

2500 scientists meeting at the the International Astronomical Union (IAU) in Prague debated and finally approved a definition of the word “planet,” redefining Pluto as “dwarf planet” and leaving only 8 major planets in the solar system.

Sadly, Xena didn’t make the cut either after much speculation that it would. According to the IAU

In short, a “planet” is now defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit…

A dwarf planet, according to the new definition, is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

Pluto was demoted because its orbit is eccentric compared to the other 8. At points, it’s actually closer to the sun than Neptune.

So now there are 8 planets and 3 dwarf planets (Pluto, Ceres, and 2003 UB313 aka Xena).