Hah! Apparently, the American Museum of Natural History gets a lot of unhappy letters from children, unhappy that Pluto is no longer considered a planet.
Interestingly enough, the IAU has had a definition for what should be considered a planet that has been used since 2006:
The definition of “planet” set in 2006 by the International Astronomical Union (IAU) states that in the Solar System a planet is a celestial body that:
1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has “cleared the neighborhood” around its orbit.