In August of 2006 Pluto was declared to no longer be a planet. This was due to a rewriting of the definition of “planet”.
According to the new definition, a full-fledged planet is an object that orbits the sun and is large enough to have become round due to the force of its own gravity. In addition, a planet has to dominate the neighborhood around its orbit.
Pluto has been demoted because it does not dominate its neighborhood. Charon, its large “moon,” is only about half the size of Pluto, while all the true planets are far larger than their moons.
Pluto is now officially a “dwarf planet”. But it’s no longer part of an exclusive club, since there are more than 40 of these dwarfs, including the large asteroid Ceres and 2003 UB313, nicknamed Xenaâ€”a distant object slightly larger than Pluto discovered by Brown last year.
Update Feb 2009: You can talk about this topic in the discussion forum at (and play video games too!):
Update May 2009: Here is information about the “Planet X” theory and information on why Pluto was initially considered a planet: http://en.wikipedia.org/wiki/Planet_x
Update August 25 2009:
The debate rages on. Did you know that 90% of the critical letters complaining about Pluto’s demotion come from North America? Food for thought, here’s the article: