From http://www.universetoday.com/13573/why-pluto-is-no-longer-a-planet/

We constantly redefine our world based on new information. Things get re-classified. Hell, I am now classified as 'old'! :-)

Is Pluto a planet? Does it qualify? For an object to be a planet, it needs to meet these three requirements defined by the IAU:

• It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet.
• It needs to have enough gravity to pull itself into a spherical shape – Pluto…check
• It needs to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule breaker. According to this, Pluto is not a planet.