Pluto

I think Pluto is not a planet. It is a dog. =D In all seriousness I think Pluto is not a planet because there was another planet discovered that was much larger than Pluto, and astronomers realize they had to reconsider their definition of a planet. They decided that a planet must fulfill three criteria, which was that a planet must orbit the sun, it must have enough mass to pull itself into spherical shape, and it must have cleared out the other objects in its orbit. So, both Pluto and Xena were named dwarf planets.