Pluto

In the astronomical world, Eris stirred up a great deal of trouble among the international astronomical community when the question of its proper designation led to a raucous meeting of the IAU in Prague. At the end of the conference, IAU members voted to demote Pluto and Eris to dwarf-planet status, leaving the solar system with only eight planets.

Because feature the Pluto not same witch Planet. So Pluto not Planet.

Well, I believe it was because they realized that Pluto is smaller than originally thought. It now longer meets the standards of size to be considered a planet. There are even moons around other planets that are larger than Pluto.