Pluto and about 50 other similar objects at the far reaches of our solar system are now called "trans-Neptunian objects". They are also sometimes called "Kuiper belt objects". The fact is, if Pluto had been discovered in the 1990s it would never have been classified as a planet. At least one other object (called Eris) is now known to be larger than Pluto. Eris, Pluto, and other such bodies are also known as Dwarf Planets. You can find out more at Wikipedia: Names of Planets.