Why isn't Pluto a planet anymore?

Pluto was once included in the list of planets, but is now considered a “dwarf” planet an object in orbit around the Sun that is massive enough to have its own gravity to pull itself into a round (or nearly round) shape. Dwarf planets are generally smaller than Mercury. Pluto's composition is similar to that of comets, and its orbit is quite different from that of the other planets. Pluto also has one moon, Charon. It is located within a part of our solar system called the Kuiper Belt.