Pluto is no longer considered a planet. Since I was in school and learned that Pluto was the ninth and smallest planet, other discoveries about the outer limits of our solar system have been made. First, Pluto lies in the Kuiper Belt, a giant ring of methane, ammonia and water ice chunks. Astronomers also found an object beyond Neptune larger than Pluto, Eris.
However, Eris is not the reason Pluto is not a planet. To be a planet an object has to 1. be spherical, 2. orbit the sun and 3. have enough mass to govern all objects in its orbit with its gravity. Earth is a planet because it meets all requirements. It is spherical, orbits the sun and controls the moon with its overwhelming gravity. Pluto, however, does not. Its mass is not enough to pull at all the many objects in the Kuiper Belt. These objects remain in their own orbits and are not disrupted by Pluto's movement and gravity.
