American astronomer Clyde Tombaugh discovered Pluto in 1930. However in 2006, the definition of a planet was changed.
According to new rules adopted by the International Astronomical Union, a celestial body must meet the following criteria in order to qualify as a planet:
1. A planet must be round.
2. A planet must orbit the sun.
3. A planet must have "cleared the neighborhood" of its orbit. This means that as a planet travels, its gravity sweeps and clears the space around it of other objects. Some of the objects may crash into the planet, others may become moons.
Pluto follows the first two rules: It is round, and it orbits the sun. It does not, however, follow the third rule. It has not yet cleared the neighborhood of its orbit in space. Because it does not follow this third rule, Pluto is no longer considered a planet. It is called a dwarf planet.