Pluto is a dwarf planet because it is not heavy enough to clear other space rocks out of its path.

In August 2006, the International Astronomical Union (IAU) created three rules for what makes a planet. A planet must orbit the Sun and be round. It must also be strong enough to pull in or push away other objects near its orbit.Pluto meets the first two rules but fails the third. Its mass is only 0.07 times the mass of the other objects in its path. In contrast, Earth is 1.7 million times heavier than the debris in its orbit.Pluto sits in the Kuiper Belt. This is a large area beyond Neptune filled with frozen objects left over from when the solar system formed. Astronomer Clyde Tombaugh first found Pluto in 1930 at the Lowell Observatory.NASA sent the New Horizons spacecraft to fly past Pluto in 2015. The mission showed that Pluto has mountains made of water ice and a large glacier shaped like a heart. Pluto is smaller than Earth's moon and has five of its own moons.