Why is Pluto not a planet?
Pluto is not a planet because it has not cleared its orbit of other objects. In 2006 the International Astronomical Union reclassified it as a dwarf planet.
After its discovery by Clyde Tombaugh in 1930, Pluto was counted as the ninth planet for 76 years. Then astronomers began finding many other icy bodies in the Kuiper Belt beyond Neptune. In 2005, Eris was found, and it turned out to be about the same size as Pluto. Scientists had to decide whether to add more planets or define the word more carefully.
In 2006, the International Astronomical Union said a planet must orbit the Sun, be massive enough to pull itself into a round shape and have cleared its orbital neighborhood. Pluto shares its orbit with many Kuiper Belt objects, so it fails the third test and became a dwarf planet.
Still a fascinating world
NASA's New Horizons spacecraft flew past Pluto in 2015 and revealed mountains of water ice, a giant heart-shaped plain of nitrogen ice and a thin blue haze. Pluto has five known moons, the largest of which, Charon, is about half its size. Some scientists still argue for a broader definition of planet.
Did You Know?
Pluto is smaller than Earth's Moon. One year on Pluto, a full orbit around the Sun, lasts about 248 Earth years, so it has not completed a single orbit since it was discovered.