Why Pluto Is No Longer a Planet

When Pluto was discovered in 1930, it was immediately celebrated as the ninth planet of the solar system. For more than seventy years, students learned that there were nine planets. However, astronomers knew very little about Pluto at first, and they had greatly overestimated its size.

Beginning in the 1990s, astronomers discovered many icy objects in the same distant region of space, known as the Kuiper Belt. In 2005, they found one, later named Eris, that appeared to be about as large as Pluto and turned out to have even more mass. This raised a difficult question: if Pluto was a planet, should Eris and other similar objects be called planets too?

In 2006, the International Astronomical Union answered this question by creating an official definition of a planet. A planet must orbit the Sun, be large enough for its own gravity to make it round, and have cleared other objects from the area around its orbit. Pluto meets the first two conditions but not the third, so it was reclassified as a dwarf planet. The decision remains controversial, and some scientists continue to argue that Pluto should be called a planet.