On August 24, 2006, the International Astronomical Union voted that Pluto should no longer be considered as one of the solar system’s nine planets. This happened following the discovery of a tenth planet beyond Pluto. The small size of this tenth planet led to a debate about what defines a planet. Eventually, the IAU, meeting in Prague in the Czech Republic, came up with three criteria that must be met before an object can be classified as a planet: 1)it must orbit the sun, 2)it must be massive enough that its own gravity pulls it into nearly round shape, and 3)it must be dominant enough to clear away other objects in its neighborhood. Pluto and other dwarf planets only fulfill the first two of these requirements. This meant that the newly discovered planet, now officially named Eris, after the Greek goddess of strife, was a dwarf planet and not a real planet, but it also meant that Pluto could no longer be classified as a planet.

Since then, scientists have classified about 12 potential dwarf planets and three official ones: Pluto, Eris, and one, named Ceres after the Roman goddess of agriculture, with an orbit in between Mars and Jupiter. Ceres had originally been named as a planet when it was discovered in 1801, but was later demoted to asteroid status.

Most other potential dwarf planets are in orbit beyond Pluto, and some astronomers estimate that the number of dwarf planets could eventually reach somewhere around 200, most of which are found in a debris field beyond Neptune known as the Kuiper belt. One, named Sedna, is the most distant known object in our solar system.