Why did Pluto stop being a planet in 2006?

Pluto stopped being a planet because in August 2006 the International Astronomical Union (IAU) voted for a definition that it does not meet. Under that definition, a planet orbits the Sun, is massive enough for its own gravity to make it round, and has cleared its neighbourhood of smaller objects. Pluto fulfils the first two, but not the third, so it became a dwarf planet.

Pluto's story started with a hero's welcome. Clyde Tombaugh discovered it in 1930, and it was immediately hailed as the ninth planet. Its name was suggested by an eleven-year-old schoolgirl in Oxford, Venetia Burney, who was interested in classical mythology.

Then it kept shrinking. In 1978 the discovery of its moon Charon made it possible to measure its mass, and Pluto turned out to be only one-sixth the mass of the Moon. From 1992, astronomers found many icy bodies beyond Neptune similar to Pluto. When Mike Brown's team announced Eris in 2005, an object more massive than Pluto, astronomers had to decide: add a tenth planet, or define the word.

On 24 August 2006, at a meeting in Prague, the IAU voted on a final draft. It created a new class, the dwarf planet, and placed Pluto in it along with Ceres and Eris. The vote passed with 424 astronomers taking part, and the decision is still argued over: some astronomers reject it, and New Mexico's House of Representatives declared Pluto a planet in its own skies.