Pluto the dwarf

Since its discovery in 1930, Pluto had been considered the ninth planet in our solar system. However, more recent discoveries of dwarf planets larger in size and mass than Pluto have made some astronomers question its status. 

In 2006, the International Astronomical Union decided upon a conclusive definition of what constituted a planet. Pluto's low mass - not even a fifth of the mass of the moon - excluded it from that definition. Now Pluto is considered a dwarf planet.