Many people will remember the controversy generated back in 2006 when the reclassification of Pluto took place. Pluto’s place as a planet at this point had been questioned since the Seventies. Other objects had been found in the Solar System of a comparable size and these objects weren’t classified as planets.
In August 2006, the International Astronomical Union (IAU) met and redefined exactly what ‘planet’ meant. It was decided that Pluto did not fit within that definition and so it was added to the new classification of dwarf planet. To qualify as a planet an object must orbit the Sun, have sufficient mass to become roughly spherical and to have ‘cleared the neighbourhood’ around its orbit. Pluto passes the first two criteria but because it crosses the orbit of Neptune it is not deemed to have cleared its neighbourhood.
Pluto is not alone in the dwarf planet category, joining four others. Of these five objects, Pluto is only the second largest with Eris just beating it to the top spot. For a few months after its discovery in 2005 Eris was referred to as the tenth planet of the Solar System, however, this moniker was promptly dropped after the IAU meeting in 2006.