Is Pluto a planet ?
Pluto's status is the subject of extensive debate among planetary scientists. Upon its discovery, Pluto was initially classified as the ninth planet of the Solar system. However in 2006, the International Astronomical Union (IAU) issued a definition for planets in the Solar system that resulted in the reclassification of Pluto as a "dwarf planet".

According to this definition, a planet is any celestial body that :
- orbits the Sun,
- has enough mass to assume a hydrostatic equilibrium (nearly round) shape,
- cleared away any other similarly-sized objects near its orbit around the Sun.

The last criteria hasn't been met by Pluto.