- is in orbit around the Sun
- has sufficient mass to possess a spheroid-shape
- has not "cleared the neighbourhood" around its orbit.
Dwarf planets are not 'true' planet only because of the last criterion, which says that a 'true' planet has "cleared the neighbourhood" in its orbital zone, meaning that there exist no objects of substantial mass other than natural satellites near its orbit.
Frequently Asked Questionsedit
Why isn't Pluto a planet?edit
Pluto was excluded from the IAU definition of a planet due to a redefinition of the criteria for being a planet in 2006. This attracted some controversy after it defined the solar system to have 8 planets instead of 9, however, it is now generally accepted that Pluto is a dwarf planet.