- 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.
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.