Dwarf planet

Explanation

A dwarf planet is an object that is not a satellite and by the IAU definition:

• 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' planets 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.

FAQs

Why isn't Pluto a planet?

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 generally accepted now that Pluto is a dwarf planet.