The International Astronomical Union (IAU) defines a dwarf planet as a celestial body that:

• Orbits the sun.
• Contains enough mass to assume a nearly round shape.
• Is not a moon.
• Has not cleared the neighborhood around its orbit.

The main distinction between a planet and a dwarf planet, is that unlike a planet, a dwarf planet has not cleared its orbital region of other objects. Dwarf planets tend to orbit regions of the solar system where other objects can cross their paths, such as the Main Asteroid Belt and the Kuiper Belt.

The term dwarf planet was adopted in 2006 and was brought about by an increase in discoveries of trans-Neptunian objects, including Eris, which was even more massive than Pluto.

Scientists estimate that there are hundreds to thousands of dwarf planets in the solar system. The IAU currently recognizes five: Ceres, Pluto, Haumea, Makemake, and Eris.

The IAU recognized Pluto's special place in our solar system by designating dwarf planets that orbit the sun beyond Neptune as plutoids.