What is a Dwarf Planet? - Definition & Facts

Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

Dwarf planets are a new class of stellar objects. They are too small to be planets, but too large to be asteroids. This lesson will cover the definition and five dwarf planets.


The International Astronomical Union (IAU) defines a dwarf planet as a celestial body in direct orbit of the Sun that is massive enough for its shape to be controlled by gravitation but which has not cleared its orbital region of other objects.


The IAU currently recognizes five dwarf planets in the Solar System: Ceres, Pluto, Haumea, Makemake, and Eris. However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition. Eris has been accepted as a dwarf planet because it is more massive than Pluto.

Kuiper Belt

All the dwarf planets besides Ceres are located in the Kuiper Belt beyond Neptune. The Kuiper Belt is an area beyond Neptune composed of countless stellar objects orbiting the sun. Objects at this distance are primarily composed of rocks and ice and are believed to have formed during the formation of the solar system. It is believed that the gravity of Neptune is what caused these objects never to form into a planet.


Ceres is the largest asteroid and only dwarf planet located in the asteroid belt. It has a diameter of 950 kilometers, making it the smallest dwarf planet. The surface of Ceres is a mixture of ice, carbonates and clay. There is some thought that it may contain liquid water under its surface.



Makemake is a dwarf planet in the Kuiper Belt smaller than Pluto. Makemake is the only dwarf planet with no known satellites. Spectral analysis shows its surface is covered with methane, ethane, and possibly nitrogen ices.


Located beyond Neptune's orbit, Haumea is about one-third the size of Pluto. It was discovered in 2004 at the Palomar Observatory.


The most massive known dwarf planet in the Solar System, Eris is more massive than Pluto, with an estimated diameter of 2,326 kilometers. Eris was discovered in January 2005 by the team at the Palomar Observatory. It is the furthest known dwarf planet, with a distance of 96 AU from the sun.

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