Dwarf Planets

Pluto, Ceres, Eris and other round worlds that are not quite planets.

Pluto with its bright, heart-shaped icy plain.
New Horizons Very Best View of Pluto. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Source, NASA media guidelines.

In 2006 the International Astronomical Union defined what a planet is. A planet must orbit the Sun, be massive enough to be nearly round, and have cleared the neighbourhood around its orbit of other objects.

A dwarf planet meets the first two conditions, but not the third. Pluto shares its region with many other icy bodies of the Kuiper Belt, so it was reclassified as a dwarf planet.

Known dwarf planets

Ceres
The largest object in the asteroid belt, visited by the Dawn spacecraft.
Pluto
Visited by New Horizons in July 2015. The image above shows its bright, heart-shaped plain of nitrogen ice.
Eris
A distant world about the size of Pluto, whose discovery started the debate about the definition of a planet.
Haumea
An elongated dwarf planet that spins once every four hours.
Makemake
A bright, reddish dwarf planet in the Kuiper Belt.