Search the Dictionary:

dwarf planet
Artist's concept of the dwarf planet Eris, named after the Greek goddess of discord and strife. Image credit: NASA/JPL-Caltech.

dwarf planet:

Dwarf planet is a new class of astronomical objects. It was created in 2006 by the International Astronomical Union so that astronomers could differentiate it from the word 'planet'. The discovery of objects in the outer solar system which were larger than or about the same size as Pluto was the reason for this decision. (This distant region is called the Kuiper Belt and extends far beyond the orbit of Neptune.) Finally the IAA voted that to be a planet in our solar system, an object must be in orbit around the Sun, have enough mass so that it has become round in shape due to its own gravity, and have cleared out all the other debris in its orbital path around the Sun (so there are not objects similar to itself at roughly the same distance from the Sun).

Dwarf planets were declared to be the class of objects which met the first 2 definitions, but failed the third. It was that definition that made it clear to astronomers that planets and dwarf planets are not the same. Pluto, Ceres (pictured here), and Eris became the first three members of this dwarf planet classification, and many others are expected to follow. In fact, there are about 70 other known objects which may be labelled dwarf planets in the future. More than a hundred other dwarf planets may still be so far undetected in the Kuiper Belt.