A dwarf planet is defined by the International Astronomical Union (IAU) as a celestial body which orbits of the Sun, that is massive enough for its shape to be controlled by gravitation, but that unlike a planet has not cleared its orbital region of other objects. The term dwarf planet was accepted in 2006 as part of a three way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of objects beyond Neptune which rivaled Pluto in size and finally precipitated by the discovery of an even more massive object, Eris. This classification states that bodies large enough to have cleared their orbit of debris are defined as planets, whereas those that are not massive enough to be rounded by their own gravity are defined as small Solar System bodies.

 

Dwarf planets come in between. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it was said to be the "right decision" by Mike Brown, who discovered Eris and other new dwarf planets, but has been rejected by Alan Stern, who had coined the term dwarf planet in 1990.It is estimated that there are hundreds to thousands of dwarf planets in the Solar System. The IAU currently recognizes five: Ceres, Pluto, Haumea, Makemake, and Eris. There are five known objects in the solar system which fit the IAU definition of dwarf planet, but it is suspected that another hundred or so objects in the Solar System are dwarf planets. Estimates are that up to 200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered.

 

Individual astronomers recognize several of these, and in August 2011 Mike Brown published a list of 390 candidate objects, ranging from "nearly certain" to "possible" dwarf planets. Brown currently identifies eleven known objects – the five accepted by the IAU plus 2007 OR10, Quaoar, Sedna, Orcus, 2002 MS4 and Salacia – as "virtually certain", with another dozen highly likely. Stern states that there are more than a dozen known dwarf planets.However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they actually fit the IAU's definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto. They subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 are to be named under the assumption that they are dwarf planets.

 

The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets. On 22 January 2014, ESA scientists reported the detection of water vapor on Ceres, the largest object and the only dwarf planet in the asteroid belt between Mars and Jupiter. The detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes".