The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, brought about by an increase in discoveries of trans-Neptunian objects that rivaled Pluto in size, and finally precipitated by the discovery of an even larger object, Eris. This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, while 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 definition officially adopted by the IAU in 2006 has been both praised and criticized, and remains disputed by some scientists.

The IAU currently recognizes five dwarf planetsdp: Ceres, Pluto, dp: Haumea, dp: Makemake, and dp: 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, but this made the mass of Pluto the cut-off point for trans-Neptunian dwarf planets (plutoids). The IAU subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude less than +1 (and hence a mathematically-delimited minimum diameter of 838 km ) 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 then immediately declared to be dwarf planets.

It is suspected that at least another 40 known objects in the Solar System are dwarf planets, and 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 might be as high as 2000 when objects scattered outside the Kuiper belt are considered. The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed, although if they were detectable they would not be considered planets.

History of the concept

main: 2006 definition of planet Before the discoveries of the early 21st century, astronomers had no strong need for a formal definition of a planet. With the discovery of Pluto in 1930, astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies such as asteroids and comets. For almost 50 years Pluto was thought to be larger than Mercury, but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure the mass of Pluto accurately and it was noticed that actual mass was much smaller than the initial estimates. It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it was one-fifth that of Earth's Moon. Furthermore, having some unusual characteristics such as large orbital eccentricity and a high orbital inclination, it became evident it was a completely different body from any of the other planets.