What is a Dwarf Planet?
From the time it was discovered in 1930 until 1978, Pluto was accepted by scientists as the ninth planet in our solar system. However, with the discovery of Pluto's moon Charon in 1978, new light was shed on characterisitcs Pluto was previously assumed to possess. More specifically, as a result of calculations related to Charon's orbit, Pluto was found to be much smaller than had been thought. Prior to this, Pluto was believed to be more massive than the planet Merucry; afterward, it was determined that Pluto was, in fact, only one twenty-fifth the mass of Mercury. Although this new knowledge cast doubt on Pluto's status as a planet, the scientific community continued to accept Pluto as a planet for lack of a better classification.
Beginning in the 1990s and continuing into the 2000s, new bodies similar in size and orbit to Pluto were found in the outer solar system. At this point it became clear to the scientific community that there were two options in categorizing these newly discovered objects: 1) if Pluto remained a planet, then these new objects would have to be called planets also, or 2) Pluto and the recently identified bodies would have to be placed into a new category yet to be officially defined.
The term "dwarf planet" was introduced in 2006 by the International Astronomical Union (IAU)―the official organization in charge of naming celestial objects―as a category for those celestial objects similar to Pluto that shared certain characteristics with the planets, yet differed significantly from the planets in other ways. The definition of a dwarf planet as set forth by the IAU is a celestial body that satisfies the following critieria:
- orbits the Sun,
- possesses mass sufficient for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
- has not cleared the neighbourhood around its orbit, and
- is not a natural satellite [moon]
Currently, there are five known dwarf planets: Pluto, Ceres, Haumea, Makemake, and Eris.