A "Dwarf Planet" is a relatively new vocabulary word on the world stage, formally introduced in August of 2006 when the International Astronomical Union (IAU) created it. It was created at the same time that the IAU formally defined - for the first time in history - what a "planet" is.
A "dwarf planet" is a celestial body that
- is in orbit around the Sun,
- has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
- has not cleared the neighborhood around its orbit, and
- is not a satellite.
The IAU also decided that it would set up a panel to decide whether or not "borderline objects" would be considered a dwarf planet, planet, or something else. Within a month of when the Resolution was passed, Ceres, the largest (former) asteroid, was promoted to a dwarf planet; Pluto, the smallest (former) planet was demoted to a dwarf planet; and Eris, an object that had been in "limbo," was classified as a dwarf planet. These form the original three dwarf planets.
On June 11, 2008, the IAU Executive Committee chose to name solar system objects that are like Pluto to be called "plutoids." A plutoid is a dwarf planet, but it has the additional property that its semi-major axis is larger than that of Neptune's, meaning that it spends at least part of its orbit beyond Neptune. moons of plutoids are not considered plutoids.
All known/named dwarf planets are plutoids except for Ceres.
The following table lists data for Pluto and the other dwarf planets for comparison purposes. Data is compiled from NASA's planetary factsheet for Pluto, and from primary sources for the other dwarf planets; see the individual pages on the dwarf planets for references.
|Perihelion (109 km)||0.381||4.436||5.260||5.761||5.65|
|Semi-Major Axis (106 km)||0.415||5.906||6.484||6.850||10.12|
|Aphelion (106 km)||0.449||7.376||7.708||7.940||14.60|
|Average Orbital Velocity (km/s)||10.587°||17.16°||4.484||4.419||3.436|
|Orbital Inclination (from Earth's Orbit)||10.587°||17.16°||28.19°||28.96°||44.187°|
|Equatorial Radius (km)||975||1195||~996 x 1518 x 1960||~2000||1300+200−100|
|Polar Radius (km)||909||1195||~2000||1300+200−100|
|Mass (1021 kg)||0.95||12.5||4.2±0.1||~40||16.7±0.2|
|Density (water=1 g/cm3)||2.08||1.75||2.6-3.3||~2||2.3±0.3|
|Sidereal Rotation Period (hours)||9.074||153.2820||3.915||?||~25.9|
|Sidereal Orbital Period (days)||1679.819||90,588||104,234||113,183||203,600|
|Apparent Magnitude||+6.7 to +9.3||+15.1||+16.7||+17.3||+18.7|
|Number of Moons||0||3||2||0||1|
|Discoverer||G. Piazzi||C. Tombaugh||M.E. Brown, C.A. Trujillo, & D.L. Rabinowitz||M.E. Brown, C.A. Trujillo, & D.L. Rabinowitz||M.E. Brown, C.A. Trujillo, & D.L. Rabinowitz|
|Discovery Date||Jan. 1, 1801||Feb. 18, 1930||Dec. 28, 2004||Mar. 31, 2005||Oct. 21, 2003|