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Plutoid Sub-Class


Planet: Current Definition - A planet is an object that (1) orbits the Sun; (2) is large enough to assume a spherical shape due to its own gravity; and (3) has cleared its orbit.

Dwarf Planet: Fits all planet criteria other than (3). Is not a satellite.

Plutoid: A class of dwarf planets that orbits beyond Neptune.

Semi-Major Axis: In an ellipse, the semi-major axis is half of the longest axis of the ellipse.

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Dwarf Planets


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.

In the same Resolution B5 that defined a "planet" (see "What's a Planet" on the main Planets page for more information), the IAU defined a "dwarf planet" as:

A "dwarf planet" is a celestial body that

  1. is in orbit around the Sun,
  2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
  3. has not cleared the neighborhood around its orbit, and
  4. 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.

Since August 2006, two additional objects, Makemake and Haumea, have been classified as dwarf planets.

Plutoid Sub-Class

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.

Ceres Pluto Haumea Makemake Eris
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°
Orbital Eccentricity 0.080 0.2488 0.18874 0.159 0.44177
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
Absolute Magnitude +3.36±0.02 -0.7 -0.48 +0.17 -1.15
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



IAU Press Release. "News Release - IAU0603: IAU 2006 General Assembly: Result of the IAU Resolution Votes." August 24, 2006.

IAU Press Release. "News Release - IAU0605: IAU names dwarf planet Eris." September 14, 2006.

IAU Press Release. "News Release - IAU0804: Plutoid Chosen as Name for Solar System Objects Like Pluto." June 11, 2008.

IAU Press Release. "News Release - IAU0806: Fourth dwarf planet named Makemake." July 19, 2008.

IAU Press Release. "News Release - IAU0807: IAU names fifth dwarf planet Haumea." September 17, 2008.

IAU Resolution. "IAU Resolution B5: Definition of a Planet in the Solar System." August 24, 2006. [Note - Link is a PDF file.]