On August 24, 2006, the International Astronomical Union (IAU) decided to reclassify Pluto as a dwarf planet, requiring that a planet must "clear the neighbourhood around its orbit." The General Assembly of the IAU further resolved to recognize Pluto as the prototype of a new (as yet unnamed) category of trans-Neptunian objects which the IAU planned on naming a "pluton" or a "plutonian object". The former was rejected, in part because "pluton" is actually a pre-existing geology term, and many geological experts wrote in complaints pointing this out, and the latter was rejected as well due to the pre-existing definition of "Plutonian". Many astronomers are continuing to use the term "ice dwarf" to describe such objects, following the precedent of calling the four largest planets "gas giants".

A Pluto-prototype trans-Neptunian object is one which:

Plutonian Dwarf Planets
Name Pluto Eris
Diameter 2306±20 km 2400±100 km
Mass in kg
compared to Earth
~1.305×1022 kg
~1.5×1022 kg (est.)
Mean equatorial radius*
in km
Density (in Mg/m 2.0
Equatorial gravity (in m/s2) 0.60
Escape velocity (in km/s) 1.2
Rotation period (d)
(in sidereal days)
Orbital radius
  • (AU)
    semi-major axis
    in km
Orbital period 248.09 557
Mean orbital speed
(in km/s)
4.7490 3.436
Orbital Eccentricity 0.24880766 0.44177
Orbital inclination 17.14175° 44.187°
Inclination of the equator from the orbit
(see Axial tilt)
Mean surface temperature (in K) 40 30
Number of natural satellites 3 1
Date of Discovery February 18, 1930 October 21, 2003[1]


  1. Discovery Circumstances: Numbered Minor Planets

Template:Footer TransNeptunian

 The Solar System v·d·e 
Solar System XXVII
The Sun · Mercury · Venus · Earth · Mars · Ceres* · Jupiter · Saturn · Uranus · Neptune · Pluto* · Eris* · Sedna*
Planets · Dwarf planets · Moons: Terran · Martian · Asteroidal · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian
Pluto' * Ceres * Eris * Haumea * Makemake
Small bodies:   Meteoroids · Asteroids (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
planets with '*' are dwarf's but listed between 'real planets'.
See also astronomical objects and the solar system's list of objects, sorted by radius or mass.
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