Pluto prototype
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On August 24, 2006, the International Astronomical Union (IAU) decided to re-classify 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 geological 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:
- is both a dwarf planet; and
- orbits the sun at a greater distance on average than Neptune.
| Name | Pluto | Eris |
|---|---|---|
| Diameter | 2306±20 km | 2400±100 km |
| Mass in kg compared to Earth |
~1.305×1022 kg .0022 |
~1.5×1022 kg (est.) .0025 |
| Mean equatorial radius* in km |
0.180 1,148.07 |
0.19 ~1,200 |
| Volume* |
0.005 |
0.007 |
| Density (in Mg/m³) | 2.0 | |
| Equatorial gravity (in m/s2) | 0.60 | |
| Escape velocity (in km/s) | 1.2 | |
| Rotation period (d) (in sidereal days) |
-6.38718 (retrograde) |
|
| Orbital radius* (AU) semi-major axis in km |
29.66-49.30 39.48168677 5,906,376,200 |
37.77-97.56 67.6681 10,210,000,000 |
| Orbital period*(a) (in sidereal years) |
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) |
119.61° | |
| Mean surface temperature (in K) | 40 | 30 |
| Number of natural satellites | 3 | 1 |
| Date of discovery | February 18, 1930 | October 21, 2003[1] |
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