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Dwarf Planets

Overview

On August 24, 2006, the International Astronomical Union (IAU) voted to designate a new class of planets - the "Dwarf Planets," of which Pluto was the prototype. At this point in time, there is no official set definition of what a dwarf planet is, other than it does not meet the third criterion for planetary status:

  1. The body orbits the Sun.
  2. The body has enough mass that it has gravitationally collapsed into a spherical shape.
  3. The body has not cleared its orbit, asserting orbital dominance.

The not in the third part is key to what a dwarf planet is, and it is why Pluto is considered to be a dwarf planet, since it has not cleared its path of Neptune. Two other bodies were granted dwarf planet status on that day -- Ceres, the (formerly) largest asteroid, and Eris, the so-far largest-known body outside of Neptune's orbit.

In time, more small solar system bodies may be granted dwarf planet status, though since there is no quantitative definition, this category is likely to remain a subject of debate.

Data for the Dwarf Planets

Ceres Pluto Eris
Mean Orbital Distance (106 km)
413.715
5906.38
10,120,000
Average Orbital Velocity (km/s)
17.882
4.72
3.436
Orbital Inclination (from Earth's Orbit)
10.587°
17.16°
44.187°
Orbital Eccentricity
0.080
0.2488
0.44177
Equatorial Radius (km)
975
1195
2400 ± 100
Polar Radius (km)
909
1195
2400 ± 100
Mass (1024 kg)
0.00095
0.0125
?
Density (water=1)
2.08
1.75
?
Escape Velocity (km/s)
0.51
1.1
?
Gravity (m/s2)
0.27
0.6
?
Length of Day (hours)
9.074
153.2820
> 8?
Tropical Orbital Period (days)
1679.819
90,588
203,500
Average Surface Temperature (Celsius)
-106°
-225°
~-240°
Number of Moons
Rings?
No
No
No
Absolute Magnitude
3.34
-1.12±0.01
Discoverer
Guiseppe Piazzi
Clyde Tombaugh
Mike E. Brown, C.A. Trujillo, and D.L. Rabinowitz
Discovery Date
January 1, 1801
February 18, 1930
October 21, 2003

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