Dwarf Planets
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:
- The body orbits the Sun.
- The body has enough mass that it has gravitationally collapsed into a
spherical shape.
- 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.
|
| Perihelion (106 km) |
380.612 |
4436.82 |
5650 |
| Mean Orbital Distance (106 km) |
413.715 |
5906.38 |
10,120 |
| Aphelion (106 km) |
446.818 |
7375.93 |
14,600 |
| 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 |
| Volume (1010 km3) |
0.20 |
0.715 |
~5.8 |
| 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 |
? |
| Surface Pressure (bars) |
0.27 |
0.6 |
? |
| Length of Day (hours) |
9.074 |
153.2820 |
> 8? |
| Tropical Orbital Period
(days) |
1679.819 |
90,588 |
203,500 |
| Albedo |
0.113 |
0.4-0.6 |
0.86±0.07 |
| 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 |
|
