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Thursday, August 24, 2006
Planets
Well, it's official. Pluto is no longer a planet. The IAU voted this to truth earlier today in Prague. The definition of a planet and other things are below.
I'm disappointed, mostly because I was looking forward to the solar system having 12 planets! The original resolution the IAU was presented with would have made Pluto and Charon (Pluto's "moon" which shares Pluto's orbit around the Sun instead of orbiting Pluto itself) double or paired planets, Ceres (the object in the asteroid belt that accounts for one quater the total mass of the asteroid belt - now that's large) a new planet, and 2003 UB313 (aka. Xena, the object larger than Pluto but much further out) a new planet also. But apparently too many astronomers baulked at the idea of letting Pluto stay a planet.
"That plan proved highly unpopular, splitting astronomers into factions and triggering days of sometimes combative debate that led to Pluto's undoing" says CNN.com.
So Pluto is now a dwarf. Charon is nothing special it seems (though really, both are because of their shared orbit). Ceres is still just a really large asteroid. And 2003 UB313 is just a really large asteroid as well, since it lives in what is called the Kuiper belt, which is actually another asteroid belt around the Sun out beyond Neptune.
From the IAU's 2006 result of resolution votes:
RESOLUTIONS
Resolution 5A is the principal definition for the IAU usage of "planet" and related terms.
Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. The IAU will set up a process to name these objects.
IAU Resolution: Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation 'planets'. The word 'planet' originally described 'wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
RESOLUTION 5A
The IAU therefore resolves that "planets" and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A "planet"1 is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects3 except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".
1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
IAU Resolution: Pluto
RESOLUTION 6A
The IAU further resolves:
Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.1
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