The Votes Are In, Pluto Is Out
It is a momentous day, August 24, 2006. Following a week of heated debate at the IAU General Assembly in Prague, the results of the voting during today's closing ceremonies are known.
The designation of Pluto has changed from "planet" to "dwarf planet," as defined in item 2 of Resolution 5A, below. Pluto is joined in this designation by Ceres and 2003 UB313. Charon will not receive this "dwarf planet" designation, remaining simply a satellite of Pluto. The current solar system planetary count is a follows: 8 planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and 3 dwarf planets (Ceres, Pluto, and 2003 UB313).
Please note that Pluto did not loose its "planet" designation because of its size, but because it did not fulfill its "planetary housekeeping" obligation of clearing out the neighborhood of its orbit. Since Pluto crosses the orbit of Neptune, it is evident that Pluto was not able to steak a claim for itself by gobbling up all of the adjacent matter and become the regional king of the hill, as the now eight solar system planets were able to do during their planetary formation.
For a look at the final draft of these resolutions, follow this link:
To view the full new release on results of the voting, follow this link:
Below is the text of the related IAU Resolutions 5A and 6A:
I should note that a proposed Resolution 5B recommend that the eight planets be described as “classical” planet, and proposed Resolution 6B recommended that the new class of Pluto-type bodies be called “plutonian objects.” However, neither of these resolutions was accepted during this meeting. The matter regarding Resolution 6B will be considered further by the IAU.
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.
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"(see footnote 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 neighborhood 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) shape(see footnote 2) , (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.
(3) All other objects(see footnote 3) except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".
Footnote 1: The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Footnote 2: An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
Footnote 3: These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
IAU Resolution: Pluto
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.