Planet definiton last draft

So the last week has been spent by a few astronomers - by no means the majority as they were busy with things that are arguably a lot more interesting such as exciting science - debating the proposed definition of a planet. There have been raised voices and angry sentiments from some, whilst the later meetings were more stricly chaired. The results are that the proposals have been modified (PDF 883kB) from those that the media pre-empted as 'final decisions' last week.

The new proposal for Resolution 5a is:

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[note 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) shape[note 2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects[note 3] orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.

Note 1: The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Note 2: An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
Note 3: These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

There is also a Resolution 5b:

Insert the word “classical” before the word “planet” in Resolution 5A, Section (1), and footnote 1. Thus reading:
(1) A classical planet (the eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.) is a celestial body . . .

...and a 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.

...and a Resolution 6b which then adds:

This category is to be called “plutonian objects.”

to the end of Resolution 6a. So, does all that make sense? We now have four parts of resolutions that will be voted on tomorrow afternoon (24th August 2006). The word "pluton" has been replaced with "plutonian objects" as that is not overusing the word and Pluto could be demoted as a planet.

These all go to a vote by astronomers and if you are around you can watch it online from 14:00 CET onwards. But, please remember folks that there is other stuff happening.

Posted in astro blog by Stuart on Wednesday 23rd Aug 2006 (22:55 BST) | Permalink
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