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Instead of one vote on a definition for the term “planet,” as has been expected, it seems the International Astronomical Union will ask members to vote on multiple proposals today.
In the scheme, astronomers would have options for layering the definition and getting specific about the status of Pluto. The thrust of it aims to demote Pluto by terming it a “dwarf” that is incapable of clearing out significant space in its region of space. [Overview of the arguments to date]
On the web site for the IAU’s meeting in Prague (thanks for the tip to a reader, George, who spotted it on OrbitSimulator.com) comes this (which I pulled from the IAU site):
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There will be separate sequential votes on Resolution 5A and Resolution 5B.
Similarly, there will be separate votes on Resolutions 6A and 6B.
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) shape [2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects [3] orbiting the Sun shall be referred to collectively as “Small Solar System Bodies”.
[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
[2] An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
Resolution 5B
Insert the word “classical” before the word “planet” in Resolution 5A, Section (1), and footnote 1. Thus reading:
(1) A classical planet1 is a celestial body . . .
and
[1] The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
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.
Resolution 6B
The following sentence is added to Resolution 6A: This category is to be called “plutonian objects.”
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Arguments for and against the proposals (by unnamed authors) are at the IAU web site here. Click on “Dissertatio cum Nuncio Sidereo III” button in the upper right, then scroll down the pdf to the last page.
A bunch of “no” votes would, of course, mean Pluto’s status would not change. If all this sounds like Greek to you, see the most recent story that summarized this whole issue and has the timing of today’s vote.
August 24th, 2006 at 4:18 am
this is unbelievable the astronomical community are making themselves look increadibly foolish . at the end of the day it ll be down to public perception of what a planet is . in the public mind its simple its a spherical body that orbits the sun . it does nt matter what the orbit is or the size . its just a different sort of planet . why stop here in demoting planets the gas giants and completly different to mercury mars venus and the earth . an absolute farce . theres a lot of …too much intelligence and not enough common sence going on here .
August 24th, 2006 at 10:16 am
1 (c) has cleared the neighbourhood around its orbit:
Does this means that Neptune needs to be demoted since Plute and Neptune swap orbits. Basically Pluto and Neptune swaps neighbourhoods.
August 24th, 2006 at 8:58 pm
[...] Poor Pluto just got demoted! Today’s vote by the International Astronomers Union has just redefined what a planet is and under the new guidelines, poor Pluto gets the boot. It will now be known as a “dwarf planet”. The ruling also reverses an earlier proposal that would have kept Pluto and added Ceres, Pluto’s moon Charon and Xena to the list. Blogger Phil Plait, who unlike me is actually an astronomer, was unsatisfied about that one, and I was too, but not for entirely the same reasons. I agree that adding lots of arbitrary definitions and exceptions to a definition confuses the matter greatly. So I figure we should simplify. [...]
August 25th, 2006 at 11:24 pm
I accept IAU’s latest definition of planet if it includes the following modification: any circular body approximately 2400 km in diameter with a rotational period of 6.4 days with an orbital inclination of 17.2 degrees and eccentricity of 0.25 that crosses the orbit of Neptune.