The Final IAU Resolution on the definition of "planet" ready for voting (Forwarded)

While it makes sense to remove Pluto from the list of planets and group
it with the Kuiper Belt objects, I don't understand the other parts of
the resolutions.

Why invent a new class of "dwarf planets" when we already have a "minor
planets" category? Who cares if an object is round or not? Why
separate Ceres from the other asteroids? Surely it would make more
sense to classify objects by composition, as we have previously done
for the asteroids with the classes C, S, M, and so on. It would have
been simpler and less disruptive to introduce a new class, say "I", for
asteroidlike bodies largely made of ice. That should take care of the
Kuiper Belt.

Even more puzzling is the replacement of the term "minor planets" with
"small solar system bodies", which says the same thing is a less
convenient form.

--- Brian

Andrew Yee wrote:
International Astronomical Union
Prague, Czech Republic

24 August 2006

The Final IAU Resolution on the definition of "planet" ready for voting


....

RESOLUTIONS

Resolution 5A is the principal definition for the IAU usage of "planet" and
related terms. Resolution 5B adds the word "classical" to the collective
name of the eight planets Mercury through Neptune.

Resolution 6A creates for IAU usage a new class of objects, for which Pluto
is the prototype. Resolution 6B introduces the name "plutonian objects" for
this class. The Merriam-Webster dictionary defines "plutonian" as:
Main Entry: plu.to.ni.an
Pronunciation: plu-'tO-nE-&n
Function: adjective
Usage: often capitalized
: of, relating to, or characteristic of Pluto or the lower world

After having received inputs from many sides -- especially the geological
community -- the term "Pluton" is no longer being considered.

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) 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".

[Footnotes]

[1] The eight planets a 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 planet [1] is a celestial body ...

and

[Footnote]

[1] The eight classical planets a 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."