[extropy-chat] Solar System Planets - Final vote

[Amara Graps]

Dear Extropes,

There needed to be a separate category for Ceres and Pluto, it
seems, so in the final vote of the International Astronomical Union,
today, Ceres, Pluto and Charon were thrown out of the basic
planet category, and now we have one less.

(Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune)


Amara
Tallinn

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http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html

The IAU members gathered at the 2006 General Assembly agreed that a
"planet" is defined as 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.

This means that the Solar System consists of eight "planets" Mercury,
Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct
class of objects called "dwarf planets" was also decided. It was agreed
that "planets" and "dwarf planets" are two distinct classes of objects.
The first members of the "dwarf planet" category are Ceres, Pluto and
2003 UB313 (temporary name). More "dwarf planets" are expected to be
announced by the IAU in the coming months and years. Currently a dozen
candidate "dwarf planets" are listed on IAU's "dwarf planet" watchlist,
which keeps changing as new objects are found and the physics of the
existing candidates becomes better known.

The "dwarf planet" Pluto is recognised as an important proto-type of a
new class of trans-Neptunian objects. The IAU will set up a process to
name these objects.

Below are the planet definition Resolutions that were passed.


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


-- 

Amara Graps, PhD      www.amara.com
Istituto di Fisica dello Spazio Interplanetario (IFSI), Roma, ITALIA
Associate Research Scientist, Planetary Science Institute, Tucson