ADOPTED
FINAL IAU 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: plü-'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 planet1 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 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.
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
1The 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."