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".


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.