IAU Resolution: Definition of a Planet
Contemporary observations are changing our understanding of
planetary systems, and it is important that our nomenclature for
objects reflects 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. (Here we are
not concerned with the upper boundary between 'planet' and 'star',
nor the distinction between stars and brown dwarfs).
RESOLUTION 5 (draft c)
The IAU therefore resolves that planets and other bodies in the planetary
systems be defined in the following way:
(1) A planet is a celestial body that (a) has sufficient mass for its self-
gravity to overcome rigid body forces so that is assumes a hydrostatic
equilibrium (nearly round) shape[1], (b) is the dominant object in its local
population zone and (c) is in orbit around the Sun.
(2) A dwarf-planet is a celestial body that (a) has sufficient mass for its
self-
gravity to overcome rigid body forces so that is assumes a hydrostatic
equilibrium (nearly round) shape[1], and (b) is in orbit around the Sun.
(3) All other objects orbiting the Sun, including most of the Solar System
asteroids, near-Earth objects (NEOs), Mars-, Jupiter-, Neptune-Trojan
asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets,
shall be referred to collectively as "Small Solar System Bodies". In the new
nomenclature the term "minor planet" is not used.
[1] An AIU process will be established to assign borderline objects into
either dwarf-planet and other categories.
RESOLUTION 6 (draft c)
The IAU further resolves:
Pluto is a dwarf planet by the above definition, as are one or
more recently discovered large trans-Neptunian objects. Dwarf-planets
that have orbital periods in excess of 200 years are designated this category
of planetary objects, of which Pluto is the prototype, as a new class
that we call "plutonids".