IAU Definition of Planet - Final Definition

Final Definition

The final definition, as passed on 24 August 2006 under the Resolution 5A of the 26th General Assembly is:

“

The IAU...resolves that planets and other bodies, except satellites, in the Solar System be defined into three distinct categories in the following way:

(1) A 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, 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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

Footnotes:

The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

 ”

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.

Footnote:

An IAU process will be established to select a name for this category.

 ”

The IAU also resolved that "planets and dwarf planets are two distinct classes of objects", meaning that dwarf planets, despite their name, would not be considered planets.

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