On 24 August 2006 the International Astronomical Union approved a definition of the term "planet". This definition excludes Pluto, for which a new category "dwarf planet" was designated. This page includes some basic information on this issue, with more to come in near future.

New classification of solar system bodies: The result of the IAU resolutions 5A and 6A, for objects in our own solar system, is as follows:

• "Planet" includes Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
• "Dwarf Planet" includes Ceres (the largest asteroid, orbiting between Mars and Jupiter), Pluto, and Eris (or 2003 UB313, the Pluto-sized trans-Neptunian object orbiting beyond Pluto). In addition, 12 other objects (3 asteroids between Mars and Jupiter, plus 9 trans-Neptunian objects) are listed by the IAU as possible dwarf planets, awaiting further observations.
• "Small Solar-System Bodies" includes everything else orbiting the Sun: the asteroids, the outer solar system objects, and comets.

So Pluto (and at least two other objects) are in an intermediate class: not full-fledged planets, but not small solar system bodies either.

(In my opinion, there are ambiguities in the distinction between "planet" and "dwarf planet" that could become issues with future discoveries, particularly with better observations of extrasolar planetary systems. Also, the distinction between "dwarf planet" and "small solar-system body" is ambiguous, because it involves a threshold of "roundness" which was not specified. But we're probably years from the IAU needing to revisit the issue. In practice, since "dwarf planet" overlaps with other categories of objects--asteroids and TNOs--and since the lower threshold for "dwarf planet" is difficult to define, it's possible the term will not catch on among astronomers.)

The IAU resolutions:

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" 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".

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.

Related information on this web site:

• Frequently asked questions about Pluto and the definition of a planet
• A size comparison for these objects
• Discussion of 2003 UB313, the other Pluto-sized object
• Pictures of Pluto and its large moon Charon
• Pictures of other planets, moons, asteroids, and comets in the solar system
• A basic review of data on important objects of the solar system

And, on a lighter note:

• The resolution that the IAU should have adopted

© 2006 by Wm. Robert Johnston.
Last modified 16 September 2006.
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