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:
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:
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".
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:
And, on a lighter note:
© 2006 by Wm. Robert Johnston.
Last modified 16 September 2006.
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