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Official Craig Research Labs Position Statement on the IAU's Definition of a Planet

On August 24, 2006 the International Astronomical Union (IAU) voted to accept several controversial resolutions, including one that defines a planet as follows:

"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."

The official Craig Research Labs position on this definition is that it is both incomplete and logically flawed, and that it is therefore invalid and should not be recognized by the astronomical community. The definition does nothing to resolve the longstanding uncertainty of what a planet actually is; on the contrary, it confuses the issue even further. We therefore propose the following definition:

"A planetary body or "planet" is any celestial body composed of ordinary matter whose mass is sufficiently great to cause it to be compressed to a spherical shape by the force of gravity, yet not great enough to cause it to burn as a star...regardless of its position, motion, gravitational association(s), and surrounding environment."

This new definition addresses the following fundamental characteristics of planets (several of which have been overlooked by the IAU):

1) Planets are spherical or nearly so.

2) An object is only a planet if its characteristic spherical shape is caused by the force of gravity -- for example, a six inch ball of orange juice floating in microgravity aboard the space shuttle is also round, but it is not a planet because its shape is caused primarily by atomic forces, not gravity.

3) Planets are not stars. Once a celestial body achieves a mass large enough to cause it to burn as a star, it ceases to be a planet. This characteristic sets an upward size limit for planets, just as items 1) and 2) above set a lower size limit.

4) Planets are composed of ordinary matter. If a celestial body is composed of anything else, then it is not a planet. For example, a neutron star is spherical and it does not burn as a star, however, it is composed of exotic matter and is therefore clearly not a planet. Similarly, magnetars, black holes, and other exotic objects are not planets.

5) Planets are planets regardless of their position, motion, gravitational association(s), and surrounding environment. For example, if the Earth were somehow ejected from the solar system and became an interstellar or intergalactic "wanderer", its fundamental nature would not change -- it would still be the Earth and it would not cease to be a planet just because of the changes in its motion and surroundings. Similarly, whether or not a celestial body has "cleared the neighborhood around its orbit" is irrelevant. A planet is still a planet whether or not it is surrounded by other objects, just as an apple is still an apple whether or not it is surrounded by, say, raisins.

Note that under this new definition, some (but not all) moons, comets, and asteroids qualify as planets with certain additional characteristics associated with their position, motion, and in the case of comets (which are generally icy), their composition. In common parlance these objects may still be referred to as moons and comets, but technically they are also planetary bodies (planets). For example, Earth's moon qualifies as a planetary body because it satisfies conditions 1), 2), 3), and 4) above, but it also gains the additional title of "moon" because it is gravitationally bound to the Earth. Likewise, Ceres qualifies as a planetary body, but it gains the additional title of "asteroid" because it is a member of the population of asteroids. We therefore suggest that currently recognized moons, comets, and asteroids continue to be referred to as moons, comets, and asteroids. Note that Pluto qualifies as a planet, and it additionally qualifies as a member of the group of objects we call Kuiper Belt Objects (i.e., comets), and perhaps one day it will be perturbed from its present orbit, fall in towards the sun, and grow a tail. However, since it is not yet in an elliptical cometary orbit and therefore is not a "true" comet, we suggest that it continue to be called what it is...a planet.

Note also that under the IAU's (flawed) definition of a planet, Neptune does not qualify as a planet because it has not cleared the neighborhood around its orbit since Pluto crosses its orbit!