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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!
8/25/2006
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