Of Stars, Pluto and Planetary Classification

The "Planet" Problem:

The IAU has come up with a planetary classification scheme that is
*not* based solely upon the physical nature of the objects being
classified. The orbital details of an otherwise qualified body can
exclude it from the "planet" classification.

This means a body can be planet if located in one orbit (and
gravitationally bound to one star) while an identical body would fail
to be classified as a planet if located in a different orbit (or
gravitationally bound to a different star).

Furthermore, the IAU classification scheme cannot be applied to bodies
orbiting stars other than the Sun. This ignores the vast majority of
potential planetary bodies.

About Stars:

Stars remain stars regardless of where they are. A star in orbit
about another star is still a star. This is the case regardless of
anything occupying a nearby orbit. This is the case even if other
stars occupy the same orbit. Even a star found between two widely
separated galaxies would still be a star.

Concerning Pluto:

Current events have placed Pluto on center stage. Either Pluto is a
planet or Pluto is not a planet. Some favor one outcome. Some favor
the other outcome. A planetary classification scheme should *not* be
doctored to specifically include nor to specifically exclude Pluto as
a planet. The classification scheme needs to be more objective.

Concerning the Number of Planets:

A planetary definition should not be judged on how many or how few
planets our solar system contains. Four planets or four hundred
planets -- it makes no difference so long as all qualified bodies are
included and all unqualified bodies are excluded.

Properties of a Good Planetary Definition:

1) It should be based solely upon one or more (preferably easily
determined) physical property of the candidate body.

2) It should permit all qualifying bodies to remain planets
regardless of where they are and regardless of where nature or future
technology might move them (with due allowances for boiling off mass,
etc.)

3) The cut-off between planet and non-planet should not be arbitrary.
It should be based upon a real, naturally occurring, physical
transition.

4) It should be applicable to our own solar system as well as to
extra-solar planetary systems.

Further Thoughts on a Planetary Definition or Planetary Classification
Scheme:

In light of the above points, the upper cut-off point could be related
to thermonuclear fusion. We wouldn't want to include stars as
planets! Optionally, (and recommended) a transition classification
could also be included.

The lower cut-off point could be related to the mass associated with
the transition between reasonably spherical and reasonably
non-spherical objects. Once agreed upon, the cut-off *mass* (not the
degree of sphericalness) will be the deciding factor. Optionally,
(and recommended) a transition classification (specifying a range of
mass) could also be included.

If so desired, the classification "Planet" could be subdivided into
Classes, with each class having upper and lower mass limits.

Pay no special attention to Pluto! Let it fall wherever in (or
outside) the definition or classification scheme. Pay no special
attention to orbits! (See "About Stars" above.). It's the physical
body we're interested in, regardless of where that body might be
found. If this means that some planetary satellites are also Planets,
then so be it!

It's not the job of science to keep the number of planets in our solar
system small for easy memorization by school children. It's more
important to have definitions and classification schemes that science
and astronomy can be proud of.

Willie R. Meghar