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Posted by spaceart on August 25, 2006, 4:15 pm
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I think the debate about what is or is not a "planet" is starting to
get a little silly. The problem I find is that most of the arguments
involve post facto reasoning...trying to come up with definitions that
will maintain traditional or historical conventions. This appears to do
nothing but lead down an ever-increasingly complex road. For instance,
astronomers who are trying to demote Pluto to non-planet status (and to
not consider any of the newly discovered large Kuiper Belt objects as
planets in the first place), face the problem of coming up with
definitions that exclude Pluto et al but include Earth, Mars, Jupiter,
etc.
Unfortunately, that is where it all becomes artificial and arbitrary.
For instance, we might say that any body with an orbit inclined more
than X degrees to the plane of the solar system is not a planet. Well,
we can then imagine a situation where there is a body orbiting at an
inclination of X degrees, making it a planet...but what of the one
orbiting at X+1 degrees? Assuming all other things being equal, does
that extra degree of inclination bar it from bearing the designation
"planet"? And if so, why? What if we get really ridiculous about it and
ask if a planet whose orbit is inclined at X.00001 degrees? What about
it? Is i not a planet...or do we fudge a little and let it join the
club?
What if instead it is agreed that any body less than X miles in
diameter is not a planet? We might then have a situation where we have
a planet with a diameter of X miles and a non-planet with a diameter of
X-1 miles. Or, if we want to get silly again, X-3 inches. Is the latter
therefore not a planet for those missing three inches? Again: why?
This is the sort of trouble arbitrary distinctions can get you into.
What about the material the world is made of? There have been a lot of
suggestions made that Pluto and its cousins are not planets because
they are icy. So what? We already have two classes of
planets---terrestrial and gas giant---planets that are composed
primarily of rock and metal and planets that are composed primarily of
gas and liquid. Those two groups could hardly be more unalike. So why
not a third group? What if an icy body like Ganymede circled the Sun at
a low inclination and in an orbit that was no more eccentric than the
earth's? It's bigger than Mercury--- would it not be a planet simply
because it's made mostly of ice?
And how much ice would be too much? Again, an arbitrary line would have
to be drawn. And as we've seen, arbitrary distinctions are, well, just
too arbitrary.
It seems as though it would be simpler to just have another category of
planet than to go through all of the bizarre and illogical convolutions
these arbitrary distinctions quickly lead to.
And that is probably getting close to the solution to the problem.
Instead of desperately trying to retrofit definitions, why not take a
clue from zoological or botanical taxonomy. I've already alluded to the
fact that this is already being done to a small degree...why not carry
it further? After all, we already refer to "gas giant planets",
"terrestrial planets" and "minor planets". Why not take a clue from
this and simply say that anything orbiting the Sun is a planet? And
that what we have are different types of planets? All that's needed, if
we admit that much, is to add to the already existing and
well-established list of three categories. We could add (just for
example) "icy planets" or "cometary planets", etc. etc. Whatever works
from a logical, taxonomic point of view based on structure and
composition rather than arbitrary delimitations regarding size, orbital
inclination and so forth. The general idea is clear enough, I think.
This would eliminate arbitrary distinctions, would follow
well-established scientific precedents and make for useful rather than
meaninglessly arbitrary distinctions.
---Ron Miller
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