A little more on Pluto

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As I stated earlier, it's a matter of near-total indifference to me whether people regard Pluto as a planet or not, nevertheless the proposed changes strike me as fairly confused. Let's recap. It's not really possible to draw a non-arbitrary-seeming dividing line between planets and non-planets that includes Pluto but doesn't bring in a bunch of other objects. This leaves us with three options:
  • Stick with the arbitrary traditional definition.
  • Pick a non-arbitrary definition that makes Pluto a non-planet. For some (irrational) reason, whenever it's suggested that we to this, some people get really upset.
  • Pick a non-arbitrary definition that makes Pluto--and a bunch of other objects--planets.

Of course none of these options are really that satisfactory, and the third option--the one the IAU has chosen--matches the traditional definition pretty badly. Really, though, there's no reason why we can't have a technical definition of a term that doesn't match the colloquial definition (think of the economist's use of the term demand or the persistent confusion over the meaning of theory, as in the "theory of evolution".) Alternatively, we can just invent a separate term with a specific technical meaning, thus minimizing confusion. But insisting that the colloquial terms have well-defined meanings doesn't really produce that great results, as we're seeing in this case.


I'm not sure how to do a non-arbitrary definition that would encompass Pluto but wouldn't catch Ceres and especially Xena. In particular, Xena is larger than Pluto (so size won't do it) and if you go by orbits in the ecliptic etc., you'll get Ceres and not Pluto.


I think if you read Eric's choices carefully, he never suggests it's possible to construct a non-arbitrary definition that includes Pluto but excludes other bodies.

He's saying it doesn't matter. Colloquial "planets" would include Pluto arbitrarily. Technical "planets" or "fooets" might or might not depending on the technical defintion.

I have to say I don't fully understand why anyone cares about whether Pluto is a planet or not either, but the objections I've seen seem to come from elementary-school-teacher types (whether actual elementary school teachers, or people who work at science museums and such) who think that kids will get confused if there aren't exactly nine planets which happen to match the set of planets listed on the current displays. I suppose there will be a retooling charge that all the various book publishers and museums will be facing, but to some large degree that's merely a side-effect of their having chosen an arbitrary definition of "planet" in the first place, which might also have something to do with the resistance -- in terms of these folks' annual budgets, the retooling might be fairly significant.

I wonder if this make-ceres-a-planet idea was intentionally advanced as something so bad that it would demonstrate to the Pluto advocates once and for all the logical consequences of clinging to that planet. My guess is that this will finally lead to Pluto being eliminated from the list of planets, as everyone recoils in horror from the alternative.

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