Planet schmanet

One thing that puzzles me about this whole pluto[1] demotion business goes back to grade school. Mrs. Fink taught us that astronomers were only able to find pluto[2] because pluto's gravitational pull disturbed Neptune's orbit. What with math and all, the astronomers were able to figure out where to look and presto there was pluto. But if pluto isn't a planet after all, but merely a Kuiper Belt Object, and there are lots of other KBOs, why is pluto the only one that astronomers found this way in the olden days?

Here's something else. Word on the street is that pluto's demotion was due to its failure to meet one of the three main criteria for a planet:
The prestigious international group in the Czech Republic today spelled out the basic tests a celestial body needs to pass before it can be deemed a planet: "A celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a . . . nearly round shape, and has cleared the neighborhood around its orbit."

It's the last part of the definition that doomed Pluto. Its oblong orbit overlaps with Neptune's.

To which I say:
  1. pluto's orbit overlaps that of neptune if and only if neptune's orbit overlaps plutos.
  2. If neptune's orbit overlaps pluto's then neptune has not cleared the neighborhood around its orbit.
  3. If neptune has not cleared the neighborhood around its orbit, then neptune is not a planet.
  4. pluto's orbit overlaps that of neptune.
  5. Therefore, neptune is not a planet.


Logic and history aside, there's some taxomaniacal hay to be made here. One consequence of our assignation of the term 'planet' to each of the major objects in the solar system is that it has encouraged us to think, wrongly, that there is some unitary class of things, planets, to which all of those objects belong. As the logical demonstration above shows, any definition we try to give of that class is going to be somewhat arbitrary. So why not abandon the effort entirely and move to a more fine grained classificatory scheme that's better attuned to the diversity of objects that the solar system actually contains?

1 Usage note: as a non-planet, 'pluto' is no longer capitalized in polite society.

2 Of course, she called it 'Pluto'.

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