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Thursday February 23, 2006
Pluto is a weird planet. So weird, in fact, that lots of respectable scientists want to call it NOT a planet.
My own view on the matter is that calling something a planet is like naming your dog. It's arbitrary. Completely arbitrary. For any definition of "planet" you come up with, you're going to find objects in this universe that fit, yet defy explanation... unless your definition of "planet" is "one of the nine celestial objects either named for the greek gods or conveniently right underfoot, those being Mercury, Venus, Earth (underfoot), Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto."
Does a planet have to have moons? Nope. Venus and Mercury are both moonless, and Pluto now has THREE.
What about atmosphere? Hmmm.... Titan has atmosphere, but it's only a MOON. Mercury has no atmosphere either. It has an exosphere -- elements from its crust that have been knocked free by high-energy radiation from the Sun, and which are so rarified that they never collide.
And don't get me started on size. Pluto is smaller than non-planetary objects, and Titan is larger than Mercury.
Well... maybe "it has to have atmosphere or exosphere, has to be at least so large, but it can't be orbiting something else (besides a star)." Okay, now it's starting to sound arbitrary.
But I'm fine calling Pluto a planet, because "Planet" these days includes objects so dissimilar that the only thing they all have in common is a name scheme. Mercury, Venus, Earth, and Mars are all rocky worlds with thin atmospheres (neither too thin nor too thick in Earth's case). Jupiter, Saturn, Uranus and Neptune are all gaseous worlds that are almost ALL atmosphere. Pluto is a Kuiper-belt object orbiting in a different plane, and which appears to actually be a clump of Kuiper belt objects caused by a collision billions of years ago.
Kids learning astronomy need to have "hooks" to hang information on. The word "Planet" is a good hook. Scientists, on the other hand, need to be able to let go of oversimplified generalizations. The current arguments about "planet or not-a-planet" are irrelevant. Pluto is a thing, different from some things and similar to others. Categorizing it or re-categorizing it will not change its nature. You can't turn your dog into a cat by calling it "Kitty." You CAN confuse your children, though.