Diane Heaton (factitiouslj) wrote,
Diane Heaton


Talking about whether Pluto should count as a planet seems to be popular again, so I figured I might as well join in. Here's why I prefer the idea of eight planets:

Imagine that you're not from this solar system, and are visiting it for the first time. Before you arrive, you prepare by looking at a map of your destination, and see that the sun is surrounded by, in order:
  • four balls of rock, one of which has interesting life
  • a belt containing hundreds of thousands of small chunks of rock
  • four gas giants
  • a belt containing thousands of lumps of ice
  • much farther out, a spherical cloud of billions of comets

But then when you get there, the locals inform you that they have nine planets. You can't tell from looking at your map what they might mean by this, so you ask for a more detailed explanation of the "planets."

"Planets are things like the Earth, where we are right now. Oh, and like Jupiter. You must have seen Jupiter on your way here, you can't miss it."

"OK, but there are only eight of those, four of each kind. What's the ninth one?"

"The ninth planet is Pluto. It's out past the gas giants, in what we call the Kuiper Belt."

"Oh, is it the biggest of the objects there?"

"No. It's probably in the top ten, but we haven't checked all that many yet, so it might not be."

"Is it the brightest one, then?"

"Well, no."

"Roundest orbit?"


"So, what's special about it?"

"Well, we discovered it quite a while before we discovered any of the others. We used to think it was the only one out there."

The point of this is that looking at the actual physical reality of the Solar System, there's no way you'll arrive at the idea of singling out Pluto specifically.

Aside from the crazy people who want to make dozens of other spherical KBOs planets, the only argument for calling Pluto a planet is tradition. It was discovered in 1930, long before I was born. There are popular mnemonics for remembering the names of nine planets. There are lots of science textbooks saying that Pluto is a planet (just as there used to be science textbooks that mentioned planets like Pallas and Juno). Star Control 2: The Ur-Quan Masters has Pluto as one of the planets. And so on. None of these objections have anything to do with the composition of the Solar System.

Any definition of "planet" we come up with will be arbitrary to some extent, because the terrestrial planets and the gas giants are so different. Including a random ice dwarf makes the situation much worse. If we don't fix this, visitors to Earth will think we're rubes who don't know what our own neighborhood looks like.

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