Vox published an article last week that made me want to slam my head against the wall, The Debate Over Pluto Will Never Die. Here’s the Latest Argument for Why It’s a Planet. Astronomer Kirby Runyon has come up with a new definition for a planet. And if we use it, there will be hundreds of planets in our solar system. For example, our Moon would be a planet. And hell, why not?
Remember: the Moon is quite a lot larger than Pluto. In fact, the largest 7 moons in the solar system are bigger than Pluto. So size isn’t the issue. Oh, you think a planet is something that orbits the sun and a moon is something that orbits a planet. Guess what? It ain’t nearly that simple. The Moon doesn’t orbit around the Earth. The two objects orbit around their center of mass. I have discussed this issue before, The Unusual Pluto-Charon Binary Orbit. It turns out that the Earth-Moon center of mass is inside the Earth. But the Pluto-Charon center of mass is way outside Pluto.
Orbits Are More Complex Than They Seem
The same thing is true of the Sun, although Jupiter is the only object large enough to make the Sun wobble. But my point is that if you looked at the Earth-Moon system orbiting around the Sun (the center of mass is effectively the center of the Sun), you would not see the Earth making an ellipse around the sun with the Moon circling it. Instead, you would see the two objects zigzagging around the sun. So if you look at it from a large scale, it looks very much like the Moon is orbiting the Sun. Because it is.
Now look at the Pluto-Charon system. It’s the same, but even more zigzag. So if Pluto is a planet, I sure don’t see why the Moon isn’t. So why not?
A New Planet Definition
Runyon and some other astronomers have suggested this as the definition of a planet:
Got a problem with that? All of these objects ultimately orbit the Sun anyway. If the Earth suddenly disappeared, the Moon would continue to orbit the Sun. So there you go: the ultimate definition of a planet that couldn’t possibly be designed just to make Pluto a planet. And I say this knowing that Brian Resnick reported, “Once he’d seen this side of Pluto, Runyon was bothered that it wasn’t a full-fledged planet anymore.”
Here’s the thing: I don’t care. I so don’t care that it’s annoying. Call Pluto whatever you want. And I’ll go further: call the Earth anything you want. I can certainly come up with a definition of planet in which the Sun has only 4 planets, knocking out the four rocky inner “planets.” Because, as Juliet put it:
What’s in a name? That which we call a rose
By any other name would smell as sweet …
Pluto Is Fascinating — Whatever You Call It
I find Pluto fascinating. I don’t need to call it “planet,” “dwarf planet,” or even “rose” to find it fascinating. Is it only the large things that are worth studying — worth caring about? From my perspective, the Earth is the most interesting plant for what I think are obvious reasons.
But here’s the thing: our solar system is amazing. Rather than fight about what category to place Pluto in, why don’t we take a moment to marvel at the fact that Triton orbits Neptune backwards? And speaking of that, how in the universe did Venus get that backwards day (which happens to be longer than its year)? Or how about if you took all the debris of the asteroid belt (including “planet” Ceres), you’d have a “planet” about 1/25th the size of our Moon?
There is something nice about the current official definition of planets: it creates 8 of them. They divide very nicely into two types: small rocky ones close to the Sun and big gaseous ones far from the Sun. They are completely dominant regarding their moons — all orbit a point inside the planet. I think the Earth has the largest moon relatively speaking, and it is only 1% of the Earth’s mass. (Charon is 12% the mass of Pluto.) But I’m willing to give it all up.
I propose the following definition:
Space garbage is anything that isn’t a star, broadly defined (including black holes, quasars, and so on).
So you say “planet” and I say “space garbage.” And literally nothing about the science of solar systems changes.