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SYFY WIRE Bad Astronomy

Is the Moon a Planet? QVC Asks, I Answer.

By Phil Plait

I’m not a big fan of definitions in astronomy. I’ve been pretty clear about this in the past; Nature is way less anal about boundary lines than humans are. Borders between categories of objects are fuzzy, and while it’s OK to put things in boxes (Jupiter is a planet, the Sun is a star, the Milky Way is a galaxy), it can get tougher when you have two similar objects that you nevertheless think should be on opposite sides of the line. It can be confusing.

And then you have what happened on QVC recently.

QVC is an online and TV shopping channel. Recently, host Shawn Killinger was featuring a line of cardigans by designer Isaac Mizrahi. She described the pattern on one: “It almost kind of looks like what the Earth looks like when you’re a bazillion miles away from the planet moon.”

To her credit she laughs at herself and says she just meant to say “looking back at the planet from the Moon,” not “planet moon.” But then the conversation takes an odd turn. Watch:

Killinger at first correctly says the Sun is a star, and the Moon is not a star. But then Mizrahi says the Moon is a planet, and she questions that, saying the Moon was never included when you learn the planets. She also goes back to saying it’s a star.

Someone offscreen then gets on Google and says “the Moon is a natural satellite.” This confuses both Killinger and Mizrahi, who then quickly move on to selling more clothes.

Let me cut through the confusion: The Sun is a star, a huge object that has ongoing nuclear fusion in its core. At the lower mass limit, the definition of “star” can get fuzzy, but the Sun is way to one side of that line, so we’re good.

Is the Moon a star? No. No fusion in its core, and not even close. It’s not a star.

Is the Moon a planet? Well, not really, as a planet is an object that orbits a star, and the Moon orbits the Earth (and yes, wannabe pedants, it really does orbit the Earth and not the Sun).

A satellite is a generic term for an object that orbits another object. You could say the Earth is a satellite of the Sun and be technically correct, though that’s not usually how the term is used. The Moon is a natural satellite of the Earth; it orbits Earth, and is not artificial. Another term for “satellite” is (lowercase M) moon, so the Moon is a moon. A weather satellite is then an artificial moon.

So there. We’re done.

… except of course we aren’t. It’s not hard to imagine if the Moon were bigger, say the same size as the Earth. Would it be a planet then? You could say we’d be part of a binary planet, two planets that orbit each other while orbiting the Sun.

Now you might remember that a few years ago, the International Astronomical Union tried to write in stone the definition of what a planet is. I think this is a mistake, and a foolish one; as my friend and astronomer (and the man who wrote the book How I Killed Pluto and Why It Had It Coming) Mike Brown points out, a planet is a concept, not a definition. It’s like “continent”; we have no definition for it—it’s more of an idea that helps you categorize things in general terms.

Is Australia an island or a continent? Yes.

By the definition decreed by the IAU, a planet orbits the Sun. But each component of a binary planet orbits the other one. So are they a planet, or two planets, or two satellites, or what? I could argue any and all of these. The fact that the definition falls apart so easily is a pretty good indication that using a definition is a bad idea in the first place.

The center of mass of the Earth-Moon system is inside the Earth, so we can safely say the Moon orbits the Earth. But if the Moon were a bit more massive, it wouldn’t be quite so clear. Ceres, the largest asteroid, was thought to be a planet for a few years before it got reclassified into the new category of “asteroid.” And it’s way smaller than the Moon.

What if the Earth didn’t exist? The Moon is pretty big, and if it orbited the Sun where the Earth is now, would we call it a planet? I don’t think so, since according to the IAU definition, a planet has to be massive enough to gravitationally affect all the objects that share nearby orbits (it’s “cleared the neighborhood around its orbit” is how it’s confusingly stated), and I think the Moon would fail that criterion. But that’s not a great definition either, to be honest. It’s complicated and weird and still mighty fuzzy along the borders. My gut feeling is that if we saw a solar system exactly like ours, but with a Moon-sized thing where the Earth is now, we might call it a planet.

Happily, I have a brain as well as my gut, and my brain says, “So what? That object is a big, round, interesting world, so who cares what you call it? Let’s study it!”

That’s science, and it’s way more interesting than nitpicky semantics.

And one final thought. A lot of folks online are making fun of the Killinger and Mizrahi for their discussion, but I think it’s fine. First of all, they’re curious about astronomy, and it led to getting an answer (even if I might quibble over how it happened). Second, it started a larger conversation about what all this means.

And third, what they were arguing over is a subtle, layered, and difficult concept that had astronomers from all over the Earth arguing for years about what it means. And they’re still arguing over it!

So if you want to feel smug and superior about the TV hosts, hey, that’s your choice. But people in glass planets shouldn’t throw asteroids.

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