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SYFY WIRE Bad Astronomy

Redefining planets: An answer in search of a question

By Phil Plait

[Jupiter and Earth to scale; we call both these objects "planets" despite being vastly different. Credit: NASA,ESA, and A. Simon (Goddard Space Flight Center) & NASA]

What’s a planet?

That seems like an easy question to answer, but really it’s not. If it were, then we wouldn’t always be arguing if Pluto were a planet or not!

This issue boiled over in the astronomical community a few years ago when astronomer Mike Brown and his team discovered an icy object orbiting the Sun well past the orbit of Neptune. Eventually named Eris, it was very close to the same size as Pluto. It has been thought for some time that there might be lots of worlds out in that part of the solar system similar to Pluto, and Eris proved it. That threw Pluto’s status as a planet in doubt; what if we had dozens more like it, or hundreds? What really is a planet, anyway?

The International Astronomical Union —the official keeper of astronomical nomenclature— stepped in. At their 2006 international meeting in Prague, they drafted a series of defining characteristics for planets and put it to a vote. The definition passed, and the outcome was entirely predictable: They made things a lot worse.

The definitions they came up with were, briefly: A planet is an object that orbits the Sun, is massive enough to shape itself into a sphere by its own gravity, and had “cleaned out” its orbit of smaller debris.

There are two problems with this set of statements. One I’ll get to in a moment (put a pin in it, as it’s important), but the more proximate problem is that these definitions are, um, silly. What if we find a planet orbiting another star, or drifting between the stars (to be clear: We’ve found many examples of both)? Well, these are “exoplanets”, but not planet planets, according to the IAU. Worse, if you moved the Earth out to where Pluto is, that region of the solar system is so large our planet — or whatever you’d call it — would never be able to clear out smaller stuff in its orbit.

The basic problem is that these definitions define a planet by where it is, not what it is. Well, the second statement does, but the other two are problematic. Making things more complicated, they also created subcategories like dwarf planet, which is where they put Pluto.

This has caused a lot of consternation in the astronomical community, as well as with the public. The IAU definition doesn’t really help, and really just complicates things.

Now, another group of astronomers is hoping to change that. For the upcoming Lunar and Planetary Science conference to be held in Houston in March, this team has proposed a new definition. They published a brief paper outlining it, and here is the meat of it:

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

Let me break that down for you. “Sub-stellar” means less massive than a star, and saying it’s never undergone nuclear fusion in its core differentiates a planet from a star (which generates energy via core fusion). Still, a planet has enough mass that its gravity crushes the shape into something resembling a sphere: A sphere is defined as a volume with three radii, all of which are the same length. If one axis is longer than the other two (like an American football), or two are longer than the third (like a beach ball someone is sitting on), or all three are different lengths, it’s called an ellipsoid. Also, that last bit removes any location-based issues. A planet is a planet no matter where it is.

I’ll be clear: This isn’t a bad way to categorize planets (at least to start). And it’s good that we have categories for objects; it helps us analyze them, look for similarities and differences. That in turn can lead to insight into how these similarities came to be; what processes led to the particular shape, size, structure, and behavior of what we’re studying.

Earth and Pluto to scale

So, what would this new definition mean for our solar system? Well, right off the bat, it means we’d have over a hundred planets! Why? Because every round object besides the Sun would be a planet. That includes Pluto, as well as Ceres and Vesta, the two largest asteroids (actually, they’re more like protoplanets), and Eris. But remember, now any round object would be a planet. That includes our own Moon, as well as dozens of other moons.

Having that many planets in the solar system doesn’t bug me; it just seems odd, because we’re used to only 8 or 9. But there’s no reason we can’t have just one, or a thousand. However, the idea that you might have a tiny planet orbiting a much bigger one is unsatisfying to me.

The authors disagree with me there. They say,

Implicitly using the geophysical planet definition in context is easy. Teachers may introduce new moon planets to their students with phrases such as, “In the 2020s, NASA will send a spacecraft to study the planet Europa, which orbits around Jupiter as one of its many moons.”

But that strikes me as more confusing than just saying it’s a moon. Perhaps it’s because, in my mind, moons and planets are two different things, but I’ll guarantee that’s true of the vast majority of people. Perhaps, in the future, that’s fine, but for right now it seems that merging these two concepts doesn’t really clarify anything (on the other hand, if you separate moon and planet entirely, what happens when two objects are close to the same size and orbiting each other? Is that a double planet? Where’s the dividing line?). Mind you, part of the impetus for creating this new definition was to clear up public confusion about what’s a planet and what isn’t. I’m not sure this new definition does that.

But there’s more to it than just the specifics. That brings me to that other point we put a pin in earlier: Why are we trying to define the term “planet” at all?

I’ve written about this extensively: The basic idea is that “planet” isn’t a definable term. That’s because it’s not a rigidly scientific word. It’s a concept. The whole idea of a “planet” is necessarily somewhat vague*.

It’s like the colors of the rainbow. Red is definitely different than blue, but there’s a continuum of colors between them. You can put rigidly defined borders on colors - say, “green is a color with a wavelength from 495 to 570 nanometers” - and that might even have some limited scientific usefulness, but the public won’t care much. They know green when they see it, and that’s good enough.

In fact, I’ve discussed this before with Alan Stern, one of the authors of the new definition (and Principal Investigator for the New Horizons Pluto mission). In a conversation we had a few years ago, he made a wonderful point: Imagine a scene in Star Trek, with the Enterprise warping into an unexplored system. Spock looks into his scanner, the blue light reflected on his face, and cooly states, “An ordinary system, Captain. Solar-type star, 11 planets, assorted rocky debris.”

In other words, Spock knows a planet when he sees one. While true, I don’t think that’s the best argument to try to define “planet.” Heck, a Supreme Court Justice made this same argument about obscenity (“I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description, and perhaps I could never succeed in intelligibly doing so. But I know it when I see it...”) and used it to show that you cannot define obscenity. It’s more of a concept, and sometimes things are obvious, but many times they’re not.

And therein lies my basic problem with all this. I’m OK with this new idea, but not if it’s called a definition! That may seem like semantics, but it’s important: Nature very rarely has vividly defined borders, even if we humans like to have them. But when we make them, they tend to be arbitrary, like defining “green”. And when you get near the borders, things get fuzzy, indeed.

So, here’s the point: Nature ain’t simple. That’s what I think is more important to teach people. If we use the new idea but call it a concept, I’m (mostly) OK with it, and then we can use it as a basis to teach about all the spectacular and amazing diversity nature uses in its palette.

If you want to use these new ideas and call Pluto a planet, that’s fine. But then, explore why we think of it that way, and why it’s different from, say, Jupiter.

Definitions are an endpoint. Concepts are a starting point. I prefer the latter.

* Mike Brown, himself, writes about this in his delightful book, How I Killed Pluto and Why It Had It Coming. If you’re still not clear where he stands on all this, check out his Twitter handle.

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