Pluto and Other Things That Aren’t Planets


Pluto: Not a Planet

While the actual data and images coming back from New Horizons have been awesome, it’s been a little frustrating that they’ve come mixed with a heaping helping of posts about how Pluto should totally be a planet again now.

And the bulk of that comes down to “Pluto is interesting, Pluto is awesome, I like Pluto,” ergo Pluto should be a planet. To be sure, Pluto IS interesting. Pluto IS awesome. I DO like Pluto.

But my dog Amos is also interesting, awesome and likable, but I don’t think he should be a planet, ya know?*

Ultimately, taxonomy is relative. Language is subjective, and we can decide how it’s used. If we want the word “literally” to mean “figuratively,” that’s within our power as a literate people. There is no cosmic absolute that determines whether something is a planet or not, it’s just our subjective view of what we want the word planet to mean and how we want to use it.

Scientifically, there’s no reasons we can’t use the term planet to mean “The nine bodies recognized as planets in the year 2000.” In which case, Pluto would officially be a planet. Or even “”The nine bodies recognized as planets in the year 2000, plus David and Rebecca’s dog Amos, who’s pretty awesome and interesting and likable.” In which case, Amos would also totally be a planet.

David and Amos at the Space & Rocket Center

Amos: Also Not a Planet

It comes down to what purpose you think a taxonomy should serve. If you want a system of categorizing celestial objects that’s “merit-based,” “rewarding” bodies that we find more “worthy,” then, sure, there’s no reason you can’t have that sort of classification system, and no reason Pluto can’t be a planet.

But if your goal is to have a taxonomy that promotes better understanding of the solar system (and thus the cosmos), it makes far more sense to use a classification system that groups like things with like things and thus encourages the use of what we learn about one body to help us better understand similar bodies. Calling Amos a planet doesn’t mean he can teach you anything about the other planets. (Trust me on this one; he totally won’t.)

Pluto is utterly unlike any of the other planets in all but the most superficial of senses. Like the other planets, it’s round, as are moons and baseballs. Like the other planets, it orbits the sun, as do asteroids and comets and the S-IVB stage for Apollo 8. (Actually, this one is less true of Pluto, which is substantially affected in its orbit by Charon, than of asteroids and comets and the S-IVB stage for Apollo 8, all of which have a greater claim to planethood under this standard.) Yes, it’s more rare for a body to meet both of these requirements, but it’s not that rare, and there are lots of other things that do that don’t make the “My Very Educated Mother…” list.

Saturn V S-IVB Stage in space

Saturn V S-IVB Stage: Also Also Not a Planet

On the other hand, from what we’ve seen, Pluto is a lot like other Kuiper Belt Objects, tiny objects that populate the far reaches of our solar system. The sad cartoons that depict Pluto as being lonely or sad for being kicked out of the planet club fail to understand or acknowledge that Pluto has far far more celestial cousins than our Earth does.

When we first discovered Pluto, we had not discovered anything else in the solar system like it, so we classified it with the things it was most like. Since then, however, in the Kuiper Belt, we’ve discovered a lot of things that are a lot more like Pluto than the other planets. So we corrected a mistake made in ignorance, and reclassified it with the new discoveries it was most like.

This exact situation happened once before, with Ceres. When Ceres was discovered in 1801, we’d not seen anything else like it. But even though it was small, it was round and orbited the sun, so the most logical classification was to call it a planet. But then we found another body near Ceres. And another, and another. And we realized that Ceres was a lot more like this new type of thing, which we called asteroids, than it was like the other planets.** And when we study Ceres, we study Ceres to learn about asteroids and their place in our solar system.


Ceres: Interesting, Crazy Mysterious, But, Yep, Also Also Also Not A Planet

Today, we have a much greater understanding of the asteroid belt, but the same is not true to nearly the same extent of the Kuiper Belt. And right now, Pluto is our best tool for understanding these mysterious worlds; which in some ways have been more alien even than planets orbiting other stars. Pluto has little to teach us about the eight planets, but it has a lot to teach us about the Kuiper Belt. So if scientific understanding is the goal of your taxonomy, you classify Pluto in the latter category rather than the former.

I was inspired to write this post by this article, which hits the nail on the head: It’s not sad that Pluto isn’t a planet. It’s awesome that Pluto is something even more valuable — our Rosetta Stone to distant worlds shrouded in secrecy that remind us how little we still truly know about our universe, and how much wonder still awaits us on our outward odyssey.

*All opinions in this post are purely my own. I make no claim of representing the views of NASA or any other organization on whether Amos (or any other body) is a planet.

**Fair or not, I judge whether someone’s desire for Pluto to be a planet or not is scientific or sentimental based on what they say about Ceres. If you weren’t posting during the Dawn approach that Ceres should be a planet, I assume your interest in Pluto’s planethood is probably based more on your childhood attachment to Pluto than in taxonomy.

3 Responses

  1. OK…here goes: Yes, I think ‘planet’ is the proper word for Pluto. Not because of nostalgia (though, honestly, I do feel a bit of that), but it’s that ‘planet’ was far too vague a term to begin with. Let’s take ‘rocket’ for example.

    Some rockets only go up a few feet (water or air-powered rockets), some a few hundred feet, some into the far reaches of the atmosphere (though without enough ‘oomph’ to get to orbit), and some that take man and machine to orbit and beyond. Heck, some rockets go from an airborne platform to the ground…or from one spot on the ground to another. Some rockets are powered by liquid fuel…some by solid fuel…some simply by air alone. Yet these are all called ‘rockets’. In order to differentiate between them, we tack-on other words: model, sounding, solid, etc..

    For me, the same logic (which I’m sure will be rapidly ripped apart) can be applied to ‘planet’. I’m not necessarily saying that Pluto isn’t a ‘dwarf planet’…but in that description alone is the word ‘planet’. I think the argument shouldn’t be “Is Pluto a planet?”; rather: “How many different classifications of planet are there?” Rocky planets? Gaseous planets? Icy planets? Binary planets? Dwarf planets? Inner planets? Outer planets? Kuiper Belt Planets? I don’t know about you, but I welcome the opportunity to “add” to our Solar System’s stable of planetary bodies.

    That’s my $.02

  2. To use your rocket analogy, if you look at a poster of rockets, it’s generally not going to include the X-15 or Tim Pickens’ rocket-bike. Not because they’re not rockets, but because they are more like other things than they are other rockets.

    Calling Pluto a planet is kind of like calling a platypus a lizard. If your goal was to add to our planet’s stable of lizard creatures, you could create a definition for lizard that includes all land-based animals that lay eggs and don’t fly. It’s a definition that includes everything we now think of as lizards, plus platypi, chickens, ostriches, etc.

    It’s a functional definition that is objective and applicable; it doesn’t require any opinions, you can just look at a creature and know if it’s a lizard or not. Snake? Lizard. Cow? Not a lizard. Iguana? Lizard. Eagle? Not a lizard. Emu? Lizard. Whale? Not a lizard. But it fails to account for the fact that there are other creatures chickens and platypi are more like. Despite the fact that no other mammals lay eggs, in far more ways, platypi are more like mammals than they are lizards. Despite the fact that most birds can fly, penguins are more like other birds than they are lizards.

    If you wanted to add to the stable of birds or fish, you could create definitions that include bats and dolphins, respectively. But we don’t. We don’t because there are more scientifically useful definitions. We understand bats and dolphins and platypi better when we compare them to other mammals than to other classes of animals.

    You’re right that “planet” is a somewhat vague term, but I don’t think the solution is to therefore move toward greater vagueness, but to move toward greater specificity. Planet may be vague, but Main Belt Asteroid and Kuiper Belt Object are much more specific. You could lump them into the vague definition and make it even more meaningless, or you could classify them with things they are specifically like, and promote classifications that actually do have meaning.

  3. The only thing Pluto doesn’t meet on the IAU definition is that it hasn’t cleared its orbital path. For me, that’s a relatively weak exclusionary reason. In fact, I’m sure that the same anti-planet argument could be made against Earth. There may be several ‘Earth trojan’ asteroids that orbit at the various Earth-Sun Lagrangian (L4 & L5) points (one has been positively identified). So, using the IAU’s definition, Earth has not cleared its orbital path and thus should not be a planet.

    Additionally, I disagree with the assertion that my goal is to add further vagueness to the definition of ‘planet’. On the contrary, I want to stratify it in much the same way that biological species are arranged. Take foxes and dogs, for example. They are both in the Canidae family, but are in different tribes within that family. To me, not classifying Pluto in the planetary camp because it hasn’t cleared its orbit would be like classifying a fox in the feline family because it may act more like a cat than a dog. Orbital “cleanliness” seems to be the least important criteria upon which to bestow/deny status.

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