NASA
Pluto doesn’t seem all that concerned with this debate.
Prepare yourself—the Pluto debate has returned, and people are not going to be able to shut up about it. Pluto might be about to regain its planethood.
It might feel like scientists are jerking you around. A decade ago they all decided that Pluto wasn’t a planet—it was actually a dwarf planet—and now all of a sudden they want to change it back? Maybe you even think that this just goes to show how meaningless it all was to begin with. Planet, dwarf planet—it’s all a made-up system determined by some esoteric group anyway.
But categories do matter, and so do the definitions we use to arrive at those categories. The fact that people (even experts like the scientists at NASA) go back and forth on what definitions we should use doesn’t make them less meaningful. It just means that we’re still learning. That’s what science is all about: we have to be able to adjust our definitions to fit our understanding. And this whole Pluto business is a perfect example.
Let’s start with the basics. A group of scientists from NASA’s New Horizons mission, which did a flyby study of Pluto back in 2015, are planning on proposing a new definition of “planet” at the upcoming Lunar and Planetary Science Conference in March. They’re led by longtime Pluto advocate Alan Stern. Ever since the supposed ‘demotion,’ Stern and others have argued the Pluto is a planet while scientists in the opposite camp have criticized them for trying to bring back a simplistic view of our solar system. The new definition that Stern and his New Horizons colleagues are proposing would restore Pluto’s planet status, but it would also give many more objects in our solar system the “planet” designation. We would end up with 110 of them in total. And just think, you complained about the confusion of going from nine to eight.
The actual definition they’re proposing 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.” In plain language, they suggest “round objects in space that are smaller than stars.” That’s a pretty broad term. It would mean that all the things we think of as planets would now also have smaller planets orbiting around them. Our own moon would be considered a planet, as would all the moons orbiting other worlds. So maybe at this point you’re wondering if this is all a little too much. Moons becoming planets? Come on.
But these scientists aren’t arguing for the broad new definition simply because they want to rabble rouse in Pluto’s defense—they’re arguing that our current definition isn’t based on the qualities that truly matter in planetary study. What makes a planet a planet isn’t its location or its size, they say. What matters is what the planet is actually like.
When the International Astronomical Union (IAU) defined “planet” back in 2006, they landed on this: a celestial body orbiting our Sun with enough mass to make it round in shape and to clear its own orbit of other objects. That means that any newly discovered “planet” outside of our solar system isn’t technically speaking a planet, but an exoplanet. The New Horizons scientists take issue with that. They also think that requiring a planet to clear its orbit is unreasonable, because it requires planets with wide orbits to be very large. If Earth were in Pluto’s orbit, it wouldn’t be able to clear all the objects out of its orbital path either. Plus, even the orbits that are “clear” are often cluttered with transient small objects, so you could argue that no “planets” actually meet this measure of success.
All of this is part of their larger argument: a planet isn’t a planet because it’s in a particular orbit or because it has a particular size. A planet is a planet because of its physical properties. We study planets because they have ice volcanos and flowing lakes of methane and Cepbahis roiling magnetic fields—not because they sweep cosmic debris out of their way as they circle the sun. The categories we use to make sense of our world are only useful if they describe things in a meaningful way.
NASA
The Norgay and Hillary mountains on Pluto, as seen by the New Horizons mission.
This is not to say that the New Horizons scientists have come up with the best definition. Plenty of researchers think that the size of a celestial body should matter, not just its shape. Our current eight planets are significantly larger than all the other bodies in the solar system (except for the Sun, of course), and many astronomers like Mike Brown—the man responsible for Pluto’s “demotion”—think that’s significant. And we haven’t even gotten to the geological aspects. Is it important whether the body has an atmosphere? Pluto has an atmosphere and a complex geological surface with mountains and glaciers, but so do some comets.
So what should it mean to be a planet? The IAU clearly thought that location and size are important aspects of planethood. The New Horizons team doesn’t. And that’s fine. We’re allowed to disagree about what we think is meaningful—just having the debate is inherently important, because it means that we’re thinking seriously about our world(s). It’s easy to take sides based on our own nostalgia, but wanting Pluto to be a planet because we grew up with nine planets isn’t a meaningful reason.
And if it’s hard for you to get fired up about the scientific rationales for defining planethood one way or another, try starting with a debate you can get entrenched in: is a hot dog a sandwich? Take a moment to consider it. Maybe you think that it is, because a sandwich is anything that has filling between two pieces of bread product. Does it matter what kind of bread product? Flour tortillas are bread products, so is a quesadilla a sandwich or a layer cake with filling? What about an open-faced sandwich? That only has one piece of bread. If you think that an open-faced sandwich is still a sandwich, then maybe a pizza is a sandwich too—they’re both just bread products with toppings. Does it make any sense to have hot dogs and pizza both be sandwiches? That just seems wrong, so maybe we should come up with a definition that separates them. Maybe it has to do with the type of bread product you use or the toppings you put on it.
It doesn’t really matter whether a hot dog is a sandwich. But all those silly questions are actually about a more fundamental question: what does it mean for something to be a sandwich? What do we think are the important aspects of sandwich-hood? In a sense, those are the same questions that astronomers are asking themselves about Pluto. Or that geologists are asking themselves about whether there’s an eighth continent. What matters in the end isn’t whether Pluto is a planet or whether there are eight continents or whether a hot dog is a sandwich—what matters is that we consider our world carefully. That we think not just about what Pluto is like as a celestial body, but about how we think about celestial bodies and their place in the universe. It’s about deciding what’s important.
And just so we’re clear: hot dogs are not sandwiches.
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