Planethood: A Debate

Two leading scientific experts consider what to call Pluto in the wake of a controversial IAU finding.

Artist’s conception of several solar system bodies shown to scale: Earth, Pluto and its moon Charon (far left), Eris (top), Eris, and Earth’s moon. Which one doesn’t fit in? (NASA)
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A&S: Although "roundness" seems to me a fairly vague term. There’s at least one large object in the Kuiper Belt that’s fairly elliptical.

Brown: That’s my favorite one in the whole solar system. It’s 2003 EL61. It’s bigger than Pluto if you look at it at the right moment in the right dimension. "Roundness," though, is only the public face of the definition. The scientific face is "hydrostatic equilibrium" [where compression due to gravity balances the outward pressure].

Stern: Yes, when properly worded, this criterion is pretty straightforward. It says that the object is large enough to become rounded by gravity. It doesn’t mean that it is round. You don’t actually have to see the object if you have a measure of its mass or radius or both.

Brown: I think 2003 EL61 is the case you’d use to explain this. Because everyone would point out, "Gee, this one’s not round." It’s hydrostatic equilibrium that’s important, not the actual shape. Even the Earth is not perfectly round.

A&S: Let’s go on to the third requirement, the one that caused much of the debate—this notion of "clearing the neighborhood around its orbit." People immediately pointed out that Jupiter, for one, might not qualify [since it has asteroids in its orbital path]. Maybe the IAU meant something more specific by this, too?

Stern: I just know what they wrote, and what they wrote is subject to exactly that kind of criticism. If Johnny’s teacher says, "The IAU says that to be a planet an object must have cleared its neighborhood," then Johnny could say, "What about the Near Earth Asteroids?" It just becomes unteachable.

My bigger criticism with this dynamical part of the definition is that it creates a situation—untenable, in my view—that a given object can be a planet in some circumstances and not in others. For example, the Earth, by this definition, would count as a planet at its current distance from the Sun. But if you moved the Earth out into the distant reaches of the solar system and discovered it there, it would not be a planet [because it wouldn’t have cleared its orbit]. And the same is true if you put Jupiter in the Oort cloud. It would not be a planet by this definition. You can play endless games with these architectures, and it’s nothing but confusing. To my philosophy, that’s disturbing. An object, it seems to me, either is a planet or it’s not. The paleontologist doesn’t care where he finds the bones of a dinosaur. It’s still a dinosaur.

Brown: All Alan has said is that he’s not interested in the dynamics of the planetary system when it comes to classifying planets. That’s a perfectly acceptable philosophical point. But it’s an aesthetic point, not a scientific point. It’s just not the way he prefers to classify things in the solar system. These examples—Earth further away, Jupiter further away—I would agree you wouldn’t classify those as planets by this definition. But that actually makes it very interesting. Why would you not classify a Jupiter in the Oort cloud as a planet? Because it behaves very differently. It has had a very different history than these other eight planets.

I hate to keep using the word "planet" when my preference is to not even use the stupid word. But these eight largest objects we are now calling planets have had very different histories than anything like a Jupiter in the Oort cloud or an Earth out at 300 times its current distance from the Sun. Dynamically, the difference between these things is like night and day.

I disagree that it’s difficult to teach. I have been having a lot of discussions about this, and I find [the IAU definition] not only easy but delightful to teach. I feel like I finally get to teach people the structure of the solar system—how planets form, why there are belts, why there are individual solitary bodies. I love the question, "What about the Near Earth Asteroids?" You can talk about dynamically unstable orbits. What about Trojan asteroids? You can talk about how Jupiter has captured the Trojans at a Lagrangian point. The quibbles are where the science gets interesting. Any definition that is so clear that there’s no room for discussion, is, I don’t think, a very interesting definition.

Stern: It’s not true that I’m unhappy with dynamics as a criterion for planet classification. I co-wrote an article after the 2000 IAU meeting with Hal Levison, who’s a dynamicist, where we coined the terms uber-planets and unter-planets in a jocular way, because we didn’t want to use pejorative terms like "major" and "minor." I do think from a dynamical standpoint, there are planets that really matter in the architecture of the solar system, and those that don’t. They’re both planets. Just as you can have wet and dry planets, or life-bearing and non-life-bearing planets, you can have dynamically important planets and dynamically unimportant planets.

I would make a term for "planetary bodies." And anything that’s large enough to be in hydrostatic equilibrium would qualify. Then, if it were orbiting a star, I would call it a planet. If it were orbiting another planet, I would call it a satellite that’s a planetary body. And if it’s ejected from the solar system— which we expect has happened although we haven’t detected objects like that—I would call it a "free floater" or "rogue planet." But they’re all planets.

With stars, we don’t classify whether something’s a star based on whether it’s in a cluster or it dominates a region of the galaxy. It’s a star because of its intrinsic properties. In astronomy objects are typically defined by what they are, not where they are or what they’re near.

Brown: I’d be much happier with Alan’s physical definition than the one the IAU initially proposed. I thought their initial proposal was philosophically flawed in that you had things that were planets only if they were in orbit around a star. But if you put it in orbit around another planet, it suddenly becomes a satellite. If you really go with the physical criteria, you don’t care where it’s located.

A&S: What’s the right number of planets? The draft IAU definition would have opened the door to dozens of planets, or even a couple hundred, in our solar system. Would that bother you? Is it important to have a number that can fit on ten fingers, or in an easy rhyme?


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