The Sciences

Pluto's big Hill to climb

Bad AstronomyBy Phil PlaitSep 30, 2008 2:26 AM


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In a post yesterday, I talked about the Moon orbiting the Earth, and the Earth's gravitational sphere of influence, called its Hill sphere. If you have an object orbiting the Sun (like a planet, say), anything inside that object's Hill sphere is more affected by that object than it is by the Sun.

I knew about this tidbit of physics, but was reminded of it by Dr. Alan Stern, the head guy of the New Horizons Pluto probe. Alan and I have mixed it up a bit before on what the definition of a planet is. He likes the idea that one criterion of planethood for an object is that the object is massive enough to modify its shape into a sphere. In other words, its gravity can overcome the tensile strength of the material making up the object, and it molds itself into a rough ball. I will admit to being swayed by this argument, though still somewhat unconvinced, because the density of the object comes into play. A smallish (say, 200 km across) ball of slushy water will have enough gravity to cause the semi-liquid water to flow, forming itself into a sphere. By this definition, therefore, it would be a planet. But if the same object were frozen solid, it wouldn't have the gravity it needs to modify its shape, because ice is so much stiffer than slush. It wouldn't be a planet! Still, this is a borderline case, and Alan and I both agree that on the borders you get some room for argument. That's where the fun is, but it's also part of the problem with trying to define what a planet is in the first place! Anyway, by this definition, Pluto is a planet since it's spherical, and several of the larger asteroids and iceballs orbiting out beyond Pluto would also fit the planetary status bill. With me so far? OK, let's look at a different definition. A current definition of "planet", handed down by the International Astronomical Union, is that a planet can sweep up most or all of the material that orbits the Sun near it. That means the Earth is a planet, but Pluto is not, because Pluto is too small to have the gravity needed to clear out its orbit. The Earth does have what it takes, and almost every rogue object that orbits the Sun near the Earth has long since been cleaned out by Earth's gravity (with the exception of the Moon, which is bound to us, and the occasional asteroid, which is important in the sense of life-sterilizing impacts, but rare enough that no one will argue with you as far as planet definitions go).

This definition, though, is silly. If the Earth were out at the distance of Pluto, it would have a hard time sweeping clear the material out there, too. The volume of space that far out from the Sun is vast, and the Earth tiny. It would be like trying to sweep your house with a tiny paintbrush. The thing is, in your gut, you know that distance from the Sun shouldn't play into the definition of a planet. If you have two objects the same size, and you call one a planet, the other one should be too, no matter where they are. Orbital dynamics shouldn't really play a role. If the Earth is a planet where it is now, it should still be called a planet even if it's 30 times farther out from the Sun at Pluto's distance. Again, Alan and I agree. This "sweeping out" rule is a bit silly. And this brings me back to the Hill sphere, the sphere of influence of a massive object. Alan emailed me the other day with an interesting little tidbit. The Hill sphere of an object depends on its mass, and how far it is from the Sun. A massive object (like a planet) can be closer to the Sun and still hold on to an orbiting object (which I'll call a moon for simplicity). The farther an object is from the sun, the less mass it needs to hold on to a moon, because the Sun's gravity is diminishing as well.

If the IAU ruled the worlds...

If you do the math, the Earth's Hill sphere has a radius of about 1.5 million kilometers. The Moon is only 400,000 km away, so it's well inside our Hill sphere, and can rightly be said to orbit the Earth. But what about Pluto? It's a lot less massive than the Earth, but it's also very far from the Sun. When you do the math, you find its Hill sphere is 5 times the size of Earth's! That means Pluto's sphere of gravitational influence has over 100 times the volume of Earth's! When it comes to planets, it sometimes pays to be a small fish in a big pond. So, does this make Pluto a planet? No, and I don't think Alan is arguing that either. The point here is that you can't really use orbital considerations when trying to define what a planet is. Even though Pluto misses out by one orbital-based definition (sweeping up material), it seems like it should pass by another (the Hill criterion). That's a pretty big hint that orbital considerations should be dropped, and planets should be judged on their own merits. We'll still argue, of course. I don't like the idea of a hard-and-fast rule of what should be a planet and what shouldn't. But I do think a general rule would prove useful, as long as -- like the definition itself -- it doesn't shape our thinking too much. Definitions tend to restrain us, and our minds like to follow the path of least resistance. And that tends to flow downHill.

Pluto and its moons.The moons are all within Pluto's Hill sphere.

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