Planet of the Apes

[notmatt]

I'd like to discuss the new "definition of the planet" being proposed by the IAU. I posted much of this to the VTSFFC mailing list yesterday, but for the benefit (or detriment) of everyone else, I'm reposting it here.

The definition is "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."



HYDROSTATICITY

A spinning fluid sphere deforms into an oblate spheroid. A planet that matches this shape is said to be hydrostatic. A planet with a rigid shell resists this deformation and is not hydrostatic. The gas giants ARE fluids and generally are hydrostatic. Earth is very close to hydrostatic because its lithosphere is broken into tectionic plates, and has no long-term strength. It however, is the exception rather than the rule. Mars, for example is not hydrostatic. Not even close really. Does that make it a non-planet? The Galilean and Saturnian satellites probably are not, except for Io. Hydrostaticity is difficult to measure. How far can something deviate from hydrostatic and still be a planet? 5%? 10%? Also, is some size cutoff needed? Otherwise the blobs of Tang the astronauts let float around the space station are technically planets. But how do you choose a size cutoff without being arbitrary?

RHEOLOGY (or material properties and deformation)

The solid bodies are primarily two materials: rock and ice. The viscosity of ice is about a million times less than that of rock, so an icy body can deform under self gravity much more easily than a rocky one. The viscosity controls the thickness of the elastic surface layer. Rigidity is really controlled by the membrane stresses in this elastic layer, which increase as the cube of the shell thickness. Then, again, the stresses also go as the fourth power of
the radius, so the rhelogical differences between rock and ice may not matter all that much. I haven't worked it out, but if it's a big difference, then icy planets have a much lower minimum size than rocky ones. How do you justify that?

DENSITY

Under the new rules, Pluto and Charon are *both* planets, because the center of mass of that system is between the two objects. The Moon, however is *not* a planet, because the barycenter of the Earth-Moon system is *inside* the Earth; that is below the Earth's surface. However, what if the Earth were slightly denser? Suppose we have a planet with an Earth mass, composed of a higher proportion of metals than the Earth. It will have a smaller radius. If it has a companion identical to the Moon at the same distance, the barycenter is *above* the surface. The smaller companion is magically a planet in this case.

TIME-DEPENDENCE

The Moon is evolving outwards from the Earth. All moons (in prograde orbits) evolve outward from their planets. Due to tidal interactions, angular momentum is transferred from the rotation to the orbits. The objects spin more slowly and move farther apart until the objects keep the same face toward each other. In virtually all cases, the moons in our solar system have already become tidally locked to their planets. At some point, as the Moon evolves outward from the Earth, the center of mass will move outside the Earth. At that point does the Moon become a planet?

Over time, as a planet's spin changes, the hydrostatic shape also changes. However, over time the planet also cools, making it difficult to deform and "freezing in" the old shape. It will depart from hydrostatic. Can it lose it's planetary status?

The real question here is "How round is round enough?"

UNIQUENESS

I think a planet should be somewhat unique. Ceres is no different than the other asteroids, it's just the biggest and first discovered. Pluto's no different than the other KBOs, it's just one of the larger ones and first discovered. Yet by the new definition they're both planets. A planet is an object that orbits a star, and is not a star itself. It is distinct in size and number. It's got to be larger than most objects in the solar system and can't belong to a large family of objects. That is, it can't be classified as an asteroid, KBO/Comet. I know that's kind of vague. Perhaps an arbitrary size cutoff is needed, although I wouldn't know what. Something where there's a clear distinction between the smallest planet and the largest asteroid or KBO. This isn't a satisfactory definition either, but it's no worse than the IAU's version.

Having said all this, I'm still going to have to abide by the decision. You're never going to get everyone to accept a definition of a planet, but we do need one. Doing this by committee is probably not a good way to do this. If you get many many brilliant people together, then they'll just come up with something stupid. But a large number of people will disagree with any definition, no matter what it is.

I'm posting this after midnight. If I said anything inaccurate or incomprehensible, please feel free to comment. Or if you agree or disagree with my reasoning.



Just remember Stephen Colbert's mnemonic. My Very Educated Mother Just Said Uh-oh! No Pluto.

Comments

[impink.livejournal.com]

Some arguments as a layman.
OK, so if the hydrostacicity argument is mistating the reasoning as to why planets are round, why not just establish an arbitrary value in the 500 mile range which seems observationally to be when most bodies start to be roundish, and that the minimum and maximum radii are within, say +/- 5% of the mean. (IIRC Mars fits in that range?)
Roundness seems to be a big distinction over the majority of asteroids and smaller stuff, so it makes sense as a characteristic, even if the process which makes them round is varied.
This also knocks some of the candidates in that list of 12 potential planets out of the running, such as Vesta which really doesn't look that round and 2003 EL61 which is big and not apparently quite ellipsoidal.

The barycenter argument bugs me too. That Ganymede isn't a planet just because it orbits Jupiter when Charon is makes no sense to me. It should orbit the sun and not primarily something else.

The time-dependence argument over the Moon doesn't bother me much, since we're talking about an astronomical body changing classification over the course of a billion years or so. But so do stars and lots of other things. The moon displacing the barycenter doesn't qualify in my mind since Charon chouldn't in the first place.

Uniqueness - that is a tough one. I've seen estimated that Ceres makes up around a third of the mass of the Main Belt, which is an advantage Pluto doesn't have in the Kuiper Belt since we've already got four KBOs in the same ballpark. Maybe that pushes it over the line? But generally speaking I agree with you.

All in all it does seem a bit of tortured logic to keep Pluto's status. And if Ceres could be demoted in the mid-19th century, why can't Pluto? Ah well.

[jameshroberts.livejournal.com]

I agree that it sounds like the definition's been concocted to preserve Pluto as a planet. I've never much liked the "double planet" thing either. I say, the biggest object in a planetary system is the planet and everything else are moons. Or ring particles.

That's another can of worms that Cassini's opening. There's a continuum of particle sizes in Saturn's rings. At what point do we call these things moons? Certainly moons don't have to be round. Do they have to be big enough to clear gaps in the rings?

Of course, if you have two bodies of nearly equal size, I guess you have to call them double planets. I don't know how often that could happen in reality. When you get to the size of things that are definitely planets (i.e. bigger than Pluto), it's dynamically improbable to have doubles. The Moon is freakishly large and it took a freakishly large impact to form it.

[cheshirecatco.livejournal.com]

Amen, brother. Although, frankly, I kind of doubt that this definition will fly and/or stick. I suspect that we're seeing the first cracking of the old guard's resistance to removing Pluto. They're trying to compromise now in a way that will keep Pluto on the rolls.

Soon, we shall remove it. Soon.

[jameshroberts.livejournal.com]

Not to belittle Pluto, of course. It's still a cool object, and it's a Good Thing (TM) we're sending New Horizons there. But it's not a planet. Francis suggested that perhaps New Horizons and Dawn have motivated these people to try to make Pluto and Ceres into planets. It *sounds* better to say we're sending these things to visit planets than to asteroids and the Kuiper Belt.