Since the barycenter of the Pluto-Charon system is outside Pluto, why aren't Pluto and Charon treated as a binary system of dwarf planets rather than a dwarf planet and a moon? Charon's certainly big enough to qualify...

Who says it isn't one?

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The IAU, actually:

Source.

They have also classified Makemake and Haumea as the fourth and fifth dwarf planets, with so far no mention of Charon being in the category.

Nick

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Nick Theodorakis

Hmm, yes interesting question. Since Pluto and Charon are a double-planet there doesn't seem to be any justification for calling Pluto a Dwarf planet and not Charon. It's not a moon and it certainly seems to satisfy the other requirements for Dwarf Planethood. Seems like a strange oversight to me, unless there's some specific reason why it is not categorised that way.

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The Sky is no longer the Limit

I think the most practical solution is just to ignore its existence. When New Horizons gets there in 2015, just make sure the camera doesn't focus on it, and we can pretend it doesn't exist. Then there is no need for a category. Kind of what like they did to Lavrenty Beria when he became inconvenient (Soviet history).

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As above, so below

Seeing as how the Pluto-Charon system is more or less a double-planet system, then it would be perfectly safe to call Charon a dwarf planet. But again, I like to think of Charon simply as "a moon of the dwarf planet Pluto."

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"Science is physics and astronomy." -Me
"There is absolutely no law in physics that prevents time travel." -Dr. Michio Kaku

Just because the IAU says something doesn't make it true. The IAU also says dwarf planets are not planets. The term "dwarf planet" was coined by Dr. Alan Stern, one of the leading astronomers who reject the IAU planet definition, and he never intended for it to mean non-planets. It was intended to describe non-self-luminous objects orbiting stars, large enough to be in hydrostatic equilibrium but not large enough to gravitationally dominate their orbits.

The IAU definition precludes any notion of a binary planet system, as by definition, the two objects in a binary do not "clear their orbits of one another."

Charon and Pluto are a system of binary dwarf planets. Hopefully, data from New Horizons will show this to be obvious. As for the IAU, the best thing is to ignore them entirely and make them irrelevant.

According to an interview I heard with an astronomer-type in the days after Pluto was declared to not be a planet, the IAU's definition of what constitutes a planet was so poorly worded that a Jupiter sized body orbiting the sun beyond Pluto would not be considered a planet. (He was in the "Pro" camp as far as Pluto being a planet, so he might be a bit biased.)

The truth is that no matter what definition we come up with, as far as what constitutes a "real" planet as opposed to a "dwarf" planet, its somewhat arbitrary. Suppose, for example, we discover an Earth-sized body orbiting a planet twice the size of Jupiter in another star system. Certainly, its a moon of that planet, but its also the size of the Earth (a body we consider to be a planet). Are we going to coin a new term to describe such a world? (Poon, perhaps?) Sure, we could call it a "moon," but that doesn't help us picture how large the body is.

Its like when I'm reading a SF story about people living on Mars and they talk about "Marsquakes." Please. Humans hang on to terms long after their relevance has faded. After all, when was the last time you "dialed" a phone? Calling Pluto a "planet" or a "dwarf planet" or even a "blerg" doesn't change what it is, namely a naturally formed object of a certain size, which orbits the sun.

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I think this is a bit harsh. On the planet naming thing, perhaps, but the IAU is very useful in other ways.

Tuckerfan has put it quite eloquently, but really, this is not so much a problem with the IAU as it is a problem of trying to use words to classify the natural world. The fact is, Pluto is Pluto, and arguing over what word best describes its category is both futile and inevitably controversial. But we humans like naming things.

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As above, so below

We could just ignore it, as has been mentioned. But the problem is that when you explain the planets and the Solar System to your friends, you'll inevitably get to a point where you need to say something about Pluto and the likes of it. Try telling someone Pluto is not a planet but a dwarf planet and count the question marks popping up from their head.

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"There is absolutely no law in physics that prevents time travel." -Dr. Michio Kaku

You can also just say that Pluto is a planet. The IAU will not arrest you. If the person argues about it, you can just say that it's a semantic issue and that the IAU used to consider it a planet but now doesn't, and who knows what they'll say next year?

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As above, so below

Plus, to be honest, "dwarf planet" is a bit insensitive. They could use a term like "diametrically challenged planet."

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As above, so below

And as Fraser said on Astronomy Cast, "Poor old Ceres, it got a promotion and nobody cares"

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What is the class that comprises: the Moon, the Galileans, Titan and Triton? Satellites of course, but that's just because the classification is based on orbit rather than mass. Arbitrary?

I know. It's still somewhat missed up, though. And I honestly don't expect them to do anything about this whole thing anytime soon. I recently bought a college text for my astronomy courses and I was actually quite surprised to see that it was all written in accordance with the new definitions. Not that I don't like it or anything, but I was sure that it was all very official already.

Watashi ha amarini mo kibou.

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"Science is physics and astronomy." -Me
"There is absolutely no law in physics that prevents time travel." -Dr. Michio Kaku

This whole thing about a "dwarf planet" is just an IAU political contrivance anyway, as opposed to scientific. they were under pressure for a compromise and invented that term to solve it. But really, to include "Ceres" as a dwarf planet is silly, because it is scientifically speaking in the asteroid belt in terms of its history of formation. Hopefully IAU will abolish this definition , and arguing about what is or isn't a dwarf planet should be viewed in this context. Ceres shouldn't be one.

Well, when Ceres was originally discovered, they thought it was a planet, and then decided, no, its not, and now its been kicked up to an "almost a planet state."

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The note you provided does not say that this list is exhaustive.

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Forming opinions as we speak

Maybe not, but by applying a sequence to it, it does mean that Charon is not as noteable as the others.

Although; I wonder if there is an implied "system" in there. Like maybe they are talking about the Pluto Dwarfatory* System.

*Fazorism.

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Numbers are not case sensitive. (me)

No, but it's clear that at the time they upgraded Haumea (Sept 2008) they still were not calling Charon a dwarf planet. They called it the "fifth" and Makemake the "fourth" and before that were Pluto, Ceres, and Eris.

Nick

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Nick Theodorakis

I don't think dwarf planet is meant to imply anything about history of formation, that is why the terms asteroid and Kuiper Belt Object/Plutino are still used about Ceres and Pluto when appropriate. There is even talk of Pluto also picking up a comet classification (it probably has a comet tail).

Why can't Pluto be like New Zealand? A north and a south island but one nation.
Pluto a two body planet?

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Then, surely, the asteroid belt can be like Indonesia - one planet in a zillion bits.

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Even Ceres should be deleted from list of Dwarf Planets. It's an asteroid.

Except there is nothing asteroidal, that is to say star-like, about Ceres.

Charon should be called a Dwarf Moon, naturally.

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There isn't? It's a distinct object floating around in space. I'd say that makes it pretty star-like.

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As above, so below

Well, no, because stars are (or at least were) primarily composed of hydrogen and capable of supporting fusion. Random hunks of rocks, like Ceres or Vesta, don't meet that definition.

Nomenclature doesn't necessarily have to make sense; it just has to be sufficiently self-consistent so that everybody knows (with some small amount of uncertainty) what each term needs.

So nobody is interested in signing the Congratulations on your promotion card we were going to post to Ceres then?

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From what I've read, it's rather indistinct, which is why it doesn't have planetary status.

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"What you think you thought you saw you did not see." Agent J, MiB - Manhatten Bureau

Since there are many conflicting definitions of what is and isn't a planet, let's call Pluto a planet, Charon a planet, Ceres a planet, Planethood for everybody!

(I personally prefer to use the older term "planetoid" instead of "dwarf planet", since if Dwarfs get an astronomical term you just know the Elves will insist on having something named after them too, and I have no wish to get involved in that whole conflict.)

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