Re: "Pluto Now Called a Plutoid"

Bryan Derksen wrote:

Erik Max Francis wrote:
Bryan Derksen wrote:
Do you know of any methods of quantifying "neighborhood-clearing" that don't make the division between Pluto and the rest obvious?

Sure, there are plenty of trivial ones. "An object has cleared its orbit if there are no orbits intersecting it (after projecting both their orbits into the ecliptic) that are greater in mass than Ceres." Or, "There are no other objects greater in mass than Ceres within 2 au." Or any number of other quantified definitions.

These are pretty silly definitions. If you include moons in consideration (since you didn't mention anything that excluded them from consideration) the first definition eliminates Earth, Jupiter, Saturn, Uranus, Neptune, Pluto, and Eris. The second eliminates every candidate _except_ Eris, and even then Eris may sometimes temporarily lose planethood when other large Kuiper belt objects pass near it.

Not considering moons, the first definition eliminates Neptune, Pluto and Eris as planets and the second definition eliminates Mercury, Venus, Earth, and Mars (both due to their nearness to each other and their nearness to the Sun, which is more massive than Ceres). In the second definition Pluto and Eris are sometimes planets and sometimes non-planets depending on how the other large Kuiper belt objects are arranged on any given day. Ceres itself is also sometimes a planet and sometimes not depending on where it is in its orbit relative to Mercury, Venus, Earth and Mars.

So clearly these are not what the IAU had in mind. Where did these come from? Or did you just make them up on the spot?

The point is, there are an _infinite_ number of possible quantifications. Anyone could come up with some that they think are useful or not. You could argue that this is part of what the Bode-Titius law is.

The fundamental point here, which you're skipping over, is the IAU has never said what they mean. You finding a few papers where people were talking about similar concepts _doesn't mean that's what the iAU had in mind when they made that declaration_, since there's absolutely zero evidence whatsoever that they did. One could certainly come up with others that don't match the list they chose.

In fact, I would doubt very much that there was a quantifiable definition that they had in mind. As I've said, all the IAU's ultimate declaration is basically a paragraph of, "Planets are big, round, and gravitationally influential on bodies around them." That much is blatantly obvious and was _already_ the de facto "definition" of a planet, for those who've tried to come up with generalizations to, say, other planetary systems, as we've seen many times over the years. Except, of course, that part of the IAU's definition was, "Oh yeah, here's an updated list; Pluto isn't on it. Sorry we had to make this list because the exclusion wasn't even obvious enough by our old definition."

Why not assume that they intend one of the previously-published methods that _do_ make this division obvious, since they explicitly say in the resolution that it makes that division? Do you have some reason to believe that they picked an ambiguous definition and then made an arbitrary decision to exclude Pluto?

Since they never gave or hinted at a definition, and excluded Pluto anyway, yes.

Could you please explain _why_, though? I already know that you're assuming this, I asked what reason you had.

Definitions of planet clearing had been previously published. The members of the IAU who voted on this resolution are, generally speaking, well-read on such matters. I don't see any reason to assume they didn't have those definitions in mind, especially since they give the same results as the IAU's unspoken definition.

That those papers exist doesn't mean that ~400 people who voted on that resolution (not voted to accept; voted total) were specifically familiar with them in detail. In fact, I'd seriously doubt that they were, as I've said above (and before), the IAU's resolution proposal was nothing more than common sense: namely, that planets tend to clear out other, smaller objects from their orbits.

I'm sure you were aware of this notion long before the IAU proposal came out, or before you found those papers after the proposal came out. Surely that doesn't mean that your (presumed) acknowledgement of that obvious abstract feature of what we tend to call planets doesn't mean that you were specifically familiar with those papers? That doesn't make much logical sense.

Remember, the only person linking the IAU's non-specific resolution with those particular papers is _you_. There's zero evidence anyone who voted for that IAU resolution specifically had those papers in mind when they voted for it, is there?

Humans are smart. I'm sure you could find evidence of _any_ vague, non-specific notion that pops up in any proposal after the fact. It ain't hard.

They were giving vague measures of what it means to be a planet. But that was stupid; we all already _knew_ vaguely what it means to be a planet -- it's something that's big, round, and gravitational influential on its neighbors. The IAU definition accomplished nothing, except strike one of the previous members off the list.

They also struck down a bunch of candidates that nobody was entirely sure would be planets or not, because the previous "definition" of planet was far _more_ vague than the one you're complaining about. It also provided some measurable basis for arguing whether new discoveries are planets - the hypothetical object responsible for the Kuiper "cliff" mentioned elsewhere in this thread, for example. Those both seem like accomplishments to me.

There was no Kuiper "cliff." The previous IAU definition was merely a list; the IAU would have to specifically _add_ objects to the list for them to qualify, and there was zero risk of this happening at any point. This is simply a false slippery slope argument.

--
Erik Max Francis && max@xxxxxxxxxxx && http://www.alcyone.com/max/
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