Re: "Pluto Now Called a Plutoid"

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?

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.

Remember, the IAU has never said word one about what definition they were using. It's pretty obvious, because just like the other definitions, they didn't give one. (What is sufficiently "round"?) All the discussion you've carried on about _which_ definition they might have meant is pure speculation, because as you've already acknowledged, there are several.

The only two orbit-clearing definitions I've seen that weren't contrived on the spot (like those other two you gave above, I suspect) give the same clear and unambiguous results, results which agree with the IAU's list of what the results are supposed to be. Perhaps it's speculation to say that they probably meant one of those but it seems like pretty well-founded speculation to me.

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.
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