Re: "Pluto Now Called a Plutoid"

In message <MtudnVr0jcjfwvnVnZ2dnUVZ_h3inZ2d@xxxxxxxxxxxxx>, Erik Max Francis <max@xxxxxxxxxxx> writes
Bryan Derksen wrote:

Erik Max Francis wrote:
Brett Paul Dunbar wrote:
No that is actually totally wrong. The definition is entirely
unambiguous, given the sheer size of the gap. On either the
Stern-Levinson parameter or Soter's Planetary discriminant, the gap is
about five orders of magnitude.

Neither of these measures is mentioned, even indirectly, in the IAU's resolution regarding Pluto or any subsequent IAU resolutions. So, so what?
They don't mention any other specific method of measuring neighborhood-clearing either. Or several other potentially ambiguous parts of the definition, such as what it means to be "orbiting the sun" (there are various co-orbital bodies that could go either way if you play with the definition a bit). I think they assumed that most people reading their resolution would not be trying to lawyerly nitpick holes in it but would instead be more interested in the basic spirit it conveyed.
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.

That is a rather contrived and obviously inappropriate definition. I.e. it is obviously wrong.

Soter's paper <http://arxiv.org/ftp/astro-ph/papers/0608/0608359.pdf> gives a fairly decent basis for determining whether the neighbourhood has been cleared in the concept of the planetary discriminant; the ratio between the mass of the body and the total mass of the other non-resonating and non-satellite bodies in the same orbital zone. He defined the orbital zone as follows "Two bodies share an orbital zone if their orbits cross a common radial distance from the primary, and their non-resonant periods differ by less than an order of magnitude." This seems a good definition for neighbourhood.


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.

They did, they resolved on clearing the neighbourhood and hydrostatic equilibrium. The two different papers give markedly different definitions of clearing the neighbourhood but identical results, indicating that what we have done is apply the label planet to a natural class. We have reason to believe that there is a critical mass above which an object will eject or absorb all of the non-resonant mass in the vicinity of its orbit. This means that once a body passes the planet threshold it will grow rapidly and quickly be an entirely unambiguous case.

What we have a clear-cut separation based on the orbital dynamics of the body.

Stern and Levinson's paper <http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf> attempted to find which objects would do that, while Soter's measured whether a given object actually had.


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.

There are two currently, it is possible that there are a few others that differ slightly, but all would produce a division at the same point


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.


Pluto didn't fit, it had been called a planet when it was first discovered as it was thought to be far more massive than it actually was, eventually it became a tiny anomaly, it didn't fit with the planets but it didn't fit any other category either, while there was only the one object it just got left. The discovery of other Plutonian bodies allowed the definition of a more appropriate category for Pluto and its removal from the class of planets, much as the discovery of other Asteroids had resulted in Ceres being re classified.
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