Re: "Pluto Now Called a Plutoid"

In message <psOdnVGGpOm07fXVnZ2dnUVZ_h3inZ2d@xxxxxxxxxxxxx>, Erik Max
Francis <max@xxxxxxxxxxx> writes
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.

Both papers were the subject of discussion at the time and do seem to
have been what was in mind. There wasn't much need to specifically
quantify the definition or to specify exactly hoe orbital dominance was
defined given that two rather different methods give a several orders of
magnitude difference between planets and everything else.


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."

Despite obviously not meeting the sort of common sense definition most
astronomers were using, Pluto was often referred to as a planet, the
redefinition restricted the name to just the class of large orbitally
dominant objects. There was a bit of a problem with general statements
about planets having to specifically exclude Pluto so much as with one
and prime numbers the definition was narrowed to exclude it.

The list was simply a footnote exhaustively stating the known bodies
that meet the definition. It is not part of the definition it is a
footnote. The IAU has a number of catalogues; the footnote lists the
members of the planet catalogue. Most classes have enough members that
this is impractical.

The definition was intended to reflect the consensus on what a planet
was while being self consistent. So it should match general usage of the
term, that was the point of the exercise. As there was a lot of press
interest it seemed to be a good idea to make sure that the press were
absolutely clear that the definition excluded Pluto, given Journalists
usual level of competence at reporting science stories this seems a good
idea.

The definition itself is as follows:

The IAU therefore resolves that planets and other bodies in our
Solar System, except satellites, be defined into three distinct
categories in the following way:

(1) A "planet"[1] is a celestial body that: (a) is in orbit
around the Sun, (b) has sufficient mass for its
self-gravity to overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly round) shape,
and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that: (a) is in
orbit around the Sun, (b) has sufficient mass for its
self-gravity to overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly round)
shape[2], (c) has not cleared the neighbourhood around
its orbit, and (d) is not a satellite.

(3) All other objects[3] except satellites orbiting the Sun
shall be referred to collectively as "Small Solar System
Bodies".

Footnotes:

[1] The eight planets are: Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus, and Neptune.

[2] An IAU process will be established to assign
borderline objects into either "dwarf planet"
and other categories.

[3] These currently include most of the Solar System
asteroids, most Trans-Neptunian Objects (TNOs),
comets, and other small bodies.

The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is
recognised as the prototype of a new category of
trans-Neptunian objects.




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.

And Pluto isn't one.

Given the nature of the discussion at the time it is fairly clear that
an object that met the definitions in those two papers was a planet. The
definition is intended to put into words roughly what everyone tended to
think of as a planet.

Rather than requiring papers to include a foot note stating the local
definition there is now a general definition which can be assumed.


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?

They were topics of discussion at the time. The definition is explicitly
based on orbital dynamics not planetary mechanics. Two markedly
different definitions of dominance give the exact same list very
clearly. That the definition might sound a little fuzzy does not make it
any way ambiguous. The definition also leaves to option of finding other
ways of quantifying orbital dominance.


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.


The IAU now has a definition with which to rule on the nature of a body.
Which class a body belongs to affects the procedure for formally naming
it. The Kuiper cliff was suspected to be real at the time and there was
speculation that a fairly massive body had created it. If such a body
exists and is responsible for the cliff then it is a planet.
--
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