Re: Pluto's Bad Year Continues

Quadibloc <jsavard@xxxxxxxxx> wrote:
Margo Schulter wrote:
The question of 4 Vesta, which has gotten a lot of
very justified attention here from an observational
point of view, is something that especially intrigues
me: but would I be correct that it might be best to
post reports of seeing it here, and pursue discussions
as to its ideal planetary taxonomy on sci.astro or the
like?
.
Here, there might well be an uninhibited discussion of the issue!

Hi, John, and thank you for some very engaging comments that helped
draw me into this thread.

Actually, one complication of this discussion is that issues
can very rapidly branch off and merge into others: maybe a bit
of delight and awe at the intricacies of this process is more
rewarding than polemic heat, but I will try to follow your example
and keep some focus.


As I recall, while the first four asteroids discovered were Ceres,
Juno, Pallas, and Vesta, while the first three of those are also the
three largest asteroids, several asteroids that are much less well
known are larger than Vesta.

But Vesta is an unusually bright asteroid, and it does outshine those
other asteroids which are larger than it.

If Ceres got promoted from "minor planet" to "dwarf planet", one could
imagine Juno and Pallas keeping it company someday, but it would seem
that of the four well-known asteroids, Vesta would be the likeliest to
stay an asteroid, and not be promoted to any kind of quasi-planet
status.

Please let me comment for now that this situation illustrates a theme you
raise very nicely elsewhere in this thread also: the question of what I
might term "internal consistency" as raised by certain possible schemes.

Thus it evidently makes sense to me, as well as you, if I understand your
point correctly, that if 4 Vesta gets placed in a certain category, then
comparably or possibly slightly more massive bodies of a similar
composition and degree of (ir)regular shape should be similarly treated.

Here I'd say that the IAU definition of "dwarf planet" quite properly
takes hydrostatic equilibrium as a vital criterion in classification.

In the adopted IAU Resolution 5A of 24 August 2006, what we have among
the asteroids is a dichotomy between "dwarf planets" in hydrostatic
equilibrium (of which 1 Ceres is the obvious and possibly sole example),
and "small solar system bodies" not massive enough to meet this criterion,
as reflected by what I might call a "graviglobal" shape.

In the kind of scheme I favor, where "planet" is defined much more
inclusively, there are three categories: "dwarf planets" in hydrostatic
equilibrium (1 Ceres); "subdwarf planets" or "mesoplanets" approaching
but not quite attaining hydrostatic equilibrium or a regular graviglobal
shape (e.g. 4 Vesta, and likely 2 Pallas, etc.); and the myriad of less
massive "microplanets" or "planetesimals" free to assume a diversity of
shapes influenced by rigid body or other forces (e.g. 243 Ida). As noted
in another post, I am indebted to Isaac Asimov for the terms "mesoplanet"
and "microplanet" -- and also "macroplanet," here meaning a planet in
hydrostatic equilibrium -- although these are different definitions than
his original ones.

One difference between this kind of inclusive scheme and the official IAU
definition is that here all asteroids are real planets of one type or
another, and more specifically "belt planets" in contrast to the eight
"dominant planets" of our Solar System; while under the IAU scheme, of
course, no belt object is a true "planet."

Another difference is that while among the asteroids the IAU approach
recognizes two categories -- either the object is in hydrostatic
equilibrium ("dwarf planet") or it isn't ("small solar system body") --
here, as proverbially in the game of horseshoes, "almost" also counts
as a distinct possibility. In terms of conceptual elegance, I might
say that this middle category at once recognizes Steven Soter's point
that hydrostatic equilibrium is a concept with a somewhat fuzzy zone
of transition, and makes the most of this by recognizing that zone as
a category in its own right.

By the way, the special attribute of 4 Vesta that might especially
catch my attention is not so much its visual magnitude as its status
as what I would call a small differentiated terrestrial planet. If
forced to classify it in a planetary scheme including 1 Ceres as a
"graviglobe" but excluding the smaller asteroids like 243 Ida, then
I might be tempted to fudge the hydrostatic equilibrium criterion a bit.
However, having a "terrestrial subdwarf belt planet" category that nicely
fits 4 Vesta while "resolving" the distinction between it and a more regular
dwarf planet like 1 Ceres -- and also including 243 Ida as a planet of a
third variety -- seems a kinder, gentler, and more precise as well as
complex solution.

But perhaps that wasn't the issue you were intending to raise.

Ah, this discussion can go in all kinds of directions: but certainly
the question of internal consistency you have raised is an important
one. Thus your nice point about the Pluto debate: to consider Pluto
a planet "for historical reasons," but exclude the larger and more
massive 136199 Eris, might seem less than ideally consistent.

Another example: to speak today of 134340 Pluto as "the ninth
planet" might raise the question: "Why the ninth rather than the
tenth, if we're counting outward from the Sun?'

In other words, a count of Pluto as one of the approximately nine
nearest "planets" out from the Sun implies that the relevant concept
is of what I would call a macroplanet: a planet sufficiently massive
to attain hydrostatic equilibrium, whether a dominant planet (IAU
"planet") or belt planet (IAU "dwarf planet"). However, if the belt
macroplanet 134340 Pluto is included, why not 1 Ceres, the fifth
macroplanet out from the Sun, thus making 134340 Pluto the tenth
rather than the ninth?

Thus I'd consider it an important point in this dialogue that people
with different perspectives on what to call objects might agree in
finding some groupings more internally consistent than others. We
have eight planets (strictly speaking) in the adopted IAU scheme;
maybe a few score or hundreds in a scheme viewing as planets either
dominant or belt objects in hydrostatic equilibrium (other than
satellites), as in Stern/Levison or the 16 August 2006 proposal
presented at the IAU General Assembly but ultimately not adopted;
and hundreds of thousands or indeed millions of planets in an
inclusive scheme of the kind I favor.

While these three varieties of taxonomies might seem quite
different, with different advantages or disadvantages, all of them
might seem more internally consistent than a "nine planets" scheme
including Pluto but excluding the larger and more massive Eris,
or the sister belt object 1 Ceres, smaller and less massive but also
in hydrostatic equilibrium.

I'm not sure how coherently this reads, and your incisive questions
might help to clarify my prose -- but the question of internal
consistency, and of different alternatives for achieving or at least
approaching it given today's understanding of our Solar System, is an
interesting one.

John Savard


Most appreciatively,

Margo Schulter
mschulter@xxxxxxxxxx
.

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