Why Xena Must Not Become a Planet!
- From: "George Dishman" <george@xxxxxxxxxxxxxxxxx>
- Date: 26 Sep 2006 07:48:23 -0700
Margo Schulter wrote:
http://www.georgedishman.f2s.com/astronomy/0608359F3.png
http://www.georgedishman.f2s.com/astronomy/0608359F1.png
Maybe you could expand a bit on the distinctions between the two
diagrams; either nicely makes the point that in our Solar System,
at least, "clearing the neighborhood" is a rather straightforward
distinction.
Sorry, I thought I had cited the source, they come
from a recent paper by Soter:
http://www.arxiv.org/abs/astro-ph/0608359
The paper makes the distinction clearer than anything
I could write.
Dear George,
Please pardon me for losing track of this thread for a while, ..
Not at all, thanks for getting back to me. I have rather
lost track myself as domestic life took over last week.
Anyway, what distracted me was finishing an article on planetary
definitions and typologies to which I posted a link here along with
an abstract.
Yes, it appears really comprehensive. I printed off
your full document but have only had time to get
through the first couple of paragraphs and this week
will be as bad (wife's birthday party at the weekend).
Also, your GAD.png is _very_ interesting, and looks quite
internally consistent. I'd love to discuss it in a new thread, maybe --
or this one, if you like.
I'd be delighted, I would like to expand it a bit into a
set of definitions and perhaps borrow some of your
citations. We have very different views so I think if
we could come to a compromise it would be quite
robust.
Keeping to the bounds of this discussion, what I am inclined to say now
(with the benefit of a month to reflect) is that the IAU debate didn't
seem to focus on what I consider a very important question: "Why not retain
the older and inclusive approach that says that a 'planet' is synonymous
with 'a major or minor planet'?"
Obviously developments of the last few decades including the realization that
Pluto is part of the Kuiper Belt and that Eris is larger (whether regarded as a
KBO or a Scattered Disk Object) raise questions about the "Nine Major Planets"
model, and call for something more consistent and responsive to our new
knowledge about the Solar System.
Further, I agree with the IAU majority position that if we are trying to decide
how to classify Pluto, the fact that it is a belt object rather than a dominant
or isolated one is a relevant criterion -- indeed as also with Ceres in the
asteroid belt.
Thus it makes sense to group Ceres and Pluto together -- so far, no argument at
all on my part. However, in response to the original 16 August proposal as well
as the actually adopted 5A, I must ask: "Why should regrouping Pluto with other
belt rather than isolated planets lead to removing planetary status from a fine
asteroid like 243 Ida -- well outside the definitional criteria for 'planet' under
either set of criteria -- when it has long been considered a 'minor planet'?"
I think that point and the next explain the IAU attitude:
My own approach, which Daniel W.E. Green interestingly advocated in a 1996 reply to
a query about Pluto, was that certainly we should regard Pluto as a planet of some
sort -- but clearly a minor planet rather than a major planet. Thus the assigning of
Minor Planet Number 134340 to Pluto seems to me quite appropriate -- but as an
occasion to celebrate the best-known of a group of hundreds of thousands of planets
in our Solar System which deserve more attention.
Historically planets were major items in the solar system,
essentially visible to the unaided eye and almost on a par
with the Moon and Sun. To suddenly create "hundreds of
thousands of planets" would utterly devalue the word. I
believe that was the key thought behind the IAU decision.
Of course, advocating that "planet" (outside of official IAU contexts, where
Resolution 5A is, of course, dispositive) carry its traditionally broad and
inclusive scope doesn't mean that we can't learn from the recent controversies
and make useful categorizations.
There I disagree, the "traditional" scope beyond the last
few centuries was just seven planets. The inclusion of
Uranus is entirely valid IMHO but the extension to the
minor planets was an understandable mistake based on
Bode's Law and the addition of Pluto similarly flawed
with hindsight. The IAU has stepped in to rectify the
situation.
One important dimension, of course, is the isolated/belt or major/minor distinction,
which Soter indeed presents convincingly. The issue, as I see it, is not whether
this "planetary discriminant" should be used, but how. Its best usage might be a
bit parallel to the discriminant for a quadratic equation: it tells us what kind
of planet we're considering (isolated or belt), just as the discriminant for a
quadratic equation tells us what kind of solution to expect (two distinct real
roots for a positive discriminant; a repeated real root for a zero discriminant;
and two complex roots for a negative discriminant).
Another dimension is the one focused on in the 16 August proposal, and incorporated
also into Resolution 5A: hydrostatic equilibrium, or a "graviglobe" shape (less poetic
than the "gravisphere" I found mentioned on Mike Brown's website as one description
for this test -- but more precise, or rather artfully less precise, since a "globe"
suggests an Earthlike shape with some allowance for oblation, etc.). We could classify
planets as macroplanets or microplanets based on this criterion: macroplanets are
large enough to be constrained by their self-gravity into near-spherical or "globose"
shapes, while microplanets might be found in a variety of shapes.
Again the question is how to use the test and really
the discussion revolves around names. What you call a
microplanet, I would prefer to call an asteroid.
However, in this case I think some thought also has
to be given to the point Thomas mentioned, that of
composition. I had wondered about density as a
discriminator but it doesn't work so I'd like to
hear your thoughts.
Looking down the images here:
http://en.wikipedia.org/wiki/List_of_solar_system_objects_by_mass
it is fairly clear that 511 Davida and below would fail
all but the most relaxed test of roundness while Ceres
and above would probably pass. On the other hand, compare
4 Vesta with Mimas. Vesta is nearly an order of magnitude
more massive yet visibly less round and both can be described
as near to hydrostatic equilibrium other than a major impact
crater. For icy bodies, tidal heating if they are satellites
gives them an advantage but clearly rocky bodies are at a
disadvantage compared to icy ones.
Anyway, this kind of philosophy might be summed up: "Include and classify."
Understood, mine might be "differentiate and describe",
the names and classifications should be unambiguous,
understandable, teachable and useful.
In this kind of approach, the term "dwarf planet" seems a felicitous synonym for "minor
macroplanet" or "belt macroplanet" as long as such planets are consistently smaller (at
least in a given stellar system) than major ones, and also not too far in size from the
lower limit for hydrostatic equilibrium. A definition that might "travel better" over a
range of hypothetical systems is that a "dwarf planet" is defined as a size category for
the smallest macroplanets, with Ceres-Eris (or possibly a bit smaller, if any other
asteroids are regarded as "graviglobes") as one illustrative portion of the range. This
means, as I remark in my paper, that "dwarf planet" as a synonym for "minor macroplanet"
could be a useful "Solar System provincialism."
Again, I'd love to discuss GAD.png more -- maybe in a new thread?
Delighted, I would be keen to refine my own views either
here or by email but time pressures will be difficult at
least until the weekend. Drop me a note at the address
in this post and I'll reply from the real one - this gets
spammed of course.
best regards
George
.
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