Why Xena Must Not Become a Planet!

Margo Schulter wrote:
In sci.astro wrote:

Margo, I can't reply in full at the moment but
this quick note will give you some reading in
the meantime:

http://www.georgedishman.f2s.com/ast.../0608359F3.png

http://www.georgedishman.f2s.com/ast.../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.

Best regards
George

In sci.astro George Dishman wrote:

Margo Schulter wrote:

http://www.georgedishman.f2s.com/ast.../0608359F3.png

http://www.georgedishman.f2s.com/ast.../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.

Best regards
George


Dear George,

Please pardon me for losing track of this thread for a while,
something I corrected by checking http://groups.google.com.
Anyway, the Soter article is indeed clear, and I should have
immediately recognized this as the source.

Anyway, what distracted me was finishing an article on planetary
definitions and typologies to which I posted a link here along with
an abstract. 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.

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'?"

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.

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.

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.

Anyway, this kind of philosophy might be summed up: "Include and classify."

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?

Best,

Margo Schulter

Margo Schulter wrote:
In sci.astro George Dishman wrote:
Margo Schulter wrote:

http://www.georgedishman.f2s.com/ast.../0608359F3.png

http://www.georgedishman.f2s.com/ast.../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.

Best regards
George


Dear George,

Please pardon me for losing track of this thread for a while,
something I corrected by checking http://groups.google.com.
Anyway, the Soter article is indeed clear, and I should have
immediately recognized this as the source.

Anyway, what distracted me was finishing an article on planetary
definitions and typologies to which I posted a link here along with
an abstract. 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.

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'?"

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.

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.

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.

Anyway, this kind of philosophy might be summed up: "Include and classify."

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?

I always refer to the Meghar scale now for these things "

http://cosmic.lifeform.org/?p=166

That being said, among the various dwarf planets, we would also have to
differentiate between icy outer dwarfs, and rocky inner dwarfs, although
Ceres appears to be at least somewhat icy. Pluto ices are expected to be
different than asteroid planet ices too. Some outer moons are almost
completely icy. Do we have to refer then to the 'purity' of the planet?

Ceres is considerably smaller than Pluto or Charon, yet it is still
gravitationally relaxed. On the Meghar scale it is an asteroid planet.
They are all still minor planets, but it's a very easy distinction.

Even a lunar sized planet would still be considered a minor planet.

http://cosmic.lifeform.org

In sci.astro Thomas Lee Elifritz wrote:
Margo Schulter wrote:


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?

I always refer to the Meghar scale now for these things "

http://cosmic.lifeform.org/?p=166

This is very interesting, and raises questions about just where we might
draw lines between categories of planetary size.

That being said, among the various dwarf planets, we would also have to
differentiate between icy outer dwarfs, and rocky inner dwarfs, although
Ceres appears to be at least somewhat icy. Pluto ices are expected to be
different than asteroid planet ices too. Some outer moons are almost
completely icy. Do we have to refer then to the 'purity' of the planet?

Certainly I agree that composition is an important distinction, and this
was one of the points of the Stern/Levison proposal suggesting a set
of categories somewhat analogous to those for stars from "subdwarf"
to "supergiant." Their system uses size as one axis, but composition as
another.

http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf

Ceres is considerably smaller than Pluto or Charon, yet it is still
gravitationally relaxed. On the Meghar scale it is an asteroid planet.
They are all still minor planets, but it's a very easy distinction.

Yes, in my proposed system, we might call Ceres an "asteroid macroplanet."
A macroplanet in this terminology is a gravitationally relaxed one.

http://www.bestII.com/~mschulter/inclusive_planet_def100.txt

Here an interesting distinction is that an asteroid like 4 Vesta, although
not quite fully gravitationally relaxed, it seems, nevertheless is now often
regarded as differentiated much like a small macroplanet like Ceres -- while
some gravitationally relaxed "ice dwarfs" might not be this differentiated.


Even a lunar sized planet would still be considered a minor planet.

http://cosmic.lifeform.org

An interesting question here is how one defines "minor" -- which in my
approach is basically dynamical -- belt planet rather than dominant or
isolated planet (or "hegemonic planet," if one likes. However, if we
take this to be a question of intrinsic size or mass, then it's interesting
that both Stern/Levison and the Meghar Scale place a categorical line at
a bit above the mass of the Moon (roughly 1/81 Earth's mass) at about 0.03
and 0.05 respectively. For Stern/Levison this is the "subdwarf/dwarf" line,
with "subdwarf" including Ceres, Pluto, and the Moon; and "dwarf" the
terrestrial major planets (a category with an upper boundary at 10 Earth
masses).

Anyway, the question of differentiation for a very large microplanet like
4 Vesta (if one concludes that it doesn't quite represent full hydrostatic
equilibrium) is one reason I favor a flexible and inclusive approach -- but
I'd agree in any event with the addition of Ceres to the Meghar scale.

Indeed I feel strongly that Ceres should be a "household word" included in
any teaching about our Solar System, which means that the asteroid belt
and minor planets generally should get more attention, since I would
consider it very important to present Ceres at once as a gravitationally
relaxed body and as "first among equals" in a fascinating society of belt
planets, so to speak.

Most appreciatively,

Margo Schulter