Toward a Rational Definition of what is a Planet

What comes below is a continuation of the thread "LARGEST ASTEROID
MAY BE 'MINI PLANET' WITH WATER ICE (STScI-PR05-27)" on the newsgroup
sci.astro.amateur, which started on September 7th as an absolutely
arbitrary babble and -so far- ended on September 10th with radical
nitpicking. The first I consider as ridiculous and the latter as
unreasonable. But if I had to choose between these two options, I would
of course not hesitate to choose the latter. And what I wrote in the
s=2Ea.a. thread was in turn based on what I wrote on August 3rd and 23rd
in the newsgroup sci.astro.research under the very same title as this
article. It is interesting to note that these articles did not cause
any controversy on s.a.r., while on s.a.a. they did (some). Let=B4s see
what happens here.

As I have already mentioned in the "mini planet" thread, I believe
that this subject -i.e. the definition of what is a planet- deserves
its own thread title. So I will reproduce that thread here starting
from the point from where it became interesting, before writing my own
comment at the bottom:


Begin quote-------------------------------------------------------

Brian Tung 8 sep 19:49
Peter Holm wrote:
I believe that the time has come to do away with the old misconception
that that planets are "big". "Big" is not a scientific term and
should therefore not be used in a scientific discussion. And this is a
science forum, or isn't it?
It's a science forum for amateur astronomy. That means that your post,

although close enough on topic for my own tastes, is probably better
disposed for another group, like sci.astro. As far as I'm concerned,
you can post it here, but others might not feel so great about it. In
particular, claims that we're not being scientific if we don't play by
your rules will not be taken kindly.
In any case, terms are not either scientific or otherwise. I'm not
really sure what it would mean to say a term is scientific; usually, we

say that a proposition is scientific if it can be tested. Although the

phrase "Planet X is really big" is not testable, and therefore can be
considered unscientific, it can be emended to "Planet X is more than
1,400 km across its smallest diameter," and then it's just fine.
Besides, the problem in definitions of the term "planet" is not that
they can't be made precise; we can make them precise without any
trouble
at all. The problem is that they are then arbitrary. Your definition
has the same root problem: it can be made either precise or not
arbitrary,
but not both.
You might not think so, but consider that if I take your definition at
face value, there are no planets at all in the solar system. No object

is perfectly spherical. Any rotating object has an equatorial bulge;
in addition, most of them have some higher-order deformation (the Earth

is slightly bulgier on the southern side than on the northern side, for

instance), and most of the rocky or icy ones, where the planethood is
in
doubt, have mountains and things poking out here and there.
Then, too, none of them has a perfectly elliptical orbit, so that you
can tell that the Sun is at one focus. They're all constantly
perturbing
each other. And the dividing line between star and planet isn't yet a
clear one: is a brown dwarf (a deuterium fusor) a star or a planet? Or

is it a star when it's fusing deuterium, and a planet afterward?
The upshot is that you have to draw a few lines in the sand to make
sure
that what we've always considered planets "for sure" (such as the Earth

and Venus and Jupiter and Saturn, and so forth) continue to be planets.

The challenge is to draw those lines in a way that is not arbitrary.
How spherical is spherical enough? Can you say how spherical it has to

be without using a number?
This is the challenge that all planet definers have had to face, and in

the end, some of us have come to a different conclusion: that it isn't
the definitions that are the problem; it's the term "planet" that
defies
"scientificness." It's simply too historically laden to be useful as a

precise term. Even the terms "major planet" and "minor planet"--for
Pluto will always be a planet, albeit possibly a minor one--are tough
to use precisely. Better to coin a new term if you want to be precise.


David Knisely 8 sep 19:47
A modification on that definition might be a body which is large enough

that gravitational forces can overwhelm its material strength, forcing
it to assume a fairly spherical shape. If such a body is in an
independent orbit around a star, then it would be considered a planet.
This would open up the classification for a number of objects in the
solar system, including Ceres and some of the large Kuiper belt
objects.
I would prefer this over some arbitrary diameter like putting the
cutoff at the diameter of Pluto. Clear skies to you.

Brian Tung 8 sep 21:19
David Knisely wrote:
A modification on that definition might be a body which is large enough
that gravitational forces can overwhelm its material strength, forcing
it to assume a fairly spherical shape.
That's still either vague or arbitrary, depending on how you define
"fairly spherical." For no other objects other than black holes do
electromagnetic forces vanish in comparison to the gravitational.

John Savard 9 sep 03:14
On Thu, 8 Sep 2005 20:19:08 +0000 (UTC), (Brian Tung)
wrote, in part:
That's still either vague or arbitrary, depending on how you define
"fairly spherical." For no other objects other than black holes do
electromagnetic forces vanish in comparison to the gravitational.
True, although with a neutron star you're pretty safe...

David Knisely 9 sep 06:55
Brian Tung wrote:
That's still either vague or arbitrary, depending on how you define
"fairly spherical." For no other objects other than black holes do
electromagnetic forces vanish in comparison to the gravitational.
It isn't arbitrary at all. The moon is "fairly spherical", even though

it has numerous bumps and bulges. However, the point isn't that a body

has a spherical form. Jupiter and Saturn both are *not* spherical due
to their extreme rotation speed. However, in both cases, if they did
not rotate, the mechanical strength of these objects would not be able
to hold them in any shape except a spherical one due to the influence
of
gravity (and no one would argue that both are planets). The tensile
strength (and/or density) of various materials can be compared to the
self gravitational forces to see which would dominate the shape of an
object. At a certain size, gravity would dominate and force the object

to stray significantly from a rather any irregular shape it might have.

*That* is where the definition would take hold. Ceres is spherical.
Vesta is not (athough it is approaching a point where gravity is
definitely starting to influence the body's final shape as it is
formed). I wish I could recall the issue of Sky and Telescope which
had
the curves, but the planetary scientist who came up with this criteria
presented a compelling argument for its adoption as the definition of
what is a planet and what is not. Clear skies to you.

Brian Tung 9 sep 08:02
David Knisely wrote:
It isn't arbitrary at all.
As stated thus far by you, it's either vague or arbitrary. If you
don't
define "fairly spherical," it's vague. If you do define it, it's
likely
going to be arbitrary. Certainly you have given me no confidence of a
non-arbitrary definition.
[snip]
At a certain size, gravity would dominate and force the object
to stray significantly from a rather any irregular shape it might have.
^^^^^^^^^^^^^
"Significantly" should be defined, or else the definition stands
self-confessed as arbitrary. I've read the Sky and Telescope article,
by the way.

Alson Wong 9 sep 20:11
"Brian Tung" wrote in message
...

- Mostrar texto de la cita -

Here's a case in point. Mimas is 392 km in diameter and is, most would
agree, fairly spherical:
http://www.nineplanets.org/mimas.html
Proteus is slightly larger, averaging 418 km in diameter, and is
somewhat
irregular, but also somewhat spherical overall:
http://www.nineplanets.org/proteus.html

David Knisely 10 sep 07:48
Brian Tung wrote:
As stated thus far by you, it's either vague or arbitrary. If you don't
define "fairly spherical," it's vague. If you do define it, it's likely
going to be arbitrary. Certainly you have given me no confidence of a
non-arbitrary definition.
Spherical is resembling a sphere. Something which visibly departs from

a sphere is not exactly spherical. Again, Ceres is fairly spherical
and
Vesta is not quite there but clearly shows evidence of at least
approaching a spherical shape. This shows that gravity is probably the

dominant force in determining the shapes of these objects. Something
like Eros clearly is not spherical, which shows that it is within the
realm of mechanical or tensile strength dominating over self
gravitation. Perhaps something with a departure from a perfect sphere
of less than 15 to maybe 20 percent would not be terribly noticable to
the eye, but this obviously will not satisfy you. In any case, this
isn't exactly what I am referring to, since I cite the cases of Jupiter

and Saturn.
At a certain size, gravity would dominate and force the object
to stray significantly from a rather any irregular shape it might have.
^^^^^^^^^^^^^
"Significantly" should be defined, or else the definition stands
self-confessed as arbitrary. I've read the Sky and Telescope article,
by the way.
Again, you may have read the article but you did not clearly understand

what the article really said. The "small" end of the planetary scale
would be determined at the point where gravity would be the primary
factor in determining the shape of the body and not mechanical strength

or other factors like surface tension or rapid rotation. The internal
strength of the material would be smaller than the internal
gravitational forces present within the body. If the body's internal
strength was less than the gravitational forces trying to make it more
symmetrical, the object would be considered a planetary body and it
would very likely be at least somewhat close to a spherical shape.
Exactly how close to spherical is not all that relevant to this
criteria. As Stern and Levinson state in their article,
"This shape need not be strictly a sphere - a rapid spinner would
conform to a distinctly oblate ellipsoid, as is the situation with
Jupiter and Saturn. Our emphasis on the rule of mass, and hence
self-gravity, has the ancillary benefit of eliminating nearly spherical

objects controlled by the surface tension or by electromagnetic or
electostatic forces. Also, we'd allow our candidates enough time
(basically, the age of the universe) to reach their equilibrium shape".

Something with a high mean density would have a larger threshold
radius for the point of internal strength vs. self gravitational
strength, while lower density objects would have a smaller threshold
radius. Look at the diagrams the author put up on the bulk density vs.

the body radius to see where he places the various small bodies of the
solar system (p. 45 of the August 2002 issue).
All in all, the definition system for a planet put forward by Alan
Stern
and Harold Levison is a more logical system than just quoting some
completely arbitrary radius like the radius of Pluto for the minimum
size of a planet. Clear skies to you.

Brian Tung 10 sep 17:41
David Knisely wrote:
Spherical is resembling a sphere. Something which visibly departs from
a sphere is not exactly spherical. Again, Ceres is fairly spherical and
Vesta is not quite there but clearly shows evidence of at least
approaching a spherical shape.
"Resembling a sphere" is a vague definition. I don't dislike it, but
it's vague. Even the Earth, even if it were not spinning, would not be

perfectly spherical. It would have random lumpiness floating around
the surface.
This shows that gravity is probably the dominant force in determining
the shapes of these objects.
I agree. It's not easy to quantify this, though. See below.
Perhaps something with a departure from a perfect sphere
of less than 15 to maybe 20 percent would not be terribly noticable to
the eye, but this obviously will not satisfy you. In any case, this
isn't exactly what I am referring to, since I cite the cases of Jupiter
and Saturn.
First of all, I'm willing to posit a non-rotating Jupiter and Saturn.
I'm even willing to consider them far removed from the Sun, so that
they
are not subject to uneven heating. Even then, they are affected by
internal heat, which causes turbulence, which leads to deviations from
a perfect spherical shape.
Secondly, what would 15 or 20 percent mean? You mean smallest diameter

deviates from largest diameter by less than 15 or 20 percent? OK,
that's
no longer vague--but it is now arbitrary. Why 15 or 20? Why not 10?
Why not 3? No matter what bar you set, that kind of bar is going to
have
a certain arbitrariness.
But you claim that the actual degree of sphericity doesn't matter that
much. OK, let's go on and see if that's actually so.
Again, you may have read the article but you did not clearly understand
what the article really said.
On the contrary, I understood very well what it said. Perhaps the
definition is better stated in the original scientific journal article.

The "small" end of the planetary scale
would be determined at the point where gravity would be the primary
factor in determining the shape of the body and not mechanical strength
or other factors like surface tension or rapid rotation. The internal
strength of the material would be smaller than the internal
gravitational forces present within the body. If the body's internal
strength was less than the gravitational forces trying to make it more
symmetrical, the object would be considered a planetary body and it
would very likely be at least somewhat close to a spherical shape.
Exactly how close to spherical is not all that relevant to this
criteria.
Is it? I think you may not have thought about some things regarding
the definition *as stated in the Sky and Telescope article*. On face
value, it's still vague. It simply states that the shape is determined

"primarily" by gravity rather than by mechanical strength or other
factors
like surface tension or rapid rotation.
First of all is that tricky word, "primarily." What does that mean?
But I won't heckle too much here--let's just assume that it means that
if we can quantify all of the factors with a common unit, the factor of

gravity is greater than all others combined.
Next, what are the "other factors"? They aren't enumerated. I've
already said that I'm willing to assume a planet far enough from a heat

source, and not rotating. You know what, I'll even throw out internal
heat--how's that for agreeability?
I'm going to guess, and it's only a guess based on what's in the
article,
that what is meant by "other factors" is electromagnetic forces, which
would include mechanical strength (whatever that means, precisely) and
surface tension, but not rapid rotation or internal heating (not
directly,
anyway).
Unfortunately, I don't have a good guess as to what the common unit of
measurement would be. Newtons, maybe? It's possible--certainly, it's
a
unit of force. But force on what? Let's suppose we start off with a
spherical body, and attach a protrusion. Can we establish the relative

strengths of the gravitational force on the protrusion and the
electromagnetic forces on it?
It turns out that we can, of course, but there's a problem. The
comparison depends on the shape and size of the protrusion. In other
words, the relative strength depends on how spherical the shape is.
This really isn't surprising: It's much easier for gravity to flatten
a tall skinny protrusion than it is for it to flatten a short squat
one.
Now, Stern and Levinson are sharp guys. I am quite certain they would
not have left their actual definition that vague. In composing their
article for Sky and Telescope, I am quite certain they ignored us folks

who want that level of precision and wrote for the vast majority who
just want to know the basic gist of their work. They must have, since
they have a graph with a specific round/irregular line.
What I am less confident about is the arbitrariness of the definition.
Note that the labels "round" and "irregular" *explicitly* indicate a
bar set at a specific level of sphericity. Again, they're pretty sharp

guys--it's surely possible that they've found a way to express that
definition in a manner that isn't arbitrary. But you *cannot*
determine
that from the Sky and Telescope article.
All in all, the definition system for a planet put forward by Alan Stern
and Harold Levison is a more logical system than just quoting some
completely arbitrary radius like the radius of Pluto for the minimum
size of a planet.
Well, we agree on that, at least.

End quote-------------------------------------------


First of all, thanks for alerting me about the August 2002 issue of
S&T. I had it in my bookshelf, but I never read those two (p.32 and
p=2E42) articles since I am far more into galactic astronomy than into
solar planets. What interests me very much though, is the irrational
nature of the human animal, which affects science just like everything
else. So this is what actually brought me into this topic, i.e. that
the issue of planet definitions actually has a deep philosophical and
anthropological aspect to it.

I believe that there is one criterion missing in Stern=B4s and
Levinson=B4s list of desiderata for a planet algorithm: The definition
of what is a planet should above all be practical so that we can work
with it. And this means more than it just being "fat free" (p.44).
It means that even though on one hand we must try to avoid
arbitrariness and vagueness in the our definitions, we must also
recognize that we will never be able to avoid it completely. Because if
we would try to completely avoid arbitrariness and vagueness in our
definitions, practically making them into purely mathematical
paradigms, then -for example- we might as well forget everything
about galaxy classification, which is pretty darn vague and arbitrary.
Scientific definitions should above all be workable. Or, as Adam
Burrows (University of Arizona) says on p.38 in S&T of 8-02: "The
fact is that there is ambiguity in the provenance of these objects and
that ambiguity will be with us for quite a while. A flexible,
open-minded philosophy toward classification is best".

When a science develops from a descriptive to an explanatory level, the
descriptive stage is characterized by beeing purely classificatory.
This is one reason why classification is essential for science.

The definition of a planet which I suggested in the "mini planet"
thread mentioned above was:
"Any spherical object of natural origin which is not a star, but which
either has a star or the gravitational center of a multiple star system
at one of the foci of its orbital motion or which is found in
interstellar space."
This is just a more elaborate version of what Gibor Basri (UC Berkeley)
suggests on that same p.38: "A spherical non-fusor born in orbit
around a fusor". And this, of course, brings up the problem of how to
define the appropriate sphericalness of a planet in quantitative terms.


Now, quantitative definitions in science should *not* orient themselves
on our theories, but on observation. Thus, if we would consider the
diameter of Mimas, the sixth moon of Saturn with a diameter of 8.0%
that of Mercury and to my knowledge the smallest natural and slightly
oval solar system body as the lower cutoff value for the diameter of a
planet, then we would run into the problem that the asteroids Pallas
and Vesta, even though bigger than Mimas, are clearly irregular.
Therefore we should choose Miranda, the 11th moon of Uranus with a
diameter 9.7% that of Mercury (472 km), to designate this lower cutoff
value for the diameter of planets. To my knowledge, there is no
obviously irregular body in the solar system bigger than Miranda.
Furthermore, a lower cutoff value of 472km instead of 500km for planet
diameters would be free of suspicions of being arbitrary, meaning based
on the number of fingers of human primates, i.e. the metric system.

Note that the diameter of Mercury is only 3.4% that of Jupiter, and
that we know of extrasolar planets which have many times the size of
Jupiter. So planets are neither "big" nor "small"-both of which
are non-scientific terms- but that they come in very many sizes.

I am not a mathematician, but I have no doubt that it would not present
a big problem to mathematicians to quantitatively describe the
sphericalness of Miranda, even though it sports the highest cliff known
in the solar system.

This would of course also mean that the "asteroid" Ceres is really
a planet. There exists a Hubble shot which clearly shows Ceres to be
spherical. And as a side note, there is one big difference between
Pluto and Ceres: Ceres has never been and will probably never be
dethroned as the king of the Asteroid belt, while Pluto has recently
been dethroned as the king of the Kuiper Belt by 2003 UB313.

For a long time have I wondered why so many scientists(?) want to deny
the status of a planet to Pluto. After all, such a distinction cannot
be based on its composition since the composition of the Ice Giants
(Uranus and Neptune) is actually more similar to that of the KBOs than
to that of the Gas Giants (Jupiter and Saturn/see also my above
mentioned article of August 3rd). Only recently did I find out that
this supposed distinction is not based on any rational criteria at all,
but simply on the *feeling* that Pluto is "not big enough" to be a
planet. So I obviously have wondered for so much time because it cost
me so much to accept how silly scientists can be.

Those who consider a diameter of 472 km to be too small for a planet
might imagine a sphere with a diameter of 472 km rolling over the
surface of Earth (hopefully not towards them). Half of this sphere
would extend beyond the Troposphere and into the Ionosphere. If it
would roll across Mount Everest you would=B4nt even notice the bump. At
night you would see the flashes of satelites smashing into it and
exploding. The ISS would hit it about in the middle. Now those who
believe that to be "too small" for a planet must suffer from a
severe case of hybris. What really is small are we piffy humans and not
planets.

But hold on: In the present situation of confusion which has been
caused by the discovery of 2003 UB313, all of what I have said above
about the quantitative definition of spericalness is really irrelevant.
Exept for Pluto and Charon, we cannot directly detect the spericalness
of KBOs. All that we can do is estimate their diameter from their
absolute magnitude, based on our assumptions of what their albedo might
be.

In the article "Pluto and New Horizons" on David Jewitt=B4s webpage
at http://www.ifa.hawaii.edu/faculty/jewitt/kb.html the typical albedo
of a KBO is assumed to be 0.04. And since, as it turns out, exact
values are really not too important in this circumstance, in order to
avoid too many interpolations I will assume it to be 0.05. Now if we
look at the conversion table of absolute magnitude to diameter (see
link on the page with the IAU KBO Table) assuming this albedo value,
then the absolute magnitude of a typical KBO with a diameter of 472 km
and an albedo of 0.05 should be about 5.4. As of the day of this
writing, 9-25-05, there are 45 objects with an absolute magnitude of
5=2E4 or brighter on the IAU list of KBOs. And on the IAU list of
Centaurs and SKBOs there are 9 objects (all SKBOs) with an absolute
magnitude of 5.4 or brighter (*). So together with Ceres this makes
APPROXIMATELY 64 KNOWN AND SUSPECTED PLANETS IN OUR SOLAR SYSTEM ON THE
DAY OF THIS WRITING. And you just have to tweak the assumed albedo of
KBOs a little bit to get to very different numbers.

I will summarize this with the same comment which I made at the end of
my above mentioned article of August 23rd:
When it comes to determining the exact number of planets in the Solar
System it is just like with determining the exact number of everything
else in the universe: We can only give an approximation. Formerly we
believed (and many people still do) that the number of planets of our
solar system is an exception to this rule. What 2003 UB313 should have
shown us, is that this is not so.

I am a public educator in astronomy. And sooner or later the IAU will
have to come up with a definition of what is a planet. Paradigm shifts
will always meet with public resistence if they run counter to social
acceptation. But social acceptation ought to be irrelevant to science.
Note that bathroom scales still indicate weight in kilograms, just like
in the 17th century. Now I sure hope that what the IAU will come up
with will be at least as rational as what I have written above, because
otherwise I might feel obliged to ridicule them publicly.


Peter Holm



*: To see for yourself, copy and paste these lists to Excel or
something similar and order them with respect to decreasing brightness.

Why not just say that anything smaller than Pluto is just a "Minor
Planet" and anything Pluto's size or larger is a Planet?

How about we forget about trying to keep Pluto as a planet. Pluto has
significantly lower mass than at least 6 of the solar systems moons and is
part of a large population of bodies (the Kuiper Belt) that includes objects
of similar size to Pluto.

Setting the lower limit of a planet to Pluto's size would put Pluto, UB313
and who knows how many other objects to a higher status than many moons.
This wouldn't be wrong if it wasn't so arbitrary. Setting the lower limit
the minimum mass for sphericity would be impractical because it would
include too many objects.

Therefore I think the mass of the solar system's largest and most massive
moon, Ganymede, (but not its size, that would exclude Mercury) should be set
as the lower limit for the mass of a planet. Anything smaller but still
spherical is a planetoid.


On 26 Sep 2005 21:42:56 -0700, wrote:

|Why not just say that anything smaller than Pluto is just a "Minor
|Planet" and anything Pluto's size or larger is a Planet?

In article ,
Daggaz answer.in.this@newsgroup wrote:
How about we forget about trying to keep Pluto as a planet. Pluto has
significantly lower mass than at least 6 of the solar systems moons and is
part of a large population of bodies (the Kuiper Belt) that includes objects
of similar size to Pluto.

Setting the lower limit of a planet to Pluto's size would put Pluto, UB313
and who knows how many other objects to a higher status than many moons.
This wouldn't be wrong if it wasn't so arbitrary. Setting the lower limit
the minimum mass for sphericity would be impractical because it would
include too many objects.

Therefore I think the mass of the solar system's largest and most massive
moon, Ganymede, (but not its size, that would exclude Mercury) should be set
as the lower limit for the mass of a planet. Anything smaller but still
spherical is a planetoid.

I read a not in my newspaper yesterday that the IAU has taken a radical
approach on this question: their latest suggestion is to get rid of the
term "planet" altogether, and to replace it with more specific terms, such
as "terrestial planet", "gas giant", "Kuiper belt object", "exoplanet".

Of course people will continue to use the word "planet" anyway, but the
IAU will not give any official definition of the word "planet" but
recommends not using it but replacing it with a more specific term which
is appropriate.

There's at least one advantage with this latest suggestion from the IAU:
there are no border cases. There's no disagreement about whether some
particular planet is a "terrestial planet", "gas giant", "KBO" or "exoplanet".



On 26 Sep 2005 21:42:56 -0700, wrote:

|Why not just say that anything smaller than Pluto is just a "Minor
|Planet" and anything Pluto's size or larger is a Planet?


--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/

Paul Schlyter wrote:

I read a not in my newspaper yesterday that the IAU has taken a radical
approach on this question: their latest suggestion is to get rid of the
term "planet" altogether, and to replace it with more specific terms, such
as "terrestial planet", "gas giant", "Kuiper belt object", "exoplanet".

Of course people will continue to use the word "planet" anyway, but the
IAU will not give any official definition of the word "planet" but
recommends not using it but replacing it with a more specific term which
is appropriate.

I did not expect that. This is what I call "chickening out". I had
some nice answers prepared for Daggaz and Edward, but I might as well
forget about them now. Especially Edward seems to not only not have
read my article, but not even the title of this thread.

But what a bunch of cowards. So they will not force the public, which
is stupid by nature(*), to -slowly- change their outdated ideas about
what is a planet. If this is true, then it will be a more than
sufficient reason to publicly ridicule the IAU.

The truth is: We human beings are in fact the most ridiculous creatures
which have ever been walking the face of earth. Challenge me and I will
give you at least two good reasons. I am so ashamed.

Peter


(*As a sidenote: The idea behind behind Democracy is a balance of
stupidity in order to avoid its extremes. It=B4s not a perfect method,
but with historical hindsight it=B4s the best we can do)

Interesting. I had been thinking somewhat along those lines myself due to
the huge diversity (in composition, internal structure, history etc) we find
amongst the objects traditionally named as planets. Throwing a whole lot of
unrelated objects into one category sounds unscientific to me.

But how about not using the term "gas giant" to describe objects like
Jupiter and Saturn. They will no longer be planets due to that term being
discarded. So they are giant compared to what? Certainly not other objects
of their composition. They are very dwarfish when compared to stars and
brown dwarf stars.

On Tue, 27 Sep 2005 11:43:18 GMT, Paul Schlyter wrote:

|I read a not in my newspaper yesterday that the IAU has taken a radical
|approach on this question: their latest suggestion is to get rid of the
|term "planet" altogether, and to replace it with more specific terms, such
|as "terrestial planet", "gas giant", "Kuiper belt object", "exoplanet".
|
|Of course people will continue to use the word "planet" anyway, but the
|IAU will not give any official definition of the word "planet" but
|recommends not using it but replacing it with a more specific term which
|is appropriate.
|
|There's at least one advantage with this latest suggestion from the IAU:
|there are no border cases. There's no disagreement about whether some
|particular planet is a "terrestial planet", "gas giant", "KBO" or "exoplanet".


______________________________________________
*
* _-_|\
* / \ Daggaz
* \_.-._/--Sydney, Australia
* v

Email address is valid (no spamblocks) but temporary.
The numbers in email address change when spam levels
become too high.

Daggaz wrote:
How about we forget about trying to keep Pluto as a planet. Pluto has
significantly lower mass than at least 6 of the solar systems moons and is
part of a large population of bodies (the Kuiper Belt) that includes objects
of similar size to Pluto.


I must preface this post by saying that I have no more than a lay
interest in, or understanding of, planetary astronomy (although it has
been a longstanding lay interest). So please feel free to take my
comments with a grain of salt.

From my position of relative ignorance (or perhaps because of it), I'm
having difficulty getting my head around the reasons for the strong
debate and lack of consensus on a planetary definition.

Based on my limited understanding, there would seem to be little issue
with the upper boundary for planetary size, with this being covered by
the IAU statement of 2001, as amended in 2003:

http://www.dtm.ciw.edu/boss/IAU/div3...efinition.html

Similarly, the distinctions within the planetary realm, between gas
giant and ice giant, and ice giant and the largest rocky/terrestial
bodies (based on composition) don't seem to be contested. Without
touching on the subject of "planet" it would seem appropriate to me if
this categorisation by composition was extended to include "ice dwarfs"
to differentiate between smaller bodies that are primarily made of ices
rather than rock.

I don't understand the reasons for considering trans-Neptunian as a
descriptor for Kuiper Belt or Oort Cloud bodies, or cis-Jovian for
terrestial planets, as some are advocating, as such definitions
obviously can't be applied to bodies outside our solar system.

In comparing those known bodies orbiting the Sun that are smaller than
Neptune to arrive at a lower boundary for a planetary definition, there
seems (from my lay position) to be only one scientifically solid point
of distinction, and that is at the point where bodies have sufficient
mass to have a differentiated interior and to tend to towards a sphere
(or whatever other oblated or extruded variation its rate of spin
dictates). While it may be difficult to set a prescriptive lower limit,
for mass or size for this point of sphericity, I expect a nominal value
could be derived (as I assume was the case for the 13 Jupiter masses
upper boundary).

Obviously, that would leave us with rather a lot of "planets", and with
a growing number of bodies that, due to our limited knowledge of them,
would be "potentially" planets. Given that we're dealing with continuum
in terms of sizes/masses, I guess we would also have transitional
bodies that are somewhat planet-like but of sub-planet mass. I don't
see the problem with this. When dealing with a continuum, why would
anyone want to pretend that they were not?

In addition to using say mass to determine planetary status, and
applying adjectives based on composition (ice giant, terrestial, ice
dwarf, etc.), if there was a need to further distinguish between bodies
that have ejected all other bodies from their orbit, or forced them
into Lagrange points, and those that haven't, the latter could I
suppose be referred to as "Asteroidal". So Pluto could be an Asteroidal
Ice Dwarf planet, and Ceres an Asteroidal Terrestial planet, or
whatever.

This makes far more sense to me than relying on an historical approach
that will someday seem merely silly or naive, or on dynamical
properties that have no context outside our solar system.

Or am I just missing the whole point?

Cheers,

Steve

wrote:
What comes below is a continuation of the thread "LARGEST ASTEROID
MAY BE 'MINI PLANET' WITH WATER ICE (STScI-PR05-27)" on the newsgroup
sci.astro.amateur, which started on September 7th as an absolutely
arbitrary babble and -so far- ended on September 10th with radical
nitpicking. The first I consider as ridiculous and the latter as
unreasonable. But if I had to choose between these two options, I would
of course not hesitate to choose the latter. And what I wrote in the
s.a.a. thread was in turn based on what I wrote on August 3rd and 23rd
in the newsgroup sci.astro.research under the very same title as this
article. It is interesting to note that these articles did not cause
any controversy on s.a.r., while on s.a.a. they did (some). Let=B4s see
what happens here.

As I have already mentioned in the "mini planet" thread, I believe
that this subject -i.e. the definition of what is a planet- deserves
its own thread title. So I will reproduce that thread here starting
from the point from where it became interesting, before writing my own
comment at the bottom:


Begin quote-------------------------------------------------------

Brian Tung 8 sep 19:49
Peter Holm wrote:
I believe that the time has come to do away with the old misconception
that that planets are "big". "Big" is not a scientific term and
should therefore not be used in a scientific discussion. And this is a
science forum, or isn't it?
It's a science forum for amateur astronomy. That means that your post,

although close enough on topic for my own tastes, is probably better
disposed for another group, like sci.astro. As far as I'm concerned,
you can post it here, but others might not feel so great about it. In
particular, claims that we're not being scientific if we don't play by
your rules will not be taken kindly.
In any case, terms are not either scientific or otherwise. I'm not
really sure what it would mean to say a term is scientific; usually, we

say that a proposition is scientific if it can be tested. Although the

phrase "Planet X is really big" is not testable, and therefore can be
considered unscientific, it can be emended to "Planet X is more than
1,400 km across its smallest diameter," and then it's just fine.
Besides, the problem in definitions of the term "planet" is not that
they can't be made precise; we can make them precise without any
trouble
at all. The problem is that they are then arbitrary. Your definition
has the same root problem: it can be made either precise or not
arbitrary,
but not both.
You might not think so, but consider that if I take your definition at
face value, there are no planets at all in the solar system. No object

is perfectly spherical. Any rotating object has an equatorial bulge;
in addition, most of them have some higher-order deformation (the Earth

is slightly bulgier on the southern side than on the northern side, for

instance), and most of the rocky or icy ones, where the planethood is
in
doubt, have mountains and things poking out here and there.
Then, too, none of them has a perfectly elliptical orbit, so that you
can tell that the Sun is at one focus. They're all constantly
perturbing
each other. And the dividing line between star and planet isn't yet a
clear one: is a brown dwarf (a deuterium fusor) a star or a planet? Or

is it a star when it's fusing deuterium, and a planet afterward?
The upshot is that you have to draw a few lines in the sand to make
sure
that what we've always considered planets "for sure" (such as the Earth

and Venus and Jupiter and Saturn, and so forth) continue to be planets.

The challenge is to draw those lines in a way that is not arbitrary.
How spherical is spherical enough? Can you say how spherical it has to

be without using a number?
This is the challenge that all planet definers have had to face, and in

the end, some of us have come to a different conclusion: that it isn't
the definitions that are the problem; it's the term "planet" that
defies
"scientificness." It's simply too historically laden to be useful as a

precise term. Even the terms "major planet" and "minor planet"--for
Pluto will always be a planet, albeit possibly a minor one--are tough
to use precisely. Better to coin a new term if you want to be precise.


David Knisely 8 sep 19:47
A modification on that definition might be a body which is large enough

that gravitational forces can overwhelm its material strength, forcing
it to assume a fairly spherical shape. If such a body is in an
independent orbit around a star, then it would be considered a planet.
This would open up the classification for a number of objects in the
solar system, including Ceres and some of the large Kuiper belt
objects.
I would prefer this over some arbitrary diameter like putting the
cutoff at the diameter of Pluto. Clear skies to you.

Brian Tung 8 sep 21:19
David Knisely wrote:
A modification on that definition might be a body which is large enough
that gravitational forces can overwhelm its material strength, forcing
it to assume a fairly spherical shape.
That's still either vague or arbitrary, depending on how you define
"fairly spherical." For no other objects other than black holes do
electromagnetic forces vanish in comparison to the gravitational.

John Savard 9 sep 03:14
On Thu, 8 Sep 2005 20:19:08 +0000 (UTC), (Brian Tung)
wrote, in part:
That's still either vague or arbitrary, depending on how you define
"fairly spherical." For no other objects other than black holes do
electromagnetic forces vanish in comparison to the gravitational.
True, although with a neutron star you're pretty safe...

David Knisely 9 sep 06:55
Brian Tung wrote:
That's still either vague or arbitrary, depending on how you define
"fairly spherical." For no other objects other than black holes do
electromagnetic forces vanish in comparison to the gravitational.
It isn't arbitrary at all. The moon is "fairly spherical", even though

it has numerous bumps and bulges. However, the point isn't that a body

has a spherical form. Jupiter and Saturn both are *not* spherical due
to their extreme rotation speed. However, in both cases, if they did
not rotate, the mechanical strength of these objects would not be able
to hold them in any shape except a spherical one due to the influence
of
gravity (and no one would argue that both are planets). The tensile
strength (and/or density) of various materials can be compared to the
self gravitational forces to see which would dominate the shape of an
object. At a certain size, gravity would dominate and force the object

to stray significantly from a rather any irregular shape it might have.

*That* is where the definition would take hold. Ceres is spherical.
Vesta is not (athough it is approaching a point where gravity is
definitely starting to influence the body's final shape as it is
formed). I wish I could recall the issue of Sky and Telescope which
had
the curves, but the planetary scientist who came up with this criteria
presented a compelling argument for its adoption as the definition of
what is a planet and what is not. Clear skies to you.

Brian Tung 9 sep 08:02
David Knisely wrote:
It isn't arbitrary at all.
As stated thus far by you, it's either vague or arbitrary. If you
don't
define "fairly spherical," it's vague. If you do define it, it's
likely
going to be arbitrary. Certainly you have given me no confidence of a
non-arbitrary definition.
[snip]
At a certain size, gravity would dominate and force the object
to stray significantly from a rather any irregular shape it might have.
^^^^^^^^^^^^^
"Significantly" should be defined, or else the definition stands
self-confessed as arbitrary. I've read the Sky and Telescope article,
by the way.

Alson Wong 9 sep 20:11
"Brian Tung" wrote in message
...

- Mostrar texto de la cita -

Here's a case in point. Mimas is 392 km in diameter and is, most would
agree, fairly spherical:
http://www.nineplanets.org/mimas.html
Proteus is slightly larger, averaging 418 km in diameter, and is
somewhat
irregular, but also somewhat spherical overall:
http://www.nineplanets.org/proteus.html

David Knisely 10 sep 07:48
Brian Tung wrote:
As stated thus far by you, it's either vague or arbitrary. If you don't
define "fairly spherical," it's vague. If you do define it, it's likely
going to be arbitrary. Certainly you have given me no confidence of a
non-arbitrary definition.
Spherical is resembling a sphere. Something which visibly departs from

a sphere is not exactly spherical. Again, Ceres is fairly spherical
and
Vesta is not quite there but clearly shows evidence of at least
approaching a spherical shape. This shows that gravity is probably the

dominant force in determining the shapes of these objects. Something
like Eros clearly is not spherical, which shows that it is within the
realm of mechanical or tensile strength dominating over self
gravitation. Perhaps something with a departure from a perfect sphere
of less than 15 to maybe 20 percent would not be terribly noticable to
the eye, but this obviously will not satisfy you. In any case, this
isn't exactly what I am referring to, since I cite the cases of Jupiter

and Saturn.
At a certain size, gravity would dominate and force the object
to stray significantly from a rather any irregular shape it might hav=
e=2E
^^^^^^^^^^^^^
"Significantly" should be defined, or else the definition stands
self-confessed as arbitrary. I've read the Sky and Telescope article,
by the way.
Again, you may have read the article but you did not clearly understand

what the article really said. The "small" end of the planetary scale
would be determined at the point where gravity would be the primary
factor in determining the shape of the body and not mechanical strength

or other factors like surface tension or rapid rotation. The internal
strength of the material would be smaller than the internal
gravitational forces present within the body. If the body's internal
strength was less than the gravitational forces trying to make it more
symmetrical, the object would be considered a planetary body and it
would very likely be at least somewhat close to a spherical shape.
Exactly how close to spherical is not all that relevant to this
criteria. As Stern and Levinson state in their article,
"This shape need not be strictly a sphere - a rapid spinner would
conform to a distinctly oblate ellipsoid, as is the situation with
Jupiter and Saturn. Our emphasis on the rule of mass, and hence
self-gravity, has the ancillary benefit of eliminating nearly spherical

objects controlled by the surface tension or by electromagnetic or
electostatic forces. Also, we'd allow our candidates enough time
(basically, the age of the universe) to reach their equilibrium shape".

Something with a high mean density would have a larger threshold
radius for the point of internal strength vs. self gravitational
strength, while lower density objects would have a smaller threshold
radius. Look at the diagrams the author put up on the bulk density vs.

the body radius to see where he places the various small bodies of the
solar system (p. 45 of the August 2002 issue).
All in all, the definition system for a planet put forward by Alan
Stern
and Harold Levison is a more logical system than just quoting some
completely arbitrary radius like the radius of Pluto for the minimum
size of a planet. Clear skies to you.

Brian Tung 10 sep 17:41
David Knisely wrote:
Spherical is resembling a sphere. Something which visibly departs from
a sphere is not exactly spherical. Again, Ceres is fairly spherical and
Vesta is not quite there but clearly shows evidence of at least
approaching a spherical shape.
"Resembling a sphere" is a vague definition. I don't dislike it, but
it's vague. Even the Earth, even if it were not spinning, would not be

perfectly spherical. It would have random lumpiness floating around
the surface.
This shows that gravity is probably the dominant force in determining
the shapes of these objects.
I agree. It's not easy to quantify this, though. See below.
Perhaps something with a departure from a perfect sphere
of less than 15 to maybe 20 percent would not be terribly noticable to
the eye, but this obviously will not satisfy you. In any case, this
isn't exactly what I am referring to, since I cite the cases of Jupiter
and Saturn.
First of all, I'm willing to posit a non-rotating Jupiter and Saturn.
I'm even willing to consider them far removed from the Sun, so that
they
are not subject to uneven heating. Even then, they are affected by
internal heat, which causes turbulence, which leads to deviations from
a perfect spherical shape.
Secondly, what would 15 or 20 percent mean? You mean smallest diameter

deviates from largest diameter by less than 15 or 20 percent? OK,
that's
no longer vague--but it is now arbitrary. Why 15 or 20? Why not 10?
Why not 3? No matter what bar you set, that kind of bar is going to
have
a certain arbitrariness.
But you claim that the actual degree of sphericity doesn't matter that
much. OK, let's go on and see if that's actually so.
Again, you may have read the article but you did not clearly understand
what the article really said.
On the contrary, I understood very well what it said. Perhaps the
definition is better stated in the original scientific journal article.

The "small" end of the planetary scale
would be determined at the point where gravity would be the primary
factor in determining the shape of the body and not mechanical strength
or other factors like surface tension or rapid rotation. The internal
strength of the material would be smaller than the internal
gravitational forces present within the body. If the body's internal
strength was less than the gravitational forces trying to make it more
symmetrical, the object would be considered a planetary body and it
would very likely be at least somewhat close to a spherical shape.
Exactly how close to spherical is not all that relevant to this
criteria.
Is it? I think you may not have thought about some things regarding
the definition *as stated in the Sky and Telescope article*. On face
value, it's still vague. It simply states that the shape is determined

"primarily" by gravity rather than by mechanical strength or other
factors
like surface tension or rapid rotation.
First of all is that tricky word, "primarily." What does that mean?
But I won't heckle too much here--let's just assume that it means that
if we can quantify all of the factors with a common unit, the factor of

gravity is greater than all others combined.
Next, what are the "other factors"? They aren't enumerated. I've
already said that I'm willing to assume a planet far enough from a heat

source, and not rotating. You know what, I'll even throw out internal
heat--how's that for agreeability?
I'm going to guess, and it's only a guess based on what's in the
article,
that what is meant by "other factors" is electromagnetic forces, which
would include mechanical strength (whatever that means, precisely) and
surface tension, but not rapid rotation or internal heating (not
directly,
anyway).
Unfortunately, I don't have a good guess as to what the common unit of
measurement would be. Newtons, maybe? It's possible--certainly, it's
a
unit of force. But force on what? Let's suppose we start off with a
spherical body, and attach a protrusion. Can we establish the relative

strengths of the gravitational force on the protrusion and the
electromagnetic forces on it?
It turns out that we can, of course, but there's a problem. The
comparison depends on the shape and size of the protrusion. In other
words, the relative strength depends on how spherical the shape is.
This really isn't surprising: It's much easier for gravity to flatten
a tall skinny protrusion than it is for it to flatten a short squat
one.
Now, Stern and Levinson are sharp guys. I am quite certain they would
not have left their actual definition that vague. In composing their
article for Sky and Telescope, I am quite certain they ignored us folks

who want that level of precision and wrote for the vast majority who
just want to know the basic gist of their work. They must have, since
they have a graph with a specific round/irregular line.
What I am less confident about is the arbitrariness of the definition.
Note that the labels "round" and "irregular" *explicitly* indicate a
bar set at a specific level of sphericity. Again, they're pretty sharp

guys--it's surely possible that they've found a way to express that
definition in a manner that isn't arbitrary. But you *cannot*
determine
that from the Sky and Telescope article.
All in all, the definition system for a planet put forward by Alan Stern
and Harold Levison is a more logical system than just quoting some
completely arbitrary radius like the radius of Pluto for the minimum
size of a planet.
Well, we agree on that, at least.

End quote-------------------------------------------


First of all, thanks for alerting me about the August 2002 issue of
S&T. I had it in my bookshelf, but I never read those two (p.32 and
p.42) articles since I am far more into galactic astronomy than into
solar planets. What interests me very much though, is the irrational
nature of the human animal, which affects science just like everything
else. So this is what actually brought me into this topic, i.e. that
the issue of planet definitions actually has a deep philosophical and
anthropological aspect to it.

I believe that there is one criterion missing in Stern=B4s and
Levinson=B4s list of desiderata for a planet algorithm: The definition
of what is a planet should above all be practical so that we can work
with it. And this means more than it just being "fat free" (p.44).
It means that even though on one hand we must try to avoid
arbitrariness and vagueness in the our definitions, we must also
recognize that we will never be able to avoid it completely. Because if
we would try to completely avoid arbitrariness and vagueness in our
definitions, practically making them into purely mathematical
paradigms, then -for example- we might as well forget everything
about galaxy classification, which is pretty darn vague and arbitrary.
Scientific definitions should above all be workable. Or, as Adam
Burrows (University of Arizona) says on p.38 in S&T of 8-02: "The
fact is that there is ambiguity in the provenance of these objects and
that ambiguity will be with us for quite a while. A flexible,
open-minded philosophy toward classification is best".

When a science develops from a descriptive to an explanatory level, the
descriptive stage is characterized by beeing purely classificatory.
This is one reason why classification is essential for science.

The definition of a planet which I suggested in the "mini planet"
thread mentioned above was:
"Any spherical object of natural origin which is not a star, but which
either has a star or the gravitational center of a multiple star system
at one of the foci of its orbital motion or which is found in
interstellar space."
This is just a more elaborate version of what Gibor Basri (UC Berkeley)
suggests on that same p.38: "A spherical non-fusor born in orbit
around a fusor". And this, of course, brings up the problem of how to
define the appropriate sphericalness of a planet in quantitative terms.


Now, quantitative definitions in science should *not* orient themselves
on our theories, but on observation. Thus, if we would consider the
diameter of Mimas, the sixth moon of Saturn with a diameter of 8.0%
that of Mercury and to my knowledge the smallest natural and slightly
oval solar system body as the lower cutoff value for the diameter of a
planet, then we would run into the problem that the asteroids Pallas
and Vesta, even though bigger than Mimas, are clearly irregular.
Therefore we should choose Miranda, the 11th moon of Uranus with a
diameter 9.7% that of Mercury (472 km), to designate this lower cutoff
value for the diameter of planets. To my knowledge, there is no
obviously irregular body in the solar system bigger than Miranda.
Furthermore, a lower cutoff value of 472km instead of 500km for planet
diameters would be free of suspicions of being arbitrary, meaning based
on the number of fingers of human primates, i.e. the metric system.

Note that the diameter of Mercury is only 3.4% that of Jupiter, and
that we know of extrasolar planets which have many times the size of
Jupiter. So planets are neither "big" nor "small"-both of which
are non-scientific terms- but that they come in very many sizes.

I am not a mathematician, but I have no doubt that it would not present
a big problem to mathematicians to quantitatively describe the
sphericalness of Miranda, even though it sports the highest cliff known
in the solar system.

This would of course also mean that the "asteroid" Ceres is really
a planet. There exists a Hubble shot which clearly shows Ceres to be
spherical. And as a side note, there is one big difference between
Pluto and Ceres: Ceres has never been and will probably never be
dethroned as the king of the Asteroid belt, while Pluto has recently
been dethroned as the king of the Kuiper Belt by 2003 UB313.

For a long time have I wondered why so many scientists(?) want to deny
the status of a planet to Pluto. After all, such a distinction cannot
be based on its composition since the composition of the Ice Giants
(Uranus and Neptune) is actually more similar to that of the KBOs than
to that of the Gas Giants (Jupiter and Saturn/see also my above
mentioned article of August 3rd). Only recently did I find out that
this supposed distinction is not based on any rational criteria at all,
but simply on the *feeling* that Pluto is "not big enough" to be a
planet. So I obviously have wondered for so much time because it cost
me so much to accept how silly scientists can be.

Those who consider a diameter of 472 km to be too small for a planet
might imagine a sphere with a diameter of 472 km rolling over the
surface of Earth (hopefully not towards them). Half of this sphere
would extend beyond the Troposphere and into the Ionosphere. If it
would roll across Mount Everest you would=B4nt even notice the bump. At
night you would see the flashes of satelites smashing into it and
exploding. The ISS would hit it about in the middle. Now those who
believe that to be "too small" for a planet must suffer from a
severe case of hybris. What really is small are we piffy humans and not
planets.

But hold on: In the present situation of confusion which has been
caused by the discovery of 2003 UB313, all of what I have said above
about the quantitative definition of spericalness is really irrelevant.
Exept for Pluto and Charon, we cannot directly detect the spericalness
of KBOs. All that we can do is estimate their diameter from their
absolute magnitude, based on our assumptions of what their albedo might
be.

In the article "Pluto and New Horizons" on David Jewitt=B4s webpage
at http://www.ifa.hawaii.edu/faculty/jewitt/kb.html the typical albedo
of a KBO is assumed to be 0.04. And since, as it turns out, exact
values are really not too important in this circumstance, in order to
avoid too many interpolations I will assume it to be 0.05. Now if we
look at the conversion table of absolute magnitude to diameter (see
link on the page with the IAU KBO Table) assuming this albedo value,
then the absolute magnitude of a typical KBO with a diameter of 472 km
and an albedo of 0.05 should be about 5.4. As of the day of this
writing, 9-25-05, there are 45 objects with an absolute magnitude of
5.4 or brighter on the IAU list of KBOs. And on the IAU list of
Centaurs and SKBOs there are 9 objects (all SKBOs) with an absolute
magnitude of 5.4 or brighter (*). So together with Ceres this makes
APPROXIMATELY 64 KNOWN AND SUSPECTED PLANETS IN OUR SOLAR SYSTEM ON THE
DAY OF THIS WRITING. And you just have to tweak the assumed albedo of
KBOs a little bit to get to very different numbers.

I will summarize this with the same comment which I made at the end of
my above mentioned article of August 23rd:
When it comes to determining the exact number of planets in the Solar
System it is just like with determining the exact number of everything
else in the universe: We can only give an approximation. Formerly we
believed (and many people still do) that the number of planets of our
solar system is an exception to this rule. What 2003 UB313 should have
shown us, is that this is not so.

I am a public educator in astronomy. And sooner or later the IAU will
have to come up with a definition of what is a planet. Paradigm shifts
will always meet with public resistence if they run counter to social
acceptation. But social acceptation ought to be irrelevant to science.
Note that bathroom scales still indicate weight in kilograms, just like
in the 17th century. Now I sure hope that what the IAU will come up
with will be at least as rational as what I have written above, because
otherwise I might feel obliged to ridicule them publicly.


Peter Holm



*: To see for yourself, copy and paste these lists to Excel or
something similar and order them with respect to decreasing brightness.

Send this over to geology.

Our planet displays an Equatorial bulge feature and as the Earth's
crust is constructed of tectonic segments,it is safe to consider the
shape of the planet as the largest known geological
feature.Unfortunately this is ignored by geologists who currently
determine the mid-oceanic ridge as the largest planetary feature

Now the thing is,that most planets and indeed any rotating object
displays differential rotation between Equatorial and polar
regions,there is little difficulty in attributing the mechanism for an
Equatorial bulge as coming from differential rotation .The greater the
differential rotation ,the greater the bulge as with Saturn.

So,being half way there to a workable definition for a planet as
displaying a deviation from spherical geometry and differential
rotation prevents planets from slipping into messy seperations between
gas giants or terrestial planets when a planet has always been
geometrically a cyclical heliocentric trajectory.

A wonderful avenue is to associate crustal motion with differential
rotation in the mantle thereby killing two birds with one stone
(Equatorial bulge).