Re: Toward a Rational Definition of what is a Planet


ph42@xxxxxxxxxxxxxxxx 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´s 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), b...@xxxxxxx (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" <b...@xxxxxxx> wrote in message
> news:dfrc2k$gut$1@xxxxxxxxxxxxxx
>
> - 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.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´s and
> Levinson´s 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´nt 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´s 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).

.