Talk:Definition of planet/definition of planet archive 2

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Contents 1 Comparative size table 1.1 The shape of Vesta 1.2 The shape of Hygiea 2 Straying from the topic 3 moons

[edit] Comparative size table

Here's my attempt at it. I've got the 5 smallest spheroids, and 6 largest irregular objects, plus Ceres for comparison. 20000 Varuna falls in between Ceres and Enceladus, in terms of mass, but the shape is unknown so I left it off. The key is that mass alone is not the key to having a gravitationally relaxed shape. Composition, Density, and the temperature during formation also have an effect, which is why this is not as clear a distinction as you would hope. Mimas and Enceladus are probably round because of tidal heating and impacts, and Miranda because of whatever catastrophic impact nearly destroyed it. Proteus and Nereid are similarly sized and composed, but are irregular because they didn't get any extra heat. Even inside the asteroid belt this distinction isn't clear, with irregular Pallas larger than the spherical Hygiea.shaggy 23:52, 23 January 2006 (UTC)

Object	Dimensions	Mass	Density	Shape
2003 EL61	~1960×1520×1000 km	(4.2±0.1)x1021kg	2.6–3.3 g/cm3	Ellipsoid
1 Ceres	975x909 km	9.5×1020kg	2.08 g/cm3	Spheroid
4 Vesta	578x560x478 km	2.7×1020kg	3.4 g/cm3	Spheroid
2 Pallas	570x525x500 km	2.2×1020kg	2.8 g/cm3	Irregular
Enceladus	505km	1.08×1020kg	1.61 g/cm3	Spheroid
10 Hygiea	407.1 km	1.0×1020kg	2.76 g/cm3	Spheroid
Miranda	471.6 km	6.59×1019kg	1.20 g/cm3	Spheroid
Proteus	436×416×402 km	5.0×1019kg	1.3 g/cm3	Irregular
Mimas	397.2 km	3.84×1019kg	1.17 g/cm3	Spheroid
511 Davida	326.1 km	3.6×1019kg	2.0 g/cm3	Irregular
704 Interamnia	316.6 km	3.3×1019kg	2.0? g/cm3	Irregular
Nereid	340 km	3.1×1019kg	? g/cm3	Irregular
3 Juno	290×240×190 km	3.0×1019kg	3.4 g/cm3	Irregular

Looks good. I see no reason not to include it, provided it is accompanied by a proviso listing all the various reasons other than gravity for which an object can be spherical. I might also include 2003 EL61, since it would tie in with the article. Serendipodous 23:27, 25 January 2006 (UTC)

When we get good mass and diameter estimates for some of the new large KBOs, They can be included for perspective. I can add 2003 EL61, but I'm having trouble thinking of a name for the shape. I'm gonna go with ellipsoid for now. There's too much uncertainty with many of the newer objects to definitively say if they're spherical. I can't quite see how to format this in with the rest of the article. shaggy 23:55, 25 January 2006 (UTC)

Ok, I went ahead and added it, I'm sure I'm duplicating a lot of information that's already in the article. I'm finding it difficult to format this section with both the table and the photograph of proteus, but they both add a lot to the section so I'm not sure what to do about it. Any suggestions? shaggy 00:19, 28 January 2006 (UTC)

Where did you get the information on 2003 EL61? --Perfection 19:14, 14 March 2006 (UTC)

It is available somewhere in one of the reference articles of 2003 EL₆₁.--Jyril 19:53, 14 March 2006 (UTC)

I'd call Mimas' shape ellipsoid rather than spherical. Difference between Mimas' longest and shortest axes is slightly greater than is the case with Proteus! Mimas is clearly non-spherical in some of the Cassini images. The famous "Death Star" Voyager 1 photo doesn't show this because of the viewing angle. The image is somewhat misleading giving an idea of orderly spherical satellite.--Jyril 19:53, 14 March 2006 (UTC)

Heh. That would require a bit of redrafting but you're probably right looking at the images. Although that would place something as smoothly rounded as Mimas in the same category as something as lumpy as say, Vesta. I think what this conversation highlights is that the differences between "sphere", "ellipsoid" and "irregular" are very much in the eye of the beholder. Serendipodous 20:08, 14 March 2006 (UTC)

Which is why I used the term spheroid rather than sphere. In fact, the three words, "Sphere", "Spheroid" and "Ellipsoid" all have very precise definitions. All three are "regular" shapes, that can be defined through simple mathematical formulae. Ellipsoidal is not an intermediate between irregular and spherical. An ellipsoid is any 3-dimensional translation of an ellipse. A sphereoid is a special case of ellipsoid where two of the three axes are equal, while a sphere is a special case where all three axes are equal. Please, take 5 minutes to follow those links and educate yourselves. Jupiter, Saturn, Earth, and Mimas are all spheroids, not spheres. The first three are oblate spheroids, where the third axis is shorter than the two that are equal, while Mimas is a prolate spheroid, where the third axis is longer than the two that are approximately equal. I intended the shapes in this table to be approximate. If you want to get really technical, all the spheroids on this list aren't true sphereoids. There is debate over the shape of Vesta being spheroidal or irregular, but it's usually considered a spheroidal body based on the fact that it's internally differentiated, like the Earth and other planets. I don't intend to be rude, but this misunderstanding is very frustrating to me. shaggy 18:43, 15 March 2006 (UTC)

Hey, no probs. Don't worry about getting frustrated. This is, for some odd reason, a very frustrating topic; something no one knows better than I. People (and I'm including myself here) can be quite passionate about their positions. I'm not sure whether the announcement from the IAU in September (assuming it comes) will make things better or worse.Serendipodous 19:23, 15 March 2006 (UTC)

Of course Mimas is regular compared to truly irregular shapes or real ellipsoids (I'd still count Mimas as a slight case of ellipsoid ;) ). I only wanted to emphasize the often forgotten fact that the shape of Mimas is far from perfect sphere. Otherwise my point was completely off-topic.--Jyril 19:16, 15 March 2006 (UTC)

Mimas is very slighly ellipsoidal, more so than any of the other things listed as spheroids, and almost more so than Vesta, which is listed as Ellipsoidal (Which I disagree with, as I'll explain in a moment). The shorter of the two short axes on Mimas is 2.5% smaller than the longer short axis, while on Vesta the difference between the two longer axes is 3.1%. Compare that, however, to 2003 EL₆₁, where the difference between the two shorter axes is roughly 50%, and the difference between the two longest is about 25%. The frustration here, for me, was the supposed ambiguity in the definitions of three very precise, regular shapes. Such is the plight of a Math major, I suppose. :oP shaggy 19:46, 15 March 2006 (UTC)

• "J. D. Drummond and W. J. Cocke Triaxial ellipsoid dimensions and rotational pole of 2 Pallas from two stellar occultations, Icarus, Vol. 78, pp. 323 (1989)." Is Pallas irregular or ellipsoid? And what about Proteus? I used to see it as spheroid, in the pic it seems a little irregular, mas not irregular. --Pedro 21:56, 15 March 2006 (UTC)

You can see the best guess at the shape of Pallas here, which uses lightcurve data to estimate the shape. Proteus is very clearly irregular, and this can be seen more clearly here. shaggy 22:21, 15 March 2006 (UTC)

I'm going with you on this; I know zip about math but I'd much prefer a mathematical than a notional basis for the argument.Serendipodous 12:51, 19 March 2006 (UTC)

There is a mathematical basis for this. All the objects classified as ellipsoids or spheroids can have thier surfaces modeled (with minimal error) by a very simple formulae, namely the one for any generic ellipsoid (borrowed from the article):

Where a, b, and c are the lengths of the three axes. This makes them regular shapes, as opposed to the extremely complex equations and algorithms used to model the shapes of irregular bodies. Surface features (such as mountains, basins, and craters) are discounted for the purpose of modeling the shape. For regular bodies, we can usually say "The shape of this body fits the ellipsoid given by this formula ±(some amount of error)". Even though the best model of Pallas and the photographs of Proteus look rounded, they are irregular, and there is no best-fit ellipsoidal solution for either. Vesta might look lumpy, but if you ignore the giant crater on its south pole, it's actually a very regular body. This classification isn't based on how they look, but on scientific and mathematical evaluation of thier shapes. It might be worthwhile to insert in-line citations for the shapes of these objects, to help clear up any confusion. The shape of Pallas and Vesta have been sourced elsewhere on wikipedia. There seems to be a complete consensus among all sources that Proteus is irregular, but I'm sure with a little digging we can find the paper. shaggy 19:49, 19 March 2006 (UTC)

I added the relevant links to the table, to show that I'm not just making this all up. The one to the vesta article is a link to the refence on the shape used in that article. It's probably a terrible way to cite something, but I have no idea how to use the reference templates properly. shaggy 19:52, 19 March 2006 (UTC)

I never said you did make it up; indeed I think that a mathematical basis for the argument is far better then debates about whether ohject X "looks more spherical" than object Y. Perhaps you could add a bit to the text, explaining your reasoning in layman's terms?Serendipodous 15:09, 20 March 2006 (UTC)

I'm working on that right now. This is an excellent example of the kind of analysis that goes into the shape of these bodies. That particular paper is on the icy sattelites of Saturn. shaggy 17:35, 20 March 2006 (UTC)

[edit] The shape of Vesta

• Whoah-ho! Hang on... does anyone else see anything wrong with the spherical / irregular chart? Although 4-Vesta is distinctly rounded by gravity, it's hardly spherical. Check out the image:

user:nclean 28 Jan 2006

This image is very misleading. The image of Vesta is not a direct telescopic image of the body itself, but rather a model of the shape, with resultant apparent smoothness of the surface. See here: [1] RandomCritic 21:12, 26 March 2006 (UTC)

Which is why I used the word spheroid. That's a special case of ellipsoid where two of the axis are equal. A sphere is a special case of spheroid where all three of the axis are equal. Looking at Vesta's dimensions (578x560x478 km) you can see that the shape of the body approximates an oblate spheroid. It has this shape not only because of the large crater on it's south pole, but because of the very fast 5.3 hour day. None of the objects in the solar system are perfect spheres. Jupiter, Earth, Saturn, and others are oblate spheroids because of the speed of thier rotations. Mimas is a prolate spheroid because of tidal distortions. If we made perfect sphericity a requirement, Earth would cease being a planet, so most arguments for this sort of cut-off are based on the shape approximating a spheroid. (I also fixed your image) shaggy 18:05, 28 January 2006 (UTC)

If you look at this animation [2] you will see that Vesta is not even approximately a spheroid. Two similar axis lengths does not suffice for qualification as a spheroid, or a cube would qualify! A section through the spheroid at any of the axes has to produce an ellipse, and a section through Vesta will not be elliptical. RandomCritic 21:12, 26 March 2006 (UTC)

Although I don't agree with the reclassification of Vesta as irregular, I'm going to leave it for now. Vesta serves as a good example of how poorly this classification scheme works, as it is a large differentiated body which hasn't quite had enough heat to become more spherical. I would, at the very least, classify it as a "honorary spheroid" based on it having any sort of geological history aside from cratering, but that's not an acceptable reason for an edit. I'll look for an authorative source before I do anything. shaggy 18:15, 28 January 2006 (UTC)

• Shaggy i support your clames and agree with you (although I also used to see it as irregular). But you must admit, Vesta is pretty wierd. They've cancelled the Dawn mission :| But in the picture a normal person can see that Ceres has nothing to do with Eros! Ceres is like Eros as much as Eros is like Earth. Now we have pics, so all these classifications should change somewhat, is that meeting of the IAU soon? Or that is about the previous meeting that decided nothing? --Pedro 22:37, 28 January 2006 (UTC)

I've changed "irregular" to "irregular/ellipsoid"; I think it hovers on the boundary between the two, though really, this does illustrate the ambiguity of the word "spherical" and why the argument doesn't really work.Serendipodous 23:21, 28 January 2006 (UTC)

• Exceptions don't make the rule. It is better to say it is just "Ellipsoid", like EL61. A closeup pic of this place would be really cool. --Pedro 17:24, 29 January 2006 (UTC)

Taking a look at the abstract for the 1997 Icarus article, Thomas et al. say that "Its shape can be fit by an ellipsoid of radii 280, 272, 227 (+/-12) km. The mean density of Vesta from the mass reported by Schubart and Matson is 3.8 +/- 0.6 g/cc. For this density Vesta's shape is close to that of a Maclaurin spheroid with superposed variations of -15 km." I.e. Vesta's shape approximates an oblate spheroid, allowing for some surface variations. However, 15 km of "superposed variations" amounts to 5-6% of Vesta's radius. In other words, if you take an oblate spheroid and knock large chunks out of it, you get Vesta. But a spheroid with large holes knocked out of it is not (mathematically) a spheroid any longer. The question is: do you classify Vesta as a spheroid because it might have been one at one time? Or do you classify its shape based on what it looks like now? This gets at the whole question of size contributing to shape. An object much larger than Vesta would either collapse back into a spherical shape through its higher gravity, or its large size would mask the "superposed variations". Vesta does neither, suggesting that (getting back to the topic of the article) it's too small to be a planet. At least, if the definition is "ability to retain spherical/spheroidal shape regardless of impact history". RandomCritic 15:58, 27 March 2006 (UTC)

Vesta is made out of rock, which is capable of supporting a more rugged topography than an icy body. Other bodies roughly the size of Vesta show similar differences in surface topagraphy (Miranda being a great example). Vesta is a spheroid, within the bars provided for variations. The same is true for Mimas, Enceladus, and the other bodies listed as "spheroids". 2003 EL₆₁ is only listed as simply an ellipsoid because there's really no other way to describe it, and I think the note about scalene ellipsoids clears up any confusion there may be about this. shaggy 16:17, 27 March 2006 (UTC)

"Vesta is a spheroid, within the bars provided for variations."

That's the tricky part, isn't it? If you give yourself enough wiggle room, pretty much anything can be an almost-spheroid. RandomCritic 01:24, 30 March 2006 (UTC)

In this case, I believe the 15-km variation is only needed because of the extremely large crater on Vesta's south pole. If that didn't exist, there might only be a few km of relief on Vesta's surface. shaggy 19:04, 30 March 2006 (UTC)

[edit] The shape of Hygiea

It appears that Hygiea is actually pretty oblong, but in the low resolution images that made it look kind of spherical it was just seen from a particular viewing angle that made it appear so. The main evidence is lightcurve analysis (check out this article, in particular pages 375 & 376. By the way, IMHO it's pretty cool how they can arrive at the shape models). I'm not sure how accurate the smaller details of the shape model are, but the strong change seen in Hygiea's lightcurves as it moves in its orbit is pretty solid evidence that it is irregular. The spherical-like low resolution Hubble images were probably taken when it was oriented like in the left-hand shape picture there. It is interesting to compare to a similar analysis of Pallas (known to be slightly irregular from occultations), which can be found here, (page 350 in particular). The change in Hygiea's lightcurve with viewing geometry is much larger than for Pallas's lightcurve, which pretty strongly indicates that Hygiea is more irregular (unless it has some pretty major albedo variations on its surface, which seems fairly unlikely). Deuar 19:50, 23 April 2006 (UTC)

[edit] Straying from the topic

As I have said, I know zip about math, but I feel this "spheroid" information is very useful in providing a quantifiable basis for the argument. However it is making the "Size" section rather unwieldy. There are two issues here; what is meant by "spheroid", and which objects in our Solar system are spheroidal. The article currently spends more time discussing the definition of "spheroid" than it does discussing Solar system objects.

I've just reworded and redrafted the section. I think it makes the argument clearer; if you feel there are any inconsistencies, please change them.

By the way, RandomCritic, when you changed Vesta from "Spheroid" to "irregular", and Mimas from "Spheroid" to "Ellipsoid" you forgot to change the colour of the fields. I swapped them for now, though I imagine this debate isn't over yet.

Question: Where does this definition leave EL61? Can it still be called an ellipsoid by this definition? It seems to me that it would be a highly oblate spheroid now. If that's the case, then perhaps we should drop it from the table. Serendipodous 22:04, 26 March 2006 (UTC)

[edit] moons

We need to add a discution over what is a moon: http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=643 --Pedro 23:14, 30 March 2006 (UTC)

I see your point, but really the argument is pretty much the same as whether Ceres is a planet or an asteroid. The Sun has several "rings" (the asteroid and Kuiper belts) and many "planetlets" within those rings, along with rubble and dust, just as Saturn's rings have moonlets. The same logic applies to planets as applies to moons: how large does a planet/moon need to be before it can be called a planet/moon?. The real question, and this is addressed in the article, is whether moons can be considered planets in their own right. Serendipodous 23:23, 30 March 2006 (UTC)

• No. The issue of Ceres not being a planet is more inaccuracy of measurements by Herschel and prejudice rather than science. The fact is that many supposed moons (in fact, satellites) have less density than water, and are, in fact, rubble badly linked together or bigger members of the rings. The problem is that these things are having the same value as real moons, like Titan, which is also a world by it self. Are Pan (moon) and Titan (moon) the same thing, just because they orbit Saturn? With this science made by probes, planets and moons are no longer dots of light in the night sky: these are real worlds or space garbage. --Pedro 19:52, 31 March 2006 (UTC)

Ceres is round, so bad example really but, as the roundess debate has clearly shown in recent days, the line between "round" and "not round" is a fuzzy one. Is Vesta round? Clearly some here think yes and others think no. Myself I have no idea; but since the purpose of this article is not to find a definition for "planet" but to illustrate just how hard it is to come up with one, that's not really a problem. The same logic applies to moons as applies to asteroids. When is a moon a moon? When it's round? How round is round? And so on. The size table doesn't distinguish between asteroids and moons, nor should it, since it deals with physical characteristics, rather than whatever other objects the asteroid or moon happens to be near. The term "moonlet" seems to be en vogue to describe these tiny moons, which is no less aprapos than, say, "planetoid" to describe a hunk of rock orbiting the Sun. Serendipodous 23:59, 31 March 2006 (UTC)

• Ok, there is always a boundary problem, the problem is not Vesta being or not round, is comparing Ceres with Eros and Pan with Titan. There is a somewhat clear boundary, although some objects are rule's exceptions. Vesta has mass enough to pull material into a sphere, Pan has not. Not even Earth is a really a sphere, but is pulls things to it with enough strength. If you read the news, you'll see the problem of these moonlets: these are different even from asteroids. --Pedro 23:55, 1 April 2006 (UTC)