Talk:List of planetary bodies

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Contents 1 Why is this article necessary? 2 Surface area (system summary) 3 Recent edits 4 planetary body cutoff 5 Fair use rationale for Image:Xenaandgabrielle.jpg

[edit] Why is this article necessary?

Nixer, why is this article necessary?

1. The title does not match the content; you have not created a List of planetary bodies, but a table of bodies in our solar system descending by diamter (without the Sun), or conceivably a List of solar system objects by radius.
2. If you want to completely recreate the article, I suggest discussing changes on Talk:List of solar system objects by radius and recreating the article first in your userspace (for an example of a Userspace "workshop," see User:Tznkai/Workshop).

I consider some of your changes meritous, but they first deserve discussion, and then if the consesus agrees they deserve a demonstrative implementation, and then lastly they deserve to be implemented into the article List of solar system objects by radius, not an article of largely the same content but different format and title. --Iamunknown 02:41, 20 September 2006 (UTC)

I agree, this is basically the same as List of solar system objects by radius. RandomCritic 21:40, 3 November 2006 (UTC)

[edit] Surface area (system summary)

This is a very interesting column. However, as it is now it seems to be a somewhat sloppy calculation.

• The surface area of smaller satellites of e.g. Saturn (that are not specified in this list) is large enough to change the 104.6 figure.
• There should be an explanation of exactly what is being counted up in a note. E.g. apparently the area of the cloud tops on the Gas giants is excluded. Also, when calculating the area, how low down do we go? For example, the particles in Saturn's rings are obviously not going to be counted, despite being solid bodies in orbit. It should be specified that we're only counting named bodies, or some such.

Deuar 13:27, 12 February 2007 (UTC)

• Couted only the listed bodies i.e. planetary-sized with solid surface.--Dojarca 18:24, 12 February 2007 (UTC)

[edit] Recent edits

This list contains only information related to planet-sized bodies. Please do not add digits that are not related to planetary bodies. Summary area of named objects has no physical meaning.--Dojarca 12:23, 14 February 2007 (UTC)

Ah, ok, at least now I know what you meant. Makes sense. Deuar 17:37, 16 February 2007 (UTC)

[edit] planetary body cutoff

A completely arbitrary cutoff diameter of 1000 km (at the top of the page), or was it 900 km (at the bottom of the page) has been assumed here. The physically most meaningful cutoff is at the size where the body's gravity overcomes its structural strength and forces its shape into that of an equipotential surface (i.e. a spheroid). This is in fact the criterion used to distinguish dwarf planets from "Small solar system bodies". We should also apply this criterion to the moons. There would be three additional inclusions: Mimas, Enceladus, and Miranda. Possibly also Vesta and various TNOs (but that's a bit of a minefield, so perhaps best not trodden). Deuar 17:48, 16 February 2007 (UTC)

Enceladus is round because it is from ice. One can find many round ice balls in Solar System in general and in Saturn's orbit in particular, which are not planetary-sized. In fact even that Enceladus is not as round as it seems and heght variations (relative to its diameter) on its surface are much greather than that of all listed objects. Besides, there is a large gap between >1000-sized satellites and ~500-sized, which allows us to clearly separate them. --Dojarca 10:57, 17 February 2007 (UTC)

• Saturn has 6 ice balls, of which 4 are included, two not. Why not be consistent and include them all.
• A lot of astronomers seem to have thought about how to best distinguish "planetary" from "non-planetary" objects, and came up with the definition for dwarf planets that puts the cutoff at the physically meaningful point of a gravitational equilibrium shape. Why now make up our own arbitrary definition of a cutoff at x=949 km?
• What difference does it make if the object is made of ice or not? A whole bunch of objects already on the list are mostly ice.
• There is indeed a lucky gap of about 440 km in diameter between Enceladus and Ceres. Another, even bigger one, is between Titania (1578 km) and Pluto (2306 km). Why not make a cutoff there? The reason is that there are clear physical distinctions between bodies that do have a gravitational equilibrium shape (i.e. planetary-shaped ones), and those that don't. For example, planetary-shaped bodies often have visible signs of ancient or present geological activity, whereas irregular-shaped ones usually don't. In fact Enceladus and Miranda are prime examples of this.
• The difference in the dimensions of Enceladus are nothing special. Plenty of bodies already on the list have similar or even larger non-sphericity. The difference in radii for Enceladus are 8.3 km (3.3% of mean radius). While for Ceres, the numbers are 33km (7.0%), For Iapetus 34.5 km (4.7%) and for Tethys 12.9 km (2.4%).

Deuar 14:59, 19 February 2007 (UTC)

First, Saturn has not 6 but thousands of ice balls. And the main reason why some body is considered planetary is its gravitation. Is the gravitation high enaugh to make the body round? And the fact is that Enceladus is round not because it has high gravity, but because it is from ice. In fact also that if has *surface features* that much larger than that of other listed bodies.--Dojarca 18:42, 19 February 2007 (UTC)

Hi, again. Responding to your points:

• I'm sorry, but your reasoning regarding being planetary or not is inconsistent. First you say that it's about "is the gravitation high enough to make the body round" (so, yes in the case of Enceladus, etc). Then, however, you change your mind and apply some arbitrary special pleading: that if the body is made of ice then being round doesn't count any more. Why not exclude all those moons between 1000 and 1500 km diameter as well, then? They're made of ice too. Such a hodgepodge of criteria is completely arbitrary.
• As was pointed out already above, no, the surface features of Enceladus are not unusually large.
• Sure, there's thousands of ice objects, but only 6 closely resemble "balls" - that is are spheroidal.

Deuar 11:50, 20 February 2007 (UTC)

There are miriads of more or less round ice balls in the Solar Systrem, and most of them very tiny. We should not include any round body here and it should be proven that their roundness is due to gravity.--Dojarca 07:27, 21 February 2007 (UTC)

Indeed − as you said, "We should not include any round body here and it should be proven that their roundness is due to gravity". I completely agree. For example, Proteus (440×416×404 km), which is even slightly larger than Mimas (414.8×394.4×381.4), should not be here. It's known that it has not fully collapsed gravitationally. On the other hand, it has been shown that Mimas, Miranda and Enceladus have collapsed. The reason, by the way, is that ice is stronger out at Neptune's orbit than at Saturn's because of the lower temperature. By a similar token, until the Dawn mission flies over to 4 Vesta (578×560×458 - larger again), it's unclear whether it has gravitationally collapsed or not (rock is stronger than ice). 2 Pallas (570×525×500), for example, is known to have an irregular shape.

Let me go a bit into some details to explain why Enceladus Mimas and Miranda are not any old round body. As you've noticed, it's not immediately obvious whether they've collapsed or not from their three-axis dimensions. The height variation over smaller scales must be taken into account. For example on good old Enceladus again, the local deviations from a spheroid are small in height and most importantly small in width. On Proteus, on the other hand, the deviations from a spheroid are of similar height to the differences in axes, and of a width comparable to Proteus's full radius. Check out this image of Proteus: . While it's "roughly" round, it's visibly not a spheroid. Sort of a squarish lump.

So, I hope we can agree that while we don't want to include any old body around the 500 km size, the gravitational criterion doesn't exclude all of them. Then we can stop our reversions, unless someone wants to add Proteus, or Vesta, etc., that is. Deuar 14:45, 21 February 2007 (UTC)

Mimas has very wide features on its surface, it has one of the largest craters compared to its diameter among any body of similar or larger size. And Enceladus's roundness is also because of cryovolcanic snowfalls. You correctly pointed out that roundness of a body depends on temperature. That is another reason why we shall not include those you proposed: if Enceladus was further from the Sun, it would be of irregular shape. That is it shall thank temperature, not gravity for its roundness.--Dojarca 10:22, 22 February 2007 (UTC)

Here we go again. I'll repeat it again just to be clear: It is not in doubt that the roundness of these three bodies is due to gravitational collapse. You might have doubts, but astronomers don't.

• Making allowance for some hypothetical temperature dependence is unobjective. What temperature is the right one to choose for comparison? How trustworthy are our guesses for how body X would behave under a different temperature?
• Roundness of Enceladus due to cryogenic snowfalls? Is this some sort of crank theory?
• The crater on Mimas is very large, but not anywhere near large enough to mask the fact that the rest of Mimas is spheroidal, with the crater just a blemish on overall roundness.

Deuar 17:19, 23 February 2007 (UTC)

[edit] Fair use rationale for Image:Xenaandgabrielle.jpg

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BetacommandBot (talk) 08:01, 15 January 2008 (UTC)