Talk:Definition of planet/Archive 2

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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):

${x^{2} \over a^{2}}+{y^{2} \over b^{2}}+{z^{2} \over c^{2}}=1$

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)

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)

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)

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)

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)

the proposed definitions

In the subheading of The IAU Debate it mentions three possible definitions of planet that they are considering. Wouldn't, under the third definition, 90377 Sedna be considered a planet? It clearly lies out of the Kuiper belt, and clearly closer than the Oort Cloud. (2003 UB313, and 50000 Quaoar are in the Kuiper belt?) Wouldn't that make Sedna a planet under this definition (and revert pluto's planetary status to that of a kuiper belt object)? McKay 07:08, 5 July 2006 (UTC)

The general consensus appears to be that Sedna is part of a population. 2000 CR105 has a similar orbit to Sedna, though not quite as extreme. Mike Brown has said that, since Sedna's orbit is so elliptical, there was only a one-in-sixty chance that anyone pointing to the sky would have seen it at this time. That means, essentially, thet there must be at least sixty more Sednas out there. Not sure if that logic holds, but then I'm not a math type. 2003 UB313 is considered part of the "scattered disc", a region of Kuiper belt objects sent into higher elliptical orbits. Quaoar is a KBO. Serendipodous 11:14, 5 July 2006 (UTC)

Re: your math. If there's a 1 in 60 chance for anyone to see an object like Sedna, and there were 60 such objects, then the probability of one of those objects being viewable is about 63%. (1 - (1 - 1/60)^60) Which does seem approximately reasonable. (i.e. Sure, I like your answer.) McKay 09:12, 22 July 2006 (UTC)

illustrating the sub-stars section

Can we get a better image for the substars section? The current one is fairly misleading in terms of scale: the planets of 55 Cancri are not equally spaced as depicted (the first three planets are bunched very close to the star, and the fourth is around 20 times further out), and it depicts a fictitious system of planets/moons around the sub-brown dwarf (not the known dust disc). Perhaps an image of 2M1207b would be appropriate here? If we do keep the current image, the caption should be changed. Chaos syndrome 15:52, 8 July 2006 (UTC)

An image of 2M1207b would not be appropriate for that section, first because there is already an image of Gliese 229B in orbit around its star in the "brown dwarfs" section, and second because it is specifically Cha 110913-773444 that is under discussion within the "sub-stars" section. If we were to locate another image, it would have to be another image of Cha 110913-773444. In light of your comments, I have already modified the caption to de-specify the larger star system within that image, and as to your concerns about the image depicting Cha's as-yet unidentified "planets", I do see your point but I am not particularly alarmed, mainly because at the px level I have set the image at, the planets are virtually indistinguishable, and the orbits merely look like rings. Also, this is an artist's impression of a hypothetical scene. Much like artistic renditions of the skies of extrasolar planets or the view from inside Saturn, a certain amount of artistic licence is to be expected. There may be no planets as yet observed around Cha, but that doesn't mean that there aren't any. Serendipodous 16:16, 8 July 2006 (UTC)

I suggested 2M1207b because it too is a planetary mass object (some argue that it is a sub-brown dwarf [3]) with a dust disc. The new wording is an improvement though. Maybe creating a new comparison image (similar to the one used near the bottom of this page on SolStation, though that image is subject to copyright) might be better - comparison of the Sun versus Cha 1109-7734 versus Jupiter. Chaos syndrome 19:08, 8 July 2006 (UTC)

That would be a good idea; though I don't really know how to do it. The reason I'm specifying Cha is because it is a planet-sized object not tied to any star, and thus blurs the line between planet and star. Any sub-brown dwarf in orbit around a star wouldn't make the same point. Serendipodous 19:17, 8 July 2006 (UTC)

I'd be able to render something in POV-Ray. Instead of rendered versions of the Sun and Jupiter, we could use actual images, though I'm not entirely sure if that can be done while still adhering to the NASA licenses. Chaos syndrome 19:28, 8 July 2006 (UTC)

That'd be great, if you could do it. I always assumed that NASA's images were in the public domain. Serendipodous 19:41, 8 July 2006 (UTC)

Ok, I've uploaded the new image. Chaos syndrome 20:00, 11 July 2006 (UTC)

Very nice! Thanks for putting the effort in. It looks great. Serendipodous 11:22, 12 July 2006 (UTC)

Merging with "Planet"

Just a headsup; when the IAU announces its final definition in September, I'm planning to merge this article with planet. Just wanted to make sure everyone's OK with that.Serendipodous 19:21, 23 July 2006 (UTC)

Can I suggest keeping it seperate for a small while, maybe 3 months or so, just to ensure that the the defintion is accepted by most astronomers... I'm sure it wouldn't be the first time that a definition from a regulatory body was rejected by the people who will use it.

After that time I'd be tempted to keep a lot of the info in an article like this as it makes for an interesting read, with a possible renaming to something like 2000's Definition of planet debate (or controversy, or some other term) - I can imagine this being an interesting matter for study in the history and/or philosophy of science, and it makes the point in an interesting manner that some things in science are not determined by nature, but by the whim of man... --Neo 21:14, 23 July 2006 (UTC)

Press coverage

The IAU meeting is underway, and press coverage is beginning. Here is an article I've encountered (Associated Press, at the website for the Boston Globe): http://www.boston.com/news/science/articles/2006/08/14/astronomers_meet_to_define_planet/. GRBerry 21:19, 14 August 2006 (UTC)

Twelve Planets

The number of planets may need to be expanded to twelve following the news that the International Astronomical Union has recommended to its members that Charon, Xena and Ceres be ratified as new planets. [4] The new definition states that a planet is any star-orbiting object so large that its own gravity pulls in its rough edges, producing a near-perfect sphere. Terjepetersen 06:30, 16 August 2006 (UTC)

What the heck? How does this work? OK. Any round object over 800 km is a planet. Fine. But according to that definition there should be at least 30 planets; not 12. Why are Pluto and ub313 added, but not Quaoar or Sedna? Why is Charon addded, but not the Moon? Both co-orbit the Sun. Why is Ceres added, but not Vesta? Serendipodous 06:25, 16 August 2006 (UTC)

Perhaps there is some variability on how strictly they want to define "round". Even Earth has mountains. As for Charon, it might not qualify as a moon because it is too large relative to the planet it orbits. Algr

I just heard Mike Brown on the radio. He says that it is likely that the current count of 12 will be increased as we determine the precise dimensions of recently discovered objects. 12 could become a hundred before too long. By the end of the 21st century it could become a million. Serendipodous 07:40, 16 August 2006 (UTC)

Yeah, we don't know as much about Quaoar and Sedna. We suspect they probably fit under the roundness definition, but we don't have enough data yet. Charon was added because the barycenter of the pluto-charon system is outside the surface of either planet. Therefore, neither is a satelite of the other, but it would be considered a dual-planet system. The moon is clearly a satelite of the earth. Ceres is added because ceres is large enough to make it's shape stable due to its own gravity. Vesta has an irregular shape, therefore its gravity isn't enough to give it a stable shape. Any other questions? McKay 13:44, 16 August 2006 (UTC)

Actually, Vesta is technically spheroidal. We have enough data on most of the "Plutons" to make a fairly definitive statement on how round they are. Sedna may be far away, but it's not as far away as Xena (not at the moment, anyway), so if we know Xena's round, I'm pretty sure we can figure out if Sedna is. Serendipodous 14:36, 16 August 2006 (UTC)

No, 4 Vesta is not spherical spheroidal. Yes, I think they will quickly add Sedna to the list. McKay 15:18, 16 August 2006 (UTC)

Vesta

I didn't say it was spherical. I said it was spheroidal. Read the subsection on sphericity. Your picture is right there. Serendipodous 15:47, 16 August 2006 (UTC)

The picture shows that vesta isn't spheroidal. Note the knob in the bottom center. This "cross section" clearly is not an ellipse, therefore, not spheroidal. This also shows that it isn't even an ellipsoid. McKay 15:51, 16 August 2006 (UTC)

Also note the shading on the front center that hints to the fact that there is another knob on the "front". McKay 15:53, 16 August 2006 (UTC)

Believe me, Mckay, I've had this argument before, and argued your side. Just look at the second archive; it's nothing but. It's perfectly possible for someone to claim that Vesta is a spheroid; if it wasn't then I would have changed that table long ago. Serendipodous 15:56, 16 August 2006 (UTC)

okay, I've been through that page, and I don't see anything (maybe I missed it) that shows that Vesta is a spheroid. I can see how it's gravity almost makes it a spheroid, but I don't think that that's enough for the IUA's distinctions. So, I can see how someone could say that it is a spheroid, because it's kinda close, but the same case could be made arguing that it isn't spheroidical, which is the claim that I'm making that I think is valid. McKay 16:04, 16 August 2006 (UTC)

They managed to boot Pluto out (hurray!) by allowing so many dumb rocks to be included, in a while nobody will care anymore. We'll go to a sensible list of 8 (major) planets, and a large number of dwarf planets. Bet you pretty soon planet will come to mean major planet, and therefore: 8 planets in the solar system. -- Jordi·✆ 13:53, 16 August 2006 (UTC)

"Dwarf planet" is to be an unofficial definition. Officially, all classical and dwarf planets and plutons would be planets.--JyriL ^talk 13:56, 16 August 2006 (UTC)

Of course. But I severely doubt the majority of these will be considered real planets by the majority. There may be twelve planets thursday, 24 before long (3 asteroids and at least 8 KOBs are large and round enough to be included), and once we discover more rocks out in the Kuiper Belt, dozens or even hundreds in a few years. With the handy unofficial (but defined by an official body) term "dwarf planet" they've given us a great cutoff point which just happens to only include the classical planets, and excludes the subject Pluto and its kin. -- Jordi·✆ 14:07, 16 August 2006 (UTC)

• that's your imagination at work, i'm happy with these twelve:

Ceres: "the little rocky planet", with a lot of debris around.

Pluto and Charon: a pair of planets orbiting each other.

UB313: the new outer planet.

the problem are those KBO's around, they need some more criteria such as minimum mass.

Plus, asteroids lost status.

See this simulation with the 3 new planets: http://www.newscientist.com/data/images/ns/av/dn9761V1.mpeg

Mass of UB313: ?! (planet)

Mass of Pluto: 1.305×10^22 (planet)

Mass of Charon: 1.52 ×10^21 (planet)

Mass of Ceres: 9.5×10^20 (planet)

Mass of Orcus: 6.2 - 7.0×10^20 (not a planet)

So anything that is round and has more mass than Ceres?--Pedro 14:36, 16 August 2006 (UTC)

That definition seems pretty arbitrary. If they're going to randomly cut off the mass limit at Ceres, why not Pluto? Or Mercury? Serendipodous 14:58, 16 August 2006 (UTC)

Yes, it it like the "Dwarf planet". Why Ceres is a Dwarf and not the Earth? --Pedro 15:02, 16 August 2006 (UTC)

Likely because the current definition of dwarf planet neatly includes all 8 classical planets, and excludes the problematic Pluto. Any other definition will be harder to sell to the public en large. -- Jordi·✆ 15:10, 16 August 2006 (UTC)

• I doubt it, people just dont know how these objects look like. If they see a picture of it, they will consider it a planet. How can you say Jupiter (a huge ball of gas) is a planet and Pluto (with a surface) is not. I don't understand your criteria. We are in the 21st centuy, probles explored these objects. --Pedro 15:21, 16 August 2006 (UTC)

  They're not "my" criteria ;)

  I can understand the rationale behind them, though. The line must be drawn somewhere: the new definitions allow for just about every roughly round rock not a moon to be a planet. We have 12 now, and hundreds before long. Arbitrarily giving Mercury's mass as a minimum not only neatly avoids all the small ones, but also is identical to the list of planets before Pluto was mistakenly added. (I call it a mistake, just as adding Ceres and the other asteroids was seen as a mistake in the 1850s: Ceres used to be a planet.) -- Jordi·✆ 15:26, 16 August 2006 (UTC)

I'm pretty sure that it has nothing to do with Ceres, it's just that Ceres has enough mass to be a spheroid on it's own right. McKay 15:39, 16 August 2006 (UTC)

• Chinese and Indians are still humans despite you can count billions of them, they have the same human rights has you. One just needs to accept the solar system is not as ordered we used to think, we have pictures and we know they are very different, we have better telescopes, etc. There's no reason to exclud Pluto and others from Planethood. People will like to know there's a tiny planet within the asteroid belt, Ceres, it will make them wonder.--Pedro 15:41, 16 August 2006 (UTC)

Exactly -- it's far more exciting to discover new planets. Very few people care nearly as much (if at all) about extra Trans-Neptunian Objects. The most significant outcome of this debate, IMHO, is that it raises public awareness and excitement about astronomy. --TiroAethra 16:00, 16 August 2006 (UTC)

Pedro, are you accusing us Earthlings of being planetist? Serendipodous 15:52, 16 August 2006 (UTC)

• Some yes: Pluto is a dwarf, Pluto is not a planet, Pluto is a rock, pluto is ice... I wounder what the Jupiterians might think of Earth... --Pedro 16:02, 16 August 2006 (UTC)

I believe the correct term is "Jovians." Most inhabitants of Jupiter would consider your word for them a racial slur. Serendipodous 16:04, 16 August 2006 (UTC)

Silly Americites :D McKay 16:06, 16 August 2006 (UTC)

• Looks like we have two new resolutions to the definition of a planet, the new one would demote pluto and leave 8 planets. [5] Tachyon01 16:36, 18 August 2006 (UTC)

Image:Newplanets.jpg

At Talk:2006 redefinition of planet#Image removal rationale, concerns have been brought up regarding the use of Image:Newplanets.jpg. I'd like to know what the general opinion here is. I personally would remove the image for the reasons given on the above page. Nick Mks 19:19, 17 August 2006 (UTC)

It's inaccurate. The new ruling by the IAU wouldn't establish nearly the number of planets as that image is claiming. Plus, it's too busy, and frankly, it's ugly. Accuracy issues aside, there has to be a much better and prettier way to show those orbits. --Cyde Weys 19:21, 17 August 2006 (UTC)

Well, as the person who imported that image I should justify my action at least. There has been a great deal of supposition and fudging released by the IAU in the last few days that has been picked up by the media as established fact; the most notable being, of course, that there are now 12 planets. This is wrong. There could be (and indeed should be, if the definition were to be scientific rather than cultural) dozens of planets in our solar system. That image demonstrates a fixed, scientific limit for the total number of planets in our system. Anything below it would have to be justified in some way other than, "But that's just too many!" Serendipodous 19:49, 17 August 2006 (UTC)

I think the image is great. Sure, it's a little rough, but I think it's cool. The rationale for the 12 planets is that those are the ones we have relative certainty of their Hydrostatic equilibrium. The other 53-12=41 Mike Brown thinks are probably round enough, but we don't have enough information. More information will most likely bring the number to around 53 quickly, and several hundred (thousand) in the future. McKay 20:40, 17 August 2006 (UTC)

Periodicity

Hello anon user. While I admire your dedication to truth, there are several rules involved, particularly WP:NOT. I would imagine that 1 Ceres.com is your page. I'm not sure how I feel about your opinions from a mathematical sense, it doesn't seem right, but it might be true. There does appear to be some credence to your claims. Alas, Wikipedia isn't as interested in Truth, as it is in Verifiability. I'm not here to say that the claim is false, but I'm saying that it isn't verifiable. When 1ceres.com gets reviewed by a major journal or website, it will go in, even if unproven, because Verifiabiliy is more important to Wikipedia than truth.

On a seperate note, reverting a revert, especially without reasoning, is considered bad form. McKay 04:10, 18 August 2006 (UTC)

Should the following information be included within the definition? See the discussion about the Titius-Bode for the actual equation.

The Periodicity of Planets Recently, a new periodicity of planets was developed by Tyler Granger from the Titius-Bode Law that demonstrates a recognizable accuracy. Because of this periodicity, planets are defined by a natural phenomena rather than an institution. This new equation was discovered in the late 1990's prior to the discovery of 2003_UB313 (Xena). The discovery of this periodicity has yet to be accepted by the IAU. Yet, the redefinition of 1Ceres and 2003_UB313 would bring the planets of the Solar System into compliance with the periodicity.

Yes or no? Fact or rule breaker? —Preceding unsigned comment added by PlanetCeres (talk • contribs)

• NO as per the entire community last night. You are also close to getting into trouble for vandalism and WP:3RR violation. Nick Mks 08:47, 18 August 2006 (UTC)
• Yes But, the equation is missing from this section. As per it was deleted from the Titius-Bode_law discussion rather than addressed with fact. Bullying does annoy me. PlanetCeres

Nothing was deleted from Talk:Titius-Bode law in the last 24 hours. Nick Mks 09:41, 18 August 2006 (UTC)

Quite simply, this is a rule breaker because it is original research. If it ever gets accepted in a peer-reviewed science journal like Nature or Science, then yes, it would be worthy of inclusion. Until then, no. Serendipodous 14:13, 18 August 2006 (UTC)

Definition of moon aka "satellite"

from IAU:

Q: What is a “satellite” of a planet?

A: For a body that is large enough (massive enough) to satisfy the definition of “planet”, an object in orbit around the planet is called a “satellite” of the planet if the point that represents their common centre of gravity (called the “barycentre”) is located inside the surface of the planet.

• so this means that objects smaller than Mimas are no longer considered to be moons?! Or I missunderstood it?!--Pedro 23:32, 17 August 2006 (UTC)

As I read it, all satellites are still satellites even if they are large enough to be planets. But I agree, that is a bit ambiguous. Serendipodous 14:13, 18 August 2006 (UTC)

Maybe the solution is more simple: they could define just two types of objects: stars (objects that sustain or have sustained some form of fusion) and planets (I would discard the term "satellite"). Then we could speak only about systems: a system is a "stellar system" if the more "representative" (normally, the more massive) object in it is a star; and a system is a "planetary system" if the more representative object in it is a planet. So, Ganimedes is a planet in the planetary system of Jupiter, which in turn is in the Sun solar system. Is there a fundamental flaw in this? I see the other definitions too arbitrary.Damien,18-08-2006.

Discussions about the issue

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Definition based on brightness?

It was said that the proposed definition would be both simple and scientifically accurate. Now the proposal is revealed. The definition based on absolute magnitude: Any object as bright or brighter than Pluto (V = −0.76) would be considered a planet.[6] Therefore Pluto would stay a planet, 2003 UB₃₁₃ would become a new one, and no other TNO found so far would qualify.--JyriL ^talk 01:35, 15 August 2006 (UTC)

It should be noted that the above is a proposal, not the proposal. Derek Balsam 01:47, 15 August 2006 (UTC)

OK, well, at least according to the small article it is just a proposal. I concede it may be the only one presented :) Derek Balsam 01:51, 15 August 2006 (UTC)

If I submitted that article for feature consideration on Wikipedia, it would get slammed. It's virtually incoherent. I can only assume whoever wrote it is not a native English speaker, because his knowledge of English syntax is shaky at best. It is also the worst definition proposal, by far, that I have ever seen. Basically, it's defining planets by their colour. Imagine if we used that criterion to define human beings!

Thankfully, after rereading it for the sixth time, I think I can rest assured that it is not THE proposal, because of this line:

"Letter has been circulated in draft form, but there has been no response from the two naming committees of the IAU."

If their letter was written by the same person who wrote that article, I'm not surprised. Serendipodous 06:55, 15 August 2006 (UTC)

You mean Tom Gehrels? It certainly looks like as if it was devised by an asteroid observer. But remember, nothing has been decided yet. This is only a draft, and as Derek Balsam said, it could be just "a" proposal. It certainly has some appeal (confirming an object as a planet becomes much easier, important as more and more distant large objects are being found), but without additional parameters it would allow huge difference in minimum sizes (1900–10000 km; from an Enceladus-style albedo to an albedo similar the darkest asteroids have).--JyriL ^talk 10:16, 15 August 2006 (UTC)

A rather foolish definition IMHO. You could have an object in an oval orbit that turns 'planet' when far from the sun, and then reverts to 'asteroid' every year when the snow melts. This whole discussion is totally backwards from a scientific viewpoint, as everyone is starting with the result that they want, (8, 9, or 10 planets, but not 30) and then trying to come up with otherwise meaningless distinctions that fit the bill. Algr

Orbit

I was wondering why the orbit is not included in defining what is a planet ? I'm just a layman but I think the following could define a planet:

a body that is in orbit around a star which is not a star itself (no internal nuclear fusion).

a body which is a spheroid due to its own gravity.

a body which is the only body in its orbit around its parent star (not including satellites).

a body which orbits its parent star in a fairly non-eccentric orbit within the star's ecliptic plane.

The third qualification would eliminate Ceres which is a member of the asteroid belt. The fourth qualification would eliminate all Kuiper Belt objects including Pluto as they have highly eccentric orbits and with significant inclinations relative to the ecliptic plane of the solar system.

All of your proposed definitions are discussed somewhere in the article. The first is discussed in the "Extrasolar planets" section. The second is discussed in the "Sphericity" section. The third is discussed in the "Shared orbit" section. The fourth is discussed in the "History and Etymology" section, near the bottom. Serendipodous 18:25, 16 August 2006 (UTC)

Specifically, the third qualification would also eliminate from the list of planets Mars, Jupiter, Saturn, and Neptune (see Lagrangian point#Natural examples) and possibly even Earth (see Lagrangian point#Other co-orbitals). Co-orbitals are quite common.Derek Balsam 18:31, 16 August 2006 (UTC)

Yes but my question is if the orbit is be considered in the official definition by the IAU and if not then why not ? That seems to be an important parameter, at least to me. The official definition shouldn't be about what's popular it should be about good science. There seems to be a bit of a desperate attempt to keep Pluto as a planet just to satisfy popular culture when it is really a Kuiper Belt object (or trans-Neptunian object).

As for the third point, the satellites are gravitationally linked to their planets so that wouldn't count. The bodies in the asteroid belt generally are not.

Which definition do you mean? The IAU specified that a planet orbit its star, be big enough to be spherical, and not be big enough to fuse (in its earlier definition concerning brown dwarfs). That's three of your criteria right there. The last one is more to do with manner of orbit and has little to do with the object itself.Serendipodous 19:01, 16 August 2006 (UTC)

Actually that's only 2.5 of my definitions as that would still include Ceres and any other asteroid which happens to be spherodial. Also why shouldn't the orbit be considered ? Kuiper Belt objects have irregular and highly inclined orbits and are at the edge of the solar system so why shouldn't they considered their own class because of this ? Clearly they're different from the eight planets which aren't in question.

No object in orbit around the Sun, planet or otherwise, as an entirely circular orbit. Some are more circular than others, but none is more "regular." Quaoar has a nearly circular orbit, almost on the ecliptic. That definition wouldn't excuse every KBO or other such object we may find. The third option, orbital dominance, is not really scientific. It's precident is largely historical. It's really just a means to keep the solar system tidy rather than an inherent scientific criterion.Serendipodous 19:16, 16 August 2006 (UTC)

Instead of "regular" I should say non-eccentric. Pluto's orbit is highly eccentric. As for orbital dominance, consider the Earth and Moon. Clearly the Earth is the planet and the Moon the satellite by virute of size. As for Quaoar then maybe it qualifies as a planet though it's not on the proposed list of 12 but can a KBO be a planet ? Also if tidyness is considered then the solar system should be as follows: Sun, inner (rocky) planets, asteroid belt, outer (gas giant) planets, Kuiper Belt objects, Oort cloud. Eight planets, lots of asteroids, probably lots of KBOs.

Again, all planets' orbits are, to a greater or lesser degree, eccentric. Orbital eccentricity is a measured planetary characteristic. As for KBOs being planets, well Pluto and Xena are both KBOs, so the answer would appear to be yes. The Moon is a satellite because it orbits the Earth. Size has nothing to do with it. Serendipodous 19:36, 16 August 2006 (UTC)

Yes that was my point about being the only object directly in orbit around a star, satellites orbit the planet. As for the IAU considering Pluto and Xena planets, it seems based much more on what the public wants (Pluto historically and Xena because of the popular culture reference). Why not Quaror as well which is spheroidal and has a much more typically "planetary" orbit (not highly eccentric and very close to the ecliptic plane) ? Pluto's orbit takes it in within Neptune's at perihelion and beyond Quaror's at aphelion. Considering Ceres a planet seems even more ridiculous, that's definitely an asteroid in the asteroid belt, albeit the largest asteroid.

I don't think that they're considering "Xena" because of the pop culture reference. Partially because the IAU doesn't acknowledge the name Xena, it is officially 2003 UB313. I think this also means that we'll have a new name for 2003 UB313 soon. I think that it's because we know more about 2003 UB313, and we have greater reason to know that it's spheroidical. McKay 20:31, 16 August 2006 (UTC)

Xena and not Quaror ? Not buying it. Xena has an even more eccentric and more inclined orbit than Pluto. All of these objects just don't seem like they should be considered planets. Certainly not Ceres which is definitely an asteroid and Charon which is a satellite. While you're probably right about the IAU not considering Xena for that reason they certainly seem to be trying to come up with a definition that fits Pluto instead of just coming up with a good definition.

Rogue planets

This may be a silly question, but what happens if a planet-like object that maybe once was in orbit around a star but was ripped out of orbit by some passing star and is now kind of floating around is discovered? What's that? --Elliskev 19:23, 16 August 2006 (UTC)

Not a planet. But we already know objects that are way too small to be even brown dwarfs (>13 M_Jupiter). There are several suggested terms like sub-brown dwarfs (hardly apt in this case), rogue planet and planemo.--JyriL ^talk 19:29, 16 August 2006 (UTC)

One thing it's not is a silly question :). In fact this is considered one of the major flaws in the IAU's definition by some see here, for example. The issue of what constitutes a star and what constitutes a "rogue planet" is not dealt with by this definition, the main purpose of which was to decide what to do about "Xena." Serendipodous 19:32, 16 August 2006 (UTC)

Proposed definition is circular

Lol, the IAU have blundered big-time: in defining a planet, check out condition (b). Hopefully, they realise this and will finally decide on a definition that is at least logically sound. MP (talk) 18:40, 16 August 2006 (UTC)

I don't see what's circular about it. Stars orbit stars. Serendipodous 18:55, 16 August 2006 (UTC)

Stars orbit stars, yes, but you can't define a star to be an object around which another star orbits (or otherwise):

“A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.”

MP (talk) 08:27, 17 August 2006 (UTC)

I wouldn't describe such a definition as "circular." It would be circular if a planet was defined as something in orbit around a star, and a star was defined as something with planets around it. But a star has always had a very rigid definition independent of planets. I agree that defining a planet by the fact that it orbits a star is certainly silly, (as do a lot of professional astronomers) since it means that if a planet were ejected from its system it wouldn't be a planet anymore, but that doesn't make it circular.

Re your edit: No, that's not circular either; it's just redundant. Satellites do not orbit stars; they orbit objects in orbit around stars, so obviously by the above definition they would not be planets. There's no need to specify that a planet cannot be a satellite. Serendipodous 08:31, 17 August 2006 (UTC)