Talk:Definition of planet/Archive 6

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Unsourced

This archive is a holding area for good content that has not been sourced, or that no longer fits the article in its current form.

Removed paragraph

"Nonetheless they could be considered as such since, though the Moon orbits the Earth, the timing of its orbit is in tandem with the Earth's own orbit around the Sun — looking down on the ecliptic, the Moon never actually loops back on itself, and in essence it orbits the Sun in its own right. This is true of any moon sufficiently far enough away from its parent body that its orbital speed round the planet is slower than the planet's speed round the Sun. The required distance from the planet to the moon depends on the mass of the planet, and the distance from the planet to the Sun, but not the mass of the moon. If the distance from the Sun to the planet increases, or the planet's mass decreases, then the required distance between the planet and moon increases. Consequently, the same argument could be used that Jupiter and Callisto or Saturn and Iapetus form double planets. "

Sphericity table

However, deciding which objects in the Solar System are spheroid is more complicated than it seems. In mathematical terms, spheroids consist of an ellipse rotated around one axis. Consequently they have two axes of equal length and one that is either longer or shorter; they resemble spheres that have been deformed (by stretching or squashing) in one dimension. A section through one axis will produce a circle, and a section through the other two axes will produce an ellipse.

All spheroids, however, have the points on their surfaces joined by smooth curves (which form the elliptical or circular sections). On a topographically irregular body this can only be an approximation; however, taking such irregularity into account, a definite contrast exists between bodies, such as Enceladus, which are essentially spheroidal, and irregular bodies, like Neptune's moon Proteus, whose limbs do not show smooth curvature.

If one uses this mathematical basis to define a spheroid, then the boundary between spheroidal and irregular objects within the Solar System frays noticeably, as this table illustrates:

---

Object	Dimensions (km)	Mass (1019 kg)	Density (g/cm³)[d]	Shape
Ceres	975 × 909	95	2.08	Spheroid
4 Vesta	578 × 560 × 478	27	3.4	Spheroid
2 Pallas	570 × 525 × 500	22	2.8	Irregular
Enceladus	513.2 × 502.8 × 496.6	10.8	1.61	Spheroid
10 Hygiea	500 × 385 × 350	10	2.76	Irregular
Miranda	480 × 468.4 × 465.8	6.59	1.20	Spheroid
Proteus	436 × 416 × 402	5.0	1.3	Irregular
Mimas	414.8 × 394.4 × 381.4	3.84	1.17	Spheroid
511 Davida	326.1	3.6	2.0	Irregular
704 Interamnia	316.6	3.3	2.0?	Irregular
Nereid	340	3.1	?	Irregular
3 Juno	290 × 240 × 190	3.0	3.4	Irregular

---

Plainly, there is no clear mass or size boundary dividing those objects in the Solar System which could be considered "spheroids" and those which are obviously irregular. The irregular objects Pallas, Hygeia and Proteus are all larger than regular objects, such as Miranda and Mimas. Also, as demonstrated by the dimensions listed in the table, the term "spheroid" is, in any case, fairly loose. Vesta, by the above formulation, could be considered either a spheroid or irregular. (see image)

The new diagram

The new diagram listing the changing numbers of planets is quite nice, but it does contradict the article slightly. Galileo considered his four moons "planets", though I don't know yet whether this term was widely used for them, or for how long. Until we sort this out it might be a good idea to take the diagram down for now. I'll place it here until I can get some better information.

Offical number of planets at various times.

Serendipodous 09:30, 22 February 2007 (UTC)

Size comparison with Jupiter

The picture comparing the sizes of Pluto with the Earth, the Moon and other KBOs suggests a false qualitative difference between Earth and the other objects. It is important to counter this with the observation that planets come in a huge range of sizes. The differences between Earth and Jupiter are much greater then the differences between Earth and Pluto, and that is important to deciding what a planet is. Algr 04:29, 15 July 2006 (UTC)

Hmm. I think the main problem is that the listed picture doesn't reflect the topic of the section anymore either. It's been part of this article for a long time and was specially made for it, so I am rather attatched to it, but the fact is that the issue raised in that section is not the small size of the KBOs/asteroids, but their large sizes relative to each other. I may swap it out with a more appropriate image, but I would like to find a way to keep it in, so I'll repost it here until I can figure out how.

The relative sizes of Earth (on the left) with (from top to bottom) the Moon, Pluto and its largest moon Charon, Sedna, Quaoar, and Ceres on the right.

Serendipodous 07:43, 15 July 2006 (UTC)

______________________________

Jupiter and Saturn as Stars

"If it is indeed possible that a gas giant could form as a star does, then it raises the question of whether such an object, even one as familiar as Jupiter or Saturn, should be considered an orbiting low-mass star rather than a planet."

Thank you! I believe this is the first time I have seen such a statement in print, and I feel vindicated. I definitely agree (disclaimer: I'm not an astronomer). When I was about 8 years old I got interested in astronomy. It seemed obvious to me from the descriptions of Jupiter and Saturn that they were unignited stars (in spite of the presence of significant amounts of methane). I've kept this opinion through the intervening 55 years, in spite of increases in my knowledge about these planets. And I would be very much surprised indeed to learn that either planet had a rocky or metallic core.

In these cases, at least, I think it is clear that an object can be viewed (and called) both a planet and star at the same time. David spector (talk) 16:18, 6 May 2009 (UTC)

At best they are "failed stars" (stretching the term) or overachieving planets. True, they do not have traditional surfaces as Earthlings define them, but neither do Uranus or Neptune. One could even claim Venus does not have a traditional surface since it is unlikely that a spacesuit could survive the pressure and heat any time soon. -- Kheider (talk) 16:40, 6 May 2009 (UTC)

For the record, I think the Juno mission in 2011 is going to be crucial in the next stage of defining a planet. For the first time we will know definitively whether or not Jupiter has a planetary core. If it does not, then I think it would be a star by all definitions save fusion, which is not true for any stars throughout their lives anyway. Serendi^podous 16:44, 6 May 2009 (UTC)

Extrasolar "Planets"

I think that, since the IAU and many astronomers insist that planets must be in orbit around the Sun, we need to coin a new word for planet-like objects in orbit around other stars.

Either that, or we should stop being so provincial and drop the requirement that a planet must orbit the Sun. We should refer to the Sun's planets as "the Sun's planets" instead of "the planets".

If we choose neither of these alternatives, as is currently the case with many astronomers, then we are forced to discuss extrasolar planets as "planets" (in quotes), since they do not fit the definition of a planet. This is unnecessarily awkward, and can only become more awkward over time as the resolution of imaging of extrasolar planets improves and the possibility of travel to them from Earth increases away from zero. David spector (talk) 16:30, 6 May 2009 (UTC)

I really do appreciate your plaudits and enthusiasm, however, I must stress that Wikipedia is not an internet discussion forum. The talk page is really more for discussing how to improve the article. If you're interested in discussing astronomy, I can recommend the Bad Astronomy/Universe Today forum, which is frequented by many astronomers. Serendi^podous 16:39, 6 May 2009 (UTC)

I appreciate that this is not an astronomical forum, but this article uses contradictory terms (a planet must orbit the sun but an extrasolar planet is presumably something orbiting some other star). In the absence of a defined term, the article uses a previously defined term to describe something that has been excluded by that definition. To my mind, totally unacceptable for an encyclopaedia. To use an analogy, it is akin to saying that an antipodean dog is not a dog. The problem is exacerbated because the article itself is about the definition of a planet. If this is considered the pinnacle of Wikipedia, then I am disappointed. Malchemist (talk) 07:27, 31 July 2011 (UTC)

So how would you suggest improving it without resorting to original research? Serendi^podous 07:58, 31 July 2011 (UTC)

Clearly the IAU has contributed to the problem by not having more than a proposed name for such an object; Exoplanet. This might be used in the article with due caution and clarification. However, astronomers have been using the term planet to refer to objects outside the solar system ever since extrasolar, non-fusing objects were discovered. The folly of rendering all these labels archaic without a clear alternative leads to the dilemma where this article finds itself. Of course the terms planet and exoplanet are mutually exclusive. There is probably a need for a term that encompasses both but 'self-rounding, non-fusing space-clearing celestial object' sounds so clumsy. Malchemist (talk) 11:48, 3 August 2011 (UTC)

Given that this article is concerned with highlighting the ambiguities of the present situation, rather than defining planet, I don't really see a problem. Planet employs a combined definition that merges the two resolutions. Serendi^podous 12:27, 3 August 2011 (UTC)

The IAU did not insist that a "planet" orbit the Sun. See the actual definition. The resolution is restricted to categorising the objects in the Solar System. For objects in that subset, it must directly orbit the Sun to be a planet. As an analogy, France has rules for who is a police officer. That doesn't mean a Tokyo cop isn't a police officer because he doesn't satisfy the French regulations: the French rules only apply to France. The IAU planet definition only applies to the Solar System. Elsewhere, different rules can be used. In particular, the Solar System definition didn't have to concern itself about any upper size limit. The IAU's working group on extrasolar planets has a "working definition" of "planet" for some (not all) other contexts. Tbayboy (talk) 00:59, 4 August 2011 (UTC)

The term "discovery" as applied to Earth's Moon

Under the section "Sattelites": "When Copernicus placed the Earth among the planets, he also placed the Moon in orbit around the Earth, making the Moon the first natural satellite to be discovered." "Discovered" is possibly a misplaced term here, since we're talking about an object that has arguably been observed by many lifeforms in detail over its history. I like the term "identified" better, but I wanted to run it past the talk page here. Stotan (talk) 03:20, 7 May 2009 (UTC)

I like it. :-) Serendi^podous 05:35, 7 May 2009 (UTC)

Moved discussion to talk page as per WP:NOTFORUM

Fair enough, but there is a genuinely relevant point here. The concept of "discovery" is simply not defined. Therefore infoboxes giving dates &c violate NPOV by giving special prominence to 1 POV. This applies to a lot of other stuff in infoboxes too. Peter jackson (talk) 11:06, 5 June 2009 (UTC)

Quotation by Alexander von Humboldt

Hi,
Helping to translate the french version of this article, it appears that the quotation from Alexander von Humboldt which supposed to be in Cosmos couldn't be found in the given link. Any information on this ( pages, traductor, edition) would be appreciated.
TIA.@+ --90.8.76.43 (talk) 13:17, 18 May 2009 (UTC)

Found it. Page 297. Lucky that was there, if it wasn't this article would have been royally screwed. :-) Serendi^podous 14:04, 18 May 2009 (UTC)

Any direct link to the page ? All I find in page 297 are about oceanic or mountain studies.--90.8.76.43 (talk) 15:00, 18 May 2009 (UTC)

It's there. Unfortunately I can't provide a direct link to the paragraph because Wiki can't handle the URL and won't accept tinyurls. When you click onto the book's hyperlink, just type "unwandering orbs" into the "search this book" engine. It will say it can't find it, but then just click "search all volumes" and there it is, in that same book. Serendi^podous 15:13, 18 May 2009 (UTC)

I found it on a french translated version (online) (french article updated).--90.8.76.43 (talk) 23:09, 18 May 2009 (UTC)

Acceptance section

What does "larger by a factor of 0.33" mean? Sounds like smaller to me. Peter jackson (talk) 10:44, 15 July 2009 (UTC)

It means that they account for 0.33 (or one third) of the mass in their orbits. That could be worded better, but I can't think how right now. Serendi^podous 15:16, 15 July 2009 (UTC)

Yes, perhaps to clarify a little, if you take all the objects in Ceres' orbit (the asteroid field) and put them all together, then Ceres would makes up 33% (or the Ceres page states it as 32%, so I don't know which is correct) of the total mass. HumphreyW (talk) 15:21, 15 July 2009 (UTC)

Extrasolar planets

Do Extrasolar planets (planets in other solar systems) fit in with the same definition as planets in our solar system, except "orbits the Sun" becomes "orbits a sun"? Jezzamon (talk) 05:37, 27 August 2009 (UTC)

The two definitions overlap, but are not the same. The extrasolar planet definition focuses on maximum size, whereas the Solar System planet definition focuses on minimum size. The exo-definition specifies minimum size as whatever is agreed for the Solar System, ie the 2006 definition, and so, in effect, contains it. It does, however, specify that a planet must be "in orbit around a star or stellar remnants." Serendi^podous 08:53, 27 August 2009 (UTC)

Dead debate?

The sources seem to cover 2006 to 2008 with a few earlier dated sources for earlier philosophizing around language and term creation. Is the debate dead, or is it just that the wikipedians have gotten enough of a hot topic? Those of you who are professionals might know, AFAIK amateurs generally just accept the definitions that professionals claim. ... said: Rursus (^mbork³) 08:44, 2 October 2009 (UTC)

Well I'm far from a professional, but the debate is not dead as long as Alan Stern et. al. still have a voice. Stern claims not to recognise the authority of the IAU, which should create some interesting scenarios when he has to register Pluto's surface features for naming. The issue is still unresolved as far as extrasolar planets are concerned, and the "Juno" mission could still throw up some surprises. Serendi^podous 12:12, 2 October 2009 (UTC)

OK, thanks! I would say some issues are also unresolved as regards to "introplanet" definition, but it is likely IAU won't to take up the issue again. The trouble is that IAU seems to be interested in their registration and naming of individual objects only, not in the complicated matter of linguistics nor science philosophy ("natural kinds" versus "measurability" and similar). In some future the later problems must be resolved however, but the dispute also seems to regard scientists genre territories, like the pretty current Quarternary dispute. ... said: Rursus (^mbork³) 12:47, 2 October 2009 (UTC)

BTW, Stern and Levinson had a better definition on what is a planet as early as 2000, taking care of the linguistics and science philosophy. I can see why he is the leading force against the current definition. Maybe we should write more about other definitions like Sterns in the sections before that from IAU. ... said: Rursus (^mbork³) 13:15, 2 October 2009 (UTC)

Levinson and Stern's definition has its own minisection. And it's not a better definition. It's the exact same definition. The only difference is that they call dwarf planets planets, whereas the IAU does not. Serendi^podous 13:59, 2 October 2009 (UTC)

Hmm? That means one distinction IAU makes is not a distinction in Stern's and Levinson's definition, which means it cannot be the same definition. Now lets give up this topic and await some novel discoveries, to see if the IAU definition is viable. The debate will then reoccur, and we can write articles about it, we're here to add to and polish the articles. Rursus dixit. (^mbork³!) 16:39, 23 June 2010 (UTC)

That depends on how much value you place on that magic word "planet". Personally I think it's an outdated word that belongs to astrologers and sorcerers. Serendi^podous 19:51, 23 June 2010 (UTC)

Discussion of the debate should include mention of the Great Planet Debate, organized by professional astronomers who reject the IAU definition, held at the Johns Hopkins University Applied Physics Lab in August 2008. Audio transcripts are available here: http://gpd.jhuapl.edu

This two-and-a-half day conference addressed not just Pluto and dwarf planets but also borderline objects such as protoplants Vesta and Pallas, exoplanets, brown dwarfs and the boundary between stars and the upper limit of planets, and teaching the controversy. Stern, Levison, and many other professional astronomers took part in panel discussions on these issues. It is noteworthy that Stern continues to refer to Pluto as a planet in his regular New Horizons update and that he and many planetary scientists have decided to simply ignore the IAU entirely. In talking about the discovery of Pluto's fourth moon, Stern said, "I believe that most planetary scientists know it's a planet, and we don't need the IAU to tell us it is": http://www.time.com/time/health/article/0,8599,2084606,00.html There is some thought that planetary scientists might formally break with the IAU and start their own professional organization. — Preceding unsigned comment added by Laurele (talk • contribs) 21:53, 31 July 2011 (UTC)

Double planet

Does the astronomical community have a formal definition for two planets orbiting a common center of mass like the Earth or moon, but of equal size? e.g. Two earth sized planets. The example of Pluto and charon is an example, although they are now called dwarf planets —Preceding unsigned comment added by 98.219.220.143 (talk) 17:34, 16 May 2010 (UTC)

No, there is no formal definition of a double planet, and there really is no need for a formal definition. When does a creek become a river? When does a hill become a mountain? -- Kheider (talk) 18:22, 16 May 2010 (UTC)

In science there is an endemic need for formal definitions, otherwise there will be no consensus about what is to be proved and, how it is to be proved. Rursus dixit. (^mbork³!) 16:41, 23 June 2010 (UTC)

After the Pluto fiasco, I doubt you will see a formal definition of a double planet in our lifetimes. A formal definition will not change the science of such an object. -- Kheider (talk) 18:00, 23 June 2010 (UTC)

There is only a need for a formal definition when a thing cannot be otherwise defined or classified. There are plenty of other ways to characterize the Pluto/Charon system ("gravitationally associated solar system bodies", as a hamfisted example). Defining double planet as a term is merely a matter of where to put things in a catalogue, not a scientific necessity. siafu (talk) 19:15, 23 June 2010 (UTC)

Well "gravitationally associated solar system bodies" of course means all bodies in the visible universe. But it depends upon how you interpret "associated". A rose by any other name ...

Anyhow, this is getting more into a forum discussion territory, let's not go there. HumphreyW (talk) 02:18, 24 June 2010 (UTC)

Clearing the Neighbourhood / Sykes

The section makes it sound like Sykes said the bit about Pluto being a potential planet in a different orbit, but that isn't in the reference, and we shouldn't be implying that he did. Sykes also didn't say the bit about Mars "therefore be surrounded by objects of similar mass", which is just wrong anyway (it would be surrounded, but the objects don't have to be of similar mass). Whether or not the isolated Pluto would be a planet depends on the definition "clearing..."; for example, a definition based on the Stern-Levison λ might still exclude it if the definition (like the IAU's handwaving) says that the object must clear the region, not just that the object currently inhabits in a clear region (hello Sedna!). However, I suspect we could find some reference for that argument. Finally, I don't think "ambiguous" is the right word here: with a reasonable definition, there's nothing ambiguous about whether an object is a planet or not. I left it in the first instance, but removed it from the Sykes paraphrase since that's not the expression used in the reference. Tbayboy (talk) 00:08, 4 January 2011 (UTC)

First paragraph

Since editing is closed, could the first paragraphs be changed to say "pluto and transneptunian objects" rather than pluto and other transneptunian objects, since pluto isn't clearly transneptunian. —Preceding unsigned comment added by 69.180.134.188 (talk) 05:34, 5 April 2011 (UTC)

By definition, Pluto is a Trans-Neptunian object. -- Kheider (talk) 05:47, 5 April 2011 (UTC)

Criteria for determining "plutoids"

This section is incorrect: per the IAU press release, the H=+1 criterion is not about "classifying" the object, but only about naming it, and there is no "demand" that an object meet this criterion to be named as a plutoid. This is explained well in the main article. Brown's objection (in his blog) is based on his own mis-reading, which is pointed out in one of the comments on the referenced blog page. I thought about fixing the first sentence, but then it becomes clear that this is not really a controversy but a misunderstanding, and the blog comment suggests it may have been cleared up (I didn't see anything more about it by Brown one way or the other, so can't say for sure). The whole section is then reduced to insignificance, so I took the bold to heart and nuked it. If it's to be revived, then it should be fixed to indicate what the IAU actually said, in addition to the misunderstanding that lead to the controversy. Tbayboy (talk) 18:42, 9 April 2011 (UTC)

Introductory language

The language of the first two paragraphs is torturous. I'd rewrite it if editing was allowed, like this: (absolute minimum to stop my eyes bleeding when I read it)

Since the word was coined by the ancient Greeks, the definition of planet has included many things now regarded as distinct, often simultaneously. In the past the use of the term was never strict and its meaning has altered to include or exclude a variety of different objects, from the Sun and the Moon to satellites and asteroids.

By the end of the 19th century, the word planet had, without being formally defined, come to cover a smaller range of objects. It only applied to objects in the Solar System; a number small enough that any differences could be dealt with on an individual basis. After 1992 however, astronomers began to discover many additional objects beyond the orbit of Neptune, as well as hundreds of objects orbiting other stars. These discoveries not only increased the number of potential planets, but also expanded their variety. Some were nearly large enough to be stars, while others were smaller than Earth's moon. These discoveries challenged long-perceived notions of what a planet could be. — Preceding unsigned comment added by 125.237.82.60 (talk) 10:11, 31 July 2011 (UTC)

OMG, I agree. On the main page, the lead starts with "The definition of planet has comprised many different things", period. What a way to start a featured article! I fixed the first paragraph, and I'll use yours for the second.

BTW, you're supposed to be able to edit the article yourself -- it's not semi-protected (yet). No such user (talk) 17:22, 31 July 2011 (UTC)

Kindly include Aryabhata's works and inferences in this topic.

I request the author to include Aryabhata's works and inferences to the list of astronomers (Planets in antiquity section). For more information, please see Aryabhata

Thank you, Funkfan 08:04, 3 August 2011 (UTC)Asatya82 — Preceding unsigned comment added by Asatya82 (talk • contribs)

Can't find anything in that article that deals with the definition of planet. Serendi^podous 08:38, 3 August 2011 (UTC)

Ongoing IAU discussion?

Is the reason that there was no improved IAU definition of planets that they wanted to pass such a decision with a large majority, but couldn't come to an agreement yet? I hope it's not that they think the current definitions are OK, because they're clearly not. For example the definition of extrasolar planet states the lower mass limit for an object to be an extrasolar planet is the same as in the Solar System... but there was no such limit given. Neither for dwarf planets nor for regular planets. There would have been no harm done if the upper limit for extrasolar planets was also applied to the Solar System, but that wasn't done either. It's like they're hiding everything behind smoke and mirrors and make up the results as they go.

It would be relatively easy to come up with criteria that could decide if a newly discovered body is a planet or not. But they didn't do it. Why? What for example is wrong with the following?
1. A planetary mass object is an object with a mass between the agreed mass limit needed for hydrostatic equilibrium (5×10²⁰kg) and the agreed mass limit needed for deuterium fusion (2.5×10²⁸kg). (Both limits might be set lower since the first would exclude Pallas and Vesta which are remaining protoplanets and the second doesn't take into account that objects with high metallicity could allow deuterium fusion at a lower mass)
2. A planet is a planetary mass object on an orbit around at least one star that also has the capability of clearing its orbit (per Stern-Levison criteria. The difference is you only need to know mass and orbital period of the object (and the star of course), whereas with Soter's criteria you'd need to know everything about the system).
3. A dwarf planet (I prefer mesoplanet) is a planetary mass object on an orbit around at least one star that doesn't fulfill the Stern-Levison criteria.

More definitions could be added to account for other types of planetary mass objects. I prefer "planetar" for those that don't orbit any star - regardless if they were formed by themselves or in a system of which they were later ejected. Also, I'd like "moon" to mean a planetary mass object that orbits a planet (since the prototype object Luna is just that) - smaller bodies would just keep being "satellites". (If you don't like to have 60 planets of which most are small, why would you allow 60 moons of which most are tiny?)

Any ideas why the IAU doesn't come up with something like that? Ambi Valent (talk) 00:25, 19 January 2013 (UTC)

First of all, there is a lower mass limit: hydrostatic equilibrium. Second, this has nothing to do with improving the article. Simply talking about the merits or lack thereof of the IAU's decision is outside the scope of Wikipedia. Serendi^podous 09:13, 19 January 2013 (UTC)

Your second point is right. My opinions are not relevant, as I'm not an astronomer, and my conclusions from the available data would be OR, so neither belongs in the article. But you don't seem to realize that your first point is extremely open to definition. It could mean the shape of the object. Then the next question is where the limit between spheroidal and irregular is. It could mean an object with this mass and composition would certainly reach hydrostatic equilibrium. Or it could mean an object with this mass would certainly reach hydrostatic equilibrium. I'm heavily in favour of the third, as it is the one that could be most easily checked (especially for exoplanets), and avoids the problem that an ice world could qualify, while a larger and denser asteroid would be rejected, since rocky worlds are much harder to pull into equilibrium. But from statements in the article, I'm guessing at least those astronomers favor the second one. (And if they're not representative, why would they be in the article?)

Basically, I want to say that the IAU should make a decision that makes it clear the astronomers know what they are doing and want to share their knowledge with the public. Because if the public decides to not follow astronomers' findings anymore, the result will likely be even less funding... and that would be a real problem. -- Ambi Valent (talk) 12:08, 19 January 2013 (UTC)

There are many ambiguities with the current definition, all of which are mentioned in the article already. Serendi^podous 13:29, 19 January 2013 (UTC)

The IAU has better things to do than debate what the general pubic should call a planet. -- Kheider (talk) 09:44, 19 January 2013 (UTC)

While you are absolutely right with that sentence, the IAU has done exactly that: They've decided that Pluto shouldn't be called a planet anymore. But the definition they made to back up the decision is unclear and hard to check. -- Ambi Valent (talk) 12:08, 19 January 2013 (UTC)