Talk:Definition of planet/Archive 1

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Archive 1 Archive 2 Archive 3 → Archive 5

Improvements

After having been working on the Planet article on and off for a long time, it occurs to me that there is a much better "definition of planet" section there than here. If somebody could find the time, could they add some (or all) of that information here? This article is severely lacking by comparison. _________________________________________________________________________

Actually, there really isn't a lot of information in that article that isn't here; it's just arranged differently. The "planet" subsection begins by mentioning the debate (still unresolved) convened by interested parties of the IAU, which is also mentioned at the head of this article. It then proceeds to say that the consensus they were arriving at was:

"A planet is a body that directly orbits a star, is large enough to be round because of self gravity, and is not so large that it triggers nuclear fusion in its interior."

All three of these criteria are mentioned in the article; the first two are mentioned just under the "issues and controversies" heading, the last is mentioned near the end of the "minor planets" section. That consensus definition, it appears, was illusory and news is that the IAU group are no closer to a resolution than before. Indeed the first "official" declaration by the group was completely different from the one listed in the "planet" section, so for now, until they make the truly final statement on the matter, it seems best to leave such speculation out.

The next section deals with the differences between planets and brown dwarfs, which is the topic of the third subsection of this article. Gibor Basri's definition is not really any different than the one mentioned above it. Although his work is phenomenal and this article has links to his essays on the subject, it seems premature to give credence to any one definition or theory until the final word is in from the IAU. However if you feel that this information could be better-arranged, please feel free to suggest alterations. Serendipodous 21:11, 21 December 2005 (UTC)

Pluto

I consider much faster Pluto as a planet than Jupiter, Saturn, Uranus or Neptune. For me a planet is a sizable mass that is so big that it starts to appear as round, and where we can land :D. My def is much better than that of those lunatics that say Jupiter is a planet and Pluto is not. A Xanax would solve their problem. -Pedro 18:40, 1 May 2005 (UTC)

Pluto is not faster than Jupiter; in fact it is far slower, because it is much further out. The further a planet is from a star's gravity, the slower it moves. Yes, being round is one criterion that some people say defines a planet, but it is by no means an established convention. And anyway, such a definition would leave us with hundreds, possibly thousands of planets in our Solar System, once the Oort Cloud and Kuiper Belt are accounted for, which is rather unworkable. As to your claim that a planet has to be solid, why do you assume that? Just because we can't land on Jupiter doesn't make it any less of a planet. [[157.140.6.143 11:06, 25 May 2005 (UTC)]]

Duh. I know Pluto is slower than Jupiter. Sorry, I didn't express myself well: I wrote "has", but I meant "as". Jupiter has a different formation, just like a star is a star. What's the problem of having several more planets? If they exist I dont see where's the problem. It wont leave us with a thousand planets, but a dozen more (today). To be round a planet must have more than 360 km in diameter when it has a tendency to become round and with that size there are just a few more.--Pedro 01:17, 26 May 2005 (UTC)

He didn't write "has," and he didn't mean "as." His native language doesn't seem to be English, and probably what he meant was that "he'd sooner consider Pluto as a planet than Jupiter." That is, that Pluto was definitely a planet but that Jupiter, one of the original seven planets which the word was coined to describe, might not be. This, like much of the author's contributions, is not helpful. 24.22.58.51 08:59, 26 November 2005 (UTC)
I think that Pedro's point is that Pluto has a lot more in common with Earth then Jupiter does. Only the lack of fusion prevents Jupiter from being a star, so what is the value of a definition that lumps Earth and Jupiter together, but excludes Pluto? The logical conclusion of Mike Brown's new position is that the word "Planet" might be unsalvageable. Algr 06:55, 31 December 2005 (UTC)

"Relative Sizes" Image

This image portrays the Moon as much smaller than it actually is. It should be edited or replaced. Pluto is actually about 2/3 the size of the Moon, not 5/6; the Earth is about 3.6 times as big as the Moon, and the picture shows it at about 4 1/2 times as big.

The image has now been retouched to reconcile the scales of the images.
Urhixidur 20:08, 21 December 2005 (UTC)
Hi there; thanks! I uploaded the prior image, with the major bodies indicated in the article (adding and resizing based on averages), but assuming that the Moon was correct; my apologies! The Moon looks larger than it should be (IMHO?), but I see the calculation that was used and take it as OK; I only suggest a mild realignment to make things ... neater. :) Thanks again! E Pluribus Anthony 03:41, 22 December 2005 (UTC)

Arbitrariness is unavoidable

The problem with "planet" boils down to one's emotional investment in the term. In times long past, there were the Seven Planets. Nowadays its just another word for a class of celestial object. Arbitrariness is unavoidable, like the minimum size cutoff of 50 m for asteroids. Urhixidur 12:57, 2005 May 26 (UTC)

I don't understand the argument that having hundreds or thousands of planets in our solar system would be unworkable. Astronomers would keep track of them anyway, regardless of whether they are defined as planets or not, so there is no extra work. In fact I think the statement "To refer to them both as planets and as members of their own distinct populations would make the word "planet" essentially useless in practice." should be removed from the article, since it's meer opinion. Horatio 08:02, 30 July 2005 (UTC)
The problem as I see it is that these populations have their own separate geological and astronomical histories. Ceres arguably has more in common with another asteroid in its belt, regardless of whether that asteroid is spherical or not, than it has with Pluto, Jupiter or Earth, just as Pluto has more in common with other trans-Neptunian objects then it has with Ceres. Therefore, to claim that, for instance, all Kuiper belt objects are part of a distinct population, except a certain number that are also planets, or that all main belt asteroids are part of a common population, except the ones that are also planets, creates unneccessary confusion (not necessarily for astronomers, but for kids in school who have to learn them) and gives the impression that sphericity is a more important shared attribute than composition, origin or location. Serendipodous 18:31, 7 September 2005 (UTC)
Sphericity in this context is only considered important because it's a physical change that occurs when an object reaches a certain size, so can form a convenient definition for "planet". Whether an object is a planet or not by this definition would in no way change its independent attributes, such as whether it's a member of a belt. For kids, I think it's always going to be easier to explain a simple rule than to expect them to understand a rule with all kinds of arbitrary exceptions, or worse, an arbitrary list of objects that needs to be memorised. Horatio 09:27, 30 July 2005 (UTC)
Perhaps. But that definition of planet is a bit like saying that a whole orange should be classified differently from an orange that has been cut into wedges. Serendipodous 18:31, 7 September 2005 (UTC)
It may be that the word "planet" now has too much history, and describes both the nature of an object and the orbit that it's currently in, with a definition that has been patched up several times for new discoveries. I think it would be nice to have a word to describe a spherical object with a solid surface, regardless of its orbit or lack of orbit, although "planet" is most likely not the most appropriate Horatio 10:07, 30 July 2005 (UTC)
Well, it took about 20 revisions but I finally found a phrase to end that paragraph that I am happy with. Personally I don't think the word "planet" has been patched up at all. It's lagging behind the observations and, really, should either be ditched or limited in such a a way that it is at least consistent. Serendipodous 18:31, 7 September 2005 (UTC)
This revision is an improvement. However I'm not convinced that there's no need for generic terms. Otherwise you end up with terms like "Trans-Neptunian object", resorting to the word "object" which is so vague as to be almost meaningless. Horatio 12:29, 30 July 2005 (UTC)
I have no problems with generic terms, only generic terms that are misleading or inaccurate. Any definition of planet that includes members of distinct populations should include every single one of those population members. To create an artificial distinction between members of a population based on shape is to give far too much importance to an inconsequential attribute. It is akin to creating separate species of human based on race. Serendipodous 18:31, 7 September 2005 (UTC)
I don't agree, because by this principle you can't have any categories of objects at all. There's no reason why this rule for planets shouldn't then be applied to other sorts of objects. If a certain cloud happened to contain stars, "planets", rocks, and dust, then you could no longer define any of these terms because they wouldn't include all members of that population. Horatio 11:23, 31 July 2005 (UTC)
Yes, Pluto had a different formation, like Jupiter had from Earth's. That's why there are the rocky planets, the gas planets /giants and the ice planets. We only had until recently one ice planet, now we have at least two. Other planetoids should have the name "planet" but they should be renamed after Roman dieties, asteroids smaller than 10 km should not be named but numbered, asteroids bigger than 360km should be renamed at least as planetoids, IMO. -Pedro 10:09, 3 August 2005 (UTC)\
Seredipitous here. Sorry for not being logged in; I am not on my computer. Pedro, you misunderstood the point I made in that paragraph. Of course gas giants are more similar to one another than they are to the terrestrial planets. But there are no objects in the solar system with the exact same composition and origin as gas giants that we do NOT call gas giants. The term gas giant is universally applied to all members of the group, and there is no arbitrariness. With the Kuiper and asteroid belts, the situation is different. Some Kuiper belt objects are round, some are not; one asteroid is round, the others are not. Yet they all have similar compositions, origins and orbits. Why claim that some parts of the Kuiper Belt are intrinsically different from others, simply because they are round?
      • answer to Seredipitous: Well, but there are terrestial asteroids, some are planets and some are asteroids.... the same occurs today with the KBO's some are planets some are asteroids. That paragrah is not very neutral and it is missleading (one thinks Pluto is an asteroid, although it is a planet). changing your question, for you to andswer: "Why claming that some parts of the "Terrestial" Belt are instrinsically different from others, simply because they are round? " There are many asteroids near the Earth and many intercept Earth orbit. So, you doubt Earth is a planet because it is round. For you what's a planet??? -Pedro 20:29, 10 August 2005 (UTC)
I believe that is the question that this article is meant to be asking :-). I did not write the concluding paragraph to this article but I agree with it; there is no one definition that anyone could create that adequately describes what is out there. My feeling is that, as our knowledge of extra-solar planets increases, we might find that there really is no difference between *gas giant* and *brown dwarf*, or between *terrestrial planet* and *moon*. It may be that the term *planet* will eventually die (it was, after all, coined three thousand years ago to describe a completely different cosmology) and be replaced by a series of terms that better describes the astronomical reality. But that is only a guess.Serendipodous 18:31, 7 September 2005 (UTC)

Some problems

The article claims, if I'm understanding it right, that no terrestrial planet other than Earth has a magnetic field. A variety of online sources suggest that Mercury and Mars both have magnetic fields, albeit weaker than Earth's. I'm inclined to cut the phrase about a magnetic field being unique to Earth among the terrestrial planets, but I don't know enough to be sure. Anyone know?

Regarding last comment, Mercury, Venus and Mars all have magnetic fields—I will remove the line. I also noted that Titan and Ganymede aren't as massive as Mercury. For what it's worth, I think Mercury rather than Pluto should be the cut-off point for planets (much as I have affection for Pluto...). Given that no moon in the solar system and presumably no Kuiper Belt object yet to be found is as massive as Mercury it allows for a neat line to be drawn. It may take a few years but given the finding of a KB object larger than Pluto recently, Pluto will almost certainly be downgraded. If so, we'll have 8 planets as it should be.

One other thing: the last paragraph reads really strangely. All astronomers "care about" is that the "concept" of extra-solar planets and belts of material be accepted. Extra-solar planets and potentially planet forming dust discs are no longer hypothetical. I don't think anyone doubts them as a "concept" and thus the paragraph seems useless. Marskell 07:55, 30 July 2005 (UTC)

I will accept responsibility for the magnetic field gaffe (my apologies), but I can say with assurance that I was not the author of that final paragraph; I have a feeling that English is not the writer's first language. I have deleted it; perhaps its author can explain its meaning, whereupon it can be reworded and returned to the article. Regardless, it should be in the "Extrasolar planets" section, rather than its own section.
Oh, and I hope you aren't offended by the discovery of the "tenth planet" :-). It'll take time, but this will get sorted out. Serendipodous 18:32, 7 September 2005 (UTC)
Sounds good. Not offended at all by recent discoveries--they will make excellent waystations and sentry posts for our interstellar expansion ;). I only suggest that the "tenth planet" will not ultimately be classified as such and that if it is larger than Pluto, Pluto itself will have to be downgraded. Marskell 09:24, 30 July 2005 (UTC)
Could I just ask you a few questions to clear up some fuzziness? First, what would happen if an object only slightly less massive than Mercury were discovered? Also, can Venus, Mars and Mercury be said to have magnetic fields, when they wouldn't have magnetic fields were it not for the interaction of their atmospheres with the solar wind? Or have I misinterpreted what I read?Serendipodous 18:32, 7 September 2005 (UTC)

Intro addition

This addition to the intro is perpetually being resurrected:

The lexical definition has changed over time, initially being an extensional definition (a list), becoming an intensional definition (the necessary and sufficient conditions for belonging to the set), and finally today in scientific use a precising definition is used which fine tunes the dictionary definition (lexical definition) of the term for the current specific issue under discussion.

I'm no expert on lexicography, but regardless, I fail to see what a paragraph about the definition of definitions has to do with planets. Also, this addition implies that there is a single definition of "planet", when none actually exists, which means it is factually inaccurate. And I'm sorry; this may be a lexicographically accurate statement, but it is poorly written. If you're going to load a paragraph with such specific jargon, you should explain what that jargon means in such a way that someone unfamiliar with your field can grasp it. If that requires that the paragraph be hundreds of words long, then the paragraph is useless. Serendipodous 18:33, 7 September 2005 (UTC)

The paragraph is not in an article about planets; it is in an article about the definition of planets. There is a difference. "What is a definition" is part of "What is the definition of a planet". So a link to definition is needed. Also, part of the problem with discussing the definition of a planet is some people come into the discussion with erroneous preconcieved notions of what a "definition" is. Another problem is that not only has the definition of planet changed, the type of definition employed has also changed.

This encyclopeedia is for everyone. Don't dumb it down. I have no doubt Wikipedia has other contents neither of us fully grasp (perhaps Navier-Stokes equations). The point of the links is so you can use them and learn. So learn and don't delete accurate relevant data just cause you can't be bothered to learn something new. It's not poorly written. It's short and to the point and has links for anyone who thinks to themself "Gee, I don't get what I just read."

"the IAU used "historical practice in accepting the eight planets that were known when the IAU was created and accepting Pluto as the ninth when it was discovered (in 1930) not long after the formation of the IAU." [1] Originally a planet was one of the permanent wandering points of light in the sky. A specific list existed that didn't change until modern times. In accepting additions to the list, the list was changed based on the necessary and sufficient conditions for belonging to the set (what you probably think of as a "real" definition) because a new object (e.g. Pluto) was discovered that was more like the current items in the list of planets than anything else known at the time. Today, we know of all manner of objects in the sky that range in tiny increments from lone electrons in space to space dust to tiny rocks to big rocks to very very big rocks, to small mooons to large planets to small stars to large stars to black holes. Scientists today carefully define their terms in their papers depending on the point they are trying to make. An object can be called an asteroid or a comet depending on what's being discussed. Same with moon or planet. Or planet vs. star. Inbetween objects are in a sense BOTH. WAS 4.250 08:52, 31 July 2005 (UTC)

Thank you for clearing that up, but I'm afraid I still do not agree with that paragraph; it says that "a" precising definition is used, but what you seem to be saying, and I believe what this article is meant to be saying, is that many precising definitions are used, each dependent on the aim of the person using it. The point of the article is that there is no one definition of planet, and that any one definition would of necessity entail a level of arbitrariness. That, ultimately, was why I deleted that addition; it contradicted the point the article was making.
Also, the initial definition for "planetes" was not a mere "list." To the ancients, the majority of observed stars were "fixed", in that they didn't move against the background of the sky (as far as they knew- proper motion had not been discovered), whereas a small number moved against the field. All the moving stars were called planetes, or wanderers. They were defined by their common characteristic of movement, hence theirs would have been an intrinsic definition, rather than extentional. The subsequent definition, after the inclusion of Pluto, is more of a list, since Pluto never did fully agree with all of the criteria shared by the other eight planets.
These issues make the inclusion of this paragraph problematical; not necessarily the way it is worded, but what it is saying. I do believe that it can be included in some form, but it needs to be modified. Serendipodous 18:33, 7 September 2005 (UTC)

What might be better

The definition has changed over time. It has sometimes been an extensional definition (a list) and sometimes an intensional definition (the necessary and sufficient conditions for belonging to the set) at various points in time, and still is for many non-scientists. Today in scientific use precising definitions are used which fine tune the dictionary definition (lexical definition) of the term for the specific issue under discussion.

  1. maybe deleting the first "lexical" would help no-name's problem.
  2. changing "initially" and "becoming" as it did go back and forth as you say (since this is the intro, I was trying to simplify things)
  3. finally being clearer that EACH differnet use can get a different precising definition.

Better? Any futher enhancements needed? Or does this just make it worse for you? (I prefer the [first A then B then C] even if it is a gross simplification.) WAS 4.250 16:41, 31 July 2005 (UTC)

That example is fine. Believe it or not, I am not trying to antagonise you about this. The brusque tone of my initial post was largely in response to being referred to as a vandal. I honestly just want to make sure this article is correct. As an aside, my apologies for not having an account name but, for some reason, my old name isn't recognised here anymore and I can't start up a new account without creating a dummy e-mail address to go with it, which is too much hassle for one username. Serendipodous 18:33, 7 September 2005 (UTC)
You CAN start up a new account. The e-mail field is optional. For example, I left that field blank on THIS account. WAS 4.250 13:10, 1 August 2005 (UTC)

I disagree

I disagree that the intro is well written. It lacks parallel structure (... [subject] has changed ..., being ..., becoming ..., and finally [second subject] is used ...). Apart from style, the problem implies a false progression of “lexical,” from “extensional” to “intensional” to “precising.” Also, “lexical definition” is initially used without explanation and later appears as a parenthetical to explain “dictionary definition.” The sentence seems to be going out of its way to modify the word “definition” as many different ways as possible, even when not saying anything new. - no-name

Magnetic Fields and Mercury

Could I just ask you a few questions to clear up some fuzziness? First, what would happen if an object only slightly less massive than Mercury were discovered? Also, can Venus, Mars and Mercury be said to have magnetic fields, when they wouldn't have magnetic fields were it not for the interaction of their atmospheres with the solar wind? Or have I misinterpreted what I read?Serendipodous 18:34, 7 September 2005 (UTC)

First, 82.45.244.104 you should create an account with a recognizable name if you want to be caretaker for articles like this. Regarding magnetic fields, I don't want to pretend to a lot of scientific knowledge I don't have but I think you've mistaken the magnetosphere for the magnetic field itself. The former is an envelope of space in which the magnetic field is dominant and through which the solar wind largly doesn't pass. The solar wind does not create the magnetic field in any sense. The magnetic field is a product of the rotation of iron in a planet's core. You may want to state that only Earth among terrestrial bodies has a substantial magnetic field. Apparently on Venus and Mars the field is extremely weak.

Regarding, potential bodies Mercury size, you are right that picking Mercury as the cutoff may be convenient but is much more arbitrary than forming into a sphere or being below nuclear fusion mass. Two ways to look at it. First, we have long devised useful measurements in which the external reference point is basically meaningless but still conventionally understandable. The foot unit is likely the length King Henry I boots. So we pick Mercury and say that's it. Eleven inches is not a foot and a body below Mercury mass is not a planet. Alternatively the IAU could pick an actual numerical marker close to Mercury size and make this the point; Merucry is 0.055 Earth masses while Pluto is more than an order of magnitude lower (Ganymede is half the amount). Thus, O.05 Earth masses could be it. I like the first alternative because it's easiest to intuitively grasp--most eight year olds know Mercury is first in line from the sun but 0.05 Earth masses is just another big number. Marskell 14:06, 31 July 2005 (UTC)

Thanks for the info! (as you can see, I am now named)

Serendipodous 18:34, 7 September 2005 (UTC)

definition of a moon

What about a moon controversy. How can space rocks around planets be classified as moons. Shouldn't we have two types of moons? maybe moons (or secundary planets or even satellite planet) for Titan, Ganymede, Europa, our Moon and another notable moons (honestely I find moons more interresting than main planets), maybe "natural satellites" for those smaller objects (or another name for these space rocks). today all terms are applied to all objects. If we only would name bigger objects with Roman dieties, we wouldnt have today's problem with so many freaky names that no one can remember. --Pedro 10:01, 3 August 2005 (UTC) [2] (see "New Names for Saturn's Tiniest Moons" section for definition of a moon controversy)--Pedro 11:24, 3 August 2005 (UTC)

--User:yisraelasper As I pointed out with the the problem of naming satellites of brown dwarfs no longer is it absolute that a satellite is a moont. The IAU is no help here as it simply avoids the term moon. I say that we should then push for the spread of the knowledge of the separation between the term satellite and moon and have it be for any planet except Mars which borders the Asteroid Belt so a special dispensation can be made for it to prevent a public outcry and still have some scientific meaning for it, that if an object would be considered a planet if what it orbits would be treated as a star or star or stellar remnant it should be considered a moon with the exception that however much what the objects orbits differs from a planet so can the moon.

Kuiper Belt vs. Trans-Neptunian region

I'm a bit confused by these two terms; one, I think, refers to a single, definable region of the solar system, while the other could refer to everything up to the Oort Cloud. I'm going to try and draft a section for the article that makes a distinction between these two terms.Serendipitous 10:32, 8 August 2005 (UTC)

Don't edit about that if you don't know what it is. And I doubt that relation you made is truthfull. --Pedro 10:18, 8 August 2005 (UTC)

I do know what they are, I am merely slightly confused as to how they are applied. The Wikipedia article on trans-Neptunian objects makes the distinction I made above, but a recent edit to the "Pluto controversy" took out any references to the Kuiper Belt, making me wonder if, by some people's standards, the term "Kuiper Belt" should not be used. I have edited the section with what I believe is the correct diffrentiation. If whoever first removed the term "Kuiper Belt" from the article disagrees with my decision they can post their reasons here, and, if I agree with their position, I would be happy to change what I wrote in accordance with what they think.Serendipitous 10:32, 8 August 2005 (UTC)

From what I've recently read: Kuiper Belt Object (KBO) is not a correct name for these objects, because someone prior to Kuiper has theorized about these objects, so Trans-Neptunian Object (TNO) is a more neutral name. If TNO includes or not the Oort Cloud I don't know. Even after reading that article. Use more than one source to validade something. -Pedro 11:11, 8 August 2005 (UTC)
Given the distance of the Oort and the smallness of the objects within, this question has never arisen, even in astronomy, as no Oort cloud objects have been identified. "TNO" refers to all objects in the solar system with their orbits at least partially outside the orbit of Neptune, but so far all of those objects have also been KBOs and a few SDOs. Because of this, there probably aren't many sources to set a precedent on this matter. siafu 14:25, 8 August 2005 (UTC)
You're forgetting about 90377 Sedna, which is considered to be an "inner Oort cloud" object. --EMS | Talk 15:36, 8 August 2005 (UTC)
Considered by some. It's hardly universally accepted as part of the Oort. See [3]. Regardless, the point still stands: it's not clear whether "TNO" indicates Oort cloud objects or not because there hasn't been much reason to decide.siafu 15:46, 8 August 2005 (UTC)
Found this table at Plutino, which ought to clarify the hierarchical classification that Serendipitous was originally asking about. --Eric Forste (Talk) 02:26, 10 August 2005 (UTC)
Types of distant minor planets
Most excellent. siafu 04:48, 10 August 2005 (UTC)


Why this article is POV

Unfortunatly wikipedia is invaded by people dethroning Pluto, in every article there's a notion abouyt Pluto not being a planet, and who (disinformed ppl) reads this article starts thinking it is an asteroid. The pluto asteroid theory (which this article enforce) is not part of a major group of scientists. Althoug it seems wikipedia is full of those scientists that say Pluto is an asteroid, plz browse planetology topics in en.wikipedia to see that.

The problem is not Pluto being a planet, but the new spherical bodies not being one (like Ceres or orcus). --Pedro 00:12, 17 August 2005 (UTC)

It's not Ceres or Orcus (which are not "new) that present a challenge to the current definition of planet, it's objects like 2003 UB313, 2005 FY9, 2003 EL61, and 90377 Sedna. Pluto is smaller in diameter than Luna, and has a relatively elliptical orbit characteristic of the other TNOs identified, which makes it difficult to label it a planet preferentially to them, particularly 2003 UB313. However, if there are errors or POV in the article, please point out specifics so that they can be worked on. siafu 00:23, 17 August 2005 (UTC)
  • The Moon doesnt define a planet. What's wrong being smaller than the moon. Titan is almost the size of Mars. All these bodies are planets, although they are secondary planets, because they orbit another planet.

Definition by NASA (which I think it is the most correct): A Planet: 

    1. orbits the Sun (rather than a moon that orbits a planet)
    2. is big enough so that its own gravity pulls it together into a sphere (it must be round)

If you read this article you are leaded to think a lot of things, and leaded to one point, Pluto isnt a planet. I think we should use info from these references also, and in their view why it is a planet. Ceres was classified as an asteroid because they couldnt see its disc, but a dot.

Planets are differentiated, if you look very carefully, by their tiny spherical shape. If you can't the tiny disc, it is a star. If you see the disc it is a planet. We know today that Ceres is spherical, then they didn’t knew because, yes, it was smaller than Mars, but with today's telescopes we can see Ceres disc.

The discs of several stars have been imaged, including Betelgeuse [4] and R Doradus. — Jeandré, 2005-08-22t20:38z

There are stars (which burn), comets (small, and have a tail when it comes close to the sun), planets (spherical, orbit the sun) and satellites/moons (orbit a planet), and asteroids (not spherical). It is pretty clear if you classify bodies by their appearance you will notice that many are at least planetoids. Plus, see this: http://www.spacedaily.com/news/outerplanets-04b.html

In the paragraph above you wrote that Ceres, an asteroid, is spherical. — Jeandré, 2005-08-22t20:38z

I've been working on creating featured articles on planetology in the Portuguese lang. wikipedia: pt:Marte (planeta) (Mars), pt:Titã (satélite) (Titan) and pt:Tritão (satélite) (Triton) and I found very hard not to use the word "Planet" in Titan and Triton. In fact, some authors use it, so I use it occasionally, but avoiding it. Triton is a very complex planet, and it is possibly similar to Pluto. I think the definition of a planet should be the responsibility of planetary scientists. Defining a Planet just by its orbit is not a very good thing to do. The Earth compared to Jupiter is just a moon... --Pedro 01:28, 17 August 2005 (UTC)

There is nothing POV at all about underscoring the debate over Pluto's status. Granted, it may be redundant as there's probably half-a-dozen pages which mention it but I think you're insistence, Pedro, that there shouldn't be a debate is off-the-mark. The Hayden Planetarium lists Pluto as KB object rather than a planet, for example.[5]
On the Planet page here you'll find this: "Mike Brown of Caltech suggested a definition which would exclude both Sedna and Pluto from being classified as planets, proposing the following: A planet is any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit [6]"
If I may be so bold, as an irregular, might I suggest the following definition I once mentioned on a Forum; "If an object, who's primary orbit is around a star, has a mass significantly greater than other objects in a similar stellar orbit, it may be considered a planet." Further to that, I might add that "If two (or more) similarly sized objects orbit each other, and the axis of their orbit is around a Star, they may be considered a Binary (or Double, or whatever) Planet". As to what 'similar orbit' and 'significantly greater' can be defined as, i'll leave up to others. 05:26, 09 November 2005 (GMT+10)
Sphericality isn't the only issue. Bulk composition (Pluto is half ice), orbital characteristics and orbital neighbourhood are in many ways more important. By all these criteria Pluto is Kuiper Belt object. Marskell 18:27, 17 August 2005 (UTC)
Sorry; forgot my password, logging under a different name.
Since I wrote about 80 percent of this article, it would unavoidably reflect my own opinion, no matter how unbiased I tried to make it. That said, as yet no one has seen fit to alter what I wrote, despite the fact that it has remained pretty much unchanged, at least in the statement it is making, for months. If someone wishes to revise the article so that it reflects a more balanced point of view (not so that it merely reflects their own point of view), I would not stand in their way.
The way I see it, there are two opposing camps in this debate: the "let's demote Pluto" camp (who want to define a planet using Mike Brown's definition mentioned above- though curiously, Mike Brown himself seems to have changed his tune since discovering Xena; wonder why...), and the "let's find a way to include Pluto" camp, who want to define a planet using Pedro's definition. I have made no secret of the fact that I dislike Pedro's definition, as it involves only the most superficial aspect of an object's identity (its shape), rather than more fundamental aspects such as composition, location and formation. However, the opposing definition isn't much better, since it too ignores such things as formation and composition. The kind of definition I would endorse, should we ever learn enough about other planets in our solar system and galaxy to create one, is one that describes what a planet IS, rather than where it is or how big it is. Serendipitus 16:17, 22 August 2005 (UTC)
  • Again. There's no reason not to classify these planets as not being planets, if Jupiter is considered one. In the popular mind a planet is a round thing that doesnt shine, dispite being mostly of rock like the terrestial planets, dispite being mostly made of gas, like the gas planets and dispite being mostly made of ice like the ice planets. And that's it. If we see the "round thing" is also a physical aspect of a planet when comparing to an asteroid (lets make the Ceres exception). Maybe the term planetoid is a good one for classifying objects as big as Ceres and smaller than Pluto, what is interestinly being done by people, because people dont see it as asteroids, because an asteroid in the popular is an object that is not round and it doesnt shine. I think that some scientist are blind, because people they didnt like discovered what they would want to discover. And for me, this seems the real centre of the debate betw. some people. -Pedro 16:43, 22 August 2005 (UTC)
  • BTW, Brown's defintion: A planet is any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit.

My contra-definition of a star: A star is any body of the galaxy that is more massive than the total mass of all of the other bodies that shine. Honestly I didnt expect that "fool" definition from a discoverer of objects in the solar system. But that's my thinking.-Pedro 16:59, 22 August 2005 (UTC)

You are erecting a straw-man here. I find the idea of comparing the mass of an object to that of all other object in similar orbits to be useful. I would make some allowance for the possibility of "secondary" planets at the Lagrange positions of "major" planets, or of two or three objects of nearly the same mass at each other's Lagrange point. However, I find this criteria to be a useful way of distinguishing between a "planet" instead of a member of a population of objects.
However, this is neither here nor there. I think that that IAU ruling needs to be awaited, and to see how it plays out. In the meantime, I just ordered two books through a book club. One is a National Geographic atlas of space. The other is called the "Grand Tour". Both question the status of Pluto as a planet, and are quite convinced that had the true nature of Pluto been known in the 1930s (based both on it's true size and on there being many other similar objects in the same region of space), then it never would have been called a planet. (I will try to post author and ISBN data for those books later today or tomorrow.) --EMS | Talk 17:14, 22 August 2005 (UTC)

Asteroid Earth

hey what are those four rocky balls after Jupiter - We know Mars since 1930s - but we now discovered an object bigger than Mars, the Earth! these are Trans-Jupiter (Belt) Objects, the TJO's! There are four large (we have also discovered more recently, one also the size of Earth (Venus) and one even tinier (Mercury), and also a medium sized (ceres) and several other tiny rocks in a very caotic belt known as Anti-kuiper Belt. Because they are so many and that space is full or rocks, Mars will no longer be considered a planet. these four (or five) planets should be removed from Planethood. And Earth is just a big asteroid!---Pedro 21:10, 25 January 2006 (UTC)

Yisraelasper

Please, be careful how you edit a Wikipedia page; be sure to read the whole article before you make an addition. Much of the information you posted was already mentioned and discussed in the article, in different sections; in particular, the links you posted were almost all already listed in the "External links" section at the bottom. On that subject, please stop posting massive external links in the middle of pages. If you want to post an external link, post it in the "external links" section, or indicate it with a number. Links do not have to have the full web address on the page- all you have to do is click the "Link" icon on the top of the page, and you can write a link like this: link title. Make the link title a simple number, and save a lot of room. Plus, I am assuming that English is not your first language, as much of what you wrote was grammatically confusing. It would help if you made clear what you intended to write on the discussion page before you posted it.Serendipitous 19:23, 28 August 2005 (UTC)

If you don't specify anything, it gets numbered automatically. This is the best way to do it. [7], [8], etc. siafu 12:52, 29 August 2005 (UTC)
WTF is the point in a linking style that adds meaningless numbers that are unsuitable to reference because they will change whenever a new link is added and what if anything is the policy document that supports using this fucked up style? Plugwash 13:53, 22 September 2005 (UTC)

You might as well ask that of the whole Wikipedia project. Wikipedia articles change, sometimes fundamentally, over time. Much of the information in the articles is ephemeral; a lot of it is here one day and gone the next, replaced by something new. There is no point in referencing a Wikipedia article because the information you refer to could be gone the next day. If you are interested in a solid source for easy reference, may I suggest a rather quaint item known as a book? Serendipodous 21:36, 22 September 2005 (UTC)

It is not usual Wikipedia style to add explicitly-numbered anonymous links into articles. If you want to add references, see Template talk:Ref. Also, the implicitly-numbered links are deprecated according to the Wikipedia Manual of Style. There's usually a better way to include links in an article. --Doradus 16:27, 9 November 2005 (UTC)

Clarification on "Roundness" Cutoff

In the article, it states that the cutoff for "planethood" should be "when an object becomes spherical under its own gravity". Later in that section, as well as on this discussion page, it has been stated that this criterion "conveys only that they are round". It also says in the article that "it would not only let Pluto into the planetary club, but also Ceres". If one reads the wording carefully, one sees that this is not what the criterion means. While this criterion would mean that all planets are round, it does not mean that all round objects orbiting stars would be considered planets. The phrase "under its own gravity" is the key point. This criterion therefore convey's more about mass than about shape. I'm sure there are many asteroids in the asteroid belt that just happen to be round. That does not mean that they are that way because of their own gravity. A baseball is round, but it is so because it was made that way, not because of the baseball's gravity. My point is that Ceres would not necessarily be deemed a planet by this criteron (I don't know if Ceres is actually big enough to be spherical _due to its own gravity_.. someone please clarify this point).

As an additional note, this criterion might actually say something about composition, since some bodies may require less gravity to become spherical than others depending on composition, though I do not have the expertise to know if this would be the case. I am only pointing out a flaw in the article's reasoning.

I think that you should be careful here, because this article is reporting the logic of others. You are right that composition will play a role, since denser substances will tend to create stronger gravitational fields, and at the same time the ability of the substance to handle a mechanical load is also an issue. However, these are interesting details. As a practical rule of thumb that does not require you to wiegh or measure the size of an obect, this definition workds quite well. As a practical matter, the smaller an object is, the less likely it is to be round to begin with.
As for Ceres, it is round due to its gravity. Many somewhat smaller moons also are also round in the same way. However, no other asteroid meets the criteria. Even the second biggest asteriod, Pallas in non-spheroidal, with its dimensions being 570×525×500. --EMS | Talk 02:59, 25 September 2005 (UTC)
The section objecting to the "sphericity" argument is the most unfair, POV and unsubstantiated part of this article. Since I wrote it, I know what I'm talking about. :) I have long wished for someone to redraft it (not me obviously, since I cannot overcome my pathological hatred of the "let's randomly select objects for planethood based on their shape" bandwagon) but so far no one has done so (at least not without completely misinterpreting it). In an attempt to find an unbiased answer to all of these questions, I posted this very question to an astronomy board, but if anything, their answers made the issue more confusing, not less: See here, I honestly don't know what to do at this point. Serendipodous 15:12, 25 September 2005 (UTC)
  • in fact the roundness is the best physical argument for planethood, there are just some people that don't accept more than 9 planets. 21st century: Dark ages for Astronomy naming and conventions, IMO. Ceres was classified as an asteroid, because the telescopes of the time werent good enough. And Ceres is a singularity among the asteroids. -Pedro 22:34, 26 September 2005 (UTC)
I have no problem with more than nine planets; I just have a problem with some objects being planets while other objects that are otherwise identical to them apart from being slightly smaller not being planets.Serendipodous 07:20, 27 September 2005 (UTC)
  • Personally I think that you do an excellent job of covering that issue. I see what you mean about there being some POV in it, but it is not as if the scientific community is running to embrace this definition. I think that the point of that section is very, very good: no matter what you do, there will be some object that does not quite fit the definition well. Even for roundness cutoff, there is a transition zome such that on either side of it you can say this "is" or "is not" a planet. However, in between there will some arbitrary standard such that if an object is just a little bigger or a little smaller od a little denser its status suddenly changes. There is not much that we can do about that. --EMS | Talk 22:54, 26 September 2005 (UTC)
  • The main objection to the roundness cut-off is that it's not exact. It varies based on composition and heating conditions. Mimas is the smallest known near-sphereical object in the solar system. 2 Pallas is slightly larger and much more massive, but is distinctly non-spherical. 4 Vesta is slightly more massive than Pallas, but has a more round shape, possibly indicating partial gravitational relaxation into a sphere. Both asteroids contain a much higher percentage of rock than Mimas, which is mostly water ice. Enceladus and Miranda are also spherical and smaller than both asteroids. Proteus is more massive than Mimas and composed of similar substances, but is non-spherical, probably because it's further from the sun. Further, some objects have a shape that isn't gravitationally relaxed, but balances gravity against the forces of the object's rotation. Jupiter has an oblate shape because of this, and in a more extreme example 2003 EL61, which has a 4-hour day, is streched into the shape of a rugby ball. Some objects, like Mimas, are distorted because of tidal forces. Most of this information is scattered about inside the article. Maybe we should add a table illustrating the size of sperical and non-spherical bodies, showing the fuzzy nature of that boundary? shaggy 20:36, 21 January 2006 (UTC)
  • I think that's a good idea; do you know how to construct a table? I don't really know how. Would appreciate the help. By the way, this section dealt with an earlier version of this article that was a lot more POV than the current one. Serendipodous 09:46, 22 January 2006 (UTC)
  • See Wikipedia:table for help. However, I scratch me head about this a bit. Enceladus is a moon and is also liquid on the inside. I find it inappropriate to compare an oversized water droplet to an ice/rock object which is circularized due to gravity itself. --EMS | Talk 05:34, 23 January 2006 (UTC)

New References section

Some external links were sprinkled randomly into the article recently. I have collected them into a proper References section, but I'm not sure they really add anything to the article in the first place. Opinions? --Doradus 14:29, 25 September 2005 (UTC)

Upa. That was me. I wrote this article a long time ago and a few days ago it suddenly occurred to me that much of what I had written had come straight out of my own head and wasn't substantiated or backed up, so I thought I'd add some references just to reassure that I wasn't just speaking personal opinions. Perhaps they don't need to be there.Serendipodous 15:03, 25 September 2005 (UTC)

I see that you have deleted them. I just thought that I should explain my "I'm not sure they really add anything" remark... They seemed to be pretty much arbitrary websites, rather than authoratative sources. Frankly, they looked like the kind of links one would find on the first page of a Google search. However, if they are authoratative sources, I have no problem retaining that References section. --Doradus 14:19, 26 September 2005 (UTC)

Well, in all fairness, they weren't the first page...

The problem is that finding something (anything) that directly and explicitly confirms what you say on the internet is actually very difficult. I don't really like the internet as a research tool, since unless you know what you're looking for, you usually have to sift through tons of dross before you find something even remotely relevant. I still prefer books. Perhaps that is a personal flaw. Anyway, I found what looks like a far better source. Pretty much answers most of my problems. Serendipodous 15:17, 26 September 2005 (UTC)

Spin axis

What if only smaller bodies can have more than one spin axis. Would that not set a natural size limit to use in defining a planet?Jan Pedersen 11:02, 27 September 2005 (UTC)

The Term Demote

I think reclassify is better, there is no ranking of heavenly bodies.--Perfection 06:28, 4 October 2005 (UTC)

Hmm. I think, given the context, it might be better to put the word "demoted" in quotation marks, since that is a word often associated with the anti-reclassification camp. The word "reclassified" is already used in the previous sentence.Serendipodous 07:04, 4 October 2005 (UTC)

Query about recent addition

There seems to be a slight contradiction in the sentance newly added by Tompw:

"The required distance depends solely on the mass of the planet. (The further the planet is from the Sun, the further the moon needs to be from the planet)."

If the planet's distance from the Sun is a factor in the required distance, then it can't solely be related to the mass of the planet.Serendipodous 17:38, 4 November 2005 (UTC)

  • Yes, you're right. I should've added a caveat about for a given distance from the Sun, but on reflection, that would be more confusing. I've changed it to read "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." Is this clearer? Tompw 14:01, 26 November 2005 (UTC)

Unsuitability of recent addition

I have cut and pasted your addition here Anthony, because I want you to fully understand why it is incorrect to use it in this context:

Planetary systems around stars, including our own, may be comprised of not only planets but other orbiting non-stellar bodies like natural satellites, asteroids, meteoroids, comets, and cosmic dust.

This article takes as its starting point that there is no fixed definition of "planet." That's its thesis. Any attempt to say "not only of planets but..." implicitly defines a planet by saying what it is not. That sentence claims, by the way it is worded, that a planet is not a natural satellite, an asteroid, a meteroid, a comet or cosmic dust, but the whole point of the article is that, depending on what definition you use, a planet could very well be a natural satellite, an asteroid, a meteroid, a comet, or cosmic dust. There is no definition of planet, and therefore it could be any of those things. Please read the article more carefully. Serendipodous 12:35, 8 November 2005 (UTC)

Excuse me? What point of this is unclear to you or you do not understand? I've read the article thoroughly (and perhaps you need to step back), because your removal is unjustified. The addition of this information, AND cited definitions for related terms regarding objects that may or may not be planets that may comprise planetary systems (some of which are not otherwise mentioned in the article) is wholly germane to this article. Just because there is a debate regarding the ambiguity of the term "planet" doesn't obviate cited definitions that may indicate otherwise, nor does this justify your attempts to maintain a mystique about the topic and sequester cited definitions by removing them. Unless you can demonstrate why this should be removed or if there's a consensus to remove it, refrain from doing so; feel free to edit it, though.
Moreover, refrain from personal attacks by characterising me as "stubborn" (is this not the pot calling the kettle black?) and use my complete alias. E Pluribus Anthony 12:45, 8 November 2005 (UTC)

You didn't phrase it that way Anthony; you said "not only of planets but..." implying that you already had a set definition of planet in your head. I fail to see where any of your list was not mentioned in the article: the ambiguity over "natural satellites" are mentioned in the "double planets" section, the ambiguity over asteroids, meteroids and comets was mentioned in the "minor planets" section, as was cosmic dust.

As for personal attacks, I do not see calling you stubborn a personal attack; it is merely an observation. And yes, I will freely admit to it myself; the difference is that you have shown, in our brief time as "collaborators" a marked inability, or unwillingness, to comprehend even the most basic thing I have said in answer to some of your edits. There are times when I wonder if we are speaking a mutually intelligable language.

But, regardless the information does deserve to be in the article; I have rephrased it and put it in a more suitable location.Serendipodous 13:04, 8 November 2005 (UTC)

I've included a more appropriate summary regarding this, and this may have superceded your edits; my apologies. Perhaps you should exercise diligence and attempt to incorporate or edit information before removing it; to do otherwise is frankly (and observedly) ignorant.
Yes: in the short time we have interacted with one another, I have observed you to be excessively stubborn, inflexible, unclear, condescending in your commentaries, and (more importantly) supportive of erroneous text or inaccurate language without cited references (in not only this article, but others). Generally, you do not clearly substantiate your arguments, thereby requiring lengthy (and time-consuming) justification. And I neither require nor seek your approval or commentary (for what it's worth) when editing. I desire to not intereact with you hereafter, but may have to to ensure information is properly relayed. Unless you accord the proper respect in your interactions with other Wikipedians, your comments will be treated and dismissed as such. End communication. E Pluribus Anthony 13:12, 8 November 2005 (UTC)
And yes: the current edition is fine. There may be hope, yet. E Pluribus Anthony 13:20, 8 November 2005 (UTC)


Mike Brown's definition of a planet.

"A planet is any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit"

While this definition excludes Pluto and Sedna, 1 Ceres _is�_ bigger then rest of the asteroid belt put together, and is therefore a planet. Am I the first person to notice this? Algr 5:42, 31 December 2005 (UTC)

According to its Wikipedia article, Ceres is about half as massive as the rest of the belt. I've asked some serious scientists to confirm that, and when they get back to me I'll let you know. Serendipodous 11:15, 31 December 2005 (UTC)

You can get into all sorts of other shenanigans with this too. How close do Earth and Venus's orbits have to get before they cease being planets? What if there is an earth-mass blob of hydrogen floating around somewhere? Since the article is "Definition of a planet", it really should list exactly what the proposals are and discuss them. Algr 15:42, 31 December 2005 (UTC)
The problem is that there are so many proposals, and so many variations thereof, it's difficult to include all of them. I've tried to pick a representative sample of the lot (A planet must be round, a planet must orbit a star, a planet must be singular, a planet must be a non-fusor). Once the official definition is agreed upon, if it ever is, we might have some firmer ground to stand on. Mike Brown's definition was never anything more than a proposal; if it was accepted, then obviously some limit would have to be set to describe a "similar orbit" (probably "within 20 percent of the object's orbital circumfrence"). And it looks like Ceres is not larger than the rest of the asteroid belt. Serendipodous 16:57, 31 December 2005 (UTC)
From the Asteroid article:
"The mass of all the asteroids of the Main Belt is estimated to be about 2.3x1021 kg, or about 3% of the mass of our moon. Of this, 1 Ceres comprises 940 to 950x1018 kg, some 40% of the total. Adding in the next three most massive asteroids, 4 Vesta (12%), 2 Pallas (9%), and 10 Hygiea (4%), bring this figure up 66%; while the three after that, 511 Davida (1.6%), 704 Interamnia (1.4%), and 3 Juno (1.2%), only add another 4% to the total mass. The number of asteroids then increases exponentially as their individual masses decrease."
shaggy 21:23, 21 January 2006 (UTC)
  • And remember that we are comparing ceres to thousands and thounsands of Asteroids, not a couple of them. ---Pedro 13:23, 23 January 2006 (UTC)

Pedro, I'm a bit confused by your comments on Herschel

in the history page. You seem to be suggesting Herschel fudged his results; I fail to see how he could have had an accurate measure of Ceres' and Vesta's sizes, given the accuracy of telescopes at the time. Also, size isn't really the point of that section; the issue is a shared orbit. Size is covered in the following section. Serendipodous 23:25, 28 January 2006 (UTC)

  • well, it was because the issue of Asteroid was in there discussed, I did not want to make the article bigger. But if you want to change the issue about asteroids (starlike) to "size" be my guest. -Pedro 17:15, 29 January 2006 (UTC)

Assessment comment

The comment(s) below were originally left at Talk:Definition of planet/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

There seem to bad formulations in the subsection Definition of planet#Double planets. See Talk:Definition of planet#Definition of natural satellite. Therefore the article currently doesn't deserve FA rating. Andres 13:20, 27 August 2006 (UTC)

Last edited at 13:20, 27 August 2006 (UTC). Substituted at 20:23, 2 May 2016 (UTC)

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