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Orbital period of the moon and the planet

The current article contains a paragraph that describes how any moon-planet system may be considered a double-planet. The paragraph goes on to say that "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."

I disagree with the statement that the mass of the moon does not change the required distance. I also disagree with the statement that as the planet's mass decreases, the required distance to the moon increases.

The period of orbit of one body about another depends upon the mass of both bodies, as this is what determines the gravitational force between the two bodies. However, as a first approximation, the mass of the smaller body is usually considered to be small when compared to the mass of the larger, and hence the orbital period calculation, in this instance, can be said to depend only on the mass of the larger body. This is usually the case in Earth's solar system, although since the article is attempting to be general and apply to any system of orbiting bodies, the approximation may not hold true in all circumstances.

On the second point, even if we assume that the mass of the moon is small, then as the mass of the planet decreases, the gravitational force between it and the moon will also decrease. Hence, the period of the moon at that orbital distance will lengthen. Therefore, the moon could orbit at a closer orbital distance and still have an orbital speed slower than the planet's speed around the star. Therefore, the required distance between the moon and planet decreases (the article as it is written says "increases").

There is no reference for the statement in the article but the article has apparently been peer reviewed so I assume someone has checked this out, and I am interested is reading what that person(s) has to write, as I am open to a complete discussion of the dyanamics of orbiting bodies and may learn something on my part. I did not make any changes to the main article for the same reason.

D. Clippinger

I'd be interested to see this issue resolved. I've been meaning to source that paragraph for a long time (for the record: I didn't write it) but lack the knowledge of Newtonian dynamics to do so. Serendipodous 19:08, 16 August 2006 (UTC)
Drat, this means that I have to break out my physics book and figure it out. Could someone get me a reference of who added that particular change while I'm doing the math? McKay 20:31, 16 August 2006 (UTC)
His name was Tompw. Serendipodous 08:38, 17 August 2006 (UTC)

Brown's POV.

There’s a preprint by Michael E. Brown and Basri, Planetesimals to Brown Dwarfs: What is a Planet?, [1]. But you’ve probably seen this...Eurocommuter 17:55, 22 August 2006 (UTC)

New IAU proposal

There seems to be a new proposal for the definition of a planet that the IAU is favoring now. See Talk:2006_redefinition_of_planet#Latest_Draft. In an nutshell, this proposal adopts the criteria that a planet be by far the biggest object in its "orbital neighborhood". In fact, one of the articles linked to in that section says that August 22 is now "the day we lost Pluto". --EMS | Talk 03:45, 23 August 2006 (UTC)

The wording seems to imply that it has "cleared out" it's neighborhood. Well, Jupiter hasn't...its orbit is filled with "trojans". As for being the biggest object in its orbital neighborhood, Jupiter is, but so is Ceres! Hopquick 14:45, 24 August 2006 (UTC)

  • they just added a lot of confusion. Balls of gas like Jupiter are recongnized as planets and real planets are not. BTW, do people know the results of the voting, there is an ongoing confusion on different wikipedias due to Pluto status. Should we change the articles to "world"?... Pluto is a world, Earth is a world... at least it would remove the term planet that is becoming a scientific term that is creating a serious confusion in people's heads. unless they will also define what's a world and add more caos, like they are doing with this and in naming celestial bodies. Is astronomy in its decadence? -Pedro 15:14, 24 August 2006 (UTC)
Yes, planets have largely cleared out their orbits (as Jupiter has), but the main issue is whether the object "dominates" its region (as Jupiter does--the Trojans only orbit where they do because of Jupiter). Ceres does not dominate in this way; you could take it out of the asteroid belt and the belt would continue to exist in much the same form as it does now. As for "world" I think this is similar to the term "continent" in geology: permanently undefined. I.e. it's a lay term, not a scientific term. --Aelffin 15:31, 24 August 2006 (UTC)
Look up trojan asteroids. Jupiter has not cleared its orbit as well as even Earth had. Pluto and Jupiter seem to be in more populated regions than Earth and Mercury. The definition is very fluid and arbitrary, IMO. But that does not affect this article. They've decided it. And I guess that's that until someone wakes up. Hopquick 23:33, 24 August 2006 (UTC)
Both Jupiter and Earth have cleared their orbits in the sense that the IAU means. Read Clearing the neighborhood.--Aelffin 12:48, 29 August 2006 (UTC)

So what now?

With the publication of the final result, this article is now as out of date as the Almagest. It needs to be completely redrafted from beginning to end, and I can't do it, as I have a masters thesis due next week.

I would like to leave a few thoughts before I depart, however:

  • The focus of this article needs to shift; the new definition should be mentioned in the opening paragraphs.
  • The IAU debate material and the stories of the discoveries of Ceres and Pluto could be merged with "History and etymology" to create a chronological "history" section.
  • As the above poster noted, this definition still leaves a great deal of ambiguity, not the least because it doesn't touch on extrasolar planets. Therefore much of the later material is still relevant. The debate about what is "round enough" should be retained, as one of the uncertainties created by the definition.

Anyway. I for one am glad this is over, and it is a definition that I agree with. I'm stepping back into the 16th century for now. See you soon. Serendipodous 15:16, 24 August 2006 (UTC)

I agree, with a caveat: we should avoid anticipating the effect of the IAU's definition among the public. So, for example, the first three sentences in the lead section are still valid, although the fourth isn't so much. Melchoir 15:42, 24 August 2006 (UTC)

"So what now?" continued

The other page is fast and furious at the moment, so I thought I'd comment here while thinking of things. First, my hat's still off to you on this FA Serendip and I hope you're not gone for long. You were into IAU debates before it was cool to be into IAU debates :). My thoughts:

Make any sense? Marskell 17:44, 24 August 2006 (UTC)

We do, however, have Second Continental Congress... I like Idea 1 in that it would invite a minimum of confusion and original research. I don't know what to make of Idea 2, because I'm not sure what a merge means to you. Is anyone currently trying this? Melchoir 19:53, 24 August 2006 (UTC)

I hope no one thinks it too forward of me, but I just reordered the article in a rather untidy attempt at how I think it should look. Let me know what you think. Serendipodous 19:48, 24 August 2006 (UTC)

The other article refers to the specific definition of 2006 and the events surrounding it, so perhaps a good article name would be 2006 IAU definition of planet. This article would contain the current definition as well as historical definitions (for historical context). It wouldn't go into the 2006 debate to nearly the same depth as the other article, naturally. --Cyde Weys 19:51, 24 August 2006 (UTC)

Er, so what's going to happen to its featured article status? Is there a precedent for this kind of situation? --Kinst 03:00, 25 August 2006 (UTC)
It can go to WP:FAR if the changes are significant enough and people can comment on its retaining status.
Regarding names, one or the other ought to denote it is the page for the definition—currently this one does, which is why I have advocated the merge. However, if this gets renamed "Historical..." or some such, I really think the other ought to lose "2006..." and try to stand as the principal sub-article for the current definition. Right now, it's a chronological blow-by-blow rather than a summary style article.
Serendip, the changes to headlines make more sense but the lead still needs to be reworked and just come out and say the new def, while the IAU section should be broken out on its own. Marskell 08:18, 25 August 2006 (UTC)
Tried to reorder the intro, Marskell, but couldn't find a way to make it work. However, since this is an article about the ambiguities of attempting to define a planet, not an attept to define it, I think it's OK if the ambiguity is mentioned before the definition. The planet article goes right out and mentions the new definition, as it should, but this article's focus is different.
Assuming everyone's OK with the alterations I've made, there are a few issues remaining that I cannot resolve right now.
  • The introductory paragraph for "Issues and controversies" needs to be expanded
  • The "clearing the neighborhood" section needs to be expanded also, with some reference made to planetary formation.
  • Some mention of Gibor Basri's planemo concept should be worked in, as it seems to be growing in informal popularity among astronomers. Serendipodous 09:18, 25 August 2006 (UTC)

I personally think that as this article will need to be significantly reviewed and will be somewhat unstable for the next few weeks/months. In the interim period because of this instability, should have its featured status temporally demoted. HOWEVER, because of my inexperience with FAR, I am not going to nominate it, but will support it if it is. Todd661 11:10, 26 August 2006 (UTC)

Vesta

After my short-lived edit, I went through the archived discussions and saw numerous comments where people assert that Vesta is spheroidal, but I did not find much external reference. Several say that Vesta really is an oblate spheroid if you ignore the big crater. But isn't that exactly the point? Vesta did not have sufficient gravity to collapse back into a spheroid. That sounds to me like it doesn't have hydrostatic equilibrium.

Google Scholar returns mixed inconclusive results, AFAICT. The abstract of the Icarus article (also mentioned in the archives) gives a statement that could be interpreted in either direction. Frankie 17:10, 24 August 2006 (UTC)

Double planet section

Are the paragraphs where moons whose orbits of their planet does not overlap when viewed from above could be considered part of a double planet system OR, or is it just me? The only cited source does show that the moon's orbit around the Earth is a wave rather than a spiral, but it makes no mention of this making it considered to be a double planet and actually says several times that the Moon orbits the Earth. Page 2 of the cite even says it orbits Earth once every 27.3 days.[2] The definition of a satellite also contradicts the inclusion of these moons as being partners in a double planet system.--Bobblehead 20:10, 25 August 2006 (UTC)

The section also says the Moon orbits the Earth. But the Moon also orbits the Sun directly. Yes, the official definition should be mentioned as contradicting this, but that doesn't lessen its value as a potential ambiguity in the agreed definition. Just edited the section. Serendipodous 20:55, 25 August 2006 (UTC)
This may be true, but unless you can find a source that supports this claim it is still OR. Take a look at the What is excluded? section. The statement seems to meet all but two of the list and as such probably should not be included, unless a reputable source saying moons that meet that criteria can be considered a double planet system, of course.--Bobblehead 21:14, 25 August 2006 (UTC)
How about this? EDIT: Couldn't wait. Swapped it out. Serendipodous 21:22, 25 August 2006 (UTC)
While I do admit that Isaac Asimov is a smart guy (I've actually read his essay books), I do find it amusing that he quoted him. Heh. That being said, even if it is a commentary, the article you provided seems to meet the Reliable Source criteria for stating that the Earth and Moon relationship might be a double planet relationship. It doesn't, however, support the remainder of the section regarding how the lack of a spiral orbit path around the sun in relation to the primary bodies orbit means any other satellites may cause it to be considered a double planet. The article seems to be saying we just don't know enough about how a moon is formed to know the origin of the moon. However, I'm off camping for the rest of the weekend. Thanks for working with me. --Bobblehead 22:08, 25 August 2006 (UTC)
If you go up top, you'll see that that subequent paragraph is already under scrutiny. Serendipodous 14:52, 27 August 2006 (UTC)

Definition of natural satellite

defining a natural satellite as any body in orbit around a planet whose barycenter (the point around which they both orbit) lies outside the planet itself

This seems to be ambiguous, bad formulation or even false. The "planet" outside which the barycenter lies seems to be meant to be the natural satellite. As English is not my first language, I don't want to change the text myself. Maybe "body" should be substituted for "planet". Further, not the barycenter of the planet or of the satellite but their common barycenter around which they orbit. Andres 12:16, 27 August 2006 (UTC)
In this article, the word barycenter seems to be used incorrectly. Andres 12:38, 27 August 2006 (UTC)
I changed it. In any case, I could not find a IAU definition of "satellite" anywhere.   Andreas   (T) 12:41, 27 August 2006 (UTC)
It seems to me that the sentence doesn't purport to refer to any definition of natural satellite by IAU; the word defining seems to mean the meaning of the word in that sentence. This should be rephrased to avoid ambiguity. Andres 13:13, 27 August 2006 (UTC)

defining a natural satellite as any body in orbit around a planet whose barycenter (the point around which they both orbit) lies inside the planet itself

It seems to me that this still is not what is meant in the text: the concept natural satellite in that sentence includes components of double planets. This is manifest from the further text. Andres 13:16, 27 August 2006 (UTC)
The original 12-planet draft specifically defined a natural satellite as an object in orbit around a planet whose barycentre lies inside the primary. The final draft does not appear to mention the definition of satellite at all; however, since one of the criteria for "dwarf planet" is that it not be a satellite, it seems fairly clear that the previous definition of satellite is implied; if it weren't, there would have been a new definition. Serendipodous 13:42, 27 August 2006 (UTC)
The further text goes as follows:

In the original draft proposal, an exception was made for Pluto and its largest satellite, Charon, which do possess a barycenter outside the volume of either body. Rather than one orbiting the other, both orbit each other like the tips of a spinning baton. The initial proposal therefore classified Pluto/Charon as a double planet: two objects orbiting the Sun in tandem.

If we proceed from that definition then where is the exception? From that definition it follows that nor Pluto nor Charon are satellites.
And here the initial draft, not the final draft is discussed. Andres 14:48, 27 August 2006 (UTC)


The interpretation of definition of "natural satellite" given in the article is incorrect. Specifically, the moons Nix (moon) and Hydra (moon) also orbit around a barycenter which is in empty space. Nix and Hydra are far too tiny to be considered as planets or dwarf planets. By the definition currently given in the article, Nix and Hydra would not be considered natural satellites. The draft definition only applied to bodies that otherwise met the criteria for being a planet (hydrostatic equilibrium, etc.) Derek Balsam 14:51, 27 August 2006 (UTC)

It's much better now. I propose to delete the sentence: It does not directly define the term "satellite". Andres 18:01, 27 August 2006 (UTC)

Does the formulation "has cleared its neighbourhood" exclude components of double planets? Why doesn't it exclude planets with satellites? Andres 18:04, 27 August 2006 (UTC)

IAU decision

The section "IAU debates" doesn't distinguish clearly between the final draft and the approved decision. There is no mention of the decision in text at all. Andres 13:23, 27 August 2006 (UTC)

What approved decision? Serendipodous 13:43, 27 August 2006 (UTC)
What I mean is that the phrase "final draft" is used but there is no mention that the draft was passed and approved by the vote. Andres 14:41, 27 August 2006 (UTC)

A bias in the structure of the article

Currently there is a section on the problems with the official discussion but there is no account of the motivation behind the official discussion (except a bit of historical background). Andres 17:50, 27 August 2006 (UTC)

The official discussion was prompted by the discovery of UB313, which the article mentions twice; once in the introduction, once in the opening sentence of the "IAU debate" section. Serendipodous 18:07, 27 August 2006 (UTC)
Yes. But what I mean is to consider the three points from the point of view why they were included to the definition. In other words, why it was decided to formulate the definition in the way it was formulated. Maybe most of this and the IAU debate should be in the other article but it is essential to explain the motivation of the choice of the definition. Currently, for each point it is only said what is wrong with it. Andres 18:18, 27 August 2006 (UTC)
This article is about ambiguity. It's purpose is to illustrate the ambiguities involved in any attempt to define "planet;" not just this one. As for the motivations behind the final draft, that belongs in the main article. There are too many shades to that issue to list here. The differences between the initial draft and the final draft, the "revolt" by dynamic physicists, etc. Serendipodous 18:25, 27 August 2006 (UTC)
Well, this goes back to the issue of whether the other article needs to exist. This doesn't have to be about ambiguity primarily. It could be split between historical ambiguities and the current definition. That's what I'd expect under this article title anyhow (though I'm beginning to sound like a broken record). Marskell 18:39, 27 August 2006 (UTC)
It is true that the tone of this article has definitely changed since the introduction of the new definition, since the definition incorporates many of the different arguments this article once debated. Since the article's main argument was that none of those definitions adequately explained what a planet was, the new article reads like an attack on the new definition. Perhaps some of counterargument is now necessary. I don't really want any of the bickering and minutiae that comprised that embarrassment of a conference to be part of this article; I'd prefer it if the article stuck mainly with theory and history. Serendipodous 18:48, 27 August 2006 (UTC)
Agree with Serendopodous that the details of the actual discussion in Prague don't belong here (and should be discussed in a special article). Here should be the historical overview (including a brief account of the re-definition and its wording) and an overview of different approaches to the definition, including their strong and weak sides. And it seems to be inevitable to present this overview against the background of the official definition.
Agree also with Marskell that anbiguity could and should addressed in this article but just as one of the aspects of the topic "definition of planet". Andres 19:06, 27 August 2006 (UTC)

Definition of satellite?

According to the current definition, our own Moon, since its barycenter lies within the Earth, is excluded from planetary status.

How can this be said if there is no official definition of satellite? Andres 01:46, 29 August 2006 (UTC)
If it weren't, there'd be nine planets, not eight. Serendipodous 06:56, 29 August 2006 (UTC)
I mean, there is no official definition of satellite. If Moon is not a satellite, then it could be a dwarf planet. Andres 15:44, 2 September 2006 (UTC)
What do you mean there's no official definition of satellite? The Moon is a satellite. siafu 15:47, 2 September 2006 (UTC)
I mean, an official definition like the official definition of planet newly approved. In that aricle it reads: The general criterion for an object to be a satellite is that the center of mass (known as the Barycenter) of the two objects is inside the primary object. But this is not official.
And I don't understand well the connection in the sentence from the article: not being a satellite is defined in the definition of dwarf planet, not in the definition of planet. Andres 16:00, 2 September 2006 (UTC)
"Not being a satellite" in this case meaning being a satellite only of the sun. siafu 16:01, 2 September 2006 (UTC)
Actually if the Moon weren't a satellite, it would be a regular planet, if it were the same distance from the sun. It's big enough to "clear the neighborhood," but the Earth is bigger. 67.168.216.176 02:11, 4 September 2006 (UTC)

its barycenter lies within the Earth

And this is certainly false, it should be the barycenter of the system consisting from Earth and Moon. Andres 16:13, 2 September 2006 (UTC)

Protest

Check out the link. Is there anything needed to be said about the strengthening rejection of the 2006 definition? http://www.ipetitions .com/petition/planetprotest/ Hopquick 17:58, 2 September 2006 (UTC)

While I agree with the principle of an open, grassroots debate leading to a consensus, and think that was how the definition should have been arrived at in the first place, I feel the need to point out that there are fewer names on that protest than actually voted for the final proposal at the IAU. Serendipodous 18:09, 2 September 2006 (UTC)
Agreed. This is just an attention-getting device. Not related to real science. Don't give them the satisfaction of being mentioned in the Free Encylcopedia.--Planetary 18:33, 2 September 2006 (UTC)
It's the Division for Planetary Sciences (DPS) of the American Astronomical Society (AAS) that is upset with the definition. Not just a few... Your quick dismissal might betray a bit of bias. ^_^ http://www.space.com/scienceastronomy/060831_planet_definition.html Hopquick 04:27, 3 September 2006 (UTC)
After reading the link, I don't really see what it is you're talking about. Sure the DPS of the AAS doesn't like the definition according to the article, but the AAS website says nothing, except referencing the IAU proceedings, the DPS site hasn't been updated since May. While I think they may have made a statement, all they did was request for clarification, not protest the decision, the article says that the DPS even "recognizes the authority of the IAU to render a decision." The group of 300 people who refuse to use the new definition is small, though probably "notable") compared to the people who actually voted in the affirmative in the IAU decision. The IAU even claims to fix these unclarities in 2009. If we find 90377 Sedna is mostly by itself before 2009, then we might be in a little bit of trouble, but we'd have to wait for 2009 anyway, because the IAU is the recognized governing body (with recognized authority that is virtually universal) with regards to astronomical nomenclature. So my vote is to recognize that there is some protest (the space.com article is notable), but it doesn't have to be much. I think the current article is perfectly satisfactory in this regard. McKay 05:46, 3 September 2006 (UTC)
I can't see any bias here. In the article the statement of one division of the astronomical society of one country is reported. And they even *supported* the decision. That the support was "lukewarm" is merely the interpretation of the author who by himself is obviously not an unbiased one. And since the statement was not voted upon by the members it's even much less a statement of *the* DPS than the IAU decisions was the decision of *the* IAU. 84.165.159.183 13:51, 3 September 2006 (UTC)
My first link was the more relevant one to the discussion. But there seems to be consensus against the inclusion of this. (That's what the talk page is for, is it not?)Hopquick 15:34, 4 September 2006 (UTC)
Heh, I decided to look at what else ipetitions.com has up, and the two most recent changes (Bring back SG-1, and bring back a shopping center in Houston), each have over 20,000 signatures. Even .com/petition/Ice_MUG/signatures.html computer users in Iceland can get over 300 signatures on a petition. And that has very limited scope. It seems as if the astronomers' petition is rather minor. McKay 15:48, 4 September 2006 (UTC)
NPR's Science Friday is reporting on the protest. Not just anyone can sign this petition. - it is 300 planetary scientists and astronomers compared to only 428 who were involved in the IAU decision in the first place. And this was in only five days. It certainly belongs in the article. Algr 17:41, 14 September 2006 (UTC)
It is mentioned in this article, briefly. Since this is a broader, more historical perspective, if that protest belongs anywhere, it belongs in the 2006 redefinition of planet article.Serendipodous 17:48, 14 September 2006 (UTC)

Grey Dwarf

Is this a term? I cannot find it. It was also removed in the Subbrown dwarf article, thanks, CarpD 9/12/06

It's in the sourced articles for that paragraph. It's the term the discoverers of those objects use for them. It does not appear to have entered common parlance, but nonetheless, that is the term they use. Serendipodous 06:20, 13 September 2006 (UTC)
Actually, I seem to have swapped out the original article. Oh well. May as well delete it then.
This is the original article. Looks like I'm going to have to eat humble pie on this; I originally thought the discoverers of these objects called them grey dwarfs, but this seems to have been just a name thrown up by a few scientists. Serendipodous 06:27, 13 September 2006 (UTC)

Serendipodous' Revert

From History: This information a) does not belong in the introduction b) is already mentioned in the article and c) is better placed at "2006 Redefintion of Planet". This article is a historical overview)

I have to disagree with you on all four points here. If this article is a historical review, then an event that happened a month ago should not dominate the introduction. Reaction to the IAU's decision is just as much a part of history as the decision itself. Most of the information you removed is not stated in the article - If the IAU's decision is stated, then the consequences of that decision must also be stated. As it stands, this article gives a false finality to the IAU's decision, despite widespread negative reaction to it in the science comunity, and almost no popular support outside wikipedia. Algr 20:12, 14 September 2006 (UTC)
The petition itself is mentioned, and until it actually has an effect, that's all this article needs. It certainly doesn't belong in the introduction. It belongs where it already is, in "Remaining controversies." This article isn't a blow-by-blow account of action-reaction in this sordid affair. If you want that, go to 2006 redefinition of planet, which is where I suggested you put your paragraph. That's what that article is for. This article is more general, dealing with events in broader historical context. If the petition is taken up and actually leads to a redefintion of planet, then yes, it should be given more prominence, but until then, no. Serendipodous 20:47, 14 September 2006 (UTC)
Also, whether you agree with the IAU's decision or not (and I do, though I feel it could be fine-tuned) is not relevant. It is the official scientific definition until another one is drafted, which, however many petitions are signed, won't happen until 2009, when the IAU next meets. So, I'd rather wait a few years before notifying the world of every shift and reaction to the definition. We have a long way to go yet. Serendipodous 21:13, 14 September 2006 (UTC)
Given that hundreds of scientists have publicly rejected the IAU's definition and stated "we will not use it", what right has wikipedia to declare the IAU authoritative? It's not our place to decide who can say what a planet is. We don't put "evil" in the definition of "Iran" just because Bush says so. Algr 21:29, 14 September 2006 (UTC)
That doesn't matter. This is the official scientific definition. Already Pluto and Eris have received their minor planet designations. Their names specifically state that they are not planets. This is moving ahead, whatever dissidents may think of it. The fact that there is dissidance is mentioned in this article, but unless the dissidents can come up with a means to make an alternate definition that is equally authoritative, their complaints won't come to anything. Serendipodous 21:57, 14 September 2006 (UTC)
Please re-read my previous post. This isn't about Pluto. This is about wikipedia uncritically accepting one organization's opinion when, the media, the public, and even much of the IAU's own membership is rejecting it. The US supreme court is authoritative because the constitution says so. Where does the IAU's authority come from? What if Merriam-Webster [3] and The American Heritage [4] continue to reject the IAU definition? What if congress acts? Algr 16:01, 15 September 2006 (UTC)
So what other definition would you cite? There has never been a definition of planet before now. Those dictionary definitions do not account for the Solar System as is. I can't see your Webster's citation, but the American Heritage definition,
A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves.
does not account for Kuiper belt objects, or define where the dividing line between planets and asteroids/comets is. Besides, it was published in 2000; until it at least recognises that the IAU's decision has been made, you can't say it has "rejected" it.
"The US supreme court is authoritative because the constitution says so" - and who gives the constituion the power to determine what is authoritative? Ultimately everything comes down to people. The IAU is made authoritative by its members defering to it. The US Constitution by the same means. If all of the citizens of the US rejected the constitution it would be as worthless as any other piece of paper. If everyone ignores the decision of the IAU then it becomes equally worthless. Currently though we have a definition by an organisation of which many people acknowledge the authority, and of which a significant number of people approve. There is no alternate definition with such gravitas. --Neo 16:22, 15 September 2006 (UTC)

Really, none of this is at issue. The issue is that your addition did not belong in the introduction. The IAU's definition belongs in the intro because it is the official definition. Right now it is the only definition. Your addition did not add anything that wasn't in the article already, apart from some incredibly POV comments ("nonsensical" is not a word to use in an encyclopedia unless you are describing a literary style) and a rather poorly worded comment about size and orbits that, at least to me, made little sense. "Mars in a 6 billion mile orbit would not be a planet"? What does that mean? The definition has nothing to do with the size of the orbit; it has to do with whether or not the orbit has been cleared of other objects. Mars in a 6 billion mile orbit would be a planet if Mars was alone. I think what you mean is that, 6 billion miles out, Mars would be in the Kuiper belt (assuming, of course, that the Kuiper belt would even exist if a Mars-sized body formed within it), and thus would no longer dominate its orbit. If I may quote this article:

by this definition, a body of Pluto's size or smaller orbiting in isolation would be called a planet, whereas larger objects close to one another would be termed "dwarf planets."

You see? What you said is already mentioned in the article. Just like I said. Now it would be nice if you could get a comment from an astronomer, along with a citation, (which, I notice, was missing from your paragraph) to make that sentence kosher; that would be good. But there's no need to place repeat information in the introduction. Serendipodous 16:45, 15 September 2006 (UTC)

"So what other definition would you cite? There has never been a definition of planet before now."

You've answered your own question - The primary idea of the introduction should be that nothing is settled. The IAU is in hot water over what they have done, [5] and I can find no one else except wikipedia who has embraced their definition. (As opposed to just reporting it.) You are also starting to repeat yourself with objections that I have already answered. The business about Mars in the Kuiper belt comes from Dr Mark Sykes. You DID ultimately understand it, so don't fault him for making you think about it - perhaps you could reword it to make it clearer. Algr 21:49, 15 September 2006 (UTC)

Others have asked me to reword the introduction making the IAU's definition more prominent. I have refused, precisely for the reasons you mention. This article's point is that the definition of planet will always be ambiguous. But I can't simply ignore the existence of an official definition. As to your added information, I don't need to reword it. It's already in the article. And until you provide a citation (preferably a webpage), there's no reason to alter it. The IAU is hardly in "hot water." Pluto has already been officially downgraded. [6] Science museums across the world are removing it from their dioramas. I can cite at least three astronomers [7][8] [9]who have embraced the new definition. Your cited article just recycles Alan Stern's objections. If you can find me an article objecting to the new definition that doesn't mention Alan Stern (good luck), maybe I would be more convinced that this represents a broad-based movement. Also, when reading something off the page, particularly in an encyclopedia, one shouldn't have to think about it. It should be clear and consise and immediately understandable. The thinking comes after you've read it and digested its implications. There should be no ambiguity. That sentence was ambiguous. Serendipodous 06:41, 16 September 2006 (UTC)
Two cents: the primary idea of the introduction is that something has settled, but not without controversy, much historical debate, and a door still slightly ajar for future revision. I think this intro more or less gets it right, though I'm still concerned about the flow, particularly introducing 24 Aug in sentence two and then not returning to it until para three. Re the revert, I think a compromise might be adding two adjectives "well-publicized and controversial"—all the summation we need. "Will not be clear-cut for some time..." does not belong. It is clear cut, and simultaneously not accepted. That's not a contradiction (think Bush v. Gore). It's not a matter of critically or uncritically accepting anything—it's a matter of reporting on what the only available decision making authority has decided. Marskell 17:06, 16 September 2006 (UTC)

IAU voting numbers

The article says that only some 400+ astronomers voted in the vote on the definition. But this report of the voting (third post) gives different figures, and says that there were a lot of abstentions. I think we can only refer to reports of the numbers voting, as I have seen no official source, rather than make a bald statement of fact. --Cuddlyopedia 16:25, 18 September 2006 (UTC)

Removed paragraph

I have removed the material in italics from the "Double planets" section

"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. "

This may be true, but it has never been sourced, and I can't verify it. If someone can, I'd be happy to see it put back. Serendipodous 22:44, 8 November 2006 (UTC)

(I added the preceding paragraph to this section to clarify what is being refered to). Serendipodous just gave me a heads up that it had been removed. The reason I never gave a source was that I felt it was self-evident. Is the problem with the following notion...
If a moon's orbital speed (around its planet) is less than its planet's orbital speed (around the Sun), then looking "down" the moon never actually loops back on itself, and in essence it orbits the Sun in its own right.
... or is it the specific examples given? Tompw (talk) (review) 22:07, 26 May 2007 (UTC)

Definitions of planets in fiction

what about adding a pra. about the Definition of planet in fiction (tv,books etc.) like Star Trek planet classifications 89.1.30.64 03:18, 3 February 2007 (UTC)

This article is about the scientific definition of a planet, so it would be inappropriate to add fictional information. However, you may be interested in reading Stars and planetary systems in fiction‎. Cheers. --Ckatzchatspy 03:56, 3 February 2007 (UTC)

Minor inaccuracy removed

Following the sentence describing Eris as a "Dwarf Planet," I removed this dubious claim: "...although the Minor Planet Center has given it a full minor planet designation: 136199 Eris." This suggests that the designation "Minor planet" is more specific than "dwarf planet" In fact, all 136199+ known small bodies are designated "minor planets." --MiguelMunoz 10:49, 12 February 2007 (UTC)

Indeed, Pluto has also recently been assigned a minor planet designation - 134340. --Neo 12:11, 12 February 2007 (UTC)


Future of the Definition?

Does anyone know if the IAU is planning to revisit or revise the new definition of the planet? I have heard of multiple petitions from both the scientific community and public that urge them to do so. --Sly 18:12, 15 March 2007 (UTC)

Tough call. Alan Stern and Mark Sykes have circulated a petition, and the definition of planet will have to be a topic at the next IAU congress in 2009, if only to deal with the extrasolar issue. However, realistically, I don't think there's any going back. Pluto was placed in the Minor Planet Catalogue, and by the time it comes up for reconsideration in 2009, 42,000 new objects will be ahead of it in the list. Since minor planets are named in chronological order, that would mean that, were Pluto removed from the catalogue, every single one of the 42,000 new objects will need to have its number changed, and not only that, but every single paper that mentioned any of those 42,000 objects would need to be altered as well. My feeling is that, eventually, "Dwarf planet" will be considered a subclass of "planet" and so Pluto will get at least some of its planetary dignity back. However it will still remain a dwarf planet. Perhaps they can add a new designation in the catalogue to list the dwarf planets as special among the minor planets. Serendipodous 19:00, 15 March 2007 (UTC)
Not to be difficult, but if the decision were made to redesignate, I doubt that something as minor as the numbering would stand in the way. There's no reason to renumber the following objects, no more than there is to retrieve and renumber your cheques if you have to cancel one. Worst case scenario, they could declare 134340 permanently empty ("retired"?) and move on. (Don't forget that Pluto - having been assigned a number 76 years after its discovery - isn't in chronological order anyways.) --Ckatzchatspy 08:56, 16 March 2007 (UTC)
Again, people forget that this is not only about Pluto. If for some reason Pluto gets back its planethood, it would mean that at least some of the other dwarf planets would also become planets. I don't think that returning to pre-2006 situation is possible. Eight or dozens of planets, but not nine.--JyriL talk 21:54, 17 March 2007 (UTC)
what the true problem is is that people are getting stuck over a definition. what you call it does not change what it is. i MY opinion, everything that directly orbits the sun and has enough mass to be round is a planet. the true root of the problem is that then there would be like 16 planets, and having a lot of planets is supposed to be bad. i dont understand why anyone thinks this way.
There wouldn't be 16 planets; there be more like 1600. We just haven't found them all yet. Try getting kids to memorise all those in school. Serendipodous 04:30, 21 March 2007 (UTC)
i doubt there would be that many, but i get your point. however, i dont see why we have to not call them planets. i dont think it would be that hard to memorize them all. i can remember the 50 states. a planet is pretty much a big thing that orbits a star. if there happens to be a lot of them, so be it.
This is a common misconception. It's not the number of planets that some astronomers consider a problem; it's their orbits - highly eccentric, or highly inclined, or both; and with huge semi-major axes that are sometimes hard to understand in their own right. Having an object is one thing; explaining how it got there is quite another. Compared to the nearly circular orbits of everything large inside Neptune, that tells you something different has been going on. So it's partially a dynamical and formation issue as well. Spiral Wave 19:49, 28 March 2007 (UTC)
That may be so of some astronomers, but the definition itself, as decided, is all about number. The distinction between a planet and a dwarf planet is the ratio of its own mass to the mass within its orbital zone. Serendipodous 09:46, 29 March 2007 (UTC)
I agree. i think that, to the astronomers, it is all about number. i think the true thing that makes this definition strange is that, suddenly, because of where it is, or the shape of its orbit, it is suddenly something else. i think we should exclude what it is DOING and focus on what it IS. i think we should take the planet as if it were alone in a void. nothing else around it. no orbits, no other planets, nothing. then, apply some test (like a size or roundness test) and see if it was a planet. if it passes, its a planet, no matter where it is or how many there are. (i.e. if a strawberry grew out of my nose, you would still call it a strawberry, and if there were suddenly 1000 George Bushes, they wouldnt suddenly not be George Bush) —The preceding unsigned comment was added by 69.177.203.87 (talk) 18:39, 29 March 2007 (UTC).
True, but then satellites like the Moon and Titan would have to be given similar consideration. I think that would be a perfectly adequate definition of planet, though again perhaps a little difficult for the kiddies to memorise in school.Serendipodous 18:43, 29 March 2007 (UTC)
Well, we just call them all planets and classify them. you have terrestrial planets, ice planets (pluto like things), gas giants, and moons. —The preceding unsigned comment was added by 64.252.194.190 (talk) 19:04, 30 March 2007 (UTC).
Not all moons, TNOs or terrestrial objects are planets though. Serendipodous 19:28, 30 March 2007 (UTC)

The thing is that there are families of objects which go roughly like this IMHO

  • Lumps of Ice and Rock which orbit the Sun
    • Giants
      • Gas Giants - Jupiter and Saturn
      • Ice Giants - Uranus and Neptune
    • Terrestials - Mercury, Venus, Earth and Mars
    • Planetoids
      • Asteroids - Ceres, Pallas, Juno, etc.
      • Kupier Belt Objects - Eris, Pluto, etc.
    • Comets

The term planet itself is somewhat outdated - refering IMHO to the terrestials and giants together when there is little to connect them beyond easy visibility from Earth. If I were to plan school curriculums then this is how I would teach it - there's not much point making kids learn the names of the nine planets as abstract concepts, but learning something of the nature of the bodies as well is surely better. --Neo 11:48, 21 March 2007 (UTC)

The "easy visibility from Earth" could well lend itself to a definition. First, let's establish two distances, the chronos (the greatest possible between Earth and Saturn), the ares (the greatest possible between Earth and Mars), and the geo (the greatest possible distance between the Sun and the Earth). How about a definition that went like this? "A planet is an object orbiting a star or stars, that is visible to an observer at a distance at 1 geo from the point being orbited. If the object in question is gaseous, it must be naked-eye visible to that observer at a distance of 1 chronos. If the object in question is rocky, it must be naked-eye visible to that observer at a distance of 1 ares." Terribly geocentric, yes, but quite in keeping with the origin of the term. El charangista 01:20, 26 April 2007 (UTC)

Recent addition

re: this addition:

Much of the work of pre-Socratic philosopher Philolaus, revolved around his struggle with the prevailing ideas surrounding the seven bodies accepted as planets during his time. When the movements of celestial objects convinced Philolaus that the world must be not only turning on its own axis but revolving around a fixed point in space, he was faced with the problem of explaining how a flat world (such as ours was thought to be) could move in this way without spilling everything on the surface into space. He came to the conclusion that the directions of up and down do not exist in space, except in that all things must fall towards the center of the universe, around which all things (including the Earth, Sun, and all the planets) must revolve. The underside of our Earth must face this fiery, central point at all times, otherwise we would fall off. This created a contradiction within the Pythagorean school of thought. Since planets, in their understanding, were composed of a fiery or ethereal matter having little or no density, they could quite easily rotate eccentric to the Earth without becoming off balance. However, the Earth was obviously made of the dense elements of Earth and Water. If there were a single Earth revolving at some distance from the center of space, the universe's center of balance would not coincide with its spatial center. Since this is the point towards which things fall, the earth must have a counter-balance of the same mass or the universe would be flung apart. This problem led Philolaus to develop idea of a Counter-Earth, a second, flat Earth, identical but opposite to ours in every way.

_____________

This addition is very interesting but it has nothing to do with the topic; the issue at hand in that section is the difference between the five planet and seven planet conception. This addition does nothing to address that. It would be better placed in the flat earth or geocentrism articles. It's also completely unsourced. Serendipodous 20:01, 16 May 2007 (UTC)

Sphericity

The article discusses at length whether objects are spherical and therefore dwarf planet candidates. But sphericity is not a criteria for that status. The criteria is "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium", which is then paraphrased as 'nearly round'. Objects can be spherical for reasons other than their mass. And objects can be in gravitational hydrostatic equilibrium and not be spherical, because of high rotation etc. I think the title of this section, and the emphasis of the content, should be changed to reflect this. Sphericity is a good first sign though, so most of the content can be kept. --Cuddlyopedia 16:25, 18 September 2006 (UTC)

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