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Planets and angular momentum

  1. Mar 31, 2009 #1
    How did the planets acquire their spin?
     
  2. jcsd
  3. Mar 31, 2009 #2

    mgb_phys

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    The gas cloud they were formed from was rotating, as the planets formed angular momnetum is conserved so a slowly rotating gas cloud condenses into a faster rotating lump of rock.
    A couple of planets rotate the wrong way due to collisions with other lumps of stuff in the early solar system
     
  4. Mar 31, 2009 #3

    DaveC426913

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    Also...


    Ah. No. mgb_phys's pretty much got it covered...
     
  5. Mar 7, 2011 #4
    Your statement about the planets, are you saying that the planets were formed from rotating gas clouds? The gas clouds we currently see, are they going to turn into planets, too? I'm having a hard time with your statement. Please think about what you're saying.

    Secondly, if the planets are rotating the wrong way because of collisions, why doesn't our moon rotate backwards from the direction of the earth's rotation? The moon was hit a lot with meteors, etc. I don't know what the other planets look like, but the moon is "pitted" for that very reason.
     
  6. Mar 13, 2011 #5

    Ken G

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    Actually, I think the correct answer to the OP is "no one knows." Yes, collisions seem to have messed with spins like Venus and Uranus, and the spin of Mercury and most moons are locked by tidal effects, but there's really no prevailing "generic rotation" that you can calculate for a planet that does not have those effects. The idea that the spin is a natural consequence of the orbital angular momentum of the disk is just not true-- because if you think about it, the orbital shear inherent in a Keplerian disk implies a spin in the retrograde direction if a strong gravity collected the planet suddenly (it doesn't), but planets not affected by the above alternate processes seem to spin prograde (if our solar system is any guide, which it might not be).

    In other words, if you follow an orbit around in a Keplerian disk, and analyze the angular momentum of the surrounding gas in the frame of reference of that orbit, you find the neighboring gas has retrograde angular momentum. There's no necessary conservation of angular momentum in a co-orbiting reference frame, because of the coriolis force, so it's just not clear which way a planet "should" spin. I believe it's a very active area of research, and there is enough complexity (tidal effects, collisions, migrations, etc.) that there might not be a "generic story" for planet spin.
     
    Last edited: Mar 13, 2011
  7. Mar 13, 2011 #6

    DaveC426913

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    mgb_phys has thought carefully about what he is saying and is still correct.

    Planets and stars form from coalescsing dust and gas clouds.
    These clouds, when the movements of their particles are averaged, will have some non-zero element of angular momentum.
    As the star and the planets coalesce, this angular momentum is conserved.

    1] The angular momentum imparted by impacts is random. It will have little effect on the Moon's direction of rotation. Unless there is one huge hit. That is what is assumed in the case of counter-rotating bodies. (Btw, it doesn;t necessarily mean it reveresed the body's rotation, it could as easily flip the planet on its head, whcih would cause it's rotation to appear backwards. Also, it only needs to flip it 91 degrees, not 180. The spin will eventually align itself.)
    2] The Moon is tidally-locked with the Earth, so the issue of which way it was rotating is pretty much moot.
     
  8. Mar 13, 2011 #7

    Ken G

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    Certainly angular momentum is conserved. Unfortunately for these rather imprecise explanations, there are two kinds of angular momentum we find in the solar system-- orbital angular momentum, and spin angular momentum. There is no particular reason that orbital angular momentum of a bunch of coalescing gas has to turn into spin angular momentum after the coalescence is over-- it is also possible for the spin angular momentum to be in the opposite direction, and the orbital angular momentum to be even larger, as a result of the coalescence. So the answer that "angular momentum is conserved explains the spin of planets" is overly simplified, it simply is an incomplete explanation, and might not even give the right sign of the spin in all cases.

    Basically, during coalescence, you have to look at the orbital angular momentum of all the gas that is participating, and then you have to look at the orbital angular momentum of the resulting body. If the two are the same, the spin is zero. If the first is more than the second, the spin is prograde, and if the first is less than the second, the spin is retrograde. So much for explaining spin that way! If you claim prograde spin should result, you need to be able to argue why the coalesced body should have less orbital angular momentum than the gas that goes into it. Does anyone want to take on that argument? I didn't think so, it's an area of ongoing research. For one thing, it seems to make a difference if the planetary body spirals in a ways as it is forming, but in any event, it is much more involved than a simple conservation story.

    Of course, after the initial spin is set, additional factors can weigh in, like tidal locking and collisions. Those are easier to understand-- tidal locking creates spin that is prograde and linked to the orbit, collisions produce a random element to the spin.
     
    Last edited: Mar 13, 2011
  9. Mar 13, 2011 #8

    DaveC426913

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    Agreed. There's still lots of details to work out. However, I don't think there's really any serious challenge to the notion that, however the details play out, ultimately, the orbits and spin of the star and planets come from the retained angular momentum of the initial gas/dust cloud.



    One thing we should not minimize is the recognition that, by far, the vast majority of angular momentum ends up in the central star. Any mass that does not play well with the other proto-planetary children ultimately ends up falling into the sun or being ejected from the system.

    i.e. it's not like a system is so delicately balanced that it becomes difficult to explain the nuances of orbits and spins - it's really more like: 99.99% of the gas cloud and its angular momentum is very easily explained, while 0.01% of it manages to ride the fine line that is stable enough to form larger bodies.
     
  10. Mar 13, 2011 #9

    Ken G

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    The total conservation isn't the issue. To understand the spin, you have to know the breakdown between spin and orbital angular momentum, and that's the part that doesn't have easy answers. The details of the formation process are going to show up in the partition between the two, and could allow no spin at all. So the conservation law really isn't the explanation for spin, though it's certainly relevant.

    No, the majority of the angular momentum can be in the form of orbital angular momentum of the disk and planets, not spin angular momentum of the star-- that is more or less the reason that there is a disk and planets in the first place. It's also the reason that binary stars are so common-- orbital angular momentum tends to overshadow spin angular momentum.
     
  11. Mar 14, 2011 #10
    I posted this thread back in 2009, but thanks for the continued response.
     
  12. Mar 14, 2011 #11

    DaveC426913

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    Hey, if the scientific community still doesn't have the answer and is still looking, it's not unreasonable to assume that you still don't have answer and are still looking... :smile:
     
  13. Mar 14, 2011 #12

    Ken G

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    My answer is that we don't have the answer to spin angular momentum and are still looking. My comments about orbital angular momentum are uncontroversial, you can check them on the back of an envelope for even our own solar system. My objection to the above answers had to do with the fact that they sounded like answers but really weren't when you look at them a little more closely. Is our goal to make questioners go away, thinking they have learned something, when something major is still missing? A cynic might say that one form of education is making people think they understand what they don't actually understand. We don't want this forum to enter into that form of education, do we? Now I'll grant you, any explanation is going to be incomplete at some level, so I'm not trying to nit-pick-- I'm just saying that "conservation of angular momentum" does not actually explain why planets spin the way they do, because there are two forms of angular momentum involved, and the total could be conserved with any spin.
     
    Last edited: Mar 14, 2011
  14. Mar 14, 2011 #13

    DaveC426913

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    There is always the issue of what constitutes an appropriately answered question. Some people like exact details, some people like overviews. It is an artform to determine, based on the phrasing of their question, which kind will best suit the OP.

    I have seen far too many basic questions answered with reams of formulae, indicating the responder misunderstands - or doesn't care about - the ability of the questioner to understand it. (That's the other side of the cynicism coin.) For example, your answer requires an understanding of the difference between spin angluar momentum and orbital angular momentum, yet you did not provide a description. Your answer may well evolve into a whole chapter.

    The OP's question was as basic as it can get, which would indicate little knowledge aforehand of stellar formation - little framework upon which to hang nuances of angular momentum. They might as well have assumed that planets formed in-place and were spun up somehow.

    Certainly, they likely did not realize that coalescing clouds of dust and gas will retain their angular momentum as they contract. That, in and of itself is a perfectly satisfactory answer.

    If they have the knowledge and inclination to pick apart the broader question, it is best to do so in a follow-up (where they will undoubtedly reveal a lot about what they know and do not know and about how much detail they want to know). Also, the information comes out in a more normal idrection: broad overview drilling down to specifics, rather than starting with specifics then having to go back and explain terminology or broad concepts.
     
    Last edited: Mar 14, 2011
  15. Mar 14, 2011 #14

    Ken G

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    Certainly, that has to be done. Most questioners don't want a complete accounting, they just want to know the basic reason. But then we have the issue of what is the basic reason! I would say that conservation of angular momentum is not the basic reason that planets spin, and for two reasons:
    1) the spin could be anything, including zero, and still conserve total angular momentum
    2) even in process that don't conserve angular momentum in a solar system, like if there is material flying off carrying angular momentum (which does in fact happen frequently in solar systems), planets would still spin, and they might spin more or less the same as they do in our solar system (perhaps in part because the disk of our solar system does not actually conserve its angular momentum, there is lots of migration going on due to angular momentum changes).
    It's difficult terrain to navigate, I agree. To me, there are two "mistakes" of giving explanation: one is saying what the questioner can't understand, but will make the responder seem smart, and the other is saying what the questioner can understand, but isn't really the answer to the question. Whether or not either of those mistakes has been made is often an undecidable issue in an absolute way, but people can have their opinion on it. I've expressed mine, that's all I was doing.
    Yes, the questioner is likely coming from a perspective of not knowing anything about conservation of angular momentum, and benefits from hearing about it. However, conservation of angular momentum does not by itself answer their question at all-- the process of forming a planet could yield either direction of spin, or no spin at all, and still conserve total angular momentum, there has to be something else going on their to explain it.
    I have given plenty of reasons why that is simply not true. I guess we'll just have to disagree on that.
     
  16. Mar 15, 2011 #15

    DaveC426913

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    I agree there are subtleties to how it will work that we haven't hashed out yet, but surely you don't suspect there is some force external the system's total energy momentum that is causing it.

    We can at least agree that it is a closed system energy- and momentum-wise, yes?
    i.e. the only forces we need to take into account are initial angular momentum, KE, PE, and gravity. The issue of how these interplay to get the observed result is under development, I can agree with. Or do you think there's more?


    A comparable example: many theories suggest that Jupiter and other Jovian planets actually start in very tight orbits and migrate outwards. We may accept that this is true, but not quite understand the subtleties of how it occurs. However, we can be sure that the only factors at play are the existing momentum and gravity within the closed system.

    Let me rephrase:

    "That, in and of itself, may be a perfectly satisfactory answer for the OP, (since it points at the root cause without getting bogged in details)."
     
  17. Mar 15, 2011 #16

    Ken G

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    I agree it is some internal process that generates spin such that any excess or deficit appears in the orbital angular momentum. But to know what spin you'll get, if any, that's the process that needs to be understood, not the fact that the process will conserve angular momentum (which it might not-- material could easily get expelled from the forming planet that carries away angular momentum in a way we are not counting in the spin).

    As another example, take the Earth/Moon system. If someone asked, "why is the Moon getting farther from the Earth with time", and I answered "because the Earth has spin angular momentum, and angular momentum is conserved", that's not much of an answer, because it doesn't even explain why moons inside their geosynchronous orbit get closer to the planet, and those outside the geosynchronous orbit get farther. How can "conservation of angular momentum" explain why the Moon gets farther without even referencing the fact that it needs to be outside its geosynchronous orbit?

    So conservation of angular momentum is a relevant issue to why the Moon gets farther, but it's not the reason. The reason has to do with the orbital torque applied to the Moon due to gravity from the distortions of the Earth caused by the Moon's tidal forces. A complicated story, but necessary to understand even the first thing about why the Moon gets farther. "Conservation of angular momentum" just doesn't cut it by itself, yet if I gave that answer, someone might think they understood the reason-- when they don't.
    I think it's all about the interplay between spin and orbital angular momentum, yes, and there is something going on that that is the real explanation for planetary spin. What it is, I don't know, and I believe it has to do with how the orbits migrate as the planets form. One could imagine either spin resulting, or no spin, depending on those additional factors that are crucial to the actual reason that planets spin. But I will agree that "conservation of angular momentum" is a useful start.

    So my answer to the OP would be: "Due to conservation of angular momentum, when planets form, any spin they get must be compensated by changes in the orbital angular momentum of the material involved in making the planet. The commerce between those forms of angular momentum has to be regulated by some ill-understood mechanism that is still a topic of research, and that process, whatever it is, is the reason that planets spin as they do-- barring tidal effects and collisions that can happen after the planet forms."

    Given the above, one can see that the answer "they spin in a prograde way because the material had prograde orbital angular momentum beforehand" is just not going to cut it-- it's too pat, and too easy to walk away thinking the reason has been understood, when it has not. I still don't understand the reason, for example, and I already know about conservation of angular momentum.
    Yes, but until we understand those subtleties, we can't claim to have the least idea why those planets have the orbits they do, that's my point.
    It isn't the root cause at all, that's my point.
     
  18. Mar 15, 2011 #17

    DaveC426913

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    I must admit, I've never questioned how planets would acquire prograde spin. As soon as one thinks about it, one realizes that normal orbital mechanics should set up a retrograde spin.

    It is an intriguing mystery.
     
  19. Mar 15, 2011 #18

    Ken G

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    Exactly, that's just the path I came by the same realization-- I thought it made perfect sense that prograde orbital angular momentum would lead to prograde spin, but thinking more about it, I realized I could argue retrograde spin even more easily, so I had to throw up my hands and admit that I don't know. And that's the current state-- I looked at some papers that talked about migration during formation, but I never really felt that I could say "yeah, that makes sense, I believe that now." It might just be a technical issue, with only a technical explanation.
     
  20. Mar 15, 2011 #19

    Chronos

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    This summarizes what I recall about planets aquiring their direction of rotation:
    http://www.sjsu.edu/faculty/watkins/revrot.htm

    A simple example is ice skating. If you skate in a circular path, jump, and spin, which direction of spin is easier - outside shoulder toward the center or inside shoulder toward the center [with or against the direction of rotation]? Nature prefers the path of least resistance.
     
    Last edited: Mar 15, 2011
  21. Mar 16, 2011 #20

    DaveC426913

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    Sorry, I don't see how that answers the question. (Not refuting it, just doesn't do anything for me.)

    The most obvious tendency for rotation is opposite the direction of orbit. In a loose mass of co-orbiting debris, the debris on the outside is in a slower orbit, while the debris on the inside is in a faster orbit. This sets up the rotation.

    I don't see how the skater is analagous to a loose collection of debris. I think it's a poor model. For one example, the skater is driving the system forward from an external force (her feet); that force must go somewhere, and it goes into the spin. That is not at all the same as a free-floating mass in a closed system of forces.

    That linked article does a lot of dancing with numbers but never spells it out. It may be right - I have no idea.
     
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