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Forestman
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How did the planets acquire their spin?
mgb_phys said: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
mgb_phys has thought carefully about what he is saying and is still correct.kishka said: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.
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.)kishka said: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.
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.DaveC426913 said: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.
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.DaveC426913 said: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.
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.One thing we should not minimize is the recognition that, by far, the vast majority of angular momentum ends up in the central star.
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.Ken G said: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?
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:DaveC426913 said:There is always the issue of what constitutes an appropriately answered question. Some people like exact details, some people like overviews. IMO, it is an artform to determine, based on the phrasing of their question, which kind will best suit the OP.
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.IME, 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.)
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.The OP's question was as broad 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.
I have given plenty of reasons why that is simply not true. I guess we'll just have to disagree on that.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.
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.Ken G said: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.
Let me rephrase:Ken G said:I have given plenty of reasons why that is simply not true. I guess we'll just have to disagree on that.
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).DaveC426913 said: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.
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.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?
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.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.
It isn't the root cause at all, that's my point."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)."
Chronos said: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.
The planets acquired their spin through a process called accretion. This is when small particles of dust and gas in a rotating disk around a young star begin to clump together due to gravity, eventually forming larger bodies such as planets. As these bodies continue to grow and collide with each other, their spin increases.
Yes, the spin of the planets was determined during their formation. As the planets were forming, they were also rotating due to the conservation of angular momentum. This means that the speed of their spin was determined by the amount of mass and distance from the center of rotation.
No, not all planets in our solar system rotate in the same direction. While most planets in our solar system rotate counterclockwise, Venus and Uranus rotate clockwise. This is due to the different angles at which these planets were impacted during their formation.
Yes, a planet's spin can change over time. This can happen due to external factors such as collisions with other objects, or internal factors such as changes in the planet's core or mantle. For example, Earth's spin has been gradually slowing down due to tidal forces from the Moon.
Yes, the spin of a planet is important for a variety of reasons. It helps to stabilize the planet's orbit, creates day and night cycles, and affects the planet's climate and weather patterns. The spin of a planet also plays a role in the formation and maintenance of its magnetic field, which protects it from harmful solar radiation.