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We inhabit a system with significant angular momentum:

http://www.zipcon.net/~swhite/docs/astronomy/Angular_Momentum.html

If our solar system formed by gravity gathering its material together to form the sun, proto-planetary disc, and eventually the planets, which all orbit in the same direction...

Where did this angular momentum come from in the first place, since angular momentum is conserved?

It does not seem possible to me that the formation of the solar system under gravity could impart this angular momentum on it, if it is a closed system. If it formed from a 'cloud of space dust', then it must have been present in that dust cloud, but where did the dust cloud get it from?

Qmechanic
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user2800708
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    Possible duplicates: http://physics.stackexchange.com/q/12140/2451 , http://physics.stackexchange.com/q/23104/2451 , http://physics.stackexchange.com/q/68646/2451 and links therein. – Qmechanic Apr 27 '15 at 09:24

2 Answers2

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A collapsing gas cloud is an open system. It loses mass, energy and angular momentum as it collapses. Even if the net angular momentum of the cloud is zero, after the collapse the final planetary disk can have a significant net angular momentum, and the ejected material will have the opposite angular momentum. What can not happen, and that's where your intuition is correct, is that all the material in the original cloud collapses into the disk while rotating in the same direction.

CuriousOne
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  • An open system, ok that makes sense, and some of the other posts such as "why does everything spin" were interesting too. – user2800708 Apr 28 '15 at 08:37
  • Why do all the planets orbit in the same direction though? Its as though almost everything with opposite angular momentum has been ejected from the dust cloud, leaving only material that orbits the same way. – user2800708 Apr 28 '15 at 08:38
  • @user2800708 this would need a model, but logic tells that if material had not been ejected with equal and opposite angular momenta it would still be a cloud, at best of milling asteroids colliding with each other and finally into the core of the star. – anna v May 11 '15 at 05:03
  • @annav: Of course this is just a hand waving explanation. The details of how planetary clouds are collapsing are probably filling a small library, by now. – CuriousOne May 11 '15 at 05:25
  • @user2800708 - The initial cloud was perhaps 3 light years in diameter. In a short time (~100,000 years), it collapsed to a central star and a protoplanetary disk that was only a few hundred AU across. Think of how fast figure skaters spin merely by pulling their arms in. The gas cloud pulls everything in. Even the tiniest bit of rotation at the start of the collapse becomes a huge angular velocity by the time the protostellar disk forms. – David Hammen May 11 '15 at 05:37
  • @CuriousOne -- I think this answer is wrong. I'm not positive, so I'm not going to downvote. If the ejected material had the opposite angular momentum, this would exacerbate the angular momentum problem. The star and protoplanetary disk dump a lot of angular momentum. They have to do so; otherwise the star and disk couldn't form. – David Hammen May 11 '15 at 05:40
  • @DavidHammen: I am sorry if there is a misunderstanding. Of course real planetary clouds start out with a non-trivial angular momentum. The OP, however, did seem to ask if a cloud can collapse if it has no angular momentum. The answer to that is affirmative. We can have $L_{initial}=0=L_{star}+L_{planets}+L_{expelled}$. – CuriousOne May 11 '15 at 06:36
  • If the cloud starts out with a non-trivial AM, that doesn't in anyway answer the question of where does the AM come from? Its conserved, so the amount at the end is equal to the amount at the start. Where did that non-trivial AM come from? – user2800708 May 11 '15 at 08:46
  • The angular momentum of a gas cloud will come from interactions with other objects, e.g. the gravity of stars, magnetic fields and radiation of very hot young stars which pushes gas and dust away from the star. Since the pressure of this radiation field depends on the distance from the star the effects will be asymmetric and they will impart angular momentum on even perfectly "resting" gas clouds. – CuriousOne May 11 '15 at 11:33
  • @user2800708 - You are assuming angular momentum is conserved. Why? All bets are off regarding the conservation laws if there are external interactions. Gas clouds interact with the nearby external environment in a number of ways, as CuriousOne just wrote in his most recent comment. – David Hammen May 11 '15 at 11:38
  • I meant universally it is conserved. – user2800708 May 11 '15 at 14:51
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Even if the original dust cloud only had a relatively small angular velocity (which it might have had for all sorts of reasons), the process of collapsing would have amplified it. That is, the collapse process preserves the angular momentum, but it translates to a much larger rotational speed in the newly-collapsed system. Think of what happens to a spinning ice skater when she pulls her arms in.

Where did the original dust cloud get its angular momentum? Since this is a relatively small amount of angular momentum we are talking about, there are lots of places it could have come from. For example, if the original dust cloud was formed by the coalescence of two smaller dust clouds which happened to collide, the collision would have imparted angular momentum to the system unless the two original dust clouds collided perfectly head-on.

Peter Shor
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