We inhabit a system with significant angular momentum:


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?


2 Answers 2


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.

  • $\begingroup$ An open system, ok that makes sense, and some of the other posts such as "why does everything spin" were interesting too. $\endgroup$ Apr 28, 2015 at 8:37
  • $\begingroup$ 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. $\endgroup$ Apr 28, 2015 at 8:38
  • $\begingroup$ @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. $\endgroup$
    – anna v
    May 11, 2015 at 5:03
  • $\begingroup$ @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. $\endgroup$
    – CuriousOne
    May 11, 2015 at 5:25
  • $\begingroup$ @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. $\endgroup$ May 11, 2015 at 5:37

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.

  • $\begingroup$ This answer is reasonable, but does not explain how angular momentum is conserved. $\endgroup$ Oct 27, 2015 at 9:40

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