# What decides the direction in which the accretion disk spins?

Planets lie on the same plane because of the accretion disk formed during the Protostar stage, as I read in this question.
I also read about the collision of particles in the gas cloud causing the overall spin to be in just one direction. But what decides the direction in which the accretion disk spins relative to the direction of the core?
(I'm thinking that it might be because of some primary conditions - maybe the direction in which the core is spinning.)

• Spin is a conserved property, so it has to be an initial condition. – Hohmannfan Apr 12 '16 at 11:43
• The cloud from which a stellar system is born is turbulent, but has some net angular momentum which to a large degree is conserved as it collapses. As your question stands now, I think the answer is given trivially by user11527, and I don't understand why it was downvoted. – pela Apr 12 '16 at 12:09
• @pela It doesn't say the cause. You should try editing that if you see the answer. – new-kid Apr 12 '16 at 12:21
• I still don't understand: The cloud has some angular momentum. As it collapses, the central star and the (proto-)planetary disk spins in some direction. Whether you call it clockwise and counterclockwise depends on how you orientate your clock. I guess I'm misunderstanding you. Do you actually mean "What decides the direction in which the disk spin with respect to the direction in which the star spins?"? – pela Apr 12 '16 at 12:29
• @pela I think that question will just answer me. – new-kid Apr 12 '16 at 12:36

Stellar systems are born from clouds of turbulent gas. Although "turbulence" means that different parcels of gas move in different directions, the cloud have some overall, net angular momentum. Usually a cloud gives birth to multiple stellar systems, but even the subregion forming a given system has a net, and non-vanishing (i.e. $\ne0$), angular momentum.

Parcels moving in opposite directions will collide, and friction will cause the gas to lose energy, such that the cloud contracts. Eventually subclouds moving in one direction will "win over" subclouds moving in other directions such that everything moves in the same direction, keeping the original angular momentum (minus what is ejected e.g. through jets).

This means that the central star will rotate in the same direction as the circumstellar disk and that, in general, the planets that form subsequently, will also not only orbit the star in the same direction, but also spin in the same direction around their own axes. This is called prograde rotation. Sometimes, however, collisions between bodies may cause a planet or asteroid to spin in the opposite direction. This is called retrograde rotation and is the case for Venus and Uranus.

• There are also plenty of examples of exoplanets that do not orbit in a prograde fashion. See Rossiter-McLaughlin effect. – Rob Jeffries Apr 12 '16 at 13:09
• @pela You explained it really well. I'm gonna mark this correct. And also thanks for helping me fix my question. – new-kid Apr 12 '16 at 15:14
• The retrograde rotation explains how the dynamic forces come to play. There are a lot of interesting references diverging from this explanation. Yes including the Rossiter-McLaughlin effect @RobJeffries – new-kid Apr 12 '16 at 15:19
• You're welcome, @new-kid :) – pela Apr 12 '16 at 18:49

Well the spin of the accretion disc is relative. We consider the top of anything as North. What if the the top was to be the South. It depends on perspective. Something rotating clockwise when seen from north would rotate counter clockwise when seen from south.

• Thoughtful fact but still there is a 3d direction of the plane in which the accretion disk, and later the planets, will align to. – new-kid Apr 12 '16 at 12:12
• Take a rotating disk, and flip your head upside down. Then it appears to rotate in the other direction. Though a bit short, this answer seems to be essentially correct. – Hohmannfan Apr 12 '16 at 12:37
• @Hohmannfan Not exactly, there still is asymmetry in the existence of the spin itself. Why did it choose that particular direction to spin in? – Manishearth Apr 12 '16 at 12:40
• As @pela said in the comment, I've edited the question a bit. Check it out. – new-kid Apr 12 '16 at 12:40

There's some angular momentum to the initial gas cloud, which is a bunch of gas particles with varying velocities. The angular momentum of this will not add up to zero usually, so as the cloud coalesces the resultant accretion disk will have a spin.

• Is this initial angular momentum of the cloud gained mostly because of the direction of movement and spin of the protostar when it catches the particles? – new-kid Apr 12 '16 at 12:45
• @new-kid IIRC yes. Usually. – Manishearth Apr 12 '16 at 12:49