# Thread: Concentric rings, but why coplanar?

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## Concentric rings, but why coplanar?

Why are coplanar discs so common in nature, like planetary rings and galaxy discs?

I can see the reason why rings consist of rings. Confocal ellipses might undergo apsidal precession, which would be at a different rate, so they would collide.

But just what is wrong with concentric rings that are not coplanar?

As they precess their planes at different rates, they still keep their different radii, so should not collide. Why are they not more common in nature?

2. The short answer is collisions. Spherical systems turn into flat systems due to collisions between particles. These collisions remove angular momentum in some directions, but can't get rid of the overall angular momentum of the original cloud.

The plane of the flattened system is defined by the direction of the overall angular momentum of the original cloud. Since the cloud had only one net angular momentum vector, it ends up with one single plane -- not a bunch of different planes at different radii, which would be required to make the concentric yet not coplanar rings you mention.

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Originally Posted by StupendousMan
The short answer is collisions. Spherical systems turn into flat systems due to collisions between particles. These collisions remove angular momentum in some directions, but can't get rid of the overall angular momentum of the original cloud.

The plane of the flattened system is defined by the direction of the overall angular momentum of the original cloud. Since the cloud had only one net angular momentum vector, it ends up with one single plane -- not a bunch of different planes at different radii, which would be required to make the concentric yet not coplanar rings you mention.
But not all parts of the cloud collide with each other.

Rings of same radius and different planes collide. Ellipses of different semiaxes collide. Concentric rings with different radii, however, do not intersect and do not collide.

4. If the system began as a spherical cloud of gas, then the gas particles would collide with each other. Result: flattened system.

If the system began as a spherical cloud of little specks of dirt or ice, those particles would collide with each other. Result: flattened system.

If the system began as a set of concentric rings with different orbital planes, then the particles in each ring would not collide with each other(*). All you have to do in that case is explain how the system starts in that way.

(*) on timescales of thousands to millions of orbital periods, the particles in one ring will influence those in other rings, causing eventual collisions. So it won't work on long time scales no matter what you do.

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Originally Posted by chornedsnorkack
But not all parts of the cloud collide with each other.

Rings of same radius and different planes collide. Ellipses of different semiaxes collide. Concentric rings with different radii, however, do not intersect and do not collide.
They dont start out asa concentric rings, but as randomized gas/particles. It is the collisions that make the rings concentric in addition to coplanar

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