# Why spherical planetary nebula flattens as it spins and ends up to a disk shaped planetary system (like our solar system)?

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Correction: Why spherical giant molecular clouds ~~planetary nebula~~ flattens as it spins and ends up to a disk shaped planetary system (like our solar system)?

In: 9

It doesn’t. I think you might be falling into the trap of assuming that planetary nebulae are related to planet formation and protoplanetary disks, they are not. They are called planetary nebulae because to early telescopes they appeared as small and bright coloured disks, like planets. There is actually no relation to planet formation.

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However your question is still valid for the nebulae that do end up forming stars and planets, giant molecular clouds. The reason is conservation of angular momentum. If you have a big sphere of gas and dust particles they will all be moving about mostly randomly, however there must be some direction which on average the cloud is rotating around. As these particles collapse and collide their random momenta in the vertical directions cancel out until the only remaining momentum is in the direction of spin of the original cloud. You’ve gone from a spherical cloud down to a disk.

Assuming you’ve learned vectors. You know how you can add up vectors together and get 1 output vector? A vector of length 1 pointing north + vector pointing east will give you a vector pointing 45 degrees, even though the vectors are totally different directions.

Now imagine the vectors were rotational (not literally, it’s hard to visualize that). If you add up all the rotational momentum of the particles (assuming the particles have a lot of time to interact with each other and homogenize their momentum), you get 1 rotation aka the disk.

(Edit: assuming you haven’t learned vectors. Imagine walking 1 block north, then 1 block east. If you draw a line between your start point and end point, this is considered adding all of your movements. This line is 45 degrees and length square root of 2 or whatever Pythagorean theorem says.

So adding all the particles’ momentums together means drawing one rotational line from start to end, and this line is the disk you see)

Makin’ pizza’s! Take that spherical dough with some angular velocity and gravitational forces and let it spread to a perfect pizza pie!