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.
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.
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