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This question already has an answer here:

If we look around the cosmos, we will see most of the systems are just flat disks. Either it is solar system, galaxies, or the rings of Saturn. What is the matter, does it depend on how they were formed, or anything else?

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marked as duplicate by Rob Jeffries, J. Chomel, peterh, Jan Doggen, Glorfindel Apr 6 '18 at 13:45

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Flatness is caused by conservation of angular momentum. As solar system/galaxy collapses under its own gravity, its velocity increases to conserve the angular momentum. This high velocity leads to formation of disc shaped structure. The similar manner in which piece of dough becomes flat when it is rotated.

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    $\begingroup$ Conservation of angular momentum isn't enough, because then dark matter structures would be flat too, whereas they are in fact much more spherical (though still elliptical). Your answer neglects a very important ingredient, namely the dissipation of energy by the collisional gas. $\endgroup$ – pela Apr 5 '18 at 12:28
  • $\begingroup$ This answer [physics.stackexchange.com/a/25954/181563] describes the gas interactions, and some exceptions [elliptical galaxies and parts of solar system]. $\endgroup$ – bitchaser Apr 5 '18 at 19:04

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