If you look at our solar system and our galaxy, the stars and planets are generally all in one plane. So, are all galaxies in one plane? And why are they in one plane?
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1$\begingroup$ physics.stackexchange.com/q/8502 $\endgroup$– BowlOfRedCommented Aug 27 at 2:14
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$\begingroup$ Possible duplicates: physics.stackexchange.com/q/26083/2451 , physics.stackexchange.com/q/8502/2451 , physics.stackexchange.com/q/93830/2451 and links therein. $\endgroup$– Qmechanic ♦Commented Aug 27 at 3:56
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3 Answers
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First, let me answer a related question. Why do the planets all orbit the Sun in (nearly) the same plane? This “co-planar” orbital motion is due to the fact that during the formation of the Solar System from a cloud of collapsing gas and dust the Sun and planets settled into a disk structure. This disk structure is the result of the conservation of angular momentum which results when a spinning cloud of gas and dust collapses, and represents a balance point between gravitational collapse and the outward force due the spin of the disk (called centrifugal force). Now, this disk could have been in any orientation, but the most likely configuration would have the residual spin of the disk, including the planets, aligned with the residual orbital spin of the Sun. This is why the spin axis of the Sun is parallel to the spin axis of the rest of the solar system.
As for the the Galaxies, the reason is similar. However, you must note that all galaxies are not in the same plane.
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As the cloud collapses under its own gravity, it shrinks in size. To conserve angular momentum, its rotation speed must increase.
The increased rotation creates a centrifugal force that acts outwards, perpendicular to the axis of rotation. This force opposes gravity in the plane of rotation but not along the axis.
As a result, the cloud collapses more readily along the axis of rotation, while the centrifugal force resists collapse in the plane of rotation. This leads to the cloud flattening into a rotating disk.
Notice that this usually happens before planets form, rather than being an attraction between planets as one other answer suggested.
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I don't agree with the idea that the solar system started out as a disk because the early universe was isotropic. I think that the planets, which were moving in different orbits, were attracted to each other by gravitational interaction, and their orbits were in a plane.
