system
Given the following post, respond as if you're commenting on Reddit:
human
Why are all the planets' orbits on the same plane?

Galaxies too...I'm guessing they are flat for the same reason, but why it that?
gpt
Because galaxies and solar systems form with some angular momentum.  In other words, they are spinning as they form.  This causes them to flatten into a disk.  In the case of a solar system, the disk coagulates into planets, but they're still in the same plane as the disk.

For a galaxy, the disk coalesces into stars (and other solar systems)