system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do all known solar systems have planets that revolve along the same plane?

Why do all of our planets revolve around the sun on the same 2D plane? Why aren't some planets orbiting the sun on another degree different than ours?

Does this also apply to planets with multiple moons? Do those moons always share the same 2D plane?
gpt
Conservation of momentum. When a dust cloud is collapsing into a star/solar system, the stuff will be moving mostly randomly, but by chance enough of it will be moving in the same direction to impart a spin on the gas and dust. As it continue to collapse, more and more of it will start to move that direction. As more stuff spins faster and faster, the stuff that's not in the center forms into a disk extending from the equator of the star. This stuff forms the planets, so they all end up in the same plane. Smaller stuff that is usually way further out isn't as affected by this gravity and spin, so that's why you have comets and stuff with wacky angles.