[geoschmo]

Originally posted by Ruatha:
But to have such a fast spin as most planets have most particles would hav eto have the same general direction, or? (Why do stars spin in the galaxy? I know what keeps them from flying away but why do they spin?)

(It doesn't seem to be any answer, most belive it's from the big bang but it seems there are no good theory as to why things spins (planets arund their axis, all stars in the same direction around the galaxy, etc))

The answer is if they didn't, they wouldn't.

That seems simplistic and counter intuitive, but to understand it you have to look at it backwards. If there were things in the solar system that didn't revolve around the sun in a particular way then over time they would fall into the sun or another planet, or be flung off out from the system entirely. Early on there were very likely many objects of varying size that were going all about in a chaotic manner. Over time all those were eliminated and what we are left with is only those things that follow a few stable orbits. Plus some small debris of course. The galaxy is the same principle as the solar system on a larger scale. It's not that something is making them revolve a certain way, it's that over time things that don't revolve a certain way get eliminated.

As far as why planets spin on their axis, I believe most of that comes not from the impact of the debris forming the planet, but from conservation of momentum of orbiting bodies. A body orbiting another will pick up a spin from this process. It's compicated orbital mechanic stuff that I don't fully understand, but I think that is the jist of it.