So I was talking with a guy last night who was trying to tell me this: Our galaxy is stirring up as it spins and this might cause another planet to come along and collide with earth. My first comment was that the stuff in the galaxy is all caught up in the same "stirring" motion, so it's not likely that solar systems would cross paths just because of this action. I also posited that if we were ever close enough to another planet that our star would also be close enough to the other star that the real story would be the collision of these two bodies—that is, even a planet in an orbit on the magnitude of Pluto's, isn't far enough to allow the planets to mix along the lines of what was being said. Anyway, can someone quickly weigh in on this debate? Does the Milky Way stir itself up, and would it ever be possible for solar systems to be close enough to allow their planets to collide while maintaining the separation of the stars?