Does Our Milky Way Stir Up Solar System Collisions?

[really.]

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?

 

[obafgkmrns]

"Not likely" is the byword. Keep in mind that if the sun is a basket ball, then Uranus is a smallish pea located a little over a half mile away, and Alpha Centauri is trundling about at a distance of 500 miles. There's simply an awful lot of space and not much matter, so collisions are unlikely. Even so, an encounter between two solar systems would probably result in severe disruptions to the orbits of their planets, possibly even exchanges of planets. It is likely that the resulting configurations would be unstable and take several million years to sort themselves out. That gives ample opportunity for interplanetary collisions. That's not to say that inter-planetary collisions never happen, just that you are unlikely to be standing on one when it does. Keep in mind that a lot can go on among 100 billion stars during 10 billion years.

On the other hand, star densities in globular clusters are hundreds or thousands of times larger than in our part of the Milky Way, and it's unlikely any stars there have long-term planetary companions.