Watch this physics simulation program:

http://www.youtube.com/watch?v=d7eGypGOlOc

I have found a java based one here to play with (doesn't quite match up with the demo):

http://people.ifm.liu.se/freka/particleworld/

Click on the file tab and then open a predefined system. Sun and earth is cool - so are the walkers.

Blacknad.