I noticed that if you place an object into an already running sim, it treats it like an explosion.
 
Which is pretty cool.  
 
All the surrounding objects fly outward.
 
...In case no one ever noticed.

Quote from Tony on 04/21/07 at 23:30:17:

 
 
My bad, it's when you take an object away.
 
I made a sim with a bunch of particles, but neglected to delete the original object (sun I guess)
 
After I had already started the sim (a minute or maybe two), I noticed the sun was still there, so I removed it, and ALL the surrounding particles fled away. It was almost like the sun imploded first, and then exploded. (not visually, but the results would have been similar in a real case)
 
The movement was definately not from the particles losing their orbit, and I say this because they fled directly outward at a high rate, as if a huge explosion had occured. (not to mention they had not really begun to orbit yet)
 
I will try to recreate the sim, and then post it.