Alright class, time for a flash game. This is "Orbitrunner" and it's reasonably addictive for such a simple concept — place your star on the grid so that the planets (and their satellites) fall into orbit without smashing into you, each other, or going out of the boundary. I'm not sure how realistic the gravity physics are, but it's very challenging, and I like the background music. The collision/explosion sound/animation is lame, however. I was hoping for a Praxis Effect. If they put this in a 3D, rotatable-camera environment, I'd play it for hours.
You can skip up to five levels if you find you're just not getting the hang of one. It stores your IP address and lets you continue.