Algodoo

A simple solar system consisting of a sun, 4 planets, and 2 moons. I started with just the first, closest planet, then added the others once my friend and I hypothesized that simply placing an object inside another's gravity well will make it be sent into that object's orbit when you click the "Send into orbit" button. What's really nice about this is that there are potentially infinite possibilities for multiple planets, as long as you keep them sized appropriately.