I would suggest to take a look at the movement/missile systems that are floating aroung in the map section. I don't know them much (Since i'm writing many systems for my map myself) but i believe some of them should be capabel to make an object fly around another one. Although: How physical/geometrical exact do you need the system to be. Theoretically you could write a symulating one. But then, without proper positioning of your planets, they could just colide with each other instead of orbiting (there are some games where you can fiddle around with orbital physics, so i can know how that could end up...)