I am thinking that while accurate physics works in the real world with real-sized objects, if you are going to use the same physics to try to simulate abnormally proportioned objects then you aren't going to see the resutls that you imagine. If you are thinking of these relatively small spheres on the screen as having the gravitational pull of a planet, surely some adjustment is going to have to be made to scale that down given the size proportions, especially if there is an unrealistic or inconsistent sense of proportions between ships and planets. I'm guessing the planets would be more like small asteroids in comparison.