Very creative concept. It was fun enough that I ended up playing to the end. I just want to share a bug I found. I noticed that energy balls could be knocked out of bounds. So I tested hitting planets around and managed to get a couple of them to go out of bounds as well. This probably won't happen to most people (since the planets have to be moving fairly fast), but you might be able to easily fix this by setting the "Collision Detection" to "Continuous" on the Rigidbody for the planets.