I would go for a system/sector menu. It doesn't need to be anything too crazy... you could even modify the current Planets screen to do so. I don't know if in the code, the planet objects have an attribute that shows what sector it's in, but if it does, you could just implement an option to sort it all by sector (grouping all the planets in one sector together), or to sort them as they are now. Further additions could then be added on top of this later on, but as is, I don't think this would be too difficult to implement.