It's fantasy right? My maxim in my games is, "Never let the rules get in the way of a good story". Real world considerations should be secondary to 'design' & 'story' elements IMO. Go with what looks good and is reasonable. All the rest can be put down to anomalies within that star system - i.e. orbiting moons and so on that don't follow the same 'rules' as our solar system...you get what I mean.