Actually the Firefly solar system was supposed to include nearly 100 terraformed planets, moons and asteroids, in a complex system that included three or four actual stars (multi-star systems are actually common) and several brown dwarves.
Yeah, it was still a stretch. The thing that got me was a line that said "giving them (planets/moons) something like earth-normal atmosphere and gravity..."
Taking a moon and giving it Earthlike gravity??? Or an asteroid?? That would be quite a trick. FTL is less of a stretch of the imagination, than believing that a STL colony that managed to lose half of their technology, still knew how to alter a moon's gravity to 1g but sent colonists out with a mule, a blanket and an axe.
THAT was a stretch. Still, the characters and dialogue, acting and plotting were SO GOOD that you really worked hard to ignore sci/tech inconsistency.
I loved that show, hated that it got canceled. Own the series on DVD, must have watched every episode 20 times.