BTW: Is Charon in a retrograde orbit or is it whizzing along so as to complete 3/4 of a revolution in 5 days?
They’re tidally locked, showing the same face to each other, with an orbital period of a bit more than six days (it sez here). Charon has a lot lower density which means it wasn’t formed at the same time and place as Pluto. Mike Brown (discoverer / co-discoverer of the larger Kuiper Belt Objects or KBOs) claims that one of those KBOs was formed in a large impact that also gave birth to Pluto and Charon.
Oh, and I screwed up, Mike Brown sez those were two different impact events. I had planned to post some stuff of his regarding the possibility of a Planet X, haven’t yet, but should probably check anyway, in case I’d posted it before. :’)