Found it. Joss explains it (almost) all in Serenity: The Visual Companion.
Not a lot, good to see you again.
Movie? 
I ask for a cite, and I get an add… 
I ask for a spell check, I get a smiley…
Hmmm.
I assumed the Chinese communities were in a different area/sector than the primarily English-speaking communities.
Has Joss spoken about where the Chinese communities are?
I think they all went home to Blade Runner.
Most of the Chinese in the 'verse ended up on Shinon, the third planet orbiting the White Sun. Mal and crew never got even close to her. It’s Inara’s home planet.
Joss’ explanation was that, after Buffy and Angel, he was allergic to latex. so there were no aliens. The 'verse was a “solar” system. It was two morphenes fewer, so it fit into snappy dialog better.
[pedantic, if obvious statement]Finally, the universe into which a TV series fits is only supposed to be self-consistent in the macro way, if that. Everything that drives the story but doesn’t make internal sense should be ignored. Shit that is excessively niggly should be relegated to Bill Shatner’s “People, get a life,” bin.[that]
Is this multiple-star thing officially sanctioned by Joss? Or is this just fanwankery? There are big problems with assuming a system could have multiple large stars in it. There are binary systems with planets, but generally those stars are so far apart and the planets in so close to one of them that the orbits are stable. Those would not be habitable planets.
The ‘official map of the verse’ shows a system that has three ‘suns’ but everthing orbits around the central star. I think the supposition is that these suns are basically Jupiter-like planets that are just big enough to become brown dwarfs. Unfortunately, one of them is supposed to be a ‘blue sun’ which isn’t really compatible with that.
I wish this hadn’t been done, in any event. I don’t see a problem with a system with only one sun and 70 habitable worlds - especially since most of them seem to be moons. If you start with a bright white star, the habitable zone would be four or five times larger than ours, and the whole system could be much bigger. Put a large planet in the habitable zone, and put 20 or 30 moons around it. Other moons and planets could be habitable through terraforming, artificial greenhouses, etc. That’s a lot easier to believe than FTL travel or a stable system of five stars and 70 planets which are still close enough to each other for non-FTL travel.
No, the multi-star 'verse was fan-wankery, and I was there* when most of the wanks were created. Think of it as a MacGuffin, just there to drive the plot along.
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- I’d asked Tuba to temporarily ban me here. Where the hell ELSE could I post?
- I’d asked Tuba to temporarily ban me here. Where the hell ELSE could I post?
Also they have advanced terra-forming that can make an airless moon into an earth like body, including gravity:
“ZOE: The border worlds – just like the central planets, they’re as close to Earth-That-Was as we could make 'em: atmosphere, gravity and such, but…”
That suggests they have or had access to ridiculous levels of tech at one point. Something that requires no maintenance that can change surface level gravity.
If they can do that they would have a lot easier time terra-forming a small moon without resorting to pressure domes.
[edit]
Oh, my point was that they aren’t habitable planets. They are cold and airless moons made habitable by super-science. For all we know there are solar reflectors bouncing sunlight to the planets even when they are behind some gas giant.
The actual form of the system was never addressed in the series or movie. More recently there was a fan wankery map with the multiple stars around one star idea. But the fan wankery is a brilliant solution and it seems to be officially sanctioned. Besides solving the problem of so many planets within non-FTL distance of each other, it also makes sense of all the references to things like “Blue Sun Corp” and why the English and Chinese populations seem to be somewhat segregated.
It’s not complete fanwank. A lot of it is supported by the data in the Visual Companion, which is “canon.” Who came up with it is irrelevant. If Joss accepts it, it’s “real.”
eta: Or, what jack said.
“Well, are there aliens among us?”
“Yes, and one of them’s a doctor.”