Think Star Trek ... but way different. Humans have colonized part of the solar system, but they have never gone beyond Saturn (at least the known colonies). They don't have ships that are fast enough to make it worthwhile.
It's way in the future. I don't know how far. Let's just say that what we know of Earth is no longer the case. Think outside the box. What exactly happened to Earth ... I don't know. That's for you to figure out.
Why did humans colonize part of the solar system? Did it have something to do with what happened on Earth? Perhaps. Perhaps not.
How many people live off world? How many people live on planet Earth? I don't know the answer to either one. That's for you to figure out as well.
Here's what I do know
Humans do not have the technology to make travel past Saturn worthwhile. That means no hyperspace (warp engines).
Perhaps they do have some sort of impulse power. Obviously not as powerful as the impulse engines on Star Trek. That would make interstellar travel possible, though perhaps still a very long trip.
Let's say that it takes at least a year to get out to Saturn—perhaps even a few years. For that reason, there haven't been manned craft to go beyond Saturn, at least in recent memory. (Hint, hint)
There are numerous off world colonies: on the moon, on and around Mars, the asteroid belt, and even a base or two orbiting Jupiter (or on one of its moons). Several nearby colonies can be part of the same country, empire, or culture, but since travel takes a while, no one entity controls the entire solar system. (Though if someone figures out how and make it work ... I'm all for it)
In fact, trade and other relations among the various sections of the solar system is somewhat limited due to travel times. In other words, not like it is for us today. Think of what it was like before the modern era. Worldwide trade was done ... but under much harsher conditions. It took months to travel along trade routes before the use of modern technology. In other words, it isn't a one day journey from the moon to Mars. Keep in mind your distances.
Aliens have thus far not been discovered. There's been no first contact. Space is pretty big, remember? And although I won't veto aliens altogether, let's keep the number to a minimum. If we do introduce an alien species (and we don't necessarily have to in order to create a good story), one species will be enough to create a ton of conflict.
Another source of new characters, almost like a new species, and I hinted at this before: though no one has gone beyond Saturn in recent memory, that doesn't mean they never did. Perhaps many generations ago there were a few pioneering vessels that ventured out of the solar system. Everyone has forgotten about them. What became of them all these years later?
Another idea is to use the alien angle along with the ancient human angle. And I'll leave that clue right there for you to explore if you so choose.
Rules of the Game
Third person. (In other words, no First Person ... except for dialog, of course)
You may write for as many characters as you like, though it is preferable to have one per page/chapter.
You may not kill another writer's character (though I have another COLAB in mind for that). Unless of course the writer is in on it, and it's a planned killing. In that case, by all means. And I encourage this since a writer can pick up another character if he/she so chooses.
I'm not going to start the story. I'd rather jump in after someone else. So, in that case ... who's first?