Forgetting the aliens, I don't know if colonizing the solar system would be a plot I would label as "Space Opera" which I generally think of as grand large scale adventure with heroes and villains and so on.
Oh, don't underestimate the scale of the Solar System. We're used to seeing it portrayed as small, but it's truly vast, with worlds that take months or years to reach, countless exotic environments that would take centuries to colonize, and whole outer reaches that we've barely even begun to discover yet. If you define the Solar System as including everything out to the Oort Cloud, that makes it about two light-years in diameter. And there's enough asteroidal material out there to be converted into thousands or millions of artificial worlds, which could easily diverge into many distinct cultures over the course of centuries or millennia.
So it would be easy to tell a Solar System-based series that's just as expansive in its effective scope as the interstellar series we're used to. Most such fiction grossly understates the sheer distances involved in space travel for story convenience, as well as grossly oversimplifying the vastness and complexity of individual planets. They treat whole planets as small places with monolithic cultures; an artificial space habitat could more plausibly serve an identical story function. They use imaginary FTL drives to allow manageable interstellar travel times; a series without FTL could have the same sense of distance (or lack thereof) for travel between worlds or habitats within a single system. They populate extrasolar planets with aliens that are basically variations on humanity; a Solar System-based series with thousands of colonies that have diverged culturally and practiced genetic engineering over the course of centuries could include numerous such "humanoid" civilizations with a much more plausible origin.
Heck, I've written an original spec novel that's set almost entirely in the Asteroid Belt, and I was able to establish numerous distinct cultures and tell a sweeping, far-ranging story. We think of the Belt as being just a uniform clutter of rocks, but there are multiple regions within it with their own distinct characteristics that would influence the cultures that settled in them.
Space opera is, generically, science fiction set in space where the stories turn on standard action/adventure or dramatic tropes and have little to do with speculation of any kind.
Interesting. I've never heard that definition of it before. The original definition, as I understand it, was space-based SF that was epic and operatic in scope, focusing on grandiose battles of good and evil, but lacking much depth to its characters -- much like
Star Wars, though of course the exemplar is the pulp fiction of "Doc" Smith and the like. These days, I've had the impression that it's become more a generic term for space-based SF in general, initially as a derogatory term but less so these days. I've heard the writings of people like Egan and Baxter and Vinge called "the new space opera," and nobody could say their fiction is lacking in speculation.
