Technically, Space Opera does not need aliens.
Firefly had no aliens and was fine. In the literature, the (real)
Foundation series had no aliens, although that was done specifically to dodge Campbell's ethnocentrism. Arthur C Clarke wrote
The Songs Of Distant Earth specifically to create a Hard SF Space Opera; others of his books, such as
Imperial Earth, could qualify as well. Ben Bova's Solar System books have no aliens as far as I know, except for some ruins on Mars (I may be wrong about that-- I haven't read them all).
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.
Exactly. In fact, this is the premise of my own Space-Opera-in-progress.