Many space operas tend to treat empires spanning multiple solar systems much like we would treat empires on our own world, complete with borders and territorial maps included on the characters’ computers. Just one look at the surrounding stars and they know that they’re in alien territory, ready to be greeted by a space-borne version of the interstellar empire’s border patrols. But considering that not only is space three dimensional, it involves stunning distances between objects, could a species carve out a large territory in space and be able to control the borders to its territory? Would it even be able to define them? And would it even matter to have a firmly delineated border between their space and the rest of the galaxy’s? Maybe borders of an alien empires would be extremely porous, extending for dozens of light years, a sort of a buffer or transition zone throughout which their presence becomes more and more prominent and they have a chance to detect intruders? And if they do spot a wandering craft, will it be worth it to them to send out an encounter team to figure out what this craft is, then drive it off rather than try to study it? In other words, how could an alien empire be defined?
One idea of how to define ownership of multiple planets may be as simple as counting only the planets which house outposts of a space-faring species. Rather than be marked by invisible lines, aliens may jumble each others’ holdings and three planets within the same solar system, or three solar systems side by side may be alternatively claimed by one of two species. For example, let’s say that future humans would lay claim to Mars, Titan, Triton, Europa, Mercury, along with the Earth and the Moon, and own several planets around two nearby stars. At the same time, another species claims Venus, Pluto, Ganymede, plus several planets around other stars. The idea is to count the worlds on which you actually have a presence and are actively inhabiting, which makes the idea of sovereign borders relatively easy to enforce. You set up patrols only around the worlds you inhabit and watch for incoming species rather than safeguarding empty space. Plus, by giving worlds to other species if you have no use for them could facilitate a sort of unspoken truce. Everyone gets want they want as long as they don’t start flashing lasers and kinetic kill vehicles and should be willing to trade for any common resource both require. Of course such commonalities could also start conflicts, but more on that in a bit.
The other, more science-fiction like scenario is one where territory is marked by considering the beginning of sovereign cosmic holdings to be the farthest outposts patrolled or explored by a species. In this scenario, any future humans landing on a planet 25 light years away have now claimed the entire target solar system along with all the solar systems along the way to their destination. It doesn’t matter how many of the worlds they will actually inhabit, all that matters is their extent. But of course this would also allow intelligent species to hold a vast cosmic empire each because the distances between them are likely to be very significant. There’s a very strong possibility that two advanced, space-faring species could live thousands of light years apart with many thousands of years separating their rise to power and acquisitions. Suddenly, as they begin to explore, rather than having to share space with hundreds of competing species, they can lay claim to several thousand cubic light years of space without the slightest challenge. Of course the big question is how they’ll mark it as theirs, especially in a way a completely alien entity would recognize as a territorial claim. One can’t just build a Great Space Wall and line it with turrets and watch towers, and detecting incoming craft with probes would require a vast swarm of robots numbering in the billions if not trillions. It could well be practically unfeasible.
And this brings us to a dilemma. What good are borders when they’re going to be that porous and the odds of another species showing up to deliberately challenge them are so remote? This is especially true when we’re dealing with immense territories claimed to unchallenged species. Thousands of light years means millions of planets around millions of stars and an empire that big simply cannot be policed. Just like some of the vast empires on our planet learned, laying claim to an enormous territory doesn’t mean you’ll ever control it. Maybe you can reach it and survey what goes on, but odds are that anything outside of your immediate habitat would just develop on its own with little to no input from you. Species could rise, leave their cradles, and fall within a wide swath of space you claim without you knowing they exist and without them ever learning that they’re your subjects, evolved on a planet you claimed millions of years ago. Even more interesting would be the question of how you would submit your claim and actually have it recognized and announced. On Earth we have maps, international organizations, and authoritative bodies which maintain official border designations, and yet even here borders are contested. What central authority would mediate border disputes between aliens, especially when, as we’ve just seen, a cosmic border is so hard to define and locate in the first place?