Unpacking some Postcards from Cutty
Cutty is exploring an Oort Cloud system similar to the Pluto system - two large bodies along with a smattering of tiny moons. Actually, it's only 550AU away, so we would currently probably call it a "Detached Object".
Why 550AU away? The week-long lightspeed delays make communications feel like "postcards", but it's still close enough to reach in a reasonable time (4,823 days, or 13 years, at 200km/s).
Cutty discovers alien robotic "life" in the system, which prompts the main story to begin - various space agencies on Earth send missions to Cutty's system.
Which is another reason for 550AU away. The alien robots do not have what we'd think of as a starship. Instead of physically travelling from one system to another, they use interstellar laser lithography to directly manufacture new robots on far away worlds. To achieve these extreme ranges, they use gravity lensing. Both the source and destination stars are used, and for our Sun the ideal gravity lens focal length is 550AU.
So, this is why the alien robots hang around a system 550AU away. This is prime siting for what they do. They have no use for inner system planetary bodies.
Still. This leaves some inconvenient questions.
1) How the heck do they know when/where to aim their beams? Obviously, they can wait for opportunities when their orbit around the Sun passes near alignment with another target star. But how the heck can they tell where a target body will be when their beam arrives? Just shooting blindly will almost certainly not hit anything.
2) What is the power source? The best I've come up with is geothermal refreshed by the occasional fusion detonation - but it's unclear how to achieve these fusion detonations without fission primaries.
I think the latter problem is not really a big deal, but the former problem is just plain mystifying. This system offers fast interstellar VN colonization without the stupendous energies required to accelerate a physical starship to relativistic speeds. But how the heck does ...
Hmm. I think I've finally figured out the answer.
The basic problem is that there's no plausible way to detect an Oort body 550AU away from a star. But there are
plausible ways to detect and characterize an asteroid much closer to the star. So, the aliens detect a suitable asteroid in a star's inner system. Then, they use a pulse laser focused by one-sided gravitational lens in order to nudge that asteroid into an orbit 550AU away from the star. This does not require the extreme resolution of interstellar lithography. Simply blast the entire side of the asteroid, and it will react by accelerating in the opposite direction. This asteroid, suffering repeated laser blasts, can be readily tracked even as it goes too far away from the star for sunlight detection.
When the asteroid reaches the desired position 550AU away from its star, the double-ended gravity lens is used for interstellar lithography. At that point, this "seed" VN probe can search for larger natural bodies nearby to sustain its mission.