I don’t really know why you would want to build a city on a Kuiper Belt Object (KBO), but then again, I’m not an alien.
Loeb and Turner (2012) make an argument that artificial lighting could be a good universal “lamppost”: something that all technologically advanced species would do so as not to be subject to the whims of a diurnal cycle. I can think of quite a few problems with that: a species that evolved in a sub-surface ocean, species on tidally-locked worlds that never had a diurnal cycle, the ever-popular “post-biological life”, etc.
But okay, sure, let’s say everyone needs street-lamps. With current technology, we would be able to detect artificial lighting (on the scale of a large terrestrial city) on KBOs in our own solar system. Regardless of plausibility, that’s pretty cool!
That part is in bold because it’s obviously the way this paper came about. I don’t think there are any good arguments for why KBOs are the best place to search for extraterrestrial life. I’m open to being proven wrong, but the paper reads as a fun thought experiment based on a new technological capability rather than any serious suggestion for how humanity can find another intelligent species.
The authors discuss a characteristic “flux-distance signature” that an artificially illuminated object would have. Based on a double r^2 relation (one in sunlight reaching the object, one for the backscattered light), a KBO that’s just reflecting naturally due to its albedo should increase in brightness by a factor of r^4 as it comes closer to the sun. Meanwhile, an object dominated by artificial illumination would only increase by r^2 as its source of luminosity approaches. Thus, if we notice any objects with this r^2 relation, we should really take note because that would be really weird.
The authors also discuss caveats and confounders of this idea (phase angles, outgassing, albedo variations, rotation, binary companions…), but indicate that all of these should be periodic with the exception of outgassing and should average out over years of observation. They also briefly talk about how this idea could be applied to exoplanets (with phase curves and such), but it doesn’t seem like the the technology is there yet.
This definitely felt to me like a good example of Davies and Wagner’s proposal of cost before plausibility in SETI work (discussed in a previous post). They outline the logic behind the method pretty well, but if they were actually interested in the results instead of the theory they should’ve looked more into which pre-existing datasets could be used to attempt this kind of work. Because if someone* was interested in actually testing this idea, and not waiting for LSST etc., it would be nice to be able to hit the ground running on it.
As I indicated in my first sentence, I don’t really see a reason why there should be artificial lights on a KBO. They’re cold, they’re small, they don’t have thick atmospheres, they have no access to non-Kuiper Belt resources (what, you wanted something other than ice and dust?), etc. That said, if the search is easy to do and we can clear out some parameter space… perhaps it’s worthwhile.