Very odd. This is the only quote I could find from Steve in the changes thread. He invaded several planets in one of his AARs, so it should be safe to assume that the feature was implemented at some point.
If only one side has ground forces present, there may be a conquest (rules and code TBD).
How long has it been since the other side's ground forces were destroyed?
Multiple months - however I've realised something. I have a ship with ELINT in orbit since the population contact is still there with around 1k thermal and EM signature. Because of my prior missile bombing the raditation and dust have rendered the planet uninhabitable. All civilian population is dead and as of yes I haven't detected any facilities either.
There being no facilities is interesting since I doubt they were all destroyed however the conquest might not be triggering because I haven't left much in the way to be conquered.
As I get more intel I might find out what is generating the signature given time.