Presuming you didn't obliterate the populations existance, how about a set of options after post-invasion both for if the invaders were unsuccessful and successful.
When a planet is captured and their exists a population still, the invaders can Assimilate, literally becoming a national planet. Like Hearts-Of-Iron, Paradox, Puppet-Regime which causes the planet to breakoff from their Empire and become Independent including politicaly and economicaly becoming closer to your Empire.
Extermination is when you wish to terminate the entire population but the results mean extended casualities to Army and small pockets of civilians meaning when colonized by vice versa they will join that population or become Partisans and cause trouble for awhile until eliminated.
Every option depending upon morale and relationship of planet and Empire affects your ability to conduct the diplomatic option, if a planet that has suffered much unrest and is invaded by your empire, their should be a lack of volunteers to the Repel effort along with postive results afterwards especially if you have used any Biological weapons, Nuclear, Bombardment, etc.
