The Problem
Players would like a method for abandoning unwanted worlds. Players do have a mechanism for doing this- by pulling off resources until the world goes into civil war and then intentionally losing. This is super awkward.
A Solution
It should be possible to abandon a world manually.
Provided that imperial might remains pegged to total population, it will be necessary to limit the number of planets that can be abandoned in a given cycle to between 1% and 5% of an empire's total planets (rounded up). Otherwise, players will exploit abandonment to blatantly manipulate their imperial might and wage penalty-free war against otherwise prohibited empires.
It seems like a betrayal of some kind to extract all the resources of a world and leave the population to fend for themselves. I think abandoning a world shouldn't destroy any improvements, and there should be a dissent penalty for nearby worlds when one abandons a world without sufficient military defenses (it certainly looks like literal abandonment too).