In Galciv II if approval rating gets to low on a planet then tiles should gradually fall into anarchy as goverment control breaks down and the citizens riot, anarchy tiles produce nothing and halve the production of neigboring tiles which are forced to divert resources to law enforcement in order to contain the unrest. Rapidly anarchy tiles, if morale in the planet is low enough should snowball out into the neigboring tiles which would then fall into anarchy themselves up until the rebels control enough of the planet to sieze control completely, at which point the anarchy tiles dissipate (which doesn't matter that much since, congratulations the planet in no longer yours) and gains independance, either spawning a civilization of it's own (extremely likely to happen if the rebellious subjects are aliens) , or defecting to an existing civilization.