Developed planets with large populations should not flip in a single turn. Let ground wars play out like ground wars.
Invasion and conquer should be a gradual process. You bring your space marines (or whatever) to the planet and they invade. Depending on the size of the population and the number of troops present, a state of war might last for a few turns or dozens. Or it might stalemate altogether, with neither side able to gain advantage. You need to get more troops to tip the odds. While the war wages on, the planet is out of commission.
And, if you are able to invade with the element of surprise, your enemy would be able to send ships to reinforce the defenders or drive off the attackers. Or research new technologies to make its fighters stronger and win the battle that way. Or appeal to the UP to stop the war. Or any number of other alternative strategies.
Think of a scenario where you've just landed marines to take a crucial planet deep in your enemy's territory, only to have the enemy destroy your fleet on the next turn. Now your invading marines are stranded, surrounded by the enemy, fighting a war of attrition in the hopes that you might be able to reinforce them.