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Planetary invasions aren't in the game yet. You can (with research in Xeno-sciences) have an enemy-held planet surrender to you (at least, the tech is in place, I haven't tried it yet) like in SotS1.
However, other than that, your options are to wipe the other guys off the planet, and then colonize it yourself. SotS doesn't have planetary "invasions" in the sense of how many other 4x games do. I kinda miss them.