I'm playing the game on Simple setting (still learning the game) and after I had a Defender ship blow away the Andinians' three scout ships it had orbiting Andinian IV

I asked the Andinian's to give me Andinian IV as tribute, and they agreed to give it to me. Great. Only when I go back to galaxy mode, it shows the planet still under the Andinians' possesion, and when I go back into diplomacy mode, it shows the planet under their possesion still as well.