Yep, though you have to have a high combat rating or whatever its called. Then you communicate with the planet and click 'demand tribute'. Either they will tell you that you are too puny and to get lost, or they will take offense and launch, as Flanjoo said, wave after wave of defense fleets. If you beat the fleet, you own the planet. After that the planet will pay you a daily tribute, and i think your combat rating becomes 'overlord' or something (been a while since i played now so sorry the details are a little sketchy). Worth doing for some places but don't even bother trying it on any home planet (eg anything in our solar system: the earth, the moon ad mars) as the fleets that come out are very very big indeed.