be on a different planet

To be completely out of touch with or oblivious to reality. Are you on a different planet? Of course you can get arrested for breaking and entering! If you're on one end of the political spectrum, people at the other end seem like they're on a different planet. A: "It's like those rich kids are on a different planet with no idea how expensive prom really is." B: "Well, yeah, because their parents pay for their tuxes, and corsages, and limos."