From Longman Dictionary of Contemporary Englishsomebody is (living) on another planet/what planet is somebody on?somebody is (living) on another planet/what planet is somebody on?spokenSTUPID/NOT SENSIBLE used humorously to say that someone’s ideas are not at all practical or sensible He thinks motherhood is glamorous – what planet is he on? → planet
Examples from the Corpus
somebody is (living) on another planet/what planet is somebody on?• As a replacement for the Bluebird, the Primera is on another planet.• People in the Antelope Valley worry that most people south of the mountains think that their valley is on another planet. 