Er lebt auf seinem eigenen Stern means “he’s living on his own planet” – almost literally, since Stern is a star. The figurative meaning is the same in German and English.
So much for translating… This is my contribution for Thursday’s Special at Lost in Translation where you can pick a word from this list: radiant | alimentary | frontal | arboreal | remote. I picked the first one, and maybe the last.
And there is also an alternative take.