COMMON You can say what planet is someone on? to show that you think someone has crazy ideas or does not know about something that most people know about. What planet are these people on? Do they not read their own headlines? Note: This expression is often varied. For example, you can ask what planet does someone live on? or what planet does someone come from?. He thinks hospitals can make major cuts to their budgets. What planet does he live on? Note: You can also say that someone is on another planet. She was just on another planet, I think. She didn't seem to understand that we are ordinary people.