To be on another planet
Definition 1 of 2
Pro Tip 1/3Daydreaming Expression
Use when someone is not paying attention, as if lost in their own thoughts. 
I called you three times, are you on another planet?
Pro Tip 2/3Informal Speech
It's a casual phrase, more suitable for conversations among friends, not formal settings. 
During the meeting, he was so quiet, like he was on another planet.
Pro Tip 3/3Figurative Language
Don't take it literally; it's a way to express someone's disconnection from the situation. 
You must be on another planet if you didn't notice the surprise party preparations.