Learner's definition of PLANET
a large, round object in space (such as the Earth) that travels around a star (such as the sun)
the planet Earth
— used in informal phrases to say that someone is not aware of what is really happening or has ideas that are not realistic or practical
You think they'll help us? Are you (living) on another planet?
What planet are you living on?
She looked at me as if I were someone from another planet.