1 [count] : a large, round object in space (such as the Earth) that travels around a star (such as the sun)2 the planet : the planet Earth
▪ We must help preserve the planet. = We must help preserve the planet's environment.3 — 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.
▪ We must help preserve the planet. = We must help preserve the planet's environment.