My friend, what's up?
Me: What's up in space is planets and stars.
My friend, what's up?
Me: What's up in space is planets and stars.
Alright listen up buttercup. The joke is about the double meaning of "what's up." Your friend is asking how you are doing, but you respond by listing things that are up in space. You think you're smart, but you're just a space cadet!