Not that planet, it's haunted

Well, a lot of people believe in ghosts, Many People also believe in Aliens.

Well I took these two quite disparate beliefs and mushed them together. You see, Aliens DO come to our planet. But only BRAVE aliens. For you see, Humans are the only species that can leave ghosts. so thusly our planet is HAUNTED! no one wants to spend a night on a haunted planet! I know I don't! and the aliens don't either! Unless they are dared to do so, just like when a child is dared to say "bloody mary" in a mirror some arbitrary amount of times. So in conclusion, our planet is haunted and spooky and nobody likes it.