Spent a moment thinking about this and I think there’s an implied definition for what “on earth” means that we intuitively accept but don’t ever really need to state.

If your projected free-fall trajectory both forward and backward in time intersects with the surface of the earth then you are “on earth”.

Standing on the ground? Intersects twice. Thrown rock? Intersects twice. Person in an airplane? Intersects twice. ISS? No intersection. Incoming impact meteor? One intersection.