Create mask so parts of background not affected

This would look so much nicer in many pics if I could mask out the part(s) of the background pic that should not be animated. I created a nice wallpaper from a drawing of a moon rising over a barren, mountainous planet. Water in the foreground has ripples, stars slowly climb in the sky with an occasional meteor flying be. But the whole impact is ruined when stars and meteors go in front of the moon or the taller mountains - there's no way to paint the area affected, or block out the area not affected, because the stars and meteors work from a horizon line.