Simple planet atmosphere

I'm making a game where the player can explore around some planets. in order to cut down on rendering times, when you enter a planets atmosphere the scene will change. but i don't want to render the entire planet underneath so i'm looking for a code which can create a simple dense atmosphere that you can't/ barely see through. i can't find anything on the subject and im hoping you guys can help.

also, i am a noob with shaders. hopefully this doesnt come across as a silly question.