Since you are looking at a warm planet you will never get your camera down to 7 K nor do you need to as all sources are close by and bright compared to the ast

Since you are looking at a warm planet you will never get your camera down to 7 K nor do you need to as all sources are close by and bright compared to the astronomical targets for JWST. So the shielding issue is somewhat different. Having Earth, Moon and Sun in the same direction so you can block all of them with a single heat shield is actually one of the reasons why JWST is going to the L2 Lagrange point.