Not much is known of the AIM-9X but it is probably similar to the AIM-9L. The AIM-9L doesn't lock on to the hottest thing, it takes an IR picture of the scene

Not much is known of the AIM-9X but it is probably similar to the AIM-9L. The AIM-9L doesn't lock on to the hottest thing, it takes an IR picture of the scene and follows the targeted "blob" in the image that's generated. Additionally it has laser proximity fusing so it knows when it is close to the shape it is tracking. By not simply picking the hottest thing in a scene you can't hide in front of the sun anymore, and the effectiveness of flares is reduced. The only thing needed to generate a "blob" good enough to target is sufficient ΔT between the scene and the targeted object. The objects, and they have been curiously consistent with calling them "objects", definitely have sufficient ΔT compared to the background sky if they are made of any solid material that has been exposed to sunlight for any period of time.