Statements :
a) The motion of the centre of mass can be studied using kepler's laws of motion.
b) The internal forces will not effect the motion of the centre of mass

For a system of particles , F_ext = M _sys . a_cm

In a system of particles , the particles may interact with each other through gravitational force , electro magnetic force and any other real force . These forces are action - reaction pairs and cancel each other . there internal forces have no contribution to the net external force . Hence statement - (b) is true