Star is far away, we only see a tiny beam of light from it. They Twinkle Because of different layer of atmosphere has different density and so the refraction of the light. With the change of the this density the refection changes and we miss this tiny light of star and receive the same from different path and feel it is twinkle.
Where as planets are very near to earth as compare to stars and we don't get the tiny beam from as the case of Stars so we don't miss this light with the change of density of atmospheric layer hence we don't see the twinkle effect.