The light coming from star get refracted through the atmospheric layers. Due to this, the star appear slightly higher than its actual position. Since the densities of atmospheric layers changes frequently and hence the position of the star appear changes continuously stars are very distant point-sized objects. As the path of rays of light coming from star is continuously changes. So sometimes light enters into our eyes and some times it faints. This is twinkling of the star. Since planets are closer to us, their size is also big. Our eyes receive light from different points of the planets. Hence planets do not twinkle.