Why do stars twinkle and planets do not?
Answer: The density of different layers of the atmosphere is constantly changing. As density changes, the refractive index of the medium also changes. Stars are far away from Earth and the rays coming from the stars pass through different density layers of the atmosphere before reaching the Earth. But because the density and refraction of those atmospheric layers change every moment, the path of the light rays as they pass through them also changes every moment. Because of this, sometimes more light and sometimes less light reaches our eyes. So it appears that the stars are twinkling.
As the planets are closer to the Earth than the stars, the light from the planets is greater. The small decrease in the brightness of incoming light due to refraction in the atmosphere is not easily understood. Hence the light of the planets appears to be constant.