How does atmospheric refraction take place? Why do stars twinkle,but planets do not?

(N/A) Atmospheric refraction occurs due to the gradual change in the refractive index of the Earth's atmosphere. As light from a star enters the atmosphere,it undergoes continuous refraction as it passes through layers of varying density and temperature,causing the star's apparent position to fluctuate and its brightness to change,which we perceive as twinkling.
Planets do not twinkle because they are much closer to the Earth than stars. They appear as extended sources of light rather than point sources. $A$ planet can be considered as a collection of a large number of point-sized sources of light. The total variation in the amount of light entering our eye from all these individual point-sized sources will average out to zero,thereby nullifying the twinkling effect.