How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?

Explanation

The refraction of light by the earth's atmosphere is due to the gradually changing refractive index of air layers. The physical conditions of the refracting medium (air) are not stationary and the apparent position of the object as seen through the hot air fluctuates. This wavering is thus an effect of atmospheric refraction on a small scale in our local environment.
Since, the atmosphere bends light from the astronomical bodies viz, stars towards the normal, the apparent position of the astronomical bodies is slightly different from its actual position. The astronomical bodies appear slightly higher (above) than its actual position when viewed near the horizon.
The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the earth's atmosphere, undergoes refraction continuously before it reaches the earth. The path of rays of light coming from the distant star geos on varying slightly, the apparent position of the star fluctuate and the amount of starlight entering the eye flickers that the star sometimes appears brighters and at some other time fainter which gives us the twinkling effect.
The planets are much closer to the earth and are thus seen as extended sources whereas stars are very distant, the approximate point-sized sources of light. If we consider a plane as a collection of a larger number of point-sized sources of light, the total variation in the amount of light entering our eye from all the individual point-sized sources will average out to zero, thereby nullifying the twinkling effect Hence, the planets do not twinkle.