why do the Planets not twinkle?.

Explanation

(i) Planets are much closer to us as compared to stars.
(ii) They, therefore do not appear as point sources but appear as a collection of point sources.
(iii) Changes in atmospheric refractive index changes the position as well as the brightness of individual point source, but the average position and total average brightness remains unchanged.
(iv) As a result of this planets do not appear to twinkle.