Question

why does the planet not Twinkle

Answer 1

planets do not twinkle because they are much nearer to the earth and are treated as a collection of large point sources of light.
the amount of light entering the eye is constant.
so planets do not twinkle

Answer 2

Planet is near to earth so it can be considered as a collection of numerous point like sources of light. So, the twinkling effect produced by some of the point sources in one part of planet is nullified by the brighter effect produced by the point sources of light in its other part. Thus as a whole, the brightness of a planet always remains the same and hence it does not twinkle.
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