I felt so embarassed that I finally had to find it out and now I am writing this short post about it. For few years, roughly, I am studying astronomy yet, I never knew why stars twinkle and planets not. I confess.
Stars twinkle because the light that reaches us goes through atmosphere and atmosphere is not very homogenous – smooth. Air refracts light and there is different temperature once in a while, humidity and so on, I think that lot of factors play the role. This causes the light of star to scatter a bit and creates the twinkling effect.
Planets do not do it. This is great because you can identify them extremely fast on the sky and you do not mistake them for some other bright star. Why? Their light still goes through atmosphere. Because they are not “point sources”. Stars are so far away that even with best telescopes we see them only as points. Planets with simple telescope on backyard already have shape. Some of their light scatters one direction, some the other and it basically cancels out creating nice image. This is also why it is better to go star-gazing in the winter, colder air does not create so much “noise” on the picture.