Why do stars twinkle but planets do not?
— Christie Williams, Springfield, Ill.
It's because of the difference in the visual sizes of stars versus planets. Although stars are much larger than the planets in our solar system, the planets appear larger because they are so much closer to us.
All of the light from any given star comes through the atmosphere and to our eyes from exactly the same direction and is bent in exactly the same way by turbulence in the air. A star will “disappear” briefly when its light is bent away from our eyes, then reappear when its light is not bent off course to our eyes.
The light from planets, traveling to our eyes along many different paths, is not all simultaneously bent away by atmospheric turbulence, and so the planets do not twinkle.