Have you ever looked up at the sky at night and wondered why stars twinkle? Its actually quite simple, they twinkle because of winds and turbulence in the atmosphere of the Earth. As the atmosphere churns, the light from the star is refracted in different directions.
As the star's light is refracted the image changes slightly in brightness and position leading to the "twinkle." The Hubble telescope is successful in space because there is no atmosphere to make the stars "twinkle", allowing a much better image to be obtained.
Planets do not twinkle the way stars do because planet's have a finite size in relation to earth. Stars are so far away that they are essentially points of light on the sky. The size of a planet in the sky is always a relatively stable image to the eye.
For a closer look at why stars twinkle you can check out the original article Cornell.edu