The twinkling of stars is caused by atmospheric fluctuations distorting their light before it reaches your eyes. Stars which appear lower in the sky, seem to twinkle more, since the thickness of atmosphere through which you are viewing them is much greater than those appearing high in the sky.

Although stars are huge fiery balls of gas; they are many, many light years away, and therefore appear as a point of light. The position of this point darts about with the atmospheric distortion, and never appears in the same place for long. It can also appear to disappear briefly, and change colour rapidly. This gives the twinkling effect.

Planets, on the other hand, are smaller than stars. They are also much closer. If you view a planet through even low powered binoculars, you will resolve a disc rather than a point of light. So, the atmospheric turbulence effect on the disc will be much smaller, as it will be averaged out across the face of the disc. You may see the edges or details shimmer slightly, but most of the disc will be fixed. To the naked eye, the edge effects are invisible.

No matter how much you magnify a star (within the capabilities of current optical systems), you will always see a point of light.