Why do stars twinkle, while the visible planets (and our Moon) shine constantly when viewed?
First, the stars themselves do not twinkle. We are seeing the effect of Earth’s atmosphere on the light they produce. The starlight is bent by moving volumes of air in our atmosphere. The bending effect makes the stars appear to be larger than points, to dance around slightly, and to change in intensity, which is what is commonly called twinkling. Planets don’t usually appear to twinkle because they are close enough to Earth that they appear as tiny disks of light. The total intensity doesn’t seem to change — hence no twinkling is apparent to the naked eye.