Why Do Stars Twinkle More than Planets?

Stars do not brighten and fade intrinsically every few seconds. Most visible twinkling occurs during the final part of starlight's journey through Earth's atmosphere. Moving pockets of air differ in temperature and density, so their refractive index changes. They rapidly alter the apparent direction and intensity of the light reaching an observer.

The crucial factor is not distance alone but angular size. A star is enormous, yet so distant that it behaves like a point source to the eye and to ordinary telescopes. When one turbulent patch bends that narrow bundle of light, its apparent position, brightness and sometimes colour change directly. A planet is much closer. Although it also looks point-like to the unaided eye, it is actually a tiny disc. Light from different parts of that disc travels through slightly different atmospheric paths. One part may brighten while another dims, and many fluctuations are averaged together by the eye, making the planet appear steadier.

This is not an absolute rule. A planet near the horizon can scintillate when the air is especially turbulent. Stars also tend to twinkle more near the horizon because their light crosses a longer path through the atmosphere. A space telescope, operating above the atmosphere, does not experience this Earth-made twinkling.

Twinkling can therefore help distinguish stars from planets, but it is not a test of whether the object itself shines steadily. It reveals how the atmosphere is disturbing an almost point-like source of light.

https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question26.html
https://imagine.gsfc.nasa.gov/ask_astro/night_sky.html