Why stars twinkle but planets don’t?I am clear with the stars but don’t know about planets

Dear Suyash
 
Stars twinkle because … they’re so far away from Earth that, even through large telescopes, they appear only as pinpoints. And it’s easy for Earth’s atmosphere to disturb the pinpoint light of a star. As a star’s light pierces our atmosphere, each single stream of starlight is refracted – caused to change direction, slightly – by the various temperature and density layers in Earth’s atmosphere. You might think of it as the light traveling a zig-zag path to our eyes, instead of the straight path the light would travel if Earth didn’t have an atmosphere.

Planets shine more steadily because … they’re closer to Earth and so appear not as pinpoints, but as tiny disks in our sky. You’d could see planets as disks if you looked through a telescope, while stars would remain pinpoints. The light from these little disks is also refracted by Earth’s atmosphere, as it travels toward our eyes. But – while the light from one edge of a planet’s disk might be forced to “zig” one way – light from the opposite edge of the disk might be “zagging” in an opposite way. The zigs and zags of light from a planetary disk cancel each other out, and that’s why planets appear to shine steadily.

Regards
Arun (askIITians forum expert)