Why Do Stars Twinkle but Planets Usually Don’t?

Stars appear to twinkle because their light passes through Earth’s atmosphere. Different layers of air have different temperatures and densities, so they continuously bend the starlight in slightly different directions.

Stars are extremely far away and appear as point sources of light, so these changes make their brightness seem to fluctuate.

Planets are much closer and appear as tiny disks rather than point sources. Light comes from many points across the disk, and the fluctuations from different parts tend to cancel each other out.