Why do stars twinkle but planets appear steady in sky?

Stars twinkle because they are so incredibly far away that they appear as point sources of light, essentially dimensionless dots. As their light passes through Earth's atmosphere, small pockets of air with varying temperatures and densities refract the light in constantly shifting directions, causing the brightness and position to fluctuate rapidly, which we see as twinkling. Planets, while also distant, are close enough that they appear as tiny disks rather than pure points of light. This disk subtends a small but measurable angle, so the light arrives from multiple slightly different directions simultaneously. The atmospheric distortions affecting different parts of the disk average out, producing a steadier, more stable appearance to the naked eye.