Why do stars twinkle but planets don't?

Stars twinkle for a reason that has nothing to do with stars. Their light burns perfectly steady; the shimmer is added in the last instant of the journey, as the beam falls through Earth's restless atmosphere. Pockets of warm and cool air drift, rise, and tumble, and each pocket bends light a little differently. A star is so far away that it reaches us as a single pinpoint beam — nudge that beam even slightly and it misses your eye, then finds it again. That rapid arrive-miss-arrive is the twinkle, which astronomers call scintillation — the flickering that turbulent air stamps onto a point of light.

A point versus a disk

Every star but the Sun is effectively a point. Proxima Centauri, our nearest neighbor at about 4.2 light-years, spans roughly a thousandth of an arcsecond in our sky — a dot with no width even the sharpest telescopes on the ground can barely resolve. Planets are trillions of times smaller than most stars, but they sit so much closer that they appear far larger: Jupiter stretches to about 50 arcseconds at its closest approach, tens of thousands of times wider than that stellar point. Your eye can't see Jupiter's disk unaided, but the atmosphere can.

Why the disk stays calm

Think of a planet's disk as thousands of separate points of light packed together. Each point twinkles on its own schedule — one flickers dim just as its neighbor flickers bright. Averaged across the whole disk, the jitters cancel, and the planet shines with borrowed steadiness. A star has no neighbors to average with. One point, one jitter, fully visible.

The exceptions that prove it

Drop a planet near the horizon and its light must plow through far more air; on rough nights even Venus can shimmer. Lift a telescope above the atmosphere and the effect vanishes entirely — Hubble sees every star as an unwavering point. The stars never twinkled. Our air did.