Why do stars twinkle but planets don't?

1steady star

A star's light never flickers. In space it shines perfectly steady. The twinkling happens at the very end of its trip, in the air above you.

2wobbly air

Air isn't uniform. Pockets of warm and cold air have different densities, and each bends light a little. They swirl, so the path wobbles hundreds of times a second.

3one thread

A star is so far away it is just a point: Sirius is only 0.006 arcseconds wide. All its light squeezes down one thread, so the whole star jumps and flickers.

4a planet is a disc

A planet is different. It is close enough to be a tiny disc: Jupiter is thirty to fifty arcseconds across. Light from each spot of the disc takes its own path.

5it averages out

Some spots flare while others fade, and the flickers average out. So the planet glows steadily. Not perfectly: low near the horizon, even planets shimmer.

6side by side

So here's the whole idea. A star is one thread of light and the air shakes it. A planet is many threads that average out. Same air, different sizes.