system
Given the following post, respond as if you're commenting on Reddit:
human
Why do stars appear to flicker but planets don't?

Is it because the planet is close and it is much less likely that another object passed in front of it before the light reached us? Or does it have something to do with emitting light versus reflecting light? or something else?
gpt
It has nothing to do with the distance the light travels, because all the twinkling we see happens in the last 50 km the light has to travel. The Earth's atmosphere is solely responsible for twinkling (and is a major advantage offered by telescopes in low Earth orbit, such as Hubble).

The reason stars twinkle and planets do not has to do with the apparent size of the object. If you look at most planets through even a small telescope, you'll see that they show a tiny disc. Almost any star in a telescope, though, will show a point, because it's well beyond the resolution limit of the telescope.

Now, picture our atmosphere as a roiling, turbulent fluid made of individual turbulence cells each moving around with their own slightly different densities/temperatures. Since stars are so small from our perspective that they can be treated as point sources, the light from a star is a pencil-thin beam that can only ever pass through one turbulence cell at a time. Whatever the temperature is of one turbulence cell along that path at any time, the light has to follow. Sometimes it refracts right, another cell quickly moves into place and it refracts left, etc. - and what you're left with is twinkling.

The light beam coming from a planet, on the other hand, is much wider, and will usually pass through multiple turbulence cells at a time. The light's direction is not completely determined by a single turbulence cell's temperature, and instead the light tends to follow the path determined by the average of all the cells' temperatures it passes through. That's much more steady, and doesn't produce twinkling.

**TL;DR**: From our perspective, stars are point sources, but planets are not. Light from stars can pass through only one turbulence cell in Earth's atmosphere at a time, while light from planets pass through multiple cells at a time, averaging out any individual variations.