Stars twinkle. Planets shine steadily. Why?
Stars always twinkle because they’re so far away from Earth that, even through large telescopes, they appear only as pinpoints. And it’s easy for Earth’s atmosphere to disturb the pinpoint light of a star.
As a star’s light pierces our atmosphere, each single stream of starlight is forced by the atmosphere to zig and zag this way and that. . . . and so stars appear to twinkle. On the other hand, planets don’t twinkle (usually) simply because they’re closer to Earth. You’d know they’re closer if you looked through a telescope. Through telescopes, planets don’t look like pinpoints. Instead, they look like tiny disks. And while the light from one edge of a planet’s disk might be forced to “zig” by Earth’s atmosphere, light from the opposite edge of the disk might “zag” in an opposite way. The zigs and zags cancel each other out . . . and that’s why planets appear to shine steadily.
It’s pretty tough to figure out which objects are stars and which are planets just by looking for the twinklers vs the non-twinklers. But if you can recognize a planet in some other way, you might notice the steadiness of its light by contrasting it to a nearby star.
By the way, if you could see them both stars and planets from outer space, both would shine steadily. There’d be no atmosphere to disturb the steady streaming of their light.
What’s more – while it’s true that, for the most part, planets don’t twinkle – you might see them twinkling a little if you spot them low in the sky. That’s because, in the direction of any horizon, you’re looking through more atmosphere than when you look overhead. Even planets can’t withstand too much atmosphere, because it’s the atmosphere that makes them twinkle!