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Because of an optical illusion. What actually blinks is not the stars, but the images we see of them. Bright light from these celestial bodies passes through more than 100 kilometers of Earth’s atmosphere before reaching our eyes. During this leg of the journey, the rays are bouncing through the air, giving the impression that the stars themselves are constantly changing their luminosity. It’s like looking down a pool drain from the outside. Doesn’t the water sway make the object image appear to shake? The atmosphere acts on starlight in the same way. As these stars look like tiny dots, the distortion of their images creates the blinking effect. With the planets visible to the naked eye – Mercury, Venus, Mars and Jupiter – this does not occur. As their images in the sky are larger to us than those of the stars, the distortion caused by the air is not enough to make them blink.
I mean, not always: “When the air is very agitated, even the planets seem to twinkle”, says astronomer Enos Picazzio, from USP. In outer space, without the influence of the Earth’s atmosphere, the brightness of any star is always fixed.