What determines the color of the sky on extrasolar planets?
The smart-alec answer, I suppose, is “the same factors that determine the sky color on Earth.” While an awfully broad question, I will give some contributing factors… The sky will be black if the planet has no atmosphere, too thin an atmosphere, or if there is no star near enough to light things up enough. The color of the star about which the planet orbits (or a moon – if bright enough, I suppose), if close enough to be a significant light source, could influence the sky color. The chemical make-up, density and depth of the atmosphere. I believe the color will be pale violet with a thin, light atmosphere and add colors toward red (down the rainbow) approximately as the density and molecular increase because that should increase the refractive index of the atmosphere. This is not exactly right, but it is close. The atmosphere may contain chemicals that absorb a particular color or or others that fluoresce in “daylight.” The color of the atmospheric gasses – if the planet has an atmos
