Our sky is blue, can we determine through atmospheric testing or something what color other planets sky would be?

Perhaps, but not as much as your might think. I took the "atmosphere like ours" into account before, but now let's tackle this idea!

If there is no atmosphere none of the light will be scattered, so we'd see a black sky and a white sun (unless the central star is hot or cold enough to be blue or red). We probably wouldn't see any stars during the day time (because the central star would be too bright).

Increasing the density and thickness of the atmosphere both have the same(ish) effect (providing more molecules to scatter light) so I'll just consider increasing them both at the same time.

As we increase the number of scattering particles in the atmosphere it starts to take on a bluish hue (because blue light is significantly more likely to scatter than red), and the sun begins to turn yellow (as the blue/green is scattered away from the direct from the sun path). Through this period of increasing atmosphere the sky would probably start out as a darker blue (think of the stereotypical blue marker color) before changing up to the sky blue color we have on Earth as green light begins to be scattered in significant portions.

Moving past Earth like conditions to a denser/thicker atmosphere the longer wavelength (red) colors are more likely to scatter and the sky will approach a white color, probably by a path of simply fading the blue, but it could take on a bit yellow/green-y tinge as it fades to white, depending on the colot profile of the central star (might also not fade if the star is really rrally hot). It'd be white light because if light from all of the visible spectrum is scattered in comparable proportons we'll see a mix of all the wavelengths (white light) where ever we look at the sky.

If we go to an extreme atmosphere where all of the light from the sun is scattered, the sky would be bright, but we wouldn't see the sun at all (all the light is scattered away from the direct from sun path), and the sky (during the middle of the day) would take on the color of the sun (usually white, but could be blue for very hot stars, and red for very cold stars) as we would be receiving all the light from the star in roughly the same proportions as it is coming from the star (all the light is scattered in all directions). The beginning of sunrise would lead to a red sky (red light scatters less, so will reach you faster) but would quickly revert back to the color profile of the star. Sunset would be blue as a result of the reverse process (after the sun has set, and no new light is "added" to the atmosphere, the blue light will be bouncing around in the atmosphere after the red has found its way out).

I ignored a couple of things to emphasize the color changes, but I'll bring those in now.

The first is that as we increase the density of the atmosphere, everything will get dimmer because we increase the amount of light getting scattered back out into space before making it to the ground. So it's possible that in the last extreme a very small amount of light would make it down to the ground (and you'd get a bit more red light, so you could have a dim dark blackish red sky).

The other important factor are the two brought up by rantonels at the top of this thread, and colored dust, or color absorbing molecules would be able to more significantly contribute to coloring the sky. For example, if you wanted a fire-red sky, you could put a bunch of molecules that absorb blue and green light into the atmosphere, and you'd be left with yellows and reds in your sky.

Now I'll try to summarize all that in case no one actually wants to read the whole thing.

Assuming a middle of the road (temperature wise) central star, and no major colored dust particles, or molecules that absorb light in the visible wavelength range, as we start from no atmosphere and increase the density until we reach an extreme level, the sky would start as black, go to dark marker blue before reaching sky blue and eventually it would fade and dim into a bland white (grey?) until the atmosphere gets so dense that no light makes it down to the surface and we would have a black sky again. During this time the star's appearance in the sky would go from white to yellow to red to nonexistent. And for the extra curious there are a few more details above.

/r/askscience Thread Parent