Terrestrial Exoplanet Skies – I've Built a Visual Sky Chart. Is it Correct?

Question

I'm an artist and I've been trying to find a comprehensive resource that would help me clearly identify likely sky colors for exoplanets that have atmospheres with a similar chemical composition to Earth. It's trickier than I expected. I've cobbled together several resources, and built what I hope is a decently accurate chart of apparent sun color along with sky color.

How accurate is my chart? Is this a good representation of sky/sun on alien worlds with heavily Nitrogen/Oxygen atmospheres? In what ways could I improve it?

UPDATE: I've updated the chart. It's based on my latest research and indicates baseline sky color, cloud color, and apparent sun color for noon/pre-sunset.

Latest science references: Reference A, Reference B

The chart is not meant to account for things like dust, the look of the sky at sunrise/sunset, or other atmospheric effects. This is meant to be a boilerplate for the baseline look of the sky during the day.

Here are the key ideas I've gleamed from my reading. If I'm wrong on any of these points I'd love to know! :)

• Earth-like atmospheres would tend to be light to dark blue at the zenith, due to the scattering of low-wavelength blue light. If the planet's sun was very hot, the sky would look a deeper blue, while cooler stars would give the sky a lighter blue to almost white look. When the sun gets to 3000k and below, the sky starts to take on an orange/brown tinge.
• Like on earth, the horizon is the lightest in color and the zenith the deepest.
• Denser atmospheres would appear brighter (more washed out) and the primary color in the spectrum more "pure" (I'm unsure what the term "pure" means exactly when it comes to optical perception). Likewise, thinner atmospheres would be less bright than earth's and the colors more "pure."
• With increasing pressure the sky color at the zenith becomes increasingly yellow. In my image this means that an earth-like sky at 10x earth pressure would appear blueish/green near the zenith.
• I'm assuming the sun would appear tinged by the color listed under "star temperature."
• I don't know how shadows would be effected, though I assume when you get down into K and M class suns, the surroundings on the planet would take on a progressively redder tinge due to the decreasing prevalence of blue wavelength light?
• I suspect the gradient of color from horizon to zenith will be steeper/gentler in some atmospheres?

I'd like to account for how the color would shift when you look in the immediate direction of the sun. Would the atmosphere surrounding the sun (like a "halo") appear tinged with the star's color?

Reference #1, Reference #2

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Original Image:

Answer 1

I think this is actually a good piece of basic artistic research (I've had professors who have attempted it too).

Sadly, I can't give you an all-encompassing answer to your question. Even though you tried to refine your scope, I'm afraid in order to get what you are looking for you have to account for things that you didn't want to get into. I know I'm going to take a hit for this, but I needed the space to help you, so I will try to address some of the problems you have posed.

(I'm unsure what the term "pure" means

In light, white light is actually an ~impure form of light in the same way that black/brown is an impure form of paint colors. Just like when you mix a bunch of paints together to get a brown-black color, so too if you mix a bunch of light colors you get a white-ish color. So "pure" in this context means closeness to an identifiable color in the spectrum.

Is this a good representation of sky/sun on alien worlds with heavily Nitrogen/Oxygen atmospheres?

It's a start; however:

• Distance from star is just as important as its solar output, if not more so.
• Chemical composition (/stage of life) of a star can also have a small impact.
• Maybe the colors are accurate at the zenith; however, once you get to horizons you enter a whole hell of factors:

An atmosphere isn't some homogeneous blend of gases; actually, it has many layers with different types and concentrations of gases, especially when you factor in organic and volcanic processes. This plays out most notably with horizons, as the sun's light has to travel farther through those more erratic layers, the ones closest to the ground.

(See also: link for why the moon appears orange)

So for your boilerplate approach, the spectrum emitted by your atmosphere should shift red as you approach the horizon from the zenith. The magnitude of this shift depends on the size of the planet/atmosphere, atmospheric density, etc.

How accurate is my chart?

I can't say for sure, but I would expect to see more reddish tones in the lower-energy atmospheres.

I'd like to account for how the color would shift when you look in the immediate direction of the sun.

Looking at the sun, the colors would be closer to the apparent light of the sun (its whitish output) and the zenith color of the sky (impure, as you would be seeing many different wavelengths of light being emitted).

Would the atmosphere surrounding the sun (like a "halo") appear tinged with the star's color?

Halos appear due to the presence of water, much the same way as rainbows appear. A halo would appear tinged with the star's color, but not to any significant degree for a human observer.

Just some useful reading if you haven't already.

I don't know how shadows would be effected, though I assume when you get down into K and M class suns, the surroundings on the planet would take on a progressively redder tinge due to the decreasing prevalence of blue wavelength light?

A shadow is a shadow, being the absence of light. The color here depends a lot on what light the object receives and emits back.

Answer 2

You could improve it by means of adding all variations of elements and compounds in the atmosphere to add the atmospheric color association to those elements/compounds. For example, a sky with sulfur would look like..., ozone with hight traces of carbon would look like...