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

I'm an artist (and science enthusiast) and I've been trying to find a comprehensive resource that would help me clearly identify likely sky colors (as perceived by human vision) 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.

UPDATE: I have revised this post and added a new, improved sky chart (below). Science references for sky chart: Ref A, Ref B

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

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. However if you'd like to comment on how the sky may change during things like sunset, or with effects like volcanism, I'm game!

Also, I'm more interested in relative color, than getting absolute color 100% right. The image was built in a vector program in RGB mode, so assuming your screen is calibrated in the standard way, we are probably seeing almost the same thing.

Here are the key ideas I've gleamed from my reading that I'm using to build this. I consider most of them tentatively held, and am very open to input:

• 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...would it look whiter?). 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.
• At lower temperatures I'm assuming the sun would appear tinged by the color listed under "star temperature." Otherwise you'll probably only see the star's color when it's near the horizon.
• I'm assuming 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've guessed that the gradient would be more apparent on a high G world (on the right).

References: Reference #1 Reference #2

Original Chart:

• It is a hard task as for here on Earth I see all types of blue in your chart. But internally your chart and the reasoning leading to it seems ok. Good job. Once I posted a Q with no success at all. Perhaps can I address you to that? As an artist and a science passionate you might help me. ... – Alchimista Dec 19 '17 at 12:18
• I think your scattering is backwards. The blue sky and yellow sun is due to scattering. A thicker oxygen/nitrogen atmosphere should be bluer than a thinner one, not less blue. A thicker atmosphere might also have more clouds, but that's also temperature dependent and I'm guessing clouds aren't part of your equation. Nice effort, by the way. – userLTK Dec 20 '17 at 7:27
• This is a really interesting question! It might be interesting to "calibrate" using a known sky besides Earth's. To that end, I've just asked the question Why would Mars' sky appear blue at dawn and dusk, but red during the middle of the day (reverse of Earth)? – uhoh Dec 20 '17 at 14:06
• @userLTK I've looked into this more, and I'm fairly confident that as atmospheric pressure goes up, the sky becomes more yellow. So at something like 5-10 bar, the sky might appear teal, and when you get to 40 bar the sky would appear a muted yellow. This happens because as pressure goes up, there is more atmosphere between you and the sun. This means the atmosphere has more molecules and other particles to scatter the short wavelengths of light, so much so that the blue starts to scatter beyond perception, and the wavelengths that reach you go from blue > teal > yellow > white (at zenith). – n_bandit Dec 20 '17 at 21:00
• @userLTK Yes. I agree with your word choice of "denser." There are apparently two types of scattering at play in the atmosphere: Rayleigh and Mei scattering. I've found two great sources that both seem to agree that as the atmosphere density increases brightness goes up, but purity of primary wavelength color goes down. So from a low bar environment to a very high bar environment the normal mid-day zenith color would be: deep blue > powder blue (earth at sea-level), > azure > teal > yellow > yellow/white > white. – n_bandit Dec 20 '17 at 21:05

(Some of) These plots are incorrect. Whilst Rayleigh scattering has a steep dependence on wavelength $$(\propto \lambda^{-4})$$, it cannot scatter what is not there. There is almost no blue light at all coming from stars with $$T_{\rm eff} < 3500$$ K.