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

Question

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:

Answer 1

I think the star,its halo and cloud can't be darker than the sky color even early M type still look blindingly bright orange-red on the sky not dim orange-red. The sky color, if heavier i think it will look more "desaturate"(brighter too but not bright to the point it's brighter than the star) and thecolor shift toward the red side(you are correct) that dense sky will be affected by the color of starlight very intensely to the point it's almost have the same color of the starlight example: 10 bars sky on a planet around K5V sun would look light orange-ish grey also the cloud would look bright orange, same color for the star's halo. example2: 10 bars sky on a planet around A5V sun would look bright pale teal/cyan with yellowish white cloud same color for the star's halo. example3: 5 bars sky on a planet around A5V sun would look powder blue with pure white cloud, same color for the star and its halo.

Answer 2

Your plots are incorrect in the segments of high pressure as well as of low star temperature. At high pressure the important factor becomes light extinction, which makes the sky yellowish unless there's really large spectral power density in blue region. At low star temperatures there's so low amount of blue light that the dependence of scattering cross section on wavelength isn't enough to compensate: the exponential factor in the Planck's law can't be beaten by the $\lambda^{-4}$ factor of Rayleigh scattering.

Also, at low pressure the extinction is much less pronounced than at normal pressure, so that even when the Sun is low, the atmosphere doesn't become yellow. This is especially easy to see when the star temperature is high: its blueness compensates the little yellowing that could happen, so we see it as whitening or lighter-bluing.

Still, I don't think one could easily guess it right, so you've actually done a good job. For the sake of completeness, below I'm including my results of numerical simulation of three atmospheres (air only, no aerosols, no ozone) lit by black-body stars with the same set of temperatures as used in your table. Simulations done by my (still WIP) software, CalcMySky.

Sun elevation 85°.

Star temperature, K	Pressure 0.25 bar	Pressure 1 bar	Pressure 10 bar
8500
7400
6700
6040
5570
5100
4350
3670
2840
2000

Sun elevation 5°.

Star temperature, K	Pressure 0.25 bar	Pressure 1 bar	Pressure 10 bar
8500
7400
6700
6040
5570
5100
4350
3670
2840
2000

Answer 3

(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.

A detailed description of the problem with your calculations(?) is given here with regard to a red giant illuminating the Earth's atmosphere; but the reasoning is precisely the same for an M-dwarf spectrum. A star (actually a brown dwarf) at 2000 K emits a negligible amount of visible light and what there is would be in the far red part of the spectrum.

Answer 4

In the Space Engine, I found a massive (6.6 Earth masses) oceanid, 2 Earths in diameter. It rotates around a white-yellow F8 star with a temperature of 5930 degrees Celsius.

Atmospheric pressure is huge! 146 atmospheres. It consists mainly of CO2 - 48.5% and water vapor - 45.8% and a small proportion of nitrogen - 5.4%

The color of this atmosphere is incredibly interesting to me, both in terms of planetary science. I want to ask if this color of the atmosphere is real at this pressure? And what can be this color of the atmosphere with other types of stars?

As you can see, at a star height of 60 degrees above the horizon, the sky looks yellowish, that is, at such a pressure, a yellow tint of color is best seen, and blue generally dissipates and is visible only at sunset, when the star has dropped to 5 degrees above the horizon.