# What color should my planet's skies be?

Assuming the skies are clear of suspended particles, what color should they be at mid-day if viewed with human eyes?

The planet is larger and twice as massive as Earth, with a nearly identical atmosphere composition but a far higher surface pressure. This implies a thicker atmosphere with more scattering, which could produce light, warm-toned skies rather than blue ones. I watched some videos by Artifexian on the subject, and used the spreadsheets and sources he had provided to determine what color they should be. However, I wasn't able to come to a solid conclusion based on some gaps and conflicting info:

• The two calculators generally point towards a light, warm-colored sky. This is based on two things: first, the approximate height of the tropopause. This isn't provided in the first calculator, but I roughly extrapolated it from the height at which air pressure is between 70-400hpa (~15591.61-30545km.) This probably isn't the best way of estimating it, and the margin is wide enough to go either way in terms of color.
• Next, I compared this and the planet's actual surface pressure (2.43atm) to the results from the second sheet; it calculates the required properties for a sky, with 1atm of surface pressure and the same scale height, to become warm-toned (tropopause at 21453.68m, ~2.36atm.) The tropopause is based on the scale height and angle of twilight. I was unable to calculate this based on my planet's radius, making the results somewhat inaccurate. It also does not take into account the density of the atmosphere, which would increase scattering at the surface.
• These results conflict with a source he cites in another spreadsheet. This guide suggests my skies will have a bluish tone at 3atm and even 10atm, given the spectral type of the parent star. It mentions that planets with higher gravity can achieve more surface pressure with a thinner atmosphere, and will appear thinner than they actually are, further pointing towards blue skies.

Planet Stats:

• Mass: 2.0MEarth
• Average Surface Temp: 25ºC
• Sea Level Pressure: 2.43atm
• Atmosphere Composition: 78.19% N2, 20.45% O2, 1.23% Ar, 0.08% CO2, .05% Trace Gases
• Star Type: K6V (.64MSol, .168LSol, Peak Wavelength ~619.94nm)

Sources:

1. Designing Earth-Like Atmospheres / Artifexian
1. SKY & PLANT COLOR ft. Worldbuilding Notes / Artifexian
1. Earth-Like Atmospheres 3.0 / Artifexian
2. Sky & Plant Color Calculator / Artifexian
3. Colors of Alien Skies / Panoptes
4. The Worldsmith v3.04 / Artifexian
5. Colors of the Sky / C.F. Bohren, A.B. Fraser

EDIT: Accidentally linked the wrong videos, corrected.

• I didn't read the linked documents and can't judge their veracity, but Earth's sky is blue due to Rayleigh Scattering. All human-breathable gas scatters light in the blue spectrum. Higher pressure means more scattering which means a deeper blue color. Some minor (very minor) shifts in color may happen due to the color of the star, but the sky will still be basically blue (the gas scatters light in the blue spectrum, the color of the star is nearly irrelevant unless it doesn't emit blue light). The only way to change the color is to contaminate the atmo (e.g. clouds).
– JBH
Feb 6 at 16:12
• @JBH The linked videos claim a sufficiently thick atmosphere with an Earthlike composition would produce conditions similar to Earth at twilight, due to the increased distance light needs to travel through the atmosphere. This would produce warm-tinted skies, albeit far lighter in tone. Feb 6 at 16:45
• Then it's wrong. Rayleigh Scattering is well understood.
– JBH
Feb 6 at 16:49
• @JBH I added a numbered list of citations, including relevant citations from the two Artifexian videos. I also included a paper by C.F. Bohren and A.B. Fraser in regards to the color of skies at different pressure levels. I believe this study may have contributed to the idea of high pressure skies having a lighter, desaturated color rather than a deeper one. I've seen it pop up in other places on the internet. Feb 6 at 18:40
• @JBH: Earth's sky is blue, or yellow, or red due to Rayleigh scattering. It depends on how much atmosphere is there to do the scattering in the path of the light. At noon, the light crosses a relatively thin layer of atmosphere, the sky is blue. In the morning, when Dawn with her rosy fingers opens the doors for the Sun to rise, the sky near the eastern horizon appears red with a gold band separating it from the rest of the rest of the deep blue dark sky. If the atmosphere were thick enough, the sun would appear red at noon with the sky around it grading from red to yellow to deep dark purple. Feb 6 at 20:36