Trappist 1e, a planet in the habitable zone around the red dwarf Trappist-1. This image is a plausible model of what the sky would really look like on Trappist-1e. To answer a few potential questions:
Why doesn't the star (Trappist-1) appear red in the sky?
The surface of trappist-1 is approximately 2600K, which means it emits light primarily in the range of a moderate / dim incandescent light bulb. Despite their name, not all red dwarfs really appear red.
Why is the disc of the star so big?
The planet, trappist-1e, is much closer to it's parent star than the Earth is to the sun. This makes the star trappist-1 appear about 6 times larger in trappist-1e's sky than the sun does in the Earth's sky.
Why is the sky green?
Rayleigh scattering tends to scatter the shortest wavelength of light hitting an atmosphere, causing the sky to glow that color. Unlike our sun, trappist-1 doesn't emit any blue light. Green is the shortest wavelength of light being emitted, and therefore that's the color that's scattered in the atmosphere.
Why is the atmosphere so thin / dark?
The atmosphere is thin because red dwarfs are extremely active stars. Because of high flare activity and how near habitable planets must be in order to be warm enough for life, it's likely that most planets in a red dwarf's habitable zone will have their atmospheres stripped off completely. I'm an optimist, however, so I've granted trappist-1e an extremely strong magnetic field and active volcanism to replenish it's atmosphere. Still, if it has any atmosphere at all, it would be thin. The sky is dim because the star, trappist-1, is dim, and because the atmosphere is thin.
Why is trappist-1 so low in the sky?
Not only is it low, it doesn't move from there year-round. This is because trappist-1e is tidally locked to it's star and the only area that is suitable for life (and liquid water) is the 'twilight' band between the area of perpetual sunlight and the area of perpetual dark.
What's the blue glow around the water?
Bioluminescent bacteria. While intelligent life in the universe might be rare (or we might be the only ones), simple life is probably more plentiful.
I noticed the lens flare / glow...
Yep, I touched up the image a bit in post. :D
Thanks for looking, enjoy!
Great lights and mood here. Very nice overall.
Very nice render, welcome to the forum and thanks for the extensive background information. Apparently, there's thought behind your work and I like that!
Echo Dune's comment, it's nice to have a framework for understanding what you're looking at...just makes it better IMHO.