Could a sky be green(day)->purple(dusk)->black(night) and still be breathable?

Question

It takes place on a planet that isn’t earth but humans would still have to be able to breathe. I’m also okay with a bluish-green. There’s two moons, typically only one is seen in any given place at once though. The sun is up for debate as I haven’t decided on that yet. However, the planet is colder in general and colder longer than earth. The length of the year is also undecided. So a smaller, dimmer, or more distant sun isn’t out of the question.

The thing Im most worried about is the green sky during the day.

Edit: I’m fine with other green things, just not a fan of the of specifically aeroplankon.

I'm sorry, but I am not sure that I grasp the difference between sky-blue and bluish-green. Isn't bluish-green or greenish-blue what sky-blue is? — AlexP, Jan 13 '24 at 16:47
Turquoise has is a lot more green? It’s darker? I’m sorry I’m confused how you can’t see the difference? — Spaghetti_Writer, Jan 13 '24 at 18:38
P.S. Here on Earth, the color of the sky varies immensely from place to place and from time to time. It also depends on where you look -- the color of the sky is much more saturated far from the sun than near the sun. For example, here is a large set of photos of minarets from Wikimedia Commons. — AlexP, Jan 13 '24 at 19:33
I gave a really long answer on if the purple sky could be purple here (https://worldbuilding.stackexchange.com/questions/236340/if-earths-atmosphere-is-technically-violet-but-our-eyes-cant-see-it-is-there/236352#236352). If the star's effective temperature was ~3000K, I think you would have a light greenish sky. Although stars of that temperature exist, I have no idea if they could support life. They would would output almost half as much energy as the Sun, so the habitable zone would be really short. — E Tam, Jan 14 '24 at 02:18
Not aurora but STEVE or Strong Thermal Emission Velocity Enhancement — user6760, Jan 14 '24 at 06:52
Yes, the planet isn't earth but it was colonized by humans. There was already life, so the stars would need to be able support some form of life. — Spaghetti_Writer, Jan 14 '24 at 21:41

score 4 · Answer 1 · answered Jan 13 '24 at 17:51

Probably no-go. Stars can't be green at all, and stars have to be yellow or colder if you want them to stick around for long enough to have life evolve on their planets. You probably need life for a long time to get a terrestrial atmosphere - Earth got its current atmosphere that way, and oxygen is a violently reactive chemical that doesn't stay free in the environment for long unless there's a process like photosynthesis going on to continually renew it.

Yellow and orange blackbody spectrum light, Rayleigh-scattered by atmosphere and particulates, can be any of the colors of the sunset: red, orange, yellow, peach, and blue. You can have purple as an optical illusion - peach near blue looks purple - but not as a true color. You can have a greenish-grey sky if you filter yellow light from sunrise or sunset laterally through dense storm clouds, but this is rare and short-lived, since it can only happen when the sun's rays are coming from the horizon and aren't already blocked by more clouds.

To get a green sky, you therefore need a sky that is actually made green stuff, full of green stuff, or under green stuff. You say you don't want plankton, which I assume also rules out other green particulates, and breathing rules out an atmosphere with a lot of chlorine gas.

I don't suppose you'd accept a planet outside an ancient alien Dyson sphere made out of something translucent and green, or a planet in the outer reaches of the solar system that's illuminated by an enormous green laser (using energy harvested by an ancient alien megastructure around the sun)?

Could a sky be green(day)->purple(dusk)->black(night) and still be breathable?

1 Answers1