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I am working on a game currently with some predetermined assets, and one of the areas I am developing takes place on a mountainous, lush jungle planet with low-hanging green clouds and a general green fog-like haze.

What factors could cause a planet to regularly develop these green atmospheric conditions while remaining habitable?

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    $\begingroup$ We usually advice to wait at least 24 hours before accepting an answer, because unsolved questions attract more attention and increase the chances of getting good answers. $\endgroup$
    – L.Dutch
    Apr 27 at 16:11
  • $\begingroup$ It's star is green? $\endgroup$
    – Richard
    Apr 28 at 0:40
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    $\begingroup$ "Green" stars are stars whose spectral output peaks in the green. They are still basically white. $\endgroup$
    – Mary
    Apr 28 at 1:03
  • $\begingroup$ Habitable by what? Humans? Just some carbon-oxygen based life? Or just anything that moves $\endgroup$
    – Hobbamok
    Apr 29 at 10:45

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Algae. You could simply state that on this planet there is some kind of micro- algae that lives in the tiny droplets of water that make up the fog and the clouds and that happens to be green.

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  • $\begingroup$ Is there any reason this doesn't happen on earth? I would recon every viable biological niche is filled on planet earth. So is there a reason why algae cannot easily live in the clouds ? $\endgroup$
    – Falco
    Apr 29 at 7:30
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    $\begingroup$ @Falco good question, I guess the water vapor that creates the clouds just cant carry them up. Or maybe its because of the big temperature change that happens in cloud formation. $\endgroup$ Apr 29 at 8:59
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    $\begingroup$ @Falco It does happen on Earth - rain, fog and indeed air are full of microbes, just not enough to be visible. I think what you need is high humidity (so the fog creates a stable environment) and some mechanism for adding nutrients to the water droplets. $\endgroup$
    – j4nd3r53n
    Apr 29 at 12:08
  • $\begingroup$ @Falco The question is about an alient planet, not Earth specifically, so whether or not this niche exists on Earth is irrelevant. A different planet that had denser, slower clouds with less turbulent air to disperse them would allow for more room for complex organisms to grow. $\endgroup$
    – D.Spetz
    Apr 29 at 17:41
  • $\begingroup$ @D.Spetz I agree with you and I think the answer can be improved by mentioning what specific differences the alien planet would likely need (like j4nd3r53n and you did in the comments) because of the science-based tag I think the OP might want to explain such details. $\endgroup$
    – Falco
    May 2 at 18:40
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Pollen. Planet has lush forests with plants spewing out green pollen year around. The pollen is carried by the winds and trapped inside water droplets and tint them green. The pollen on this planet should be lighter than our pollens or your world should have a thicker atmosphere for this to work.

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  • Aerosols of iron rust could produce a green haze or green clouds.
  • Aerosols of chromium oxide (Cr2O3) do the same but look nicer. It is used as pigment.
  • Among gases the only one I can think of is chlorine. But if it is concentrated enough to be green the conditions would not be habitable any more, at least not for you.

I would probably stick with the microalgae.

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    $\begingroup$ The OP explicitly stated "...while remaining habitable?". All the substances you mentioned can cause problems when inhaled by humans (I understood the OP meant habitable by humans). Inhaling rust causes siderosis. Chlorine is toxic even in modest quantities. Chromium compounds have various degrees of toxicity: the worst are hexavalent Cr compounds, but even trivalent Cr (like in Cr2O3) can cause problems. $\endgroup$ Apr 28 at 14:34
  • $\begingroup$ Not sure if a bit rust or chromium oxide in the air renders it uninhabitable. Depends on how much of it you need to get a green haze. As for chlorine, I explicitely mentioned that it would render the environment uninhabitable, so there is no point in repeating it $\endgroup$
    – Avun Jahei
    Apr 28 at 15:08
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    $\begingroup$ Pulling up a random safety data sheet for chromium oxide gives a maximum permissible level in the atmosphere of 0.5 mg/m^3. $\endgroup$
    – Mark
    Apr 28 at 23:34
  • $\begingroup$ Thanks. I have found a paper from the 1960s about aerosol concentration and visibility. 0.5 mg/m^3 corresponds to a visibiity range of 2 miles. That's a hazy day actually. academia.edu/47066338/… $\endgroup$
    – Avun Jahei
    Apr 29 at 0:40
  • $\begingroup$ Aerosols here on Earth are generally a mix of dust (silicon oxides), soot, and hydrocarbons, all of which are lighter than chromium oxide. I'd expect a chromium oxide haze to correspond to a visibility of around 4-5 miles, which would be more of a green tint to the sky rather than a proper haze. $\endgroup$
    – Mark
    Apr 29 at 21:22
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It would help if there were an artist's conception of what this greenish haze and surrounding landscape would look like, but one additional explanation I'd like to add is maybe light reflected from bioluminescent plants?

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Aesthetics from some sentient life.

Some particular culture visited this planet and decided that green fog was better than plain fog, and so they decided to add green fog.

The normal, simple compounds that make a particular pigment are usually toxic, but some modern pigments are advertised as non-toxic, regardless of their shade.

As such, large fogs(presumable water based) colored by pigments similar to these should do no harm at all.

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    $\begingroup$ "Aliens did it" isn't exactly what we're looking for in answers to "science-based" questions. $\endgroup$
    – Mark
    Apr 28 at 23:35

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