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This question has been asked in similar forms on this site already with some good answers like here:

What Color Can the Sky Be?

How could the sky be orange in a breathable atmosphere?

What would make a sky appear purple during the day?

but I want to put a special emphasis on compatibility with life as we know it. So the question is: What possibilities exist to color the sky of an earth-like planet differently from our blue sky under the condition that life as we know it is supported?

This means:

  • The atmosphere consists of about 20 % O2, some N2 (for our dear plants!) and has a pressure of about 1 bar at sea-level

  • None of the atmospheric gases must be poisonous for (most) animals and plants

  • Photosynthesis must be possible

  • All constituents of the atmosphere must be chemically inert with respect to each other (e.g. large amounts of CH4 would combust with O2 at the earliest convenience and set the world on fire)

  • The color is seen by a human, so no fancy photoreceptors.

I'm open to suggestions about more conditions I may have overseen.


Now the possibilities of changing the color are to my knowledge:

  • Rayleigh scattering: Since the gases of our atmosphere possess resonance frequencies in the UV region, violet light is scattered more than red light. A quantitative deficiency in violet light (at least percieved by us) leads to blue being the dominant color. Are there gases with different resonance frequencies that could alter the scattering behaviour?

  • The sun: Emits a color spectrum according to it's temperature. By changing the temperature a different color can be obtained, but would this be possible in an extent that still satisfies the condition of permitting life? A strong red-shift would cause a UV-deficiency (photosynthesis) while a blue shift would increase the UV radiation to possibly hazardous levels for everyone.

  • The sun again: Next to the continuous spectrum defined by Planck's formula elements have strictly defined elemental lines. Could an enrichment of a certain element like Ca lead to a strong Ca-line that changes the sun's apparent color?

  • Particles in the atmosphere (like on Mars or even in Peking): In this case I am concerned about getting enough light to the surface to let plants live. Are there different colors possible or would it always be something red-ish?

Edit: I really don't know why this question has been flagged as duplicate. The linked question only covers one coloring mechanism and completely lacks the compatibility with life which is emphasis of my question as I clearly stated.

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    $\begingroup$ Please remember that the human visual system has a very good automatic white balance correction functionality, which means that it automatically adapts and corrects for the ambient illumination. At dawn, at dusk, in rooms illuminated with incandescent light bulbs, the illuminant is redder; under an overcast sky, the illuminant is bluer: and yet our built-in white balance adjustment immediately kicks in and we don't see the world around us bathed in red or in blue. $\endgroup$ – AlexP Feb 4 at 13:40
  • $\begingroup$ This is an interesting note. But does this also affect the sky's color itself? I would be quite satisfied with a "normal" looking world and a discolored sky. $\endgroup$ – And Feb 4 at 13:45
  • $\begingroup$ I'm going to vote to leave open because the duplicate question does not account for things like photosynthesis and the support of current Earth lifeforms. $\endgroup$ – Cyn says make Monica whole Feb 4 at 16:27
  • $\begingroup$ Not a duplicate... has a quite different emphasis. $\endgroup$ – Monty Wild Feb 5 at 11:32
  • $\begingroup$ does perception count? (i.e. ask certain types of color-blind people what color the sky looks). I ask because you said you would be satisfied with a normal world but discolored sky but only human eyes (no fancy photo-receptors) and it made me think of "why isn't the sky purple?" or as one of my friend's once asked me "what's blue look like anyway?" $\endgroup$ – LinkBerest Feb 5 at 13:24
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Just about anything, if the conditions were just right.

First, about the color of the sky itself, in general:

The other questions you link to in your own question do a fairly comprehensive job describing factors that affect how we perceive the color of the sky. Insofar as what possibilities exist for an Earth-like atmosphere specifically, the basic principles of those answers still apply: the thickness of the atmosphere, the type of star, and so on. In other words, having an Earth-like atmosphere doesn't really change the main considerations, so in large part you can still refer to those other answers. In particular, there are several answers here that should directly interest you.

Second, specifically regarding plant life:

Since the atmosphere is going to be hospitable (and assuming they get all the other nutrients and hydration they normally need), the main concern is the amount of light they will receive. Different plants require different amounts of sunlight, so some species may have difficulty flourishing if your planet experiences too much difference in light level.

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