If an Earth-like planet had an older, red sun — or maybe just a reddish sky due to some property of the atmosphere — how would that affect the growth and/or adaptation of plant and animal life on the planet? Would plants still be green? Would the darker sky affect the eyesight of animals?

Please describe any notable differences in flora/fauna between Earth and a comparable planet with a red sky. For the topic of this discussion, assume all other properties of the planet are similar or identical to those of Earth.

In case it's important, I'm building an alien world with a red sky and mountainous, spiky terrain. Note that I'm not looking for a list-format answer, but rather a description (and explanation) of the differences I should implement when designing the organisms on the planet.

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    $\begingroup$ This album helps you: imgur.com/a/fS7hV --- More information here: reddit.com/r/worldbuilding/comments/1dui0v/… $\endgroup$
    – kexx
    Oct 30, 2014 at 20:32
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    $\begingroup$ Unintuitive as it might sound, a good start may be studying a bit about why the sky is blue, also over here. Then, it might help focus answers if you are able to answer what factor causes the sky to be reddish on your planet. $\endgroup$
    – user
    Oct 30, 2014 at 22:16
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    $\begingroup$ What is the link between the color of the star and the color of the sky ? $\endgroup$
    – Vincent
    Oct 31, 2014 at 1:46

2 Answers 2


If sunlight was red then the red sun would be long living allowing evolution to progress longer on that planet. That sun would have been born before our own sun as well, so if compared to the current Earth Date then an Earth-like planet could easily have life that's been around way longer and maybe more advanced. If sunlight was red the sky most likely wouldn't be blue. Problems involving gravity would put a livable planet farther away from the star which creates its own problems. The reduced energy from a bluer spectra would probably not provide the necessary amount to support life which raises issues if we consider the sun to not be a contributing factor of the redness. Our red light would follow a black body curve centered on red, so the plants and animals wanting to reflect the light would be cyan or blue, and would blend towards white as they reflect more and more sunlight. Ones wanting to absorb the light would be red and blend more towards black as they absorb more and more light. Green always has a more efficient alternative so there would be no pressure for green and probably very little of it, if it appeared at all, Colors as viewed by the natives would probably be viewed in shades of red due to the lack of other color components in the sunlight. Similarly the rainbow would be red-shifted.

A red sun could be cooler than our own and last much longer allowing life to evolve to higher levels. Solar heating would decrease faster with distance so the net effect would be that the sun would appear much larger in the sky (probably at least twice as large). The decreased strength of sunlight would place limits on the food chain. Less species in a web means more specialization and less burden. Less input energy means less plant growth per unit area. Less growth means slower metabolisms, tricks to find food, and longer lives have selective pressures. Low metabolism, slow-paced (culturally), long-lived, intelligent creatures would be selected by evolution (k-selection). Note that intelligence would be more likely to develop.

Vision would be adapted for red. This means the natives would most likely see in shades of red with maybe some yellow mixed in for shades of orange. Likely their visible spectrum would be shifted to see infrared and so heat would be visible to you average creature. This may help accelerate intelligence or hinder it on caloric, etc. If it was useful to distinguish between plants and animals that might use the other colors they could end up with a visible light range similar to ours, plus heat vision.

The increased likelihood of predator and prey both having heat vision results in typical camouflage adoptions not being very effective. You might see more fauna with thick fur and similar features, to insulate their heat signature.

  • $\begingroup$ Prevalence of heat vision in an ecosystem could probably span a question of its own. $\endgroup$
    – Black
    Oct 31, 2014 at 10:07
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    $\begingroup$ Would it not be important to note that, if you used a Red Sun, it's either going to be a "Red Dwarf" sun (Very small, cool, long lived) which brings with it, its own set of problems (Tidal locking, variations in output, very close orbits for liquid water) or a "Red Giant" sun (very large, cool, very short lived, not ideal for life even if in the habitable zone)? $\endgroup$
    – Raisus
    Aug 25, 2016 at 10:29
  • $\begingroup$ @Raisus Maybe not by name but I touched on that at the start. There's really only so much I can cover without writing a thesis on this. Your comment adds to the answer though. $\endgroup$
    – Black
    Aug 27, 2016 at 4:11

Under a red sun, there will be a higher output of red light, but as red stars are the coolest, they don't produce as much energy as say, yellow stars. Thus, plants that evolved for life under a red sun would have dark pigments, so that they can absorb as much radiation as possible.


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