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Question in the title. A Rogue planet is a sunless world that exists in interstellar space. It might be possible for life to develop on one, but what happens when that happens?

I've seen people say that plants could not develop on these worlds. But there's the theory of Thermosynthesis, which is almost what I'm looking for. But would it be possible to absorb infrared photons? What is the most efficient color to absorb infrared light? I've heard different things about nonvisible light's albedo, and can find nothing to confirm one way or another.

I saw this, and I can see nowhere on it that mentions the color of plants.

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    $\begingroup$ Plant and fauna on Earth can have leaves of a huge variety of colors. There is no reason to consider this less likely elsewhere. However I'd direct you to the answer by Ville Niemi for why there's a basic problem with thermosynthesis. $\endgroup$ Commented Jan 18, 2018 at 8:32
  • $\begingroup$ Black, of course. The whole surface is illuminated only by starlight, so even a dayglow orange safety cone and a Vantablack pit will appear the same. If there is no light to see by, does color really exist? $\endgroup$
    – PcMan
    Commented May 3, 2021 at 8:15

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In a world like that the primary sources of energy for organisms would be chemosynthesis or, hypothetically, thermosynthesis. Neither relies on electromagnetic radiation and as such have no effect on the color of the organisms.

As such organisms would have no pigmentation as such and would probably be colourless or transparent "things" living in lightless subsurface oceans. You can look for speculation on possible life forms on Europa (“ALL THESE WORLDS ARE YOURS, EXCEPT EUROPA. ATTEMPT NO LANDING THERE.") for inspiration.

Absorbing infrared photons would not really work. Photosynthesis relies on receiving photons from higher energy level than your ambient energy level. This is the energy flux that does chemical work. Receiving photons from your own or lower energy level is not really useful as it cannot generate a positive energy flux that does work. The exception would be scavenging waste energy from your own energetic processes. Typically we would be talking about power plants, engines, chemical factories, or high performance micro-processors, not life forms.

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  • $\begingroup$ Love the 2001 reference. $\endgroup$ Commented Jan 18, 2018 at 13:47
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Plants on the surface of the Earth appear green to our eyes because they're reflecting the green spectrum and absorbing (basically) everything else ... including infrared.

If we look to plants surrounding geothermal vents we find a wide variety of colors, I expect that is because they evolved to rely on something other than sunlight to provide the energy they need.

A rogue planet has a similar problem.

  • Some plants may evolve to take advantage of what radiative energy exists in the void. As these plants have no dependency on visible-spectrum light, I would expect them to develop a wide array of colors depending on their chemical composition (assuming they could withstand the visible light spectrum). I expect this would mean the same plant located in an iron-rich soil would be more red than a plant in a magnesium-rich soil (white) or a plant in a cobalt-rich soil (blue).

  • It should be noted that on a rogue planet, plants are more likely to evolve to take advantage of a wider array of energies (including geothermal) than was necessary on Earth. As with all things, evolution tends to take the path of least resistance. An abundance of sunlight would preclude the evolution of chemically-dependent coloration.

Except for flowers, of course... and the red-barked dogwood. Ware all ye who adventure beyond the void, the red-barked dogwood....

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    $\begingroup$ Green is only the dominant color in earth plants by accident, any color could have (and in many cases did) evolved. $\endgroup$
    – John
    Commented Jan 18, 2018 at 15:40
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There is no evolutionary advantage to evolving pigmentation (for the visible spectrum) if there is no source of visible light. "Thermoflora" would be whatever colour is innate to their molecular substrate. Colour also does not play a factor in absorbing or permitting infrared, so your plant's appearance will be dictated by the wavelength-blocking materials their surfaces are made of.

If you want to get really alien, leaves of vanadium and sapphire would be one of the most efficient absorbers nature could hope to produce – sapphire blue and grey suspended in layers within the leaf structure.

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I have heard a theory that on such a rogue planet the radiation from transuranics or anything radioactive would be used as an alternative to sunlight. This may seem weird, but there are Earth fungi that can do the same thing (Google it. It's weird how easy it is to find stuff on them). I'm guessing such lifeforms wouldn't have any particular color at all but just focus on moving through the ground, looking for more radioactive ore to feed on, like slime mold seeking food.

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I believe it's safe to assume the flora would evolve to absorb every photon they can

But let's begin with a disclaimer. Unless there's a source of heat and energy available on the rogue planet, it's highly unlikely anything other than bacteria would evolve — and probably not even bacteria. Heat and energy are the basis of life. If the planet spun fast enough to keep the core hot and volcanism active, then it's believable that the flora would draw most of it energy from those sources.

And when you think about that, the idea of leaves begins to fall out of favor. What the plant would want is strong roots. If stems, trunks, branches, and leaves did form, they would do so to pull energy and nutrients primarily from the atmosphere near the geothermal vents or, if the conditions were right, moisture from the air.

Knowing this, the idea of terrestrial flora simply doesn't conform. I can imagine fungi or massively willowy plants. Trees don't make sense at all (from the perspective of my short-of-research answer).

But, that doesn't answer your question

let's ignore all that and simply answer the question. What color would the leaves of most plants be?

Answer: Black.

If electromagnetic radiation is important to the plants in a similar way that it is here on Earth, then the most likely color would be black. No light would be reflected in any or most spectra. On a rogue planet where every photon would convey precious life-giving energy, I can easily believe not a single one would be reflected. They'd all be absorbed. Which means a human looking upon the plants would only see the flattest, non-reflective black.

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