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In my story, there is an Earth-sized planet orbiting a red dwarf, tidally locked to its parent star, with large oceans like Earth, two moons similar to ours and a thick atmosphere to distribute heat around the planet.

On the dark side of the planet there are two small continents, each about the size of Greenland, as well as other smaller islands. Now, I've been wondering, would it be possible for plant life to exist on the dark side of the planet?

I mean, considering that the planet doesn't rotate, then obviously plants on the dark side can't photosynthesize, but what about the sunlight bouncing off from the moons surface. Could that be enough sunlight for anything to grow? And if plant life could exist on the dark side what would it be like? Considering the fact that even though the atmosphere does distribute heat around the planet, the dark side is still slightly colder.

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  • $\begingroup$ Welcome to Worldbuilding.SE Johnara, glad you found us. Please check out our tour and help center. $\endgroup$ – Cyn says make Monica whole Sep 2 at 15:29
  • $\begingroup$ Is your ocean frozen? If so how deep, if not why not? $\endgroup$ – James Jenkins Sep 3 at 16:55
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    $\begingroup$ How do you apply Taxonomic terms like "plant" to an alien ecology? $\endgroup$ – Aron Sep 4 at 7:14
  • $\begingroup$ "slightly colder"? You are aware the temperature differences would be bigger than on Earth, so that if your hot side is below boiling [of water], the cold side would be below freezing? So even if you had "continents" and "isles" on the cold side, the point would be moot because everything would be connected by kilometer thick ice sheets? $\endgroup$ – Nobody Sep 4 at 13:39
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    $\begingroup$ I would perhaps edit this Q to define what you mean by plant life - perhaps any multicellular non-motile organism lacking a nervous system would suffice. Mushrooms (fungi) are phylogenically closer to animals than plants, but would likely fit the kind of "flora" that you are seeking. $\endgroup$ – DoctorWhom Sep 4 at 22:59
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Photosynthesizing plants will have serious troubles and are already well covered by the other answers.

But what about other kinds of plants?

Life is not limited to using the sun as a power source (it just happens to be most abundant one on earth, so most organisms using it easily outcompete others in ecological niches where sunlight is available) - many bacteria can live off hydrogen or other electron rich elements (sulfur or elemental metals). Traditionally higher organisms like us humans, but also plants (or fungi) like to cooperate with microorganisms for various reasons.
I see no reason why some plants that evolved on the day side of your planet (and already had the ability to attract bacteria for symbiosis) couldn't have made it to the dark side and once there establish a symbiosis with microorganisms that can extract energy from non-solar sources.

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    $\begingroup$ In essense, the light/dark side of the planet are no different from ecological biomes (desert/tundra/temperate forest/jungle/...) where different plants thrive because of different environmental conditions. $\endgroup$ – Flater Sep 3 at 13:51
  • $\begingroup$ @theonlygusti I wanted to say electron-rich elements/substances, which actually can be oxidzed and not reduced (this is where I went wrong), the choice of words was also probably not the best. (will edit this now) $\endgroup$ – Nicolai Sep 3 at 16:27
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    $\begingroup$ It's life, Jim - but not as we know it! $\endgroup$ – Peter Wone Sep 3 at 23:57
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Not all plants photosynthetise. About 1% of all angiosperms are freeloaders.

Your plants might have evolved from regular photosynthetizing plants from back when the planet was not tidally locked. Millions of years in the eternal dark will have caused the ones on the dark side to evolve an absence of chlorophyll.

Such plants may thrive by parasiting fungi and/or animals.

Alternatively, if there is a lifeform on the planet that thinks of itself as intelligent, plants may take advantage of their technology in order to get a light source for photosynthesis:

SMBC

Source: https://www.smbc-comics.com/comic/mimic

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One possibility could be wind-powered plants.

A tidally locked planet with a stable atmosphere must necessarily have powerful winds transporting heat from the day side to the night side. (Ocean currents can also contribute to the heat transport, but you're still going to have a significant temperature differential between the two sides, which is going to generate winds.) A plant growing near the terminator could supplement the meager sunlight available there by extracting energy from the wind, and once such a mechanism evolved to be sufficiently efficient, it could even allow those plants to grow without any sunlight at all.

What would such plants look like? I'd imagine a primitive (or simply convervative) form might look like a tree with leaves that flutter in the wind, just like they do on trees here on Earth. The differences would be at the cellular level, where the hypothetical wind-trees would have some way of converting mechanical oscillation into chemical energy — perhaps by using the alternating compression and decompression of their cells to drive an ion pump and create an ion concentration gradient across the cellular membrane, which they could then use to drive chemical reactions (like ATP synthesis in an Earth-like biochemistry).

All of this seems perfectly doable using little more than standard Earth biochemical building blocks. The fact that no plants on Earth have evolved to exploit this power source is probably due to the combination of insufficient selection pressure (there being few places on Earth with lots of wind and little sunlight) and simple lack of time for evolution to explore this particular corner of the fitness landscape.

Of course, once a plant on our hypothetical planet becomes fully wind-powered and fully loses its dependency on sunlight, there's likely to be lots of rapid morphological evolution and divergence as the descendants of these plants spread and adapt to their new niche. For one thing, these "plants" would most likely lose their photosynthetic capability entirely — it's complex, expensive to maintain and useless on the dark side. This in turn will free their leaf morphology to evolve to fully maximize wind capture efficiency, with no constraints on having to also collect sunlight.

So I'm imagining something like tall rigid masts rising up into the windstream, with pale white banners fluttering behind them like sails on a boat turning into the wind. Or perhaps the semi-rigid stems could themselves be the vibrating elements, like giant cat's whiskers standing upright and humming in the wind. Or perhaps some varieties could look like long streamers, anchored to a suitable terrain outcrop (or to another plant!) at one end, with the rest of the plant just freely fluttering in the wind behind it.

What I don't really expect to see would be rotors of any kind: while efficient at capturing wind energy, they'd necessitate the evolution of a freely rotating biological axle, something that evolution on Earth never seems to have managed. If such a mechanism did evolve, the rotating part would likely be fully composed of dead tissue — the rotors would grow while still attached to the main plant, then harden and detach so that they can start rotating. But I'd still find a purely vibrational ("fluttering") mechanism more likely.

Also, while I've been imagining all this evolution occurring on land, there's really no reason why it couldn't happen underwater as well. Actually, now I'm wondering why it apparently has never happened that way on Earth, given that there are surely lots of places underwater with plenty of currents but very little light. Perhaps it's just the difficulty of converting the existing photosynthetic machinery over to a new energy source, coupled with the fact that this source can really only be efficiently exploited by multicellular organisms (which tend to have longer generation times, and consequently slower evolution, than unicellular microbes) with advanced materials like cellulose to stiffen their structure (which, on evolutionary timescales, have only appeared relatively recently).

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The dark side honeymoon hideaway!

  1. Bright moon. Here is a question about a bright moon driving photosynthesis.
    Nocturnal Photosynthesis

In my answer I used data from cave plants fed by artificial lights. Your moonlight needs to be 100x brighter than on our world. I humbly propose metallic, reflective moons, one silver and one golden. Aesthetics, you know.

  1. Coldness. Yes the dark side will be cold. But maybe you can compensate with internal heat! Your oceans are warmed from below with geothermal fires. They steam and sometimes bubble in the light of the gold and silver moons. Places close enough to the ocean to be warmed by steam will be good places for plants. But as you climb in altitude it gets very cold. And snowy - the steam falls as snow and produces glaciers which gradually push their way downslope back into the oceanic hot tub.

A great place for a ski vacation!

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    $\begingroup$ With large oceans and a thick atmosphere cycling heat, geothermal warming on the dark side isn't necessary. Current models of this sort support largely ice-free antistellar surfaces. $\endgroup$ – rek Sep 2 at 16:53
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    $\begingroup$ The problem with a bright moon is that it's kind of incompatible with a tidally locked planet. Specifically, the tidal force exerted by the moon is proportional to the cube of the angular diameter, while brightness is proportional to its square. So a bright moon needs to loom big in the sky, and that means it's going to exert a strong tidal force on the planet, preventing the planet from tidally locking to anything else. $\endgroup$ – Ilmari Karonen Sep 3 at 10:09
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    $\begingroup$ (A very high albedo moon would help some, since it could be smaller while still being bright. But it's hard to see how a moon around a planet warm enough for life could have such a high albedo, since most of the low molecular mass volatiles that could form a high albedo coating — like water, ammonia, etc. — would evaporate and escape so close to the sun. Unless, of course, the moon was big enough to retain them gravitationally — but then we'd basically have a double planet, and again run into the same tidal issues. Unless the planets were really far apart, but then they wouldn't stay bound...) $\endgroup$ – Ilmari Karonen Sep 3 at 10:16
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    $\begingroup$ @IlmariKaronen - what about a metal moon? Metal can be shiny. Like this: en.wikipedia.org/wiki/16_Psyche $\endgroup$ – Willk Sep 3 at 13:48
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    $\begingroup$ Also possible (and brighter than a moon) is a binary star system, with the second star larger and brighter than the one that the planet is locked to, but distant. (It can't be close for reasons of orbital stability). $\endgroup$ – nigel222 Sep 4 at 9:04
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It's possible, but complicated, and probably limited to very simple things at best.

The first thing to keep in mind is that tidally locked planets are going to have crazy weather, due to the massive energy differential in the atmosphere. I'd expect strong winds to be common, especially around the more temperate transition zone where you'd expect life to concentrate.

While the hot winds will keep the dark side warmer than it would otherwise be, it will still be extremely cold - think the months-long Antarctic nights on Earth for an idea of how cold. If the oceans flow freely between the light and dark sides, they would also have strong currents powered by the huge energy difference, but the precise geography would determine how much this can warm the dark side.

Plants on Earth absorb light within specific wavelengths, mostly but not exclusively red. The star will be emitting more in the red, but that has to get to the plants to be used. For the moon to do this, it needs a very high albedo, and to reflect in wavelengths that the plants can use. It's possible for the plants to be adapted for different wavelengths, so color doesn't have to be an issue, but the moon needs to be extremely bright. The closer to the planet, the better, but beware how its gravity and orbit would affect the planet (could it even orbit like this, or would it need to also be tidally locked? If so, would the planet not block most of the light?)

Even with all that, supplementary lighting would be helpful, if you want more than the sorts of lichens and such we find in Antarctica. You could add other light-sources, such as nearby gas-giants. If the setting allows for it, orbiting mirrors could be used to focus more light on the dark side of the planet. Artificial lights and greenhouses would also help.

Realistically, though, getting anything more than Antarctic levels of growth on the dark side would be very difficult. The closer to the transition zone, the better, just because it would be warmer there, but you're still looking at mostly mosses and lichens, or setting-appropriate analogs.

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On Earth, Fungi are not plants (nor animals. They're a third major branch of the tree of life).

Before modern understanding of DNA it used to be thought that fungi were plants that first became parasitic on other plants, and then lost the ability to photosynthesize because they no longer needed it. Finally, they evolved the ability to live off the bodies of dead plants, as well as parasitizing live ones.

Elsewhere, this may be exactly what happened.

On the dark side, are plant-fungi, getting their energy from stuff brought in on ocean currents such as driftwood. They'll be found in a thin zone close to the shore (no further than their roots / hyphae can stretch).

It's also possible for life to use geothermal energy, such as the life on Earth around deep volcanic vents "black smokers". Again, elsewhere, this may be evolved from plants rather than animals.

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I am afraid your main problem will be the low temperatures.

Lunar light is sufficient to have a visible effect on vegetation (the old school farmers knew that seeds grew better with rising moon, but not because the rise influenced the growth, as they believed, but because of the additional dose of photons supplied by the moon), however the temperatures staying on the low side will severely affect the growth of any plant.

With the right conditions you might get some moss or lichens to grow, but nothing more, I am afraid.

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    $\begingroup$ How do you know that the dark side temperature will be low enough to preclude plant life? A tidally locked planet will have a very large temperature range assuming it has an atmosphere. Why couldn't there be a band that was habitable? $\endgroup$ – Mike Nichols Sep 2 at 15:47
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    $\begingroup$ I'm talking about heat distribution via winds and currents from the light side to the dark side. The planet will be hottest in the middle of the light side and coldest in the middle of the dark side, but every temperature in between those extremes will exist at some band along the planet. Air will expand on the hot side and condense on the cold side driving massive winds that will redistribute the heat from the light side to the dark side and reduce the temperature differential. $\endgroup$ – Mike Nichols Sep 2 at 16:04
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    $\begingroup$ @MikeNichols and the condensed air on the cold side will magically melt again and go back to hot side, I guess... $\endgroup$ – L.Dutch - Reinstate Monica Sep 2 at 16:14
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    $\begingroup$ There are models of tidally locked worlds with large oceans and thick atmospheres supporting the idea of ice-free (or largely ice-free) antistellar surfaces; the larger/thicker the more heat will be transported and the farther the warming will extend. The dark side as a permanent icebox slowly locking away all moisture and freezing out the atmosphere is no longer a given. $\endgroup$ – rek Sep 2 at 16:57
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    $\begingroup$ The preconditions of the question don't imply this answer. What if the whole planet is really hot (due to being close to the sun or somesuch), and the dark side is in fact the only side with a temperature low enough to allow plant life? $\endgroup$ – T.E.D. Sep 3 at 20:26
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Why not plants that can use the radioactive decay to power photosynthesis? (see this great answer for the chemical feasibility)
Since the planet is tidally locked, the tidal heating could cause strong volcanic activity. These magma hotspots could provide heath (still necessary for the plant in order not to freeze) and a steady flow of radioactive elements to the surface. Plants could then grow and activate photosynthesis thanks to the gamma radiation from these elements.

Regarding the evolutive path, the best explanation is that some plants near volcanoes in the dusk zone (the part of the planet in the transition zone from day to night) developed a mutation that allowed their leaves to exploit the gamma radiation (directly, or through a scintillator layer, that is a substance that can convert gamma photons to visible-wavelenght photons).
Such a mutation would be very advantageous, since it allowed them to colonize the dark side of the planet. They could indeed spread their seeds in the atmosphere (exploiting the strong winds), so that the ones landing near another volcano could germinate and slowly colonize the dark zone.

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Carnivorous Plants

We have them here on Earth. Sure, Venus Fly-Traps, Pitcher Plants, and the like still rely on photosynthesis in addition to their steady diet of insects, spiders, and small birds, but had they evolved on a world without a regular source of sunlight, they might have become more specialized at catching and digesting unwary animals for nutrition, possibly becoming large enough to even pose a danger to humans (or whatever other sapient race(s) inhabit your planet). It's possible that on the borders of the dark side you'd see some less dangerous carnivorous plants, since they still have some access to light at certain times of the year, but on the full-dark side, it would be truly perilous for adventurers to tread - the less light the plants get, the more they rely on meat to survive, so the more vicious they become. This would especially be a problem for any animal that relies on eyesight to identify potential threats, as they'd be effectively blind on the dark side.

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