Putting the life back in science fiction


Vegetation on a Red Dwarf world

I’ve been running a blog post on Antipope while the owner is otherwise occupied.  Part of that posting was a short riff on what it would be like to colonize an earth-like world that orbits a red dwarf star Rather than bore that (largely techie) crowd over there to tears with an extended botanical geek-out, I figured I’d post it for the smaller, more discerning group here.

Here’s the question du jour: what would plants look like on a red dwarf world?

The first, lazy reference I’m going to use is Wikipedia’s article on the habitability of red dwarf planets.   SFReader also flagged The Many Worlds post on the subject. The reason we care is that red dwarf stars are over three-quarters of the stars in the galaxy.  Assuming it’s possible to travel to other stars and colonize planets around them, red dwarfs, assuming they can support an oxygen atmosphere and our kind of life, may hold the bulk of the available real estate.

That said, such systems are very different from our solar system.  Red dwarfs are tiny: Trappist-1, which has seven planets around it, is over 90% smaller than the sun, while the biggest are less than half the sun’s size.  As a result, they’re cooler, so their light peak production is in the infrared, probably meaning that it would take more photons to power each photosynthetic reaction.

The planets have to be really close to the star.  The orbital periods (years) of the Trappist brood are in the same range as those of the Galilean satellites of Jupiter: 1.5 to 18.6 days.  Since they’re so close, the planets are almost certainly tidally locked, at least within the habitable zone around the star.  What this means is that the planet’s day is the same length as its year.  On the planet side facing the star, the star doesn’t move in the sky, while the planet’s back side is eternal night.  Similarly, there’s an ocean bulge of a kilometer or more at the subsolar point directly under the sun.    The result of this is a global atmospheric Hadley cell, where air on the day pole of the planet gets heated and picks up water from the ocean almost certainly under it.  That humid air rises until it cools enough to form clouds (frozen water crystals) at which point the clouds move out, as does dry air above them.  These don’t fan out equally, because the planet is rotating slowly.  However, eventually the air ends up on the dark side, sheds any remaining moisture it has, falls down to the surface, and the, because air is piling in on top of it, blows back towards the day side to start the cycle over again.  The modelers argue about how thick an atmosphere is necessary for this global cycle, and how windy it gets.

While red dwarfs are cool and very long-lived (trillions of years), they’re also highly unstable, producing huge sunspots that can drop their light emissions by up to 40%, and blowing out huge storms of X-rays and energetic particles.  While they settle down over time, the first few billion years of a red dwarf’s life is rather violent, and it is thought that they can blow a lot of atmosphere off the planets orbiting them through their flares.  The X-rays are also much higher than solar x-rays experienced on Earth, to the extent that trekking on a red dwarf planet probably requires a radiation shield and a dosimeter.

So what about them plants?

First off, I’ll bow to the standard cliche and assume that the foliage is black, although any color other than red is possible.  Plants leaves are the color of the light they reject.  On Earth, our plants use red and purple light the most, reflecting green–the conventional color of leaves, although if you actually look, leaves come in many colors.  On a red dwarf world, the idea is that leaves would try and snag every photon, so they’d be black. Maybe, maybe not, but I’ll go with it here.

Second off, I’m going to ignore the radiation hazard.  On Earth, tardigrades and bacteria like Deinococcus radiodurans (aka “Conan the Bacterium”) survive radiation that would be lethal to humans, through really good cell and DNA repair proteins.  It’s fair to assume that any life under a red dwarf star has come up with similar adaptations or been relegated to the dark depths.  That doesn’t mean they don’t respond to huge solar flares, but it does mean that the planet is not sterilized by the radiation it experiences.

At a first pass, the biggest forces shaping the vegetation are  competition for light and wind.

The thing to remember is that the light is always coming from the same angle.  While that light varies due to things like clouds and sunspots, there aren’t any seasons on a red dwarf planet, even if the orbit is fairly eccentric.  Or rather, there are seasons, but if the year is four days long (as on Trappist 1d), each 24 hours is a different season, suitably modified by solar flares and sunspots.  We get similar weather in every part of Earth where the local saying is “wait five minutes and the weather will change,” (continental interiors) or “we get four seasons every day” (tropical highlands).  In other words, not a problem on Earth, exactly, so probably not a problem elsewhere.  Anyway, getting back to leaf angle, this means that leaves are optimally held perpendicular to the sunlight, whatever that angle is.

This is very different from what plants experience on Earth, where the sun moves 47 degrees across the sky north/south over the course of a year, and 180 degrees across the sky east to west every day.  On Earth, a tree canopy is optimized to some sort of roundish blob, to catch light from all angles.  On a red dwarf world, a tree should hold all its foliage perpendicular to the light.  While there are good reasons to put multiple layers of leaves up to intercept the light, there’s little reason to put up leaves at any other angle, because the red sun doesn’t move.

There’s still fierce competition for light, and the plants will be trying to outgrow each other in every sunny spot.  Still, the canopy layers will be at perpendicular to the sun angle, not perpendicular to the ground as in terrestrial forests and jungles.  On flat ground, this may well lead to the usual carpet of trees, except that all of the foliage will be on one side of the tree.  Probably the sites for greatest competition will be on fertile, slopes where the slope angle is perpendicular to the sunlight.  That will lead to both a lot of competition for light and lot of tree-falls, leading to a lot of local disturbance and succession.

As for the wind, that’s a force that shapes plants on Earth.  On a red dwarf world, with its global air circulation, some places will be windier than others.  The cool thing on a red dwarf world is when there’s a steady wind from one direction, and the light’s only coming from some other direction, then the tree will be shaped in a profound and unearthly way by both forces.  Remember, on Earth trees get light from many directions, while on a red dwarf world, they only get lit from one direction.  It makes a difference.

As for the diversity of vegetation: I’ve talked about trees above, and since Earth has been producing trees for 350 million-odd years, I think trees are a safe bet anywhere the climate is predictable and plants compete for light.  That’s what trees are about, to a first approximation: competing for light.  On a red dwarf world, the shadows tend to be permanent.  I suspect some plants might be able to live off the backscattered light in the sky, but that’s not much to work with, and terrestrial plants that do this (like the fern <i>Hymenophyllum</i>) tend to be pretty tiny.  I’d expect biological crusts and moss mat equivalents in the deep shadows.  Similar low plants might be found in the understories of some red dwarf forests, too, especially near the terminator where the sun is perpetually low in the sky.

Would there be deserts?  Yes, but they would be in rain shadows (where mountains squeeze the rain out of clouds), because the other cause (subtropical Hadley Cells on Earth) doesn’t happen on a red dwarf world.  The problem with a rain shadow is that the mountain blocking the rain also blocks sunlight, so the deserts would be rather narrow compared to those on Earth, in areas that are in the rain shadow of mountains, but not in their light shadow.  Death Valley or the Great Basin in the US would be terrestrial analogs.

One thing to remember is that vegetation doesn’t change by latitude as on Earth.  It’s easier to consider that red dwarf worlds have “day poles” (the subsolar point directly under the star) and “night poles” (the point directly opposite the star).  If you mapped “latitude” from day to night, that would be what plants responded to, with plants not growing much past the terminator line where the sun is not above the horizon.  The north and south rotational poles would be on the terminator, and experience similar light environments.

Finally, there is how the plants deal with radiation storms.  I suspect their primary method for dealing will be rebuilding the damage, as do tardigrades.  During unfavorable space weather, I’d expect plants to simply wilt their leaves, drooping to get their surfaces parallel to the incoming light, so they take a bit less damage.  My squash plants are doing that now during hot afternoons.  Of course, if a plant part takes too much damage, it will die and be discarded, so a severe solar storm might be accompanied by widespread leaf shedding, followed by a “reblackening” (instead of regreening) as new growth is produced.

It may turn out that it’s possible to create a decent radiation shield out of biological metamaterials, akin to the foam metals that appear to be good radiation shields now.  If so, I’d expect analogous structures, perhaps a thick, foam-like bark, may evolve naturally on red dwarf worlds.  Repairing cellular damage takes time and resources, and if there’s a way to make a durable structure that absorbs the damage, then it might be advantageous to have it.  Bark, on terrestrial plants, is a structure made of dead cells, so dead bark that that acts as an x-ray shield would be very useful for protecting the live tissue underneath.  Perhaps animals will sport radiation shields too.

A final note: I’m not sure how deep red dwarf light will penetrate into water, because water absorbs red light fast and the stars don’t produce a lot of blue and purple light.  On Earth, photosynthesizers are limited to the top 100 meters of the ocean.  On a red dwarf world, it might be limited to the top ten meters or less.  That profoundly makes life in the oceans profoundly different than what we see on Earth–photosynthetic reefs and deep kelps wouldn’t work on a red dwarf world, and there would be no reason for animals to shelter in the deeps during the day and rise at night, because there’s no day/night cycle.  Perhaps there would be more floating biomass, akin to sargassum, but perhaps more developed?

Anyway, that’s my first take, off the top of my head.  Was that sufficiently confusing?  What did I miss?

 

 

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4 Comments so far
Leave a comment

Thanks for this.

According to the Wikipedia article red dwarfs stop flaring after a billion years or so. If true, radiation ceases to be such an issue.

Given the need to collect as much light as possible I would imagine photosynthetic stems as well as leaves. There is also a reasonable possibility of yellow or orange leaves. Paralleling Earth leaves might grab the most energetic light (blue) and the least (infrared) allowing for redder leaves

Also, given an eccentric orbit you would not have 1:1 tidal locking but a resonance (e.g 3:2), which is what has happened with Mercury. See the wikipedia article on Gliese 581c (https://en.wikipedia.org/wiki/Gliese_581c.)

I have no idea what proportion of planets would experience tidal locking as opposed to a resonance, but I would think tidal locking is more likely to be common. You could certainly have a non-tidally locked planet if you wished – but that implies an eccentric orbit.

Comment by PubliusJay

You’re welcome, and thanks for your comments too. Actually, in most plants, the primary stem tissue is green. Because of the way bark is formed, it’s harder to get photosynthetic bark, but that gets done too (as in Palo verde).

I’m not sure what goes on with red dwarfs flaring. My impression is that they don’t stop entirely, they just flare less frequently. After all, Earthly life took 4 billion years to evolve an oxygenic atmosphere. If red dwarfs only flared for a billion years, I don’t think it would be seen as the problem researchers think it is.

Comment by Heteromeles

Regarding longevity of red dwarfs, I have always through that that is probably not the limiting factor for longevity of a biosphere. Here in our solar system, with our rapidly brightening Sun, it is solar effects that will squash our biosphere eventually, but Mars died on its own before the Sun could do anything to it. Around a very long-lived star, I would think that the planetary geospheres and atmospheres would probably die long before the lifetime of the star became relevant.

Comment by Tony

Probably right. There’s a paper by O’Neill et al. (A window for plate tectonics in terrestrial planet evolution?) that you might want to hunt out, modeling the evolution of planetary crusts as a function of how hot the core is. That’s one limit. The other limit for red dwarfs appears to be how turbulent the star is, and how capable the planet is of resisting damage from stellar emissions. These emissions decline over time, but remain nastier than our Sun. Therefore, I’d guess that the window for complex life on a red dwarf exoplanet is bracketed by when the star behaves itself sufficiently for the surface to be habitable (the beginning), and when plate tectonics ends (the end). Microbial life will last for billions of years before and after this, and I suppose it’s possible that a Barsoon-like condition may persist for some unknown time after plate tectonics ends. With a system like TRAPPIST-1 with seven planets in really close proximity, their gravitational interactions add some energy to prolong plate tectonics (Barr et al., Interior Structures and Tidal Heating in the TRAPPIST-1
Planets).

Comment by Heteromeles




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