I’m looking for speculative but scientific answers about plants on other planets.
Plants on Earth are typically green, so I understand it, because they absorb light in the blue and red part of the spectrum well, but in the green part poorly. Since green light is reflected, not absorbed, plants appear green. Sunlight has a lot of red and blue light in it, which is why plants primarily focus on it.
So what about possible plant life on other planets? Speaking from a science-fiction standpoint, could there be other worlds with predominantly red plants, or black plants, or yellow?
Would all the planets in the solar system have plants the same color (because the star emits a certain type of light)? Or might one planet have red plants, and another planet in the same star system have blue plants? Is there a connection between the color of the sky and the color of the plants?
I haven’t read the NASA link, but the color of photosynthesizing organisms would be dependent upon the chemical the happened to be used. On Earth, organisms using chlorophyll (which is green) gave rise to multi-cellular forms, which we notice since we are multi-cellular forms ourselves.
There are any number of potential chemicals which could work to capture light energy to use for food. The perceived colors would depend on the combination of the chemical, the spectrum of the star, and the atmosphere of the planet.
There is nothing magical or inevitable about green plants.
In fact, it’s believed the first photosynthetic organisms on this planet were purple because they used retinal to absorb light at the sun’s maximum (in the greens), and reflected blues and reds. You have retinal in your eyes as well.
At some point organisms (the ancestors of modern plants) evolved to not compete with retinal-based organisms, and collected light in the blue/red ends of the spectrum, reflecting green. It’s also thought that maybe they didn’t collect green to avoid heating up. Regardless, chlorophyll-based organisms eventually out-competed the retinal-based organisms, probably because chlorophyll has one of the highest molar absorptivities known, and retinal has to drain nutrients from other biosynthetic pathways.
I think if you go to other planets, you will usually find plants absorbing in the range of the maximum output of the sun (I think our sun’s is 555 nm), or near it. I think it’s very random that plants on this planet don’t absorb that wavelength, but there were specific reasons for it. Then again, maybe chlorophyll is so advantageous that the evolution of it would occur over time on any planet.
To clarify, sunlight has more green in it than red or blue, so Earthly plants are actually reflecting away a fair portion of what hits them. It just so happens that chlorophyll is very good at getting usable energy out of what it doesn’t reflect away, so it ends up being more efficient overall than the other competing substances. There may well be some chemical which would have chlorophyll’s efficiency while also absorbing more in the green, which would then be even better than chlorophyll, but it just so happens that evolution never stumbled upon that chemical, so it never got the chance to prove its worth.
It’s quite plausible, though, that on some other planet, chlorophyll might not have evolved, or some other chemical might have evolved that’s better than chlorophyll, so there’s no real reason to expect that alien plants would use the same stuff as Earth’s.
In fact, there are other chemicals in Earth-evolved life that also perform photosynthesis, including xanthophylls (yellow or orange), anthocyanin (red, purple, brown), and the carotenes (typically orange or yellow-orange) found in photosynthetic plants, algae, and cyanobacteria and their precursors. You see this every autumn when seasonal plants ‘turn’ their leaves, going from green to yellow to orange and red, and occasionally in plants with variegated leaves. So, in fact, we would entirely expect plant-like phototrophic organisms evolved on other worlds to be colors other than green.
Am I correct that the general consensus for this sort of question is that extraterrestrial plants (how’s that for an interesting term?) will be under more or less the same kind of selective pressure to become the leanest, meanest most efficient photon-eating badasses possible? If so then it seems the standard answer would be “whatever spectrum is most energy-efficient to absorb” and doesn’t necessarily have to be chlorophyll.
Would it be at all advantageous to develop the ability to absorb high-energy radiation like deep UV or Xray? If so, what color would they be?
I’m not sure whether that would be possible or advantageous. But if a plant was getting all its energy from non-visible light, then its color wouldn’t have anything to do with the light it absorbs. Instead, it could be whatever color lets the plant blend in with its surroundings to avoid predation, or stands out to attract pollinators (if some of these E.T. plants use pollinators?), or just whatever the rest of the stuff in the plant happens to be colored (for what it’s worth, Monotropa uniflora is a parasitic flowering plant which doesn’t have chlorophyll and is white.)
Minor addendum: “Whatever is most energy-efficient out of the set of things that evolution happened to try”. Remember, evolution is not prescient. It can only work on things that arise randomly, so if something never arises randomly, natural selection never gets a chance. So even in identical circumstances (same kind of star, same atmospheric absorption, etc.), you might not necessarily get chlorophyll again.
In fact, it’s probably even fairly unlikely that you would get the exact structure of the chlorophylls again, although given the same preconditions there is a not insignificant chance that you’ll get something resembling the central chlorin ring, albeit perhaps using something other than magnesium as the central ion…maybe calcium or zinc. On the other hand, the abiogenesis and early evolution of fundamental structure is probably highly perturbative and there are plenty of other ways to convert external energy, so extraterrestrial phototropic organisms are likely to have some completely novel ways of absorbing sunlight depending on the spectrum, element concentrations, and utter random chance.
Absorbing X-rays is unlikely; UV is somewhat less so, although most life that we know cannot withstand large amounts of UV (but non-Earthlife may be more robust in this regard). Infrared is a possibility, but the problem there is the low energy plus the need to exhaust waste heat back to the environment, so that would have to occur in a very low temperature environment. Most likely phototropic organisms as we would recognize them as plant analogues would convert radiative energy in the visual spectrum.
Thank you for all the informative replies so far. This is a fascinating thought exercise, I can see that now.
I have another question, though. Is there any necessary link between the color of the sky, and the color of the plants? Would certain combinations be more likely? (I saw the NASA wavelength chart but there isn’t much explanation what it might mean.)
Sky colour and plant colour link? Only in that the photosynthesizers are limited to frequencies of electromagnetic radiation that the given atmosphere lets through.
I do not have the math or the imagination to make the connection between light absorbtion of an atmosphere and the perceived sky colour generated by scattering.
Well, our thin nitrogen-oxygen atmosphere scatters blue and violet light due to Rayleigh scattering, so these would not be the most efficient wavelengths for plants to absorb. Presumably, an atmosphere of different composition–say, one that scatters yellow and green light per the Lorenz-Mie solutions to Maxwell’s equations–would cause organisms to evolve to utilize red or violet wavelengths instead.
But if the atmosphere were a different composition, it would be unbreathable, I presume. That suggests that any habitable planet would necessarily be similar to ours, yes?
Not that our planet is filled with 100% green chlorophyll-using plants, of course.
Possibly pollutants in the upper atmosphere would filter certain wavelengths of light — volcanic debris, perhaps, on a highly unstable world — that would cause the plant colors to change. Hmm…
Unbreathable for us; not necessarily for organisms evolved in an extraterrestrial environment. Chlorine is toxic to us (and oxygen at atmospheric levels is toxic to many primitive organisms) but for an alien species it might be the ideal respiratory medium.
For that matter, as far as most Earth life is concerned, the nitrogen in the atmosphere is just filler. You could replace it with any other nontoxic gas and be just as breathable, while possibly changing the Raleigh scattering properties and thus the color of the sky.
Not entirely; the nitrogen cycle depends on the fixation of atmospheric nitrogen by other organisms, mostly cyanobacteria, but there are a few archaea and the leguminosae plants, and other plants that have developed symbiotic relationships with diazotrophic bacteria). While animals and most plants don’t use nitrogen in their respiratory cycles, nitrogen is nonetheless an important element in amino and nucleic acids for all Earth life.
