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What colors and attributes could alien flora & fauna have?

  1. Mar 29, 2016 #1
    So I'm writing this comedy Sci-Fi story and I want it to have some realism. One of the main characters is a sentient alien, and I want to do a little better on the Sci-Fi hardness scale than "rubber forehead alien." The chances are high for any alien life we encounter to be synthetic, so to clarify, I'm talking about organic alien organisms and their planets. I was a social science major so I don't have a lot of background in the hard sciences, but I've done a little research.

    What I'm looking for is a sort of guide for coming up with alien planets and the alien races and animals that live on those planets. In order to do that, I need to fill some gaps in my knowledge about why things are the way they are here on Earth.

    Since a lot of this is TL;DR, I've put my big questions in bold. I hope your expertise can help, internet!

    From what I've gathered, the majority of plant color is determined by the dominant wavelengths on the electromagnetic (EM) spectrum emitted by their planet's star, which is determined by its temperature. On the extremes, plants on an exoplanet with a sun lower on the EM spectrum (a reddish sun) would need to soak up as much light as they can muster, and may be darker, maybe black, and could potentially host a rainbow of colors. On an exoplanet with a brighter star, the plants may be brighter to reflect some of the more energetic light back, maybe even white (and reflect "excess high energy photons and appearing slightly blue" according to this). In-between planets (like ours) may have plants somewhere in the middle of the spectrum. Here seems to be a handy, slightly more detailed guide on star-temps/plant-colors.

    But there is an issue: plants on Earth would be more efficient if they were black, right? I'll quote this article:
    "Origin of chlorophyll- The purple pigment absorbed all wavelengths of light except for the reds and violets. Thus, any bacteria using purple pigments that lived deeper in the water than the purple bacteria on the surface would have no light to use because it had all been absorbed by the surface bacteris (exploitative competition). Because red and violet wavelengths pass through to deeper water, bacteria that contained a pigment that was able to absorb these wavelengths would be able to coexist with the purple bacteria. This was the origin of chlorophyll... chlorophyll is the dominant photosynthetic pigment today (there are still examples of photosynthetic bacteria with purple pigments, but they are limited to very harsh environments). Interestingly, chlorophyll came to dominate, not because it was a better at absorbing light energy, but rather because the cyclic flow machinery associated with chlorophyll was more efficient at producing ATP than the machinery associated with the purple pigment was. Thus, it is an evolutionary accident that modern plants are green." (note that there are other theories as well). Also troublesome is that some plants here on Earth are colors other than green.

    ... so what does all this mean for exoplanet flora?

    A thought occurred to me that most animals try and blend in to their environment in some way. Soils on our planet are mostly variations of reddish, yellowish, brown and beige, and a lot of natural camouflage reflects that. Aside from a few exceptions, sand and soil on Earth isn't very colorful. I know colors can vary, for example Mars has red soil due to iron that may be able to support life. The reflective properties may play a factor in what they look like, but I'm wondering about the soil itself, say under perfect white light. Organic life, minerals, etc. can affect soil color, but is planet-wide colorful soil that can still support life possible?

    Our normal daily sky is blue due to Rayleigh scattering, and our ocean color has something to do with the sky... but could it be different on other planets?

    • According to this, "stars with surface temperatures of 3,300 kelvins or lower (red dwarfs of spectral type M2.5 such as Gliese 581, or redder) would emit so fewer photons towards the bluish wavelengths compared to Sol that the sky would appear whitish down to reddish to Human eyes (more from Earth Science Picture of the Day). If comparatively more bluish or reddish light reaches a planet's surface than on Earth, photosynthetic plant-type life may may not be greenish in color, because such life will have evolved to different pigments in order to optimize their use of available and so color the appearance of the planet's land surfaces accordingly."
    • According to "jjdebenedictis" here: "The sky is blue because the molecules that make up most of it are "dipoles", i.e. there's an electrically negative end and an electrically positive end to the molecule.
      Light is electromagnetic radiation. That means it will interact with these electrical dipoles. Specifically, there's a small chance that light passing close to a dipole molecule will bounce off that molecule. Furthermore, light with a short wavelength (i.e. bluer end of the spectrum) is more likely to bounce off than light with a long wavelength (redder end of the spectrum.)
      So when you look at the sky, and see all the sunlight that has bounced off the air itself, there's more blue light reflecting down to you than red. Hence, a blue sky.
      Another point: the light, when it bounces off a dipole, tends to bounce sideways.
      And this explains why sunsets and sunrises look red. At those times of the day, the light has to travel through a lot more air because it's coming in at a glancing angle through the skin of atmosphere on our ball-shaped world. Thus, at sunrise and sunset, most of the bluer light scatters out sideways and there is only the redder light left to reach your eyeballs.
      How does this impact alien worlds? Well, there are a lot of different gases that form dipoles, and those that don't form dipoles don't scatter light very effectively. So most atmospheres, regardless of the composition of the air, would produce a blue sky.
      ... however, impurities do have an effect. The sky of Mars looks red due to all the dust in it, and! (this is really cool) the sunsets and sunrises look blue because so much red light gets scattered off the dust that there's only blue light left to get through...
      One way you could play around with sky colour on an alien world would be to have the star, i.e. the planet's sun, have a different spectrum than our Sun does.
      If the planet's sun is a red star, then the sky might look green or yellow to human eyes. If it's a blue star, the sky might look purple or even black with the stars showing through because the light reflected is mostly in the non-visible ultraviolet. (Ouch. Sunburn city.)
      Note that this is only how humans would see it, however. Creatures that had evolved on that world would have eyes that see best at wavelengths of light that their sun puts out. To them, the sky would look blue; it's just their idea of "blue" would be at a higher or lower wavelength than what we call "blue".
      By the way, the colour of the sky is a darker blue at higher elevations because there's less air above you to scatter light. You're effectively seeing a bit of the black of space through the blue.
      So a thicker or thinner atmosphere would be a way to change the colour of the sky also.
      As for water, it forms a dipole too, so you should get similar effects from it, but that's not something I ever learned about specifically.
    • on the same site, YeonAh says: "White clouds are caused by CIE scattering, which is larger particles that scatter ALL wavelengths in all directions so they combine as white in our eyes.
      With all that said, by changing up the particles in the atmosphere (as mentioned above), you can easily change the color of the sky. Want a green sky? Knock off wavelengths on both extremes of the spectrum to get that green color in the middle. What happens if the clouds get rid of all the blue in the light and turn red? What if there's a gas between the sun and the planet that takes all the red wavelengths out of the light, so absolutely no red reaches the planet?
      Also, about the ocean. According to my university teacher at least, the ocean is NOT blue because it's reflecting the sky. It's because the particles in the water absorb small amounts of light, starting with the wavelengths on the red side of the spectrum. In something like a glass of water, this amount is so small it doesn't matter and the water will appear clear. But in large, large amounts like the ocean, you can see the water as a deep blue because all the red's gone. When water appears green, it's mostly because green is the mid-point between red and blue in the spectrum, and the water isn't deep enough to be blue. Go deep enough, and all wavelengths will be absorbed so the water will be...dark."

    So are these quotes correct? What could cause the sky to be purple, pink, yellow or green?

    Plants are crazy, even on Earth. They adapt to their environment in every way, shape, and form, it seems, depending on all sorts of environmental factors - sources of energy, On an alien planet, there could be seas of algae (not a plant, granted), giant lillypad things, black trees, etc. I haven't even scratched the surface in my research for possible plant characteristics. Here are some ideas.
    What do you think is most likely? Earth-like plants, or what?

    Information on skin color was tough to come by. I believe the way feathers, scales, and whatever things like Jellyfish have are structured is what allows them to get so colorful. I know that in humans, the lightness and darkness of pigmentation is caused by melanin and relates to the amount of UV light the area gets - more and the skin is darker, less and the skin is lighter. It can't be too dark in areas with less UV because we need some of those rays in order to get Vitamin D, and those with light skin are more prone to skin cancer in areas more exposed to UV. I get that, but what I have more difficulty finding is why our skin isn't more colorful - why isn't our skin, say, blue, green, bright fuchsia, red, purple, yellow, etc. (excluding disorders)?

    The closest I found was this site, which says "Pigments that contribute to skin color are called carotene, a yellowish hemoglobin, in blood vessels (pink-red), and melanin (black, brown, red). Darker skins are dominated by melanin, which is produced from the amino acid tyrosine, by pigment cells (melanocytes) in the skin. Melanocytes are characterized by long, fixed extensions of the outer cell membrane. In humans, other mammals, and birds, melanin is dispersed permantely throughout each melanocyte, including the extensions, and is also, transported to nearby skin cells. In other words, if you increase the amount of melanin in the skin you become darker and vice versa." This leads me to believe that our skin is almost translucent, and it's the color of the stuff under our skin (like blood vessels) that lend it the pinkish color... but then why does our skin tend to get dark brown instead of black (like some pigs)? Since our bloods have iron carried by hemoglobin, our blood is red; if what's under the skin can lend the creature its color, is it possible for an intelligent alien species, especially something mammalian, to have blood a color other than red?

    All kinds of variables exist of course, such as differences in gravity, that would affect organisms - low gravity could mean weaker and taller aliens, high gravity could mean stout and strong aliens, for example. silicone-carbon hybrid life forms may exist. Here on Earth, young planthopper insects actually have mechanical gear wheels in their legs to help with jumping. The environment may promote things Earth doesn't seem to have, light a thick atmosphere causing creatures to be able to communicate non-visually (sensing electrical pulses through the thick humid air) or aliens with bio-luminescent patches on their skin. Especially for non-intelligent animals, the possibilities seem endless.

    There are some specific things that intelligent life most likely possesses. I have a whole document of notes from a variety of sources.

    • They likely need to be land-based because fire is the basis for the development for a lot of technology (water doesn't mix with fire); this means they would probably have something like lungs to breathe the atmosphere and an internal (skeletal) structure to support life on land.
    • Being on land, they probably wouldn't be too squishy so they would have something protecting their internals (chitin or bones and a coating like skin).
    • They need delicate grasping manipulators (like hands), and thus probably aren't quadrupeds (we can't manipulate with our feet very well, they're our workhorse limbs).
    • Most creatures are bilaterally symmetrical. They need good enough senses to detect predators and prey from a distance (probably needing light and auditory sensors).
    • They would need to be social and have the ability to pass on information to younger generations (a decent memory and a complex way to communicate).
    • Eyes on the side of the head allows you to spot predators from a greater viewing angle, but forward binocular eyes give one better depth-perception (which also means they need at least two eyes, probably with a lens and an iris), which aids in capturing prey; predators are more intelligent than prey and since an intelligent, sapient species would likely be an apex predator, they may be more likely to have forward-facing eyes.
    • delicate parts are generally protected, which is why our eyes are recessed, our brain is within a skull, we have a rib cage, etc.
    • Since evolution doesn't like having things that are unnecessary, creatures like Minotaurs are unlikely (why have 4 legs when you can have 2?).
    • The main sensors would probably be on or close to the head, like most creatures on Earth, because it provides the brain with the fastest and best signal.
    • It would need a brain and nervous system of some sort in order to feel, control its body, and process information.
    • If it had an olfactory sense, especially in the form of a nose like ours, it would probably be above the mouth and under the eyes, for the same reasons ours is (better for swimming, doesn't get in the way when consuming, nasal drainage isn't pleasant if it gets into the eyes, sharing the windpipe allows us to breathe with our mouth closed, etc.).
    • Its eyes would be above the mouth and nose, because when consuming nutrients, you don't want the food to fall into the eyes or block the field of view.
    • Auditory sensors (ears) would likely be on the sides of the head, close to the brain to ensure a good signal, and on the sides because this is the best way to detect the directional source of a sound.
    • It would need something to spurn a bigger brain, so it probably wouldn't be stationary, overly slow (like armored animals) or plant-based, as those forms of life do not promote complex survival strategies (and thus, a bigger brain).
    • A feathered, flying creature would probably need to be light, and thus have a small brain, making them less likely candidates. Scales are generally useful for ectothermic organisms, which seem less likely to be intelligent. Some combination, like scaly skin may be possible.
    • Eyes would probably be near the brain to reduce time delay and degradation of the signal
    • an elevated head is useful because it allows one to spot predators - the head and eyes being on the bottom of the body, for example, is unlikely.
    • They would need a means to extract and consume energy (meaning they're probably endothermic) from their surroundings to power their brains and limbs and excrete the waste from the energy somehow (probably a mouth or some analogue). Plants became fibrous to prevent being consumed, so their predators developed teeth to bypass that, and so on.

    One thing I don't know though is how aliens would see. Evolution seems to like 2 things: using only what it needs, and adapting to suit the environment. Since humans only need the visible light we see in the EM spectrum to survive, that's all that most of us see (notwithstanding Tetrachromacy). Some animals, like butterflies and insects, have reasons for seeing what we can't - bees navigate with UV vision for example. Since we don't need to be able to see (or sense) infrared, our bodies don't waste the extra energy and resources it would require to have that ability. BUT... on a planet with a sun emitting different wavelengths of light, would it be possible that the fraction of the EM spectrum that's most adaptable to be able to see would be greater or lower than on Earth? On, for example, a planet with a red sun would the inhabitants be able to see in the infra-red spectrum (and maybe they couldn't see the color violet as a result?)?

    • http://blog.sciencefictionbiology.com/2007/04/color-of-alien-flora.html
    • http://biol1404hon2015.blogspot.com/2015/02/why-arent-plants-black.html
    • http://www.nasa.gov/centers/goddard/news/topstory/2007/spectrum_plants.html
    • http://www.giss.nasa.gov/research/briefs/kiang_01/
    • http://www.astrobio.net/topic/deep-space/alien-life/colors-of-alien-plants/
    • http://www.nasa.gov/centers/goddard/news/topstory/2007/spectrum_plants.html
    • http://www.nature.com/news/2007/070409/full/news070409-7.html
    • http://www.solstation.com/life/a-plants.htm
    • http://www.ext.colostate.edu/mg/gardennotes/142.html[/URL]













































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  3. Mar 30, 2016 #2


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    It means you can make the plants just about any color you like.

    Possible? Almost certainly. Probably? No one knows.

    There's no way to know. Personally I tend to think that life on other planets would be vaguely like life on Earth, but when it comes to the details all bets are off.

    There's probably little to no evolutionary benefit for humans to have colorful skin. Not to mention the fact that we evolved from species who also didn't have colorful skin. I doubt any species within our lineage in the last several dozen million years or more has had the necessary adaptions to have very colorful skin. It may not be possible to re-evolve it without some substantial modifications to the molecular machinery in our bodies.

    Not sure. Perhaps the pigment in pigs is a different type of melanin?

    Sure. Some creatures here on Earth have blood color other than red.

    Probably. Though there would probably be a limit, as large amount of UV radiation can cause tissue damage, and if the radiation peaks too far into the IR range then the photons do not have enough energy to induce chemical or structural changes in the molecules used to detect light (assuming the aliens don't use some other method of detecting light)

    It's possible.
  4. Mar 30, 2016 #3
    Earth has white plants. Some plants, like some cacti, have leaves covered with dense hairy thorns, which reflect the incident light.
  5. Mar 30, 2016 #4
    Your plants will probably be the most efficient color for the star they are in orbit around. If your atmosphere is dense though, that may change what the most efficient wavelength is lower to the ground. For example: the green plants on earth would not like the orange light of Titan's sky. You may get a shading of fauna radiating away from the equator like humans and skin color. The equatorial plants may reflect more light to prevent themselves from baking, and the polar plants could be almost black.

    Dirt will be brown on all planets with life. Life is made of complex hydrocarbon, complex molecules tend to be brown. Enriching the soil with something like iron would make it reddish, however, after only a few million years, all of that iron would be buried because plants would continually bring everything else to the surface and leave the iron.

    Your plant shape would depend on your environment. In order to have big trees, you need low enough gravity for them to support themselves. If water is rare, they'd protect it, if nutrients are rare, they may be carnivorous. Your fauna can change depending on where you are, as long as it's well suited to survive and reproduce in that environment. For example, don't put a huge fern in a cave underground, evolution will force efficiency over size.
  6. Mar 30, 2016 #5
    By which reasoning?
  7. Mar 30, 2016 #6
    https://www.heliospectra.com/sites/default/files/general/What light do plants need_5.pdf Jump to 1. What is Absorption Spectrum?
    Plants on earth do photosynthesis with light that's primarily on the blue end of the spectrum. Makes sense considering that Earth's sky is blue. Earth plants barely absorb orange light at all, and Titan's sky is orange.

    This is true because of evolution, I see no physical reason that plants can't live under an orange sky, just not Earth plants.
  8. Mar 30, 2016 #7
    Chlorophyll, the primary pigment for photosynthesis in plants on Earth comes in various types which are good at capturing light at particular wavelengths.
    The mainly used wavelengths are at the blue end and the red end of the visible spectrum, and not at the yellow/green wavelengths where sunlight is most intense.
  9. Mar 30, 2016 #8


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    The color of the sky has little to do with the color of chlorophyll. The intensity of light scattered by the sky onto the plant is minuscule compared to the intensity of unscattered light from the Sun.

    Not true. Plant leaves absorb light across the entire visible spectrum. Note that in the graph at the bottom of this post (which is also found in your link) the pigment extract by itself absorbs much less light in the green region than the others regions, but when bound inside of the cells of whole leaves the absorption spectrum of the leaf is much flatter.

    That depends on what you mean by "primarily". From page 5 in your link:

    The areas of the spectrum that drive photosynthesis are highest in the red end (600-700 nm), followed by the blue region (400-500 nm) and lastly, the green region (500-600 nm). These data show that between 50 and 75% of the green light is used in photosynthesis.

    The percent of the incoming light used for photosynthesis is highest when the light is in the blue or red part of the spectrum, but a significant amount of the incoming green light is used as well. Also note that the intensity of light from the Sun peaks in the green area (500-600 nm), so this partially offsets the lower efficiency of photosynthesis with green light. Especially compared to the blue region, where the intensity is roughly half to three-quarters of what it is in the green region.

    Graph from page 5 page 4 in your link:

    Last edited: Mar 31, 2016
  10. Mar 31, 2016 #9
    "Absorb" does not equal "use". Leaves absorb a lot at 1450 nm, but this only heats the leaf and does not drive reactions (it's an absorption band of water).
    Which is a graph of absorbance - not actual use/photosynthetic reactions made by the leaf.

    Also note the contrast between the two environments of coloured light on Earth.

    There is little red light, but more blue, in deep clear waters.
    Plants in deep clear water adapt to it by actually rejecting and reflecting what red light does reach them: the red algae.
    They still have chlorophyll, but supplement it with phycoerythrin.

    Whereas green light is common - shade of other plants. A lot of plants are adapted to grow in limited light of forest understory.
    But they are still green! No one adapts by preferentially absorbing green light rejected by green plants.
    Why? Because no viable pathway for green light photosynthesis exists?
  11. Mar 31, 2016 #10


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    Read the link provided in post #6 please. Specifically pages 4 - 7.
  12. Mar 31, 2016 #11
    The graphs of absorption of light by leaves can be different from site to site, so it is difficult to distinguish what is what.
    There is some noticed difference from that of the extract of chlorophyll and that from the actual plant.
    The difference can be described as being from the carotenes and other pigments.

    And some have noted that further penetration of green light into the leaf would excite the lower chloroplasts, such as from,

    Hyperphysics gives some more graphs regarding this issue,

  13. Mar 31, 2016 #12
    Which is exactly what I did.
    I was responding to your choice of graph, which you claimed to be from page 5, but which actually is on page 4 - and is exactly absorption rather than action spectrum.
  14. Mar 31, 2016 #13


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    If you read it, then I have no idea what you're getting at, as the article explained my position pretty well. And I realize the graph itself is about absorbance. I included it to show that leaves absorb a large amount of light all across the visible spectrum, including orange and green. But you are correct in that the graph is from page 4, not 5. Thanks for catching that.
    Last edited: Mar 31, 2016
  15. Apr 2, 2016 #14
    Your choice of graph did not show that realization.
    The article contains 2 graphs which actually are about action spectrum, and which actually are on page 5, that you did not choose.

    Now, my point is that the effect of light on plants depends on action spectrum, not absorption spectrum.

    Are the lights from various parts of action spectrum freely interchangeable?
    The action of red light drops of steeply redwards of 680 nm - but only there.
    "Orange" light on Titan should provide the light in 600...680 nm range, even if the blue light is absorbed away in the organic haze.
  16. Apr 2, 2016 #15


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    *shrug* Okay. If that's what you believe, then that's what you believe.

    Indeed it does.

    That I can't answer.
  17. Apr 3, 2016 #16
    It is not just red algae which have phycoerythrin and phycobilisomes.
    Blue algae also have them. And blue algae are able to adapt to light they encounter, being quite green in suitable light.
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