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I’m creating an Earth-like world where the sun appears blue to people from the planet’s surface. I think I’ve narrowed down the cause of this effect to the presence of 1 micron particles in the atmosphere, which would scatter and absorb the longer wavelength red light from the planet’s sun. This would mean that people on the surface would primarily see the blue light waves from the sun, making it appear blue. So now that I’ve explained my planet’s blue sun, my question is, would this scattering and absorption of the red light in the atmosphere harm Earth-life forms. In other words, if lifeforms just like the ones on Earth lived on this planet, would the difference from the scattered/absorbed wavelengths of light kill them? Here’s a few points of clarification:

  • By Earth-life, I mean not just animals but plants and vegetation as well.
  • The forms of Earth-life I’m referring to are lifeforms that rely on sunlight. I’m not concerned about cavefish, etc. Any creatures or plants that could actually be negatively affected by changes in sunlight.
  • The temperatures are similar to those on Earth.
  • This planet’s sun is similar to our own. It isn’t a blue star, it only appears blue from the planet’s surface.
  • I think I should also clarify that some red wavelengths of light are still reaching the planet, it’s just a small enough and scattered enough amount for human eyes to perceive the sun as blue.
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    $\begingroup$ Given that life on earth can exist in total darkness what makes you think that lack of only red light would have a lethal impact on all earth life? Are you specifically asking about photosynthetic plants? Try narrowing the scope to ask about that and only that. $\endgroup$
    – sphennings
    May 1, 2023 at 14:34
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    $\begingroup$ Blue light as such may or may not be a killer, but an atmosphere full or micron-sized particles sure is. $\endgroup$
    – AlexP
    May 1, 2023 at 15:46
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    $\begingroup$ @IanKemp Because everything I’ve heard about blue stars is that the would cook a habitable planet, and UV and other harmful of wavelengths of light would be much stronger. Most of the answers to questions here asking about whether a habitable planet orbiting a blue star would be possible say that the planet would get cooked, or it would be too far away to get enough light for life, etc. Though that was my original idea. $\endgroup$
    – Kal Madda
    May 2, 2023 at 12:05
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    $\begingroup$ Do you have a link on the discussions relevant to a blue star being necessarily a killer? The star would be hotter and shorter-lived, certainly, and 10 Myr isn't much time for intelligent life to evolve, but absent other evidence, I'd assume the "hotter" could be solved by putting the planet's orbit further from the star and that shorter-lived is potentially compatible with your story if you're dealing with a civilization that travelled there from some other star. $\endgroup$ May 2, 2023 at 14:04
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    $\begingroup$ @SarahMesser Here’s a couple of links to discussions I saw: worldbuilding.stackexchange.com/questions/66155/… & worldbuilding.stackexchange.com/questions/213770/… $\endgroup$
    – Kal Madda
    May 2, 2023 at 15:50

5 Answers 5

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Let's invite an expert's opinion

Dr William Stiles of Aberystwyth University wrote the following (emphasis is mine):

Light is an essential component of plant development and is a key driver of plant physiology and morphology. Light is crucial for photosynthesis, the chemical reaction that fixes CO2 for the purposes of food production, but it also acts as an environmental prompt, informing plants about the world in which they exist. Plants detect information from incoming light using sophisticated multi-functional sensory proteins called photoreceptors. Plants possess at least five classes of photoreceptor: phytochromes, cryptochromes, phototropins, Flavin-binding F-box proteins, and UVR8. Actual signalling pathways, and the interaction between different receptors, are complex, but in essence phytochromes perceive red and far-red light, cryptochromes perceive blue and UV-A light, phototropins and Flavin-binding F-box proteins blue light, and UVR8 perceives UV-B light.

...

Light from within the visible spectrum drives photosynthesis, particularly light from within blue and red wavelength ranges, but the potential for photosynthesis will be governed by the amount of energy available in the form of photons that a plant can absorb. Light intensity, and its potential for driving photosynthesis, is referred to as the photosynthetic photon flux density (PPFD). The higher the PPFD, the higher the potential for photosynthesis. Plants absorb light energy via the light-absorbing pigment chlorophyll. Chlorophyll appears green as it absorbs all visible light except green wavelengths, which are reflected. Chlorophyll A and B absorb red and blue light strongly, and as such these wavebands have been considered the only portion of light that truly matters for plant production. However, increasingly it is recognized that plants make use of all available light to at least some degree, including green light, and that presence (or absence) of different wavebands influences plant development.

...

Each of the wavebands of the light spectrum, and their relative proportion in the available light, will trigger a response in the plant. The different wavebands are:

Red light (600-700 nm) – light from the red wavelengths is the main driver of vegetative growth. This means more leaves and more biomass. But growing in the absence of other spectra may result in a phenomena referred to as red-light syndrome, where leaf photosynthesis can become impaired. Without the presence of blue light, the form or morphology of plant tissues may also result in unfavourable growth profiles, where plants become stretched and tall, with thin leaves, which is a typically unfavourable growth profile. It may also mean plants cannot utilise all available light energy, leading to overall inefficiency. Overall, red is the most important wavelength for plant growth and development, but not in isolation.

Blue light (400-500 nm) – light from the blue portion of the spectrum has a large effect on plant morphology. It can increase the ratio of root to shoot in plant development, promoting root growth and plant compactness, which has certain implications depending on production goals. Blue light also promotes more stomatal opening, which means more stomatal conductance and gas exchange. This is typically considered favourable from a plant health perspective but may result in greater humidity potential, which is a consideration for controlled environments. Blue light is absorbed readily by plant photoreceptors, and is an important factor in plant environmental perception. For instance, increasing the percentage of blue light will convince plants that there is more available light overall, which will change plant behaviour.

Green light (500-600 nm) – green light is weakly absorbed compared to red and blue wavelengths, but is increasingly recognised as important for overall photosynthesis potential. Green light is reflected and scattered within leaves and the canopy, which increases the potential for total absorption. Green is particularly important in dense-growing scenarios where there is a large amount of shading, as it drives photosynthesis in lower or shaded leaves. Green light also affects morphology via the green to blue light ratio. This acts as a signal to indicate shade conditions, informing the plant and leaf of its position in the canopy, initiating growth behaviour associated with shade avoidance. This can include extra growth or stretching of the internode and leaf length, and the angle of the leaves may also change to capture more incidental rather than direct light.

Far-red (700-850 nm) – this portion of light is referred to as super-visual, as the majority of this waveband is outside the visible portion of the spectrum. Far-red is not considered conventionally photosynthetically active and it only weakly drives photosynthesis, but adding far-red will change how plants grow as this light is absorbed by phytochrome photoreceptors, which are involved in the regulation of leaf expansion, flowering, internode extension, and the partitioning of resources between organs. Far-red will also have the opposite effect to blue light on root to shoot ratio, resulting in higher shoot to root distribution. Yet, as with all elements of the light spectrum, there is a balance to be struck between a beneficial amount of far-red light and too much. Plants grown under high levels of far-red light will appear tall and stretched, with lower chlorophyll content resulting in yellowing of the leaves, which is perhaps unfavourable from a marketability perspective. In addition to direct effects, the ratio of red to far-red light is also an important mechanism for governing plant responses. Far-red penetrates the canopy more than red light, so plants receiving a higher amount of far-red relative to red will interpret this as a shading effect, and increase shade avoidance responses such as increased upwards growth.

UV spectrum (100-400 nm) – UV light is also outside of the PAR wavelengths, but this light will still affect plant development. Plant responses to UV-A light are similar to blue light. UV-B is higher energy and has its own photo receptor in plants, called UVR8. Adding UV-B to the spectrum will change the morphology in ways which are not considered essential for survival, but which may affect the potential for production. For instance, under UV-B light plant cuticles can grow thicker, making the plants generally more robust, and UV-B exposure will positively regulate stomatal development, but hypocotyl and petiole length may be shorter and rosette leaf expansion may be impaired. Secondary metabolite production is also higher under UV-B, which is typically a favourable response, particularly for production systems focussing on pharmaceutical production. UV-C is not believed to be directly perceived by plants, but it can be highly useful for the control of pests and disease in controlled environments.

Summary

Light across the so-called visible spectrum is used by plants — not just the red spectrum. Losing the red spectrum would have a detrimental affect on the plants, and therefore any animals dependent on the plants. But would your world kill the plants?

NO

Blocking and/or scattering enough light to let the sun appear blue to human eyes does not mean that no red light is striking the surface. There would still be sufficient red light to promote plant growth and, obviously, plenty of the other spectra of light to positively affects plants.

As a quick aside... if you did block all the red spectra light, that means that NOTHING on your planet would appear red because there's no red light to reflect. Chalk that one up to the Law of Unintended Consequences, so it's good that some red spectra light is getting through.

However, and as a viable and interesting part of your story, plants would suffer. Or, perhaps said better another way, they would not be as prolific under the conditions you propose as they would be on Earth. What that means is that they'll need to adapt. Human history and science has demonstrated that biology is really, really, good at adapting. It will take some time (if not helped by intentional human effort), but I don't see a reason why the plants wouldn't be adequate in the beginning and fine over time. (OK, thousands if not millions of years worth of time... but still....)

What about the animals? Tell me about UV...

I don't think your animals have any problems at all. The presence of red-spectrum light doesn't mitigate the effects of UV light. It could be true that if you require a brighter star in order to have the same luminosity on the surface of your world as we do here on Earth, that would up the UV. But that also ups the heat and a lot of other things (like upping the solar wind...). That means plant and animal life are threatened for reasons that have little to do with the question you asked.

If, indeed, the sun is "similar to our own" in that the star's output is similar to Sol's output, then you have no problems at all. The world would be (frankly, imperceptibly) dimmer and the star would look blue, but that's it. Nothing would die.

Now, a particulate count high enough to achieve what you're proposing, depending on what those particles are... that could kill everything. Or at least give everything with lungs and sinuses the worst case of chronic hay fever ever heard of. But I'll leave that issue up to you.

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    $\begingroup$ Thank you, that is very helpful! 👍🏻. So the particles are the main thing I have to worry about in this system. I’ll have to look more into how to mitigate the effects of the particulates. Maybe somehow they stay in a higher strata of the atmosphere or, I kind of hate to go with biospheres and such, but I suppose that could possibly help with the particles. Thank you for this answer, that’s very helpful for the photosynthesis part! I wasn’t able to find anything before that listed spectra-specific effects like this does! 👍🏻 $\endgroup$
    – Kal Madda
    May 1, 2023 at 18:34
  • $\begingroup$ There could probably be effects similar to what makes our ozone layer stay in the upper atmosphere, which would limit the particles mainly to the upper atmosphere. $\endgroup$
    – Falco
    May 4, 2023 at 9:04
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I don't think any of the answers have addressed this point yet: if the effect is caused by scattering it will not actually stop much red light from reaching the surface. (Some will be scattered back into space, but not much.) The sun will appear blue but the sky will appear bright red, and the red light from the sky plus the blue light from the sun will add up to white.

The opposite happens on Earth when the sun is low in the sky. Direct light from the sun appears yellow, but shadows appear bluish because they are lit by blue light from the sky. A white piece of paper in indirect light will still appear white.

This means there won't be any problems for photosynthesis on your planet at all. Plants will be getting the same spectrum of light they get on Earth, it's just that some wavelengths will be coming from different directions than normal.

I leave you with this photo of a sunset on Mars, taken by Nasa's Curiosity rover. Mars' atmosphere scatters red light. You can see the blue light from the setting sun as well as the red sky further away from it.

enter image description here

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    $\begingroup$ Thank you for the answer, that photo from mars is helpful! 👍🏻. I’ve seen photos from Beijing in 2021 during a sandstorm, but oddly enough I’ve never seen that photo from Mars. I’ve had a hard time trying to find Mars pics in the past. $\endgroup$
    – Kal Madda
    May 2, 2023 at 13:24
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Scattering will not turn light bluer. Smaller wavelengths scatter more than bigger ones. Absorption, in which your atmosphere works as a kind of blue-pass filter, might be possible (if you can get the chemicals or particulates that are absorbing the redder spectra to stay in the upper atmosphere somehow), but will probably kill everyone, because:

1: A blue-pass filter is a greenhouse effect generator. The energy from incident red light is going to still get to the planet as IR, and up to half of the re-radiated IR from the planet's surface is going to come back to the planet, since the blue-pass filter will absorb it and re-radiate it in all directions... Things are going to get really, really hot.

2: As Nosajimiki notes, you'd need to increase the star's brightness if you want terrestrial plants that need full sunlight to grow - which would have the noted UV hazard effects, but would also make (1) even worse.


Below: a response from when the question indicated a blue star, not a blue-pass filter.

Terrestrial surface life would have trouble. Plants and microbes would fare worse than animals, who can have nonliving shells or fur between them and UV, or can hide under things during the day and come out at night. UV is harmful because it is readily absorbed by DNA, not because it penetrates deeply (hence burns and skin cancer, but not radiation sickness).

Subterranean life would be fine. UV does not penetrate rocks.

Aquatic life would be fine below a depth of several meters, because UV is absorbed more by water than visible light; however, life that lives at the surface of the water would have the same problems as on land.

A much bigger problem is time. Blue stars live fast and die young - so young they may not even have planets that have swept out their orbits and cooled off enough for liquid water, let alone billions of years after that for abiogenesis and evolution. You might be able to get away with having a planet captured from another, much older star system, although I'd guess that the chances of that happening in a way that leaves anything macroscopic alive are vanishingly small.

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  • $\begingroup$ @g s Sorry I forgot to clarify this well enough in my question, but this planet’s sun isn’t a blue star, it’s similar to our own sun. What makes it appear blue from the planet’s surface are the particles in the atmosphere absorbing the red light. $\endgroup$
    – Kal Madda
    May 1, 2023 at 16:27
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    $\begingroup$ @KalMadda ah, I see you changed the question. I'll add a new section to my answer, but it won't be what you were hoping for, I think... $\endgroup$
    – g s
    May 1, 2023 at 17:10
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My apologies for tossing off a quick answer this morning. Here's more details.

Photosynthesis is generated at multiple wavelengths by two different forms of chlorophyll. Chlorophyll A has its peak absorption in the 429nm (almost purple) and 659nm (red-orange) wavelengths. Chlorophyll B has peak absorption at 455nm (indigo) and 642nm (orange). Chlorophyll B doesn't actually complete the step from light to sugar, it just supplements A's range of energy. If there were enough blue light, the plants could still generate enough energy to survive.

Chloropyll A and B sensitivity ranges

The real problem you will find is that plants use red and blue light for different purposes. This isn't a matter of what it CAN receive, but how it's bred to use it. Blue light is used by plants to encourage fast growth, longer stems. Red light is used to make thicker stems and induce blooming. This is due to the increased prevalence in red light during the autumn months.

Plants uses red light as a signal to maintain its annual cycle in the same way we use blue light to maintain our diurnal cycle. Thus, you would have issues with plants never blooming (or fruiting), and not knowing when winter was coming in the temperate zones. I believe this would lead to extinction of many species.

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  • $\begingroup$ So if I’m understanding you correctly, plant-life would die with the lower levels of red wavelength light? Or would it just mean plant-life would take longer to grow? I’m a bit confused by the “it wouldn’t hurt them” part. Are you talking about other lifeforms besides plants there? $\endgroup$
    – Kal Madda
    May 1, 2023 at 16:40
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    $\begingroup$ @KalMadda Most photosynthesis takes place in the red, but some takes place in the blue, very little happens in the green range. So, if the light is bright enough, the plant will get enough energy in a blue light spectrum to survive (in theory), but that much blue light will likely cook it in the process. Those few plant that could perhaps survive would likely struggle to much to be agriculturally useful. $\endgroup$
    – Nosajimiki
    May 1, 2023 at 16:48
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    $\begingroup$ @KalMadda, I don't think this would work with an Earth-like planet. You'd be surprised at how little light-reduction it takes for plants to die off. Think of it like an energy budget. If you cut your financial budget in half, would you be able to survive? What would you have to give up? Plants would immediately lose the ability to reproduce, and would eventually starve. $\endgroup$ May 1, 2023 at 20:39
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    $\begingroup$ @KalMadda, As a writer, that's your call. Rayleigh scattering does result in less red light hitting the surface, but that's what our plants are adapted to. If this was a persistent condition, then you can guarantee that the plants would adapt. $\endgroup$ May 2, 2023 at 0:43
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    $\begingroup$ @RobertRapplean Thank you for your input, I really appreciate it. 👍🏻. So you think plants could be adapted to survive in these conditions? If that’s the case, then perhaps the humans engineered the plants to survive in these different conditions. That could work as long as that is possible. 👍🏻 $\endgroup$
    – Kal Madda
    May 2, 2023 at 1:03
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Different setups create different solutions.

There are a few basic ways your hypothetical setup could exist.

For starters, either the upper atmosphere is reflecting shorter wave lengths, or it is absorbing them. This is an important distinction because if it is reflecting them, it means your planet needs to be much closer to the sun to get Earth like surface temperatures from only the short wave length light. If it is absorbing them, then you can have the same orbital distance, and your lower atmosphere can maintain a similar temperature with less light actually reaching it.

The other possible variable is if you are filtering just visible light in the 700-500nm range, or if you are filtering all wave lengths shorter than 500nm. Since the goal is to kill Earth life, I will assume you are filtering everything shorter than 500nm (green, red, infrared, microwave, etc.) since this will produce a more extreme effect.

If you are reflecting the longer wave lengths

For this to work, the sun has the same apparent magnitude as ours, but the sun is much closer to the planet such that it is the same amount of energy reaching us, but only from short wave lengths. For a filtered light to keep the planet warm, it needs to carry a lot more short wavelength light. So, for the star to be the same as our sun, it must also be closer.

Infrared, red, and green light collectively make up about 81.25% of the sun's total radiation with ~10.75% being in the blue spectrum and the remaining 8% being UV through gamma. This means that the blue, UV, and Gamma Radiation of the sun will need to be about 5 times as intense to maintain the planet's surface temperature. That said, the blue spectrum makes up 25% of visible light; so, it will also be 25% brighter outside despite the missing red and green light.

Despite seemingly tolerable levels of heat and light, the UV radiation would sunburn us several times as quickly, we'd develop skin cancer very easily, the blue light would damage our retinas making us go slowly blind, and plants would whither and die. Terrestrial desert life might be able to survive in this planet's rainforests where the heat is high enough, but there is little direct exposure to the sun... but this would depend on if we can eat the local flora since growing our own agriculturally useful plants anywhere they would get enough light and not too much UV would be very difficult to balance.

If you are absorbing the longer wave lengths

In this scenario, you get the same heat. With the red and green light filtered out, you would only get blue light photosynthesis. While blue light has a higher RQE than green light, it is lower than red. This should all balance out to being about equivalent 25% of normal photosynthesis.

While this might be enough for certain forest floor plants to survive, if planted in direct sunlight, it would significantly reduce how quickly and fully any plants grow which would make growing enough food for humans to survive very difficult.

This would be a less deadly option than reflection, but it would certainly make the planet difficult to survive on as well.

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  • $\begingroup$ he does state that the star is similar to ours in all regards, it's just the way the light is perceived on the planet's surface that's different. $\endgroup$
    – jwenting
    May 2, 2023 at 8:00
  • $\begingroup$ @jwenting I see there are 2 ways to read the OPs question. Temperature must be the same which can be arrived at by more than one possible setup. I've updated my answer to address both scenarios. $\endgroup$
    – Nosajimiki
    May 2, 2023 at 15:46

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