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Okay, I'm trying to make a planet that has at least part of its surface, or at least part of its underground, habitable for humans (breathable atmosphere at a suitable pressure, a suitable level of gravity and radiation, and a suitable temperature). The plants here are white in color. What kind of star(s) will it need to orbit, in order for white plants to evolve there?

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    $\begingroup$ There are white plants and white flowers on Earth. Could you elaborate on how your plants would be different? $\endgroup$
    – kaya3
    May 8, 2023 at 4:28
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    $\begingroup$ "White" is a human concept, it's basically "all the light frequencies we've evolved to see". So the question isn't really what would cause plants to be white, it's "what would cause plants and people to co-evolve so the latter don't see any of the frequencies plants use to power their photosynthesis" $\endgroup$
    – biziclop
    May 8, 2023 at 22:11

9 Answers 9

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A Red or Brown Dwarf

Red and Brown dwarfs produce very little light in the visible spectrum meaning that plants would get most of thier energy from infrared light. Since visible light is not a major factor, plants don't really have a reason to specialize in the visible spectrum. That said, low exposure to visible light can also lead to a low resistance to it. Just like Earth life has a limited resistance to UV and greater, your world's sensitivity to short wavelength EM radiation may start in the visible spectrum.

So plants, especially those that live in the brighter parts of the world, will evolve to reflect harmful RGB radiation making white a dominant color on your world.

Also of note: Red and Brown Dwarfs are very long lived, stable stars. As a general rule of thumb, you don't really want a star bigger than a Type F Main Sequence star if you want to give life any chance of evolving into anything plant life.

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  • $\begingroup$ Dwarves are little men with beards. Dwarfs is the right term for stars. $\endgroup$ May 8, 2023 at 23:24
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    $\begingroup$ @fluffysheap Technically, Dwarves also emit light that is mostly in the infrared spectrum... $\endgroup$
    – Nosajimiki
    May 9, 2023 at 0:11
  • $\begingroup$ Wrong. the answer is quite the opposite. A plant growing on a planet orbiting a red dwarf would be completely dark, as they would have to absorb as much light as possible. This answer is in all aspects totally inaccurate, and the fact that this inaccurate answer was accepted kinda makes me deeply disturbed. $\endgroup$
    – Alastor
    Sep 7, 2023 at 12:05
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    $\begingroup$ @FuriousArcturus You are assuming an orbit of 1 AU. A closer to star orbit could still receive the exact same amount of total EM energy as a plant on Earth, but very little of that radiation would be in the visible light spectrum by comparison. A primary producer will generally make chemicals to target the sweet spot of where the energy of a wavelength corresponds to its predominance in the spectrum. If plants need to make a separate chemical just to capture a tiny bit of visible light, then it's a waste of resources $\endgroup$
    – Nosajimiki
    Sep 7, 2023 at 13:18
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As @L.Dutch pointed out in his answer, white is not a single wavelength, but a mixture of all the colours in the spectrum of visible light.

However there is a way you can get white, or atleast, very pale plants. A plant, as pointed out, absorbs most of the light that falls on it (excluding the green part of the spectrum). However, if by chance, you managed to make the parent star extremely bright, then the plant would have to reflect off most of the sunlight, so as to prevent it from getting damaged. There is even a real life example of this. Earth plants have been known to turn pale and die upon exposure to too much sunlight. So, if you had a star that was extremely bright, then that would cause your evolving plants to be very pale, almost white and form an analogue to chlorophyll for energy harvesting.

However there is a massive drawback to this. Really bright stars, especially above F-type spectral class stars, have a nasty reputation for releasing a b***load of UV-rays. (Let's ignore UV flares from smaller stars like M-type and K-type stars for now, these are only rare events). So the planet's water would have been already have been disintegrated into hydrogen and oxygen and blown away before you even got simple bacterial life. I do not know for how bright would it have for your plants to be really pale, so as to reflect nearly all starlight that falls on it, but a certain-something tells me that you would need something like a A-type star for your plants to appear pale.

Even if you managed to keep the water to the surface (thanks to ozone shenanigans blah-blah), you would face a big problem. Life is rather slow, like hella lot slow. It took nearly 3 billion-something for life to evolve from simple bacteria to walking bald apes who waste life making memes- no, we are getting off-topic.

Anyways, the point is the same. Since our Sun is a long-lived star (As of 5th May 2023, it has about 7 billion years before it goes on steroids and expands into a red giant), life has a ton of time to spend to grow. But, boy, if you want white plants, then you have got only a trickle of time left (Ranging from a few million years to nearly just 1 billion years), because you need a bright star, and well, bright stars' lives aren't, well, very "bright". So you have got very little time for your white plants to quickly evolve.

So, although you can get "white" plants, the possibility of getting them is very, very low. Your star will go supernova in just a few million years (Yes, a "supernova"), and life has got only that much time to quickly evolve into "white" plants. So, no white plants for you :(

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  • $\begingroup$ That's exactly what I'm thinking. $\endgroup$ May 5, 2023 at 18:10
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    $\begingroup$ Can "very bright" not simply be replaced by "closer to the planet", removing the drawbacks about short life and UV? $\endgroup$
    – wimi
    May 7, 2023 at 17:02
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    $\begingroup$ The other option is to go the other direction: a red dwarf puts out most of its energy in the infrared. Since there isn't much selective pressure driving plants to absorb in the visible spectrum, you could get white plants entirely as an accidental side effect of optimizing for infrared absorption. $\endgroup$
    – Mark
    May 7, 2023 at 20:03
  • $\begingroup$ Leafs actually absorb most green light. They just absorb even more red and blue light. See here (note this goes into the infrared): link.springer.com/chapter/10.1007/978-3-030-33157-3_14. This does not include transmittance but transmitted light can be absorbed by other leaves. $\endgroup$ May 7, 2023 at 21:28
  • $\begingroup$ @wimi: look at Venus for example for what closer to the planet might do to planets. $\endgroup$ May 8, 2023 at 12:13
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None. White is not the color of a single wavelength. It's just the impression of all the perceived frequencies at the same time.

A white plant would absorb very little light, which is what a plant needs to be a plant.

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    $\begingroup$ That makes a lot of sense. $\endgroup$ May 5, 2023 at 16:55
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    $\begingroup$ Mostly true. A star that provided a great deal of light might require the plants to develop highly reflective coloration in order to avoid chromosomal damage. This would also probably result in a planet that was too hot for water, but maybe the planet had a naturally high albedo for other reasons? Planet of chalk? $\endgroup$ May 5, 2023 at 17:08
  • $\begingroup$ It could also be a response to a change in the planets atmosphere, or a flare heavy star. $\endgroup$
    – John
    May 5, 2023 at 20:58
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    $\begingroup$ The visible spectrum is a tiny tiny piece of the entire electromagnetic spectrum. Being white would only mean that it could absorb little light from that tiny slice of wavelengths. That would matter little to a plant that absorbed light from the ultraviolet, infrared, or any of the many other frequency bands that aren't visible. $\endgroup$
    – nben
    May 6, 2023 at 1:38
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    $\begingroup$ This seems convincing but it's not true. You can get white from just blue and yellow. The plants could be absorbing everything else. $\endgroup$
    – user86462
    May 7, 2023 at 8:15
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I'm interpreting your question to be equivalent to "what kind of light would cause white plants to evolve on a planet?" I make this distinction from that of the star because my understanding is that this is mostly a question of the atmosphere—most stars produce light across a very broad range of the electromagnetic spectrum, but atmospheres absorb pieces of the spectrum. Our atmosphere absorbs most light outside the visible spectrum and so (one can hypothesize that) plants on our planet have evolved to use light in the visible spectrum for photosynthesis.

White plants would reflect light across the visible spectrum. This is a very small fraction of the total electromagnetic spectrum, so that shouldn't necessarily matter as long as there is enough usable light for photosynthesis outside of the visible spectrum. If the planet's atmosphere absorbs most visible light and lets pass a substantial amount of light outside the visible spectrum, then (one can hypothesize that) anything photosynthetic that were to evolve would be likely to use non-visible light for photosynthesis while reflecting visible light. This all requires hypothesizing that a photosynthetic mechanism could evolve that uses non-visible light; for what it's worth, I do not personally find this to be a very big leap for a science fiction world.

Unfortunately, I don't have the expertise to tell you precisely what gases (or gas plus particulate) are needed to block visible light while letting pass other parts of the electromagnetic spectrum, but whatever the atmosphere was made of, it would almost certainly look black on account of its absorbing visible light.

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  • $\begingroup$ This is along the lines of what I was thinking. White is simply stimulating all the receptors pretty much equally, In fact, since our plants generally reflect Green, all you'd have to do is get them to reflect Red and Blue for us to see it as white. Perhaps a dim star and the planet's atmosphere combine to put the most energy in the infrared part of the spectrum and plants are not interested in most of the visible spectrum because it is too low-energy. $\endgroup$
    – Bill K
    May 8, 2023 at 18:44
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A plant on earth converts CO2 and H20 + sunlight to food. The earths atmosphere blocks much of the energy that is outside of the visible spectrum (ozone < 300nm, H20 and C02 > 750 nm). Ozone doesn't block radiation < 200nm, ultraviolet. So a star that emits a lot of ultraviolet (bluer) that had plant life that evolved to use ultraviolet for photosynthesis could conceivably produce plants that are white to us.

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It isn't the star, but biology. You should be able to achieve purely white plants with our sun.

Chlorophyl is green, absorbing both blue and red while reflecting green, giving that color to plants. Yes, plants are throwing the most abundant light away for some reason - I have no idea why. Getting white color requires reflecting at least two wavelengths - check CIE color chart and drag any line that goes through the white spot and those are the two wavelengths you require.

For example, your plants reflect blue/violet and yellow. They would want to get rid of blue and lower to limit UV damage (perhaps there is little to no ozone?); reflecting yellow for those white plants and green of our plants seems about the same (= I have no idea why, but ...)

Note that while light reflected off plants looks white, anything illuminated by it will look weird. Say green stuff will look kind of grey, red stuff will look grey and similar. Perhaps that is desirable.

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  • $\begingroup$ Chlorophyl does not so much "reflect" green as it just absorbs red and blue better. Chlorophyl still absorbs up to 80% of green light, but its better efficiency in the high and low ranges make it look green. As for why the importance of green is neglected, most of the total energy from the sun comes from infrared and red light; however, shortwave light plays an import part in the production of resin and root growth apart from any benefit it may have for energy production. In otherwards, green absorption is slightly neglected because Red and Blue are more important. $\endgroup$
    – Nosajimiki
    May 9, 2023 at 0:29
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As mentioned in other answers and comments, greater sunlight intensity could result in plants developing pale colorations to protect themselves from excessive light. Brighter stars are an unlikely mechanism due to their short lifetimes, but a closer orbit will have the same effect, and a close binary would allow you to double the luminosity without shortening the lifespan of the stars. However it's achieved, it would need to be a pretty fine-tuned system to produce pale colored plants but not just overheat the planet beyond being capable of supporting complex life.

One thing that helps: the stress of excessive sunlight will be stronger toward the equator. There might be more colorful life toward the poles, "bleached" varieties being able to spread closer to the equator. Or there might be a strong axial tilt, with the "bleached" varieties being able to grow further into the hot season, and winter life perhaps being more colorful.

Also, yellow stars become more luminous as they age, swelling into red giants. A very old system might have a much brighter star, the plant life having had plenty of time to adapt to the increasing luminosity, and the light-colored plant life might actually be the only thing keeping the planet cool enough to support life.

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Probably: any.

It is a misconception that Earth plants are green. They come in red, green, and brown.

It is really just a matter of what energy process or processes evolve in the plants. So it is all a matter of luck.

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As so far everyone has mentioned, earth based plants are mostly green because there has been evolutionary pressure that lead to green being a dominant color. For earth this is mostly due to the spectrum of light (a lot of orange and red) but there are other colors in plants and these plants have survived the evolutionary pressure, so they are possible.

What if there is another source of pressure that made these plants white? What if there is a reason that the other colors no longer exist?

Maybe there was a plant-based disease that killed all non-white plants? Perhaps there was a mass extinction event that made all plant-eaters that saw color disappear and white plants are (or were) harder to see? Maybe white offered better camouflage for pollinators? Maybe there was a science experiment gone wrong by a (previous) intelligent inhabitant that made white the dominant plant color? Or the previous inhabitant had religious reasons to strongly prefer white plants and remove all others?

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