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Before I continue, yes, green stars don't really exist visually. Although stars like the Sun are green stars, they appear white (and are known as yellow dwarfs) because it doesn't only emit green light (which is the most intense set of wavelengths it emits) but also the colors around it. Through color addition this makes white. In this situation however let's suppose this star MAGICALLY is green.

However I want to focus on plants. Star type is a major factor in determining what the plant color is (it is not the only factor but let's skip all those exceptions). Under a red star, plants would appear very dark, while under a bright blue star they would rather be white colored. On Earth, plants are green since the Sun mostly emits green light, and thanks to chlorophyll, plants absorb red and blue light while reflecting the green light.

Since the star is green, it emits almost entirely green light, and I'd expect that only a tiny amount of blue and red wavelengths are emitted. I would expect the plants to absorb the green light and only reflecting the blue and red, and perhaps appearing magenta/violet.

Would magenta be the correct color of plants if they grew under a green sun? If not then what color would plants be under a green sun?

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    $\begingroup$ A google search for "purple leaf plant" images turns up plenty of photos, so yes, it is possible $\endgroup$ – nzaman Nov 21 '18 at 15:23
  • $\begingroup$ @Renan sunlight is most intense at a wavelength that seems green to us. It is blue-green outside of the atmosphere at around 500nm and is highest at around 520-540nm at ground. Thus the highest amount of any individual color that reaches us from the sun is "green". However the sun emits a spectrum over a vast range and thus its light appears "white" to us. $\endgroup$ – Adwaenyth Nov 21 '18 at 15:29
  • $\begingroup$ I see some problems that need a bit more clarification. (1) Colours are a subjective artefact of human eyes because of the limited number of colour receptors we have (2) Colours of objects change when we see them in different coloured light or in white light next to other colours. (3) Do you want them to appear purple when someone is viewing them whilst illuminated by the star, or purple to humans when examined in white light on their spacecraft? $\endgroup$ – chasly from UK Nov 21 '18 at 15:31
  • $\begingroup$ From a human point of view it's white-yellow, from a scientific point of view you would not use such a vague description and you'd use e.g. it's B-V index and color temperature. I've never seen the Sun described as "green" in a scientific context. $\endgroup$ – StephenG Nov 21 '18 at 16:10
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    $\begingroup$ Star type is a major factor in determining what the plant color is Given we have absolutely no scientific knowledge of plant life in relation to any star other than our own (or any any other planet for that matter) this claim is simply not verifiable. However evidence from Earth suggests that plants have leaves in a wide variety of colors (including purple) and there's no reason to think that life, assuming it exists elsewhere, is not just as diverse. $\endgroup$ – StephenG Nov 21 '18 at 16:16
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To determine the color of the plants most likely to arise on a planet orbiting the star, we have to consider the wavelengths of peak absorption of different photosynthetic pigments. I would argue that different star types do indeed influence pigment choices.

Chlorophylls respond well to light around 435 nm and 650-680 nm; 500 nm - green light, approximately - falls in the middle. This helps the development and growth of green plants and green/red algae. If we wanted a different color for our plants, we'd need a different pigment. Purple is a sign of either bacteriochlorophyll a or bacteriochlorophyll b. The former has absorption peaks from 800-900 nm, while the latter has peaks at 435 and 740 nm. Spirilloxanthal, active in the 400-550 nm range, can also produce purple coloring in bacteria.

To have the majority of your plants be purple would require peak wavelengths for the star in the 800-900 nm range. This corresponds to a late K-type star or an early M-type star, which on the main sequence be red-orange in color, low-mass, and long lived. On the other hand, if you wanted magenta, you'd also need red pigments to be active. There are a long list of these, including the chlorophylls. Emission at 500 nm would indeed encourage the use of spirilloxanthal, depending on the exact wavelength. The problem is, it would also encourage the use of all the other pigments active in the range, including the chlorophylls. Some plants will be purple or magenta; most won't.

It's worth noting that Sun-like stars - as well as more massive stars - may grow redder as they evolve, ascending the red giant branch. Red giants aren't particularly conducive to life, but they certainly have wavelength ranges that would encourage bacteriochlorophyll production and use.

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If we assume you have a green star, that emits mostly in green light. (magic)

So all the light we have on that planet is green. So everything on that planet looks a green shade or black.

  • Green if it reflects significant amount of the light.
  • Black if it absorbs most of it.

If your plants don't emit their own light.

For note, your plants will have to make use of something else, as there is no blue or red to absorb for the chlorophyll. You may use Oenin. For us it is red-purple, on your world - reddish-black.

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  • $\begingroup$ What you eventually see is a result of absorption followed by emission of light, not reflection. It's not as trivial as what comes in must come out. $\endgroup$ – StephenG Nov 21 '18 at 18:55
  • $\begingroup$ Yes, I know. But here we have only green or close to only green light to absorb or reflect. So in most cases, things will be green shades or black. $\endgroup$ – Artemijs Danilovs Nov 21 '18 at 19:12
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Change chlorophyl

The absorption spectrum says that chlorophyl has absorption peaks in red and blue, basically reflecting green, hence the color. You have some kind of inversion of this situation. I would actually think that short-wave (blue) absorption will still happen, because of more energy at the wavelength. Hence, rather red "chlorophyl", the foliage might look differently, however, because of superposition of color in other cells or structures. Say, the veins in the leafs have some further color for whatever reason.

Change Rayleigh scattering

The next point is that a green star is quite impossible in the main sequence, as already mentioned. But we can have a green sun in the skies! We just need to tweak the scattering of the light in the atmosphere. Add some nano-particles from the Old Ancient Ones, or too much of ammonia in the upper atmosphere. Or ozone. Or whatever. I cannot say for now which gases exactly would shift the irradiation of which star class to green in the atmosphere, but it appears doable to me.

Change the perception

As a final resort, let the inhabitants of the planet have different eyes. What appears green and violet to them, might not have these colors for us.

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Frame challenge: You can't have a planet around a green star

There is another question which specifically addresses how you can have a green star. Two of the answers to that question are relevant here.

First is that a 'normal' star will never appear to be green. The reasoning is the same reason that an iron bar heated from a blowtorch will never appear to be green, our eyes are just not that sensitive to green. Wavelengths that activate the green cones in our eyes usually also activate the red or blue ones as well, since green in in the middle of the visible spectrum. Add on to this the fact that Rayleigh scattering in the atmosphere tries to shift sunlight towards blue as it passes through the atmosphere, and the ultimate effect is that a star can't be percieved as green as long as it is also emitting plenty of red or blue light...which all 'normal' stars are.

The second answer does give a plausible mechanism for a green star to develop: that it is surrounded by a cloud of interstellar gas with a high oxygen content. But, this gas cloud must be very large. If the gas cloud was surrounding a star and roughly the size of our solar system, then it would act like...our solar system: it would coalesce through gravitational attraction into planets. If the gas cloud still exits around a star, that implies that planets have not yet formed.

On the other hand, if the gas cloud was enormous like a nebula, it could remain stable for some time, and perhaps catch a rogue planet that was shot into interstellar space by some long ago migration of giant planets. Unfortunately, now we run into the problem that this gas cloud does not emit very much light. The who principle behind the green light is that the cloud absorbs most of the radiant energy from its interior, and only emits the light in the green band, which is why it appears green.

The gas cloud in the linked question, NGC 6826 has an apparent visual magnitude of 8.8 at a distance of 2000 ly. This translates to an absolute magnitude of -0.1, about 100 times as bright as our sun. But, with the inverse square law for luminosity, a planet would need to be about 10 AU from this object to receive the same light energy as our planet (at 1 AU) does from our sun. However, NGC 6826 has a radius of about 0.2 light years, around 13,000 AU. Thus, a planet getting the required light radiation for photosynthesis would be well inside the gas cloud. In short order, the gas cloud with leach the planet's orbital energy through collisions and that planet will end up approaching or crashing into the sun.

Therefore, I conclude that there is no reasonable mechanism to have a planet circling a green star, so the answer to the headline question is: No, you can't have purple plants (or any plants) on a planet orbiting a green star (because that planet does not exist).

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  • $\begingroup$ I already knew that and in fact clearly said it at the very beginning of my question. I don't care if it exists or not. All I want to know is what color the plants would be. $\endgroup$ – Victorbrine Cassini Nov 21 '18 at 16:02

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