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Basically, in a sci-fi roleplaying group I am apart of, I have began to work on a long-standing human colony world. The tech-level is pretty much bog-standard sci-fi, and such, colonisation is possible and is already widespread.

This planet is known for its fall/autumn tones and colours within its plant life, orange leaves, red leaves, red grass, red forests. Is it possible to have such a world orbiting a k-type star? If not, how could I explain these floral differences while remaining decently habitable?

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  • $\begingroup$ If these plants are deciduous, then these colors have nothing to do with photosynthesis. They're just the colors of the proteins of the structure of the leaf... those could be anything. Really, you can have the non-photosynthesis pigments be nearly any color. Some develop even in late summer, so you don't have to worry about the chlorophyll analog needing to be able to "cover" the others. $\endgroup$ – John O Feb 24 '20 at 17:37
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    $\begingroup$ Trees with purple leaves are extremely common in Earth gardens cities. And some plants, e.g., Plectranthus scutellarioides, are known for the great variety of cultivars with beautiful leaf colors. Moreover, here on Earth, the green plants, Viridiplantae, are but one of the photosythetic lineages; red and brown lineages exist too. Accident is an important factor in evolution. $\endgroup$ – AlexP Feb 24 '20 at 20:48
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Here's an example of how many elements allow you to plausible handwave and get your gorgeous orange-red plants. Feel free to invent others, it's really only an exercise of imagination.

It is true that K-Type peak spectral emission is in red-very near IR, but let's start waving our hands:

  1. there's a non-trivial amount of light in the upper spectrum too. Even if not making the maximum intensity, the speed of biochemical reactions can benefit from more energetic photons (as long as they are not excessive). Look, the Sun's max spectral happens at between 500-550nm, wavelengths that chlorophyll mostly reflects - that is to say: "harvesting" light at non-peak radiance spectrum is proven to have some advantages.

  2. the star's emission spectrum and the spectrum of light at the planet's sea level are two different things and it's the latter that actually matter. A thinner atmosphere will scatter the blue light less making the intensity at closer to-the-normal-incidence for blue light a tad higher. True, a thinner atmosphere would raise the problem of maybe an insufficient UV light absorption, but...

  3. ... if you make the magnetic field of the planet stronger, a thicker ionosphere will help absorbing more energetic radiation.

  4. a stronger magnetic field may also be necessary to prevent a thinner atmosphere being blown away by the solar wind - make it strong enough and you may have in the solar wind a way to grow your atmosphere. The auroras there must be fantastic. Maybe, unlike Earth, they are a permanent feature and are visible even during daytime? With intensity high enough to count as non-trivial amount in the balance of light available to photosynthesis? 'Cause, you know, most of the light in the aurorae is in the green/blue spectrum - perhaps the plant life can tune to an "always on" green/blue light bulb.

  5. It is very likely that a necessary condition for life to appear is the existence of tides (alternation between wet/dry, solvation/crystallization). In Earth case, the biggish Moon does it and moons of this size are unusual. For that planet, a closer proximity to the star may be the one to trigger higher "star-caused tides"? K-type stars are smaller/cooler but do emit more in IR - so they have larger Goldilocks zones. What if we dial down the size of the K-type star and decrease the planet proximity to the start, then we'll get an increased amplitude for the "tidal sloshing" (and aren't we lucky we handwaved a higher magnetic field higher, so a closer proximity doesn't hurt that much when the star throws a tantrum?)

  6. about that higher magnetic field, how it came to be? Does a higher magnetic field sorta imply a more mobile planet core, thus a hotter/younger planet? Not necessarily. Cobalt has a higher Currie point and a slightly lower melting temperature, so if we handwave a higher concentration of cobalt on that planet, the mantle may be less viscous and the magnetic core stringer, so we're good.

  7. if you don't particularly like cobalt for some reasons, you can still make the core purely iron and make the planet younger. How to allow the life emerging faster? Well, we can place everything inside a nebula resulted by some supernovae explosions in their star cluster. Not only this will increase the probability of higher concentration of organic compounds, but a slightly higher concentration of transition metals with low ionization potentials, heaps of oxidation numbers and maybe catalytic activity may have kickstarted the life sooner and pushed it faster. A slightly higher dose of actinides may provide a small boost of energy for biosynthesis even in the present.
    Maybe an interesting reading on how Earth’s chemistry and life coevolve to be nearly in tune

  8. all good and dandy, but how this relates to the colour of those damn'd leaves?
    Well, the photosynthesis (and maybe radiosynthesis) is mediated by "organic dyes" and protein-complexes - capable of absorbing energy in large bands and "delivering it" to the inside of the cells at energy levels just fit for the synthesis (in one or, usually, multiple steps). The most efficient light absorbents are those dyes that absorb higher energy photons (simply because there are more ways to give that energy back), but other other factors play a role too (the abundance of substances to synthesize or replenish them, the power spectrum available, their stability vs activation, etc). Two such large spectrum "dyes":

    • rhodopsin - that thingy in your the rods cells. Their absorption spectrum is mostly in the green, which makes it appear redish-purple when not activated (becomes bleached when activated);
    • Ruthenium based dyes that the Grazel dye-sensitized solar cells are using. Absorbs mostly in blue and green (but some reds as well).
      Isn't it lucky that we placed out planet in a post-supernovae nebula so ruthenium is not that scarce? And we are on a youngish planet so ruthenium didn't totally migrate towards the core?

So, what do you see when you absorb blue and green? Well, mostly orange and reds.

If you want, you can use melanin in animal lifeforms and make them obtain their vitamins by radiosynthesis? Why not the plant life too? If we made the plans use retinal dyes, let's throw in a small quantity of melanopsin for deeper/darker red leaves?

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In theory, ideal leaves would be black, to absorb as much light as possible. The fact that they're green, because chlorophyll was what nature stubled on, ought to tell you all you really need.

If you want to go further though, look at why there is such an incredible diversity of earth flora colours and the pressures driving than. And call foliage colours aren't even adaptive anyway as far as I know.

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There was a 2007 study on alien plant colors: https://pubs.giss.nasa.gov/docs/2007/2007_Kiang_ki09000a.pdf.

The author's conclusions are that terrestrial plants would likely absorb more light in the peak wavelengths of the star. Plants on a planet orbiting a K-type star would likely absorb light in the redder part of the visible light spectrum and hence would be more blue-green in color, getting darker and closer to black for smaller and redder stars.

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    $\begingroup$ A little bit simplistic to use as a criterion. Counter-example: peak Sun emission is between 500-550nm. Chlorophil is reflecting most of it and absorbs red and blue. $\endgroup$ – Adrian Colomitchi Feb 25 '20 at 0:11
  • $\begingroup$ @AdrianColomitchi Right, I was wondering about that myself. Kiang et al. argue the surface incident spectral photon flux density is strongest at 670-680 nm (see their second article on this topic: pubs.giss.nasa.gov/docs/2007/2007_Kiang_ki08000d.pdf). $\endgroup$ – RobertF Feb 25 '20 at 0:54
  • $\begingroup$ @AdrianColomitchi Another issue is overheating - absorbing too much radiation can be bad. So plants exposed to light from say A-type stars might reflect strongly in the blue wavelengths. $\endgroup$ – RobertF Feb 25 '20 at 1:07
  • $\begingroup$ yes, there are so many factors in play, not at the least the composition of the atmosphere (what it does absorb/scatter), the thickness of it (how much), what other sources of energy for biosynthesis (beside light) are available, how much liquid water is available (as a reaction and cooling medium). Even for A-type stars, a very thick atmosphere may absorb the hard-UV and X-rays on the top and "fluoresce" it down at non-bio-harmful wavelengths (like have the entire sky glowing yellow, not ever seeing the sun or any star directly and an non-existing or very short deep-dark night). $\endgroup$ – Adrian Colomitchi Feb 25 '20 at 2:56

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