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Primary question

If this world averages a visible light level of civil twilight (about 1000x dimmer than full daylight, or 500x brighter than full moonlight) during the day, what kind of plants can I get away with?

Information

The planet as determined by my previous question here and my personal solidifying of elements has these characteristics so far. Flexible characteristics are things I am currently going with but are not vital to any story or world-building elements yet.

Flexible Characteristics:

  • No moon
  • very little tectonic movement, so (relatively speaking) quite flat (this would probably mean shallower water and few mountains).
  • Gentle seasons (Similar to near equator on Earth).

Important Characteristics:

  • Majority of, or all year, is above freezing
  • Almost constant fog.
  • Frequent rainfall (usually light drizzles)
  • Light exposure much lower than full sunlight on Earth

Additional Details

  • The planet was not always dark like this. It has been dark for a very long (undetermined) period of time but was not always so dim. It is as dim as it is because of astrological events and its incessant fog.

  • Because of the planet's flatness, fog, and frequent drizzling, I am assuming lots of shallow, stagnant, or slow moving, bodies of water (marshes) and plains.

  • I can assume that fungi and mushrooms will be fine with the low light levels and I may end up using them as my "woody" plants, but I am not confident in determining what other types would be reasonable. Mangrove trees maybe?

  • I am hoping for leafy plants or even trees even if they grow very slowly.

Answers just need to be plausible. Some magic is totally acceptable. I am looking for a spectrum of plants with a few more specific examples.

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    $\begingroup$ Is there a possibility of incoming light outside the visible spectrum? Maybe the fog/clouds are transparent to UV. Then plant life could absorb that non visible light. $\endgroup$
    – Lex
    Commented Jan 17, 2018 at 20:15
  • $\begingroup$ @Lex Yes, that is certainly a possibility. However, at least some of the dimness is caused by astrological occurrences (I have chosen a large star-ring of dust) which would probably block a percentage of the entire light spectrum. $\endgroup$
    – BornToDoStuff
    Commented Jan 17, 2018 at 20:19

3 Answers 3

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Your plants must metabolize higher energy light waves

Higher energy light penetrates more deeply through particulate matter. In the case of both various astronomical phenomena from the other question and incessant fog, infrared light will be totally blocked, visible light will be heavily attenuated, and ultraviolet light will be attenuated very little.

enter image description here

You can see there that visible light is in an absorption 'gap' in water. That means visible penetrates through water much more than IR radiation or UV light smaller than about 200 nm. This is why both plants and our eyes use visible light. But notice that the absorption is even lower in the near-UV spectrum.

Photosynthesis using chlorophyll is optimized for the light conditions on Earth. However, there are extant alternative pathways for metabolizing UV radiation. Carotenoids are accessory pigments that work alongside chlorophyll, extending its range as low as 400 nm, just into the UV range. Another accessory pigment, Oenin, can absorb as low as 200 nm. There may be other undiscovered or extinct light absorbing pigments as well. You can have plants pick up accessory pigments, such as Oenin and Carotenoids, to up their energy intake in the low light conditions.

However, there is one more catch. Here is plot of received EM radiation on Earth by wavelength:

enter image description here

Look to the far left in the 200-400nm range (the near UV area). Just as water's absorption goes down on the specturm, the sun's radiation also goes down. That is due to the sun's ~6000 K surface temperature. If you really want to make UV light a thing, you are going to have to find a way to increase the Sun's radiation levels at smaller wavelengths. The only way to do this (in a science based manner) is for the sun to get hotter.

enter image description here

Bump up your sun's surface temperature to about 8000 K as an F-type Main Sequence star, like Procyon. Now, your new UV driven plants can flourish!

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  • $\begingroup$ Alright, I am learning a lot of science today. I didn't quite understand everything here but from what I am getting there are other pigments besides chlorophyll that help plants absorb other wavelengths of light (I didn't know this! So cool!) and this would make plants more yellow/red which is super rad. In order to help these other wavelengths of light along though the sun should be hotter than Sol and this would make it slightly whiter? $\endgroup$
    – BornToDoStuff
    Commented Jan 18, 2018 at 6:11
  • $\begingroup$ @BornToDoStuff That is all right except for the plants being different colors. They could be different colors, but since the goal is absorb UV light, and we can't see UV light anyways, this won't have much of an effect on the visible spectrum. But if you want yellow or red plants, I say go for it. $\endgroup$
    – kingledion
    Commented Jan 18, 2018 at 14:46
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    $\begingroup$ This also implies that despite the darkness you would get wicked sunburn on this planet. $\endgroup$
    – Joe Bloggs
    Commented Jan 18, 2018 at 15:05
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    $\begingroup$ @kingledion: I more meant that you'd accidentally overexpose yourself without thinking about it because 'hey, everything is dark', rather like how sunburn is always worse when there's a breeze because the cool air tricks you into thinking it's fine. Cool numbers though! $\endgroup$
    – Joe Bloggs
    Commented Jan 18, 2018 at 17:00
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    $\begingroup$ With this knowledge about UV light I can actually grow things as large as redwoods in this world. Yesterday I learned that redwoods can grow to the size they do because of the consistent and frequent fog of the areas they thrive in and that seems to match this world pretty well. $\endgroup$
    – BornToDoStuff
    Commented Jan 22, 2018 at 17:14
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You could take cues from the flora of the rainforest floor.

forest floor source

I have read that as little as 2% of sunlight penetrates to the forest floor. Vegetation is sparse; a lot of ferns and mosses (I wonder why these more primitive plants do better in low light areas?). Here is a schematic for flora of the different levels of the Australian rainforest.

schematic for flora of the NSW rainforest

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  • $\begingroup$ Oh! I had looked up rainforest floor diagrams but did not find one with names of actual plants I could look up, good find! Do you know what the references next to the plants are? "shield fern (43L)" $\endgroup$
    – BornToDoStuff
    Commented Jan 17, 2018 at 21:00
  • $\begingroup$ Although the source of this PDF is this site nationalparks.nsw.gov.au only google turns it up - I could not find it searching on the site. My suspicion given L and R suffixes is that they are markers on a walking tour of the Sea Acres site. $\endgroup$
    – Willk
    Commented Jan 17, 2018 at 22:00
  • $\begingroup$ Note that 2% is still 20x brighter than civil twilight, which is 0.1% the brightness of full daylight. $\endgroup$
    – RonJohn
    Commented May 18, 2018 at 18:34
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What if your inhabitants are recent arrivals, and have vision that is not matched to what gets through the atmosphere. E.g. if they have eyes that see only medium and far UV, an earth type planet will be very very dim on the surface, because most of that is blocked by the atmosphere.

Similarly if they saw by near infrared.

Or give them vision across a large range of infrared -- they see many more colours than we do, but vision is blurrier due to the longer wave lengths.

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