How dark can a habitable planet be?
I'm picturing a world with a permanent, dark, overcast sky, but I'm wondering how dark it can be before photosynthesis is no longer an option.
It looks like there are a good number of plants that have good shade tolerance, but I can't find any real information on the bottom limit. There also appear to be, a few plants that don't need to photosynthesize at all.
The closest analog I can think of is the oceanic zones, photic and aphotic.
Photic being the top layer of a body of water where most, if not all, of your photosynthesis is happening. Aphotic being too deep for enough light to penetrate.
Apparently the photic zone:
extends from the surface down to a depth where light intensity falls to one percent of that at the surface
So everything else being equal, temp, atmosphere, etc. could a thriving planetary ecosystem exist at say slightly above 1% normal daylight?
Failing near total darkness, how dark can it be?
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Fungus-like plants grow well without light; they just need something to feed on.
So there are insects that feed on the fungus, and larger animals that feed on the insects. The fungus feeds on the droppings and bodies of the insects and larger animals.
We have whole ecologies that exist in total darkness, so it could work on a planetary scale.
Heat is the big thing, but you could solve that several ways.
Easiest way is to have the planet orbiting a rogue brown dwarf through interstellar space.
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The second law of thermodynamics dictates that without a decent amount of energy, existing physical structures will decay because there are more ways of arranging an unordered structure than one carefully constructed to fit a purpose. That is, $ \text{order} \longrightarrow \text{chaos if energy} \lt \text{required} $.
I won't go into calculating the exact energy requirement to stave off thermodynamics right now, but suffice to say it's fairly low otherwise life would not exist at the bottom of oceans with just 1% of the light at the surface.
Thus, we can infer that a habitable planet can at least be as dark as the bottom of an ocean; if this is the true limit then it will simply hVe no submarine life. If, however, the same 1% rule still works out, we can suddenly infer that the planet could have 0.01% of Earth's light. This cycle repeats: decrease the light, and if the rule applies divide the minimum light by 100.
Of course, it also depends on how science based your setting is. If you're open to creating new types of plants that don't require so much light but instead of photosynthesising get their energy from another source, then the minimum light level can be as low as you want to make it possible to be.
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