# 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?

• There are extremophiles that live in total darkness on the ocean bottom right now. They get their energy at geothermal vents. – candied_orange May 30 '15 at 19:47
• @CandiedOrange I considered that, but geothermal would probably offer rather small habitable regions. – apaul May 30 '15 at 19:49
• Life needs energy. If you take away light and aren't happy with geothermal you're going to need plants that figure out how to split the atom. ; ) – candied_orange May 30 '15 at 19:56
• @CandiedOrange I'm not trying to remove light completely, I'm trying to figure out how far light can be realistically reduced. – apaul May 30 '15 at 19:59
• So, dim like on Pluto? It's about as bright in the day as midway between moonlight and sunlight. But with that little light you also get very little energy. Like Earth's atmosphere would freeze and fall to the surface kind of little energy. – Samuel May 30 '15 at 21:52

You'd be surprised at how versatile nature is

I would expect you could get remarkably dim. Really all nature is looking for is an energy gradient which is sufficient to stave off the effects of entropy.

In the early days of life, energy came mostly from purely chemical processes (themselves empowered by light, but over a very long period of time, so dimness is not a big issue). The initial genesis of life is probably rather independent of light levels, save for perhaps the need for enough ionizing radiation to lean through to jostle things around a bit at the molecular level.

Once life takes off, its greatest foe is not the environment, but the other lifeforms themselves, so they will naturally form a balance to match whatever the environment allows.

Now if your goal is recognizable life, you may have to keep things bright. A low energy environment is going to reward different tracts of life differently. As a general rule, you can probably use the 10% rule: in a food chain, you need 10 masses of foodstock to support 1 mass of predator that eats that foodstock, but the methods used by the predator to prey will shift. We won't see as much attention on movement, because movement costs calories, and they will become increasingly valuable. Rather, there will be more focus on lying in wait, preparing for prey to arrive, and then attacking them slowly (after all, the prey will be slow, themselves).

If you want modern Earth style photosynthesis using chlorophyll, your lower bound on how dim it could get is really based on the energy required to produce chlorophyll versus the energy it can harness before breaking down. I don't think there's a well known biology formula for that, because chlorophyll breakdown will certainly depend on the particulars of your solar spectrum. More UV would probably cause the molecules to break down sooner.

Why do you assume the prime source must be photosynthesis?

You could have a planet as black as a coal mine that was habitable, it just needs a different prime energy source.

I'm thinking of Io—tidal flexing causes substantial geothermal activity. Now, picture a planet in a fairly elliptical orbit quite close in degenerate star. Of course it's tidally locked but since its not getting its energy by radiation the fact that what little the star puts out hits mostly one side doesn't matter. The primary source of heat is tidal flexing. This also provides the energy source upon which the ecosystem is based—think of the ecologies around the volcanic vents on the deep ocean floor. Obviously this will be a pretty sparse ecology but that's not the same as saying it's uninhabitable.

Given the much more stable and widespread energy source this is in comparison to the volcanic vent systems I would expect life to develop far better ways of exploiting the energy. The vent ecologies are based on reactive ions in the water and do nothing with the heat—a more sophisticated organism could exploit this temperature differential.

• If the orbit is not nearly circular, it would seem to rock back and forth since the rotation rate is constant while the orbital rate is not. The planet will rotate too far when at the slow end of the orbit, and vice versa. So the sun would move and introduce stresses. BTW, a half-integer multiple is more stable in that case (as with Mercury). – JDługosz Jun 1 '15 at 1:43
• @JDługosz But stresses are exactly what we want here--more stress, more vulcanism. With a circular orbit there would be no vulcanism. – Loren Pechtel Jun 4 '15 at 3:24
• My point is that tidal locking doesn't necessary mean that the sun doesn't move in the sky, and can still have tides. Your use of But makes me think you think you're disagreeing. – JDługosz Jun 4 '15 at 3:35
• @JDługosz Yeah, there would be tides. So what? They're not high enough energy to power an ecology. – Loren Pechtel Jun 4 '15 at 3:43

Earthy plants would have a problem in a permanently overcast sky (think of the effects of the Tambora volcano and the "year without summer", or for the more extreme case, the Dinosaur killer asteroid filling the sky with cubic miles of pulverized rock dust).

Depending on the nature of the dark sky, it may be possible to hand wave a plant which can photosynthesize using infrared radiation. Since it is using a much longer wavelength than current plants, these plants would probably look black rather than green, and since the wavelength of light would be longer, they would probably need much larger leaves or other light gathering organs than what we are used to seeing now. Since the dark planet will probably be colder, other adaptations like waxy coverings, deep, starchy roots or even reflective leaves to focus "light" onto a photosynthetic organ might all be possible.

The other thing to remember is that infrared light is lower energy than visible light, so the plants will grow much more slowly and the entire ecosystem will be limited by the amount of energy the plants can convert into sugars, starches and other edibles. On Earth, with lots of sunshine, most plants actually convert as little as 1% of the sun's energy into food, so your planet will be very calorie limited indeed.

• Such a lifeform might use quantum photon combining or other way of absorbing lower energy radiation and pumping up enough energy to drive the reaction. This could be done electrically using phonons in mollecular orbitals, or mechanically where it pumps up vibration until the swing hits hard enough. Using dim red sunlight could be very different from what we understand as photosynthesis, but still exploit the incoming light directly within the living tissue. – JDługosz Jun 1 '15 at 1:50
• That would be very interesting indeed. Not exactly a "plant" in the sense that we would understand. Would such a thing have the analogues of leaves or would it have to use a different mechanism to utilize the available light? – Thucydides Jun 2 '15 at 0:00
• Alge and cyanobacteria are billions of years older than plants. A planet that doesn't get the next big step, being generally slower going or just hasn't happened yet, will be full of microbes, not plants. – JDługosz Jun 2 '15 at 1:32

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.

• Pluto is brighter than the bottom of the ocean and there is plentiful life inside of lightless cave systems. Your inference is not valid at all. – Samuel May 30 '15 at 22:41
• @Samuel on the contrary, I don't see how that contradicts at all. Assuming photosynthesising plant life, this simply means a possible minimum is now none (though the photosynthesis process would need modification for this) – ArtOfCode May 30 '15 at 22:44
• The abyssal plain is fed from the lighted world above, from "marine snow" and even whale carcasses. – JDługosz Jun 4 '15 at 6:32

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.