I'm curious if it's possible for human life to survive in both of the following cases:

1. A fantasy world with only sunlight (no night).

2. A fantasy world with only night (no day).

Note: let's ignore astronomic concerns for now (things like tides or gravity if, say, the world was a planet that didn't spin) unless you strongly believe it's an issue no matter the details of the world structure.

If you think it's possible, explain how. If it involves ingenuity on the inhabitants' part, bonus points for "fitting it" into a fantasy realm (vs. science fiction). For example, your average fantasy peasant in the "darkness world" probably doesn't know about Vitamin D deficiency. However, it's possible that they learn (somehow) that "healthy" people eat fish.

I'm sure more clarification will be required on my part (and I'll edit my answer accordingly), but I'm trying to keep it pretty open for now.

I'm not requesting hard science--just plausible answers.

To start, a couple issues that I'm interested in ideas for:

Both worlds:

  • Lack of a day/night cycle for humans and other creatures to sleep
  • Extreme hot/cold temperatures (thanks, santyclause!)

Darkness world:

  • Oxygen production (no photosynthesis)


For clarification:

Both worlds:

  • The worlds are linked: light world inhabitants who are in charge of governing / policing the dark world will travel between worlds often
  • Portals: the only way that the worlds are connected are through "small" (a few meters in height and width) portals. All transportation between worlds must be done through one or more of these portals.
  • Portal technology. The portals were not created by the light world inhabitants or the dark world inhabitants, and largely remain a mystery to both (other than that they know, generally, which portals lead to which places). Thus, they cannot be manipulated, but only used for their "intended" purpose. Another point that may be important to note: all portals are one-way. For example, if you take a portal from the light world to the dark, you'll need to know where to go to get to a portal that leads back to the light world. There are many more "intra-world" (only lead to other same-world locations) than there are "inter-world" portals.
  • Light cannot pass through portals.
  • Distance between worlds. They are very far apart, so much so that transportation outside of the portals is impossible for either civilization (even though the light world civilization is considered "advanced", they aren't advanced enough to transport materials / energy / etc. to the other world without the portals).

Dark world:

  • One moon and stars (think nighttime on Earth), but many storms and clouds. I'm open to multiple moons if it would increase plausibility.

Light world:

  • Inhabited by a relatively advanced civilization capable of building large structures and retrieving the dark world's resources (water, for example) via portals to sustain itself. NOTE: this does not mean that the civilization is very advanced technologically. Their scientific progress is still semi-medieval (a little more medical knowledge), though aided by magic.
  • 3
    $\begingroup$ Temperature will be a concern. The planet that is always day will become a roasting wasteland while the planet that is always night will become a freezing wasteland (assuming they are earth-like). $\endgroup$ Feb 13, 2017 at 19:30
  • $\begingroup$ Please elaborate: Is the light world a world without ANY shadows? How dark is the dark world? What would the light sources of the dark world be? Would there be a moon? Would the moon always be visible? Multiple moons? $\endgroup$
    – user20762
    Feb 13, 2017 at 19:41
  • $\begingroup$ Please consider posting one case per question - unless these worlds are connected in some way that will make it really relevant to human survival? $\endgroup$
    – Mołot
    Feb 13, 2017 at 19:44
  • 1
    $\begingroup$ @Molot The worlds are connected--the "light world" people have essentially enslaved / rule the "dark world" people and do not allow them out of their world. The people of the light world that actively govern the dark world will go between the worlds often. $\endgroup$ Feb 13, 2017 at 19:49
  • 1
    $\begingroup$ The novel Dark Eden by Chris Beckett describes one take on your second (no sunlight) scenario. I recommend it. If I could describe the ecosystem without spoilers it would be that "life finds a way". Without sunlight (or photosynthesis), the plants dig down towards the planet's core for energy. $\endgroup$
    – GSP
    Feb 13, 2017 at 21:43

5 Answers 5


Here's what I have for starters.

Since the "Light" world is advanced, they could have gardens/farms growing in areas that could mechanically be placed in darkness for periods of the day (in the shadows) Which would also take care of oxygen production.

Since they also are technologically advanced, they could bring light to the dark side either through mirrors or other means, scientific or magical.

Heat would be a problem for both worlds. If the light world was sufficiently advanced, they could vent the heat to the darkness world in a symbiosis that would heat the dark world and cool the light one.

Fungi could be easily be farmed on the dark world, and a glowing fungi could provide more light, perhaps amplifying the light from the moon and stars.

EDITED to add:

The mystical heating and cooling could function through the same magic as the portals, but with a much different conduit, it would be one-way with all the heat and light being absorbed from heat-sinks on the highest elevations, so that as these heat-sinks sent off heat to the other planet, they would be significantly cooler than the surrounding air, which would then drop down off of the mountains helping to create a circulation system of the cooler air. One the dark world, the heat dispursion areas could be underwater, making hot-springs or heated lakes, with the light being sent to various areas with a fiber-optic type network.

This would create a symbiotic relationship with both planets making ruling the dark world be more of a beneficial dictatorship than an iron-fisted one, much the way Rome expanded by building cities in lands outside the empire which became major hubs of trade, and remain so today.

  • $\begingroup$ Richard, see my clarification edit regarding portals and distance between worlds. Still, +1 for a generally helpful answer. $\endgroup$ Feb 13, 2017 at 20:37
  • $\begingroup$ @avidspartan1 I revised and expanded on the concept a bit. let me know if you like it or want more detail. $\endgroup$
    – user20762
    Feb 13, 2017 at 21:02
  • 1
    $\begingroup$ I have used my newly-gained upvoting powers to give you a true +1. @RichardU Helpful suggestions--I'd never thought of heat / cold going through the portals (basically a mystical, symbiotic A/C system for both worlds), and it makes a lot of sense. Great stuff! Oh, this also made me remember one more edit that I'll add to the original post: light can't travel through the portals. $\endgroup$ Feb 13, 2017 at 21:22

Yes to both!

And despite what you would first think, the planets can absolutely maintain pleasant temperatures.

First lets look at the bright and shiny day planet.

Sure, you're first instinct is that it's gonna get pounded by sunlight and turn into a scorching ball of gas, but that's not necessarily true. This one is actually relatively easy for humans to thrive on. All you need is lots and lots of pretty white clouds.

Assuming the atmosphere is somewhat similar to Earth's, then give it a few (relatively) small tweaks and turn the sky white. This greatly increases the planet's albedo and reflect most of the radiation back up into space, and if a nice balance is formed then the surface of the planet can remain relatively earth-like.

Now for the dark planet.

This one is a little bit tougher, but still doable thanks to the dreaded and evil Greenhouse Gases! Since it's always in the dark I'm assuming something must be close enough to block out the light. So there's our first piece of heat in the form of tidal heating. Next there's the heat that emits from the the planet's core. Then there's however much light that may reflect from the planet's moon. And then there's another source of tidal heating depending on how close that moon is to the planet. So all in all, you got something to work with but if the atmosphere was exactly like Earth's then probably still wouldn't be enough.

But we can add some lovely lovely greenhouse gases. We can safely breath multitudes more CO2 than what's in our atmosphere. We can also add in a little bit of methane and, my personal favorite, toss in a decent amount of ammonia for good measure. And who says the atmosphere has to be at 1atm. We can safely crank that up higher so lets say the planet is closer to 1.5atm. That's more gas near the surface which means more trapped heat. Toss in a good an ocean as well and yes this planet could remain relatively warm.

What about food though? No fear, the moon is here. Earth's moon, interestingly enough, is capable of providing enough light to support some small phytoplankton. And our moons albedo is only 0.12, and it's very possible your moon could have a significantly higher albedo which would mean it would reflect more light onto the planet which could potentially grow more plankton which could form the basis of a food chain. So the humans would most likely be coastal and live primarily on fish. There are also plants here on Earth that can grow in very small amounts of light, so it's entirely possible that the land could be dominated by plants that also grow from the moons light.

So, in closing, it's very much possible that both these planets could be habitable for even primitive humanity. Though I would expect those on the daylight world to have an easier time of it than those on the dark world.

Hope I helped!


Did you know that there is a real world equivalent to what you are talking about? Well there actually is. If you have a planet in the habitable zone of a red dwarf star it will likely be tidally locked, then you have one side that is always night and one side that is always day! And since you know they live on the extreme sides of the world anybody that tries to travel outside of civilization will die before they get far. You could also have the portals (and your magic system) be created by a previously super advanced civilization that got sent to the extremes of the world after a war. And for the moon? Just place it at L2, if it's roughly the same color as the day side then people on the night side might think they are seeing the day side, you could do something like that by placing a similar disk at L1.

Just think about, everything that you ever wanted but it's in the future, and sticks with what could be possible.

Edit: What I'm recommending is something that requires clark-tech (technology so close to magic it's indistinguishable from it), and a ruling civilization that nobody is aware of making sure that they stay alive (to prevent morality wars) and tech-backwards. It's basically sci-fi but really fantasy.


The light may help some visually and certainly life is sustainable with minimal to no light. Look at the deep ocean waters where the light cannot reach. most of those fish are blind or create their own form of light. Temperature will still be an issue. A planet that has total darkness will be frigid. Take a look at Pluto (you can argue if it is or is not a planet later :P), minimal sunlight reaches and it is basically a floating iceball. Since the light source will be from moon(s) which is an indirect reflection of sunlight, there will almost be no external heat being provided.

The other issue you have to consider is that you would have to probably create your own storm/cloud system. Clouds form due to moister evaporation. A planet darkened out will most likely be completely frozen. Also with the lack of heat, the air particles are more than likely going to be rigid so there will be very little room for friction to be created and lightning will also not be possible or rarely seen.

Even so, this wouldn't be scientifically possible as all planets revolve around a star which means they have more or less sun exposure depending on how far away they are. There could be a state where the planet is far away enough which minimal light get's through, but it would be more like a permanent dawn/dusk rather than the deep of night.

You would be better off just doing some handwaving or have part of the mystery of why the planet is in total darkness is because there is some object in the way or some magic barrier placed by the people of the light which won't be revealed till what ever plot in the story makes someone curious as to why they are always in the dark.

As far as the light planet goes, as everyone said, it would pretty much be like living in the Sahara desert anywhere you go all day every day but it seems like you have this one mostly thought out with underground shelters (and water wells would be possible to find in the ground as well). The only issue would be a source of food because plant life as we know it requires some light and proper temperatures to grow. Anything above the surface would die from being too hot and the shelters are more than likely going to be too deep to create skylight windows. Meat would also be hard to cultivate but would be possible to create underground rooms for raising livestock.

Cool concepts to play with but in terms of a reality check it wouldn't pass. That doesn't mean though the idea cannot hold water, you just have to do some handwaving so that it can seem plausible. Like maybe the people in light have replicators for food since they are advanced and don't need to grow crops or livestock.

  • $\begingroup$ Thoughts on a "fantasy cooling system" suggested by Richard U's answer? Also, +1 for "handwaving" ideas--I had been thinking of having the light world "in the way" of the sunlight getting to the dark world. $\endgroup$ Feb 13, 2017 at 20:36
  • $\begingroup$ @avidspartan1 I think it would definitely fit the theme you are looking for and a solid starting point. It could also add to the subplot of why the lightworld is using this darkworld and it turns out that they are eventually going to super heat the planet and it will be destroyed or something. I think it would also help make it so the dark world can have more "normal " temperatures. The only thing would be explaining how the portal got there in the first place if their technology is unable to go there by other means. $\endgroup$
    – ggiaquin16
    Feb 13, 2017 at 20:42
  • 1
    $\begingroup$ See my edits on portal technology and how "advanced" the light world civilization is. In general, I agree, though--nefarious purposes on the part of the light world inhabitants are a great plot point to help smooth doubts over. $\endgroup$ Feb 13, 2017 at 20:57
  • $\begingroup$ @avidspartan1 Sounds like you got a great story so far. A lot of mysterious pieces to add intrigue. Many ways you can take this for sure. You will still have to answer at some point why the portals are there though with it being such a huge focal point of the story. $\endgroup$
    – ggiaquin16
    Feb 13, 2017 at 21:06

The light world is easy. Simply make the sun enough dimmer than ours that the planet maintains a reasonably constant temperature and does not overheat. It can be any temperature you want it to be, from Arctic to Sahara, or cooler or hotter than either of those, by just adjusting the sun's output appropriately. A bit of cloud cover from time to time helps make things more variable.

As for dark, the big problem is that the world is going to freeze unless you provide some source of heat. Possible sources include:

  • Geothermal (ultimately coming from nuclear reactions in the planet's core: volcanoes and such, geysers, or just heating through the ground)
  • Tidal heating (for instance, a moon closely orbiting a gas giant gets constantly wrenched by gravity, causing earthquakes, friction and heating. Take a look at Io, for instance.)
  • Solar (a planet orbiting a brown dwarf, dying red dwarf, a star surrounded by opaque-to-visible-light dust clouds, etc. might mostly receive infrared radiation, rather than visible light)
  • Environmental (like Solar, except that infrared comes from nearby space dust or maybe matter orbiting a dormant black hole)
  • Meteorological (lots of meteors fall, and each contributes heat as it burns up in the atmosphere)
  • Electrical (lots of thunderstorms due to something (what?) moving air around)
  • High Energy Physics (Aurorae? A constant wash of subatomic particles from a nearby star or black hole? Could fry people with radiation if you're not careful.)
  • And, of course since this is fantasy, some type of Magic.

Note that on the dark world, the weather patterns are going to be due to whatever's causing the heat, not to sunlight like on ours. So you might see clouds or winds forming around volcanoes for instance.

It would help to have an atmosphere that traps heat (Greenhouse gases).

Another possibility is that the planet is actually in a fairly well lighted area outside a star, but an upper layer of its atmosphere blocks most visible light, letting only infrared through. Perhaps somewhat like the planet Venus, but even darker. This means you can't see the moon or stars, though.

How life would evolve on a world mostly or entirely without light is an interesting question, of course. I would imagine a lot of bioluminescent creatures, like we see in the deep ocean or in caves where similar situations exist. Also, creatures that "see" using other senses, like echolocation, excellent hearing, excellent smell, feeling vibrations through the ground, sensing electricity, and so forth.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .