# What could reduce sunlight on an Earth-like planet?

I want to create a world where there is very little sunlight during the day -- at noon the light would be about the same as an Earth sunset. The planet would be Earth-like until something causes the sunlight to be reduced (so that life can evolve normally first).

So, what are my options? What could possibly prevent most of sunlight (not ALL) to reach the planet's ground, considering these requirements:

• It must be stable, meaning, continuously block some sunlight during millions of years, if not forever
• It can be something completely natural or "kickstarted" by humans (because of their environmental damage for example)
• The planet setup and the star must stay the same: an Earth-like planet with 1.4 earth masses, in a double planet system with another, smaller Earth-like planet. The star is sun-like. Anything can be added to this as long as it doesn't hijack the setup.
• It should keep the climate as close to Earth-like as possible. I read that dense clouds can have this less-sunlight effect, but I don't want dense clouds everywhere all the time. I want to keep the Earth's diversity in climate and environments.

My goal is an alternate Earth with just the sunlight being missing, I'm trying to get everything else as normal as possible, so I need to know what effect each option will have on the planet (ex. temperature drop) so I can work on solving those problems one by one later.

So it's alright if life is supposed to go extinct, the humans will have plenty of time to find ways to avoid that. I also expect life to evolve very differently with less sunlight, even if the rest is the same, so no issues about that.

Magic or a bit of handwavium is acceptable, as long as there is a somewhat clear understanding of how it works and its consequences.

• You can't have less sunlight and the same climate as in the normal Earth: either you drink the beer or keep the bottle full.
– L.Dutch
May 24 at 14:10
• @L.Dutch of course, I know. This is why I explicitely put it in the question. I'll find ways around that (I try to keep everything as plausible as possible scientifically, but it's still a fantasy world). I just don't want answers that are related to climate (like dense clouds). May 24 at 14:13
• @L.Dutch The planet would be Earth-like until something causes the sunlight to be reduced... Lulullia isn't asking "what happens next?" (which would affect the climate), but "how did this happen?" I expecially like So it's alright if life is supposed to go extinct... which sounds like the beer's gonna get drunk.
– JBH
May 25 at 0:35
• en.wikipedia.org/wiki/Space_sunshade May 28 at 2:55

Orbit change

https://solarsystem.nasa.gov/news/925/what-does-a-sunrise-sunset-look-like-on-mars/

A rogue planet plunges through the systm and alters the orbit of your planet. Your world moves farther from its sun and enters a new stable orbit. Other planets in the system did not fare so well, one plunging into the star and the other being thrown out of the system.

It is darker farther away. It is colder. That is the new way of things. Life adapts.

OK - exactly how far away does the planet need to move? Farther away than the orbit of sunny Mars, it turns out.

https://en.wikipedia.org/wiki/Daylight

Illuminance Example 120,000 lux Brightest sunlight

111,000 lux Bright sunlight

109,880 lux AM 1.5 global solar spectrum sunlight (= 1,000.4 W/m2) [3]

20,000 lux Shade illuminated by entire clear blue sky, midday

1,000–2,000 lux Typical overcast day, midday

400 lux Sunrise or sunset on a clear day (ambient illumination)

So sunrise / sunset at 400 is 0.003x of bright sun at 120,000.

https://en.wikipedia.org/wiki/Sunlight

1 AU is the distance from the sun to Earth. Saturn is at 10 AU and is 1.1% of Earth sunlight. Uranus at 20 AU is 0.2%. You want 0.3%. So somewhere between 10 and 20 AU will reduce sunlight of your planet to the desired amount.

• Combine your idea with mine (dramatically increasing greenhouse gases e.g. via a deliberately triggered methane clathrate gun or SF6 emission) and you've got a full solution with less light but an otherwise similar climate. May 26 at 11:46
• This is very interesting! How much farther away from the sun would the planet need to be in order to have sunset-like light during daytime? May 26 at 12:24
• Not sure. I guess look up the ratio of light intensity at noon vs sunset and take the square root? Someone here will know. It's bedtime here. May 26 at 12:39

With reduced sun light, plant life won't develop as it did on Earth. With different plants there will be different plant eating animals. Similarly, reduced sun light will also reduce the temperature of the planet, which again will affect both plant life and animal life.

One way to get reduced sun light, compared to Earth, is to have a cosmic dust cloud permanently between the star and the planet. Alternatively, the star could be a reduced energy output star, such as a red dwarf.

An artificial way to get reduced star light on the planet is to have Dyson spheres or a Dyson swarm in between the star and the planet. Energy from the Dyson spheres/swarm can then be directed to the planet so intelligent life there can use it.

• I suspect any dust cloud would be blown away by solar wind in days or maybe weeks.
– Tom
May 25 at 1:44
• Dyson spheres are incredible! I didn't know about them! I may choose something else for reduced sunlight, but these are definitely going to be used to redirect sun energy. As for the dust clouds, they're an interesting option. I'll consider it (and also ways to keep it in place). Thanks for the answer! May 26 at 10:38

## Change the humans, not the sunlight

My goal is an alternate Earth with just the sunlight being missing.

You still need the light of the sun for heat and growing food and stuff; so, instead of eliminating the sunlight, simply eliminate our ability to see it. For about 0.5% of the human population, this is already the case. We call it blindness. There are many ways to cause blindness. Parasites, viruses, drug overdoses, genetic disorders, and physical trauma are the big ones though.

To modify humans to be blind for millions of years you can't rely on parthenogens like parasites and viruses since human will eventually become either resistant to, or remove these things from our environment. You also can't rely truma or drugs since those only effect a single generation.

Your best bet here is a genetic bioweapon. Genetic bioweapons are the weaponized form of gene therapy where you design an weapon to manipulate the genetic composition of it targets. Such a weapon could in theory inflict a blindness by inserting a genetic disorder like norrie disease into all of its victims causing rapid retinal detachment. Unlike traditional bioweapons, this weapon would cause not just blindness in its victims, but in all of their descendants as well. If such a bioweapon were to get out of hand, you could eliminate the genes from the human races required for seeing permanently.

I want to create a world where there is very little sunlight during the day -- at noon the light would be about the same as an Earth sunset

Many vision disorders do not cause complete blindness. Something that clouds the corneas or kills off all of your rod receptors could cause the world to appear much darker without actually blinding you completely.

Normally, any civilization with the advanced bio-engineering skills required to make such a weapon, could also fix the damage given enough time, but the problem is that once you blind all of the people with the skills required to fix it, there will not be anyone left to read the computer screens needed to fix it. Most of human knowledge will be lost simply because it is not available in braille or any other handicap accessible format. And even if you do have it in braille, 99.5% of the population would suddenly become illiterate; so, the people who need to read it will be unable to do so. In all probability, civilization will collapse long before most people can learn to deal with their new found blindness, and civilization will have to be rebuilt from the ground up were blindness is the new norm.

• Thanks a lot for this detailed answer! It certainly is interesting. However, in the context of my world this simply isn't suitable. I want reduced sunlight because as I'm illustrating my world, I can have a big emphasis on lighting. People would adapt by cultivating bioluminescent plants for example, then putting them in lanterns to light their way. Since light is scarce, it'll pop out more. That's the kind of end result I have in mind, and I can't do that if people are blind (that would also screw up all my stories haha) I really like the concept though, I might even use it somewhere c: May 26 at 10:26
• Ancient dying star, a la Tales of a dying Earth.
• Spacetime singularity which is sucking up the star's light.
• Epidemic of opthalmological disease, reducing the visibility of light by the population.
• Welcome to WB. You've posted four quick answers in a handful of minutes. Gratefully, the answers are (for the most part) pertinent. However, from the Help Center we learn, "Brevity is acceptable, but fuller explanations are better." We get that it takes a bit of rep to earn the privilege of commenting, but it's preferred that an answer explain why it's valuable. The point of an answer isn't simply to throw some ideas at the wall, but to educate the querent. Answering one Q well would have been better than four quick comments.
– JBH
May 25 at 0:31
• I find your comment confusing. The question here explicitly asked for a list of options, which is what I have provided. It is not a comprehensive list nor something that will win any awards, but hopefully something the questioner might find helpful and which would be received in the spirit it was given. May 25 at 0:40
• @Zinklestoff In very short : Explain > Tell. But let me... Explain it instead of just telling it :p : If you're a rookie in a topic, it's much easier to understand if we get more bits of what your thoughts are ^^. For instance, what do you mean by "dying star"? For some, a dying star can be a huge red giant (before it goes supernova)... or a tiny dwarf (after the supernova), or... Something else. I lack sun expertise as much as the asker I bet, otherwise they wouldn't need to ask the question (the sun solution would have come to mind very quickly otherwise!). May 25 at 12:20
• Fair enough. Thanks for the feedback. May 25 at 14:25
• Um I'm not sure what you want me to say to this. I agree entirely that better answers would be better. I am definitely sure that scolding people (repeatedly) is not a good way to get them to do what you want. For an example of constructive criticism see @Tortliena above. I am straining but failing to see the self advertised tolerance and patience. May 25 at 22:10

## Decrease the amount of light arriving but increase greenhouse gases

Earth would be about 30°C colder if it was at its blackbody temperature, i.e. received the same amount of light but didn't have a greenhouse effect. The core idea here is to decrease the amount of light arriving while increasing the amount of greenhouse gases to maintain your planet's temperature.

One option is to be like Venus and have double digit percentages of CO2 instead of a few hundred parts per million.

There are also several relatively non toxic greenhouse gases that are hugely more potent than CO2: methane (23x more potent), CFCs (1000x more potent) and SF6 (A whoppimg 20000 - 50000x!)(1).

If methane or CFCs or SF6 are present in significant (not enormous) quantities, you could increase the temperature of Earth hugely.

Now, the amount of heat absorbed/emitted by a planet is proportional to temperature to the 4th power; if you can increase the greenhouse effect by 40 degrees, then you could have the amount of sunlight reduce by 50% and still have the same temperature; increase it 60 degrees and you can have it reduce by 65%. 100 degrees allows 85%.

See e.g. https://www.astro.indiana.edu/ala/PlanetTemp/index.html or other planet temperature calculators out there; this forum doubtless knows many.

Maybe mankind could deliberately set off a methane clathrate gun or produce ludicrous amounts of SF6 to compensate for some event that knocked the planet further away from the sun. Willk's orbit change suggestion would work nicely.

(1) SF6 is incredibly inert chemically but is very heavy so it builds up in the lungs of animals if present in any significant quantity, eventually choking them. Your fauna would need some way to expel it from their lungs, maybe l by totally displacing all the gas in their lungs when they breathe out, or by means of an enzyme that binds to it and transports it to the digestive tract. I assume CFCs would have the same problem. Methane won't; it's light.

Edit: I'm guessing that seasons and maybe polar-equator temperature differences get minimised by doing this.

• While this is a very insightful answer (really, it's tremendously helpful for my temperature drop problem), it's not answering my question at all... The point is not how to avoid/fix the consequences of less sunlight, but really how to have less sunlight 🤔 May 26 at 11:28
• @Lulullia Sorry, I've been editing it and just saw your comment. Combine mankind doing this deliberately or it happening naturally to compensate for any of the other answers here (I like Willk's orbit change) and you have your lower sunlight, similar climate solution. That online calculator suggests quintupling the greenhouse effect compensates for moving from 1 to 1.5 AU orbital distance. May 26 at 12:00
• it's alright, in fact I'm very grateful for your answer! You're right, changing the orbit combined with your solution seems like a good deal, it's even simpler and tighter than I expected (I was ready to make a lot of changes/workarounds/handwavium for this idea to work). I'll experiment with it and see what I can come up with, thanks again! May 26 at 12:28

Reducing Sunlight

There are many ways to reduce sunlight (as told in other answers):

• Move world to a farther , stable orbit.
• Some kind of sunscreen in between.
• Light is sucked away
• Fusion reaction on the sun becomes slow.
• Some phenomenon disintegrates the sun, taking many smaller fragments away, and size of the sun is reduced.
• Small shiny spherical particles float in upper atmosphere which reflect, refract, disperse light (like water molecules after rain).
• Some kind of light emitter, which emits light causing destructive interference.

Bad Effects of Reducing Sunlight on Humans

• Low Vitamin D causing weak bones

• High Blood Pressure

• Low levels of serotonin causing higher risk of major depression leading to mental health problems

• Reduced sunlight exposure during childhood increases the risk of developing multiple sclerosis

• Studies have shown that children in dimly-lit rooms suffer learning deficits

Bad Effects of Reducing Sunlight on Plants

• If plants do not receive enough light, they will not grow at their maximum rate or reach their maximum potential, regardless of how much of any other variable – water, growth medium or fertilizer – they receive.

• 1% less light will give 1% decrease in plant growth, resulting in a 1% lower yield. Fruits or grains need sunlight to get ripe.

Conclusion

You may make a wold with low sunlight but the life will not be the same. It may finish at all or may become something entirely different.

# Tidally locked double planet

Early during this planet's history, its rotation was at a different rate than the orbit of the smaller planet around it. However, its rotation rapidly slowed due to tidal effects. Because the smaller planet is much heavier than the Moon, this didn't move its orbit outward nearly as much as the Moon's. Because of that, your planets have now settled into a long day that is still much less than a month, with each presenting the same side toward the other all the time.

That by itself means that there is no midday sun. Note, however, that the other planet, large in the sky, is as warm as the ground, so the loss of heat into space is also reduced.

Light can also be reduced by other effects. In our solar system, the "zodiacal light" is produced by dust from Mars that extends far away from it. Suppose your smaller planet releases lots of dust due to the combination of weaker gravity, perhaps an arid landscape (all its water and air having escaped its gravity only to land on the larger world), and maybe some extreme volcanic events (maybe it isn't totally tidal locked relative to the larger planet, which would be odd, but we just suppose it originally rotated unreasonably rapidly?). This dust might fog up the local neighborhood and block the light of the star. I admit that's quite a bit of dust, but we have the example of Saturn to suggest that such astronomical phenomena could be highly visible with the right set-up.

## Exodus-style strange darkness

God is angry at some atrocity that mankind has perpetuated for too long. He sends warning and then alters physics overnight so that fusion produces 50% less light. There's probably chaos throughout the rest of the universe but most effects except the light level itself more or less cancel out in Sol-like systems. He sent a detailed enough warning that it's basically understood what has happened, at least once reality is acknowledged. People respond by attacking the messenger, probably.

Edit: Just saw the 'science based' tag. Substitute in some undiscovered physics integer constant halving if you really dislike a theological solution. Less satisfying, IMO, but does the job. Astronomers can infer what has happened over a few decades as the night sky changes.

• Nice, but too bad my world doesn't have any confirmed God! x) I think I prefer cause-and-effect solutions since having one all-powerful being doing whatever they want is, contrary to your opinion, less satisfying in my eyes. That's part of the reason there's no theology-related things in my world. Being able to explain how a fantasy world works using real-life principles, blows my mind (and is the main reason I love worldbuilding for the sake of it!). Still, nice answer ;) May 26 at 12:37