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A century ago, after a catastrophic event on our planet, many of us were were forced to find a new home. There was one world that caught our attention: a tidally-locked blue planet orbiting a red dwarf. Since the planet was 70% ocean, had an ideal atmosphere (thanks to its strong magnetic field), and orbited a low-intensity star, our astrophysicists predicted that the sun-side would be more or less habitable.

The wind and the heat took a bit of getting used to, but it was better than the nuclear winter back home. Solar panel and wind turbine industries are booming nowadays, so at least the environmentalists are pretty happy.

There's just one teeny tiny problem we discovered recently... the dark side of the planet is populated by hideous Lovecraftian monstrosities. Just our luck, right? They avoid the light, but we've found archaeological evidence to suggest that they might have migrated through our side of the planet at multiple points in the past. We're wearing our brown pants.

What could have possibly casted darkness on the sun-side of our world long enough for the Horrors to be able to visit? We're thinking for days at a time, or even possibly weeks.

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    $\begingroup$ The other question is, what kind of monstrosity could cause serious fear to a civilization, so advanced that they can emigrate to another planet of a different star. These creatures don’t like light, we are able to generate enough energy to move an entire society to another star and we know how to make light, doesn’t sound like a big deal, does it? $\endgroup$ – Holger Mar 3 '17 at 10:57
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    $\begingroup$ Are days and weeks really enough for any sizeable migration? This would require the monsters to have some well-powered long range vehicles. Sailing ships took weeks or months to cross the atlantic. $\endgroup$ – Innovine Mar 3 '17 at 14:30
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    $\begingroup$ You've seen Pitch Black, right? $\endgroup$ – Whelkaholism Mar 3 '17 at 16:12
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    $\begingroup$ Note that any extraplanetary shading will give you bigger troubles than some monsters. Your dayside will rapidly cool, leading to devastating storms, ruined crops, etc. $\endgroup$ – ths Mar 3 '17 at 19:10
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    $\begingroup$ Given that we are talking about Lovecraftian monsters here, non-euclidean space could cause the light to bend round the planet. Also that giant thing you think is a Sun... try to not to look at it because it just might look back. If you don't want to plunge your planet into darkness, whatever you do, don't make the "Sun" blink. $\endgroup$ – gmatht Mar 5 '17 at 14:02

16 Answers 16

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Ash clouds from a supervolcano or many volcanoes

This one has all the flexibility and trappings that you need:

  • Increasing seismic activity, smoke and/or minor eruptions as ominous warning signs
  • Storms now bring darkness or blow it away at peak drama.
  • It can cover as small or large of an area as you want
  • The eruptions and ash clouds can last as long as you want.
  • It can optionally be triggered by humanity's hubris (giant Geothermal Powerplant supposed to power the whole planet)

Lovecraftian Periodic Cicadas

If volcanoes are just too mundane...

For a completely different approach, you can have the sky fill with clouds of Periodic Cicadas. The Earth versions live underground for 13-17 years and then the entire species/brood emerges at once and takes to the skies, to meet, mate, lay eggs and die.

Your planet's bugs are more... Lovecraft. They have been sleeping underground for hundreds or even thousands of years, waiting for a sign of some kind. It might be a big solar flare, passing meteor or mortals disturbing the ground with their construction activities. Now, they rise from the ground, take to the skies and head for the light side, blocking out the sun with their swarms.

Oh btw, they are also the size of cars, not coins.

A comet

If you want the darkness to be external to the planet, here is a somewhat weaker scenario.

The solar system has a big outer planet that keeps slinging balls of dust and ice at the sun. Big ones. None of these was or is on a collision course with the planet, but they have the unfortunate tendency to break apart into clouds of highly reflective dust somewhat inside the planet's orbit, leaving the planet in the shadow until they disperse.

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    $\begingroup$ +1 for the Lovecraftian Periodic Cicadas alone... but I like the rest of it as well. $\endgroup$ – Ghotir Mar 3 '17 at 15:07
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    $\begingroup$ I don't fancy the comet idea (comets come from the outer solat system, so surely sooner or later one should hit the planet), but I love the cicada idea... $\endgroup$ – John Dvorak Mar 3 '17 at 17:06
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    $\begingroup$ The Lovecraftian Periodic Cicadas are a satisfyingly terrifying answer. If they come out with volcanic eruptions, that'd be even more dark. Thank you. $\endgroup$ – kathyra Mar 3 '17 at 23:01
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    $\begingroup$ Fun fact: Cicadas like prime numbers. So maybe it's 101 years or 1009. $\endgroup$ – Pharap Mar 4 '17 at 0:47
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    $\begingroup$ @VilleNiemi: Did I mention flying or breathing? They don't really do either. Their vibrating tentacles may sound like buzzing wings, but they move through the skies by not being entirely in this world. They inhale reality and exhale darkness. Having them in all sizes is awesome though... Just thinking about it makes me want to claw my eyes out. Good thing I have a therapist... so I could claw his eyes out instead. $\endgroup$ – Cyrus Mar 7 '17 at 5:53
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Rings

First, observe this picture of Saturn.

Saturn, with its rings casting a shadow on the cloud tops

This is a picture of Saturn's rings casting a shadow upon the cloud tops. If your planet:

  1. Has rings

  2. The rings are tilted slightly relative to the planet's orbital plane

The rings will cast a shadow taking the form of a long arc stretching from sunrise to sunset, while the rest of that side of the planet is lit. This will take the form of a very narrow line twice per year when the rings line up with the star, and will then transition into an arc, growing thicker and more arced while migrating northward, then it will retreat slowly back towards the equator and continue the same thing in reverse over the southern hemisphere, and back. This would cause the dark regions near the equator to be very short lived and narrow, while further from the equator the shadow will be very wide and last much longer.

The exact arc depends on the size of the planet, size of the rings, how wide the rings are, and how tilted they are with respect to the planet's rotation axis and orbital plane, how long the planet's year is, etc. But this would be a cool effect if you need your monsters to have made this trek across the light side often, and in specific paths. The particular scenario I've described would favor paths away from the equator, further north or south.

Edit: @David Dubois seems to have found a cool simulation of this, if you are willing to download and run Wolfram Alpha's software. See his comment below. If you don't want to or don't have time, I found a picture that might be helpful. It shows how Saturn's rings are oriented as seen coming from the direction of the sun as it goes about its year. You can imagine that the shadows are directly behind the rings in each image. Original source here.

A picture of Saturn's rings, as viewed from the Sun, during different parts of Saturn's year.

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    $\begingroup$ I've been trying to find a video simulating how the shadow would change over the course of a year, but so far have been unable to find one. If anyone knows of one, please edit it into this answer. $\endgroup$ – Cody Mar 3 '17 at 21:02
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    $\begingroup$ Also, as batches of ring particles de-orbit they are like hundreds of comets hitting the atmosphere. This could cause thick cloud cover. $\endgroup$ – Jamie Cox Mar 4 '17 at 13:18
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    $\begingroup$ Rings, like moons, works be extremely unstable around tidally locked planet $\endgroup$ – Swier Mar 5 '17 at 10:32
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    $\begingroup$ I've found an interactive visual demonstration of how ring shadows move throughout the year from Wolfram Alpha. You'll need to download their player to see it in active. demonstrations.wolfram.com/SaturnsSeasonalSundial $\endgroup$ – David Dubois Mar 5 '17 at 13:27
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    $\begingroup$ @Swier this is true, but the existence of a belt region created by the rings probably precludes the need for tidal locking. $\endgroup$ – the dark wanderer Mar 5 '17 at 20:07
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You've already written the answer

If you're in orbit around a "low-intensity star" then that, to me, sounds like a Red Dwarf. Red Dwarf sunspots can dim the effective brightness by 40% for anything from days to months. Make your planet the right distance away or make the creatures tolerate low light levels and you're sorted.

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What you need is an eclipse. However being a tidally locked planet you're not going to have a moon, at least your people would have been idiots for settling on a tidally locked planet with a moon as it would be unstable as discussed in this question: (https://physics.stackexchange.com/questions/25577/stability-of-moons-around-tidally-locked-exoplanets)

Edit: As mentioned in the comments; early in the planet's life it would be stable enough. But at the point that you have day to week long eclipses you can be fairly sure that the planet's biosphere is dying. Depending on the size and distances between the planet, moon, and sun; the planet and moon might even hit the Roche limit and be destroyed before this ever happens.

The next possibility for an eclipse is a large planet inside the orbit of your planet. But if this planet was close enough to cause an eclipse then the two planets' gravity would likely be ripping each other apart every time they passed. Again if you settled on a planet like this you'd be an idiot.

A third and probably most plausible possibility would involve a space mega-structure near to the sun. Something like a Dyson swarm might do it if the swarm members were large enough but you'd need to do some size and distance calculations that I can't do at the moment to confirm. Personally if someone had built a mega-structure in a star system I would stay well away until I had done some investigation, however, if you were desperate enough you might settle on this planet.

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    $\begingroup$ If, as per the question, people were forced to flee "after a catastrophic event on our planet", then picking a tidally locked planet with a moon that is perfect in every other sense is not very stupid. It's not ideal in the long term, but it allows temporary shelter for reasonable investments. $\endgroup$ – Falc Mar 3 '17 at 8:51
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    $\begingroup$ The movie "Pitch Black" could be a fun/good example $\endgroup$ – Jeff.Clark Mar 4 '17 at 20:59
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    $\begingroup$ It's vaguely possible that two large planets in orbit around each other and inside the orbit of the planet in question could move in such a way that one was in retrograde for a significant length of time and hence could cause a prolonged eclipse, even though their orbit is significantly smaller than your planet's. $\endgroup$ – Hot Licks Mar 5 '17 at 3:38
  • $\begingroup$ The "instability" being discussed in regards to moons around tidally locked planets is on the order of millions to billions of years. If the moon seems to be stable right now, the civilization will very likely be either eaten by grues, or moved on to another planet, long before any issues arise. $\endgroup$ – MichaelS Mar 6 '17 at 7:02
  • $\begingroup$ The op asked for an eclipse that lasted days, long enough ago that you could find archaeological evidence of it. By the time the moon gets that close I'm figuring that the planet or at least it's biosphere is in it's death throes - I'll edit to reflect that. $\endgroup$ – Static Mar 6 '17 at 21:11
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You have wind and heat, therefore sand and dust are widely available. Planet wide sandstorms, the same kind happening on Mars, can cast darkness over long time, independently from astronomical situation.

“Every year there are some moderately big dust storms that pop up on Mars and they cover continent-sized areas and last for weeks at a time,” said Michael Smith, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

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    $\begingroup$ Martian storms, though, hardly obscure the sunlight. Such thin atmosphere can't hold dust in high densities; sure some sunlight reduction occurs, but it's far from "darkness". $\endgroup$ – SF. Mar 3 '17 at 14:03
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    $\begingroup$ But sand storm in the dust bowl, back in the '30es, did...en.wikipedia.org/wiki/Dust_Bowl $\endgroup$ – L.Dutch Mar 3 '17 at 14:09
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    $\begingroup$ Yes, these lasted a couple days at most though. The planet would need a very hostile weather for dust storms both this intense and this long - certainly milder events would happen quite frequently. $\endgroup$ – SF. Mar 3 '17 at 14:15
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    $\begingroup$ @sf are we talking the real-world Mars, or the one Matt Damon got stuck on, with a much denser atmosphere and Earth-like gravity? $\endgroup$ – John Dvorak Mar 3 '17 at 17:10
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    $\begingroup$ @JanDvorak: a) real Mars, b) "a tidally-locked blue planet orbiting a red dwarf [that] was 70% ocean, had an ideal atmosphere (thanks to its strong magnetic field)". A tidally locked planet will have pretty persistent winds along the terminator line, and nearly no winds near the centers of sunlit and dark side, as there's neither surface dragging the air along, daily temperature change, nor variable tidal forces circling it once per revolution. $\endgroup$ – SF. Mar 3 '17 at 17:16
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The planet may only recently have become tidally locked.

Tidally locked planets don't start out that way. All planets lose their rotational momentum over time as they orbit their star. Planets that are smaller and closer to their host lose it more quickly, so it not inconceivable that while your planet may be tidally locked now, it may not have been so in its recent past.

Even the Earth's current 24 hour period is decreasing, resulting in the length of a day getting longer by 1.7 milliseconds per century. Nothing to worry about there, then. But for a smaller planet close-in around a red dwarf star, the decrease in spin would be many times quicker than that. Having a moon (or moons) will also affect it.

Your planet is tidally locked, but has only become so very recently. In fact, it isn't really tidally locked yet; it still had some residual spin, resulting in a "day" that lasted maybe several thousands of years. It would be moving almost imperceptibly; to the point that the settlers may not even realise at first, but it might affect their long-term planning -- ie that city you build now will start moving into the night zone after a couple of hundred years time.

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  • $\begingroup$ +1 This is an interesting concept. But I actually like your deleted large moon/eccentric orbit idea even better. $\endgroup$ – kingledion Mar 3 '17 at 21:01
  • $\begingroup$ @kingledion - I deleted that post because I realised other people had already mentioned eclipses. But as you've referenced it, I have now undeleted it :) $\endgroup$ – Simba Mar 6 '17 at 9:07
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A space sunshade in orbit around the red dwarf that periodically comes between the planet and its primary star.

A space sunshade or sunshield is a parasol that diverts or otherwise reduces some of a star's radiation, preventing them from hitting a spacecraft or planet and thereby reducing its insolation, which results in reduced heating. Light can be diverted by different methods. First proposed in 1989, the original space sunshade concept involves putting a large occulting disc, or technology of equivalent purpose at the L1 gravitation point between the Earth and Sun.

A sunshade is of particular interest as a climate engineering method for mitigating global warming through solar radiation management. Such shades could also be used to produce space solar power, acting as solar power satellites. Proposed shade designs include a single-piece shade and a shade made by a great number of small objects.

This should be capable of bringing darkness to the planet's dayside and allow the Lovecraftian monstrosities ease of access. I most definitely agree with Static's comment about not settling in a planetary system where someone or something has installed or constructed a megastructure. This should be taken as a sign that effectively says "Lesser lifeforms keep out. Enter at your peril." As your settlers do seem to have out.

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Giant Lovecraftian Space Monster

You're already going with Lovecraftian Horrors as your antagonists, use one as your catalyst as well. This system is home to some massive Space Kraken that has periods of dormancy and activity. While dormant it curls in on itself and lazily orbits the star, perhaps farther out than your planet, nearly invisible against backdrop of space.

When it wakes it uses it own unknowable propulsion to migrate closer to the star in order to feed on the stellar radiation/commune with the other cosmic entity that lives within the stellar mass/etc... In order to more thoroughly feast upon the radiation of the star it unfurls it's bulk, stretching massive fleshy sails/wings/polyps and creating a massive shadow between the planet and the star. Make this happen close enough to the planet to cast an eclipse and you've got your darkness.

Plus this feeding could take as long as you wish and because it's not a regular spherical mass but rather an undulating writhing creature you can also have irregular period of light when some part of the creature moves and lets light past.

Extra points for the beasties on your planet being cast offs shed by this monster as it flies past on it's periods migration thus also allowing for an occasional re-population of monsters if your humans decide to get organized about wiping out the monsters.

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Eclipse from another planet in orbital resonance.

There's a large planet - like a gas giant - on a lower orbit. The orbit of that planet has a rather high eccentricity; in particular such, that when near aphelion it moves at a speed very similar to ours.

Its gravitational influence put our planet in a resonant orbit: every n orbits of the central planet, they make "closest approach", and that results in an eclipse that lasts several days or weeks, until the giant begins its descent towards its perihelion, and our planet (in a circular orbit) continues on its merry way.

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    $\begingroup$ Hm, around a red dwarf, that seems problematic. The orbits are just too small. All of the TRAPPIST-1 planets have orbits of days. $\endgroup$ – ths Mar 3 '17 at 14:51
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    $\begingroup$ Trappist-1 is exceptionally small even for a red dwarf though. And orbital resonance may make the conjunction quite infrequent. $\endgroup$ – SF. Mar 3 '17 at 15:08
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    $\begingroup$ Keep in mind that the planets are not exactly in the same orbital plane. From Earth, we only see transits of Venus crossing the sun every 243 years, most of the time it goes above or below the sun from our perspective. Depending on the length of years and orbital plane inclination, this can be regular but infrequent. $\endgroup$ – Cody Mar 3 '17 at 18:05
  • $\begingroup$ @Cody: Planets may be in the same orbital plane. Nothing forbids that. The individual deviations from the plane of ecliptic are completely random, a deviation of 0 is definitely within the realm of possibility. $\endgroup$ – SF. Aug 23 '17 at 13:21
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Every few thousand years, one of the Horrors sheds its skin. It casts it into space, and it drifts in orbit around the planet until it de-orbits. This creates repetitive periods of darkness, not a continuous dark period.

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Very occasionally, the creatures go to war on each other along factions, at large scale.

One of their wartime activities bring up gargantuan amounts of dust. Nuclear explosions come to mind, maybe a giant creature that eats and purifies uranium until it reaches supercriticality. But could be something else.

The dust is spread by the wind, and blocks the sun.

The war then spills to the entire planet.

The war ends, and the dust gradually settles.

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How about a swarm of asteroids/comets on an elliptical orbit that crosses between the star and planet every "so often". The swarm wouldn't need to be dense - it could be quite sparse - it would just need to create opacity between star and planet. If the trajectory passed the planet at "steep" angle (nearly straight into the star) it could stay between star and planet for a while.

I realize the swarm would need to be "quite big" to do this, considering that the further it got from the planet, the larger it would need to be to shade the planet, and the velocities it would be traveling would work against a long duration of shade.

Also, you would expect a lot of surface impacts with that much crap in overlapping orbits - maybe the locals can deflect those destined to collide with the planet.

It's a little thin, but maybe someone can save (or condemn) the idea with hard physics.

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Non-euclidean space

The space around the lovecraftian monsters is reputedly quite non-euclidean. This could bend light away from the sun.

Giving a more detailed science based answer to questions involving lovecraftian monsters is a bit hard since scientists who study such things tend to be a little odd. Best guess is that the non-euclideaness results from a cluster of black hole like objects in a chaotic orbit. Mostly this just results in flickering, and moving the stars round, but like most chaotic systems it is hard to say how long the blackness would last, or when it would start.

When pressed as to when the great darkness might begin the scientist would just say things like "when the stars are right", "probably in the past; objectively we are almost certainly dead, just don't look at the sun, lest it looks back and collapses the wave function.". Eventually they just insist that they are a cat in a box, and laugh maniacally until the people in white coats come to take them away.

On the other hand, Lovecraftian monstrosities can be quite crafty. Why would they need the whole planet to be dark, when it is quite dark inside human skin? Think carefully, do you remember ever seeing your neighbours blink?

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As others have mentioned, an inner planet causing an eclipse would work. The TRAPPIST-1 system has seven tidally locked planets. See here for general information on the system.

If one of the inner planets is sufficiently large enough, it could cause the eclipse. The TRAPPIST-1 planets likely encounter frequent eclipses and transits. The question is, how far out is the resident planet (how long does it take to make one complete revolution around its host star)? If it takes a year or two, then the inner planet would be able to block the star light long enough to allow Lovecraft's hordes plenty of time for a little light-side exploration/destruction.

Mercury is tidally locked, but it is small and close in. All of the TRAPPIST-1 planets are closer than that to their host star. The longest TRAPPIST-1 "year" is only 20 days, meaning that any eclipses would last only a few minutes.

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A large moon with an eccentric orbit

The planet has a moon that is large enough to cause a total eclipse. The orbit of this moon is eccentric, meaning that actual eclipses are rare events, but when they do happen, the monsters can break out of the dark side by following the path of the eclipse and wreaking havoc along the line of totality.

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  • $\begingroup$ Note: I previously deleted this post because others had already mentioned eclipses, but it was referenced in a comment elsewhere, so I have undeleted it to avoid confusion. $\endgroup$ – Simba Mar 6 '17 at 12:25
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Some kind of light on the dark side

So you say the monsters don't like the light and stick to the dark side. Fair enough, but what happens if the dark side were to become illuminated?

If that happened, I can imagine the monsters fleeing across the whole planet, even the daylight side, desperately trying to escape the light and causing chaos as they go.

So what could happen to light up the dark side of the planet? Here are some ideas:

  • Fire: every few hundred years, a massive fire sweeps across the dark side of the planet. Lighting it up and driving the monsters across to the opposite side of the planet for a week or so until it subsides. The fire could also generate a global smog cloud that would darken the whole planet and make the light side more tolerable for the monsters.

  • Asteroid impact: Similar to the fire above; major planetary disturbance with ash blocking out the sun for weeks or months. The whole planet becomes dark so the monsters get free reign

  • Solar flare: a major solar flare could cause bright aurora even quite a long way round the back of the planet. They may not be as bright as daylight, but in a world of perpetual darkness, and where they don't happen often they could easily be the trigger to unleash the fury of the monsters.

  • Binary star. Maybe the red dwarf is just one star of a binary system? The layout of the system could mean that the 'dark side' of the planet alternates between being in complete darkness and being illuminated by the second star. Each could be last for a period of several thousands or even tens of thousands of years at a time, but the switch-over between the two states could be fairly quick (a sunrise of the second star that lasts maybe a few days or weeks? The second star would give much less light than the main star even if it was a brighter star, simply due to the distance, but it would be enough to create a gloomy daylight on the dark side, but that should be enough to stir the monsters into action.

  • Other significant light source elsewhere in the system: You could also achieve a similar effect with one of the outer gas giants in the system. You could have a planet that normally just looks like Jupiter, but every now and then can be triggered to produce its own light for a time by some external event; maybe a solar flare or comet impact. So normally it just looks like another point of light in the starfield, but randomly every fifty or five-hundred years, it bursts into light and illuminates the dark side of the planet for a few weeks.

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protected by Community Mar 5 '17 at 17:57

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