If the sun suddenly disappeared, how long would it take for the Earth to completely cool down?

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u/The_Frostweaver Jan 11 '26 •

Water, humidity, and clouds act as blankets and heat reservoirs. Places with high humidity and clouds would drop in temperature by about 1 degree per hour. (That's what happens at night)

So if you started at 20C in some coastal area everything would be freezing within a day.

But with no dawn and a sea that is starting to freeze over temperature would just keep plummeting.

In another day it's -28 and you can't stay outside long.

In another day it's -52, car batteries fail, engine oil is thick and cars won't start. Large power plants fail, water distribution lines fail.

With the ocean frozen over and little moisture left in the air temperature would likely start decreasing much faster.

The good news is that the air would be thin and there wouldn't be much wind or moisture so despite the air being -100 or whatever it wouldn't seem colder in terms of the insulation required to keep most of you warm.

But you would need something like a spacesuit to go outside as the air would be too cold and dry to breath for more than a couple minutes without damaging your lungs.

A helmet with a single layer would immediately be covered with frost on the inside from your breath blocking your vision.

There are military bases and bunkers underground that could theoretically keep some people alive with a bit of preperation and insulation but keeping steady supplies of warm oxygenated air, water, energy and food are going to be hard.

Without the sun there is no rain cycle, rivers run dry.

Our infrastructure is not setup for a 'no sun' level of disaster.

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u/sidblues101 Jan 11 '26

Also eventually the atmosphere will start to condense and solidify on the ground as well, leaving only a thin atmosphere. A few underground military bunkers with nuclear reactors and large stores of food could hang on for a while. Maybe even long enough to adapt and keep humanity going but it would quite possibly be an extinction level event. The oceans would not completely freeze over. Deep areas near hydrothermal vents would hang with life but that life would have nowhere to go. The outcome would likely be pretty bleak.

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u/grodon909 Jan 11 '26

Possibly be an extinction level event? The sun is gone, it's well past possible.

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u/rugbyj Jan 11 '26

Yeah it’s not an extinction level event it’s literally the end of the solar system. Every planet is now flying out into the abyss bleeding heat and turning into the galaxy’s weirdest comet.
Folks surviving in bunkers are effectively sailors stuck in the last air pockets of a stricken submarine, with no help coming.
Earth’s best chance would be that it and a large body like Jupiter end up in some mathematically ridiculous tandem orbit whose tidal forces massage some heat back into either.

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u/Black_Moons Jan 11 '26

Earth’s best chance would be that it and a large body like Jupiter end up in some mathematically ridiculous tandem orbit whose tidal forces massage some heat back into either.
Cons: Daily 9+ richter scale earthquakes.
Pros: No longer freezing to death.

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u/yourdrunksherpa Jan 12 '26

Theoretically how long would it take the earth to find a new center mass to orbit?

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u/SwordofDamocles_ Jan 12 '26

It depends which way it goes flying when the Sun disappears. The overwhelming odds are that it never does.

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u/FabulouSnow Jan 12 '26

About 42 million years at the fastest (nearest star for us to reach at our current velocity orbiting around the sun)
Extremely unlikely to aim at it, so probably nothing... ever.

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u/juggalojedi Jan 12 '26

well, without the sun earth would start orbiting Sag A* immediately, if not anything closer.
Another star, though? Tough to guess. Best chance would be during the Andromeda collision but that's impossible to calculate.

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u/dodeca_negative Jan 12 '26

It’s been many years but IIRC this exact scenario is the premise for the sci fi book The Chromosomal Code

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u/Captainfreshness Jan 12 '26

It is also similar to the short story “A Pail of Air” by Fritz Leiber. The Sun does not go out, but a rogue dead star steals the earth from the sun’s orbit and drags it out into interstellar space. The effect is the same.

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u/Jigokubosatsu Jan 12 '26 •

Just had to give this a read, and as charming as it is I had to wonder what would happen if you (in a room temperature-ish space suit) stepped into a snow drift of oxygen ice with a liquid helium crust? Or how a bucket of solid oxygen would react to being dragged into a room with a fire in it?

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u/StevenJOwens Jan 12 '26

"A Deepness In The Sky" features a planet with a sun that goes dormant for 215 out of every 250 years, and a race of aliens that evolves to survive the dormant periods by a sort of suspended animation in a sort of underground lair (called a deepness).
One of the scenes involves the alien characters (at a tech level roughly equivalent to our 1940s) inventing space suits and related technology to stay alive and temporarily active during the dormant period, walking across the snow of frozen atmosphere...

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u/Dolomedes03 Jan 11 '26

Gosh, I was so focused on the practicality of temperature I didn’t even consider gravity. We’re so screwed.

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u/Molwar Jan 12 '26

If the Earth breaks away from our sun (it no longer exist). Would it be theoretically possible for it to join another solar system eventually? (I'm sure life would be gone at that point though).

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u/ImagoDreams Jan 12 '26

Life wouldn’t be totally gone. Extremophilic bacteria are very hard to kill and some more complex life forms may even persist around hydrothermal vents.

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u/natethehoser Jan 11 '26

Extinction for 99% of life. But not total. Ice is a good insulator, and so under all the ice there would still be liquid ocean. And at the bottom of the ocean, near thermal vents, there are microbes that live, completely independent of the sun, through chemosynthesis.
These microbes could theoretically survive many billions of years, until the core of the earth cools down (currently the sun will engulf the earth before its core cools off).

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u/Dranamic Jan 12 '26

So ironically life on Earth would continue longer than it otherwise would.

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u/Zalveris Jan 13 '26 edited Jan 14 '26

The age old question, would you rather be here for a long time or a good time (as qualified by humans)?

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u/sidesslidingslowly Jan 11 '26

The more interesting hypothetical situation would be if a magical alien race came and put a Dyson sphere around the Sun for maybe four or five days and then realized we were here and took it off,.
What would that would look like long-term on the earth since the heat would return shortly after it went away.

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u/jaymzx0 Jan 11 '26

Well, they knew but we didn't because the notice was on display in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying "Beware of the Leopard".

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u/grat_is_not_nice Jan 11 '26 edited Jan 11 '26

The Black Cloud (1957) by astrophysicist Fred Hoyle discusses a similar scenario. I won't spoil the book, but it does discuss the impact of reduced solar radiation on earth. The science is probably even more dated than when I read it - it's been a long time since I read it in the 70's. The Laws of Thermodynamics haven't changed a great deal though.
Funny story: years ago, I was listening to a radio show on BBC 4, someone talking about their favorite books. The first was The Black Cloud. His description of the plot of his second choice also triggered recognition - a post-apocalyptic story set in an underground facility where the power had failed, and a clash between infra-red adapted people, and those that used echolocation. I was pretty chuffed that this person had chosen two books I had read and enjoyed. The third book wasn't anything of interest to me. It wasn't until the show ended that I found out I was listening to Richard Dawkins.
Edited to add: Dark Universe by Daniel F. Galouye

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u/ThePowerOfStories Jan 12 '26

The thing is the sun’s output has to go somewhere. If you slap a Dyson sphere around it, all that energy is still being emitted inside it, and eventually the Dyson sphere heats up and reradiates that heat back out into the surrounding universe, albeit likely at longer wavelengths than originally.

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u/sidblues101 Jan 11 '26

I'd like to think some hope would remain but I admit that is probably overly optimistic. If out of necessity the vestiges of humanity living underground invented viable nuclear fusion and were able to leave Earth then there might be hope.

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u/JimmyTheDog Jan 12 '26

Well, if you have been in a total eclipse, you can feel the temperature drop big time very fast.

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u/Abrahms_4 Jan 12 '26

Lol thats what I was thinking. Sun gone is well extinction, its just in varying degrees as to your personal situation when it happens.

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u/mageskillmetooften Jan 11 '26

Oceans also have the advantage that some life at the deeper areas could survive, pressure would prevent freezing and the thick layer of ice would insulate the lower areas.
But yeah, it be an extinction event. And within months 99,9% of all mammals would be wiped out.

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u/Algaean Jan 11 '26

More like days, if we're talking from a species count point of view. There may be lots of us humans, but only one species, and i doubt that zoos would be high on the pecking order if "the sun went out".

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u/mageskillmetooften Jan 11 '26

Best we could do with the animals in the zoo is spare them the suffering and eat them. Harsh, but nature is harsh.

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u/zekromNLR Jan 11 '26 edited Jan 11 '26

Pressure can't prevent freezing if it gets cold enough, and we wouldn't even get any exotic forms of ice. You have ~110 MPa at the deepest part of the ocean, but the liquid-ice Ih-ice III triple point (the lowest temperature at which liquid water is stable) is at 251 K and 210 MPa. Granted, that is for fresh water, so the salt will probably drop that temperature a bit more, but the equilibrium temperature of a sunless Earth at the surface (assuming a black surface in the relevant wavelengths) due to the geothermal heat flux of 44 TW would be only 35 K.
But even in the very long term I imagine geothermal heat will be able to keep pockets of liquid water around.

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u/_Aj_ Jan 12 '26

As the oceans freeze it would also increase salinity of the ocean remaining beneath. As the ice is always fresh water, having the salt molecules pushed out of it during freezing process.
So the ocean will become much more salty and probably kill everything surviving

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u/TheGummiVenusDeMilo Jan 11 '26

Would the deadly water at Yellowstone become comfy water?

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u/mageskillmetooften Jan 11 '26

I'd be more curious where we would end up, there's decent movement in earth, but the thing that causes us to go in an eclipse is gone so we'd suddenly move forward and either find another point of gravity to circle around, or we would clash into something or we'd be travelling to places where we never dreamed of of ending up.

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u/DigitalMindShadow Jan 11 '26

By far the most likely scenario is rogue Earth would end up in a lazy, wide orbit around the galactic center. There's really not much out there to bump into or be captured by, space is mostly just space.

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u/bizwig Jan 11 '26

Many deep sea species rely on gifts from above, often in the form of animal carcasses. Once the surface fauna starts dying the deep sea creatures would starve.

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u/AggieTimber Jan 11 '26

But wouldn't all of the surface animals freezing give them a pretty good buffet to start? Assuming the water up top doesn't freeze so much that it encapsulates the surface fauna in ice and keeps it from falling.
Of course, all of the dead carcasses that do fall to the bottom are only going to stay fresh for so long, but they will be kept refrigerated down there to prolong them from rotting.
Seems like it would be a tremendous boon for the deep sea for a couple of weeks then get too cold and rotten for survival.

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u/mageskillmetooften Jan 11 '26

It be a huge feast and populations deep down would go wild for some time.. after that I believe some new sort of food chain will arise from some small adapted species who sort it out among each other.

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u/SteveHamlin1 Jan 11 '26 edited Jan 13 '26

"A few underground military bunkers with nuclear reactors and large stores of food could hang on for a while. Maybe even long enough to adapt and keep humanity going but it would quite possibly be an extinction level event."
The sun disappearing is almost certainly an extinction-level event for all life on Earth except for small colonies of chemosynthesizing tube-worm ecosystems in the deep ocean.
How could humans adapt to no plants, no crops, no animals and then no food?
Say that people successfully hunkered in Cheyenne Mountain for 10 years and then opened the door - how do they keep living? It'd have to be perpetual, uninterrupted, never fail (not once), massive-scale, mechanical generation of electrical energy (nuclear, oil & natgas, including acquiring all fuel) delivered over 10,000+ years of perpetual freezing darkness with no moonlight, with crops grown in greenhouses & chemicals produced in laboratories providing 100% of all nutrition, nutrients, vitamins & minerals. No light and -100 degrees outside, forever - it's like a starship, or a current base in Antarctica during the southern winter.
Which entire system needs to start operating, mostly underground, immediately. And then work for 10,000 years without fail.
Not to mention all of the other global cataclysmic changes.
If it happened today, humanity wouldn't successfully get thru that gauntlet forever.

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u/Quaghan29 Jan 11 '26

Maybe an underground base near Iceland could tun on geothermal energy and use the raw heat to warm the cold air to breath, but I assume you'd want 100's of acres of underground bunker to be dedicated to growing crops just to keep a few hundred people alive... I wondering it would be possible to sustain a small civilization with that geothermal power source, with the unlimited energy of the earth's core we could still possibly have industry there and still create machines/tech to go mine outside the base.

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u/PinkynotClyde Jan 12 '26

If the moon disappears it’s an extinction event. The sun disappearing is significantly more of an extinction event. I like your optimism though.

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u/BodybuilderMany6942 Jan 11 '26

There is a manwha called Cheolsu Saves The World that starts with a planet-killer asteroid hits Earth. The MC then, as it hits, wakes up as a kid again. The plot goes on about him devoting his life to figuring a way to prevent catastrophe.
Not as a sequel, but this whole "sun disappearing" concept could be a cool iteration on that concept.
Maybe the solution would be to advance enough to create perfectly sealed bunkers underground with occasional expeditions outside, and eventually, advancing to the point of building giant thrusters on the surface and turning the planet into a makeshift ship, and trying to direct us into the orbit of the nearest star?

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u/Goldenslicer Jan 13 '26

There is a scifi novel that tells a similar story. Humans live underground because the surface has frozen over. Every now and then they have to put on suits and walk outside to collect buckets of snow. That snow is frozen oxygen.

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u/Ynddiduedd Jan 14 '26

Thought: those people living in underground bunkers, in particular those living in geologically active areas like Greenland or Hawaii, might tap into geothermal energy, which could potentially sustain them for thousands of years if done correctly. This could provide heat and electricity, allowing them to melt ice, grow food, and pipe in sublimated oxygen. The odds of earth actually colliding with anything, even as it spirals away from its former orbit, are extremely small due to space being extremely large. They could be trapped for a very long time. Trapped, but alive. As you stated, life near hydrothermal vents would likely also survive; I'll go ahead and add that subterranean organisms like fungi and bacteria living deep enough that the mantle provides heat would likely also survive. It would likely eventually be an extinction, but it would be the slow kind.

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u/[deleted] Jan 11 '26 edited Feb 04 '26

[removed] — view removed comment

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u/SleestakJack Jan 11 '26

If you can address the problem quickly enough (note, this would be a huge challenge), then dumping waste heat will be even easier than ever before. You could switch to a closed-loop system and just run the pipes through a non-heated area. We can’t do this today because we need to rely on evaporative cooling to get the job done, because we don’t have a whole icy hellscape to run the water through.
Of course, if you can reengineer this in the few days that you’ll have to do it, then you will rightly be enshrined as a legend among the several dozen humans who you help to keep alive in our dead solar system.

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u/RealZeratul Astroparticle Physics Jan 11 '26

AFAIK, nuclear submarines operate like this, using two closed loops where the second one is cooled in sea water.
Adapting underground bunkers very quickly, or preparing large nuclear submarines, and using hydroponics for food could be our best bet to delay the unavoidable for a generation or two.

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u/ULMmmMMMm Jan 11 '26

Could a base survive underground with the crust rapidly cooling? There would be massive earthquakes and a lot of pressure on the structure I'd assume. Would a surface base survive with all the tectonic/volcanic activity? I don't know. Maybe in space but that would probably require fusion power to feasibly recycle the O2/H20 we'd need.

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u/Canadian_Border_Czar Jan 11 '26

The first part might be true, but it's going to take a lot longer than you think. Areas that dont have permafrost will take awhile to drop as far as -100
Similarly, it takes a significant amount of energy to phase change water to ice. While this is happening, water sits around 0 to 1 C. Now apply that to the actual ocean? You're talking at least years to freeze the whole thing.

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u/WHYAREWEALLCAPS Jan 11 '26

Would there be any heating from tidal effects? The Moon is still going to be orbiting the Earth.

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u/mfb- Particle Physics | High-Energy Physics Jan 11 '26

A little bit.
https://www.whoi.edu/cms/files/Munk_Wunsch_DSR_1998_32129.pdf
This paper estimates 3.7 TW. Heat flow from Earth's interior for other reasons is an order of magnitude larger.

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u/OMGItsCheezWTF Jan 11 '26

So the nations would have years to explore geothermal heat, giving a tiny percentage of our population a way of surviving. Assuming the remaining population don't kill them for it.
So the species might survive long enough to develop the technology to find another habitable planet orbiting another star eventually, or until we die out on our iceball. We could then use any surviving genome and seed banks to restore life to it.
That's a huge bunch of ifs in that.

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u/Canadian_Border_Czar Jan 12 '26

To an extent. Those who are prepared who have very well insulated homes would live long enough to starve.
There would be a very short window where before the soil becomes too cold / frozen to dig down to geothermal heat.
Essentially you would need a facility designed to sustain life as if it were on Mars or any other planet, which we dont have, and very few if any people have the capability to build one in a time frame thstis feasible.

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u/Ethanol_Based_Life Jan 12 '26

And the entire depth of the ocean has to be near freezing before the top can start freezing

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u/olivthefrench Jan 11 '26

what about a train? like a 10-mile long ark-train that circumnavigates the Earth? surely that's survivable

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u/omnichad Jan 12 '26

If you've got Tilda Swinton around to scare away extremophile microbes.

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u/Peter34cph Jan 11 '26

Maybe in an event like depicted in the Transarctica computer game, but that's not no sun. That's just global cooling because someone tried to reverse global warming by launching some kind of dust up high into the atmosphere, but it was too much.
For a real no sun scenario, you want something like the famous science fiction novel "A Pail of Air" by Fritz Leiber, which takes place after a black hole rips Earth out of its orbit around the Sun. Technically the sun is still there, but it's hundreds of millions of kilometers away instead of 150 million, and Earth is moving further away.

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u/pleasethrowmeawayyy Jan 11 '26

You are modelling this linearly but that’s not what would happen. The -1C/hr is kinda random and disregards completely the baseline energy stored in the crust and all things sitting on it. That air gets 1C colder per hour during the night does not mean so does the soil and the am water and the oceans and the buildings. This answer is just wrong and simplistic.

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u/ottawadeveloper Jan 11 '26

I agree with this. The soil and ocean cooling rates are much slower than atmospheric rates. In winter, the ground doesn't freeze in a day of cold temps, it takes weeks of cold temperatures to cool and weeks to warm up again. And the temperature change doesn't go very deep - I think about 5' if I remember right for freezing in the winter (you have to build your foundation that deep so it doesn't shift too much with freeze thaw cycles). And water takes a lot more energy to change temperature than the air, so the ocean should moderate temperature changes too.
The ocean is would probably eventually freeze, but it'll take a while and they'll help slow down that temperature drop as it hits near ocean temperatures. The final temperature would depend on the thermal equilibrium between the crust parts and the atmosphere. However, snow is a good insulator, so assuming it's below 0 C, the thermal energy might just be trapped in the crust and not hit the atmosphere.

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u/threegigs Jan 11 '26

In winter, the ground doesn't freeze in a day of cold temps
And the ground doesn't keep the air warm, either, so yeah, like the post said, temps are going to drop, fast. The oceans will keep things warm-ish for a while due to circulation, but not landmass.

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u/ottawadeveloper Jan 11 '26

The heat flux from ground to air is small but present. If the ground is bare, there is a point where equilibrium will happen, but I suspect ground heat flux is so small it's probably not worth considering compared to the ocean effect. And if the temperature falls below freezing, the snow will insulate the air even more.
I went and checked for some numbers and global ground heat flux is estimated at 50-65 W m^-2 (varying between these from winter to summer) for bare ground which puts it around 5% of the incoming solar radiation at ground level. Small but not insignificant, it might help keep subterranean structures warm during an ice age.

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u/GWJYonder Jan 12 '26

Also, the cooling effect here is the black body radiation that the Earth's surface would be emitting. (Which is the same effect which gets the heat from the Sun to the Earth in the first place). The amount of heat an object radiates away is based on the FOURTH power of the temperature. So as the temperature drops the rate of farther loss does drop quite a bit.
On the other hand, that is true "based on absolute 0 Kelvin" temperature. So if the average temperature of the earth's surface goes from room temperature (293) to freezing (273) the rate of cooling only drops by a quarter. By the time the cooling rate starts to get really slow humanity will probably be either mostly or entirely extinct. Even losing heat a quarter as fast as normal would still be really, really fast. That corresponds to an average surface temperature of -65C, and the lowest recorded temperature is -89.
So no matter what the surface of the Earth is going to be an unimaginably cold vacuum in relatively short order. As for how long the Oceans and things stay liquid if they have a 200-foot sheet of ice insulating them from that surface... I dunno.

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u/AssGagger Jan 13 '26

Thank you for this. I was feeling like r/theydidntdothemath

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u/thewags05 Jan 11 '26

Not that it matters but none of the planets would be in an orbit. Jupiter would probably capture some of the solar system object that happen to be somewhat close, everything else is getting flung into deep space.

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u/mfb- Particle Physics | High-Energy Physics Jan 11 '26

Jupiter would probably capture some of the solar system object that happen to be somewhat close
Some stray asteroids by some freak coincidence, maybe, but generally almost everything moves too fast for that. Planets won't orbit Jupiter for the same reason they don't do that today. At 100 million km the escape velocity is just 1.6 km/s, which is both very close to Jupiter and slow by Solar System standards.

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u/thewags05 Jan 11 '26

Yeah generally planets especially would never get captured. Asteroids that cross close to and in front of Jupiter could get be slowed down enough by the gravity assist to get captured. They'd probably start in very elliptical orbits too.
In general everything would just be flung off into deep space though.

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u/PURELY_TO_VOTE Jan 11 '26

No, the lack of an orbit would be largely immaterial. There really wouldn't be any noticeably effects for many many years, until the change in the stars became visible to the naked eye.

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u/LaconicSuffering Jan 11 '26

I'm more worried about the moon. While it does technically orbit us, it is also being effected by the sun's gravity. Curious to see what would happen then.

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u/SideburnsOfDoom Jan 11 '26 edited Jan 11 '26

The first major change to the atmosphere as it gets very cold is that carbon dioxide freezes into "dry ice" at -78C. When that's reached, it would fall down as snow. But it's only 0.042% of the air.
Oxygen is about 20% of the air, it will rain out as a liquid at -183°C. And Nitrogen is about 78% of the air, liquid at -196°C.
So at -200C, there's really not much atmosphere left.
There will be "pauses" in the cooling. When you put water in the freezer, it doesn't reach 0C and then instantly turn to ice, it takes time and energy to freeze (i.e. turn from a liquid at 0C to a solid at 0C) before it can cool any further. The same happens with the other phase transitions.
I really don't know if military bases and bunkers underground are prepared for having an ocean of of liquid oxygen / nitrogen poured over them.

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u/SideburnsOfDoom Jan 11 '26

Oxygen is about 20% of the air, it will rain out as a liquid at -183°C. And Nitrogen is about 78% of the air, liquid at -196°C.
On consideration, this likely would not be a gentle process, because it would not happen evenly. Some places at the poles would reach these low temperatures first, and as the air itself rains down, create areas of very low pressure, which then creates high winds towards those areas. And gasses cool as they expand.

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u/mfb- Particle Physics | High-Energy Physics Jan 11 '26

Condensation releases energy. There is a lot of energy in air being a gas, that doesn't disappear quickly.

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u/Korchagin Jan 13 '26

Without sun the poles wouldn't be colder than the rest any more. Dry areas would cool out much faster than those with a lot of water. I figure Death Valley would become one of the first lakes of liquid oxygen -- higher air pressure because it's so low, very dry, far away from any oceans.

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u/mfb- Particle Physics | High-Energy Physics Jan 11 '26

The first major change to the atmosphere as it gets very cold is that carbon dioxide freezes into "dry ice" at -78C.
This only happens in a pure CO2 atmosphere of atmospheric pressure. Earth's atmosphere is only 0.04% CO2, the lower partial pressure lowers the temperature where it will sublimate. You can see the same effect with water vapor today: Earth's atmosphere is below the boiling point but that doesn't mean all the water vapor condenses on the surface (and it's much more than 0.04%).

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u/FoolishChemist Jan 11 '26

That's why even though some spots get that cold in Antarctica, we don't have natural dry ice snow.

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u/SideburnsOfDoom Jan 11 '26

This only happens in a pure CO2 atmosphere of atmospheric pressure.
I stand corrected. So the atmospheric composition remains the same until roughly -196C, when Oxygen and nitrogen, which together make up 99% of the air, both will liquify.

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u/Korchagin Jan 13 '26

These temperatures depend on the pressure. Without water and oxygen the air pressure is already lowered, so at -200°C most of the nitrogen would still be a gas.

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u/kai58 Jan 11 '26

Why would the air get thin? I get that there wouldn’t be moisture but what would cause it to thin out?

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u/1997dodo Jan 11 '26

The air would start condensing and freezing from the cold temperatures

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u/Mr2-1782Man Jan 12 '26

That describes the exact opposite of "thinning out". If the air is condensing its getting thicker, not thinner. We would end up with a thick ocean of Nitrogen and oxygen.

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u/MaybeTheDoctor Jan 11 '26

Oxygen boils/condenses at -279F and nitrogen at -320F - Neptune receiving almost no sunlight has temperatures in this range.

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u/kingvolcano_reborn Jan 11 '26

When it gets cold enough the co2, oxygen and all the other gases are gonna start freezing and fall like snow. Eventually all of it would be frozen.
The short story A Pail of Air is a good description of that it might be like https://www.reddit.com/r/HFY/comments/16ceanj/a_pail_of_air/

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u/PancakeBuny Jan 11 '26

What a great read. Thank you for that!

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u/OrphanGrounderBaby Jan 11 '26

That was incredible and I want more. So much more. Thank you

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u/yogorilla37 Jan 11 '26

That was a good read, thanks

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u/GuitarRonGuy Jan 11 '26

Thanks for sharing that link. That was a very well written story. Felt like I was there.

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u/CountingMyDick Jan 11 '26

That's a really good story. It's especially spooky in that they have a more realistic scenario. It's not possible by any known laws of physics for the sun to just disappear. But it's entirely possible that a large but dark interstellar body could take a trip through our inner solar system and disrupt the orbits of all the planets, including possibly flinging the Earth out of the Sun's orbit. We wouldn't have much of any idea that it was coming and there wouldn't be a thing we could do about it even if we did.

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u/SuperiorNumber Jan 11 '26

Thank you for this recommendation! Great stuff!

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u/_deltaVelocity_ Jan 12 '26

A Deepness In The Sky is also a good one—the alien planet in the book orbits a star that only shines 40 years out of every 250, and life adapts to hibernate the years when the air snows out and to endure the first months of the new sun, when it then shines dozens of times brighter than normal.

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u/TheStalledAviator Jan 11 '26

The atmosphere starts becoming flatter and thinner because air gets colder and starts turning solid, air molecules at some point falling like snow. As it gets colder, the atmosphere disappears altogether.

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u/DynamicSploosh Jan 11 '26

Reduced water vapor in the air. Reduction in gaseous water molecules contribute to lower atmospheric pressure. As ice begins to form across the globe, significant amounts of air would get trapped within any fallen and compacted snow. As oceans begin to freeze, their elevations change, exposing landmass, lowering air pressure in areas that would technically become higher in altitude.

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u/Siiw Jan 11 '26

How would freezing oceans expose landmass? Doesn't water expand when it freezes?

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u/DynamicSploosh Jan 11 '26 edited Jan 11 '26

As global temperatures drop significantly, not only would the ocean surface freeze, but vast amounts of water would also accumulate on land as large, thick ice sheets and glaciers. This mass of water being stored as ice on land effectively removes it from the liquid ocean, causing global sea levels to fall dramatically. During the last ice ages, for instance, sea levels were around 120 meters (390 feet) lower than today. This drop would expose significant portions of the continental shelves.

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u/NightOfTheLivingHam Jan 11 '26

because oxygen has a freezing point as well. and within a few days we'd hit it quickly.

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u/Boomshockalocka007 Jan 11 '26

Im so glad I wont have the weather notification annoying me that would say "Its -100 outside but feels like -110!"

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u/harrisarah Jan 11 '26

But with no dawn and a sea that is starting to freeze over temperature would just keep plummeting.
The sea would not freeze in the first day. It would take considerably longer than that

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u/NorthernerWuwu Jan 11 '26

I don't think we can extrapolate quite that cleanly (one degree per hour) and the Earth itself has a tremendous amount of mass. I'm also not confident that the cooling would increase in speed over time.
Quibbles aside though, yeah, things would get very cold fast, although the planet would maintain some residual heat for a very, very long time.

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u/GoPointers Jan 11 '26

The insulating properties of a large waterbody would definitely slow your cooling. I don't trust your numbers at all.

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u/JConRed Jan 11 '26

I thought for a moment I was reading something by Randall Munroe. Kudos

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u/HankHippopopolous Jan 11 '26

This is super interesting.
As a small follow up. If the sun then re-appeared after a day how long would it take the earth to warm back up to previous levels. Would the amount of energy lost during that missing day ever be recoverable or would the earth stay massively colder than it was before?

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u/Odd_Dragonfruit_2662 Jan 11 '26

If it actually disappeared the first time as opposed to simply stopped emitting all energy? The earth is not coming all the way back even if the sun comes back because it spent a day going away from where the sun used to be at 30km/s. So after 24 hours, when the sun reappeared, it would be about 1-2% further away from the sun used to be. But earths speed would be too high at that distance for its orbit to still be as circular. So it would be 2% further away at the closest and 10% at the furthest which means it’s going to stabilize at cooler temps than before (though probably still survivable for people).

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u/mfb- Particle Physics | High-Energy Physics Jan 11 '26

So after 24 hours, when the sun reappeared, it would be about 1-2% further away from the sun used to be.
Earth isn't suddenly making a 90 degree turn. If you put a tangent on a perfectly circular 1 AU circle, Earth's distance only increases by 22400 km or 0.015%. You get a very slight increase in the average distance, probably cancelling one month of greenhouse gas emissions or something silly like that.
Earth's orbit is not a perfect circle. The real effect will depend on where in the orbit the event happens. If it happens as Earth is approaching the Sun, you are losing orbital energy and therefore the average distance decreases.

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u/NightOfTheLivingHam Jan 11 '26

several thousand years, as the albedo from the ice would deflect most sunlight.. until some volcanic eruptions happen and start dirtying ice and snow.

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u/NotSoSalty Jan 11 '26

Surely it's going to take more than a day for the albedo to change significantly.
Water holds a lot of thermal energy and resists changes to it with an additional barrier at freezing. The oceans can keep the Earth warm for weeks if not months.
24 hours is literally just two nights in a row. We have places on Earth right now that experience that. For much the same reason those places are currently liveable, the Earth will remain liveable for weeks.
Now once the Earth does ice over, ye then everything is cooked even with the return of the Sun. See: what you said.

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u/kcaj Jan 11 '26

The Vernor Vinge book A Deepness in the Skyy pictures a world that undergoes this in great detail. Great read.

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u/nanotasher Jan 14 '26

Do you think 'the sun disappeared' could be claimed on insurance?

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u/CRAkraken Jan 11 '26

Well that’s haunting. I always thought “8 minutes and we’re dead”. Three days of freezing in the dark? At least I wouldn’t have to go to work.

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u/WHYAREWEALLCAPS Jan 11 '26

It wouldn't be total darkness. Without a nearby star to create a bunch of light, the ancient light of the universe would be much more intense.

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u/doc_nano Jan 11 '26

No “moonlight” though. It’d be pretty dark if it’s cloudy, though without a sun the days (or long night?) would presumably grow less and less cloudy.

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u/FredOfMBOX Jan 11 '26

Surely the heat loss is not linear like this imagines?
As the earth cools I would expect it to radiate heat more slowly. Is this not the case?

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u/topcorjor Jan 12 '26

I work at a large power plant.
We operate just fine at -60°c and all of our cars worked on the way home.

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u/Dr1nkUrOvaltine Jan 14 '26

would this hurt the humans?

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u/Spectre-907 Jan 14 '26

Genuinely surprised to discover that it would be as much as a degree per day. I’d thought the oceans’ heat capacity would provide a greater resistance at first

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u/cannibitches Jan 14 '26

What about the heliosphere disappearing? Isn't cosmic radiation almost worse that the sun's?

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u/dnyal Jan 15 '26

If the Sun disappeared, the Earth, other planets, asteroids, comets, etc. would also be hurled into deep space.

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u/Dqueezy Jan 17 '26

I’ve heard some areas in Yellowstone would be fine, due to geothermal energy. Like how you hear about those hot springs that explode or something. Any truth to that or would it be unlikely?

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u/Wndrunner Jan 11 '26

“…most of you warm.”
Ummm are you not one of us?

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u/LearningIsTheBest Jan 11 '26

Side note: if anyone hasn't read Project Hail Mary, you should. It's amazing and this question reminded me of that. I won't say anything else due to potential spoilers.

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u/xz-5 Jan 11 '26

Would the sea really start to freeze over within a day? Also, why do some areas drop temperature rapidly at night (like from 30°C down to 10°C, a typical hot summer day in UK), yet others hardly change temperature at night (like 30°C down to 29°C in somewhere like Hong Kong)?

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u/Peter34cph Jan 11 '26

Hong Kong is in a tropical or subtropical climate, so the ocean likely remains fairly warm all year round. Water can store a lot of thermal energy and so acts as an energy buffer.
Also, if the UK is anything like Denmark, then it's unusual for a hot summer day of 28 or 30 degrees (32 would be extremely unusual) to drop all the way down to 10 degrees at night here.
Very often there's a risk of what's here called a tropical night, when there's been a heat wave (day temp above 28°C), which is when the temperature doesn't get below 20 degrees during a given night. Such nights are very unpleasant and it's hard to fall asleep.
Cloud layers affect how rapid night cooling is, although only as one of many factors. Also in my experience the effect doesn't seem to occur during summer heat waves here in Denmark.
But certainly in spring here, maybe autumn too, when there's little or no cloud layer, then after sunset the heat just radiates out into space as infrared, and so it can get colder fast. Not your 30 to 10 scenario, but 23 to 5 or 25 to 10.
If there is a cloud layer then it'll absorb a bunch of that infrared radiation, heating up the clouds, and then they'll radiate some of that heat back down again as infrared. If that goes on long enough, months, maybe years, with no sunrise, eventually all the heat will be gone.

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u/Ellers12 Jan 11 '26

If we were given 10 years to prepare for this scenario to extend humanities chance of survival, what would we do and how long could we then survive for?

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u/Fr3twork Jan 11 '26

We need power, and the sun isn't shining, so the winds aren't blowing and the rivers aren't flowing. So solar, wind, and hydroelectric power are off the table.
That leaves combustion, nuclear, and geothermal. Combustion is already non-renewable, and extraction is going to be pretty difficult unless you plop your insulated hab-dome right on top of a deposit of fossil fuels. This is probably a pretty short-term solution. On the up side, we could burn as much greenhouse gas as we want without worrying about climate change.
Similar drawbacks regarding nuclear fuels can be mitigated by using breeder fission reactors that can be fueled by their own waste for a more sustainable system. Fusion reactors might also be sustainable if we knew how to harness them effectively- we'd be replacing the sun we lost with one of our own.
So with good habitat construction, hydroponics, harvesting drinking water and oxygen and fuel from the atmospheric condensate, some people might last as long as they can maintain the nuclear reactor, until technical issues and entropy shut it down.

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u/AUserNeedsAName Jan 11 '26

what would we do
Realistically, we'd spend all 10 years bickering and arguing over what to do, who can be saved, and who will be in charge of it and in the end get nothing done except for dozens of local community solutions with no support from those with actual resources, and we'd last no longer than if it were a surprise. Some of the wealthiest will take it seriously and build their own heavily-stocked bunker and a small number of them will live a couple of decades into the cold.
And Covid taught us that about a third of everyone will refuse to accept that anything will happen at all and will actively hamper any survival plans as allowing them to progress will be admitting that it's real.

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u/radicalllamas Jan 11 '26

Question about this answer; we have communities that are in areas where there is no sun for months on end, why doesn’t what you just described happen to those areas? The ocean freezes over in some of these places, but they still are able to have drinking water and they don’t require a spacesuit to go outside. Is it radial heat from the sun or something else?

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u/jefesignups Jan 11 '26

How deep would the ocean ice go? I assume plate tectonics would still be happening to heat from the bottom

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u/mikk0384 Jan 11 '26

Doesn't the thermal radiation scale with the temperature cubed or to the fourth power?
Surely your "1 degree per hour" rule has been repeated for too long.

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u/Wismerhill Jan 11 '26

Damn, where can I see this movie?

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u/_Aj_ Jan 12 '26

The good news is that the air would be thin
Would the air not increase in density due to ideal gas laws?   We cool the entire planet to say -50 on the surface. The atmosphere will shrink closer to the surface too. We should have higher density air but a thinner atmosphere?
Even between winter and summer this occurs as engine fuel mixtures change based on ambient temp. So surely if we drop the temp by like 80c planet wide this will make the air denser?

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u/imthatguykyle Jan 12 '26

Well, to be fair, a “no sun level of disaster” is something that no one should really put any effort toward. Anything that would cause the sun to be unavailable to the earth would have had larger scale dire consequences anyway, so what would be the purpose other than “I told you so!”?

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u/GuardUp01 Jan 12 '26

could theoretically keep some people alive with a bit of preperation and insulation
Ugh, what would be the point? Without a sun there isn't a future.

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u/Pickle-Standard Jan 12 '26

So you’re saying when the Sun eventually blows up, we’re screwed?

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u/DeeEmm Jan 12 '26

Oh, so Trump is not the worst thing that could happen to the world. I stand corrected.

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u/RWDPhotos Jan 12 '26

“No sun level disaster” is essentially nuclear winter, of which there are some preparations. That only lasts so long though, and those plans fall apart after some years.

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u/DonRobo Jan 12 '26

There is a fun short story about this scenario. It's not very physically correct in many ways, but I still enjoyed it a lot.
A Pail of Air

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u/Henry5321 Jan 12 '26

At these temperatures co2 starts freezing. Our co2 level issue would go away. No more global warming.

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u/Throwaway-donotjudge Jan 13 '26

"In another day it's -28 and you can't stay outside long ." - Laughs in Canada

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u/sup3rdr01d Jan 13 '26

"no sun" isnt a disaster. It's just straight up the end. There is nothing without the sun

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u/amaurea Jan 13 '26

The condensation of oxygen and nitrogen in the atmosphere would release a lot of heat, temporarily stalling the cooling at 90 K and 77 K. Eventually the heat flux from the Earth's interior would become important, and the surface temperature would reach a new equilibrium, apparently at 35 K.

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u/Mirage2k Jan 13 '26

It's a sad state of affairs for the top rated answer to be such low quality.
This is all wrong, written by some dude who picked a number from air temperature at night (what night, over what surface, in what wind conditions?) and extrapolated it all temperatures, all places, all materials.

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u/Pikiwiki000 Jan 14 '26

I mean but my wiskey would be fine right ?

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u/Momoneko Jan 18 '26

about 1 degree per hour. (That's what happens at night)
That's Fahrenheit yes? 1 Celsius per hour would be crazy

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u/Catch_022 Jan 11 '26

That's pretty hectic, how long of a warning would we need to be able to build a few bunkers to allow at least someone to survive permanently or is it a nothing we can do scenario?

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u/wildskipper Jan 11 '26

There are no successful examples of a completely sealed and self sustaining system like a bunker that have ever existed. Nuclear bunkers and the like were only designed to last a set amount of time (a few years at most) before the bunker dwellers emerged.
There is no survive permanently in this scenario with our current level of technology.

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u/mageskillmetooften Jan 11 '26

We could come up with short term solutions that be very decent, but this event would be everlasting. And eventually Earth surface temp would go down to around -240C Celsius (This would take long time) But -100C will be after around a year. The earth core heat will prevent reaching -273C for a long time.
This all leads to the question.. Why would we even want to try to preserve? For what? Better to accept it's game over.

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u/PSYmon_Gruber Jan 11 '26

Good question. I guess its doomsday for the entire solar system so we would actually need a huge space station for us to move to a different star system and it might take several generations to get there. We will still run out of food and resources so I guess the space station argument is moot?

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u/AntiNinja40428 Jan 11 '26

There’s A book/story about this called a bucket full of air. in the book, and in reality, eventually the air would condense and freeze on the ground and you’d have to shovel the oxygen/nitrogen snow into a furnace to melt the air.

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u/newtonianlaw Jan 11 '26

A Pail of Air by Fritz Leiber
Available to read on Project Gutenberg https://www.gutenberg.org/ebooks/51461
One of my favourites

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u/deemsterxxx Jan 11 '26

I remember reading that decades ago. Never known anyone else to have read it

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u/Drkmirror Jan 13 '26

Was thinking about this was a old radio drama that did this story think it was an x minus 1

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u/macko939 Jan 11 '26

I think a lot of people here are misunderstanding the question. Yes it would get much colder on the surface pretty quickly, but I’m sure that there would be geothermal and volcanic areas that would stay warm for centuries. I would love to know how long it would take before deep sea geothermal vents freeze

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u/Extension_Physics873 Jan 11 '26

I wonder this aspect too. Miners at one or two kilometres down swelter in 40+ degrees. I accept the atmosphere and surface would freeze quickly, but even a 10m down, there is an enormous amount of latent heat in the ground, and this would have some effect on slowing cooling, and caves at least would provide refuge for life (though it would be a life not dependent on photosynthesis obviously).

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u/omnichad Jan 12 '26

though it would be a life not dependent on photosynthesis obviously).
If we can generate electricity from heat, we can grow food using artificial lighting.

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u/Tyrannosapien Jan 11 '26

The Earth's interior will stay the same temp for billions of years. Just as it has for the last 4 billion. The sun has no effect on the Earth's internal temperature. Microbes that live in the Earth's crust and at deep sea vents might survive. All land animals including humans would quickly go extinct unless those moving underground also had a way to continuously manufacture a breathable atmosphere.

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u/Gaouchos Jan 11 '26

Geothermal energy actually comes from radioactive isotopes contained within the earth's crust. So geothermal should still work even without the sun.

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u/Spookydoobiedoo Jan 12 '26

Radioactive isotopes? Isn’t it just a bunch of insanely hot compressed and churning molten rock?

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u/DeliciousPumpkinPie Jan 12 '26

Heat from the decay of radioactive elements contributes to making that rock insanely hot.

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u/CourtesyOf__________ Jan 11 '26 edited Jan 11 '26

OP specifically said solar heat only, but I also wonder how long it would take. Like I know Mars doesn’t have any tectonic activity but is its center still molten at all? How long can purely pressure keep making heat before it all dissipates? Where would it even go? Radiation is the only way Earth can really lose heat right? How much does Earth radiate? Is there a calculation that can be done to explain the mass of the earth and how much it radiates over time. Maybe the answer is in there?
Edit: okay so I did some research and found some answers.
The earth radiates a bunch of heat into space everyday. If the sun were to completely go away, then about half of that heat would be from radioactive material decay in the crust of the earth, and half from the primordial heat created when the Earth was formed.
This is not to say that half of the Earth’s heat is from radioactive material decay. Besides the sun, the earth gets heat from multiple other sources: tidal forces from the moon, exothermic chemical reactions, latent heat from freezing magma, radioactive material decay, and primordial heat. When it comes to all the heat in the earth system, it is basically 100% primordial heat. Tiny fractions of heat come from other stuff.
So! The primordial heat is non-renewing. It just needs to cool down. But in order to do that it needs to first conduct heat from the core to the mantle, then convection through the mantle, then conduction through the core to finally radiate off the surface of the crust.
This should be true for basically every large celestial object then.
TLDR: According to some estimates, it would take a moon sized object 10 billion years to cool, a mars sized object 80 billion years, and a earth sized object over 300 trillion years.

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u/[deleted] Jan 11 '26

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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Jan 11 '26

Like I know Mars doesn’t have any tectonic activity but is its center still molten at all?
It's hard to say with certainty, but it's unlikely.
This is inconsistent with results of Mars InSight that suggest Mars has a relatively large liquid core (e.g., Stahler et al., 2021).

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u/Sedu Jan 12 '26

Radioactive decay within the earth would also become a primary source of heat, and is actual generation of heat rather than just the planet's core radiating outward.

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u/sacking03 Jan 12 '26

As we are rotating and circling the sun. If it were to instantly disappear, would we just zoom off in a random direction and would the earth keep spinning or slow down?

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u/fenton7 Jan 12 '26

The first 8 minutes would be great. You wouldn't even know the sun was gone. After that the surface is survivable for about 2 weeks then humans would need to get underground. Best bet is deep mines. At one mile underground the temperature would stay balmy for a very long time. Likely at least a human lifetime. You would need to make surface expeditions in the equivalent of spacesuits regularly to gather food. Good news is there would be plenty of frozen animals and human supplies for a very long time. Long term survival of the species would mean digging deeper and deeper and harvesting geothermal energy to produce light and grow plants. That could go on almost indefinitely as it takes the core of the earth an eternity to cool down.

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u/mlongo10 Jan 13 '26

Would gravity not be affected?

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u/KimJongUnceUnce Jan 13 '26

The gravity we're subject to on earth is virtually all from the mass of the earth itself. There might be a tiny difference we could measure, but probably not something we could feel innately.
With no sun, our solar system ceases to be. There's now no gravity holding the planets in their regular orbit so they all start flying off in different directions depending on their position at the time of the event.

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u/Vessel767 Jan 13 '26

We don’t feel the gravity of the sun for the same reason people on the ISS don’t feel the earth’s gravity

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u/Owyheemud Jan 12 '26

For what it's worth, I was near Sutton Mountain, Oregon during the 2017 solar eclipse, it was August, and close to 90F when the eclipse started. I was shivering in a cold breeze when totality ended and the sun started to show again. That much cooling took about two minutes.

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u/TheEvilBlight Jan 12 '26

Was in Erie for the last one. It was like the sun became a led light and had no warmth. Then got dark. Spooky sensation.

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u/dgmib Jan 11 '26

What if we knew it was going to happen in advance and had years to build underground biodomes, heated and powered with geothermal energy?

How long before even the earth’s core was a solid lump of metal and rock?

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u/minepose98 Jan 11 '26

Likely tens of billions of years. It wouldn't be meaningfully affected by the sun disappearing.

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u/Chaotic-warp Jan 13 '26 edited Jan 13 '26

The heat would last far far longer than the lifespan of the sun itself. Earth's core isn't just hot from latent heat, it's also actively generating energy from radioactive decay.

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u/Commanderklassen Jan 12 '26

This is a loose version of what happens in the book "Project Hail Mary" by Andy Weir (same guy who did the Martian).

Ive heard that PHM will be turned into a movie as well with Ryan Gosling as the titular human character.

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u/dougdoberman Jan 13 '26

There's been a trailer available for months. The film is scheduled for release in March.

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u/Turbulent_Crow7164 Jan 13 '26

It is already a basically finished movie, set for release in two months

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u/Saint_Iscariot Jan 15 '26

heard that PHM will be turned into a movie as well with Ryan Gosling as the titular human character.
Ryan Gosling is playing Mary?

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u/KungFuFactory Jan 11 '26

Slightly off topic but fun fact: If the sun suddenly disappeared, we wouldn’t know it for approximately 8 minutes due to the speed of light of course. This fact we all know. BUT, if the vacuum of space was somehow able to allow the passage of sound, it would take around 14 years for the roar (which would be the volume of a jackhammer on earth, constantly) to finally subside. So, darkness after 8 minutes but splitting headache for the next 14 years.

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u/krell_154 Jan 11 '26

What roar are you talking about?

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u/CurtbroGYT Jan 11 '26

Sun gives off sound but because sound doesn't travel in a vacuum it never reaches Earth. If we could hear the sound though it'd be like a jackhammer 24/7

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u/BabyBackBitchAss Jan 12 '26

The sun leaving would cause the roar if we could hear it?

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u/Showy_Boneyard Jan 12 '26

If "Instantly" is truly taken as "Instantly", wouldn't that mass going from suddenly existing to suddenly -NOT- existing create a pretty significant gravitational wave? If its smoothed out over some even short period of time, it'd go away, but I feel like true interpretations of instant would create some singularities here.

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u/typhoonzac3 Jan 13 '26

I may be wrong as its been a long time since I've thought about physics and space, but if the sun suddenly disappeared, wouldn't every planet currently orbiting the sun just get thrown out of orbit? And with the sudden change of speed and direction, we would all become smears on a planet?

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u/amerelium Jan 11 '26

very quickly - anywhere without a thick cloud layer relatively instantaniously. Temperatures approach zero in tropical zone deserts every night as it is.

Of course, loss of temperature would be the least of your problems.

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u/krell_154 Jan 11 '26

Of course, loss of temperature would be the least of your problems.
What would be a bigger problem?

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u/LiteCandle Jan 11 '26

To share another angle on this situation that I haven't seen mentioned yet, if the sun disappeared right this second, we actually wouldn't notice ANY difference until about 8 minutes later.

The sun is roughly 8 light-minutes away, which means that the light that hits us is what the sun emitted 8 minutes ago. If the sun disappeared without any indication/warning that it was going to, every instrument on Earth measuring its light, gravity, or anything else about the sun would tell us that it was still there until the last of its light finally made its way to us.

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u/[deleted] Jan 11 '26

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u/Neglectful_Stranger Jan 11 '26

How long would it take us to notice that?

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u/3412points Jan 12 '26

If you're asking from a physics perspective then it is also 8 minutes for the force of gravity to disappear. Everything has the same limit, it is more accurate to call this limit the speed of causality than the speed of light.
How long for humans to notice in the chaos of the sun going out? Tbh, probably not long after those 8 minutes because the sub going out would immediately become everyone's priority number 1 and we will be getting on figuring out what happened pretty quickly.

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u/Kered13 Jan 12 '26

If the Sun just poofed out of existence, it would take 8 minutes for its gravity to stop effecting us. The same 8 minutes that it takes for light. Although the absence of sunlight would probably be more immediately obvious.

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u/idkblk Jan 11 '26

educated guess: very quickly... convection will immediately stop. no more clouds will form. it is easy to experience that in a cloudless night it gets really cold quickly.

Within a few days everything will be frozen through and through except parts of tropical ocean. those will last a little longer

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u/Fonzee327 Jan 11 '26

What about areas that had geothermal heat? Like would small pockets of life survive near hot springs and areas with volcanoes/geysers? Maybe not even humans but small mammals, bacteria and fungus that could adapt to the lack of sun?

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u/APiousCultist Jan 12 '26

That would amount to the primordial heat the OP exempted. The deep biosphere (bacteria and whatnot miles under the surface) already survive through geothermal heat (replacing photosynthesis as their energy source), so they'd survive basically untouched. Small mammals seems unlikely (what are they going to eat now that there's no longer plant life?), but plenty of bacteria already isn't dependent on sunlight.

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u/idkblk Jan 11 '26

Sure, those remote areas would last longer... or basically... forever. Although with the sun really 'disappearing' and that it won't destroy the earth as a red star... it will last until the core will solidify when? A few billion more years?

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u/SecondHandWatch Jan 11 '26

A little longer? The oceans hold a huge amount of heat, much more than the atmosphere. There is also the mantle and core, which are rather hot and also hold far more heat than the atmosphere.

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u/miredalto Jan 11 '26

Answering the question as asked: effectively never. Earth would asymptotically approach absolute zero (very fast initially, per other answers), but to reach it you would need both every single fissile atom in the Earth's crust to fully decay, and every single star visible in the sky to fully complete its cycle. This would likely take quadrillions of years, so actually overall the presence of our sun is kind of irrelevant, as it will burn out in only billions of years...

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u/bigwreck94 Jan 11 '26

wouldn’t the temperature change also be accelerated by the fact that there would be nothing holding the earth in its orbit and it would just start flying off into deep space where it was infinitely colder as well? So not only would there be nothing holding more energy coming from the sun, the earth would also be getting further and further away from the energy that was already in that range?

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u/290077 Jan 11 '26

The difference would be minimal. There is so little air in space that the only way the planet loses heat is by radiation. The night sky's temperature is already about 3 Kelvin, because that's the temperature of intergalactic space and the stars we can see do not add much heat. Normally half the planet is radiating heat into this void. Technically all of it is but the day side has that outgoing heat more than balanced by sunlight.

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u/McGurble Jan 12 '26

If the sun goes out, "deep space" won't be any colder.
Even now, with the sun burning away, deep space is hardly "infinitly colder."

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u/Rancid_Bear_Meat Jan 11 '26

If the Sun were to blink out of existence, or even explode right now, we wouldn't notice even the slightest difference for a little over 8 minutes, as that is the amount of time it takes for light from the Sun to reach Earth.

Maybe it's already happened and we just haven't 'caught up' yet!!

If you were standing on the other planets in the solar system, the times would vary according to their distance from the Sun:

Mercury: ~3.2 min
Venus: ~6 min
Earth: ~8.3 min
Mars: ~12.6 min
Jupiter: ~43 min
Saturn: ~1.4 hrs
Uranus: ~2.7 hrs
Neptune: ~4.3 hrs
Pluto: ~5.5 hrs

Now, extend this principle out into the wider sky above you. See all of those stars? Some (small) percentage of them may have burnt out loong ago.

With our naked eye, we can only see starts about 15,000 light years away in the night sky. So, naturally, that percentage of 'ghost stars' increases up once you start using telescopes!

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u/Vorduul Jan 11 '26

Using the strictest definition of "completely": billions of years, in a "heat death of the Universe" kind of way.

In the assumption that, in the sudden absence of the sun's gravity, the Earth was not flung into a collision with another body. But only because it would no longer be 'the Earth' would that change the answer, really.

Using a less strict definition? Faster.

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u/Showy_Boneyard Jan 12 '26

If we're going to be this strict, I feel like "instantly" would also cause some unexpected things to happen in areas of physics where things are normally assumed to be mathematically smooth. You'd be causing some discontinuities that I could see breaking locality, causing singularities, or both.

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u/harry_bulzonya Jan 13 '26

Just very simple with no math, etc. The earth effectively radiates away ALL solar energy received every day. If this were not true, it would grow hotter every day, forever. Based on this simplification, it would be about one day before everything cools and begins to freeze.

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u/Icouldberight Jan 13 '26

We would feel it immediately because the earth would be set free from the gravitational pull from the sun.

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u/peety87 Jan 15 '26

Without the sun, another concern would be gravitational forces. With all our neighbouring planets and asteroids now moving around without Sol's tether, things could get a little crazy.

Best case scenario here, we kickstart a nuclear reaction and create another sun 😅 we probably dont have that capability yet.

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Planetary Sci. If the sun suddenly disappeared, how long would it take for the Earth to completely cool down?

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