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I just had an idea in a sci-fi setting for an alien race that has re-emerged, but I don't know how plausible it is or if there is anything to make it more plausible.

Consider a conventional universe, just with an FTL method of travel, on another plane of existence as not to overlap with reality.

The alien race years ago went underground as their sun was dying and managed to survive in the subterranean setting. Their sun died, and all was good. Fast-forward now to their re-emergence. They came out of their subterranean realm because life had returned to the surface thanks to their/a sun now being there.

So how does this happen, and how likely is this to happen?

I have a few ideas:

  • Their solar system drifted through something that added enough matter to their sun and displaced enough of what was there to give it enough fuel to reignite. Mental simplification: a snooker ball smashes into another and effectively replaces it.

  • Their planet, once the sun had died, drifted out of orbit and through dead space for a time as a rogue planet before being captured and settling into orbit around another sun.

  • The Wizards did it. I don't like this option as it's just lampshading it, and also potentially introducing a power capable of stellar engineering. That concept alone outweighs the reappearance of this ancient race unless it was the ancient race, and I don't want them to be at that advanced level of technology.

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    $\begingroup$ When you say the sun died, what do you mean? Did it become dim? Or do you envision it completely cold? $\endgroup$
    – UVphoton
    Aug 6 at 18:15
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    $\begingroup$ Suns don’t die like a person where they just stop living but more or less still exist. Suns lose mass and all manners of stuff changes within them that makes them “die” sometimes they even shoot off a load of their mass creating nebulas. $\endgroup$
    – Topcode
    Aug 6 at 18:45
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    $\begingroup$ @Topcode Oh yeah, the planet also needs to not have been sterilized when it goes red giant. $\endgroup$
    – DKNguyen
    Aug 6 at 18:45
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    $\begingroup$ (a) If you read the roll-over text or wikis for science-based and science-fiction, you'll learn that they are mutually exclusive. Please pick one and delete the other. (b) The help center states, "To prevent your question from being flagged and possibly removed, avoid asking subjective questions where … your answer is provided along with the question, and you expect more answers." You've already answered your question three times. (c) You are allowed to ask one and only one question (see VTC reason "Needs More Focus"). $\endgroup$
    – JBH
    Aug 7 at 6:38
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    $\begingroup$ Well, I'm sorry to tell you that you would never be able to re-ignite a star, since all it's fusion-ready elements (aka hydrogen) fused to helium, which is not so happy to fuse with it's cousins. Oh, and also the sun shed off it's outer hull, which leaves it naked with a core full of iron that also does not want to ignite. Still not convinced? After the elements fused and hull is dropped, sun has a different gravitation field, so your planet will probably float off and deep-freeze forever $\endgroup$
    – clockw0rk
    Aug 9 at 12:49

11 Answers 11

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Your planet was shaded by a ring in its orbital plane.

From the persepective of your planet, the star gradually dimmed and went out. What actually happened is that the star tore apart a planet that got too close and acquired a ring.

https://www.cs.mcgill.ca/~rwest/wikispeedia/wpcd/wp/r/Roche_limit.htm

roche steps

This planet was between your world and the star. As it dissolved into the ring, the ring gradually shaded your planet more and more and it got cold and dark.

Rings do not last forever.

https://www.wionews.com/science/saturn-is-eating-its-own-rings-they-may-disappear-altogether-466925

Saturn is unlikely to lose its grandeur during our lifetimes. At the current rate of degradation, it is estimated that the core of the ring would disappear in 100 million years and the rings will completely disappear in 300 million years

The rings shade your planet for a time but gradually fall into the sun or ablate away in the solar wind. The sun re-emerges.

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    $\begingroup$ Quick back of the envelope calculation suggest this could envelope the sun with something on the order of 1 000 kg/m^2 to 100 000 000 kg/m^2 layer of dust if the broken up planet is the size of Jupiter, depending on how close the belt will be. The higher end would make quite thick dust belt - very close to the sun (few solar radii). $\endgroup$
    – tylisirn
    Aug 7 at 11:46
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    $\begingroup$ Nice answer, solves both the re-birth problem and the even harder "sun died" problem with no hand-waving and a minimum of coincidence. $\endgroup$ Aug 7 at 14:40
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    $\begingroup$ Upvoted, and it still leaves a massive mystery as to how a formed planet somehow got anywhere near it's roche limit. Especially considering the formation of such a monstrous gas giant shouldn't have occurred anywhere inside the orbit of a habitable rocky planet. $\endgroup$
    – Nohbdy
    Aug 7 at 21:21
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    $\begingroup$ @Nohbdy - here is how : en.m.wikipedia.org/wiki/Hot_Jupiter $\endgroup$
    – Willk
    Aug 8 at 2:26
  • $\begingroup$ I forgot about those. But they can't originate too near their Roche limit, right? They'd get scattered during early planetary evolution. A cursory literature check says we're still establishing the lower limit of how close to the limit they will form, and we've found at least one only 20% percent out from it's Roche limit. On the face of it at least, sufficient acceleration to bring one of these monsters in range of tidal death would require spooky action or catastrophic amounts of energy. The existence of the planet itself suggests a stable system. $\endgroup$
    – Nohbdy
    Aug 8 at 3:27
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Rumors of the sun's death were greatly exaggerated.

The sun didn't die. Instead, it entered a quieter phase. Unfortunately, this began at the same time as a global cooling event (damn those evil chemicals for eating away the beloved greenhouse gasses we rely on!) was triggering an ice age.

”The sun will be dead in 25 years!” was an easier way to convince everyone to move underground than ”We don't really know how far those glaciers will go, so start digging today just to be safe.”

The glaciers didn't go as far as expected and partially pulled back after a few hundred years. It was still cooler on the surface, but well within habitable ranges. However...

Once the great lie about the sun dying started, all contrary evidence was suppressed. Over time, the ruling class destroyed more and more of the actual data and each generation of rulers became more convinced that the sun really was dead and the world above truly was nothing but icy darkness.

This could have continued for many more generations, but 500 meters of limestone above one of the shallower shelters ended up getting slightly acidic runnoff from the hills to the east. Over the centuries, water filled caverns grew. Then the rivers changed course and things began to dry out. Sinkholes opened allowing sunlight (now almost as intense as when the shelters were built) to reach greater and greater depths.

Near the bottom of one of the deeper sinkholes, a tree root worked its way down. It hit a point of great resistance. The root spread and tiny rootlets forced their way into cracks and pried them open wider. All was going well until a large piece of the resisting material gave way, dropping the tree into the center of a public plaza and exposing the people to sunlight and fresh air for the first time in many generations.

People in other shelters considered the news to be a hoax, but as more and more came to visit the hole in the roof, they began to believe. Soon, steeply ascending tunnels were dug from shelters arould the world.

Eventually, the last scraps of evidence of the great lie were pieced together. "NEVER AGAIN!” became a mantra of all the people.

Then a terrible question was raised. "How can we be sure another event can never force us away from the sun again?"

Science had not been dormant during those long centuries underground. It wasn't long before a presentation titled ”One sun in not enough. Let's spread out and get more.”led to global support for turning some theoretical physics into reality. Other suns would soon be in reach.

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    $\begingroup$ I love that tree story! I love the idea of a tree falling in a burst of sunlight down into a cave! I picture it as a painting by an Italian master. $\endgroup$
    – Willk
    Aug 9 at 19:14
  • $\begingroup$ @Willk - Thanks! Now I just need an Italian master and some method to keep a publisher from changing the tree into a pack of large alien mole rats. 😂 $\endgroup$ Aug 10 at 11:15
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TLDR; Things are not what they appear to be.

Stars don't just reignite. They very rarely die without destroying literally everything in their vicinity. Nothing could possibly have refueled it - it would either finish exploding faster, or outright collapse into something properly dangerous to orbit. The libration (Lagrange) point between this planet and the sun couldn't capture something massive enough to blot out the sun, and a debris ring from an planet closer to the sun exploding would be too sparse and too far away to have a significant effect on a fusion source containing more than 99 percent of all mass in the solar system, for similar reasons that Venus doesn't cast a noticeable shadow on Earth.

All Readings Normal

Stars contain genuinely insane amounts of matter. Our ability to somewhat accurately describe the size, makeup and inner workings of a star is packed with error bars and speculative high energy physics. Looking through bits of polished glass and using math can tell us a lot, but biological brains can't actually contain a real sense of something like how large a star is, or how long 8 light-minutes really is. There's simply nothing to compare it to.

We've Found an Anomaly

After emerging from hibernation, you really have quiet a lot going on. Only a dedicated few are going to start trying to point instruments at the sky. Science still works, and the math starts telling them things. Some things take time. Getting an accurate range on what appears to be a star in your own system takes a while, especially if you have no moon with a shadow to triangulate off of. You're largely left trying to sort out orbitals of other neighborhood objects.

What if there aren't any?

That's No Star

No other natural satellites? None? It takes months to confirm that your planet is no longer experiencing parallax - your planet's not even orbiting. The red shift and blue shift make sense, and the star isn't growing in aspect ratio, so you aren't falling into it.

Something is holding you there. On purpose.

Now tell me a story.

Lots of options to go from here. Mind bending cosmological horror comes readily to mind, but given your setup, you might prefer that your intrepid planet was rescued by a high energy starfaring society with handwavium tech that is now evaluating them for potential inclusion in their daily lives. The handwaving would be pretty heavy (moving stars is pretty uneconomical), but if large scale matter to energy conversion is within your threshold of believability, you could have quite a time writing this.

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Rogue planet

Planets do not always have a star. Some float in the depth of space all alone. They might have formed like this, bit a more likely scenario is that they came from a star system. Thanks to any amount of factors they can be thrown away. Their orbit isn't stable. They receive an impact and lose/gain material or speed. Some (massive) object comes too close and pulls the planet out of orbit. Each can be responsible. Even a rogue planet can go through a solar system and pull out more planets.

Lastly, a rogue planet can be captured by another solar system. This can be by impact, like our moon is thought to have crashed into Earth and then stayed in an unstable orbit, though stable enough to stay with us for a bit. It can also be that the speed, angle and other celestial bodies pull it into an orbit around the star.

So the alien planet can be pulled out of orbit at one star. They survive the long journey to another star, where they are fortuitous enough to get into orbit again in the goldilocks zone.

Better alternative: clouds

Much easier is not making the sun go away, but to prevent the rays from reaching the surface. Bug asteroids can prevent light to reach the surface for some time. Nuclear winter as well. Sone climate catastrophes can do the same. This can be long processes of years, decades, centuries or longer depending on what happens. Hide in the bunkers and hope Vault Tech was kind with yours...

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    $\begingroup$ Orbit-changing impacts won't work, as anything with enough momentum to achive that will just utterly destroy the planet, or at least fully melt it. Being pulled out of orbit by a near miss would work, be highly unlikely, and it would be even more unlikely to be reinserted into a new (and habitable) orbit of the same or another star, but it would at least be possible. $\endgroup$
    – toolforger
    Aug 8 at 7:17
  • $\begingroup$ @toolforger anything is unlikely with the question asked. It is just something that can happen and I say has happened. The universe is a big place. Iirc the debris of the planet after a collision can reform into a planet, causing it to become a rogue planet. It isn't automatically dust. $\endgroup$
    – Trioxidane
    Aug 8 at 7:33
  • $\begingroup$ I'm agreeing with the dust idea. It's the idea that an inhabited planet might be thrown into interstellar orbit via a collision and stay inhabited that I don't think will work. $\endgroup$
    – toolforger
    Aug 8 at 7:54
  • $\begingroup$ @toolforger I agree. It is just one of the ways how to get a rogue planet. I do not use it in the last paragraph, saying the planet is pulled out if orbit. The whole idea of a civilization surviving without the sun is far fetched anyway. $\endgroup$
    – Trioxidane
    Aug 8 at 8:15
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    $\begingroup$ +1 for Nuclear Winter, the most plausible suggestion so far by a wide margin. Was going to make a whole answer about it if someone hadn't mentioned it already. I'd expand on that idea, maybe super-volcanoes if you want a more natural disaster rather than a man-made one. $\endgroup$ Aug 8 at 13:19
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Changing stars in a binary system

Their sun was dying, becoming a red giant and losing mass. The civilization saw the writing on the wall and hid underground, prepared to stay there until the conditions were better. As the sun lost mass, the planet was not as strongly gravitationally bound to it's star, and its orbit widened and eventually broke away from the sun! Fortunately the planet was flung towards the sun's binary companion star, and after a series of bounces, the planet slowed down due to the dust and ice surrounding the companion star, and settled in a new orbit. More details.

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  • $\begingroup$ That "planet hoppers" scenario could even make the cold interregnum a recurring event. I do see highly elliptical orbits though, so the planet would have extreme seasonal variation. I guess with some parameters that could be worked around, but the orbits and the star distances and masses would have a much narrower tolerance I suspect. $\endgroup$
    – toolforger
    Aug 8 at 7:26
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The Success of Project Prometheus

Project Prometheus:

In brief, an expedition to a relatively nearby starsystem to acquire sufficient stellar material to reignite the home-star.

This mission was a hail-mary attempt prior to the Great Cowering and the Big Freeze destroying all possibility of interstellar travel.
An unmanned, artificially intelligent space-probe was dispatched to a neighbouring star system with all the tools needed to construct a massive Dyson Swarm from the various moons and rocky bodies there.

It took a long time, there were false-starts, the tech malfunctioned here and there, but eventually the swarm was completed.
With that, the englobed star was induced to generate solar flares steadily, altering its trajectory and guiding it to intercept the Home system.

It took a further X-Thousand years to actually reach Home, and much of the stellar mass was expended as reaction-mass to get it there. The final part was the most delicate piece. Combining the two stars without catastrophically destroying the entire Home system. Fortunately with so long to travel, the artificial intelligences controlling the swarm were able to work out every detail and successfully merged the two stars together, even managing to contain the massive solar flaring this produced so it didn't fry the system.

The final state? The husk of the Home star is revitalised as a vibrant new Main-Sequence star, wrapped in the remains of a swarm of artificially intelligent Dyson Statites.

The sky is lit with comets and meteors from the Oort cloud for centuries to come.
The Homeworld is thawing and its inhabitants are emerging from their buried cities and cryo-stasis vaults to a new day.

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    $\begingroup$ The other object do not have to be another star, could very well be a super jupiter. Added mass could very well increase throughput of the star for a while. $\endgroup$ Aug 9 at 14:20
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The problem the natural events you describe occur on stellar time scales. Gas or even another star enters into decaying orbits for tens of millions of years before merging. And if they were on trajectories fast enough to to literally collide with each other without orbiting, they would just pass through each other leaving cosmic destruction in their wake. Do you need to explain this? Can the aliens just not know the cause?

The captured rogue planet might be more plausible but needs a very lucky set of circumstances: star systems passing through each other slowly enough for a proper capture without a useless rogue orbit but fast enough the planet's geothermal energy did not run out to power life.

Unless they have a crazy power source sustaining not just civilization but all other underground life not under their purview (since you mention life re-emerges on the surface of its own accord). Unless your intention is they seeded that surface life.

In any case, surface life would not survive in the mean time. It would take time for the surface to stabilize and for chemical cycles to restart. I have no idea how long that would take on a frozen planet that used to support them. For all I know it might be like starting from scratch similar to trying to terraform a dead world except all the required materials were already present.

Then it might take a hundred millions of year after that for subterranean life to re-emerge and adapt on the surface. I imagine most of that time would be spent restarting all the biological cycles and foundations (primary surface producers) that were broken. Only a relatively small amount of time would be required for consumers to adapt to the surface.

The planet also needs to not have been sterilized when it goes red giant when the star died. That also lasts millions of years.

Why did they go subterranean to escape the red giant? Were they not capable of space colonization? It all pretty much requires the same tech to survive without a sun whether underground, on the surface, or in space.

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They had no cosmology to speak of (think Ptolemaic), and what truly happened is that their planet is orbiting a really, really bright star (or smething more exotic, like a quasar) on a really, really big orbit, taking many millennia. And what happened was just that they left the periastron and their planet is now coming back. Evidence in favo(u)r: they managed to survive underground, obviously the life is well adapted by evolution for this. The previous orbit just disappeared from the collective memory (or even preceded their recorded history). There were maybe just some legends of an eternal night surviving but being dismissed as, well, legends, until the sun "died".

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Mental simplification: a snooker ball smashes into another and effectively replaces it.

Actually, not that hard. Stars rarely collide because they are small compared to the vast amount of space. They usually "scatter" - it is in fact a parabolic orbit, but looking from a distance it looks like an elastic bounce.

So yes, if you align your vectors and masses right, it is actually quite possible to replace a star. If the process is quick enough, the planets' orbits won't twist much. You are 50/50 whether this small orbit correction is good or bad for your plot.

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If you want to strictly stick to the sun specifically died slowly, that poses an issue. The light spectrum from most kinds of stars makes life as we know it an issue, but even ignoring that, the process through which most types of stars die not a simple fizzle-out if starting from main sequence. Your planet is likely to be consumed by the star expanding. In addition, without some mechanism for the star to lose mass, the planet's orbit wouldn't change.

I think the ideas posted here are a good start where it has to be something to the effect of the star didn't necessarily die so much as some other force interfered with it. Blocked by a planetary ring is a great answer. It could also be that an object from outside the solar system disrupted the orbit, casting the planet out of its orbit. Then through some mechanism, it is caught by another solar system or maybe even the same one.

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The sun didn't die, rather there was so much pollution caused by the society's industrial activities (or, perhaps, war) that the whole atmosphere became all but impenetrable to light. With no heat from the sun, they retreated underground to take advantage of the heat nearer the planet's mantle.

The underground society rarely ventured to the surface as it was too cold, and so eventually it faded into their cultural understanding that "up there" is just a cold, desolate place that people have no reason to go to other than curiosity - just as our society understands this about Antarctica and space, despite probably none of us ever having met anyone who has been there. And after many thousands of years, nobody really believed in those old myths about "the sun" any more.

With almost no biological processes occurring on the surface, the smog lasted for a long time. There was always the occasional intrepid adventurer who wanted to make a trip to the surface to see for themselves, but eventually there came a point when what one of those adventurers saw was a sky that seemed to get a little less dark every 24 hours or so, and maybe, just maybe there really was a sun up there.

Scientific experiments confirmed that planting lots of vegetation on the surface did slowly cause the "bright times" in those areas to get brighter. And so the society covered as much of the planet as they could with plants and trees, which cost an awful lot of labour and precious water, but in the end, the optimists were right.

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