I'm writing a story where aliens are using humanity like a botnet to do hyper complex calculations - each infested human has a program running in their subconscious brain that does a small part of the overall computation. They're doing this in order to find a habitable planet, since they've more or less depleted their homeworld. Their system of FTL travel requires nearly perfect accuracy in terms of coordinates, so they're running simulations to be absolutely certain the next planet they reach is habitable before using up expensive resources to get there. At the moment, most of the aliens live on a mothership orbiting Earth, while a few are disguised as humans and some have been on Earth for 20 years.

That last point is where my problem lies - if some aliens are on Earth, why don't they all settle there permanently? Clearly they're capable of living on Earth for extended periods of time, but for the story I need them to have ruled out Earth as their home the moment they arrived. Also you would assume they ran some simulations prior to coming here, since their FTL is so expensive. What about Earth or our solar system would be hard to predict and makes living here untenable, without being impossible in the short term?

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild
    Commented Nov 28, 2019 at 12:30

25 Answers 25


"It's the smell!"

-- Smith

I think the easiest answer is that there is something in Earth's biology that disagrees with them... something that can be "managed" with continuous medical intervention, but would be really annoying for them to try to settle permanently without completely terraforming the planet. Imagine, for example, that 99% of the aliens experience the equivalent of severe allergies when in contact with our biome. Sure, maybe they can damp it down to a manageable level with the equivalent of antihistamines, but could you imagine living for generations under those conditions? Well, neither can they.

So why don't they just terraform the planet? Well, aside from the possibility that they'd rather not exterminate us (as mentioned in other answers), maybe they don't have the technology, or even if they could, maybe it would take thousands or even millions of years for the planet to become tolerable again after sterilizing it (and this assumes that they're sure they could fix whatever caused the problem in the first place) before the planet would be worth living on. It's quicker and easier to just roll the dice again.

It seems plausible that there could be some subtle biological thing that their models didn't predict that would be suitable. I also think suffering from near-constant hayfever would make for good story-telling.


The aliens are humans.

The aliens did do the calculations and they did find a habitable planet, 100,000 years ago. They are depleting their homeworld - here.

On arriving on Earth and settling the colonists, the mothership immediately started working on finding the next world. These calculations take a very long time.

Almost none of the humans on earth remember their alien heritage. Those that do are part of a secret society.

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    $\begingroup$ Scientologists are real? $\endgroup$
    – nzaman
    Commented Nov 23, 2019 at 5:59
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    $\begingroup$ This seemed really formulaic to me until the last sentence, which had me immediately hooked! Nice save :P $\endgroup$ Commented Nov 23, 2019 at 23:47
  • $\begingroup$ I came here wondering if someone had already posted this idea +1 $\endgroup$
    – IT Alex
    Commented Nov 26, 2019 at 14:04

The aliens evolved on a planet on which photosynthesis never developed. Oxygen is a poison to them. Some of their least susceptible adults can handle it for limited periods of time with medical support. Earth is not a healthy place for them long term and they cannot raise their children in our atmosphere.

Removing the oxygen would be particularly difficult, because it is continually replaced by photosynthesizing organisms. It would not be enough to wipe out green plants. The upper layers of the oceans, and earth has a lot of ocean, are teeming with Cyanobacteria.

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    $\begingroup$ “Raising a child in an oxygen rich environment?? Are you insane!!?!” $\endgroup$
    – Joe Bloggs
    Commented Nov 24, 2019 at 17:34
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    $\begingroup$ This answer can be extended by postulating that the aliens have to wear suits 24/7 which are equipped with chameleon technology so that they appear as human to the natives. $\endgroup$
    – EvilSnack
    Commented Nov 24, 2019 at 20:33
  • $\begingroup$ @EvilSnack The exact inconveniences are left as an exercise for anyone using this idea. The children being susceptible to oxygen poisoning alone would make Earth undesirable. I wonder if the aliens will think to paint any exposed iron in their equipment? $\endgroup$ Commented Nov 25, 2019 at 5:30
  • $\begingroup$ "Some of their least susceptible adults can handle it for limited periods of time" - hmm. I think that's about as plausible as "less susceptible" humans coping with an atmosphere containing 20% chlorine. (Chlorine is fatal within a few minutes at 0.1%) $\endgroup$ Commented Nov 25, 2019 at 13:15
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    $\begingroup$ @MartinBonnersupportsMonica To be fair, oxygen is toxic to humans as well. We have a good tolerance to those toxic effects, but it's still one of the things that's slowly killing us. All you need is for their oxygen tolerance to be significantly lower. It doesn't kill you as soon as you breathe in, it just causes accumulating damage in your cells that you can't deal with continously. But put them back in their natural environment, and their cells have a chance to repair the damage and be mostly fine in a few days. It can even do permanent damage and be mostly fine (humans do those for fun:P) $\endgroup$
    – Luaan
    Commented Nov 25, 2019 at 13:26

1. They actually cannot live on Earth for extended periods of time

Your premise is that Earth should be a candidate, since:

"[...] some have been on Earth for 20 years. [...] Clearly they're capable of living on Earth for extended periods of time, but for the story I need them to have ruled out Earth as their home the moment they arrived. "

However, you seem to be ignoring their alien-ness and looking at things from a human perspective. I propose they're not capable of such feat, and there could be several reasons why this is so, I'll try to brainstorm some.

Living here would shorten their lifespan

The 20 years they've been here, in their perspective, is a very short time, and the ones who are on Earth are committing an act of self-sacrifice. It could be that oxygen/water/sunlight/some form of radiation common around the Sun's class of star, but uncommon around other classes (call your astrophysicist buddy, maybe he knows what it is) causes their bodies to deteriorate much faster than they're supposed to, and in less than a century, they will die a heroic death. A mere century, I say, for a species whose usual lifespan is measured in millenia!

If you need a motivation, think of humans who volunteer for disaster intervention in high risk areas. They know it'll shorten their lifespan and they'll do it for the greater good. Perhaps they're already very old. Or they're convicts who expect their lifespan will be shortened much less than their sentence.

Earth is almost perfect—if only it had some unobtainium...

Even though a small colony can live in Earth perfectly, their technology needs an energy source much larger than anything available on the Solar System. Perhaps they'd require a star with much higher energy output than the Sun, around which they can build their Dyson Sphere. Or perhaps their technology relies on some type of natural resource that isn't abundant on Earth. Maybe they're not even looking for a new planet to live in, just one to mine.

They're prospectors, Earth failed the test

Perhaps Earth was their initial candidate—or one in a string of planets they're visiting. And it failed some test. Perhaps they detected some natural catastrophe that would happen too soon to make the trip worth it, like the cooling of the core, or the death of the Sun, but they had to run a test in loco to be sure.

Maybe humanity didn't even exist before they came. Once they realised they couldn't colonise, they just seeded the planet with literal "bots", and our concept of history is part of the process: implanted memories necessary for the algorithm to run. Mere code.

Earth is too small

Maybe their species is so numerous that the planet's surface simply isn't enough. Perhaps their home planet didn't have so much water, so it could fit a lot more people comfortably (c'mon, over 70% of the surface? Who even needs that much water?).

Earth is too dry

Or maybe it's the opposite: Earth doesn't have enough water. How could they have guessed that less than 3% of the water would be fresh? Their most pessimistic models predicted 10%, this is unheard of, and the investment to desalinise this much water and bring everyone over is just untenable. Back to the drawing board.

They haven't actually been down here

We're talking about a species that can remotely harvest our brainpower for their own purposes. Why would they even need to come down here? Perhaps they have synthesised human-looking fleshbots that they can control remotely to do their bidding—like deploying the thought-harvesters and relaying the data back to the mothership.

2.They've already found a better place

You said it yourself:

Their system of FTL travel requires nearly perfect accuracy in terms of coordinates, so they're running simulations [...] before using up expensive resources to get there.

Maybe they already found a planet they want to go to, and it has more amenable climate, all the unobtainium they need, is just the right size, with just the right amount of water, and the right amount of hoomans. And much better location, you know what they say about the planet estate market.

But it's a very long haul, so they stop every now and then to recalibrate, just to be extra sure they won't end up in the middle of nowhere—where there isn't even a semi-rational civilisation like for them to run some code. How awful would that be, never getting to your new home because you didn't stop for directions and now you're lost? No sir, I'll take 20 years on Earth any day.

And as for why they don't simply stay, just think with me: if you were moving to live by the beach and made a stop in a sleazy motel along the way for a little shut-eye, would you suddenly decide to live there just because it has a pool? And what, ignore the cockroach infestation? If they do have an extra long lifetime, it becomes even more drastic. Ever stopped at a gas station for directions and just decided to live there? No? Figures.

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    $\begingroup$ +1 - raises some great points and goes in totally different directions than the other answers. You also anticipated a major plot point with your thought-harvester comment :) $\endgroup$ Commented Nov 24, 2019 at 2:46

The aliens may have decided that their time on Earth should be minimally invasive to humanity. Although they're tagging humans and using us to solve their distributed computing problems, for the most part they're allowing us to go about our day-to-day lives without undue interference. Obviously, colonizing our planet is going to be much, much more invasive and will probably make our existing lifestyle impossible in one way or another.

Some potential reasons they might want a minimally invasive Earth program:

  • Science. When humans study lower life in the wild, we generally try not to disrupt it too much. The alien scientific community may want to study us in our natural environment, or they may feel an obligation to leave the environment pristine for others who do.

  • Ethics. Whether they consider us lesser or not, there's no doubt that an invasive intervention on Earth would have profound effects on humans, and a lot of those effects could be negative. The aliens may feel ethically bound to leave us alone, or they may feel that too much interference in our affairs reflects badly on their own morality.

  • Isolationism. On the flip side, the aliens may be worried about our influence on them - whether that's the influence of barbarians bringing out the worst in the aliens, or the influence of free humans prompting a revolt among alien serfs, or any number of other potential threats. A small number of agents can be carefully chosen and inoculated to prevent any ideological threats; a large colony is much harder to manage.

  • Medicine. The aliens may be worried about cross-contamination between us and them, which could potentially introduce devastating diseases to one or both populations. Fewer contacts means fewer people to monitor and less equipment to sterilize, with fewer chances for mistakes.

  • Warfare. The aliens may consider that (whatever their stated motives) if they try to colonize Earth, it'll come down to a fight. They could probably win a war with humanity, but if it kills too many people and inflicts too much damage on the biosphere, it would all be for nothing.


Why go to science fiction, when there are real-world examples today, why humans don't want to go to the Moon or Mars for extended times?

Bone strength.

The alien's home is a 7g planet. They can live on 1g Earth for a while, but over time their bones get too weak, their blood vessels expand, and all the other ailments that humans have to recover from when they were too long on the ISS.

No, it is not an option for them to walk around with 16tons weights as their Earth-shoes to compensate :-)


A) The aliens are hypersensitive to gravitational changes, and find the moons slight gravitational effects (which can lift water, dont forget that) nauseating. So after the invasion, they discovered, that every half moon orbit, they conquered a planet that makes them vomit. They try to protect themselves in there ships, but if you are living in a ship anyway- whats the point of conquering a planet?

B) They where already here- they released a biological weapon to get everything go extinct (oxygen) - and due to a bureaucratic lapse, they come back to find a world, where there most toxic weapon has become vital. Basically- a species that needs nanotechnically produced mustard gas for survival and could spread everywhere if stirred. We are the dangerous ones.

  • $\begingroup$ Somewhat implausible (especially for a species that can withstand the considerable g-force changes required for space travel... but ok, maybe they somehow anesthesize themselves for launch etc. and only wake up in artificial gravity). Note that the change in gravity induced by the moon is actually really weak, about 1 part per million. It's just the vastness of the oceans that makes this small change quite noticeable to us. $\endgroup$ Commented Nov 25, 2019 at 12:16
  • $\begingroup$ Have you ever wondered why there are tides in the oceans, but not in the pond in your garden? If tides were caused by the Moon pulling water up towards itself, your pond would regularly jump up and down. Of course, the real explanation does have relation to the gravitational pull of the Moon, but not in any way relevant to the aliens in your answer :) The change in gravity from the Moon being overhead (or underfoot) is tiny compared to you just walking up a hill. $\endgroup$
    – Luaan
    Commented Nov 25, 2019 at 13:33

This was the result of First Contact

For whatever reason, these aliens do not want to invade, conquer, or otherwise subjugate us under their rule. However, they need our brainpower in order to move on. So our first contract with them resulted in a trade once the language barrier was resolved: They assist us in advancing our technology along reasonable lines, and we permit them to botnet a percentage of humans to help solve their FTL Math problem. Since us understanding enough of their tech to be able to mass produce it and them getting all the calculations perfect will take years, this is a mutually beneficial trade for all parties in the longer term.

In the short term, they can adapt to our planet through their alien technology -- tech that they have a limited amount of on their mothership and is onerous to mass produce on the Death World known as Earth. Or for that matter, on its moon not too far away (by space standards).

In the long term, they will be unable to stay planet-side. This could have potential consequences based on the plot you are envisioning.

This grand bit of (possibly unprecedented) cooperation could lead to a supercomputer based on human brains and alien computing technology that surpasses current computing from either race though on different metrics.


Just fast forward our current real-life state of Earth a little bit:

Pollution. The Earth is so full of plastics and trash and smog everywhere, and getting worse daily. Also the resources of Earth are too mostly depleted, making it not a viable target for mass colonization. (why expend all the FTL resources to get to place that is in same of even somewhat worse than your current homeworld?)

While it is quite possible for quite a few alien individuals to live there for extended periods of time, their life on Earth is not something they'd choose if they had any other options. You know, overall poverty, and cancirogens and other pollutants in the air and the water and food... Not to mention all those pesky humans doing collateral damages by waging wars etc.

  • $\begingroup$ Trash is just a resource. Remember, we're talking about an alien civilization that cannot live on Earth as-is, regardless of the minute differences caused by the human presence so far. We only have trash because the stuff in the trash isn't useful enough to warrant its reclaiming right now (look throughout history to see how often that kind of thing changes!), but compared to xenoforming the entire planet, it's peanuts. They probably can't tell the difference between grasses and our most durable plastics - it's pretty much the same thing from a chemical perspective. $\endgroup$
    – Luaan
    Commented Nov 25, 2019 at 13:30
  • $\begingroup$ @Luaan judging from the question, the aliens don't seem to be near advanced enough for xenoforming the entire planet (and/or they lack the time to wait out for xenoforming to finish). They are looking for planet that is already habitable for them. If they could just xenoform the entire planet quickly regardless of it's composition, they presumably wouldn't be looking for habitable planets at all. They'd just take the closest planet (probably one in their solar system, so they wouldn't even need the FTL) and xenoform that. $\endgroup$ Commented Nov 26, 2019 at 17:49
  • $\begingroup$ Which is exactly why you want a good reason why that isn't easy. Removing pollution is easy. Burning trash works just fine. It doesn't really hurt you unless you're close by, so you don't need to clean the entire planet at once. It's a trivial task for a star-faring civilisation. But we'll never fix Mars' gravity. Even terraforming a planet like Venus is orders of magnitude more effort than fixing whatever bad things we could possibly do to Earth. This has actually been a stumbling block for a story of mine - it's hard to come up with reasons to leave Earth, no matter how much we screw up. $\endgroup$
    – Luaan
    Commented Nov 26, 2019 at 20:01

Perhaps microbiological life is more rudimentary or non-existent on their home planet. When they arrive on Earth the microbiological biosphere becomes very apparent to them and they can’t compete with it.

Perhaps the aliens who do come down to Earth can survive but there are big issues with contamination that they have great difficulty in overcoming. Maybe a significant number of the aliens die from diseases and they spend a lot of their time and energy in studying bacteria and trying to figure out how to beat the bugs.

  • $\begingroup$ "Perhaps the aliens who do come down to Earth can survive but there are big issues with contamination that they have great difficulty in overcoming." That sounds suspiciously like "We From Arcturus"... $\endgroup$
    – Matthew
    Commented Nov 24, 2019 at 0:12
  • $\begingroup$ or war of the worlds even... $\endgroup$
    – Slarty
    Commented Nov 24, 2019 at 1:23
  • $\begingroup$ Well, sure, except for the "survival" bit 😉. The Arcturans don't die from exposure the way the WotW Martians do. $\endgroup$
    – Matthew
    Commented Nov 24, 2019 at 2:00

Earth makes them sterile

It's a variation on other answers, but something about Earth, its atmosphere, magnetic field, radiation, bacteria etc makes the aliens sterile.

So while a sacrificial few can live down on the surface for the sake of the mission with no other noticeable ill effects long term colonisation is out of the question.

These surface dwellers could be aliens who have already had offspring or a celebate caste etc.

  • $\begingroup$ I was going to suggest they discover some basic biologics are long-term carcinogens -- close enough to your answer that I won't post it separately $\endgroup$ Commented Nov 25, 2019 at 19:06

Answering the question "Why would most of the aliens live on a mothership orbiting Earth instead of on the surface?" from the OP, not the question "Why would Earth be long-term unsuitable for these aliens?" from the title.

Their ship might simply be more comfortable for them, with all their alien structures and alien luxuries. They might not be able to reproduce said luxuries on Earth because of:

  • A lack of materials (something that doesn't exist on Earth),
  • A lack of knowledge (ask an astronaut to build a spa and see what happens, then ask them to build a computer from scratch, no pre-manufactured parts),
  • A lack of incentive (why build the same thing down there, when we already have it up here and the commute is only 0.1 seconds?).

You might also want to explain why they don't land the ship.

  • The easiest answer is that it lacks atmospheric capabilities (e.g. its engines won't work in an atmosphere).
  • But you could also go with structural integrity deterioration due to extended exposure to gravity (unlikely if they only moved at sub-c since they would want to accelerate past 1g often, but with an FTL drive they might do 0.5g for a couple of hours then FTL all the way)
  • Or hull oxidization
  • Or a potential damage to the Earth's magnetic field due to the interference caused by the (inactive) FTL drive
  • Or it's for a fast getaway
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    $\begingroup$ There are innumerable reasons not to land a ship. Designing starships that can land is actually a major pain, even if your tech is advanced enough that the deep gravity well isn't a show-stopper on its own. There' are reasons why you rarely see starships make planetfall (aside from ships that land exactly once, anyway) after the finned rocket era (which was before we knew better). $\endgroup$
    – Matthew
    Commented Nov 24, 2019 at 0:19

The aliens evolved on a planet without a magnetic field, and Earth’s magnetic field makes them falling-down dizzy. They can manage temporarily by wearing equipment that senses and cancels Earth’s magnetic field in their immediate vicinity, but it’s bulky and imperfect and not a good solution for a long term colonist.


Solar flares

The problem is not Earth, but the neighborhood.

In the search for perfect place to settle, so much attention got on the planet side of things. The alien's sun has the most perfect, calm and stable sun -- and everyone's sun is like yours, right?

So it's come to a shock when they arrive and warp stop working and communications is unstable, all because all their computer technologies cannot work well in this crazy oscillating solar system.

Good thing we got these biological computing virus at hand.


Other posters here have come up with reasons not to colonize that are friendly to Earth's resident human population. I'm going to take a darker tack:

Earth is doomed

Why colonize a planet, if it's only going to be around for the next 1000 years?

  • Human scientists haven't figured out that the sun will go nova shortly, but the aliens have.
  • The aliens passed Planet 9 on the way into the solar system, and ran a few calculations. Humans haven't seen Planet 9 yet, because it's so far away, so small, and so dark, but it's getting closer...and will collide with and destroy Earth soon.
  • Arriving on Earth sent a signal to the aliens' implacable enemy, who will destroy the aliens if they can find them. The signal travels at lightspeed, so your aliens have a little time, enough to run their distributed computing project...but not enough to settle down and build defenses.*
  • In the history of galactic civilization, planets that have built more than three nuclear weapons inevitably end up sterilizing themselves in a global war a few centuries later. Your aliens don't expect us to be any exception.

Yes, these circumstances don't end well for the aliens' computing machinery, but as the aliens tell themselves: "The computers are just some dumb animals; they're not really people. Maybe, if we're feeling nice, we'll leave the humans a little note of warning right before departure."

*This plot device is borrowed from a very popular, very well-reviewed recent sci-fi series. I'm not going to say which one to minimize spoilers, but if you go with it, you may want to tweak it a little to seem less derivative.


I would think there are two reasons why the aliens have not colonised earth.

  1. While technologically superior, the aliens are biologically inferior to humans, and humans exist in much greater numbers. Meaning that there are equal chances of either losing a war, or destroying earth in the process, or winning. So they would rather not risk a confrontation.

  2. Even if they won a war, the aliens want to terraform the earth so they need to get rid of all man-made structures and lifeforms, that would take too long and frankly speaking, no one likes to clean up other people's mess. And the cleaning process might end up making the earth unusable.

The aliens are looking for a planet that holds life, but want to find a planet that is at a relative young developmental age so that they can perform the least terraform necessary and populate the planet with their own plants and lifeforms. Perhaps they are looking for a planet that is at the beginning of the Paleozoic Period so that they can properly seed the planet with the aliens own flora and fauna.

These aliens have not just left their planet themselves. I would assume their spaceship is a giant ark that carries the genetic data and material of their original homeworld. So it is reasonable to expect them to want to populate their new home with their stuff as well.

When they found earth they could not properly estimate the level of biological life and when they arrived they found out that the life forms on earth had advanced too much and could pose problems for them.

So why risk it? Just use the humans to calculate with greater accuracy their next destination and run simulations on what developmental period the new planet will be at.

If you ever get that book out please send me the title. It sound like an interesting story.


When the aliens "discovered" and calculated the habitability of earth (from x light years away) - even with FTL we can assume that travelling here took them years - if not hundreds of earth years. At that time earth was in our "medieval age" - total population probably around a low hundred million - no noteworthy military danger - intact ozone layer, clean environment.

After arriving the aliens detected that humanity made the planet not as desirable as it first looked to them (military strength noteworthy or even overwhelming, damaged ozone layer, thus higher risk of cancer and gene defects, heavily damaged environment with many near deadly poisonous places, nuclear technology in use, borderline overpopulation) - which lead to the conclusion of departing only an "expedition" force to install and maintain the described humanoid botnet.


They are altruistic

They recognize that life deserves a chance to thrive, and an intelligent species deserves the right to choose its own path. Humanity is still a fledgling race, so the aliens do not intervene. The computations done subconsciously don't affect our genome, so have no long-lasting negative consequences. Their calculations are for a place without intelligent life, but with ample resources, so that they can continue to thrive.

They have, from our point of view, mastered and entirely embraced empathy. 'Do unto others as you would have others do unto you' is actually an alien saying.

  • $\begingroup$ That, and they've watched enough of out movies and read enough of our history books to know better than to ask us if they can stay and live in peaceful co-existence. $\endgroup$
    – Nosajimiki
    Commented Nov 25, 2019 at 22:08

Earth was never the final destination

It was designated for a stopover, for refueling, servicing, or recuperation.

This is continuing and inspired by the kadu's answer, but adding quite enough ideas to be a separate answer.

Main condition

Perhaps FTL travel technology allows travelling only a limited distance. And even with FTL travel, the travel may be so long that the whole distance can't be made in one jump, or even in a single generation.

Some conditions that might force a stopover

  • the spaceship itself might need servicing that can't be done in flight (or perhaps the hyperspace drive can make just one just a single jump, and needs a complete replacement afterwards?),
  • even vitrified bodies can't reliably survive the harsh conditions inside the FTL travelling spaceship for long enough,
  • reliability of repeated vitrification is orders of magnitudes worse, so aliens need to procreate and send only the next generation to their actual destination,
  • that's given that vitrification is possible at all. If it's not, then it's a much bigger obstacle. Perhaps even short travel may take a huge toll on individual's health and put a hard limit the distance that one can travel in the lifetime, and this limit may be too short to reach the final destination,
  • kadu's original suggestion: recalibration is necessary to get to destination within reasonable distance,
  • possible explanation connecting above points: the precision of FTL travel is quite bad: you can end up several light-months from you destination, and if there are any bigger celestial bodies around, you are relatively likely to hit them (cause gravitation something something), so you need to aim quite far into the empty space on the edge of the system, and so on each jump you expect many years of conventional travel before reaching the habitable planet inside.

These conditions could compel aliens to colonize Earth as the place to recuperate after the first part of the journey, even though the environment is far from perfect, and the planet is way too small.

Alien representation currently on Earth

It may be that the crew at the Earth and the number of travelers present at any time are actually a small number, while the rest of the species either still preparing for departure on the home planet, or in transit. To add some spice: it's possible that to make the whole operation feasible, the spaceships need to come to Earth in intervals relatively short to the travel time and the light-distance from the home world, so while the initial crew prepares the place and bootstraps the calibrations for the next jump, most or all the rest of the migration spaceships are already in transit, ready to arrive every few months/years for the next few centuries of even millenia.

Aliens lifespan and telepathic abilities

While some answers suggest that the aliens are mighty individuals of very long lifetime, I'd suggest just the other direction: aliens are actually rather weak and short-lived individually. However, they possess some sort of shared or telepathic mind (allowing them to effectively transfer knowledge and experience between generations, and advance quickly as a civilization). They do that naturally or with relatively simple technological aid. That's why they used their collective mind as supercomputer, and that's why they chose to use humans (because transferring enough of their own folk in the first wave would be infeasible). This means that likely the technology they need to access people's minds is also pretty simple.

The short lifespan usually correlates with smaller body size. This might allow Nomes... sorry, the aliens, to easily sneak between humans undiscovered. And instead of disguising themselves as humans, they might just slightly manipulate some people, either completely taking control of them, or (more likely to me) simply planting (incepting?) in their heads the necessary ideas. Or, you know, talking to theirs ears veeeeryy slooooooowly.

The short lifespan plays well with the idea of stopovers forcing travel to be multi-generation due to perishable nature of the travelers. Perhaps neither the first generation of travelers nor their children born on the way to Earth can make it safely to the Promised Land, and only their children born on Earth will go.


There are a number of factors that could make Earth not suitable to be their long term home.

  1. Gravity - too low or too high could cause significant biological changes over time to the point where they don't consider themselves the same specie any more. That could be a very high psychological / ethical threshold to cross and might be enough to send them packing.

  2. The Sun is G2V type of start. Most stars are smaller and dimmer (they are called red dwarfs) and will produce light in different wavelength. Thus Earth can be too hot, too bright, or they might not be able to see very well due to the wavelength.

  3. Red dwarfs are highly variable - they can develop flares or spots that significantly change their brightness. A life form that evolved around such a star might have biological functions that depend on that kind of variability, so while they can survive on Earth, long term, they might not be able to live here.

  4. The biology might be hostile - there is tons of pollen, virus, bacteria, etc here on Earth. If their sun put out more ultraviolet light they might've evolved in much more sterile environment. While it seems unlikely that they would be affected by any of Earth's pathogens that's been a common trope.

  5. Finally there might be cultural reasons. They want to recreate "paradise" as it existed on their home planet so their new home has to match exactly in terms of climate, type of star, gravity, etc. This could be anything - the moon is too big, they want 2 smaller moons, they want rings, etc, etc.

That last point might sound unreasonable but if you are powerful alien race with FTL and you lived on the planet Mul from Valerian, would you settle for Earth?


Because they need the humans as they are

What if the aliens realize that using human subconscious makes for the best computational device they could ever develop? The human brain is particularly efficient in running the computational models that these aliens have invented, to the point that none of their computers would ever be able to outperform it.
In this case, it would be counterproductive to try to colonize Earth. This would require to fight or anyway reduce the human population of the planet to make room for them, but this would also disrupt the computation power that they have available.
In this case, a wiser approach would be to keep the status quo, keeping a secret permanent base in the nearby, and exploit the mankind-processor to discover even more habitable planets or new technologies, in order to maximize the spreading and well-being of their species


The program included in humanity has, in reality, infected all of it.
In fact, it was included in some of humanity precursors, hence the missing link that will never be found.
The aliens know that.
They just don't understand how a though-calculator that render its victims somewhat unpredictable and violence-prone could create such a fast-evolution specie (their point of view). And, given the actual state of things, how this specie is still alive at this point. But that's cool for them.

The problem is : humanity is deemed too crazy to subjugate or for a neighboring settlement.

They can be talked to.
They can be bartered with.
Some aliens can emigrate.
But colonization would be answered in both a costly (for the aliens) and probably auto-destructive (for the humans) way.
The aliens don't really need Earth, they mainly need the computation result. No reason to linger any more than necessary.
They'll just need to get a couple corpse when the computer goes ding to read the result in their brain, H2G2 style.


Long story short:

From their home planet, the aliens can only determine if a star system contains a planet with liquid water and carbon occurence, but they can't see if the planet is really suitable for colonization. When the aliens hide from humans, they can live on earth for a long time, but since humans are a pretty aggressive species, the aliens wouldn't survive a colonization. So they move on to the next star.

Detailed story:

Like most living beings, the aliens have a survival instinct. They want to have a "backup" of them, so they start several spaceships from their home planet to the nearest star systems. Their technology is only a few hundred years ahead of human technology, so they share some of the problems that humans have with interstellar travel. FTL travelling isn't possible, cryogenic tanks are science fiction or can at least not be used for hybernating an adult alien for several years and transporting living aliens over several generations isn't possible, too (they can't produce enough food for all aliens and the risk of one alien going crazy and destroying the ship during flight is too high).

Fortunately, the aliens got an idea on how to avoid those problems. They simply don't transport living beings on their spaceships. The spaceships are automated transporters that contain storage devices in which the DNA of the alien flora and fauna is stored, alongside many books, videos, websites etc. from the alien planet. Furthermore the aliens edited their DNA to become more intelligent, peaceful and able to settle on planets that are not exactly like their home planet. For example they can withstand wide varieties of gravitational forces, temperatures, poisons etc. They not even need a mother to breed. They edited their DNA so they can reproduce like a cross of a jellyfish and a mushroom. They only need to plant a pollen in a flat riverbed, where it begins to grow to some kind of spawn that extracts nutrients from the earth and can breed the aliens. After four months they are ready to be born.

So this is what happened: The aliens started 16 spaceships from their home planet. One of them with the earth as destination. The spaceship is a small automated transporter controlled by a simple artificial intelligence. It only measures around twice the size of a Boeing 747 airplane. When it arrived in earth orbit, it launched three landing vehicles that were distributed evenly over the earths surface. After that, the spaceship proceeded to the Lagrange point between mercury and sun to fill up its antimatter tanks (what takes a few decades).

The landing vehicles successfully landed in the Gila National Forest in New Mexico, the Black Forest in Germany and the Bureya Nature Reserve in Russia. The vehicles contained everything that is needed to raise 8 aliens per ship. For example some robots that are able to plant the alien seeds in nearby riverbeds, carry the alien babies to the landing vehicle and prepare food, some toys, entertainment and learning devices, VR-nanny projectors and so on.

The aliens grew up in the landing vehicles and the surrounding forests. They learned very fast and with the age of three years they started to research which species are living on the planet. They soon found out that there are lots of humans around and that they are using radio broadcast. The aliens were able to receive the broadcast signals and soon understood the language of the humans. They learned about society, and that in the human calendar they seem to be in the year 1932. They also learned that there just recently has been a world war, so humans seem to be aggressive and they have weapons that could also kill the aliens. The aliens decided not to stay on earth and that they will use humans for their "wetware botnet".

The aliens started using their landing vehicles to capture humans. Since they didn't know much about human biology, the anesthetic didn't work as expected and some humans were still conscious when the brain interface was installed. The interface chips were installed on the backside of the tailbone, because the nerves can be accessed easily there. That's why some of the conscious humans reported of having had a rectal examination by the aliens.

The botnet is not only usable for finding the next star to head to. The observations needed for that could easily be made by computer controlled telescopes. The main reason for the human botnet is their problem solving capability. During the flight from the home planet to earth, the spaceship got hit by many micrometeorites. Some of the repairs can be done automatically but some need the ability to find different solutions. The connected humans can easily do the remote controlled repairs while sleeping, but there was a problem. Since the humans were connected to the alien-Wikipedia, they started to dream of alien technology. Most humans weren't able to remember their dreams, but a few started to research their dreams. In the USA, scientists started to build jet engines. In Germany, Konrad Zuse built a first programmable binary computer.

Then things started to get out of hand. The germans started another world war and soon found the alien landing vehicle that landed in the Black Forest. They killed the aliens, took the ship to a military base and tried to figure out how it works. They started the "Haunebu" project which stands for "Halbautomatische Untersuchung natürlicher Eigenschaften bemannter Untertassen" (half-automatic research of natural properties of manned saucers). They also tried to gather informations about alien tech from the VR-nanny projectors, which they called "Vril" ("virtuelle rechnerinduzierte Lehrerinnen", virtual computer-induced teachers). There are rumors, that they even bypassed the controls and flew those saucers by hand, what didn't work well (same as if you tried to fly a Joint Strike Fighter by manually tilting the control surfaces).

The aliens began to become nervous. They were 24 and now a third of them were killed. The spaceship wasn't fully fueled up yet and only the repairs that could be done remote controlled were made. They still had to build some spare parts on earth and install them in the spaceship. The aliens disconnected the human interfaces, but since one of the landing devices was found and partly disassembled, it wasn't remote controlable anymore. So the interface to the alien database still existed near the german landing device. It didn't take long for the german scientist Otto Hahn to dream about nuclear fission. He started to research that topic and gave lectures in the princeton university what lead to the Manhattan Project and later to the drop of nuclear bombs over Hiroshima and Nagasaki.

The aliens couldn't hide anymore. All the years they lived in remote forests but now they had to stop the humans and try their best to hinder humans from nuking the whole planet. In russia, the alien Bō Prad (known as "Skinny Bob") revealed himself to the KGB and offered Boris Jelzin informations about space travel in exchange for the promise to not launch a nuclear rocket.

The next years went better. The war was over, the disassembled landing vehicle was repaired to a state that it could be remote controlled. Since it wasn't save to use for a flight to the main ship, the aliens intentionally crashed it at a remote location near the south pole.

Sadly, one of the New Mexico aliens suffered a stroke while flying over Roswell and crashed. The aliens only have half of the human lifespan, so a 28 year old alien has to expect such illnesses. Since only one of landing vehicles was left and the aliens didn't trust humans enough to commission them to build more landing vehicles (that would mean, that they had to grant humans access to the knowledge of how to build and control an anti-gravity device. The humans would most likely use it for another war.), they repaired the main ship, attached the remaining landing vehicle to it and launched it to the next promising star system.

The earth was very promising but the humans are too dangerous. Since the aliens are a peaceful race, they wouldn't kill all humans. They already have a bad conscience because they frightened some humans with their failed anesthetic. They would have liked to live in a peaceful coexistence but that didn't work out.

The aliens that remained on the earth, helped humans to build better computers and advised them to build a global network of university computers in the future, so humans will get easier access to knowledge. Some of them wrote letters to science fiction authors, to give them some inspiration for their books. The last one of them died 47 years ago in November of 1972. Rest in peace.


Alien's biology is very different from Earth lifeforms.

Their disguise is a very complex process requiring a different biological body, similar to the premise in "Avatar". They can't live on Earth, and they don't envy human bodies either, so it's better for them to stay isolated.


Our environment is lethal to them. Too hot? Too cold? Oxygen as one poster suggested? Some pollen or the like, it doesn't matter.

The ones here running things are in MOPP-5 (chemical warfare suits) or even space suits any time they aren't in their base.

  • $\begingroup$ This is a bit to broad, methinks. There's plenty of environmental issues they could fix relatively easily (compared to another transstellar journey). Maybe they wanted to fix all those issues by influencing humans to build an industrial civilization to xenoform Earth for them, and they have been foiled in their evil plans by a band of renegades or the humans realizing what they're doing :P $\endgroup$
    – Luaan
    Commented Nov 25, 2019 at 13:38
  • $\begingroup$ @Luaan It was meant to be broad. The basic idea is that they can't live here without major protective gear. They're after a what is for them a shirtsleeve environment. $\endgroup$ Commented Nov 25, 2019 at 16:31

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