In my novel, an alien species has defeated Earth and its sister planets, but rather than outright slaughter, subjugation or slavery, they want to remove all (or effectively all) Humans from Earth so that they can have a healthy planet to their own ends. The end goal is to remove 99% of Humans and relocate them to Mars or nearby colonies like Ceres, Europa and Titan.

How would this be feasible? In the end, any displacement and relocation of mass amounts of people is going to result in all sorts of catastrophes, but the alien victors do think of themselves as merciful and ethical. Would Humans be gathered in open spaces beside large population areas and shuttled off-world in massive haulers? Would space-elevators and a staggered removal over a year or two be more effective?

For the sake of this question, the aliens have highly-advanced starships which can enter a planetary atmosphere if need be, but no FTL travel and no teleportation.

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    $\begingroup$ You'll need to define "ethical." I'd argue that forcibly depriving people of their property and liberties falls under "unethical." History tends to agree. en.wikipedia.org/wiki/Trail_of_Tears $\endgroup$ – Ranger Jun 20 '19 at 17:20
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    $\begingroup$ Viewing oneself as merciful and ethical has no relation whatsoever to the victim's opinion, or to an outside observer's opinion. Andrew Jackson thought he was doing wonderful things. His human slaves and the thousands of tribesmen who died from his actions might see things differently. Your aliens need simply delude themselves. $\endgroup$ – user535733 Jun 20 '19 at 17:23
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    $\begingroup$ The human ego, thinking our planet is specially valuable to lifeforms capable of interstellar travel. Also, moving humans to anywhere but Earth is probably going to kill them, so why not just do it? $\endgroup$ – Muuski Jun 20 '19 at 17:23
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    $\begingroup$ The answer is "any way you like." You didn't tell much about aliens' tech level. Maybe they need to round up all humans and herd them to transport ships. Maybe they can lift humans from Earth with levitation tractor beams. Maybe they can place humans in stasis during this process. Maybe they will place humans in stasis indefinitely, thinking that this is the most humane approach. $\endgroup$ – Alexander Jun 20 '19 at 17:25
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    $\begingroup$ Sorry for any lack of clarity - the aliens are definitely NOT ethical in most regards, they simply believe themselves so. Much like Jackson and other colonizers. The comparison isn't off target. $\endgroup$ – Kabob Maraca Jun 20 '19 at 17:34

The only "ethical" solution is if the aliens are actually trying to help us to begin with.

Let's assume the Earth is experiencing a runaway greenhouse effect that was already killing us, but the aliens thrive on worlds that are hotter than modern Earth. They have the technology to warm a planet up and build up it's atmosphere, but not to go in the other direction to be able to fix what we have done. When the aliens come, they see what we are doing to ourselves and try to help by terraforming other planets in our solar system to make nice cosy human friendly colonies. Not only did they not come to harm us, but they really like us. We're like a bunch of cute little kittens to them with our little fuzzy heads, and watching us play with our toys (like internal combustion engines and computers) brings them great joy.

Once the new worlds are ready, the aliens decide it's time to relocate us for our own good. The aliens understand that removing humans from our homeworld is a lot like trying to get a stray cat out of a tree. It's dangerous work, they might get bitten, but in the end, it's the right thing to do for everyone involved. They disable our defences with EMPs and then use tasers and sedatives to round us up into internment ships. They have the tech to vaporize us all from orbit, but their goal is simply to disarm us while doing as little harm as they can. What we see as an unfair war, they see as a mission of mercy.

When all is said and done, they save the human race, and the Earth with its average temperature now above human tolerance levels makes a cozy new world for the aliens to begin developing.

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    $\begingroup$ This is along the lines of The Arrival (1996). The aliens want hot planets, saw what we were doing to ours, and decided to help the whole process along. They knew they were killing humans - and I'm sure they thought what they were doing was ethical. After all, considering what we were (are) doing to our planet, we obviously don't want it, right? +1. $\endgroup$ – JBH Jun 21 '19 at 2:22

How quickly do the aliens need to get rid of humans? The simplest solution, which has been exercised in works of fiction like Half Life 2, is to inhibit human reproduction, either through sterilization or the potentially more peaceful route of simply chemically inhibiting the urge to reproduce. This gives you a pretty good guarantee that all humans, or the percentage that you want, will be gone within the century. Of course this solution is not ethical, but I think it will be far less disruptive than forced relocations. Forced relocations have been tried in human history, and they have historically gone extremely poorly for those relocated. We often underestimate how much our culture, our social and economic order, and our mental stability are tied to the land.

If the aliens are advanced enough to try and do this ethically, they’ll probably have to give up on relocating the current crop of humans. They’ll have to start educating and training the young humans and the newly born after their takeover that one day they will all be taken to somewhere else. For those who expected a whole life on Earth, the move will simply be too traumatic, but by preparing a new generation from a young age or from birth, and waiting for the generations with animosity from the war and attachment to Earth to die naturally, the aliens can manage expectations and exert their will over a populace who have only ever known this reality. This of course will require a long and sophisticated indoctrination process and a long wait for the old humans to die, but again should be accomplished within one century. These two strategies could be combined, producing a smaller number of new humans to move offworld.

The specific mode of transport becomes less important, probably whatever is will simply be what is most efficient, as the problem is now much more logistical rather than one based in conflict and ethics.


TL;DR: with the best will in the world and a totally willing populace, its probably gonna take you decades to move everyone from earth. You can't just build new sustainable environments on barren lifeless radiation swept balls of rock for 8 billion people overnight, and you won't be picking all those people up and moving them to their new homes at once.

Even assuming the feasibility of sublight spacetravel and wars of conquest, your aliens are going to have to be happy building infrastructure for angry apes for years, and years, and years.

Honestly, I expect they'll get sick of the whole thing and just kill everyone.

Would Humans be gathered in open spaces beside large population areas and shuttled off-world in massive haulers? Would space-elevators and a staggered removal over a year or two be more effective?

The principle problems are ones of scale. Even if everyone on earth were super enthusiastic about leaving, the logistics of the whole enterprise will be unbelievably difficult.

Lets say that there are 8 billion people on the planet. If you could evacuate ~22 million people per day, it would still take a year to get rid of them all, and that doesn't even take into account population growth. Probably the most efficient way to get people into space is via space elevator. If you build 20 of them, and you shifted 1.1 million people per day per elevator, you'd need to be throwing 12-13 people per second up the cable and in to space.

A million people a day is the size of the commuter traffic of a pretty sizeable city, so it is achievable with modern technology... oh, but city commuters generally come from satellite towns and suburbs, but the catchment area for each space elevator has commutes of thousands of kilometres. You're going to need to build mass transit systems to funnel people in from all over the place to your departure points.

At the top of each elevator, you'll need a way station that is capable of supporting a throughput of over a million people per day. Each of those people will need feeding, and housing. They'll need to be entertained. They'll need healthcare, and all the usual paraphernalia of civilisation... don't forget police, and jails, and criminal justice. Babies will be born at the waystations, every day, and people will die up there, every day.

Next, for each of those waystations, you need to load those million people into spacecraft and launch them to their destinations. Atlanta international airport had 107394029 people moved through it in 2018, with over a thousand flights a day. Your spaceports will be more than 3 times busier, and you'll have 20 of them. ATL has 63000 people working in it by the way, and it has ready access to the rest of the civil infrastructure of Georgia.

When you've staffed these waystations, and solved the logistic issues of the continuous movement of people through them, congratulations! You now need infrastructure that is equally as capable at the receiving end. But what about all that space in between? You've just thrown 22 million people out into the big black. Even with super ultra futuristic handwavium alien space rockets, they're going to be spending a few days out there. You'll need to house, feed, entertain and protect them during their flights. Your ships will need fuelling and servicing. Did the aliens bring enough capacity for 22 million humans a day? You'll need a shipbuilding industry.

You might not need actual space elevators (though they would be vastly easier to build anywhere in the solar system other than venus and earth) but you can't just throw a million people down in an aerobraking lander every day, unless those landers are in fact completely self-contained and ready-to-go towns for their occupants.

So now you've made a bunch of new towns, all over the solar system, every day. Great! What are those people going to eat? What are they going to drink? What are they going to do? How are they going to get power for all of the above? Hell, what are they going to breathe? Think quick, there's another few million people on the way!

So, seems crazy trying to do it in a year, doesn't it? Well, you say, maybe you could do it in ten. Only 800 million people per year! What's that, the whole of the conteinental US in about 5 months? Seems pretty easy by comparison. Now you only have to deal with a couple of million people a day. Surely that's straightfoward, just creating a brand new empty city and populating it every 24 hours. Hardly anyone need die on the mass transit systems which I'm sure will be super reliable alien technology.

They will be super reliable alien technology right?

Wait, have these people travelled using sublight spacecraft, for years, to defeat us in combat, just to spend a decade or two building trains and space elevators all over the planet they've just spent all this mindboggling amount of effort to acquire?

One wonder what on earth they're getting out of all this.


Let them choose to leave. A simple way of convincing yourself of being ethical in depopulating the Earth is letting people choose whether they want to leave or not. Create a somewhat better place for them on Mars and then launch a large marketing campaign asking people to sign up to move to Mars. If your campaign is good enough many people will leave. That could cause some problems on Earth, so you could campaign again that Mars is better and everybody should go (adding that their relatives are waiting for them there, and so on). After a few decades of this you should remain with only a few stubborn communities on earth, I would say with a population of a million or so. Moving those people would be ethically hard to justify, but waiting a few centuries might decreas the size of that population. On the other end many people will want to go home to Earth. To be reasonably ethical you should make that process hard, with multiple years waiting for the trip, but not impossible. If you achieve this you might seem correct and reasonably moral and ethical, although many might argue that you were not for various reasons (for example your marketing campaign contained lies, some people did not get what they hoped for when they chose this, you knowingly created a global crisis on Earth which forced more people to "choose" what you wanted them to).

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    $\begingroup$ @KabobMaraca I think this is a good idea. But if the aliens are making another place even better (for humans), then you'd want to explain why the aliens really specifically want the Earth enough to go through the trouble. $\endgroup$ – Xplodotron Jun 20 '19 at 19:46
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    $\begingroup$ @Xplodotron, "better" is relative. If we offered 21st century lower middle class accomodations and amenities to royalty from a few centuries ago, they would probably take it and think that they were getting the best part of the deal. Indoor plumbing trumps jewels and cable-tv is better than even the best minstrel. If our alien conquerors are even just a few hundred years ahead of us technologically, then their table scraps are going to look better than our banquets. They want Earth because of what they can turn it into. Not for what we have been able to do with the place. $\endgroup$ – Henry Taylor Jun 20 '19 at 21:07
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    $\begingroup$ @HenryTaylor No I just meant that you'd have to explain why they are working to make another planet "better" and then move the humans there so they can take the Earth, instead of just making that other planet better for them. Like Nosajimiki had a great reason, which is that the Earth is too hot for humans but just right for them. That makes sense for altruistic aliens to go through the trouble. $\endgroup$ – Xplodotron Jun 21 '19 at 16:31
  • $\begingroup$ @Xplodotron, I work with wood as well as words and worlds. In woodworking, the skill of the carpenter affects the end product as much as the quality of the raw materials. If we consider Earth and the future human homeworld to both be the raw material, the inferior world (not-earth) should go to the inferior carpenters (us), so that the superior world (earth) can become even greater in the hands of the more skilled carpenters (the aliens). $\endgroup$ – Henry Taylor Jun 21 '19 at 17:08

Like botava above said, make them want to leave. I just want to add that the "carrot" he describes should be supplemented by a "stick".

Aliens could make Earth less pleasant to live on. Military approach is very crude (look up Holocaust). Spreading a disease is more subtle, but can backfire if discovered (and it will be discovered with so many curious and educated humans). Best solution is to make it look like earth is running out of resources, choking in pollution, being wrecked by changing climate, etc. They can manipulate media, force people into crowded reservations, and maybe even secretly cause some of environmental calamities.

And, at the same time, promote new planets as a world of opportunity (like America was in the 19th and 20th centuries). Let the first relocated settlers live as kings, and send back stories of unlimited space and resources, advanced alien technology, even freedom from oppressive elites of old Earth.

PS As Cumehtar said in his comment, if your aliens have technology to let large number of humans live on Mars Ceres, Europa and Titan, they do not really need Earth. They can build a habitat for themselves on those planets. But you can assume they want Earth for some sentimental reason like eco-tourism.

  • $\begingroup$ How exactly is making it look like or making Earth suffer from resource scarcity less likely to be a disaster than a discovered plague? People will be upset about any manipulation if it is discovered. $\endgroup$ – TheDyingOfLight Jun 20 '19 at 19:39
  • $\begingroup$ You have a point. If aliens engineer a virus to look like a natural mutation of native virus, it will be very hard to trace. I was thinking more along the lines of using a purely alien organism to infect people. Likewise, environmental intervention can be more or less obvious. Feel free to edit my answer (i gotta go) $\endgroup$ – Bald Bear Jun 20 '19 at 20:30

First, one have to wonder just how ethical any sentient species is if they still engage in warfare. After all, war does involve killing lots of people, & the destruction of property & wealth, both of which leads to chaos & misery.

Second, one would have to think in terms of this non-human species. For example, their civilization might be highly regimented, & commands from the superior ranks or castes is usually obeyed without question. (Or at least the form of command given to us humans to leave earth is never disobeyed.) Finding that even a defeated & demoralized humankind is not eager to leave Earth would be baffling to them as they could not imagine such resistance.

Which leads to the simplest solution for this forced evacuation of Earth: this victorious alien race simply orders everyone who is not otherwise identified to remain on earth to leave. A system is set up where on specified days the various humans are to board transport units at specified places. (These transport units are provided by the aliens.) The emigrants would be allowed to take with them some amount of personal belongings. (Say 100 pounds, or 50 kg., or 144 alien weight units of stuff; the rest would be destroyed in order to make room for the Rehabilitated Earth.) This alone would remove a large share of humans from Earth.

(Note: I'm side-stepping the issue of technology, which may be the actual point of your question: how to physically transport billions of people from Earth's gravity well to the gravity well of another planet. The only good answer I could offer is to use the technology that enabled them to defeat us Earthlings. If you don't have an idea for that, then you're not going to write a plausible story to begin with.)

So how are the rest of the identified forced immigrants removed? As Botava & Bald Bear note above, the aliens would use a carrot & stick strategy. However, what these aliens consider to be "a carrot" & "a stick" is most likely not what we humans would consider to be "a carrot" & "a stick". In any case, this would lead to a few more emigrating.

Now we come to the point where force is needed. It makes logical sense to start with the large urban centers that survived the war. Cut off their power & water -- if that hasn't been done before this point -- & send units of aliens & human collaborators thru these urban centers to systematically empty the buildings. (Of course, some locals will evade these units, but 99+% success would likely be acceptable.)

Next come the smaller urban centers. Repeat as above.

Last are the rural areas. What I would do is to section off these areas so that the inhabitants of one couldn't escape to another, & systematically empty each sector. Of course, the aliens being aliens will come from a different mindset & might come up with a different strategy. (Maybe they started with the rural areas, penning the remaining humans into the urban areas, which they forcibly emptied one by one.)

At the end of this exercise -- which would conceivably take years -- well over 95% of Earth's population would be removed. The last percentage would need to be removed thru various stratagems that can include (1) identifying all humans permitted to remain on Earth, & those found without proper authorization are summarily transported off-planet; & (2) hunts for groups of humans living without authorization on Earth.

Of course, all of this assumes this exercise proceeds without any problems, that all of the mechanical stuff works flawlessly, & this disruption does not lead to any unintentioned famines or epidemics.

In short, one could write a novel on how this was done alone. Based on this, if I were in your place I'd work this part out first because of the effects it would have on your novel.

Good luck.


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