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Like our good SciFi prophet Isaac Arthur postulates, for a civilization with a decent space presence planets are overrated and rubbish.

  1. They're down there in the deep gravity well.
  2. There is no resources on them that can't be reached far easier on asteroids or tiny planetary bodies, or be made up in space via factories or 3d printers (and nano-assemblers if need be) from raw materials mined in asteroids.
  3. There is no environments there that can't be simulated to convincing enough degree on a large O'Neil station habitat with bonuses of being predictable and completely controllable (Gravity, light cycle, geomagnetic fields, climate, atmosphere, and so on).
  4. A cloud of space station habitats could easily provide living space several million times larger than the surface area of a planet, while using just a very tiny fraction of its mass for the construction.
  5. And even appeal to "we like it better that way" would work only for that generation who were born on planets to begin with - for those born on space station or space ship and who lived all their life there, it would be unnatural and weird to live under an open sky.

Not to mention that in a realistic scenario only one planet is truly habitable for humans, and even those other planets that have biospheres of their own would require decades or centuries of terraforming efforts to make them comfortable for our species. For planets that don't, those terraforming projects would last millennia instead.

Yet, there is a huge human presence in space across many solar systems with habitable or not so much planets in them (setting with a decent but not dirt-cheap or ultra-fast FTL), and these humans decided to not convert these solar systems into swarms of rotating habitat modules with population of trillions, and appear to have at least 40% of their population be living down the gravity wells, for reasons that aren't just "we like planets". What those reasons could be?

n.b.: technically there's not only humans but all sorts of aliens as well, but I'd guess that any reason valid for humans I could apply to the aliens more or less effortlessly as well.

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    $\begingroup$ "There is no resources on them that can't be reached far easier on asteroids or tiny planetary bodies" - the logistics of retrieving everything that we need from different asteroids is very challenging. $\endgroup$ – Alexander Dec 17 '20 at 19:58
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    $\begingroup$ if we need to lift all of that material. If I understand correctly, we are comparing self-sufficient planets against space habitats here. $\endgroup$ – Alexander Dec 17 '20 at 21:04
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    $\begingroup$ Try putting a decent mountain range in your O'Neill cylinder. Better to ask why anyone would want to live permanently in one if there's a halfway decent planet anywhere nearby. $\endgroup$ – jamesqf Dec 18 '20 at 3:23
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    $\begingroup$ Humans are quite capable of living in spacious mansions with swimming pools, so why do so many choose to live in slums? $\endgroup$ – Andy Newman Dec 18 '20 at 21:35
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    $\begingroup$ "They're down there in the deep gravity well" - how is that still a problem when you have solved interstellar travel? $\endgroup$ – Aetol Dec 19 '20 at 23:53

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Stability

Nothing beats the radiation shielding of 1,000 kilometers of magnetic field + 100 kilometers of air + (at night) 6,000 kilometers of water and rock.

For heat, nothing beats 22 billion cubic kilometers of atmosphere to dissipate, by whatever means necessary, heat concentrations. You can have 1 terajoule nuclear blasts, or multi-terajoule meteor airbursts, and life goes on with barely a blip (unless you are immediately at the location being affected).

You have 24.5 million billion kilograms (pentillion) of glacial heat reservoirs that can sink 24.5 hexillion joules of excess heat, and keep the mean temperature pleasant.

Your oxygen and water is recycled automatically by the biosphere. The oxygen quality, with very few exceptions, is stable. Same for the water.

Infrastructure

With a lot of your life support taken care of you, nothing beats how many people you can cram into a cubic kilometer of earth. You can stack people horizontally, on the roof, or beneath the surface. You can do this with materials as flimsy as particle board. No need for structures that can hold atmospheric pressure, fire + leak detectors, or maintenance kits. In the 2,600 square feet (240 m²) of the International Space Station (hosts 6 people), planetside you could easily fit 50.

Likewise, you have to travel a few tenths of an astronomical unit to pick up a body of water. Planetside, it falls from the sky, or you can pump it up from a well -- it is where you are.

Need minerals? They are often laying on the ground, just begging to be picked up and put to use.

Need biologically engineered construction materials? "Wood"™ just grows right next to you. It might drop bits of bio-engineered hardware on your roof, or you can collect in bulk with a saw.

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Unless I'm living underwater, if a large hole appears in the side of my home due to some incident, my immediate emergency response is put on warm clothing. My second is to cover the hole with anything that might be handy: piece of wood, a piece of plastic, just enough to keep the heat in and weather out. It might be uncomfortable until more permanent repairs are made, but I'm not going to die of minor things like rapid depressurization, asphyxiation, falling into an endless void until the life support runs out...

Heck, in many place or parts of the year, I might not even have to be in any rush to fix the hole and decide to take care of it in the morning.

Or if there's a fire, well, I can run outside buck naked and survive until help comes. In certain places, this might only be a few minutes because of cold, but in others, you might survive for quite some time until assistance arrives.

People vastly underestimate how much life support and safety they take for granted by living on an earthlike planet.

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Planets are fail-safe.

A space colony has to be operated and maintained by competent and responsible people. Most people aren't and this is not entirely bad (we need the other kind of people to make innovations here and there or to run the administration). Well, a planet can be screwed up as well, but it takes a lot of effort. A space colony can be destroyed in a hearthbeat.

And where do you evacuate a failing space colony? All of them are expensive as hell so they are at capacity. Building a spare one is long and expensive as well, keeping a separate one "just in case" for an insurance-like model is a logistical nightmare.

Planets are sustainable.

You don't leak atmosphere on a planet. Everything you waste is waiting for a technology or economy to improve and becomes a resource. A planet has an ecosystem and a water cycle that recycle most of the stuff by itself.

Planets are big

Bilions of people can live on one of them, benefitting from network effects, urbanisation, social, cultural, political and genetic diversity. A large economy can power research, education, military and other kinds of development. This makes planets competitive - just like a large city or a country is competitive against the small ones.

Most people are OK with the gravity well and they can contribute to the economy and other things just as well from down under.

Making a space colony for even a milion people is an engineering and safety nightmare.

Planets are cheap

You can develop a space-hungry, workforce-hungry and mineral resource-hungry industry that is impossible in space. Think Taiwan Semiconductor Manufacturing Company (TSMC).

Mineral resources

SOME things may be easier to mine on asteroids as on Earth they may be concentrated deep underground (heavy metals), but you'll need a complex and expensive enrichment/separation industry. On Earth, the water cycle and the biosphere concentrates (or did so in the past) a lot of things. Here, we have copper mines with ~0.5% copper. I doubt we can find an asteroid with a deposit that rich (or, for that matter, 1/10 of that). And good luck digging down to the core (easier than on Earth, but still quite hard).

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    $\begingroup$ You can also add there is not enough loose mass in asteroids to build enough colonies to compare to a planets surface, so you would need to be mining planets to build space colonies. which means your costs skyrocket, literally. $\endgroup$ – John Dec 18 '20 at 14:59
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    $\begingroup$ You seem to be under the impression that space colonies are expensive. They're not. They're basically free once you've got the technology to make them viable. Your numbers are way off. There's enough material in the asteroid belt to make 3000 Earths' worth of living area. $\endgroup$ – Yurgen Dec 18 '20 at 17:17
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    $\begingroup$ @Yurgen Your numbers are way off either. Asteroid belt is quite poor in light elements. Nitrogen, Hydrogen, etc... You'll have to harvest comets (a limiting factor) or mine Pluto and likes (slow and expensive). And these light elements will keep escaping so the whole thing is not sustainable. You may more or less build a habitat, but you will lack the important materials needed to make humans. Not a deal, is it? $\endgroup$ – fraxinus Dec 18 '20 at 21:38
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    $\begingroup$ You're still thinking too small. Titan likely has hundreds of cubic kilometres of liquid ammonia in it, and pumping it up isn't a technical challenge for anyone considering mass-producing space habitats. Sure, eventually one will reach the stage where that isn't enough, but by that time you can probably afford an even bigger operation, like mining gas giants with fusion-powered statites or starlifting. $\endgroup$ – Yurgen Dec 19 '20 at 2:11
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    $\begingroup$ "Titan likely has hundreds of cubic kilometres of liquid ammonia in it, and pumping it up isn't a technical challenge for anyone considering mass-producing space habitats." So, what you're saying is that your space colonies are dependent on planet-sized bodies with gravity wells to supply necessities. So why ask why people wouldn't skip the middleman and just live where those supplies already are? $\endgroup$ – Keith Morrison Dec 19 '20 at 21:09
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Beavers that have never seen running water in their entire lives will start gathering twigs on hearing it. We aren't that programmed, but we may not prove to be absolutely blank slates. Living in space may prove to be subtly (or egregiously) badly suited to human beings, even those born on space station or space ship. Perhaps we need genuine gravity, and the subtle discrepancies produced by rotation-simulated gravity prey on our bodies and minds.

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Cthulhu dislikes Dyson Swarms

Any argument about planets being safer of more stable assumes that your automation and general engineering capabilities are rubbish (on the current day level instead of on one one might reasonably expect form an interstellar civilization in other words). The environment might kill you isn't a good reason not to settle a place, it hasn't even kept prehistoric humans form adapting via technology (clothing for the northern hemisphere, boats for Polynesia). It won't keep an advanced civilization from mastering new technologies and building space habitats. The only way I see to get the classical "planetary" Scifi-Setting is that something that humans can't solves keeps them planet-bound.

I propose that your setting has some kind of K3-civilisation level "gods".

Maybe they are sleeping, or waiting or have a political system that is so gridlocked that they haven't done anything for a billion years. No one really knows. Asking stupid questions results in vaporization. The only thing that is certain is that they handed you, and any other civilization a list of ten(-thousand) commandments. Something like this.

Don't make pizza with pineapples, that's just disgusting.

Don't research zero-point energy, you morons might cause a false vacuum collapse.

Don't develop a solar system beyond K1.2 (obviously relative to a K-type star!!!), this means no Dyson Swarms, you simpletons.

...

If you disagree with or break any of these rules, we will collect your entire civilization, crush you into a black hole and let you orbit the galactic core along the the [23463] others.

- sincerely, the K3 civilization in whose backyard you are playing

Something like this would allow you to have any number of otherwise illogical rules or limitations in your setting. From weapons restrictions, over a prime directive to a strong preference for inhabiting planets.

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Supply

Logistics of mining in space is a lot more complex as mines will be small and distanced. Some mines will not be worthy of a big mining ship with a strong long term life support system. Those ships, workers will need shipments from other places. It will consume a lot of time and workpower.

Comfort and Freedom

Everything in a space station is a utility service provided by the station. So, you need to pay for the air you breathe. Water might get rationed from time to time.

If you are born in space you will hear stories of planet living where the land is vast air is free, water falls from the sky. You will hear about cheap excess amounts of animal products and magnificent animals themselves. Large trees you can climb. You are free to light a fire on a planet!

On a space station, as you stated, EVERYTHING is controlled and needs to be controlled in order to stay alive. Planets, thanks to their size and complexity, are much more forgiving in the sense of resources.

Competition between species

Also since planets are excellent environments to reproduce with enough space to supply the population your people will gain advantage over other alien species. High population can be a burden but it is needed to survive againts other technologically equal alien species.

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    $\begingroup$ i love "You are free to light a fire on a planet". A space culture would need to be really advanced to have locations where an actual open flame fire is allowed, especially something in the 'bonfire' class. Spacers would learn history, and one of the first inventions that raise Humans above the animals is the use of Fire. Imagine being taught in spacer school that Mankind started civilization by mastering Fire, and little johnny raises his hand..."Teacher, what is Fire"? "Sorry johnny, we are unable to make fire here". "but... aren't we civilized?" $\endgroup$ – PcMan Dec 18 '20 at 7:03
  • $\begingroup$ "You are free to light a fire on a planet" ...unless you live in a city with clean air laws, an apartment building or an area prone to wildfires. There's also lots of places where the water that falls from the sky isn't enough and people need to desalinate or draw from nonrenewable aquifers. And if you've ever looked at the costs of running a large building's HVAC system, you'll know that air isn't free either. If you've got billions of people living first-world lifestyles, it turns out that all the resources you mention aren't as plentiful as you claim. $\endgroup$ – Yurgen Dec 18 '20 at 17:04
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    $\begingroup$ On a space station, you will rent air the same way we rent land space on Earth. And look where that got us. $\endgroup$ – user253751 Dec 18 '20 at 20:19
  • $\begingroup$ @PcMan I guess they would have some sort of fool/children/fireproof device for this exact sort of demonstration. $\endgroup$ – val is still with Monica Dec 19 '20 at 10:46
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    $\begingroup$ @Yurgen The HVAC may be expensive, but the air itself is free. More importantly, there is much more freedom of movement on a planet than a space station - if you don't like paying for your utilities, it is actually possible to move someplace they're cheaper on a planet, not so on a space station. $\endgroup$ – No Name Dec 20 '20 at 4:54
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Space habitats cost an enormous amount of money. Who's gonna pay for that? The people imagining O'Neil cylinders have been dreaming about bustling Moon cities and rotating habitats in Earth orbit by 1980 since the 50's. We don't have any of those simply because nobody is willing to finance them.

Those up-front investments would only start to pay off after probably hundrets of years, and no corporation is thinking that long-term.

In contrast, a colony requires quite little up-front cost, depending on how basic you are willing to get and how friendly the planet is. You basically can just drop a few hundred settlers with seeds and saws on the planet and let them do their thing wild west style. They'll even pay for it themselves, they won't need their earth money anymore anyway.

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    $\begingroup$ Biology is a little more finicky than that. Even a living biosphere based on proteins will be unlikely to be compatible with unmodified humans due to amino acid differences. I like to add panspermia to my stories so I can handwave this very factor away. $\endgroup$ – DWKraus Dec 17 '20 at 22:04
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    $\begingroup$ that's a lot easier to handwave than FTL... $\endgroup$ – ths Dec 17 '20 at 23:32
  • $\begingroup$ I've also gone with medical self-replicating nanites passed from mother to child. Ultimately a little more vulnerable to eventual degradation, but workable nonetheless. Even thrown those into a few answers on this site... $\endgroup$ – DWKraus Dec 18 '20 at 1:05
  • $\begingroup$ If you're even considering shipping a few hundred settlers to another earthlike planet, your industry has advanced to the point where personal-scale manufactured goods are effectively worthless. If anything, building on a naturally-ocurring habitable planet to an interstellar civilisation would be like burning ancient historical artefacts for fuel to us: Destroying the only thing they can't make in near-unlimited quantities. $\endgroup$ – Yurgen Dec 18 '20 at 16:48
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    $\begingroup$ @Yurgenyur you mean, like oil? $\endgroup$ – ths Dec 18 '20 at 21:33
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Homesteading and Independence

On earth, we see this phenomenon currently. Some people are attracted to the idea of settling on a plot of land and doing as much as possible themselves. Some are able to live "off-grid" independently from the rest of society, in various community sizes and various levels of import/export of goods and services with others. Some humans would rather work harder doing manual labour on their homestead and live in comparatively lower luxury just so that they don't owe anything to anyone. No mortgage, loans or debt of other kinds.

This is near-impossible to do in a space colony, because a small pod for one person (or family) just doesn't work. The systems that keep it stable are so advanced you need multiple specialists to keep it in working order. A larger station is a better solution and getting it working is an effort by the entire society to keep its members alive. There is no way to control everything yourself, you have to depend on others. The "Do It Yourself" days are gone.

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  • $\begingroup$ I dunno. Just fly a rocket to an uninhabited asteroid and set yourself up using the materials there. Seems like it could be doable - maybe even moreso than homesteading on Earth. $\endgroup$ – nick012000 Dec 19 '20 at 11:28
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A space colony is like a hive. If you're part of it, you must work for it.

There was this story I read, settled in a post-apocalyptic world. Some of the world's most socially powerful people, made use of their influence to build a safe haven.

However, if you wanted to be part of it, there was a condition. By working for them, in return, they allowed you to stay. So basically they maintained social hierarchy, even if there was no money anymore.

A space colony is one gigantic beast to maintain. Even if a lot of things are automated, it might not be truly become fully autonomous. So you require people to maintain it. Beside that, any person living on it, is one more mouth to feed.

In that kind of situation, I can see at least 3 group of people that would live on the planet:

  1. If you are unlucky and a colony deemed that you were not necessary to them (in making them thrive), then you have no choice but to try your luck elsewhere. Either knock on another colony, or live on a planet.
  2. Related to the first group are those who would want to join them. Whether because they're family members, or because they believe it was unfair.
  3. And last, you also have lone wolves who would rather live on their own. But that can hardly be done on a space station. No matter where you go, you will always be part of the colony. So the only way out is on a rock.
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Isn't this kind of like asking why fish prefer to live in the ocean?

Humans evolved on Earth, so we evolved quite specifically to live in Earth-like conditions.

In general, the more Earth-like an environment is, the more humans will find living there to be comfortable/healthy/enjoyable. Sure, you might be able to approach Earth-like conditions in a sufficiently high-tech space habitat, but it's likely you will never be able to 100% replicate "the real thing", and any deviations away from Earth-normal will result in some level of discomfort/dissatisfaction amongst many of the people who live there, which will in turn drive a certain amount of demand for on-Earthlike-planet real estate.

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Because in the final analysis Earth like worlds and space stations/ships are all just habitats i.e. places that are capable of sustaining human existence. The only difference between them (apart perhaps from scale)? Habitats have their environment on the 'inside' while Earth like worlds have theirs on the 'outside'. So location is simply a matter of need and opportunity - choice.

(Edit; and part in fun) So its simply a case of choosing the right type of real estate for your current lifestyle and moving in. (Footloose and single) a small apartment on Centauri Station close to bars and the entertainment district. (Married with kids) perhaps something with a big yard and a pool down on Deneb 4

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In aquarium hobbyist culture there's a rule of thumb that larger fish tanks are less maintenance and more resilient to change. This is because they're just so big that the environment is self-correcting and is a lot more tolerant to local fluctuations. Nano tanks, on the other hand, are a huge amount of maintenance and often have problem with oxygen depletion, ammonia buildup, and other fun problems. This applies to all self-contained environments and is why, for example, humans can pollute the oceans, kill off the megafauna, cause global warming, and burn the Amazon but still have time to reverse their environmental destruction rather than the environment turning into an outright hellscape. By contrast, compare this how often "we need to fix this because otherwise life support will go down and we have no margin for error" issues come up in human space travel either as theoreticals or actual issues like with Apollo 11.

O'Neill cylinders and other space stations are basically microtanks designed for the human species. They have little margin for error, are super vulnerable to radiation and micrometeoroids, and would require your population to spend a significant amount of time performing maintenance that they could spend on leisure, research, or cultural pursuits. They also have the issue that precious water, atmosphere, and oxygen can be very easily lost through breaches or venting, and in many cases it may not be possible to move your space station to absorb an asteroid to get some back. By contrast, because of gravity and magnetic fields those resources tend to stay attached to a planet. You could build a really, really large space station that bypasses these problems by having a larger environment but by that point you're just building an artificial planetoid like the Death Star.

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Simplicity

If you live on Earth, your address may be something like:

2020 Somewhere st
Some City, ST, 00000
Country

And you only need to care about North, East, South and West. For example if you wish to visit the bakery, you go south for two blocks and then turn right.

Now if you live in space, your address needs to contain the following data, from the point of view of a base planet in the system:

  • Eccentricity
  • Semi-major axis
  • Inclination
  • Longitude of the ascending node
  • Argument of periapsis
  • Mean anomaly

(I am assuming that a small space station in orbit around a star would be catalogued the same way that asteroids in a belt are).

Might look like this:

e 0.22997227±0.00000003
a 2.773841434±0.000000004 AU
i 34.832932°±0.000003°
Ω 173.024741°±0.000006°
ω 310.202392°±0.000009°
M0 144.97567802176°

This is the address for 2 Pallas in our own solar system, by the way.

Your "cardinal directions" are prograde, retrograde, radial in, radial out, normal and anti-normal.

To visit aunt Bertha, you need to wait for the perfect launch window so that you can do a rendezvous. Might take months and you'll need an immoral amount of fuel.

Since I'm not a fan of astrogating just to buy toilet issue, I'll choose living in a good old planet any day.

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    $\begingroup$ Think about all the calculations computers needed to do just to work out which computer to talk to to connect your computer to the one that's serving "worldbuilding.stackexchange.com". Now thing about how many times per day the average person visits a website. The problem you're talking about was solved in the 90s to the point where most people have forgotten it was ever a problem. $\endgroup$ – Yurgen Dec 18 '20 at 16:40
  • $\begingroup$ @Yurgen for the address part, yes, we could use monikers instead of numbers. It would still be a female dog to get a pilot license and to go from one place to another (in terms of time and fuel, not calculations). $\endgroup$ – The Square-Cube Law Dec 18 '20 at 17:19
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    $\begingroup$ I don't expect "drivers' licenses" to be a thing by the end of the century. AI is bound to advance to the point where driving your own car on a public road will be considered reckless and irresponsible. Why do you think people will be flying their own ships from place to place millennia into the future? $\endgroup$ – Yurgen Dec 18 '20 at 17:24
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    $\begingroup$ The alternative described by the OP consists of "a cloud of space station habitats", which don't need to be very far from one another, nor would they need to have any velocity relative to one another (no worrying about launch windows). The stations could be large enough that you won't need to leave very much, and when you do travel to another station, it might actually be much closer, faster, and cheaper than some terrestrial travel - flying to the other side of the planet is 10k+ miles away through a speed- and efficiency-limiting atmosphere, takes a day, and uses an immoral amount of fuel. $\endgroup$ – Nuclear Hoagie Dec 18 '20 at 17:40
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    $\begingroup$ Doubt it. They'll come up with names for common orbits and numbers for the space stations in that orbit - same as we do for roads. Your comparison isn't fair unless you also address planet-dwellers with latitude and longitude and X/Y/Z offsets within the building. $\endgroup$ – user253751 Dec 18 '20 at 20:20
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Beyond whether or not it's practical, it can be something as simple as one's principles or beliefs

There could be a group of people. They may be a few handy, or they may be a vast majority of those who still live on a planet. They have in common that, they believe walking on earth is the right thing to do from their perspective.

Maybe it's because they must remain connected to their roots, maybe because they do not want to anger their gods, as they're afraid going so far away into outer space would be like challenging them.

Or, if you are also interested in fantasy stuff, maybe they are shaman, and being on earthly grounds allow them to better connect with the elements.

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Violence and Economics

O'Neil cylinders, Dyson spheres, Ringworlds and other space colony megastructures indeed have many benefits, as you indicate. They have vast population densities and don't have annoying gravity wells to contend with. But they have some drawbacks: They take forever to build and they are relatively fragile. If you can either amplify these drawbacks by presenting hostile circumstances, or negate these benefits with the right technology, then space mega-colonies will be the exception rather than the norm.


War

One reason your civilisation does not build many space mega-colonies is because they are in a constant state of war, or at least suffer major wars on a regular basis. Wars present numerous pressures against space colonies.

Weakness

Firstly, space colonies are fragile. Consider an O'Neil cylinder or a Ringworld. If you can make a large enough hole in the hull in just one point, the entire thing can tear itself apart. A Ringworld or a Dyson sphere can be pushed off axis and crash into their star (as long as you can push harder than any restoring forces). These space colonies all present large areas of exposed hull which can be targeted by weapons, and the amount of shielding is limited.

Compare with Earth-like planets. While planets are still easy to shoot at, planets automatically come with protective atmospheres, and the cautious can put a further few hundred metres of rock between them and the sky at little additional cost. Planets are also far harder to destroy, being bound together by gravity and having vast amounts of matter to distribute shocks through. Space colonies have no such gravity or spare matter.

While shields and point defences can reduce some of the risks, such defences can apply equally well to planets and space colonies, or even better to planets if such defences require vast heat sinks.

This assumes you do not invent some easily-mass-produced indestructible building material. If you can cheaply convert matter into unobtanium hulls, then space mega-colonies become better protected than planets. But even with unobtanium, there are other reasons not to build mega-colonies.

There's no time

Secondly, space mega-colonies take forever to build and use up the engineering capacity of your solar system. In war, you don't have time or production capacity to waste.

You could disassemble a planet and build an O'Neil cylinder, or you could build your colonies on a planet in a fraction of the time and cost. Then the population of your solar system will be able to direct their energy towards the war effort. If you have time to build a mega-structure, the Empire could do with a few more Death Stars, not frivolous housing.

There's no need

While planets have lower potential population densities than mega-colonies, this is not a concern for your war-torn civilisation. They are constantly acquiring new solar systems to inhabit, or losing solar systems (and reasonable chunks of their population) to their enemies. As such, the principle benefit of these mega-colonies is never needed.

Additionally, if territory changes hands on a regular basis, then there is no sense spending vast resources to create a mega-colony when there is a good chance it will fall into the hands of your enemies in the near future. You might build these mega-colonies in your core territory, but territory with even a remote chance of becoming contested will use the cheaper planetary colonies.


Space Elevators and Expansionism

Another reason your civilisation might not build many space mega-colonies (independently of war) is that the technologies and space available to your civilisation make mega-colonies redundant.

The first benefit of living in space is not having to climb a gravity well to travel. However, if your civilisation has a cheap way to ascend (or descend) gravity wells, then expensive space mega-colonies lose this advantage. This method might be space elevators, or a reactionless drive, or a (cheap) torchship, or teleportation, or antigravity, or anything else you can imagine.

The second benefit of space mega-colonies is efficient use of mass in terms of livable surface area. However, it may be cheaper to find another planet to live on than to disassemble your current planet and turn it into a mega-colony. If you have decent FTL travel and communication, and the galaxy is not yet fully occupied, then finding a new world and maintaining a connection with the rest of the civilisation is easy.

In the core worlds you may get some of these space mega-colonies, unless galaxy-wide travel is cheap and instantaneous. These mega-colonies would be the Kardeshev-Type-2.5 equivalent of our skyscrapers, used to produce a high population density close to central infrastructure. But you would still have a substantial amount of the population living in the space equivalents of suburbia and rural regions, which use cheap and readily available planets for their colonies.

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To address a small portion of your question, terraforming a planet, assuming that is a thing, need require essentially no time at all to accomplish.

Send out a terraforming suite ahead of the colony ship. Have the colony ship travel at extremely near the speed of light on a trajectory which puts the colony ship at the new planet in whatever number of centuries is required for the terraforming suite to accomplish its task. Relativistic time dilation will ensure that that very little time will have passed for the colonists (basically acceleration and deceleration), despite the passage of centuries (as needed) for the terraforming suite and the new planet.

This removes the barrier of "Un-inhabital Planets" from the list. The answers others have given become that much more feasible. A universe could have thousands of planets in the process of terraforming with colonists in transit, even if FTL is involved.

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Under the Principle of Non-exceptionalism, the reason populations do not expand much beyond the planets has to be something which applies to all systems, and all sophont species.

A single culture with an inclination for expansion will do so quite well with FTL ... unless it cannot.

Perhaps the Peak Phosphorous limit is quite low in this setting? Or some other phlebotinum limit caps the population in each system.

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Your civilization has a large presence in space, out of planets, but those stations are inhabited by only robots and a temporary or semi-permanent maintenance crew. These space stations would be like modern oil rigs or ships. They would have a relatively comfortable environment for the engineers to live, with indoor tennis courts, living quarters, cafeterias, greenhouses. Employees could live and work there for months or even years if such habitats have artificial gravity, but (most of them) wouldn't want to spend their entire life there.

People would want to live a normal healthy live on an actual planet with an atmosphere, with nature. While it is possible to live in an artificial environment for extended periods of time, it is not healthy in general. It is not the best solution.

And the more automation and robotics you have, the less reasons you need people to be there. You could eventually reach a point where most of the space facilities are automated, and have automated repairs and maintenance as well and you have only a small group of engineers overseeing the systems.

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There will be a large presence is space, but with mastery of space, the energy requirements to drop into/climb out of a planetary gravity well will be small.

Earth will be a tourist destination, a repository for original ecologies, and general gene bank. Moderate numbers of people will choose to live on Earth, but most will prefer the higher wages, and controlled lifestyle without hot weather, blizzards, mosquitoes, and general unpredictable environment.

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All right, lets make an artificial planetoid or space station suitable for human habitation. First off, we're going to need an atmosphere to allow people to breathe. You could enclose everything inside a metal and plastic shell, but that's very vulnerable to puncture by micrometeorites. An alternative way to do that is build something with a lot of mass so that gravity will keep the atmosphere attached and...oh.

However, we don't want our atmosphere to be stripped off by solar winds or our population to be bombarded by cosmic radiation. Space stations tend to have very thin walls (compared to a planet) so what we can do is generate a magnetic field to...wait a minute.

Additionally, our population is going to need a lot of resources, as well as extensive environmental controls in order to maintain the environment at a stable level. So to do that, we'll have extensive ecoscapes spread across the surface to provide wood, food, and oxygen just...like...forests...yeah.

We also need to be at the correct distance from the sun to grow our food and power our solar panels, which means we need to place our space station within the habitable zone and...why aren't we just colonizing the planets in the Goldilocks Zone again?

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