The concept of a city that is so large it engulfs an entire planet is one of the most common tropes of science fiction, especially space opera. After all, population growth and urbanization throughout history, particularly in the last two centuries, have driven cities to become ever bigger. Cities today harbor populations measured in the tens of millions sprawled over regions dozens of miles in diameter. Extrapolating historical and present trends into the future and exploring the consequences is fundamental to science fiction, and bigger cities is one of those trends.
As I have outlined previously in my posts on the likely demographics of the solar system and galaxy of the future, as well as the likely effect of near-future transportation technologies, the urbanization trend may well reverse altogether in the future due to the urban sprawl trend de-concentrating metropolitan areas so much they become rural. This ruralization (or deurbanization) will be greatly helped by the likelihood that the human population will shrink rather than grow in the next few centuries, compounded further by the fact that vast expanses of land will be opened up for habitation throughout the solar system. Planetary population densities in the spacefaring future will be far closer to the Siberian wilderness than Hong Kong for a very long time to come.
The City of the Future
Over the longer term or in a hypothetical scenario fit for space opera worldbuilding, however, what are the prospects for a city-planet to emerge? It is important to note that future ruralization will not cause cities to disappear altogether; gathering places for business, pleasure, tourism, and socializing will still be needed, and cities serve these purposes very well even if none of the gathering masses actually live there. The seeds of this future type of city are already here in many places in the form of the “central business district”, a section of a city harboring businesses and tourism but few permanent residents. The hallmark of the business district is high structural density coupled with low population density, usually also accompanied by high “daytime” population density.
Interestingly, as economies develop and become more complex central business districts tend to expand, both in absolute terms and relative to the whole city, which may be related to the suburbanization (and what will eventually become ruralization) trend. Manhattan, host to New York’s largest central business district, has a current population (1.6 million) that is much smaller than its peak in 1910 (2.3 million) due to central business district expansion. The City of London, a square mile entirely within a central business district, presents a much starker example, harboring 7,000 or so people today compared to a peak of 132,000 in 1851.
So even if cities’ permanent populations empty out they will find a new function as daytime gathering places for rural denizens. This will manifest in the future as suburban populations dissolving into the vast countryside as central business districts expand until they entirely cover city centers, greatly marginalizing the high-density residential type of built environment.
The Dawn of urban Worlds
What this means for city-planets is that there will still be a place for areas of high structural and daytime population density even in an extremely rural galaxy that makes the Siberian wilderness look crowded, assuming there is (effectively) faster-than-light travel (wormholes, warp drives, etc.) in the setting of the science-fiction or space opera world that enable these far-flung masses to easily get to these gathering places. Even a sparse galaxy that averages 10 people per star system would still in the case of the Milky Way have a total population of 4 trillion. If only 1% of these people are in the galaxy’s planetary equivalent of New York or London at any given time that is still 40 billion people on the most populous planet, more than 5 times Earth’s current population.
Although a good start, a daytime population of 40 billion is nowhere near ecumenopolitan density. It might be closer than you think, since with ever-greater wealth a given population density will economically support ever-higher structural density. Nevertheless consider that Earth’s current population density, averaged over both land and ocean, is 39 people per square mile, which is pretty rural; 5 times this figure would still only amount to 195 per square mile, on the high end of the rural range. This density won’t be distributed evenly, of course; many “megaregions” will become truly merged urban areas with this sort of population increase, even if it’s only transients, businessmen, and tourists, and downtowns will become far larger, especially with a greater level of per-capita wealth in the spacefaring future. Still urbanity will not cover even most of the planet, let alone all of it.
A true City-Planet
I suspect that you would get into true city-planet range starting at 5 to 10 times this amount, or 25 to 50 times Earth’s current population; Earth’s density then would be 1000 to 2000 per square mile; while this is suburban rather than truly urban, the unevenness of development implies that urban-style structural density would cover a large fraction of the planet’s surface. Regions comparable in size to Europe might be covered end-to-end in high-rise buildings. Regions comparable in size to ocean basins or mountain cordilleras may be spared from development to serve as appropriately super-sized urban parks.
The busy areas, this world’s equivalent of our ordinary downtowns, would achieve truly futuristic levels of structural density, harboring clusters of skyscrapers reaching well over a mile in height. The busiest districts of such a planet, this world’s equivalent of Manhattan or Hong Kong, would harbor the tallest buildings, possibly stretching many miles into the sky, with mile-tall skyscrapers stretching from horizon to horizon as viewed from the centers of these districts.
Now there’s a setting that fits into the ecumenopolitan mold of Coruscant or Trantor. Coupled with mass personal flying cars, not to mention airborne mass transit, there would be little need for residents and transients in these tallest skyscrapers to ever travel to the surface; indeed, from a penthouse window at an altitude of several miles it may be difficult to even see the ground.
These sort of planets, and particularly these densest and richest districts, would make for a fascinating setting; the plausible cities of the far future or of space opera lacking much of a permanent population might even enhance intrigue, as such city-planets would be the crossroads of the galaxy, a place where countless millions travel in and out every day. Diverse and interesting characters of unfathomably different origins may intersect there, and these sort of planets will likely host the largest and most fascinating gatherings and social circles in the galaxy.
All this would require a daytime population of 200 to 400 billion people; only a few billion (or even less) would actually live there. With a galactic population of 4 trillion, such a planet would be what Greater New York is to the United States in terms of the share of total population (10-20% of the total); if you’re worldbuilding a space opera you can certainly posit that current urbanization patterns stay stable or will return. After all, almost anything could evolve after untold centuries and millennia. However, if we are to hold to the realistic near- and even far-future population deconcentration trend, a galactic population much higher than 4 trillion would be needed to support such a city-planet. Taking my guess of 1% of the galactic population being present there at any one time, we come to a figure of 20 to 40 trillion for the whole galaxy.
That sounds like a lot of people, but spread over the Milky Way galaxy it only amounts to 50 to 100 people per solar system. Not quite as sparse as the light-years of continuous wilderness that would be all-dominant in a less-populated galaxy, but overall still surprisingly sparse. What would this same sort of city-planet look like in a much more crowded galaxy? Let’s say the average galactic population density rises to 100,000 people per solar system. Note that the same effect may be created by populating 1,000 galaxies at the 50 to 100 per solar system density mentioned earlier while retaining easy commuting distance to the central planets. 1,000 times more population means the human population swells from 4 trillion to 4 quadrillion.
One possibility is that the minimal city-planet mentioned earlier is about the densest development that is technically or economically viable; in this case you would get 1,000 such planets at roughly the same density. While this might be an interesting scenario in its own right, we’ll assume 1,000 times the population can be harbored on a single planet.
A high-grade Ecumenopolis
The 200 to 400 billion people roaming the ecumenopolis in the daytime now swell to 200 to 400 trillion people, a titanic number. For an Earth-sized planet this means an average population density of roughly 1 to 2 million per square mile; in contrast to our previous scenario which was in the suburban range, this is deep within the urban range. In fact no urban district on Earth today is this dense, taking us into the truly science-fictional realm.
At 1 to 2 million per square mile, it’s safe to say the entire planet will be covered in high-rise skyscrapers, barring areas specifically preserved for parks or wilderness. The sleepiest areas, this world’s equivalent to the taiga, may have a density of around 20,000 per square mile, which is right in the middle of the modern urban range. If the busiest areas are as popular relative to the rest of the planet as today’s downtowns, densities may run into the billions per square mile.
This might sound crazy, but 10 square feet of area for every person is just enough to permit movement in a crowd; spread out over a square mile each square mile can accommodate 2.78 million people in such conditions. It would take 360 levels to hold a billion people; at 10 feet per level that is 3600 feet, less than one mile! Admittedly that doesn’t take into account the open space needed for the infrastructure, but towers only 5 to 10 miles high wouldn’t have that much trouble holding this sort of population. It would be crowded and choked with traffic, but if something drew that many people there the formation of such a horde is possible.
Towers ten miles high teeming with traffic and endless crowds milling in and out, with the middle levels of these skyscrapers only being able to see urban jungle from horizon to horizon and from zenith to nadir is at least as extreme a vision as even the densest districts of Coruscant in Star Wars. It would be a spectacular place to visit, though even connoisseurs of urbanity might find it overwhelming.
The ultimate Limit of Population Density
This exercise does suggest a fundamental or at least practical limit to population density. 5 to 10 miles is pretty high up; while buildings may well eventually reach those heights even in the near future, once you reach past 60 miles or so on an Earth-like planet the pinnacle of the skyscraper extends into outer space. At that point you might as well build a space elevator, since that might be the only type of structure that can be made habitable over such great heights. More to the point, the part of the building past the atmosphere and into outer space arguably wouldn’t count as being on the planet, even if it’s physically attached to the ground. Otherwise you would count inhabitants of an artificial orbital ring as inhabitants of the planetary surface.
Taking 60 miles as the limit for being “on the planet”, a population density of 1 billion for 5-to-10-mile-high towers becomes 6 to 12 billion for 60 mile high towers. This suggests that the practical “structural limit” for population density on a planet is around 10 billion per square mile. An Earth-size planet at that density therefore would support a population of 1.69 quintillion.
For humans as we know them on an Earth-size planet with an Earth-analog atmosphere that is the ultimate limit, which would apply no matter what technology was available. That’s several thousand times even the 400 trillion population the high-grade city-planet has. Scaling up the galactic population accordingly, this would imply a galactic population of around 170 quintillion; while a very large absolute sum that still works out to 420 million per solar system in our Milky Way galaxy. That’s less people than Earth alone has today; spread out over a whole solar system even this figure would be rather comfortable.
Obviating other Obstacles
Other obstacles would probably come into play long before the structural limit is reached. Exotic agricultural and life support systems similar to what is sometimes posited for space colonies may become required at the mid to upper range of city-planet populations (the full range being from 200 billion to 1 quintillion). I would contend, however, that any setting that features cheap and fast travel across the galaxy would likely rely on more conventional foodstuffs grown in space habitats and on other planets; the amount of land taken up by agriculture to feed a quintillion people even at today’s crop yields, let alone what they might be in the distant future, is actually very small on a galactic scale. Still, the greater the population the greater the amount of food that must be imported, which will inevitably give these planets long and presumably less efficient supply chains; while not a serious problem for lower-grade city-planets it might make the highest-grade densities uneconomical long before the structural limit is reached.
It is also sometimes thought that waste heat from such a titanic population and economy would become a major problem; one estimate is that the waste heat from a population density greater than that of Barcelona would warm the planet too much to be habitable. This, however, neglects the possibility of active cooling of the planet; if coolant could be imported from cryogenic environments and fed into a planetary-scale air conditioning system almost any amount of waste heat could be countered. The only fundamental limit would be how fast the required volume of coolant could be transported on and off the planet. Although I haven’t done any precise calculations, it seems the population that could be accommodated with such cooling systems would be very large even by city-planet standards.
Much of the technology needed even for a downtown of mile-high buildings, let alone a whole planet of 10-mile-high buildings, overlaps with the technology used for “arcologies”, large buildings that are self-contained and self-sustaining, but that is a topic for another post. Much of even the most basic infrastructure of these towers would be rather impressive; imagine the pumping system needed to send water even a mile straight uphill, let alone 10 miles.
Along similar lines, imagine how fast the elevators would need to go up and down to enable access to any of the building’s floors in a reasonable time. For such tall buildings the elevator tubes would almost certainly use maglev technology, possibly with the air evacuated from the tubes to achieve greater speed.
The transit system must also be fairly impressive; even if only a tiny minority of the transients and residents on a city-planet use mass transit it would still be an immense horde compared to any modern system. The vast majority, especially considering they are businessmen, tourists, and commuters in this scenario, would almost certainly have personal aircraft. Traffic congestion in all but the high-grade city-planets would likely be less severe than Earth cities today, since the traffic will be spread over three dimensions as opposed to two. An Earth-analog atmosphere is immense enough to make traffic volumes even many orders of magnitude greater than Earth’s today comfortable.
All of this may be a topic for another post, but for now we have explored how a city-planet may realistically fit into a likely human future of ever-thinning populations as mankind spreads across the galaxy and even beyond; if we ever attain affordable and rapid interstellar travel areas of higher structural density hosting business, tourism, social gatherings, and other pleasurable activities are almost certain to emerge, even if no one is a permanent resident in these regions. Given a sufficiently large galactic population, these regions would expand enough to encompass an entire planet, possibly multiple planets.
Many of the limits to population growth often discussed in science-fiction worldbuilding circles don’t even necessarily apply, at least not on a the technical or fundamental level. The only real limit is how much people can be physically crowded; at that point it becomes a question of how tall buildings can be built and at what height a floor counts as a space as opposed to planetary habitat. The real limits are likely to be economic feasibility as well as the level of desire to congregate in such massive cities in the first place.
If a science-fiction or space-opera setting has populations remaining or reverting to predominately urban, city-planets for obvious reasons become even more likely, and will emerge even earlier in galactic civilization’s development. However, we have seen in this post that large populations living in urban centers is not a prerequisite, and an ecumenopolitan setting without this aspect might be a much more fascinating place to explore in space opera and science-fiction, whether for pure worldbuilding, for a story, or for other works of art.