A city planet is a subtrope of Absurdly Cool City, Mega City, and Single-Biome Planet, because you just took an entire planet and made the entire planetary biome a single city. In other words, this is what happens when someone takes Planetville a little too literally: there is only one "city" on the planet, and it covers the entire planet.
Sometimes referred to as a planet city, world city (though "world city" has also been used to mean other things), completely urbanized world, omniopolis / omnopolis, or ecumenopolis. While most examples are recent, the concept dates as far back as the nineteenth century work of Thomas Lake Harris, and the term "City Planet" dates at least as far back as the first draft of the script for Star Wars: A New Hope.
This trope occurs as the apparent result of a civilization, presumably over centuries or millennia of expansion, converting the entire surface of a world into one vast city. To be sure, many City Planets are divided into "administrative sectors" or other such local government institutions to keep order manageable, but effectively it's all the same city. Generally, this trope implies that the only biome of importance on the planet is urban jungle. Taken to an extreme, it may be implied the locals even paved over volcanoes and oceans in the process of creating the City Planet.
In works not all the way to the less realistic end of the Sliding Scale of Realistic vs. Fantastic, such a world can present a Mega City-sized ball of Fridge Logic. After all, what do people eat if there is no farmland? And wouldn't so much urban space all over the planet heat it up until the point of ending all life?note Often it's simply handwaved, but other times it's revealed that:
- The local Starfish Aliens don't need food as we might understand it.
- Food and other supplies have to be imported from elsewhere at great expense (or not-so-great expense, if they've got some form of Teleportation — be it cool gates or... something more unusual).
- Massive greenhouses for crops and battery farms for livestock. Not as scenic as acres of farmland, but it works.
Because Sci-Fi Writers Have No Sense of Scale, City Planets tend to have implausibly low populations, even if the number quoted seems high. Assuming a city literally covered the surface of a planet the size of the Earth, with no large bodies of water or other uninhabited areas, a population of 1 trillion would have a density of around 5000 people per square mile, or five times smaller than that of New York City (and fourteen times smaller than Manhattan). Considering that such sci-fi cities are often said to have multiple levels as well, either fair portions of this "global city" must be largely empty or the planet must have a population density equivalent to that of a rural town. Even assuming that the census data is off doesn't work as you would run into problems overheating the planet once you got past 10 trillion people, still far below having to give people less than 1000 square feet of living space, much more than the cramp conditions shown in most science fiction.
Alternately, the trope will be subverted, introducing the audience to a seemingly endless city, and only later revealing that there are, in fact, vast areas of truly rural or wilderness areas remaining, the locals just don't like to talk about it.
Another subversion can be that the city, despite covering an entire world, is no bigger than an "ordinary" city — because the planet is so small. In the past, such stories seemed more realistic than they do today; however, many serious hard — Sci-Fi tales involve colonizing an asteroid or a city-sized space station. In recent decades, computer image manipulation technology has resulted in these occasionally showing up in video games or humorous images.
City planets will often have immense industrial bases by necessity, sometimes enough to become full Industrial Worlds.
Finally, in some settings, a city occupies not a planet, but an entire plane of existence, a layer of a Layered World, or other planet-like... thing.
- Blame is set in a City Dyson Sphere that is implied to have long since consumed the solar system with its sheer enormity.
- The Five Star Stories: Pestako is a small, pluto-like planetoid with no natural atmosphere that was originally used for mining, the planet has glittering bands of light stretching across its surface that are actually highways so big you can see them from space.
- The Authority: In the distant future of Earth, the entire surface of the planet, from the tallest mountains to the deepest oceans, has been converted and into a city. There people who derive power from urban development like Jack Hawksmoor are essentially Physical Gods and are treated as such.
- The DCU:
- Legends of the Dead Earth:
- Wonder Woman: Whichever of Saturn's moons the Empire of Saturn rules from in Wonder Woman (1942) (the view makes it abundantly clear they're not meant to be on Saturn itself since it's visible in the sky) is depicted as covered entirely with city.
- Issue 28 "Last Man on Earth": The Wheel, the fifth planet of Alpha Centauri, is a planet-city ruled by a dictatorial Artificial Intelligence computer.
- Issue 29 "The Moonstealers": The planet Joaphat is covered entirely by a gigantic city with no areas of countryside.
- Issue 59 "Starseeker Squad": The planet Bessel has an Earth city/colony that covers the entire planet.
- Issue 64 "The Exterminator": Vanderdecken is a vast, ancient city-planet. Although this mysterious world is millions of years old, everything on it is in perfect operating condition.
- Issue 208 "Planet of the Dead": The planet Vegas Prime is entirely covered in all manner of vice dens.
- Captain Eo: The Supreme Leader lives on one until Eo gives it a Genesis Effect at the end.
- Guardians of the Galaxy Vol. 2: The Sovereign Homeworld is covered in a beautiful golden metropolis so large it encompasses multiple planets anchored to the main planet by Anulax Batteries.
- Jupiter Ascending: The planet Ourus, the birthplace of humanity, has buildings that stretch above the atmosphere and two artificial rings.
- Star Trek: First Contact: Earth has become this under Borg rule in an alternate timeline. Oddly enough the population consists of only nine billion Borg even though the planet's entire surface seems to have been completely urbanized and technified. The planet might be one huge automated factory.
- Star Wars
- Coruscant (a Canon Immigrant from Legends' The Thrawn Trilogy), the capital of the Galactic Republic and later the Empire, is a very layered version of this trope. The parts of its seen in the movies are chiefly its upper layers, where the bulk of its population lives — the uppermost spires are the homes of the wealthy elites, while the areas directly below them house the middle-class population. Below that are kilometers of infrastructure leading down to the crust, where life is dark and dangerous.
- Nar Shaddaa (another Canon Immigrant, originally from Legends' Dark Empire series), also called the "Smugglers' Moon", is a large moon of the Hutt capital Nal Nutta, making it basically a narcostate. In the Legends novel The Hutt Gambit, Han Solo notes that the uppermost levels of Nar Shaddaa look like the lowermost levels of Coruscant, and promptly resolves never to visit the lowermost levels of Nar Shaddaa. Its reputation in the present Star Wars Expanded Universe is no better.
- Come The Force Awakens, Hosnian Prime, center of the Hosnian system and current capital of the New Republic, is shown to be a city planet (though it still has oceans).
- Aeon 14 has several of these. The most important one that is detailed is Alexandria, the capital of the Scipian Empire in the 9th Millennium, which is specifically noted to be unique in that it is self-sufficient.
- Bill the Galactic Hero features a world-covering city, Helior. While visiting, Bill suffers a grievous mishap: the map of the city chained to his arm is stolen leaving him one of the desperate Unplanned, with no hope of ever discovering where he is or where he needs to be. It's pointed out that Helior imports all its food from agricultural planets in exchange for fecal waste to use as fertilizer. Apparently, they use the same transport ships for both. Bill also finds and nearly joins the organization responsible for waste disposal and recycling. They're desperately trying to find ways of re-using the stuff people throw away or, at least, prevent people from throwing away things, like plastic coffee cups that turn into music discs when they're empty. Harrison also points out that, without forests and jungles, oxygen has to be imported as well.
- "The Cosmic Express": The Earth of 2432 is well on its way to becoming one of these. Much of the world is covered by cities, often domed cities, with most of the remaining space being used for farms, parks, and resorts. Wild Nature is gone, which is one of the principal complaints of main characters Eric and Nada.
- Ecumenopolis: The author, architect, and urban planner Constantinos A. Doxiadis suggests the Earth's fate is to a mild version of this, with rural and wilderness areas reduced to islands between the interconnecting "bridges" and "nodes" of a single city.
- Hyperion Cantos: Tau Ceti Center and Renaissance Vector. Simmons at first explains away the food problem by means of the interstellar Portal Network (the farcasters) that make the transport of food from offworld a trivial matter (this same technology, after all, allows you to have a single house on twenty-plus worlds—if you're rich). After the network disappears, Tau Ceti Center collapses and becomes entirely deserted, while Renaissance Vector is able to hold on due to the existence of another planet with good agricultural land, Renaissance Minor, in the same star system.
- Isaac Asimov:
- Foundation Series:
"Its urbanization, progressing steadily, had finally reached the ultimate. All the land surface of Trantor, 75,000,000 square miles in extent, was a single city. The population, at its height, was well in excess of forty billions." — Encyclopedia Galactica
- Trantor, capital of the Galactic Empire and one vast city (also one of the first depictions of this idea), is completely covered by domed urban areas during the start of the Foundation series. It persists in this image until it is sacked and looted, which begins its transformation into an agrarian world. The planet is based on Rome, as described by Edward Gibbon's The Decline And Fall Of The Roman Empire. Ancient Rome was the world's largest metropolis, with a population of over a million people, something which would not be matched again until 19th Century London. The Roman Empire had so many people at the capital that it could not sustain itself with only local agriculture, so food was constantly imported from northern Africa (at first from Egypt, then from what is now Tunisia and Algeria) to supply the city. The seat of government was forced to relocate to Constantinople due to Rome being burned down. Trantor is likewise overpopulated, with a population of 40 billion. It is dependent on imports of food from other planets (specifically twenty Farm Planets), which made it vulnerable to a space warlord coming along to conquer the capital. The "emperors" evacuate and move to Neotrantor, ruling only a few dozen worlds.
- "The Psychohistorians": Trantor, capital of the Galactic Empire, has converted every landmass into a single interconnected city (making it one of the first depictions of a planet-wide city). Because its population is in the tens of billions, it depended on outer worlds for food. Fleets of ships deliver produce from twenty different farm planets.
- Prelude to Foundation: Trantor, capital of the Galactic Empire, is divided into approximately 800 domed cities, each with their own subcultures, with some open space in-between used for transportation, communication, cooling towers, etc. The twenty nearest inhabited planets are all agrarian economies whose primary export market is Trantor. (Prelude can be considered Asimov's attempt to reconstruct the idea of a planet-wide Mega City.) Hari Seldon goes on The Quest to find a model for psychohistory by exploring multiple areas of Trantor.
- In the robot novels, Earth has become a planet of underground or domed "Cities". There's still wilderness outside, but very few people ever go there, so from the protagonists' point of view, they live on a City Planet. In fact, airplanes no longer have windows because most people are agoraphobic and would have panic attacks during flights if they could see outside of the plane. All of this is contrasted by the Spacers, whose worlds have a very low population density and as a result they prefer to live on estates with a lot of open space. This presents a real challenge to the agoraphobic protagonist Elijah when he visits the Spacer Worlds. Incidentally, Asimov himself was a claustrophile (i.e. he liked small, enclosed spaces) and hated flying.
- Foundation Series:
- Paradox Trilogy: Aeons have a flocking instinct which makes them prefer living with a high population density, thus planets colonized by them tend to be city planets.
- Riesel Tales: Two Hunters: Riesel is an ecumenopolis with not only a miles-high skyline, but mountain-sized masses of detached cityscape that float within the polluted skies. However, the planet's glory days are long gone. Most of the cityscape is dirty and rusting, and vast swaths have been outright abandoned; while there are a few good expanses of up-to-date, crowded cityscape. Crime is rampant, and the planet itself is run by a powerful mafia. While it has a population of twelve trillion, this is considered anemic in comparison to other urban worlds in the galaxy, which tend to have at least ten times as many people. A more comprehensive description can be found in the Riesel Tales wiki.
- Sheri S. Tepper:
- Star Wars Expanded Universe includes at least eighteen of these planets. Each has populations in the trillions and requires several nearby planets to support it. A complete list can be found here.
- The Stainless Steel Rat Wants You. The aliens have covered their entire planet this way because they hope to wipe out the humanoid races, but they've adapted by moving into the city and hiding there like stainless steel rats themselves.
- Uncharted Stars has a planet completely covered by city.
- Star Warped, a Star Wars parody, has a Coruscant parody called Metropolanet in addition to Earth having become one of these with a population of a trillion.
- The Worthing Saga shows in passing how one of these came to be. The short story "Skipping Stones" begins on a world that's heavily developed, but is still famous for its wildlife, particularly its "whiplash trees" that bend all the way to the ground in windstorms. As the skyscrapers go up, the trees go extinct, and eventually all the planet has become the "Capitol" that is the focus of later stories.
- Doctor Who:
- Firepower: The alien warworld is completely covered with assorted metal structures.
- Dying Earth: In Turjan's Tome of Beauty and Horror, the planet of Merchdilan is entirely devoted to business/commerce and entirely covered by city.
- SLA Industries: All Industrial Worlds, including Mort, have their land surface (and most of their seas) completely covered with industrial production facilities and cities.
- Warhammer 40,000: Specialized, city-covered planets aren't rare, and Imperial ones tend to be completely dependent on nearby agri-worlds for food imports.
- Hive Worlds are only partial examples, consisting of enormous Hive Cities dotting a vast polluted wasteland, but Holy Terra is an example where the Hives have completely covered the planetary surface. It's home to billions of people, its oceans are long since drained away and its surface is completely overgrown with cathedrals and administrative buildings (the Imperial Palace itself covers most of Asia, while the Astronomicon is inside a hollowed-out Mount Everest).
- The Forge Worlds of the Adeptus Mechanicus are planet-sized industrial sectors.
- Necron Tomb Worlds are covered in eons-old alien ruins, beneath which deathless metal warriors slumber in stasis... at least until interlopers wake them up.
- Fading Suns: Emperor of the Fading Suns features the planets of Byzantium Secundus and Leaguehiem. In terms of game rules new cities can't be built on these worlds (not that the Imperial Guard on Byzantium II would allow any units outside the embassies) as every hex is already covered with cities.
- Knights of the Old Republic: Taris is another one of these. Interestingly, it's not quite as ridiculously tall as other examples noted in the Film folder, as you do get to take an elevator to the "Lower City" and then another one to the "Undercity", where you actually get to stand on solid ground. It makes up for it in lower levels being Wretched Hives, however, with the Lower City being almost entirely comprised of swoop gangs and a powerful crime lord, and the Undercity being filled with dangerously poisonous predators called rakghouls, with the only people living there having been banished to the Undercity for some crime or another, or born to people who were banished there long ago. The ecumenopolis is subjected to a global Orbital Bombardment by Darth Malak at the end of the first act of the game, and returns in Star Wars: The Old Republic as a planet of Ruins of the Modern Age that the Republic is trying to resettle.
- Marco and the Galaxy Dragon: The Machine Planet is implied to be one. Scenes set on the planet showcase a futuristic urban sprawl that stretches as far as the eye can see, without even a hint of greenery or undeveloped land.
- Mass Effect: Feros is home to a sprawling ruin that covers at least two-thirds of the planet's surface. The remaining third is presumably the planet's ocean, though because of the dust and debris thrown about in the storms that plague the ground level, it's apparently hard to tell.
- Phantasy Star Universe: Parum. Although large sections are given over to nature reserves, the majority of the planet is urbanized.
- Ratchet & Clank tends to feature at least one city-themed planet per game, and as such features many variations on the theme. The straightest examples are Planets Oltanis and Endako from the original trilogy, along with Igliak and Terachnos from the Future trilogy. Interestingly, however, the two most recurring examples, Planets Kerwan and Rilgar, are subversions. Kerwan is primarily covered in urban sprawl, but features plenty of green space nestled within the buildings, which may explain why it appears fairly Earthlike when viewed from orbit in Up Your Arsenal and Tools of Destruction, which also revealed it has oceans (in contrast with its orbital appearance in the original game, where it looks more like Coruscant). Meanwhile, Rilgar is focused on Blackwater City, but the planet as a whole appears to be primarily covered in oceans, and Blackwater City itself features a number of caves filled with coral-like formations that are accessible by a short elevator ride, implying that the city is very close to sea level; in a similar vein, Planets Kalebo III and Damosel also take place in urbanized areas built above massive bodies of water.
- Sid Meiers Alpha Centauri: A possible scenario. If you achieve a Conquest victory and decide to keep playing after that, you can cover practically the whole of Planet in bases, with all resources (food, minerals, and energy) supplied by orbiting satellites and mines on Planet's moons. It's an interesting Self-Imposed Challenge.
- Starcraft features several. Korhal, the Dominion homeworld, bears a resemblance to the Star Wars planet Coruscant when seen from outer space.
- Star Ocean: Till the End of Time: Earth. Cities still appear to have individual regional styles, like New York, but together seem to cover the whole planet.
- Stellaris: With the MegaCorp DLC, you can build these. A city world cannot generate basic resources on its own, which means that it'll need outside planets to send it things like food and energy. They are, however, amazing at Refining Resources into more complex goods and research.
- Wing Commander: Privateer:
- New Detroit was one of the first industrial hubs in the Gemini Sector, resulting in a massive industrial boom that saw the initial settlement spread to cover the entire planet in only thirty years. The combination of a planet-wide city and heavy industrialization essentially destroyed the ecosphere and resulted in a near-perpetual cover of acid rain clouds.
- During the events of Wing Commander: Prophecy, the Nephilim invaded the Gemini sector and bombarded New Detroit from orbit. The majority of the population was killed and the planet has since been abandoned as the cost of cleaning up would be astronomical.
- Irregular Webcomic! spends a number of Star Wars strips deconstructing the idea, such as food, sewage, power, heat output, space and cooling. Unsurprisingly, Darths & Droids also touches on the issue. Parodying the deconstruction — even David Morgan-Mar knew he was running things into the ground — the last IW strip to touch on this "changes channels" halfway through to the Pirate storyline, where the captain is flogging a dead horse.
- Schlock Mercenary: Subverted with Earth, one of the most heavily populated planets in the galaxy. While it's known for being a heavily urbanized world, advanced technology means there's plenty of breathing room. Energy production and "agriculture" are so advanced that they can fit two-hundred billion people on the planet using only ten percent of the available landmasses (and some of the seas) for megacities that are measured in cubic kilometers instead of square kilometers. The remaining ninety percent of the land is preserved sort of like continent-sized national parks.
Footnote: There are other ways to fit 200+ billion people on a planet, but this is one of a very few ways to pull it off while still having it be useful as a planet.
- Steven Universe: The Gem Homeworld has been cracked and split into multiple chunks, cradled by a set of two rings and covered in dense cityscape.
- Transformers: Cybertron, a machine world from core to surface. The usual food/water/air problems are averted thanks to the population being fully robotic and powered by Green Rocks. The only exception is Beast Machines, which shows that Cybertron once supported organic life and ends with the planet transformed into a techno-organic Eden. In most series however, much of the planet is loosely, if at all populated, and it's City Planet appearance largely comes from simply being a metal planet, dotted with semi-independant mega-cities.
- Transformers Cybertron: In addition to Cybertron, over thousands of years of relentless construction, Gigantion has become several massive layers of city surrounding an Earth-like inner planet.
- Transformers: Generation 2: The concept is taken even further with the Hub, a vast, physically connected network of Cybertron-type worlds that serves as the center for power of the Cybertronian Empire.
"Baby Planet" Examples:
- Dark City: The city is a rather unique example — it is a City Planet because it's a world unto itself.
- Aeon 14: "The Cho" (short for "Callisto Orbital Habitat") is a gargantuan Hive City of concentric habitation rings that extends outwards from the surface of Jupiter's moon Callistonote , which used to be a terraformed world but whose surface has become devoted entirely to waste-processing for the Cho. Its population of over three trillion as of Outsystem comprises almost half of all humans in existence as of 4183 CE.
- "Strikebreaker": The setting is a hundred mile asteroid with a colony — barely started, but already with a fifty thousand people population, fully self sufficient. The story is centered around a supply problem — the man responsible for recycling the waste decides to go on a strike.
- Ratchet & Clank: Going Commando: In addition to the series' more conventional City Planets, the games feature a couple of Baby Moons, orbiting Dobbo and Damosel, that fit this trope perfectly. And you get to smash them to pieces with Giant Clank.
- Prospit and Derse are small, spherical agglomerations of gothic architecture floating through space. While only the surface is usually visited in the comic, it's shown that the buildings go all the way down into the planetoids' cores.
- Some of the kids' planets are also covered in artificial constructions of various sorts, such as the Land of Tombs and Krypton, which is covered in spire-like mausoleums; the Land of Wrath and Angels, which is covered in gothic cathedrals; and the Land of Tents and Mirth, which is covered in tents and circuses.
"Plane of Existence" Examples:
- The Mighty Thor: Asgard is a city that encompasses another plane of existence.
- The Concentration City: The densely crowded residents can't find an end to the upper and lower floors of the buildings that they live in. Train rides out of the city end with you coming back in. Coming back on the same day, the exact same time, as your departure. This is a city that has swallowed everything.
- Dungeons & Dragons:
- Dis, the second layer of Hell, is a single, vast city.
- Mechanus, the heaven for Lawful Neutral beings, consists largely of clockwork structures. Some of the gears alone are said to be the size of small continents, and many have buildings built on top of them.
- Planescape: Sigil, City of Doors, is a world that happens to be a city, although it's not a planet — it's Another Dimension. The city of Sigil doesn't cover the entire plane either, but the city is so distinct from the rest of the Outlands, and there is no effective way of traveling from one to the other without magic, so it is effectively this trope.
- Ravenloft: Paridon became this trope after the Great Upheaval stripped away the countryside that used to surround it, reducing its Island to a pocket of streets and buildings adrift in the Mists. (Not quite a plane, but Land of Mists certainly isn't a planet, so...)
- Magic: The Gathering has a high fantasy version of this in Ravnica. The city covers the primary planet in one plane of an infinite Multiverse.
- The illustrations on some of the Basic Land cards are particularly impressive, depicting Forests as expansive gardens, Islands as the city's waterways and reservoirs, Mountains as the tallest spires and centers of heavy industry, Plains as the suntouched rooftops, and Swamps as the sewers.
- The plane didn't use to be that way; in fact, there are still remnants of the wilds on Ravnica, most of them in Ravinca's expansive and poorly explored Undercity or in ruined and overgrown areas of the world-city, and are primarily the home of the Gruul Clans and some members of the Selesnya Conclave.
- To solve the food issue, one of Ravnica's ten Guilds is devoted to spreading and preserving growth on every available inch of the city. Another is devoted almost entirely to various forms of compost and recycling, including necromancy. Yet another is constantly developing new organisms that will thrive in the endless urban sprawl, and a fourth smashes old areas and lets the wilderness return in their area.
- Return to Ravnica addressed the question of why Ravnica doesn't have any large natural bodies of water. Turns out that they paved over the oceans. After the failure of the guildpact the merfolk living beneath Ravnica began to sink huge sections of the city.
- Both New Capenna and Amonkhet are semi variants, which a single massive city is all there is outside of vast wasteland (in the former's case the reuslt of a phyrexian invasion, in the latter's an unclear calamity). The former's inner structure is currently unexplained, but the latter is sustainable thanks to the Luxa River, which provides water and crops. Mummies provide all the work force, leaving the living free to pursue other activities.
- Warhammer 40,000 has Comorragh, the city of the Dark Eldar. It's roughly the size you'd expect from this trope, but it's not on an actual planet — rather, it's a collection of ports, fortresses, lesser settlements and assorted strongholds and refuges built by the Dark Eldar within the Webway (essentially a sort of pocket dimension consisting of a galaxy-spanning maze of tunnels and passages used by the various Eldar peoples for FTL travel) all closely linked by a large number of portals and passages to the point that, for all practical purposes, they act as a single, vast city.