A city planet is a subtrope of Single-Biome Planet and Mega City, in which said biome is said 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 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, but for all practical purposes, it's all the same city. Generally, this trope implies that for all practical purposes, 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 Mohs Scale of Science Fiction Hardness or the Sliding Scale of Realistic Versus Fantastic, such a world can present a Mega City-sized ball of Fridge Logic. Perhaps most importantly, what do people eat if there is no farmland? 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.
- Massive greenhouses for crops and battery farms for livestock.
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. 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 township. Even assuming that the census data is off doesn’t work as you would run into problems overheating the planet once you got passed 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.
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.
- Tsutomu Nihei's Blame! is set in a City Dyson Sphere that is implied to have long since consumed the solar system with its sheer enormity.
- Pestako from The Five Star Stories. 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.
- From DC Comics Jack Kirby's Fourth World series, Apokolips.
- The eponymous planet in Alejandro Jodorowsky's Megalex.
- British 1980's science fiction comic Starblazer.
- Issue 28 "Last Man on Earth". The Wheel was the fifth planet of Alpha Centauri. It was a planet-city ruled by a dictatorial A.I. 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.
- The Superman: The Man of Steel "Legends of the Dead Earth" annual had the world-city Metropole, capital of Lex Luthor the 60th's empire.
- In Jupiter Ascending, the planet Ourus, the birthplace of humanity, has buildings that stretch above the atmosphere and two artificial rings.
- Star Wars has Coruscant, the capital of the Galactic Republic. Other, lesser-known planets and moons also fit the bill, such as Nar Shaddaa.
- It is interesting to note that Coruscant escapes some of the population questions specifically because it is depicted as having extensive areas dedicated to purposes other than habitation. In Attack of the Clones and Revenge of the Sith, industrial zones and landing areas for spaceships more than a kilometer in length are shown. Everything is bigger on Coruscant! Even with vast areas that are unpopulated, Expanded Universe sources and one of the movie novelizations peg Coruscant's population at upward of 1 trillion permanent residents plus billions of transients. As a result, the planet has to import vast amounts of food from elsewhere.
- In The Hutt Gambit, Han Solo notes that the uppermost levels of Nar Shaddaa look like the lowermost levels of Coruscant. He promptly resolves NEVER to visit the lowermost levels of Nar Shaddaa.
- Come The Force Awakens, Hosnian I, center of the Hosnian system and current capital of the New Republic, is shown to be a city planet. It gets destroyed by Starkiller Base.
- In Star Trek: First Contact, Earth has become this under Borg rule in an alternate timeline. Oddly enough the population consists of only 9 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.
- In the Super Mario Bros. Movie, Koopa's globe of the dinosaur-ruled parallel Earth includes a stylized depiction of a country-sized city as an island in a worldwide desert (which he counts as under his rule). As far as logistics, there's a severe shortage of food, clean air, and water, but it's largely blamed on Koopa's 20-year regime (and is the driving force behind his planned conquest of our Earth).
- In Isaac Asimov's novels:
- Trantor, capital of the Galactic Empire, is apparently completely covered by urban areas by the time of the Foundation series. Until it's sacked and looted into an agrarian world once the Empire falls.
- In the prequel Prelude to Foundation, it is revealed that the planet 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 this trope.)
- In the Foundation series Asimov stated that he was drawing a direct historical parallel from Gibbon's Decline and Fall of the Roman Empire. Ancient Rome was then the world's largest metropolis, with a population of over a million people, something which would not be matched again until 19th Century London. Rome was so densely populated that it could not sustain itself off of local agriculture and food was shipped constantly from North Africa (at first from Egypt, later from what is now Tunisia and Algeria—or in other words, the old heartland of the Carthaginian Empire) to supply the city. Trantor likewise is dependent on imports of food from other planets, a point which becomes historically important in the fall of Trantor as the capital of the Galactic Empire. The final claimants to the title of Emperor are eventually forced to move to another planet, much as the government of the Western Roman Empire relocated to Ravenna during the transition to the Dark Ages.
- In his 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)—he wasn't exactly an agoraphobe—and hated flying.
- Trantor, capital of the Galactic Empire, is apparently completely covered by urban areas by the time of the Foundation series. Until it's sacked and looted into an agrarian world once the Empire falls.
- Andre Norton's Uncharted Stars has a planet completely covered by city.
- Bill the Galactic Hero by Harry Harrison 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 worse than that. Losing a map is a crime on Helior. If you're caught without one, you're arrested.
- 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.
- In the works of Sheri S. Tepper:
- In Beauty, the Earth has had all its wilderness wiped out, followed by any and all crop growing facilities.
- And in Shadow's End, the governing planet of an entire solar system is a City Planet.
- In Ecumenopolis, 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.
- 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.
- In Emma Clayton's The Roar, "The Upper Half of the World".
- From Dan Simmons' 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.
- The 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 titular planet of Riesel in Riesel Tales: Two Hunters 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; though 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.
- In Jack Williamson's "The Cosmic Express" (1931), 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.
- In the 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.
- Doctor Who:
- The American Space Patrol has Terra, the man-made planet (although that's more of a "Space Canberra").
- The alien warworld in Firepower is completely covered with assorted metal structures.
- A specific example from Warhammer 40,000 is Holy Terra, home to billions of people its oceans drained away and its surface completely overgrown with cathedrals and administrative buildings (the Imperial Palace itself covers most of Asia, while the Astronomican is inside a hollowed-out Mount Everest). The Forge Worlds of the Adeptus Mechanicus are planet-sized industrial sectors, while Hive Worlds are only partial examples, consisting of enormous arcologies dotting a vast polluted wasteland. Finally, 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.
- It's explicitly mentioned there that all these specialized worlds do import their food from nearby agri-worlds. Well... probably not the Tomb Worlds.
- Dying Earth RPG supplement Turjan's Tome of Beauty and Horror. The planet of Merchdilan is entirely devoted to business/commerce and entirely covered by city.
- In Dragonstar, Draconis Prime.
- Magic: The Gathering has Ravnica, a rare fantasy example. 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.
- SLA Industries. All Industrial Worlds, including Mort, have their land surface (and most of their seas) completely covered with industrial production facilities and cities.
- 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.
- Total Annihilation: Subverted, Core Prime is a giant computer containing the conscience of the whole population and the entire Core culture.
- From Freelancer, the planets Manhattan, New Berlin, New Tokyo and New London.
- Starcraft also features several.
- From Meteos, the oddly-shaped world of Grannest.
- Zerard from Rogue Galaxy.
- Sunder from Anachronox.
- Parum from Phantasy Star Universe. Though large sections are given over to nature reserves, the majority of the planet is urbanized.
- Earth in Star Ocean: Till the End of Time. Cities still appear to have individual regional styles, like New York, but together seem to cover the whole planet.
- Feros from Mass Effect is home to a sprawling ruin that covers at least two-thirds of the planet's surface. The remaining third is presumably that of 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.
- New Kroy (invert the letters in Kroy, by the way, and see what you get) from Fury.
- Z: One of the five planets in the game is of this type.
- Taris in Star Wars: Knights of the Old Republic 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 Pirate Homeworld, a.k.a. Urtraghus, from Metroid Prime 3: Corruption. The only non-organic patches found there, such as the Metroid nests or the Leviathan Seed, were introduced to the planet by the Space Pirates very recently. As a result of heavy pollution, the planet is also covered in a Perpetual Storm of extremely corrosive acid rain that will kill Samus in seconds without a Hazard Shield.
- New Detroit in Wing Commander: Privateer 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.
- The Ratchet & Clank series tends to feature at least one city-themed planet per game, and as such feature many variations on the theme. The straightest examples would probably be 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.
- 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.
- The city planet of Acmetropolis in Loonatics Unleashed.
- Cybertron in Transformers, 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.
- In Transformers Cybertron, in addition to the title planet, over thousands of years of relentless construction, Gigantion has become several massive layers of city surrounding an Earth-like inner planet.
- The concept is taken even further with The Hub in Transformers: Generation 2 a vast physically connected network of Cybertron-type worlds that serves as the centre for power of the Cybertronian Empire.
"Baby Planet" Examples:
- Isaac Asimov has a story called The Strikebreaker about 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 the supply problem - the man responsible for recycling the waste decides to go on a strike.
- In addition to the series' more conventional City Planets, Ratchet & Clank: Going Commando features 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 from Homestuck.
"Plane of Existence" Examples:
- In The Concentration City by J. G. Ballard, the densely crowded residents can't find an end to the upper and lower floors of the buildings 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.
- In 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's Sigil, City of Doors is a world that happens to be a city, although it's not a planet — it's Another Dimension.
- The Ravenloft city of 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.
- 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.
- 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.