City planet, frequently known as ecumenopolis, is a theoretical class of terrestrial planet with most or nearly all of the surface covered in cities, meaning there must be intelligent life roaming on those planets since cities are only built artificially. Much of the surface of city planets would be occupied by buildings, including skyscrapers like we see in cities on Earth. The color of city planets as viewed from space usually range from steel gray to brown. Population of city planets the size of Earth tend to be more than a trillion and humans may not be the only creature in our galaxy capable of building into city planets.
Like forest planets, city planets are never defined in science as they are only seen in science fiction, such as Coruscant in Star Wars. At present, the prospects of finding evidence of real city planets are very slim.
The color of city planets depend on the materials used for buildings, streets, etc. If city planets are brown, it means that cities are made mainly of wood, while gray means that building would be made of metal such as steel. Most city planets would be gray since metal is preferred in buildings since metal is stronger material than wood, and it thus lot less flammable than wood. Most city planets that was formerly forest planets tend to be brown due to vast amounts of wood available. Desert, ice, and ocean planets becoming city planets would tend to be gray with buildings and other structures made of metal due to little or negligible variability of wood.
The planet should be located inside the habitable zone with a very circular orbit (e<0.04) in order for life to properly evolve into intelligence capable of converting nature-wide world into city-wide world. The planet should also have at least one large moon to reduce the extremity of the planet's axial tilt and therefore climatic variations.
The star type planet orbits affects the evolution of life into intelligence. Since cooler, redder stars have longer lifespans than hotter, bluer stars, habitability would last longer and thus lengthen their evolution, making it more likely to evolve into intelligence. City-wide worlds are believed to be most commonly found orbiting around K-type stars. Habitable planets around M-type stars tend to be tidally-locked making it more difficult for forming intelligent life.
Evolution of life into intelligence usually takes three to five billion years. Habitable planets orbiting around short-lived stars especially A-, B-, and O-type stars would have no chance for life to evolve into intelligence before stars die. To evolve into intelligence, planets should orbit around stars with masses less than 1.5 M☉.
If there are city planets around red dwarf stars, then intelligent civilization would most likely have moved from another planet around another star. This move is what human civilization might do far in the future when Earth becomes overpopulated or uninhabitable. If there is a rocky planet in the habitable zone around Barnard's Star with water on its surface, then this would likely be the next home for us, called Earth 2.
City planets, unlike all other classes of planets, is not natural since city components like buildings, skyscrapers and streets are not formed naturally. Life must evolve into intelligence, and those life must have capability to build cities. This process requires clearing of land and loss of habitats, which can cause extinctions to other organisms. As world population grows, so does numbers and sizes of cities. Eventually all the cities and metropolises merge to form ecumenopolis, which is a single, worldwide city. In order to qualify as a city planet, at least 50% of the planet's surface must be covered by cities, but ecumenopolis is not required. City planets can have many metropolises that are not merged together but are separated by rural areas (towns) or countryside in small widths. Even city planets have natural (non-city) terrains such as forests, bodies of water, ice caps, deserts, mountains, etc.
Intelligent civilizations can turn other planet class into a city planet. Desert planets, ice planets, or even forest planets can become city planets when they build cities on top of sand/rock or ice/snow, respectively, and on forest planets, most of the forests must be cleared for urbanization. It is extremely challenging to turn ocean planets into city planets, as floating cities would have to be built over much of the planet's surface. If cities cover most of the surface of ocean planet, then there is a term "ocean city planet," hybrid between ocean planet and city planet. Another possible method is to drain oceans and then build cities on seafloor like it has done on Coruscant.
Humans may not be the only intelligent beings in the universe. Believably in the universe, type of organisms most likely to have capability of building cities are primates or the like. On many city planets, dominant lifeforms would tend to be primates, meaning creatures that can walk on two legs and use of two arms for grasping or operating tools. It can be humans or human-like organisms depending on the evolution pathways. City planets can even have many species and global populations are often numbered in the trillions. Since first cities on current city planets are often built thousands of years ago, one dominant species can evolve into different creatures mostly by genetic engineering. This evolution can also caused indirectly by different environments, or even naturally regardless.
On many city planets, intelligent species don't just contain one type, but it can cross-breed with other organisms to form hybrids using genetic engineering. One common method is to cross-breed with flying creatures so that intelligent beings can develop wings for flying, which would be important for quickly navigating to another nearby building without even using vehicles nor walkways, and also for entertainment especially in conjectured flying sports.
In the universe, biological life is not required to become city planets. Machine life can even have such capability, often superior to humans. Such life, including robots, would be extremely useful that it reduces human workload. Machines don't just use tools and building materials, but it can grow and self-replicate to assemble buildings.
Cities on those planets are used for employment and entertainment. Mayor is a boss ruling in certain areas and is used for city planning and management and police is used to make people safe. Members living in cities are called city goers or more familiarly, resident. In addition to featuring buildings and other components by advanced engineering, city planets often come with computerized technologies, used specifically for navigation, communications, and entertainment important for society. City planets tend to be orbited by satellites, used for advanced navigation and communications and serve as lifeblood of society. It is also helpful for designing, modifying and building bigger cities that would eventually become city planets beforehand.
Politics is the controlling of land used for civilization. On city planets, almost the entire surface are influenced by civilizations. Surface of planets in different regions are influenced by different groups, known as political divisions. On Earth, the most important political split is countries, which are further divided into states, provinces, or territories. Different planets with civilizations may well split lands into so-called countries often with subdivisions. Metropoleis on Earth are often split into districts, boroughs, and suburbs. Since they're worldwide city on city planets, subdivisions like districts, boroughs, sectors, and suburbs tend to be superceded by larger political splits, including quadrants as is the case on Coruscant.
Politics influence cultures in different ways. Different political areas would experience some different arrangements of occasional events, especially holidays.
People use language on city planets for communication since it is a universal tool of communication. On some city planets, one worldwide language can be spoken for entire global population while others can speak in several different languages. Even though English is most commonly spoken on Earth and most commonly used on other planets in science fiction including on Coruscant, English is not the universal language in reality, meaning none of extraterrestrial life speak English or any of the other languages used on Earth.
Timekeeping is a universal tool for intelligent life. It is used for wide variety of purpose, including sequence and arrangement of events and for scheduling. Methods of timekeeping can be concepted in variety of ways depending on the world where intelligent beings live. For us on Earth, we use timekeeping based on the Earth's rotation (days), orbit of the Moon (months), and Earth's orbit around the Sun (years). This method of timekeeping may be very common among intelligent life in the universe. For timekeeping less than planet's complete rotation, they tend to split into units based on specified fraction of time it takes the planet to rotate 360°. For us on Earth, days are split into 24 parts, well known as hours, hours is further divided into minutes then seconds. This practice is very useful for specifying daily events such as sleep cycle, meals, employment, education, and others.
For us, tools of timekeeping are calendars (for specifying days, months, and years) and clocks (determining 'time location' in a day). Alien civilization may also use these tools. All days are not equal, days may contain special events such as holidays, a reason why calendars are so useful. Clocks are also important, which help people coordinate activities, especially daily activities.
Much of the surface of city planets are covered with buildings, streets, parks, and other urban components. Most buildings on most such planets tend to be skyscrapers such as featured on Coruscant. Tallest skyscrapers would tend to be several times taller than the tallest skyscraper on Earth, often miles high, since city planet typically have many more skyscrapers than there are on Earth. Another common feature of buildings would be interconnections, which can be used to go from building to building without going outside. This feature can even feature indoor streets, as well as outside. Streets are important urban component used as pathways to go from one block to another via on foot or by vehicles. Parks can be found in open spaces between the building or even on the roof of buildings. City areal components include downtown, neighborhoods, undercities, alleyways, courtyards, tracks, and parking lots.
Cities on Earth influence environments in many ways, ranging from greenhouse gas emissions causing global warming to light pollution caused by abundance of city lights. Environment impacts would certainly be lot greater on city planets in pretty much every factor.
Cities on Earth have factories that can emit wastes which can cause pollution or even long-term effects such as global warming. On city planets, environmental effects would be lot greater than experienced on Earth due to having many more factories and powerplants. However most city planets have life intelligent enough to respect the environment by using renewable energy sources to power the city like from its sun. On city planets, many buildings would have solar panels or windmills on rooftops used to generate electricity.
City lights cause skyglow at night, an effect known as light pollution. Observing stars or other objects on city planets would not be as good because the brightened skies would wipe out most of the fainter celestial objects. Brightened night skies on city planets are seen almost everywhere around the globe, whereas on Earth, night skyglow are seen only in localized or regional scale. As a result, there would be few or virtually no dark sky locations, depending on the proportion of surface covered in cities and area of non-city terrains. Correspondingly, observatories on city planets would be few or nil, and astronomical studies are mostly done in space or on one of its moons.
Bright nights on city planets would be so great that it would strongly cast light and shadows onto the surface. From space, details on the night side of city planets can be easily seen and nightside often have considerable proportion in brightness to the star-lit dayside.
Global climates on city planets are different than Earth's, because numerous skyscrapers almost everywhere affect global wind and stellar heating patterns, as well as less surface water. Skyscrapers casting shadows on the streets below combined with wind blockage mean that street level would be cooler than on rooftops, but decreasing temperature with increasing altitude would minimize the effect. As a result, winds would be light or almost calm in gaps between skyscrapers, depending on distributions, while winds are higher on rooftops. On top of the tallest buildings, winds would be strongest due to the natural rule that wind speeds increase with increasing altitude.
Precipitations like rain or snow on city planets are rarer than Earth's, because they are mostly land producing less evaporation from bodies of water. This results in dryer, more desert-like climates.
Demise and survival
Like on Earth, natural disasters, such as storms, earthquakes, and volcanic eruptions can cause destruction and deaths on city planets, although on localized or regional scales. Big enough asteroid or cometary impact can cause global destruction of city planets as well as loss of civilizations. Intelligent beings on city planets usually have capability to prevent such impacts by deflecting asteroids, like what humans will be doing in the future. Another cause of global destruction can be a worldwide war, especially with the use of nuclear arsenals. Alien invasion can also result in destruction of ecumenopolis. A worldwide epidemic can also result in loss of civilization but not ecumenopolis, allowing city planets to exist without civilizations. Loss of civilization but with cities standing can also caused by dramatic changes in climate and cosmic events such as nearby supernova or gamma ray burst.
For civilizations to survive the aging of the parent star, they may need to move the planet to more distant orbit to stay habitable, but when parent star is dying, they may need to move to around another star. If their technology is advanced enough, they can move the planet to around another, younger star using wormhole or teleporter instead of leaving ecumenopolis behind and rebuild it on another planet. In alternative to moving a planet, they can create an artificial star when a natural star dies.
SETI is currently searching for artificial radio signals from exoplanets. However, this method would not be a city planet finder, as technique only measures the concentration and frequency of E.T. signals. SETI has yet to detect E.T. signals despite over 50 years of searching, initiating Fermi paradox about "Where Is Everybody?." Maybe it is possible that beings on city planets evolve away from leaking signals out into interstellar space.
A possible method to find city planets is the detection of artificial lights on the night side of the planet. This can be done with next generation of telescopes such as European Extremely Large Telescope (E-ELT) and perhaps James Webb Space Telescope (JWST). Detecting artificial lights from exoplanets would be extremely hard since lights would be very faint as seen from Earth with magnitudes beyond current capabilities. The easiest solution is to isolating the light of the transiting planet by subtracting starlight (when the planet is behind the star) from the light of the star and planet's reflection combined. A more far-fetch technique is to use Sun as a lens, this works by using gravitational lensing of the Sun to brighten faraway planets, making city lights easier to detect. But in order for this method works effective enough, a telescope would have to be located at least 550 AU from the Sun, roughly 17 times the current distance between Pluto and the Sun.
Another way to find city planets is the study of angles of reflection due to structures, the same method used for finding forest planets. This would be carried by Bidirectional Reflectance Distribution Function (BRDF), a project used to study shadow variations or changes of planetary albedos throughout the planet's orbit, thus determining what's on the planet's surface. Shadows seen on the planet's surface would most often be caused by structures such as mountains, hills, boulders, cliffs or other geologic features. Unique variations of shadows cast on the surface would be used to determine what structure it is casting shadows and would be used to detect signatures of buildings or other structures that make up the city. By studying number and density of buildings, it would be used to determine if it is a city planet.
Intelligent life with capabilities of building cities in the Milky Way Galaxy would likely be extremely rare, to go along with Fermi paradox. Due to rarity, city planets are extremely rare, lot rarer than even forest planets required by complex life. It is estimated that in our galaxy alone, there are only 16 million city planets out of 820 billion total planets (including giant planets) and 524 billion terrestrial planets, making it the rarest physical class of terrestrial planets. It is so rare that we need a sample of roughly 52,400 planets and 33,500 terrestrial planets just to find an average of one city planet, which means that such planets are unlikely to be discovered anytime soon giving that there are only about a thousand planets identified and about 200 terrestrial planets! However, city planets are still more abundant than four of nine chemical classes of terrestrial planets, especially chlorine planets as the rarest class of terrestrial planets, and twice as abundant as carbon giants, the rarest classification of giant planets.
Will Earth become a city planet?
In 1967, city planner Constantinos Doxiadis represents the possibility that every city would eventually merge to form one worldwide city called ecumenopolis. Since most of the Earth's surface are covered by oceans, Earth becoming a city planet would be impossible, however it is possible to build floating cities on water, thus making the city planet Earth possible. It is projected that almost all of Earth's surface would be covered by cities by middle of this millennium assuming present trend of population growth. However, population growth is projected to stop in about 60 years, meaning ecumenopolis on Earth would be far-fetch. So Earth becoming a city planet may not happen for at least millennia.
As of now, city planets are only in the realm of science fiction. The most notable fictional city planet is Coruscant, home planet of Jedi in Star Wars. Coruscant is also known as the Imperial Center and was the center of the Galactic Republic, and located in the hyperspace coordinates (0,0,0). The name of a single, planet-wide city on Coruscant is the Galactic City.
Town planet is a less populated subclass of city planet with most or nearly all of the surface covered in towns usually with few isolated urban areas. It is believed that town planets are often precursors of standard city planets due to more efficient developing societies and politics. Town planets would be much more difficult to detect those than standard city planets due to lot fewer artificial lighting.
- Forest planet
- Ocean planet
- Extraterrestrial life
- Planetary habitability
- Habitable planet
- Extrasolar planet