Ecumenopolis

An ecumenopolis, also called a cityworld, urban planet, urbano, and other names, is a planet or moon whose entire surface is covered by cityscape. The word originates from Greek, an ancient language of Earth, and is derived from οικουμένη (oikoumene), meaning "world", and πόλις (polis) meaning "city"; thus a "world-city" (pl. ecumenopolises or ecumenopoleis). Ancient works of science fiction in most civilizations have depicted ecumenopolises.

History
Over time, a species may end up transforming their homeworld into an ecumenopolis. This is not always the case; sometimes urbanization is restricted for the sake of preserving natural areas. Planets that do undergo complete urbanization often transport the remaining surface wildlife to wildlife sanctuary planets or facilities, or simply enclose such areas in climate-controlled domes and build over them. In more extreme cases, the oceans themselves will be built over or even drained entirely.

Definition
According to most academics, the landmasses of a planet or moon must be at least 90% covered by cities in order to class as an ecumenopolis, although that definition is challenged by critics who say that an ocean world with a single urbanized island would technically fall into that category. An unofficial but generally accepted opinion is that a planet which has at least 20% of its surface covered by dry land, and that land is at least 90% urbanized, is considered an ecumenopolis; and that an ocean world with a few urbanized islands would still be considered oceanic.

Variants
Garden-variety ecumenopolises feature dense cityscape with an average rooftop altitude ranging from half a mile to below two miles, and their populations normally sit between one and ten trillion. Older, more advanced ecumenopolises have skylines that stretch five to ten miles above the surface, and can have upwards of 500 trillion people. Aerial structures, such as sky continents, are often employed to expand capacity.

A hyperopolis is a significantly more advanced ecumenopolis in which a significant portion of its skyline surpasses the boundary of space, typically concentrated at the equator. This often allows starships to dock at spaceports without needing to enter the atmosphere. Hyperopolises are usually more than 10,000 years old and have populations that sit in the upper hundred trillions to low quadrillions.

An urban giant is another advanced variant, in which the habitable altitude of a gas giant's atmosphere is occupied by interconnected aerial structures. Such worlds often host populations surpassing a quadrillion people.

A ruined ecumenopolis, also called a rustball, necumenopolis (from necro-), or ghostopolis, is a devolved variant that suffered from a disaster or economic decline, resulting in total or partial depopulation and disrepair. Riesel is technically considered a ruined ecumenopolis since roughly 90% of its cityscape is in disrepair, and most of its remaining population of 12 trillion resides in the 10% that still sees proper maintenance.

Sustainability
The standard ecumenopolis will often feature gargantuan facilities (which can be dozens of miles in width) devoted entirely to food production, water storage and purification, and waste treatment. Massive food shipments from outside sources are also common.

Waste heat is handled by various means, often by the implementation of heat dissipators in each structure. Other ecumenopolises may resort to using massive and powerful refrigeration towers, as seen on Riesel.

The buildings themselves are commonly constructed with lightweight and durable metamaterials, some of which naturally negate gravity. Gravity repellers are also employed for stability, acting similar to vernier thrusters or invisible guy-wires to prevent the buildings from swaying excessively.

Occurrence in the galaxy
Urban worlds are not particularly uncommon and usually occur along major trade routes. There are thousands of lesser variants (i.e. most of the cityscape is below a mile in height), and a few hundred whose towers stretch up beyond a mile.