Artificial planet. An artificial planet (also known as a planetary replica or a replica planet) is a proposed stellar megastructure. Its defining characteristic is that it has sufficient mass to generate its own gravity field that is strong enough to prevent atmosphere from escaping, although the term has been sometimes used to describe other types of megastructures that have self-sufficient ecosystems. The concept can be found in many works of science-fiction.

In science

Replica planet 
Mark Hempsell suggested that an artificial replica planet could be created in the Solar System as preparation for future space colonization, probably in the habitable zone between the orbits of Venus and Mars. It could evolve from the construction of a smaller space habitat. They would have similar purpose to other large scale megastructures intended as living spaces (such as O'Neill cylinder) or the concept of colonizing (or terraforming) existing planets. Unlike a space habitat, the artificial planet would be large enough to create its own gravity field that would prevent its atmosphere from escaping, and atmosphere would also serve to protect the world from radiation or meteorites. However, an artificial planet would have a much worse mass invested to usable surface area ratio.

Material for artificial planet construction could be extracted from stars or gas giants or from asteroid mining. A sufficiently advanced civilization could use those resources to mass-produce artificial planets using a stellar factory that itself would likely be the size of a large planet.

Construction of an artificial planet has been described as scientifically plausible but likely taking thousands of years and would be highly expensive. It has also been suggested that such an endeavour would be more challenging than terraforming existing planets, although both ideas are mostly speculative at this point of human history.

Other 
The term artificial planet has also been used to describe other types of megastructures, such as large spherical space stations. D. R. Glover defined artificial planet as "a self-sufficient, independent ecosystem in space", noting that size of such an entity is less relevant and that it could be much smaller than what is traditionally defined as a planet. Glover sees development of such a station as a precursor step for development of ships capable of interstellar travel. 

Paul Birch has used this term to describe a concept of a supramundane planet. Such a structure would resemble the concept of a Dyson sphere, as the habitable surface would exist on the inner side, but it would be built  around a massive stellar body, such as a giant planet or a black hole.

In fiction and popular culture 

The concept of artificial planet can be found in many works of science fiction. An artificial planet is the main setting of several science fiction series, such as Philip José Farmer's Riverworld series (1971–1983), Jack L. Chalker's Well World series (1977-2000) and Paul J. McAuley's Confluence trilogy (1997-1999). Iain Banks' novel Matter (2008) is set on a shellworld (an artificial planet with several habitable layers).

The concept of artificial planets is also found in, among others, The Hitchhiker’s Guide to the Galaxy franchise created by Douglas Adams, where one of the characters is a "planet designer". The Death Star from the Star Wars franchise has been called an artificial planet as well.

In the 2000 film Titan A.E., a groundbreaking scientific project known as "The Titan Project", was designed to create new man-made, habitable planets in space.

See also 

 Atherton: The House of Power
 Dyson sphere
 Generation ship
 Ringworld
 Shellworld

References 

Megastructures
Space habitats