Living World

The concept of Living Worlds refers to planets, moons, stars, asteroids or other celestial bodies which are also living beings, either literally or in practice due to being covered by a single superorganism. The idea of living worlds is incredibly prevalent in science fiction and fantasy. Generally speaking these worlds are sapient but this isn't necessarily always true.

In real life
Applied to the real world, there is an idea known as the Gaia Hypothesis, as proposed by James Lovelock and defended by other scientists including Lynn Margulis. They propose that Earth's biosphere itself may be considered; if not a real superorganism; at least a self-regulatory system with features of a living being itself. According to this idea, the biosphere tends to have a stabilizing effect upon the habitability of Earth, in terms of regulating environmental factors such as temperature and atmospheric composition; the result being that the presence of life helps create an environment adequate for life. Therefore: the biosphere itself is self-perpetuating. The opposite idea; the Medea Hypothesis, as proposed by Peter Ward; claims instead that life tends to destabilize its environment in the long-run, therefore: the biosphere is self-destructive. It is also possible that neither Gaia nor Medea represent an accurate picture of Earth, and that reality would fall somewhere between the two scenarios.

In popular culture
Usually, even when part of a hard sci-fi universe, living worlds will tend to have an element of mystery and even mysticism, since it's hard to see how they could have originated. Generally speaking, though, there are three basic situations that can lead to the formation of a living world. Some cases may draw elements from two or more of these situations.
 * 1) The world is indeed a form of life, in this case, a space-borne organism so huge that it essentially becomes a world.
 * 2) The world is an artificial construction which possess some form of artificial intelligence.
 * 3) The world is a naturally-formed celestial body which is covered by a single organism or superorganism, or rather, a world in which the entire biosphere has evolved into a single superorganism, which is very common in science fiction.

It should be noted that the definition of "living world" is vague at best, impossible to define at worst. For instance, the capacity for reproduction, which is usually considered a fundamental property of life, is rarely shown in such beings. However, if a living world is considered a superorganism, perhaps the perpetuation of its own biosphere would be considered reproduction, rather than the creation of other living worlds. It's also possible that many of the examples listed below are capable of reproducing, in some way of which we are simply not aware of. Saybrook's planet's attempt to convert Earth into another living world, for instance, could be considered an attempt at reproduction.

In addition, the definition of "world" itself may be problematic. It is intuitive to think of a "world" as referring to any celestial body which either is or could be made habitable. Still, for the purpose of simplicity, living stars will be listed on this page. The distinction between stars and planets is made difficult by the existence of brown dwarves, multiple star systems and rogue planets. Living clouds of gas, on the other hand, although quite commonly featured in science fiction, are usually not considered "worlds" (despite that they may be inhabited by some exotic forms of life) and will not be listed.

Finally, it is fair to note that practically all living creatures are inhabited by smaller forms of life (parasites, symbiotes and commensals). Therefore any life form that is space-borne could technically be regarded as a living world from this point of view.

Moons

 * Alter-Ego from Marvel, Ego's brother and eventually moon.
 * Brethren Moons from Dead Space
 * Doctor Moon from Doctor Who
 * Erys from the Doctor Who audio "The Brood of Erys"
 * Erythro from Isaac Asimov's Nemesis
 * Eywa, a.k.a. Pandora, from James Cameron's Avatar
 * Id the Selfish Moon from the Marvel Universe
 * Indobok from Star Wars

Planets

 * Acheron from Unreal
 * Akhaten from Doctor Who
 * Alcoreth from Nat Schachner and Arthur Leo Zagat's The Menace from Andromeda
 * Alyx from Murray Leinster's The Lonely Planet
 * Cheetah World from Doctor Who
 * Chiron, a.k.a. just "Planet" from Sid Meier's Alpha Centauri
 * Dahak from David Weber's Mutineers Moon
 * Dark Bramble from Outer Wilds
 * Earth in Sir Arthur Conan Doyle's When The World Screamed
 * Ego the Living Planet from the Marvel Universe
 * Evil from The Fifth Element
 * Eylor from Rifts
 * Fairy from Sentō Yōsei Yukikaze
 * First Sirian Bank from Terry Pratchett's The Dark Side of the Sun
 * G889 from Earth 2
 * Gaea from John Varley's Gaea Trilogy
 * Gaia from Isaac Asimov's Foundation's Edge
 * Ghroth from the Cthulhu Mythos (a creation of Ramsey Campbell)
 * Gozmastar from Dengeki Sentai Changeman
 * House from Doctor Who
 * Kathulos from the Marvel Universe
 * The Krang from Alan Dean Foster's The Tar-Aiym Krang
 * Totus from IDW's Teenage Mutant Turtles
 * Laylora from the Doctor Who novel The Price of Paradise
 * Mogo from the Green Lantern comics (a creation of Alan Moore)
 * Ōban from Ōban Star-Racers
 * Pandarve from Storm comics
 * Penelope from Kir Bulychev's A Million Adventures
 * Petaybee from Anne McCaffrey's Petaybee Series
 * Phaaze from Metroid Prime
 * Primus from Ben 10
 * Primus/Cybertron from Tranformers
 * "Planet 7 of Star System 84" in Ray Bradbury's Here There be Tygers
 * Planet Remina from Junji Ito's Hellstar Remina
 * Raga from the Green Lantern Comic in DC
 * Ronyards from Star Wars
 * Safenesthome from the Doctor Who audio "Earth Aid"
 * Saybrook's Planet from Isaac Asimov's Green Patches
 * Scub Coral from Eureka Seven
 * Solaris from Stanislaw Lem's Solaris
 * Symbios from the Doctor Who audio "The Crimes of Thomas Brewster"
 * Thallon from Star Trek: New Frontier
 * Triton from Muhammed Zafar Iqbal's Triton is a Planet's Name
 * Unicron from Transformers
 * Venus in Stephen King's The Cursed Expedition
 * Worm Planet from Ken Follett's The Power Twins
 * Wormwood from Rifts
 * Yggardis the Sorcerer Planet from DC
 * Zonama Sekot from the Star Wars universe
 * The unnamed world from Arthur C. Clarke's "Crusade"
 * The world of the Hive Mind Bacteria from J. Michael Straczynski's Genius Loci
 * Possibly the world of the "Symbiotic Lifeform" from Enterprise

Stars

 * Calebans from Frank Herbert's Whipping Star
 * Torajii from Doctor Who

Others

 * Various worlds spread across the galaxy in Alastair Reynolds's Revelation Space universe are inhabited by Pattern Jugglers, networks of marine organisms which serve as vast information-cataloging systems, recording the minds of various star-faring races which have visited the planets. It is unclear to what extent Juggler worlds are independently sapient.
 * Stars and nebulae are presented as living, intelligent life forms in Olaf Stapledon's Star Maker.
 * The Luma from Super Mario Galaxy might literally be baby stars.
 * There are creatures as large as planets or even larger, which could probably qualify as worlds due to their sheer immensity, even though they haven't been referred as such. Examples, for instance, the Planet-Eating Monster from Megas XLR, or the Darmats from Robert J. Sawyer's Starplex.
 * Living nebulae, or dust and gas clouds, are a similar concept although they aren't considered worlds as detailed above. Such creatures are quite common in fiction, see for example: Fred Hoyle's Black Cloud.