Fictional planets of the Solar System

The fictional portrayal of the Solar System has often included planets, moons, and other celestial objects which do not actually exist. Some of these objects were, at one time, seriously considered as hypothetical planets which were either thought to have been observed, or were hypothesized to be orbiting the Sun in order to explain certain celestial phenomena. Often such objects continued to be used in literature long after the hypotheses upon which they were based had been abandoned.

Other non-existent Solar System objects used in fiction have been proposed or hypothesized by persons with no scientific standing; yet others are purely fictional and were never intended as serious hypotheses about the structure of the Solar System.

Vulcan
Anomalies in Mercury's orbit around the Sun led Urbain Le Verrier to propose the existence of an unseen planet with an orbit interior to Mercury's exerting gravitational influence in 1859, similar to how irregularities in Uranus' orbit had led to his discovery of Neptune in 1846. This hypothesized planet was dubbed "Vulcan", and featured in several works of fiction including the 1932 short story "The Hell Planet" by Leslie F. Stone where it is mined for resources, the 1936 short story "At the Center of Gravity" by Ross Rocklynne where its hollow interior is visited, and the 1942 short story "Child of the Sun" by Leigh Brackett where it is inhabited by intelligent life. Mercury's orbital anomalies are now understood to be caused by the effects of general relativity. In science fiction, the name has since come to be more associated with the extrasolar planet Vulcan in the Star Trek franchise.

Counter-Earth
The most popular hypothetical planet in fiction is Counter-Earth—a planet diametrically opposite Earth in its orbit around the Sun. The underlying concept was originally proposed by the ancient Greek philosopher Philolaus in the fifth century BCE, working in a pre-heliocentric framework where the Earth, Sun, and Counter-Earth (called Antichthon) all revolve around a "Central Fire" and Counter-Earth is perpetually hidden from Earth's view. The position of Counter-Earth on the other side of the Sun corresponds to the Sun–Earth L3 Lagrange point. In reality, the two planets would not remain hidden from each other as the gravitational influence from other planets would perturb their orbits, altering their relative positions. Conversely, Counter-Earth would reveal its existence indirectly by exerting gravitational influence on other celestial bodies.

Counter-Earth is variously depicted as very similar to Earth or very different, and often employed as a vehicle for satire. Its earliest appearance in fiction may be 's 1896 novel From World to World (later expanded into the 1913 novel The Love of Meltha Laone). In Paul Capon's 1950 novel The Other Side of the Sun and its sequels in the Antigeos trilogy, there are two societies on Antigeos—one of which is utopian—separated by extreme tides caused by the planet's moons. The Doctor Who villains Cybermen, first introduced in the 1966 serial "The Tenth Planet", originate from a Counter-Earth known as Mondas. John Norman's Gor series, starting with the 1966 novel Tarnsman of Gor, uses the titular planet as the setting for planetary romance stories.

Counter-Earth being inhabited by counterparts of the people of Earth is a recurring theme. In Edison Marshall's 1916 short story "Who Is Charles Avison?", the two versions of the title character depart from their respective planets by spaceship but inadvertently both return to the same one. In Edgar Wallace's 1929 novel Planetoid 127, both individuals and events are identical between the two worlds, though with a slight and variable time difference in either direction that enables the inhabitants to gain foreknowledge by communicating with the other planet. In Ben Barzman's 1960 novel Out of This World (a.k.a. Twinkle, Twinkle, Little Star; a.k.a.  Echo X), Counter-Earth displays an alternate history where World War II never happened. In the 1969 film Doppelgänger (a.k.a. Journey to the Far Side of the Sun), Counter-Earth is the mirror reflection of Earth, but is otherwise identical. The theme also resurfaced decades later in the 2011 film Another Earth.

Variations on the concept have also appeared. Extrasolar examples of planets on opposite sides in the same orbit around their star appear in the 1976 episode "The Last Enemy" of the television show Space: 1999, where one planet has an all-female population and the other an all-male one, and the two planets are at war; and 's 1981 novel A Gift of Mirrorvax. A planet hidden on the other side of the Moon, rather than Sun, appears in Paul Ernst's 1931 short story "The World Behind the Moon" and W. J. Passingham's 1938 short story likewise titled "The World Behind the Moon". The Mars equivalent, Counter-Mars, also appears occasionally.

Phaëton
A now-superseded theory to explain the existence of the asteroid belt that was popular in the 1800s was that it consists of the remnants of a planet predicted by the Titius–Bode law to exist between Mars and Jupiter that had somehow been destroyed. The idea was originally proposed by German astronomer Heinrich Olbers, who discovered the asteroids Pallas and Vesta in 1802 and 1807, respectively. In astronomy, this hypothetical former fifth planet is known as Phaëton; in science fiction, it is often called "Bodia" after Johann Elert Bode. An early science fiction work that mentions this explanation for the origin of the asteroids is Robert Cromie's 1895 novel The Crack of Doom, which describes the release of energy stored in atomic nuclei a few thousand years ago as the culprit.

By the pulp era of science fiction, Bodia was a recurring theme. In these stories it is typically similar to Earth and inhabited by humans, often advanced humans and occasionally the ancestors of humans on Earth. Interplanetary warfare with Mars causes the destruction of Bodia—and indirectly, the end of civilization on Mars—in Harl Vincent's 1930 short story "Before the Asteroids". An internal disaster resulting in the explosion of the planetary core is responsible in John Francis Kalland's 1932 short story "The Sages of Eros". In Leslie F. Stone's 1934 short story "The Rape of the Solar System", war with Mars over the colonization of then-uninhabited Earth results both in the partial destruction of Bodia, thus creating the asteroids, and the displacement of the largest fragment to a much wider orbit to create Pluto, while the settlers on Earth eventually become humanity.

Following the invention of the atomic bomb in 1945, stories of this planetary destruction became increasingly common, encouraged by the advent of a plausible-seeming means of disintegration. Robert A. Heinlein's 1948 novel Space Cadet thus states that the fifth planet was destroyed as a result of nuclear war, and in Ray Bradbury's 1948 short story "Asleep in Armageddon" (a.k.a. "Perchance to Dream"), the ghosts of the former warring factions infect the mind of an astronaut stranded on an asteroid. Several works of the 1950s reused the idea to warn of the dangers of nuclear weapons, including Lord Dunsany's 1954 Joseph Jorkens short story "The Gods of Clay" and James Blish's 1957 novel The Frozen Year (a.k.a. Fallen Star). In Jack Williamson's 1942–1951 Seetee series an antimatter explosion is to blame, and in Theodore Cogswell's 1955 short story "Test Area", the destruction results from a nuclear test conducted by the inhabitants of Mars, while in Heinlein's 1951 novel Between Planets the technology that caused the destruction has been lost to time. The planet's destruction by Martians is also mentioned in Heinlein's 1961 novel Stranger in a Strange Land, and implied to have been caused using supernatural powers. The 1977 novel Inherit the Stars, the first in James P. Hogan's Giants series, revisits the theme of the fifth planet—here called "Minerva"—being destroyed by war fought with advanced weapons.

In Raymond Z. Gallun's 1950 short story "A Step Farther Out", valuables from the destroyed civilization are recovered, and in Harry Harrison's 1969 novel Plague Ship, an ancient virus is found in the asteroid remnants. Paul Preuss's 1985 short story "Small Bodies", where fossils are found on an asteroid, is a late example of the destroyed planet theme; it has otherwise largely been relegated to deliberately retro works such as the 1989 tabletop role-playing game Space: 1889. A variation on the theme appears in Clifford D. Simak's 1973 short story "Construction Shack", where the asteroids are leftover material originally intended for the construction of a fifth planet.

Trans-Neptunian planets
Planets beyond the orbit of Neptune (or even Pluto) appear in several works of science fiction. The most common name for this hypothetical ninth (or tenth, if Pluto is counted) planet is "Persephone".

Elsewhere in the Solar System
Science fiction bibliographers E. F. Bleiler and Richard Bleiler, in the 1998 reference work Science-Fiction: The Gernsback Years, list various imaginary constituents of the pre-modern "science-fiction Solar System". Among these are planets between Venus and Earth, planets on the inside of a hollow Earth, and a planet "behind the Earth".

Rogue planets
A rogue planet colliding with Earth is a recurring motif in apocalyptic fiction.