Fictional planets of the Solar System

Fictional planets of the Solar System have been depicted since the 1700s—often but not always corresponding to hypothetical planets that have at one point or another been seriously proposed by real-world astronomers, though commonly persisting in fiction long after the underlying scientific theories have been refuted. Vulcan was a planet hypothesized to exist inside the orbit of Mercury between 1859 and 1915 to explain anomalies in Mercury's orbit until Einstein's theory of general relativity resolved the matter; it continued to appear in fiction as late as the 1960s. Counter-Earth—a planet diametrically opposite Earth in its orbit around the Sun—was originally proposed by the ancient Greek philosopher Philolaus in the fifth century BCE (albeit in a pre-heliocentric framework), and has appeared in fiction since at least the late 1800s. It is sometimes depicted as very similar to Earth and other times very different, often used as a vehicle for satire, and frequently inhabited by counterparts of the people of Earth.

Following the discovery of the first asteroids in the early 1800s, it was suggested that the asteroid belt might be the remnants of a planet predicted by the Titius–Bode law to exist between Mars and Jupiter that had somehow been destroyed; this hypothetical former fifth planet is known as Phaëton in astronomy and often dubbed "Bodia" (after Johann Elert Bode) in science fiction. Bodia was popular in the pulp era of science fiction, where it was often depicted as similar to Earth and inhabited by humans who might occasionally be the ancestors of humans on Earth, and stories about its destruction became increasingly common following the invention of the atomic bomb in 1945.

Additional planets in the outer reaches of the Solar System, such as a ninth planet beyond Neptune or especially a tenth beyond Pluto (between the 1930 discovery of Pluto and its reclassification from planet to dwarf planet in 2006), appear regularly. Many different names for this hypothetical outermost planet have been used, the most common being "Persephone". Some stories depict so-called rogue planets that do not orbit any star entering the Solar System from without, typically on a collision course with Earth. Less frequently, fictional planets appear in other locations, such as between Venus and Earth or inside a hollow Earth. Similarly, fictional moons appear in some works; fictional additional moons of the Earth largely fell out of favour with the advent of the Space Age.