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A hypothetical Solar System object is a planet, natural satellite or similar body in our Solar System whose existence is not known, but has been inferred from observational scientific evidence. Over the years a number of hypothetical planets have been proposed, and many have been disproved. Some were proposed early in philosophical history, and perhaps belong more to protoscience than science. However, even today there is scientific speculation about the possibility of planets yet unknown that may exist beyond the range of our current knowledge.


  • Counter-Earth, a planet hypothesized by the Greek philosopher Philolaus, who reasoned that, in order to keep the universe in balance, there must be an antichthon, a second Earth, identical but opposite to ours in every way, on the other side of the Central Fire. Abandoned with the acceptance of heliocentrism.
  • Fifth planet (hypothetical), historical speculation about a planet between the orbits of Mars and Jupiter.
    • Phaeton, a planet situated between the orbits of Mars and Jupiter whose destruction supposedly led to the formation of the asteroid belt. Nowadays this hypothesis is considered unlikely, since the asteroid belt has far too little mass to have resulted from the explosion of a large planet.
    • Planet V, a planet thought by John Chambers and Jack Lissauer to have once existed between Mars and the asteroid belt, based on computer simulations.
  • Planet X, a hypothetical planet beyond Neptune. Initially employed to account for supposed perturbations (systematic deviations) in the orbits of Uranus and Neptune, it has been disproved to cause any such perturbations, while the belief in them inspired the search for Pluto. The concept has been re-applied to account for subsequent observations of Kuiper Belt objects, however.
  • Theia, a Mars-sized impactor believed to have collided with the Earth roughly 4 billion years ago; an event which created the Moon.
  • Vulcan, a hypothetical planet once believed to exist inside the orbit of Mercury.
    • Vulcanoids, a ring of asteroids which may exist within a gravitationally stable region inside Mercury's orbit.
  • Chiron, a moon of Saturn supposedly sighted by Hermann Goldschmidt in 1861 but never observed by anyone else.
  • Other moons of Earth, a second moon of the Earth, thought by Frederic Petit, director of the Observatory of Toulouse, to have been observed three times on March 21 1846. [1]
  • Mercury's moon, hypothesised to account for a sudden burst of radiation detected by Mariner 10. It was disproved by the spacecraft's subsequent flyby. An object thought to be orbiting Mercury eventually revealed itself to be the star 31 Crateris.
  • Neith, a purported moon of Venus, falsely detected by a number of telescopic observers in the 17th and 18th centuries. Now known to not exist, the object has been explained as a series of misidentified stars and internal reflections inside the optics of particular telescope designs.
  • Themis, a moon of Saturn which astronomer William Pickering claimed to have discovered in 1905, but which was never seen again.[2]
  • Nemesis, a large planet or brown dwarf whose existence is suggested by physicist Richard A. Muller, based on purported periodicities in mass extinctions within Earth's fossil record. Its regular passage through the Solar System's Oort cloud would send large numbers of comets towards Earth, massively increasing the chances of an impact.

See also


  1. ^ Bakich, Michael E. The Cambridge Planetary Handbook. Cambridge University Press, 2000, p. 146, ISBN 0521632803 , see
  2. ^ Hypothetical Planets


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