List of hypothetical Solar System objects

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This article is about hypothetical Solar System objects in astronomy. For hypothetical Solar System objects not recognized by science, see Planetary objects proposed in religion, astrology, ufology and pseudoscience. For hypothetical planets outside the Solar System, see List of unconfirmed exoplanets. For other hypothetical astronomical objects, see Hypothetical astronomical object.

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. However, even today there is scientific speculation about the possibility of planets yet unknown that may exist beyond the range of our current knowledge.


  • 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, belief in its existence ultimately inspired the search for Pluto. Though the concept has since been abandoned following more precise measurements of Neptune's mass, which accounted for all observed perturbations, it has been re-applied to account for supposed deviations in the motions of Kuiper belt objects. Such explanations are still controversial, however.
  • Theia, a Mars-sized impactor believed to have collided with the Earth roughly 4.5 billion years ago; an event which created the Moon.
  • Vulcan, a hypothetical planet once believed to exist inside the orbit of Mercury. Initially proposed as the cause for the perturbations in the orbit of Mercury, some astronomers spent many years searching for it, with many instances of people claiming to have found it. The perturbations in Mercury's orbit were later accounted for via Einstein's General Theory of Relativity.
    • Vulcanoids, a ring of asteroids which may exist within a gravitationally stable region inside Mercury's orbit.
  • Tyche, a hypothetical planet in the Oort Cloud supposedly responsible for producing the statistical excess in long period comets in a band.[1] Results from the WISE telescope survey in 2014 have ruled it out.[2][3][4]
  • A hypothetical fifth gas giant originally in an orbit between Saturn and Uranus, which was subsequently flung out of the Solar System into interstellar space after a close encounter with Jupiter, resulting in transferred angular momentum which caused Jupiter to recede from the Sun and may have ensured the orbital stability of the inner terrestrial planets. It may have also precipitated the Late Heavy Bombardment of the inner Solar System.[5]


  • Chiron, a moon of Saturn supposedly sighted by Hermann Goldschmidt in 1861 but never observed by anyone else.
  • Other moons of Earth, such as that thought by Frederic Petit, director of the Observatory of Toulouse, to have been observed three times on March 21, 1846.[6]
  • Mercury's moon, hypothesised to account for an unusual pattern of radiation detected by Mariner 10 in the vicinity of the planet. Subsequent data from the mission revealed the actual source 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 not to 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.[7]


  • Nemesis, a brown or red dwarf whose existence was suggested in 1984 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. Also believed to be the cause of dwarf planet Sedna's unusual elongated orbit.

See also[edit]


  1. ^ The Independent, "Up telescope! Search begins for giant new planet", Sunday 13 February 2011, Paul Rodgers
  2. ^ K. L., Luhman (7 March 2014). "A Search For A Distant Companion To The Sun With The Wide-field Infrared Survey Explorer". The Astrophysical Journal 781 (1). Bibcode:2014ApJ...781....4L. doi:10.1088/0004-637X/781/1/4. Retrieved 20 March 2014. 
  3. ^ Matese, John J.; Whitmire, Daniel P. (2011). "Persistent evidence of a jovian mass solar companion in the Oort cloud". Icarus 211 (2): 926–938. arXiv:1004.4584. Bibcode:2011Icar..211..926M. doi:10.1016/j.icarus.2010.11.009. 
  4. ^ Helhoski, Anna. "News 02/16/11 Does the Solar System Have Giant New Planet?". The Norwalk Daily Voice. Retrieved 10 July 2012. 
  5. ^ Lisa Grossman: "Lost planet explains solar system puzzle" New Scientist: 01.10.2011: 14-15
  6. ^ Bakich, Michael E. The Cambridge Planetary Handbook. Cambridge University Press, 2000, p. 146, ISBN 0-521-63280-3 , see
  7. ^ Hypothetical Planets