List of hypothetical Solar System objects

A hypothetical Solar System object is a planet, natural satellite or similar body in the 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.

Planets

• Counter-Earth, a planet situated on the other side of the Sun from that of the earth.
• 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. These days, 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 Nine, a planet proposed to explain apparent alignments in the orbits of a number of distant trans-Neptunian objects.
• 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, asteroids that 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]
• In the Five-planet Nice model a fifth giant planet originally in an orbit between Saturn and Uranus is ejected from the Solar System into interstellar space after a close encounter with Jupiter, resulting in a rapid divergence of Jupiter's and Saturn's orbit may have ensured the orbital stability of the terrestrial planets in the inner Solar System. It may have also precipitated the Late Heavy Bombardment of the inner Solar System.^[5]

Moons

• Chiron, a moon of Saturn supposedly sighted by Hermann Goldschmidt in 1861 but never observed by anyone else.
• Other moons of Earth, such as Petit's moon, Lilith, Waltemath's moons and Bargby's moons.
• Mercury's moon, hypothesised to account for an unusual pattern of radiation detected by Mariner 10 in the vicinity of Mercury. 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.^[6]

Star

• 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 minor planet Sedna's unusual elongated orbit. The existence of the Nemesis in the modern Solar system was ruled out in 2014 after the infrared survey performed by WISE spacecraft found no brown dwarf up to 10,000 astronomical units (0.16 ly) from Sun.

See also

• Planet formation
• Theoretical planetology
• Oort cloud
• Planets beyond Neptune
• Tenth planet (disambiguation)
• Ninth planet (disambiguation)
• Trans-Neptunian object
• Trans-Neptunian objects in fiction

References