List of Hypothetical Solar System Objects - Planets

Planets

• 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 4.5 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.
• Tyche, a hypothetical planet in the Oort Cloud supposedly responsible for producing the statistical excess in long period comets in a band.
• A hypothetical fifth gas giant has been mooted 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 insured the orbital stability of the inner terrestrial planets. It may have also precipitated the Late Heavy Bombardment of the inner Solar System