List of Hypothetical Solar System Objects

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 that thought by Frederic Petit, director of the Observatory of Toulouse, to have been observed three times on March 21, 1846.
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 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.
S/2000 J 11 was an object reportedly situated in the vicinity of Jupiter, but its orbit was never determined and later searches could not find it. It is no longer considered a likely Jovian satellite.