See also: List of hypothetical planetary bodies
A hypothetical planet is a planet 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 discarded. However, even today there is scientific speculation about the possibility of planets as yet unknown that may yet exist beyond the range of our current knowledge.
Within the solar system
The solar system is by no means fully mapped and charted. Much of its territory is still unknown, and many astronomers have hypothesised from indirect observation that other substantial objects could still exist undetected in its farthest reaches.
- Main article: Vulcanoid asteroid
In the 19th century, the astronomer Urbain Le Verrier, credited with the discovery of Neptune, attempted to locate a hypothetical planet within the orbit of Mercury that he believed was causing perturbations in its orbit. This planet, which he named Vulcan after the Roman god of the forge due to its closeness to the Sun, was never observed, and Einstein's general relativity theory subsequently resolved the issue of Mercury's orbit.  However, a gravitationally stable region does exist between Mercury and the Sun, and some astronomers, notably Alan Stern, contend that a field of small minor planets, the Vulcanoids, should exist within it. However repeated observations of the region have yet to yield any results, and the Vulcanoids, if they exist, must be rather small and few in number.  Some conclude that the existence of the Vulcanoids is impossible, as any minor planet within the orbit of Mercury would eventually be destabilised by the Yarkovsky effect; motion by the force of its own heat. 
- Main article: Planet X
In the early 20th century, astronomer Percival Lowell's observation of apparent irregularities in the orbits of Uranus and Neptune led him to conclude that a distant planet, which he called Planet X, must lie beyond them. The Lowell Observatory's long search for this planet ultimately led to the discovery of Pluto. However, Pluto's mass was found to be too small to disturb the other planets' orbits significantly, and subsequent measurements by the Voyager 2 spacecraft showed that earlier calculations of Neptune's mass had been in error, leading to the irregularities observed. Today, few scientists accept Lowell's theory; however, a number of recent observations have reopened the debate on the existence of a "Planet X", even if it would bear little resemblance to that invisioned by Lowell.
- The Kuiper Belt has a very sharply defined edge. At around 49 AU, a sharp dropoff occurs in the number of objects observed. This dropoff is known as the "Kuiper Cliff", and as yet its cause is unknown. Some speculate that something must exist beyond the belt large enough to sweep up the remaining debris, perhaps as large as Earth or Mars. This view is still controversial, however. 
- Physicist Richard A. Muller has hypothesized that the Sun may be part of a binary star system, with a distant companion named Nemesis. Nemesis was proposed to explain some timing regularities of the great extinctions of life on Earth. The hypothesis says that Nemesis creates periodical perturbations in the Oort cloud of comets surrounding the solar system, causing a "comet shower". Some of them hit Earth, causing destruction of life. This hypothesis is no longer taken seriously by most scientists, mostly because infrared surveys failed to spot any such object, which should have been very conspicuous at those wavelengths. 
- Dr. John Murray of the Open University and John Matese of the University of Louisiana at Lafayette believe that the motions of long-term comets in the sky suggest the existence of a large, distant planet, or, more likely, a small substellar companion such as a Brown dwarf, in the deep solar system. This hypothetical substellar object is not Nemesis, since its existence is inferred from a different set of data; however there is the possibility that both sets of data could be true for the same object. 
A onetime Trojan to the Earth that, once its size grew to roughly that of Mars, became unstable in its orbit and collided with the Earth, transforming its crust and upper mantle into a magma ocean, and ejecting massive amounts of light material into orbit, which eventually coalesced into the Moon. This theory is known as the giant impact hypothesis. 
Hypothetical extrasolar planets
- PSR 1829-10's planet, proposed by Andrew Lyne of University of Manchester on July 24 1991, was retracted in 1992. A combination of an inaccurate position for the pulsar and a timing model approximating the Earth's orbit about the Sun with a circle yielded processed data resembling that which would have been expected from a pulsar planet with an orbital period of half a year. 
- PSR 1257+12 D, the proposed fourth planet in the first extra solar planetary system, was retracted due to further detection refinements. (It has subsequently been replaced by a proposal for a comet)
- A microlensing event in 1996 of the gravitationally lensed quasar Q0957+561, observed by R. E. Schild in the A lobe of the double imaged quasar, has lead to a controversial, and unconfirmable speculation that a 3 Earth mass planet is possibly in the unknown lensing galaxy, between Earth and the quasar. 
- ↑ Paul Schlyter. Hypothetical planets. saaf.se. Retrieved on 2006-07-23.
- ↑ Durda D.D.; Stern S.A.; Colwell W.B.; Parker J.W.; Levison H.F.; Hassler D.M. (2004). A New Observational Search for Vulcanoids in SOHO/LASCO Coronagraph Images. Retrieved on 2006-07-23.
- ↑ Vokrouhlický, David; Farinella, Paolo; Bottke, William F. (2000). The Depletion of the Putative Vulcanoid Population via the Yarkovsky Effect. NASA Smithsonian. Retrieved on 2006-06-25.
- ↑ Croswell, Ken (1997). Planet Quest: The Epic Discovery of Alien Solar Systems. Oxford University Press p. 66 (ISBN 0192880837).
- ↑ Michael Brooks (2005). 13 Things That Don't Make Sense. NewScientistSpace. Retrieved on 2006-06-23.
- ↑ J. G. Hills (1984). Dynamical constraints on the mass and perihelion distance of Nemesis and the stability of its orbit. Nature. Retrieved on 2006-06-23.
- ↑ Cometary evidence of a massive body in the outer Oort cloud. The University of Louisiana at Lafayette. Retrieved on 2006-07-22.
- ↑ Daniel Fischer (2002). A comet orbiting a pulsar?. The Cosmic Mirror. Retrieved on 2006-07-25.
- ↑ A. G. LYNE & M BAILES (1992). No planet orbiting PS R1829−10. 'Nature'. Retrieved on 2006-07-25.
- ↑ Carl H. Gibson, Rudolph E. Schild (1999). Theory and observations of galactic dark matter. University of California at San Diego. Retrieved on 2006-07-25.