Hypothetical trans-Neptunian planets

Ever since the discovery of Pluto, the existence of a tenth planet has been speculated by astronomers and the general public alike. Tenth planets have frequently appeared in fiction, and media reports of new trans-Neptunian object (TNO) discoveries have also used the label with some frequency.

Tenth planets in astronomy

The Planet X hypothesis, first put forth by Percival Lowell in the late 19th century, stated that there was a fifth gas giant beyond the orbit of Neptune, providing the most common basis for the existence of a tenth planet. The X stands for unknown, rather than the Roman numeral for 10 — when Lowell postulated Planet X, the known planets totaled eight. Planet X was intended to explain perceived anomalies in the orbits of the outer planets. More accurate measurements taken by space probes such as Voyager 2 caused the anomalies to vanish without the need for an extra planet, and the hypothesis was discredited in the 1990s.

With the discovery of the Kuiper belt, the astronomical community began instead to increasingly expect the status of tenth planet would most likely be awarded, if ever, to some other Pluto-like object. The newly-discovered TNO 2003 UB313 is the most popular candidate for status as the tenth planet. Whether or not it is elevated to this status will depend largely on clarifications to the definition of the term "planet" presently being undertaken by the International Astronomical Union (IAU).

Discovered candidates

Sedna and Quaoar

Two sizable TNO discoveries in the early 2000s prompted media usage of the term "tenth planet," although there has not been tremendous appetite for their elevation to planethood in the scientific community. 50000 Quaoar was discovered in 2002 by California Institute of Technology scientists. Although significantly smaller than Pluto, Quaoar was the next-largest TNO by a considerable margin at the time of its discovery.

Another object, 90377 Sedna, was discovered in 2004, at the time the most distant known object in the solar system. It is speculated to be an inner member of the Oort cloud, which is a distant reservoir of comets created when most of the original population of the Kuiper Belt was scattered by the outer planets early in the history of the solar system. Sedna is believed to be intermediate in size between Pluto and Quaoar.

2003 UB₃₁₃

The Kuiper Belt object designated 2003 UB313 was discovered in early 2005 from imagery dating to 2003 [1]. The newly-found object appears to be larger than Pluto, the first such object to be found. Many media outlets have reported that 2003 UB₃₁₃ is now the tenth planet in the solar system, and this classification has been backed by NASA [2]. However, the IAU has not weighed in on the issue and the status of the new object remains officially unclear.

One of its discoverers noted that although it is in an eccentric and highly inclined (44°) orbit, any definition of "planet" that includes Pluto should also include this new object [3]. Deciding just what constitutes a planet has been an ongoing problem for astronomers, and 2003 UB₃₁₃ is expected to restart this debate [4].

Persephone and Proserpina

Persephone is the most frequently-suggested name for a tenth planet. The major planets are, by tradition, named after Greco-Roman gods; in Greco-Roman mythology Roman Pluto corresponds to Greek Hades, and Persephone is Hades' wife, held in the darkness of the underworld, hence hers is thought an appropriate name for the 10th planet.

The name was previously used for the asteroid 399 Persephone, discovered in 1899 (before the planet Pluto, which was found in 1930); since the International Astronomical Union (IAU) Committee on Small Body Nomenclature, which is charged with naming minor planets, has a policy against names that are too similar, it is unlikely that any Trans-Neptunian Objects (TNOs) will be named "Persephone". If the IAU resolves the dilemma of defining what is and is not a planet and determines that the new TNO 2003 UB313 is a major planet, this same policy will not strictly apply; in any case, the discoverers of 2003 UB₃₁₃ have proposed a name from "another tradition".

An alternate proposal would use Proserpina, wife of Pluto, the Roman name for Persephone; but 26 Proserpina received that name in 1853, presenting the same nomenclature policy problems as "Persephone".

It should be noted that the mother of Persephone, Demeter; and of Proserpina, Ceres; have been used as names of asteroids, 1 Ceres and 1108 Demeter respectively; while Hades has not been used as a name for a body in the Solar System. See Eleusinian Mysteries for details of the relationship between these three dieties from Greek and Roman mythology.

Tenth planets in fiction

Persephone is a name frequently put forward for a future tenth planet.

In Larry Niven's Known Space universe, Persephone is a small gas giant with a single moon, Kobold.
In Douglas Adams's novel Mostly Harmless, the tenth planet is officially called Persephone, but nicknamed Rupert, and is inhabited by the crew of a spaceship who have forgotten almost everything about their mission, except that they are supposed to be "monitoring" something.
Although not considered canonical, a 1970s publication entitled Star Trek Maps suggests that, in the Star Trek universe, the solar system has a tenth planet called Persephone that orbits at a great distance from the Sun. No canonical film or TV episode has yet supported this, however.
Arthur C. Clarke makes several references to a tenth planet called Persephone.

In Doctor Who, the solar system contains several additional planets. One, dubbed The Tenth Planet is called Mondas, home of the Cybermen. Mondas is/was a twin planet of Earth which left the solar system. Another planet is Cassius which is located beyond Pluto.
The TV series Space: 1999 features the tenth planet, named Ultra, in one episode.
The 1984 cult film The Adventures of Buckaroo Banzai Across the Eighth Dimension has aliens from "Planet 10" crossing the 8th dimension and invading the Earth.
In Robert Anton Wilson's Schroedinger's Cat trilogy, the tenth and eleventh planets are named Mickey and Goofy respectively.
On the 80's sitcom ALF, the Alien Life Form called the tenth planet "Alvin" (and the eleventh "Dave").

Tenth planets in myth

The Sumerian scholar Zecharia Sitchin, in his book The 12th Planet, discusses the planet Nibiru, a tenth planet (twelfth to those who included the Sun and Moon). Nibiru allegedly follows a long, elliptical orbit, which reaches the inner solar system every 3600 years. This is considered to be pseudoscience by mainstream scientists and historians. See Nibiru (myth).