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tenth planet

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For the Doctor Who serial, see The Tenth Planet.

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 Template:mpl 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. 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, 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.

Template:mp

The Kuiper Belt object designated Template:mpl was discovered in early 2005 from imagery dating to 2003 [1]. Its size has now been inferred at approximately 3000 km, which is 50% larger than Pluto. Because Template:mpl is too far away to measure the size directly this estimate is base on how much light it reflects to Earth and how much light it absorbs to keep it warm. Many media outlets have reported that Template:mp is now the tenth planet in the solar system, and this classification has been backed by NASA [2]. However, the IAU has now weighed in on the issue to try to define exactly what consitutes a planet.

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]. Another issue is that the object is not alone, but orbits in the Kuiper Belt along with Pluto and at least 700 other objects the status of the new object remains officially unclear and Pluto's status may be in jeopardy. Deciding just what constitutes a planet has been an ongoing problem for astronomers, and Template:mp is expected to restart this debate [4].

Nicknamed Xena after the lead character in the TV series Xena: Warrior Princess, its moon is nicknamed Gabrielle after Xena's sidekick in the series. However, it is unlikely that "Xena" will be the object's final approved name.

Suggested names: 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 Template:mpl is a major planet, this same policy will not strictly apply; in any case, the discoverers of Template:mp 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". However, several asteroids share names with the moons of the outer planets, which might allow the use of Proserpina or Persephone (as well as Ceres and Demeter) as the name of the tenth planet. The use of the name Proserpina would also be consistent with the tradition of naming major planets after Roman (Latin) gods, rather than Greek ones.

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 (Demeter was also briefly used as an unofficial name for the moon of Jupiter now called Lysithea); while Hades has not been used as a name for a body in the Solar System (except, briefly, as an unofficial name for the Jovian moon now known as Sinope). See Eleusinian Mysteries for details of the relationship between these three deities from Greek and Roman mythology. It is a historical curiosity that in the seventeenth century, Ceres and Proserpina were sometimes used as astronomical names for the Earth and Moon, respectively.

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 (after "some astronomer's parrot"), 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.

See also

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