Hypothetical trans-Neptunian planets

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Artist's rendition of what a ninth planet might look like.
Artist's rendition of what a ninth planet might look like.

Since the discovery of the eighth planet Neptune in 1846, there has been considerable speculation that another planet might exist beyond the orbit of Neptune. In the years since then, various discovered objects and hypothesized bodies from the trans-Neptunian region have been identified as the "ninth planet" or "tenth planet'".

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[edit] Nomenclature

Although historical changes in the enumeration of planets (particularly whether or not to count Ceres or other asteroids as planets) would have variously made Jupiter or Neptune the "ninth planet", the terms "ninth planet" and "tenth planet" are generally used to refer to hypothetical trans-Neptunian planets. Pluto's status as a planet from 1930 to 2006 made "tenth planet" the predominant term for such objects during this period, while "ninth planet" has been more technically correct since then.

[edit] History

[edit] 1846 to 1930: Search for a ninth planet

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 ninth planet. The X stands for unknown, rather than the Roman numeral for 10 — when Lowell postulated Planet X, Pluto (counted as ninth planet until 2006) had not been discovered. 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.

[edit] 1930 to 2006: Search for a tenth planet

Pluto, which was found during the search for Planet X, was counted as the ninth planet from shortly after its 1930 discovery until August 24, 2006, when it was reclassified by the IAU as a dwarf planet.

From the discovery of Pluto in 1930 to the establishment of a formal definition of planet in 2006, the existence of a tenth planet was speculated by astronomers and the general public alike. Tenth planets frequently appeared in fiction, and media reports of new trans-Neptunian object (TNO) discoveries used the label with some frequency.

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 Eris was the most popular candidate for status as the tenth planet, but the 2006 International Astronomical Union (IAU) definition of the term "planet" considers it a dwarf planet.

[edit] Recent

The 2006 redefinition, however, made it less likely any body will ever be termed a "ninth" or "tenth planet" by establishing orbital dominance as a principle. As Pluto has been eliminated from the list of planets, any object that meets the criteria will be classified as the ninth, not the tenth planet, assuming no other changes are made to the definition of a planet.

However, based on modern theories of solar system formation some astronomers believe that bodies up to the size of Mars are yet to be discovered orbiting the sun. If one is found that fits the modern definition, it would constitute a ninth planet. [1] At this time, no such true ninth planet has been found, nor are any specific hypothetical bodies identified as potential planets.

In 2008, Tadashi Mukai and Patryk Lykawka from the University of Kobe published a hypothesis that the patterns in the orbits observed in the trans-Neptunian population argue for the existence of a ninth planet. According to models, the planet would be located at around 100 AU, and it's mass would be about 0.3 to 0.7 of that of the Earth. Due to the very low temperature, its surface could be cover of ice, ammonia and methane.[2]

[edit] Discovered candidates

[edit] Sedna and Quaoar

Two sizable TNO discoveries in the early 2000s prompted media usage of the term "tenth planet," although there was no 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.

[edit] Eris

Main article: Eris (dwarf planet)

The trans-Neptunian object named Eris was discovered in early 2005 from imagery dating to 2003 [3]. Its size has been inferred at 2398 km, which is 4.81% larger than Pluto, after the Hubble Space Telescope was directed at the planet in 2006. [4] Because Eris is too far away to measure the size directly, this estimate is based on how much light it reflects to Earth and how much light it absorbs to keep it warm. Many media outlets reported that Eris is now the tenth planet in the solar system, and this classification was backed by NASA at the time [5].

One of Eris' discoverers noted that any definition of "planet" that includes Pluto should also include this new, larger, object [6]. While Pluto orbits in the Kuiper Belt along with at least 700 other objects, Eris also has an eccentric and highly inclined (44°) orbit. Unlike Pluto, Eris is not situated in the Kuiper Belt, or the traditional gravitational orbit of any other known planet, and is therefore even more unusual than Pluto. Eris was eventually classified under the new category of dwarf planet; its discovery was one of the primary factors for the reclassification of Pluto as a dwarf.

[edit] Suggested names: Persephone and Proserpina

Persephone has been the most frequently-suggested name for a new major planet. The major planets are, by tradition, named after Greco-Roman gods. The Roman god Pluto, the name of the outermost known planet of the solar system for the 76 years it was defined as such, corresponds to the Greek Hades. Persephone, as Hades' wife, was therefore a fitting name for the 10th planet.

The name is currently 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".

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". Several asteroids however do share names with the moons of the outer planets.

The mothers of Persephone, Demeter, and of Proserpina, Ceres, have been used as names of asteroids: 1108 Demeter and Ceres respectively. (Demeter was also briefly used as an unofficial name for the moon of Jupiter now called Lysithea). 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.

The novel Mostly Harmless, fourth sequel to The Hitchhiker's Guide to the Galaxy involves aliens landing on the newly discovered tenth planet (the novel was written before Pluto was officially redefined, and before Eris was discovered), which is officially called Persephone (and unofficially known as Rupert).

[edit] See also