Tenth planet
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- This article is about the historical speculation about the existence of a tenth planet in the solar system while Pluto was considered a planet. For modern hypotheses of planets beyond Neptune, see ninth 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. The 2006 redefinition, however, made it highly unlikely that any body will ever be termed a "tenth planet" by establishing orbital dominance as a principle and moving the number of planets back down to eight, meaning that a hypothetical ninth planet will now need to be discovered first and meet the new criteria.
During the mid-1800's when the first four asteroids discovered were considered planets, Saturn was the tenth planet.
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[edit] 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, 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.
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] 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
The trans-Neptunian object named Eris was discovered in early 2005 from imagery dating to 2003 [1]. 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. [2] 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 [3].
One of Eris' discoverers noted that any definition of "planet" that includes Pluto should also include this new, larger, object [4]. 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 bizarre than Pluto. The status of the new object was later that of a dwarf planet, and the discovery of Eris was one of the main factors for the new classification that also demoted Pluto to 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 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".
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.
[edit] See also
- Outer planets of the Solar System in fiction
- Fifth planet (hypothetical), historical speculation about a planet between the orbits of Mars and Jupiter.
- Ninth planet
- Planet X
