Planet X is the definition of a large hypothetical planet that supposedly was thought to have existed beyond Neptune before Pluto was discovered. The X means unknown. Planet X was first applied to Pluto before its discovery in 1930 (as the ninth planet) and then to Eris before it was discovered in 2005 (as the tenth planet). However, as of 2006, both former planets were reclassified as dwarf planets, resulting in only eight planets in the solar system. Today, Planet X is the general term for any undiscovered planet in the solar system.
Is there another planet beyond the dwarf planet Eris (2003UB313), the recently discovered dwarf planet that lies beyond the recently demoted dwarf planet Pluto? Prior to 1781 that question could have been asked concerning Saturn. In that year, German-born English astronomer Sir William Herschel (1738–1822) discovered Uranus, after detecting what he believed to be a comet. Calculations to determine the orbit of Uranus were made, and the planet was found to conform to the law of planetary distances suggested by German astronomer Johann Elert Bode (1747–1826).
However, a problem later arose. After 60 years, it was noticed Uranus was not following its predicted orbit, evidence that suggested another planet, the gravity of which was perturbing Uranus, must exist beyond it. Calculations for the position of this planet were made by French mathematician Urbain Jean Joseph Le Verrier (1811–1879) and British mathematician and astronomer John Couch Adams (1819– 1892). Then, in 1846, Neptune was discovered by German astronomer Johann Gottfried Galle (1812– 1910) and Prussian astronomer Heinrich Louis d’Arrest (1822–1875). Neptune’s gravitational pull accounted for most of the differences between the predicted and observed positions of Uranus, but there was still a discrepancy.
The search continued for yet another planet. American astronomer Percival Lowell (1855–1916) expended a great deal of energy looking, but came up empty-handed. However, Lowell’s calculations laid the groundwork for the discovery of Pluto, which was finally found by American astronomer Clyde Tombaugh (1906–1997) in 1930. However, Pluto turned out to be such a small, low-mass object that it could not possibly account for the perturbations. Some astronomers argue that another planet, with a mass of three to five times that of the Earth, might be out there.
Before 2005, astronomers said that if there is a Planet X beyond Pluto, it will be very difficult to find. Calculations showed it would have a highly inclined (tipped) orbit, and would take 1,000 years to complete a trip around the sun. At that distance, the amount of sunlight it would reflect would be very small, making it a very dim object. Worse yet, one calculation placed it within the constellation of Scorpius, which has a dense concentration of stars. Finding a faint planet there would be comparable to identifying a particular grain of sand on a beach.
To make a bad situation worse, there was no agreement on where in the sky to look; some astronomers had suggested the constellations Gemini and Cancer. It had also been suggested that the gravitational tug of a Planet X could perturb material in the Oort cloud. This cloud, suggested by Dutch astronomer Jan Hendrik Oort (1900–1992), is one source of comets. Planet X, if it existed, could deflect some of this material, causing it to fall into the inner solar system and become new comets.
Most astronomers argued before 2005 that there is no Planet X. Tombaugh’s search for Pluto was very extensive; he found Pluto and nothing else. The argument was stated that nothing else was found because nothing else exists beyond Pluto of a size to be called a planet. As far as the remaining perturbations, scientists surmised that perhaps they were just errors in the imperfect calculations made in the nineteenth and twentieth centuries.
On July 29, 2005, American planetary astronomer (California Institute of Technology) Michael E. Brown (1965–), American astronomer (Gemini Observatory in Hawaii) Chadwick A. Trujillo (1973–), and American researcher (Yale University) David Lincoln Rabinowitz (1960–) announced that they had discovered a large trans-Neptunian object (TNO) past the orbit of Pluto based on images taken on October 21, 2003. The TNO initially called 2003UB313 was found to be larger than Pluto. The discoverers called it the tenth planet in the solar system: the newest Planet X.
In April 2006, based on images from the Hubble Space Telescope, 2003UB313 was measured to have a diameter of about 2,400 kilometers, slightly larger than the diameter of Pluto. However, in August 2006, the International Astronomical Union (IAU) officially defined the term planet. Based on this definition, 2003UB313 was designated a dwarf planet, as was Pluto. The dwarf planet, know called Eris and designated (136199) Eris or 136199 Eris, is the largest dwarf planet currently known in the solar system.
"Planet X." The Gale Encyclopedia of Science. . Encyclopedia.com. (February 15, 2019). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/planet-x-0
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