Wormwood Planet X Red Kachina Nibiru Tyche Nemesis Hercolubus Tiamat?

This article is about hypothetical planet(s). For known objects beyond Neptune, see trans-Neptunian object.

"9th planet", "10th planet", "Planet X" and similar titles redirect here. For other uses, see Planet X (disambiguation), Ninth planet (disambiguation), and Tenth planet (disambiguation).

Percival Lowell, originator of the Planet X hypothesis

Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and culminated at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the gas giants, particularly Uranus and Neptune,^[1] speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.^[2]

Clyde Tombaugh's discovery of Pluto in 1930 appeared to validate Lowell's hypothesis, and Pluto was officially considered the ninth planet until 2006. In 1978, Pluto was found to be too small for its gravity to affect the gas giants, resulting in a brief search for a tenth planet. The search was largely abandoned in the early 1990s, when a study of measurements made by the Voyager 2 spacecraft found that the irregularities observed in Uranus's orbit were due to a slight overestimation of Neptune's mass.^[3] After 1992, the discovery of numerous small icy objects with similar or even wider orbits than Pluto led to a debate over whether Pluto should remain a planet, or whether it and its neighbours should, like the asteroids, be given their own separate classification. Although a number of the larger members of this group were initially described as planets, in 2006 the International Astronomical Union reclassified Pluto and its largest neighbours as dwarf planets, leaving only eight planets in the Solar System.^[4]

Today, the astronomical community widely agrees that Planet X, as originally envisioned, does not exist, but the concept of Planet X has been revived by a number of astronomers to explain other anomalies observed in the outer Solar System. In popular culture, and even among some astronomers,^[5] Planet X has become a stand-in term for any undiscovered planet in the outer Solar System, regardless of its relationship to Lowell's hypothesis. Other trans-Neptunian planets have also been suggested, based on different evidence.

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Nemesis is a hypothetical red dwarf star^[1] or brown dwarf,^[2] originally postulated in 1984 to be orbiting the Sun at a distance of about 95,000 AU (1.5 light-years),^[2] somewhat beyond the Oort cloud, to explain a perceived cycle of mass extinctions in the geological record, which seem to occur more often at intervals of 26 million years.^[2][3] As of 2012, over 1800 brown dwarfs have been identified and none of them are inside the Solar System.^[4] There are actually fewer brown dwarfs in our cosmic neighborhood than previously thought. Rather than one star for every brown dwarf, there may be as many as six stars for every brown dwarf.^[5]

More recent theories suggest that other forces, like close passings of other stars, or the angular effect of the galactic gravity plane working against the outer solar orbital plane, may be the cause of orbital perturbations of some outer Solar System objects.^[6] In 2011, Coryn Bailer-Jones did an analysis of craters on the surface of the Earth and reached the conclusion that the earlier findings of simple periodic patterns (implying periodic comet showers dislodged by a hypothetical Nemesis star) to be statistical artifacts, and found that the crater record shows no evidence for Nemesis.^[7] However, in 2010, A.L. Melott and R.K. Bambach found evidence in the fossil record confirming the extinction event periodicity originally claimed by Raup & Sepkoski in 1984, but at a higher confidence level and over a time period nearly twice as long.^[8] The Infrared Astronomical Satellite (IRAS) failed to discover Nemesis in the 1980s. The 2MASS astronomical survey, which ran from 1997 to 2001, failed to detect an additional star or brown dwarf in the Solar System.^[9]

Using newer and more powerful infrared telescope technology, able to detect brown dwarfs as cool as 150 kelvins out to a distance of 10 light-years from the Sun,^[10] results from the Wide-field Infrared Survey Explorer (WISE survey) have not detected Nemesis.^[11][12] In 2011, David Morrison, a senior scientist at NASA known for his work in risk assessment of near Earth objects, has written that there is no confidence in the existence of an object like Nemesis, since it should have been detected in infrared sky surveys.

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