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 continued 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 giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.
Sedna is a dwarf planet in the outermost reaches of the Solar System discovered in 2003. Spectroscopy has revealed that Sedna's surface composition is largely a mixture of water, methane, and nitrogen ices with tholins, similar to those of some other trans-Neptunian objects. Its surface is one of the reddest among Solar System objects. Sedna, within estimated uncertainties, is tied with Ceres as the largest planetoid not known to have a moon. It has a diameter of approximately 1,000 km, with an unknown mass.
Chadwick A. Trujillo is an American astronomer, discoverer of minor planets and the co-discoverer of Eris, the most massive dwarf planet known in the Solar System.
Michael E. Brown is an American astronomer, who has been professor of planetary astronomy at the California Institute of Technology (Caltech) since 2003. His team has discovered many trans-Neptunian objects (TNOs), including the dwarf planet Eris, which was originally thought to be bigger than Pluto, triggering a debate on the definition of a planet.
(148209) 2000 CR105 is a trans-Neptunian object and the tenth-most-distant known object in the Solar System as of 2015. Considered a detached object, it orbits the Sun in a highly eccentric orbit every 3,305 years at an average distance of 222 astronomical units (AU).
The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small Solar System bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units (4.5×109 km; 2.8×109 mi). These extreme orbits are thought to be the result of gravitational "scattering" by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune.
229762 Gǃkúnǁʼhòmdímà (provisional designation 2007 UK126) is a trans-Neptunian object and binary system from the extended scattered disc, located in the outermost region of the Solar System. It was discovered on 19 October 2007 by American astronomers Megan Schwamb, Michael Brown, and David Rabinowitz at the Palomar Observatory in California and measures approximately 600 kilometers (400 miles) in diameter. This medium-sized TNO appears to be representative of a class of mid-sized objects under approximately 1000 km that have not collapsed into fully solid bodies. Its 100-kilometer moon was discovered by Keith Noll, Will Grundy, and colleagues with the Hubble Space Telescope in 2008, and named Gǃòʼé ǃHú.
Gonggong (formally 225088 Gonggong; provisional designation 2007 OR10) is a dwarf planet, a member of the scattered disc beyond Neptune. It has a highly eccentric and inclined orbit during which it ranges from 34–101 astronomical units (5.1–15.1 billion kilometers; 3.2–9.4 billion miles) from the Sun. As of 2019, its distance from the Sun is 88 AU (13.2×10^9 km; 8.2×10^9 mi), and it is the sixth-farthest known Solar System object. According to Deep Ecliptic Survey Gonggong is in a 3:10 orbital resonance with Neptune, in which it completes three orbits around the Sun for every ten orbits completed by Neptune. Gonggong was discovered in July 2007 by American astronomers Megan Schwamb, Michael Brown, and David Rabinowitz at the Palomar Observatory, and the discovery was announced in January 2009.
(470308) 2007 JH43, provisional designation 2007 JH43, is a trans-Neptunian object in the outer regions of the Solar System, approximately 500 kilometers in diameter. It was discovered on 10 May 2007, by the U.S. Palomar Observatory in California. The team of unaccredited astronomers at Palomar consisted of Megan E. Schwamb, Michael E. Brown and David L. Rabinowitz
(528381) 2008 ST291, provisional designation 2008 ST291, is a 1:6 resonant trans-Neptunian object located in the outermost region of the Solar System that takes almost a thousand years to complete an orbit around the Sun. It was discovered on 24 September 2008 by American astronomers Megan Schwamb, Michael Brown and David Rabinowitz at the Palomar Observatory in California, with no known earlier precovery images.
471143 Dziewanna, exact:, provisional designation 2010 EK139, is a trans-Neptunian object in the scattered disc, orbiting the Sun in the outermost region of the Solar System.
(523639) 2010 RE64, provisional designation 2010 RE64, is a trans-Neptunian object in the scattered disc located in the outermost region of the Solar System, approximately 570 kilometers (350 miles) in diameter. It was discovered on 11 July 2010 by the Pan-STARRS-1 survey at the Haleakala Observatory, Hawaii, in the United States.
The Palomar Distant Solar System Survey (PDSSS) was a wide-field survey aimed at finding distant trans-Neptunian objects that used the robotic 1.2 m Samuel Oschin Telescope at Palomar Observatory and the QUEST large-area CCD camera. The survey was specifically designed to identify putative members of a Sedna-like population with perihelia greater than 45 AU. The limiting magnitude of this study was 21.3 in the R-band, it was sensitive out to distances of 1000 AU, and 12,000 square degrees of sky were searched. This observing program was responsible for the discovery of 25 minor planets including trans-Neptunian objects and centaurs. (309239) 2007 RW10 and Gǃkúnǁʼhòmdímà ((229762) 2007 UK126) were among the objects discovered by this survey. It redetected Sedna but no other objects in Sedna-like orbits were identified.
2012 VP113, also known by its nickname "Biden", is a trans-Neptunian object of the sednoid population, located in the outermost reaches of the Solar System. It was first observed on 5 November 2012 by American astronomers Scott Sheppard and Chad Trujillo at the Cerro Tololo Inter-American Observatory in Chile. The discovery was announced on 26 March 2014. The object probably measures somewhere between 300 and 1000 km in diameter, possibly large enough to be a dwarf planet.
A sednoid is a trans-Neptunian object with a perihelion well beyond the Kuiper cliff at 47.8 AU. Only four objects are known from this population: 90377 Sedna, 2012 VP113, 541132 Leleākūhonua (2015 TG387), and 2021 RR205, but it is suspected that there are many more. All four have perihelia greater than 55 AU. These objects lie outside an apparently nearly empty gap in the Solar System and have no significant interaction with the planets. They are usually grouped with the detached objects. Some astronomers, such as Scott Sheppard, consider the sednoids to be inner Oort cloud objects (OCOs), though the inner Oort cloud, or Hills cloud, was originally predicted to lie beyond 2,000 AU, beyond the aphelia of the four known sednoids.
Suzanne W. Tourtellotte was an American astronomer and discoverer of minor planets, a researcher at Yale University in the United States.
S/2015 (136472) 1, nicknamed MK2 by the discovery team, is the only known moon of the trans-Neptunian dwarf planet Makemake. It is estimated to be 175 km (110 mi) in diameter and has a semi-major axis at least 21,000 km (13,000 mi) from Makemake. Its orbital period is ≥ 12 days. Observations leading to its discovery occurred in April 2015, using the Hubble Space Telescope's Wide Field Camera 3, and its discovery was announced on 26 April 2016.
Michele Bannister is a New Zealand planetary astronomer and science communicator at the University of Canterbury, who has participated in surveying the outermost Solar System for trans-Neptunian objects.
541132 Leleākūhonua, provisionally designated 2015 TG387, is an extreme trans-Neptunian object and sednoid in the outermost part of the Solar System. It was first observed on 13 October 2015, by astronomers at the Mauna Kea Observatories, Hawaii. Based on its discovery date near Halloween and the letters in its provisional designation 2015 TG387, the object was informally nicknamed "The Goblin" by its discoverers and later named Leleākūhonua, comparing its orbit to the flight of the Pacific golden plover. It was the third sednoid discovered, after Sedna and 2012 VP113, and measures around 220 kilometers (140 miles) in diameter.
2018 VG18 is a distant trans-Neptunian object that was discovered well beyond 100 AU (15 billion km) from the Sun. It was first observed on 10 November 2018 by astronomers Scott Sheppard, David Tholen, and Chad Trujillo during a search for distant trans-Neptunian objects whose orbits might be gravitationally influenced by the hypothetical Planet Nine. They announced their discovery on 17 December 2018 and nicknamed the object "Farout" to emphasize its distance from the Sun.
