541132 Leleākūhonua

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 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 migratory birds. It was the third sednoid discovered, after Sedna and 2012 VP113, and measures around 220 kilometers (140 miles) in diameter.

Discovery
Leleākūhonua was first observed on 13 October 2015 at the Mauna Kea Observatory (T09), by American astronomers David Tholen, Chad Trujillo and Scott Sheppard during their astronomical survey for objects located beyond the Kuiper Cliff. The unofficial discovery was publicly announced on 1 October 2018. The survey uses two principal telescopes: For the Northern hemisphere, the 8.2-meter Subaru Telescope with its Hyper Suprime Camera at Mauna Kea Observatories, Hawaii, and for the Southern hemisphere, the 4-meter Blanco telescope and its Dark Energy Camera at Cerro Tololo Inter-American Observatory in Chile. For follow-up observations to determine an object's orbit, the astronomers are using the Magellan and the Discovery Channel telescopes. The survey's discoveries include 2012 VP113, 2014 SR349 and 2013 FT28.

Orbit and classification
Leleākūhonua orbits the Sun at a distance of 65–2000 AU once roughly every 32,000 years (semi-major axis of ~1080 AU). Its orbit has a very high eccentricity of 0.94 and an inclination of 12° with respect to the ecliptic. It belongs to the extreme trans-Neptunian objects defined by their large semi-major axis and is the third sednoid ever to be discovered, after Sedna and 2012 VP113 ("Biden").

Implications of orbit
Along with the similar orbits of other distant trans-Neptunian objects, the orbit of Leleākūhonua suggests, but does not prove, the existence of a hypothetical Planet Nine in the outer Solar System.

As of 2019, the object is inbound 78 AU from the Sun; about two-and-a-half times farther out than Pluto's current location. It will come to perihelion (closest approach to the Sun) in 2078. As with Sedna, it would not have been found had it not been on the inner leg of its long orbit. This suggests that there may be many similar objects, most too distant to be detected by contemporary technological methods. Following the discovery of Leleākūhonua, Sheppard et al. concluded that it implies a population of about 2 million inner Oort cloud objects larger than 40 km (25 mi), with a combined total mass of 1×1022 kg, which is several times the mass of the asteroid belt.

Numbering and naming
This minor planet was numbered by the Minor Planet Center on 10 October 2019 (M.P.C. 117077). In June 2020, it was formally named Leleākūhonua, allegedly a lifeform mentioned in the Hawaiian creation chant, the Kumulipo, though the name/phrase does not actually occur there. Like other recently coined Hawaiian names for astronomical objects, it is more likely a descriptive phrase. The name was suggested by the Hawaiian-language program A Hua He Inoa, which state (in English) that the name "compares the orbit to the flight of migratory birds and evokes a yearning to be near Earth" and (in Hawaiian) me he manu i ke ala pōʻaiapuni lā, he paʻa mau nō ia i ka hui me kona pūnana i kumu mai ai (like a bird on a path circling the sun, it is forever seeking a leeward wind back toward home).

Physical characteristics
The size of Leleākūhonua depends on the assumed albedo (reflectivity); if it is a darker object then it would also have to be larger; a higher albedo would demand that it be smaller. The faint object has a visual magnitude of 24.64, comparable to the visual magnitudes of Pluto's smaller moons. It was initially estimated to be 300 km (190 mi) in diameter under the assumption of an albedo of 0.15, though observations of a single-chord stellar occultation at Penticton, Canada on 20 October 2018 suggested a smaller diameter of 220 km (140 mi), corresponding to a higher albedo of 0.21.