2018 AG₃₇ is a distant trans-Neptunian object and centaur that was discovered 132.2 ± 1.5 AU (19.78 ± 0.22 billion km) from the Sun,^[8] farther than any other currently observable known object in the Solar System.^[3][9] Imaged in January 2018 during a search for the hypothetical Planet Nine,^[10] the confirmation of this object was announced in a press release in February 2021 by astronomers Scott Sheppard, David Tholen, and Chad Trujillo. The object was nicknamed "FarFarOut" to emphasize its distance from the Sun.^[3]

At a very faint apparent magnitude of 25, only the largest telescopes in the world can observe it.^[1] Being so far from the Sun, 2018 AG₃₇ moves very slowly among the background stars and has been observed only nine times in the first two years.^[5] It requires an observation arc of several years to refine the uncertainties in the approximately 700-year orbital period and determine whether it is currently near or at aphelion (farthest distance from the Sun). JPL Horizons computes an aphelion around the year 2005 at about 133 AU,^[11] whereas Project Pluto computes aphelion around the year 1976 slightly further out at 134 AU.^[12] Its perihelion is a little less than Neptune's.

2018 AG₃₇ was first imaged on 15 January 2018 by astronomers Scott Sheppard, David Tholen, and Chad Trujillo when they were surveying the sky using the large 8.2-meter Subaru Telescope at Mauna Kea Observatory, Hawaii, to find distant Solar System objects and the hypothetical Planet Nine, whose existence they proposed in 2014.^[1][10] However, it was not noticed until January 2019, when Sheppard decided to review the Subaru images taken in 2018 after having an upcoming lecture delayed by weather.^[10][13] In two of these images taken one day apart in January, he identified a very faint apparent magnitude 25.3 object that moved slowly relative to the background stars and galaxies.^[1] Based on two positions of 2018 AG₃₇ in those images, Sheppard estimated its distance was roughly around 140 astronomical units (AU), farther than 2018 VG18 which was discovered and announced by his team one month earlier in December 2018.^[13]

In his rescheduled talk on 21 February 2019, Sheppard remarked on his discovery of 2018 AG₃₇, which he jokingly nicknamed "FarFarOut" as a succession to the nickname "Farout" used for the previous farthest object 2018 VG₁₈.^[13] Following 2018 AG₃₇'s discovery, Sheppard reobserved the object in March 2019 with the 6.5-meter Magellan-Baade telescope at Las Campanas Observatory, Chile. Additional observations were then made in May 2019 and January 2020 with the Subaru Telescope at Mauna Kea.^[1]

These observations over a two-year period established a tentative orbit solution for 2018 AG₃₇, permitting it to be confirmed and announced by the Minor Planet Center.^[14] The confirmation of 2018 AG₃₇ was formally announced in a press release by the Carnegie Institution for Science on 10 February 2021.^[3]

The object was nicknamed "FarFarOut" for its distant location from the Sun, and particularly because it was even farther than the previous farthest known object 2018 VG₁₈ which was nicknamed "Farout".^[3] It is officially known by the provisional designation 2018 AG₃₇ given by the Minor Planet Center when the discovery was announced.^[1] The provisional designation indicates the object's discovery date, with the first letter representing the first half of January and the succeeding letter and numbers indicating that it is the 932nd object discovered during that half-month.^{[lower-alpha 2]}

The object has not yet been assigned an official minor planet number by the Minor Planet Center due to its short observation arc and high orbital uncertainty.^[2] 2018 AG₃₇ will be given a minor planet number when its orbit is well-secured by observations over multiple opposition and will become eligible for naming by its discoverers after it is numbered with a well-defined orbit.^[14][16]

As of 2021^[update], 2018 AG₃₇ has only been observed nine times over an observation arc of two years.^[2] Being so far from the Sun, 2018 AG₃₇ moves so slowly that two years of observations have not adequately determined its orbit.^[3] The nominal orbit is highly uncertain with a condition code of 9.^[2] Several years of additional observations are necessary to refine the orbital uncertainties.^[3][16] It comes to opposition each January.^[8]

Only 2018 AG₃₇'s distance and orbital elements that define its position (inclination and longitude of the ascending node) have been adequately determined by its two-year observation arc.^[5] The orbital elements that define the shape and motion of 2018 AG₃₇'s orbit (eccentricity, mean anomaly, etc.) are poorly determined because its observation arc does not provide sufficient coverage of its wide-ranging orbit, especially when it moves very slowly due to its large distance.^[5] The nominal best-fit orbit solution provided by the Jet Propulsion Laboratory (JPL) Small-Body Database gives an orbital semi-major axis of 80.2±4.5 AU and an eccentricity of 0.655±0.02, corresponding to a perihelion and aphelion distance of 27.6±0.2 AU and 133±7 AU, respectively.^[2][5] The orbital period of 2018 AG₃₇ is poorly known, but it probably lies around 700 years.^[5][3]

Given the uncertainty of 2018 AG₃₇'s nominal perihelion distance, it likely crosses Neptune's orbit (30.1 AU) with a nominal minimum orbit intersection distance (MOID) around 3 AU (450 million km; 280 million mi).^[2] 2018 AG₃₇'s small perihelion distance and elongated orbit implies that it has experienced strong gravitational interactions with Neptune in past close encounters.^[3] Other trans-Neptunian objects are known to have been scattered onto similarly distant and elongated orbits by Neptune—these are collectively known as scattered disc objects.^[4]

Based on 2018 AG₃₇'s apparent brightness and projected distance, the Minor Planet Center calculates an absolute magnitude of 4.2.^[2] It is listed by the Minor Planet Center as the 12th intrinsically brightest known scattered disc object.^[4]

The size of 2018 AG₃₇ is unmeasured, but it likely lies between 400–600 km (250–370 mi) in diameter assuming a geometric albedo range of 0.10–0.25.^[20] Sheppard estimates that 2018 AG₃₇'s diameter lies at the lower end of this range, as he concludes that it has a highly reflective and ice-rich surface.^[3] Johnston assumes a dark albedo of 0.057 and a larger diameter of 797 kilometres (495 mi), and classifies 2018 AG₃₇ as a centaur. If correct, that would make it the largest known centaur.^[6]

• 2018 VG18, the next most distant known object discovered in 2018, nicknamed "Farout"
• 2020 FY30, the third most distant known object discovered by Sheppard's team in 2020
• 2020 FA31, the fourth most distant known object discovered by Sheppard's team in 2020
• List of Solar System objects most distant from the Sun