2018 AG37
is a distant trans-Neptunian object and centaur that was discovered from the Sun, farther than any other currently observable known object in the Solar System. Imaged in January 2018 during a search for the hypothetical Planet Nine, 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.
was discovered when it was near aphelion, the farthest point from the Sun in its elliptical orbit. The object is estimated to be at least in diameter. Because of its extreme distance, appears extremely faint with an apparent magnitude of 25—only visible to the largest telescopes in the world.
Discovery
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. However, it was not noticed until January 2019, when Sheppard reviewed the Subaru images taken in 2018 after having an upcoming lecture delayed by weather. In two of these images taken one day apart in January, he identified a faint apparent magnitude 25.3 object that moved slowly relative to the background stars and galaxies. Based on two positions of in those images, Sheppard estimated its distance was roughly around 140 astronomical units, farther than which was discovered and announced by his team one month earlier in December 2018.In his rescheduled talk on 21 February 2019, Sheppard remarked on his discovery of, which he jokingly nicknamed "FarFarOut" as a succession to the nickname "Farout" used for the previous farthest object. Following '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.
These observations over a two-year period established a tentative orbit solution for, permitting it to be confirmed and announced by the Minor Planet Center. The confirmation of was formally announced in a press release by the Carnegie Institution for Science on 10 February 2021.
Name
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 which was nicknamed "Farout". It is officially known by the provisional designation given by the Minor Planet Center when the discovery was announced. 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.The object has not yet been assigned an official minor planet number by the Minor Planet Center due to its short observation arc and orbital uncertainty. 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.
Orbit
, has been observed nine times over an observation arc of two years. Being so far from the Sun, moves so slowly that two years of observations have not adequately determined its orbit. The nominal orbit is highly uncertain with a condition code of 9. Several years of additional observations are necessary to refine the orbital uncertainties. It comes to opposition each January.Only 's distance and orbital elements that define its position have been adequately determined by its two-year observation arc. The orbital elements that define the shape and motion of 's orbit are poorly determined because its observation arc does not provide sufficient coverage of its wide-ranging orbit, especially when it moves slowly due to its large distance. The nominal best-fit orbit solution provided by the Jet Propulsion Laboratory Small-Body Database gives an orbital semi-major axis of and an eccentricity of, corresponding to a perihelion and aphelion distance of and, respectively. The orbital period of is poorly known, but it probably lies around 700 years.
Given the uncertainty of 's nominal perihelion distance, it likely crosses Neptune's orbit with a nominal minimum orbit intersection distance around. 's small perihelion distance and elongated orbit implies that it has experienced strong gravitational interactions with Neptune in past close encounters. 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.
Distance
The object was initially estimated to be roughly from the Sun, but this estimate was uncertain due to the very short initial observation arc. When it was announced in February 2021, had an observation arc of two years. Based on this, it was from the Sun at the time of its discovery on 15 January 2018., it is the farthest observed object in the Solar System.However, over a hundred trans-Neptunian objects are known to have aphelion distances that bring them farther from the Sun than and many near-parabolic comets are currently much farther from the Sun. Comet Donati is over, and Caesar's Comet is calculated to be more than from the Sun. However, none of these more distant objects are currently observable even with the most powerful telescopes.
Physical characteristics
Based on 's apparent brightness and projected distance, the Minor Planet Center calculates an absolute magnitude of 4.2. It is listed by the MPC as the 12th intrinsically brightest known scattered disc object.The size of has not been measured, but it likely lies between in diameter assuming a geometric albedo range of 0.10–0.25. Sheppard estimates that 's diameter lies at the lower end of this range, as he concludes that it has a highly reflective and ice-rich surface. Johnston classifies as a centaur and thus assumes a very dark, centaur-like albedo of 0.057, which translates to a much larger diameter of. If correct, that would make it the largest known centaur.