2018 VG18


is a distant trans-Neptunian object that was discovered well beyond 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 may be gravitationally influenced by hypothetical Planet Nine. They announced their discovery on 17 December 2018 and nicknamed the object "Farout" to emphasize its distance from the Sun.
the object is at an observed distance of from the Sun, which is more than three times the observed distance of the dwarf planet Pluto from the Sun. was the most distant natural object ever observed in the Solar System, until it was itself supplanted by an object initially estimated at nicknamed "FarFarOut". However, is not close to being the object with the most distant orbit on average, as its semi-major axis is estimated to be only about ; in comparison, the semi-major axis of the planetoid 90377 Sedna is about.
is considered to be a dwarf planet candidate, as its absolute magnitude implies that it is around in diameter. Assuming that the object is predominantly icy in composition, it is expected to be large enough to attain a gravitationally rounded shape, and thus be a dwarf planet. Observations of show that it is pinkish in color, which indicates that it has an ice-rich surface.

Discovery

was discovered by astronomers Scott Sheppard, David Tholen, and Chad Trujillo at the Mauna Kea Observatory in Hawaii on 10 November 2018. The discovery formed part of their search for distant trans-Neptunian objects with orbits that may be gravitationally perturbed by the hypothesized Planet Nine. The search team had been involved in the discoveries of several other distant TNOs, including the sednoids and. was first identified as a faint object slowly moving in two images taken with the 8.2-meter Subaru Telescope on the night of 10 November 2018. At the time of discovery, was located in the constellation Taurus, at a very faint apparent magnitude of 24.6, approaching the lowest detectable magnitude limit for most telescopes.
The very slow movement and low apparent magnitude of indicated that it is very distant, which prompted additional follow-up observations to constrain its orbit and distance. The object was reobserved in December 2018 with the 6.5-meter Magellan-Baade telescope at the Las Campanas Observatory in Chile, with observation times spanning ten days. However, due to 's slow motion, its orbit could not be accurately calculated from the short observation arc. Despite this, the discovery of along with a preliminary orbit solution was formally announced in a Minor Planet Electronic Circular issued by the Minor Planet Center on 17 December 2018.
Since the announcement of 's discovery, the object has been occasionally observed by Scott Sheppard using the Magellan and Subaru telescopes in January, February, March, and November 2019 and also by the Gran Telescopio Canarias located at the Roque de los Muchachos Observatory on the island of La Palma in November 2019 and January 2020. These observations helped reduce uncertainties in 's orbit., has been observed for over three oppositions, with an observation arc of 3 years. Only two precovery images of have been identified, and both were taken by the Cerro Tololo Observatory's Dark Energy Camera on 16 January 2017.

Nomenclature

Upon the announcement of 's discovery, the discovers nicknamed the object "Farout" for its distant location from the Sun, and particularly because it was the farthest known TNO observed at the time. On the same day, the object was formally given the provisional designation by the Minor Planet Center. The provisional designation indicates the object's discovery date, with the first letter representing the first half of November and the succeeding letter and numbers indicating that it is the 457th 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. is expected to receive a minor planet number once it has been observed for over at least four oppositions, which would take several years. Once it receives a minor planet number, the object will be eligible for naming by its discoverers.

Orbit and classification

is the second-most distant observed Solar System object and is the first TNO discovered while beyond 100 astronomical units from the Sun, overtaking the dwarf planet in distance. 's distance from the Sun is, more than three times the observed distance of Pluto from the Sun. For comparison, the distances of the Pioneer 10 and Voyager 2 space probes were approximately 126 AU and 124 AU in 2020, respectively. At such distances, is thought to be close to the heliopause, the boundary where the Sun's solar wind is stopped by the interstellar medium at around 120 AU. The new orbit determination indicates that this object is currently very close to or at aphelion, and that it is a member of the scattered disc.
At the time of discovery on 10 November 2018, 's distance from the Sun was 123.4 AU, and has since moved 0.1 AU from the Sun. As it is approaching aphelion, is receding from the Sun at a rate of 0.03 AU per year, or. was the farthest TNO known until February 2019, when another distant object dubbed "FarFarOut" was discovered at about 140 AU by the same discovery team. While and "FarFarOut" are among the farthest Solar System objects observable, some near-parabolic comets are much further from the Sun. For example, Caesar's Comet is over 800 AU from the Sun while Comet Donati is over 145 AU from the Sun.
On average, orbits about 81 AU from the Sun, taking approximately 727 years to complete one full orbit around the Sun. With an orbital eccentricity of about 0.53, it follows a highly elongated orbit, varying in distance from 37.8 AU at perihelion to 123.8 AU at aphelion. Its orbit is inclined to the ecliptic plane by 24.5 degrees, with its aphelion oriented below the ecliptic. At perihelion, approaches close to Neptune's orbit without crossing it, having a minimum orbit intersection distance of approximately 8 AU. Because approaches Neptune at close proximity, its orbit has likely been perturbed and scattered by Neptune, thus it falls into the category of scattered-disc objects. last passed perihelion in the late 17th century.
s, with scattered-disc objects and shown for scale.
Despite being one of the most distant TNOs known, does not have the largest average orbital distance among all known objects. In comparison, the semi-major axis of the planetoid 90377 Sedna is about 480 AU. In an extreme case, the scattered-disc object has a semi-major axis around 1,500–1,600 AU, though its distance from the Sun is only 63.5 AU, which is approximately half 's current distance from the Sun.
Prior to orbital refinements in 2019–2020, was thought to have a smaller orbit and perihelion distance, based on a preliminary orbit calculated from a short span of observations. The preliminary orbit solution implied that crosses Neptune's orbit, which would have fit under the description of the centaur classification. Regardless, the Minor Planet Center generally classifies as a distant object since it orbits the Sun in the outer Solar System.
was discovered in a particular region of the sky where other extreme TNOs have been found, suggesting that its orbit may be similar to those of extreme TNOs, which characteristically have distant and highly elongated orbits that may have resulted from the gravitational influence of the hypothetical Planet Nine. 's nominal orbit appears to be anti-aligned with Sedna; the longitude of pericenter of 's orbit is oriented about 198 degrees from Sedna's orbit.

Physical characteristics

The size of is uncertain, though it is likely large enough to be a possible dwarf planet, based on its intrinsic brightness or absolute magnitude. Based on its apparent brightness and large distance, the JPL Small-Body Database calculates 's absolute magnitude to be about 3.7, though this estimate has a large uncertainty. The Minor Planet Center calculates 's absolute magnitude to be 3.5, in close agreement with the former estimate. Based on the Minor Planet Center's estimate of 's absolute magnitude, it is listed among the top six intrinsically brightest scattered-disc objects.
The albedo of has not been measured nor constrained, thus its diameter could not be calculated with certainty. Assuming that the albedo of is within the range of 0.10–0.25, its diameter should be around. This size range is considered to be large enough such that the body can collapse into a spheroidal shape, and thus be a dwarf planet. Astronomer Michael Brown considers to be highly likely a dwarf planet, based on his size estimate of calculated from an albedo of 0.12 and an absolute magnitude of 3.9. Unless the composition of is predominantly rocky, Brown considers it very likely that has attained a spheroidal shape through self-gravity. Astronomer Gonzalo Tancredi estimates that the minimum diameters for a body to undergo hydrostatic equilibrium are around and, for predominantly icy and rocky compositions, respectively. If the composition of is similar to the former case, the object would be considered a dwarf planet under Tancredi's criterion.
Observations of with the Magellan-Baade telescope show that the object is pinkish in color. The pinkish color of is generally attributed to the presence of ice on its surface, since other ice-rich TNOs display a similar color. Apart from its color, the spectrum and surface composition of have not yet been measured in detail and will require further observations.