2018 VG₁₈

Animation of two discovery images taken by the 8.2-meter Subaru telescope on 10 November 2018. The moving dot in the center is 2018 VG₁₈ at magnitude 24.6.

2018 VG₁₈ was discovered by astronomers Scott Sheppard, David Tholen, and Chad Trujillo at the Mauna Kea Observatory in Hawaii on 10 November 2018.[1][3] The discovery formed part of their search for distant trans-Neptunian objects (TNOs) 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 2012 VP113 and 541132 Leleākūhonua.[3][13] 2018 VG₁₈ 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.[1][3] At the time of discovery, 2018 VG₁₈ was located in the constellation Taurus,[lower-alpha 2] at a very faint apparent magnitude of 24.6, approaching the lowest detectable magnitude limit for most telescopes.[1][11]

2018 VG₁₈'s low on-sky motion and brightness indicated that it is very distant, which prompted additional follow-up observations to constrain its orbit and distance.[3] The object was reobserved in December 2018 by Sheppard at the Las Campanas Observatory, with observation times spanning ten days.[1] However, its orbit remained with a significant uncertainty due to its short observation arc. Nonetheless, the discovery of 2018 VG₁₈ 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.[1]

Since the discovery announcement, 2018 VG₁₈ has been periodically observed by Sheppard at the Las Campanas and Mauna Kea observatories.[14] Additional observations were also made at the Roque de los Muchachos Observatory in November 2019 and January 2020.[15] As of 2022, 2018 VG₁₈ has been observed for over five oppositions, with an observation arc of 16 years (5,900 days). Several precovery observations of 2018 VG₁₈ have been identified in images taken by the Cerro Tololo Observatory's Dark Energy Camera on 11 March 2015 and 16 January 2017, as well as images taken by the Canada-France-Hawaii Telescope and Subaru Telescope in November 2003 and September 2005, respectively.[2]

Upon the announcement of 2018 VG₁₈'s discovery, the discoverers nicknamed the object "Farout" for its distant location from the Sun, and particularly because it was the farthest known TNO observed at the time.[3] On the same day, the object was formally given the provisional designation 2018 VG₁₈ by the Minor Planet Center.[1] 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.[lower-alpha 3] 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.[2] 2018 VG₁₈ is expected to receive a minor planet number once it has been observed for over at least four oppositions, which would take several years.[16][3] Once it receives a minor planet number, the object will be eligible for naming by its discoverers.[16]

2018 VG₁₈ is the second-most distant observed Solar System object from the Sun and is the first object discovered while beyond 100 astronomical units (AU), overtaking the dwarf planet Eris (96 AU) in observed distance.[15][3] As of 2021 2018 VG₁₈'s distance from the Sun is 123.5 AU (18.5 billion km; 11.5 billion mi),[10] more than three times the observed distance of Pluto from the Sun (34 AU during 2018).[3] For comparison, the distances of the Pioneer 10 and Voyager 2 space probes were approximately 128 AU and 126 AU in 2021, respectively.[17] At its nominal distance, 2018 VG₁₈ 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.[13] The new orbit determination indicates that this object is currently very close to aphelion which it should reach in mid-2067,[6] and that it is a member of the scattered disc.

At the time of discovery on 10 November 2018, 2018 VG₁₈'s distance from the Sun was 123.4 AU, and has since moved 0.2 AU from the Sun as of 2021.[10] As it is approaching aphelion, 2018 VG₁₈ is receding from the Sun at a rate of 0.06 AU per year, or 0.3 km/s (670 mph).[10] 2018 VG₁₈ was the farthest TNO known until February 2019, when 2018 AG37 (nicknamed "FarFarOut") was discovered at about 132 AU by the same team led by Sheppard.[15] While 2018 VG₁₈ and 2018 AG₃₇ are among the farthest Solar System objects observable,[15] some near-parabolic comets are much further from the Sun. For example, Caesar's Comet (C/-43 K1) is over 800 AU from the Sun while Comet Donati (C/1858 L1) is over 145 AU from the Sun as of 2021.[18][19]

The orbit of 2018 VG₁₈[5]

Ecliptic polar view of 2018 VG₁₈'s orbit (top)

Side view of 2018 VG₁₈'s orbit

Retrograde motion in the sky through Taurus

2018 VG₁₈'s average orbital distance from the Sun is approximately 81 AU, taking approximately 738 years to complete one full orbit. With an orbital eccentricity of about 0.53, it follows a highly elongated orbit, varying in distance from 38.3 AU at perihelion to 125.0 AU at aphelion. Its orbit is inclined to the ecliptic plane by about 24 .3 degrees, with its aphelion oriented below the ecliptic. At perihelion, 2018 VG₁₈ approaches close to Neptune's orbit without crossing it, having a minimum orbit intersection distance of approximately 8 AU.[2] Because 2018 VG₁₈ 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.[4][13] 2018 VG₁₈ last passed perihelion in the late 17th century.[5]

Orbit diagram of several extreme trans-Neptunian objects, with scattered-disc objects Eris and 2018 VG₁₈ shown for scale.

While 2018 VG₁₈ is one of the most distant objects observed, it does not have the largest orbital semi-major axis.[4] For comparison, the semi-major axis of the planetoid 90377 Sedna is about 500 AU.[12] In an extreme case, the scattered-disc object 2014 FE72 has a semi-major axis around 1,400 AU,[20] though its distance from the Sun as of 2021 is about 64 AU, approximately half 2018 VG₁₈'s distance from the Sun in that year.[21]

2018 VG₁₈ 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.[3] 2018 VG₁₈'s nominal orbit appears to be anti-aligned with Sedna; the longitude of pericenter of 2018 VG₁₈'s orbit is oriented about 193 degrees from Sedna's orbit.[lower-alpha 4]

The size of 2018 VG₁₈ is unmeasured, though it is likely large enough to be a possible dwarf planet, based on its intrinsic brightness or absolute magnitude.[3] Based on its apparent brightness and large distance, 2018 VG₁₈'s absolute magnitude is estimated to be in the range of 3.4–4.5.[5] According to the Minor Planet Center, it is the ninth intrinsically brightest scattered-disc object.[4]

The albedo (reflectivity) of 2018 VG₁₈ has not been measured nor constrained, thus its diameter could not be calculated with certainty. Assuming that the albedo of 2018 VG₁₈ is within the range of 0.10–0.25, its diameter should be around 500–850 km (310–530 mi).[22] 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.[9][23] Astronomer Michael Brown considers 2018 VG₁₈ to be highly likely a dwarf planet, based on his size estimate of 656 km (408 mi) calculated from an albedo of 0.12 and an absolute magnitude of 3.9.[9] Unless the composition of 2018 VG₁₈ is predominantly rocky, Brown considers it very likely that 2018 VG₁₈ has attained a spheroidal shape through self-gravity.[9] Astronomer Gonzalo Tancredi estimates that the minimum diameters for a body to undergo hydrostatic equilibrium are around 450 km (280 mi) and 800 km (500 mi), for predominantly icy and rocky compositions, respectively.[23] If the composition of 2018 VG₁₈ is similar to the former case, the object would be considered a dwarf planet under Tancredi's criterion.

Observations of 2018 VG₁₈ with the Magellan-Baade telescope show that the object is pinkish in color.[3] The pinkish color of 2018 VG₁₈ is generally attributed to the presence of ice on its surface, since other ice-rich TNOs display a similar color.[3][13] Apart from its color, the spectrum and surface composition of 2018 VG₁₈ have not yet been measured in detail and will require further observations.[13]

• 2018 AG37, nicknamed "FarFarOut" by its discoverers
• List of Solar System objects by greatest aphelion
• List of Solar System objects most distant from the Sun
• List of trans-Neptunian objects

Wikimedia Commons has media related to 2018 VG18.

• Discovered: The Most-Distant Solar System Object Ever Observed – Carnegie Science press release
• Farout Images Archived 24 April 2019 at the Wayback Machine – Discovery images of 2018 VG₁₈ taken with the Subaru Telescope
• It’s the Solar System's Most Distant Object. Astronomers Named It Farout. – The New York Times article by Kenneth Chang
• Meet your very, *very* distant solar system neighbor 2018 VG18 – Bad Astronomy blog by Phil Plait
• List Of Centaurs and Scattered-Disk Objects – Minor Planet Center
• 2018 VG18 at the JPL Small-Body Database