2018 VG₁₈

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 2015 TG387.[3][11] 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 1] at a very faint apparent magnitude of 24.6, approaching the lowest detectable magnitude limit for most telescopes.[1][8]

The very slow movement and low apparent magnitude of 2018 VG₁₈ 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 with the 6.5-meter Magellan-Baade telescope at the Las Campanas Observatory in Chile, with observation times spanning ten days.[1] However, due to 2018 VG₁₈'s slow motion, its orbit could not be accurately calculated from the short observation arc.[11] Despite this, 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 announcement of 2018 VG₁₈'s discovery, the object has been occasionally observed by Scott Sheppard using the Magellan and Subaru telescopes in January, February, March, and November 2019[12][9] 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 2018 VG₁₈'s orbit.[12] As of 2020, 2018 VG₁₈ has been observed for over three oppositions, with an observation arc of 3 years (1,095 days). Only two precovery images of 2018 VG₁₈ have been identified, and both were taken by the Cerro Tololo Observatory's Dark Energy Camera on 16 January 2017.[2]

Upon the announcement of 2018 VG₁₈'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.[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 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 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.[13][3] Once it receives a minor planet number, the object will be eligible for naming by its discoverers.[13]

2018 VG₁₈ is the second-most distant observed Solar System object and is the first TNO discovered while beyond 100 astronomical units (AU) from the Sun, overtaking the dwarf planet Eris (96 AU) in distance.[9][3] 2018 VG₁₈'s distance from the Sun as of 2020 is 123.5 AU (18.5 billion km; 11.5 billion mi),[7] more than three times the observed distance of Pluto from the Sun (34 AU during 2018).[3] In comparison, the distances of the Pioneer 10 and Voyager 2 space probes are approximately 124.4 AU and 123.6 AU as of January 2020, respectively.[14][15] At such distances, 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.[11] 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, 2018 VG₁₈'s distance from the Sun was 123.4 AU, and has since moved 0.1 AU from the Sun as of May 2020.[7] As it is approaching aphelion, 2018 VG₁₈ is receding from the Sun at a rate of 0.03 AU per year, or 0.14 kilometers per second (310 mph).[7] 2018 VG₁₈ 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.[9] While 2018 VG₁₈ and "FarFarOut" are among the farthest Solar System objects observable,[9] 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 2020.[16][17]

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

On average, 2018 VG₁₈ 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.[5][2] 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][11] 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.

Despite being one of the most distant TNOs known, 2018 VG₁₈ does not have the largest average orbital distance (semi-major axis) among all known objects.[4] In comparison, the semi-major axis of the planetoid 90377 Sedna is about 480 AU.[10] In an extreme case, the scattered-disc object 2014 FE72 has a semi-major axis around 1,500–1,600 AU,[18] though its distance from the Sun is only 63.5 AU as of 2020, which is approximately half 2018 VG₁₈'s current distance from the Sun.[19]

Prior to orbital refinements in 2019-2020,[12] 2018 VG₁₈ was thought to have a smaller orbit and perihelion distance, based on a preliminary orbit calculated from a short span of observations.[1] The preliminary orbit solution implied that 2018 VG₁₈ crosses Neptune's orbit, which would have fit under the description of the centaur classification.[11] Regardless, the Minor Planet Center generally classifies 2018 VG₁₈ as a distant object since it orbits the Sun in the outer Solar System.[2]

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 198 degrees from Sedna's orbit.[lower-alpha 3]

The size of 2018 VG₁₈ is uncertain, 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, the JPL Small-Body Database calculates 2018 VG₁₈'s absolute magnitude to be about 3.7, though this estimate has a large uncertainty.[5] The Minor Planet Center calculates 2018 VG₁₈'s absolute magnitude to be 3.5, in close agreement with the former estimate.[2] Based on the Minor Planet Center's estimate of 2018 VG₁₈'s absolute magnitude, it is listed among the top six intrinsically brightest scattered-disc objects.[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).[20] 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.[6][21] 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.[6] 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.[6] 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.[21] 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][11] Apart from its color, the spectrum and surface composition of 2018 VG₁₈ have not yet been measured in detail and will require further observations.[11]

• List of possible dwarf planets
• List of Solar System objects by greatest aphelion
• List of most distant trans-Neptunian objects
• List of Solar System objects most distant from the Sun in 2018
• 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 – 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
  • Close approach · Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters