2018 VG18

2018 VG18 is a distant trans-Neptunian object that was discovered well beyond 100 AU (15 billion km) from the Sun.[8] 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 might be gravitationally influenced by the hypothetical Planet Nine. They announced their discovery on 17 December 2018 and nicknamed the object "Farout" to emphasize its distance from the Sun.[3]

Not to be confused with FarFarOut.
"Farout" redirects here. For other uses, see Far Out (disambiguation).
2018 VG18
Discovery [1][2]
Discovered byS. S. Sheppard
D. Tholen
C. Trujillo
Discovery siteMauna Kea Obs.
Discovery date10 November 2018
(first imaged)
Designations
2018 VG18
"Farout" (nickname)[3]
SDO [4] · TNO [5]
distant [2]
Orbital characteristics [5]
Epoch 31 May 2020 (JD 2459000.5)
Uncertainty parameter 7
Observation arc3.00 yr (1,095 days)
Earliest precovery date16 January 2017
Aphelion124.877±0.524 AU
Perihelion37.796±0.101 AU
81.336±0.342 AU
Eccentricity0.53531±0.00310
733.56±4.62 yr
158.009°±5.953°
0° 0m 4.837s / day
Inclination24.403°±0.012°
245.363°±0.005°
16.753°±2.129°
Physical characteristics
Mean diameter
656 km (est. at 0.12)[6]
500 km (est.)[3]
24.6[7]
3.719±0.524[5]
3.5[2] · 3.9 (est.)[6]

    2018 VG18 is the second-most distant natural object ever observed in the Solar System, after the trans-Neptunian object "FarFarOut" (≈140 AU) discovered by the same team in 2019. As of 2020 the object is at an observed distance of 123.5 AU (18 billion km) from the Sun, more than three times the observed distance of the dwarf planet Pluto. 2018 VG18 is not close to being the object with the most distant orbit on average, as its orbital semi-major axis is estimated to be only about 81 AU. For comparison, the semi-major axis of the planetoid 90377 Sedna is about 480 AU.[9]

    2018 VG18 is considered to be a dwarf planet candidate, as its absolute magnitude implies that it is around 500 km (310 mi) 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.[6] Observations of 2018 VG18 show that it appears pinkish in color, indicative of an ice-rich surface.

    Discovery
    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][10] 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]

    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.[11] Additional observations were also made at the Roque de los Muchachos Observatory in November 2019 and January 2020.[12] As of 2020, 2018 VG18 has been observed for over three oppositions, with an observation arc of 3 years (1,095 days). Two precovery observations of 2018 VG18 have been identified in images taken by the Cerro Tololo Observatory's Dark Energy Camera on 16 January 2017.[2]

    Nomenclature

    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 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]

    Orbit and classification

    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.[12][3] As of 2020 2018 VG18's distance from the Sun 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] For comparison, the distances of the Pioneer 10 and Voyager 2 space probes were approximately 126 AU and 124 AU in 2020, respectively.[14] 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.[10] 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 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 team lead by Sheppard.[12] While 2018 VG18 and "FarFarOut" are among the farthest Solar System objects observable,[12] 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.[15][16]

    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 730 years to complete one full orbit. With an orbital eccentricity of about 0.54, it follows a highly elongated orbit, varying in distance from 38 AU at perihelion to 125 AU at aphelion. Its orbit is inclined to the ecliptic plane by about 24 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][10] 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 480 AU.[9] In an extreme case, the scattered-disc object 2014 FE72 has a semi-major axis around 1,400 AU,[17] though its distance from the Sun is 64.1 AU as of 2020, approximately half 2018 VG18's distance from the Sun in that year.[18]

    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 3]

    Physical characteristics

    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).[19] 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][20] 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.[20] 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][10] Apart from its color, the spectrum and surface composition of 2018 VG18 have not yet been measured in detail and will require further observations.[10]

    See also

    Notes
    1. The celestial coordinates of 2018 VG18 at the time of discovery are 04h 49m 36.353s +18° 53′ 10.69″.[1] See Taurus for constellation coordinates.
    2. In the convention for minor planet provisional designations, the first letter represents the half-month of the year of discovery while the second letter and numbers indicate the order of discovery within that half-month. In the case for 2018 VG18, the first letter 'V' corresponds to the first half-month of November 2018 while the succeeding letter 'G' indicates that it is the 7th object discovered on the 19th cycle of discoveries. Each completed cycle consists of 25 letters representing discoveries, hence 7 + (18 completed cycles × 25 letters) = 457.[13]
    3. The longitude of pericenter is the sum of the argument of pericenter (ω) and the longitude of ascending node (Ω) of the orbit of a body. For 2018 VG18's orbit, ω=16.75° and Ω=245.36°,[5] thus 16.75° + 245.36° = 262.11°. For Sedna's orbit, ω=311.53° and Ω=144.33°,[9] thus 311.53° + 144.33° = 455.86° or 95.86°. The difference between Sedna's and 2018 VG18's longitude of perihelion is about 198°: 455.86° – 262.11° = 193.75°.

    References
    1. "MPEC 2018-Y14 : 2018 VG18". Minor Planet Electronic Circular. Minor Planet Center. 17 December 2018. Bibcode:2018MPEC....Y...14S. Retrieved 29 December 2018.
    2. "2018 VG18". Minor Planet Center. International Astronomical Union. Retrieved 10 January 2020.
    3. "Discovered: The Most-Distant Solar System Object Ever Observed". Carnegie Science. 17 December 2018. Retrieved 29 December 2018.
    4. "List Of Centaurs and Scattered-Disk Objects". Minor Planet Center. International Astronomical Union. Retrieved 29 December 2018.
    5. "JPL Small-Body Database Browser: (2018 VG18)" (2020-01-16 last obs.). Jet Propulsion Laboratory. Retrieved 20 January 2020.
    6. Brown, Michael E. (13 September 2019). "How many dwarf planets are there in the outer solar system?". California Institute of Technology. Retrieved 5 November 2019.
    7. "Horizon Online Ephemeris System for 2018 VG18" (Settings: Sun (body center) [500@10]; Start=2018-11-10, Stop=2020-12-31, Step=1 d). Jet Propulsion Laboratory. Retrieved 9 May 2020.
    8. Chang, Kenneth (17 December 2018). "It's the Solar System's Most Distant Object. Astronomers Named It Farout". The New York Times. Retrieved 29 December 2018.
    9. "JPL Small-Body Database Browser: 90377 Sedna (2003 VB12)" (2018-12-07 last obs.). Jet Propulsion Laboratory. Retrieved 18 January 2020.
    10. Plait, Phil (18 December 2018). "Meet your very, *very* distant Solar System neighbor 2018 VG18". Bad Astronomy. Syfy Wire. Retrieved 17 January 2020.
    11. "MPEC 2020-A128 : 2018 VG18". Minor Planet Electronic Circular. Minor Planet Center. 14 January 2020. Retrieved 14 January 2020.
    12. Voosen, Paul (21 February 2019). "Astronomers discover solar system's most distant object, nicknamed 'FarFarOut'". Science Magazine. Retrieved 26 February 2019.
    13. "How Are Minor Planets Named?". Minor Planet Center. International Astronomical Union. Retrieved 5 January 2017.
    14. Peat, Chris. "Spacecraft escaping the Solar System". Heavens Above. Retrieved 9 July 2020.
    15. "Horizon Online Ephemeris System for -43K1". Jet Propulsion Laboratory. Retrieved 16 November 2018.
    16. "JPL Horizons On-Line Ephemeris for Comet C/1858 L1 (Donati)". Jet Propulsion Laboratory. Retrieved 23 February 2019.
    17. "JPL Small-Body Database Browser: (2014 FE72)" (2018-05-15 last obs.). Jet Propulsion Laboratory. Retrieved 28 November 2020.
    18. "AstDyS-2, Asteroids - Dynamic Site". Asteroids Dynamic Site. Department of Mathematics, University of Pisa. Retrieved 28 November 2020. Objects with distance from Sun over 63 AU
    19. Bruton, D. "Conversion of Absolute Magnitude to Diameter for Minor Planets". Department of Physics, Engineering, and Astronomy. Stephen F. Austin State University. Retrieved 20 January 2020.
    20. Tancredi, G.; Favre, S. (2008). "Which are the dwarfs in the solar system?" (PDF). Asteroids, Comets, Meteors. Retrieved 16 October 2019.

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