2018 AG37

Lore & Background

2018 AG37 was first imaged on 15 January 2018 by astronomers Scott Sheppard, David Tholen, and Chad Trujillo during a survey using the Subaru Telescope to find distant Solar System objects and the hypothetical Planet Nine. It was not noticed until January 2019, when Sheppard reviewed the images after a lecture delay caused by weather. He identified a faint object of apparent magnitude 25.3 moving slowly relative to background stars, estimating its distance at roughly 140 AU. Sheppard jokingly nicknamed it 'FarFarOut' in a rescheduled talk on 21 February 2019. Subsequent observations in March 2019 with the Magellan-Baade telescope and in May 2019 and January 2020 with the Subaru Telescope established a tentative orbit, leading to its confirmation and announcement by the Minor Planet Center in a press release on 10 February 2021.

The object's provisional designation 2018 AG37 indicates its discovery date, with the first letter representing the first half of January and the succeeding letters and numbers showing it is the 932nd object discovered that half-month. It has not yet been assigned a minor planet number due to its short observation arc and orbital uncertainty. As of 2021, it had been observed nine times over two years, but its orbit remains highly uncertain with a condition code of 9. The nominal best-fit orbit gives a semi-major axis of 80.2±4.5 AU, eccentricity 0.655±0.02, perihelion 27.6±0.2 AU, and aphelion 133±7 AU, with an orbital period probably around 700 years. Its small perihelion distance and elongated orbit suggest past strong gravitational interactions with Neptune, classifying it as a scattered disc object.

Physical characteristics are uncertain. The Minor Planet Center calculates an absolute magnitude of 4.2, listing it as the 12th intrinsically brightest known scattered disc object. Its diameter is estimated between 400–600 km assuming an albedo of 0.10–0.25, with Sheppard favoring the lower end due to a likely highly reflective, ice-rich surface. Johnston classifies it as a centaur with a very dark albedo of 0.058, yielding a much larger diameter of 790 km, which would make it the largest known centaur.

Reader's Guide

2018 AG37's significance lies in its status as the farthest currently observable known object in the Solar System, discovered at 132.2 ± 1.5 AU from the Sun. Its discovery was part of a search for hypothetical planets beyond Neptune, and it was nicknamed 'FarFarOut' as a successor to 'Farout' (2018 VG18). The object's orbit is poorly determined due to its slow motion and short observation arc of two years, with a condition code of 9. Its nominal orbit suggests it crosses Neptune's orbit, with a minimum orbit intersection distance around 3 AU, indicating past gravitational scattering by Neptune. This places it among scattered disc objects. The object's size is uncertain, with estimates ranging from 400 km to 790 km depending on assumed albedo. If Johnston's centaur classification and dark albedo are correct, it would be the largest known centaur. The object has not yet received a minor planet number, pending further observations to secure its orbit. As of January 2024, it remains the farthest observed object in the Solar System, though many trans-Neptunian objects have greater aphelion distances and comets like Comet Donati and Caesar's Comet are currently farther but unobservable.

Did You Know?

• 2018 AG37 was discovered at a distance of 132.2 ± 1.5 AU from the Sun, farther than any other currently observable known object in the Solar System.
• It was nicknamed 'FarFarOut' by discoverer Scott Sheppard as a succession to the nickname 'Farout' for the previous farthest object 2018 VG18.