# 2015 TH367

2015 TH367 is a trans-Neptunian object approximately 220 kilometers (140 miles) in diameter. As of 2021 it is approximately 90 AU (13 billion km) from the Sun. At the time of its announcement in March 2018, it was the third most distant observed natural object in the Solar System, after Eris and 2014 UZ224.
At a visual apparent magnitude of 26.2, it is one of the faintest trans-Neptunian objects observed and only the largest telescopes in the world can observe it. Being so far from the Sun, 2015 TH$_{367}$ moves very slowly among the background stars and has only been observed eight times over 355 days. It requires an observation arc of several years to refine the uncertainties in the approximately 700-year orbital period and determine whether it is currently near or at aphelion (farthest distance from the Sun). As of 2023 the nominal JPL Horizons solution has it coming to aphelion around the year 2238, whereas Project Pluto (which only fit 5 of the 8 observations) shows it reached aphelion around 2015.

## Discovery

2015 TH$_{367}$ was first observed by Scott Sheppard, Chad Trujillo, and David Tholen on 13 October 2015 using the Subaru Telescope, a large reflecting telescope at the Mauna Kea Observatories on the summit of Mauna Kea with a primary mirror 8.2 meters (27 ft) in diameter. In 2015 it was only observed for 26 days, which is a very short observation arc for a trans-Neptunian object as objects far from the Sun move very slowly across the sky. It is calculated that it will remain in the constellation of Aries from 1994 until 2077. It was announced on 13 March 2018 alongside several other trans-Neptunian objects with a current heliocentric distance greater than 50 AU. The trans-Neptunian objects 541132 Leleākūhonua and 2015 TJ$_{367}$ were also discovered by this team on 13 October 2015.

## Orbit

The orbit of 2015 TH$_{367}$ is poorly constrained, as it has only been observed 10 times over 2 years due to how dim it is. At a visual apparent magnitude of 26.2, it is about 75 million times fainter than what can be seen with the naked eye, and it is one of the dimmest trans-Neptunian objects ever observed, only being able to be seen by the largest modern telescopes. The JPL Small-Body Database estimates that it came to perihelion (closest approach to the Sun) around the year 1888±15. JPL estimates aphelion (farthest distance from the Sun) to be in 2238 at 128 AU whereas Project Pluto (which only fit 5 of the 8 observations) estimates aphelion was in 2015 at 86 AU. As the JPL solution fits all 8 observations, it is a better orbit determination. When dealing with statistics of small numbers, automation can reject some data unnecessarily.

## Distance from the Sun

The precise distance of 2015 TH$_{367}$ still remains unknown due to its poorly understood orbit and the fact it has not been observed since 2016. It is currently outbound roughly 90±4 AU from the Sun, and will require further observations to better refine the orbit. At magnitude 26, it is only observable with a small number of telescopes that are capable of following it up and refining its orbit. It is expected to come to opposition in the constellation of Aries around 3 November 2021 when it should have a solar elongation of roughly 175°.
As of February 2021, there are only five known minor planets further from the Sun than 2015 TH$_{367}$ under its nominal orbit: Eris (95.9 AU), 2020 FA31 (97.2 AU), 2020 FY30 (99.0 AU), 2018 VG18 (123.5 AU), and 2018 AG37 (~132 AU).
Observed Solar System objects that periodically become more distant than 89 AU from the Sun include Sedna (which is much larger in size), 2000 CR105, 2012 DR30, 2013 BL76, and 2005 VX3. There are 804 known objects that have aphelia more than 89 AU from the Sun as of March 2018. This distance is about double the outer limit of the torus-shaped Kuiper belt that lies outside Neptune's orbit. Far beyond this region is the vast spherical Oort cloud enshrouding the Solar System, whose presence was deduced from the orbits of long-period comets.
Another distant object publicly known as V774104 was purportedly discovered at around 103 AU on 13 October 2015 by the same team, but public press releases may have confused its distance with Leleākūhonua (V302126, then known as 2015 TG$_{387}$). 2015 TH$_{367}$ is believed to be V774104.
Study of the population of Solar System objects that are significantly more distant than 2015 TH$_{367}$ will likely require new instruments. The proposed Whipple spacecraft mission is designed to determine the outer limit of the Kuiper belt and directly detect Oort cloud objects out to 10,000 AU. Such objects are too small to detect with current telescopes except during stellar occultations. The proposal involves use of a wide field of view and rapid recording cadence to allow detection of many such events.