2015 TH367

Lore & Background

2015 TH367 was first observed on 13 October 2015 by Scott Sheppard, Chad Trujillo, and David Tholen using the Subaru Telescope at the Mauna Kea Observatories. It was announced on 13 March 2018 alongside other trans-Neptunian objects with heliocentric distances greater than 50 AU. The same team also discovered 541132 Leleākūhonua and 2015 TJ367 on the same night. At a visual apparent magnitude of 26.2, it is one of the faintest trans-Neptunian objects observed, requiring the largest telescopes in the world. It has been observed only eight times over 355 days, and as of 2023 the nominal JPL Horizons solution has it coming to aphelion around the year 2238, whereas Project Pluto (fitting only 5 of 8 observations) shows it reached aphelion around 2015.

Reader's Guide

2015 TH367 is notable as one of the most distant observed objects in the Solar System at the time of its announcement, ranking third after Eris and 2014 UZ224. Its extreme faintness (magnitude 26.2) and slow motion across the sky make it observable only with the largest telescopes, and its orbit remains poorly constrained due to a short observation arc. The uncertainty in its orbit is highlighted by the disagreement between JPL Horizons (aphelion around 2238) and Project Pluto (aphelion around 2015). The object's distance of about 90 AU places it far beyond the Kuiper belt, and study of such distant populations may require new instruments like the proposed Whipple spacecraft. The confusion with 541132 Leleākūhonua in media reporting underscores the challenges of communicating preliminary discoveries of extremely distant objects. As of February 2021, only five known minor planets are further from the Sun under its nominal orbit.

Did You Know?

• 2015 TH367 was the third most distant observed natural object in the Solar System at the time of its announcement in March 2018.
• It has a visual apparent magnitude of 26.2, making it about 75 million times fainter than what can be seen with the naked eye.
• The object was initially confused in press reports with the sednoid 541132 Leleākūhonua, leading to incorrect claims about its distance.

Discovery and the Faintness Problem

On the night of 13 October 2015, a team led by Scott Sheppard, Chad Trujillo, and David Tholen pointed the Subaru Telescope at the sky above Mauna Kea. The 8.2-meter reflecting instrument, perched atop the Hawaiian summit, captured a faint smudge that would later be catalogued as 2015 TH367. That same night the team also identified 2015 TJ367 and the sednoid now called 541132 Leleākūhonua. Yet the discovery of TH367 was anything but straightforward. At a visual magnitude of 26.2, the object is roughly seventy-five million times fainter than what the unaided eye can detect, meaning only the largest telescopes on Earth can even register its presence. In 2015 the team managed just twenty-six days of tracking—an extraordinarily short arc for a body so distant that it barely shifts against the star field. Over the following 355 days only eight additional observations were recorded. The formal announcement did not arrive until 13 March 2018, bundled with several other trans-Neptunian objects beyond fifty astronomical units. The object is expected to linger in the constellation Aries from 1994 through 2077, a slow drift that makes each new measurement precious.

The Aphelion Puzzle

Perhaps the most tantalizing question surrounding 2015 TH367 is whether it has already turned around its roughly seven-hundred-year orbit or is still climbing outward. The JPL Small-Body Database, working from its nominal solution, places perihelion in 1888 and predicts aphelion at 128 AU around the year 2238. Project Pluto, however, which incorporated only five of the eight available observations, suggests the object reached its farthest point from the Sun around 2015, with perihelion occurring in 1886. The discrepancy is a direct consequence of the extremely sparse data: ten observations spread over two years is a thin foundation for pinning down an orbit of this scale. As of 2021 the object sits approximately 90 AU from the Sun, outbound, with an uncertainty of about four AU. Refining that distance and resolving the aphelion question will demand years of additional tracking, and only a handful of the world's most powerful telescopes are capable of following a body this dim. Until then, 2015 TH367 remains one of the Solar System's great orbital mysteries.

A Body at the Brink of the Known

At roughly two hundred kilometers across, 2015 TH367 is a modest world by Solar System standards, yet its position places it among the most remote natural bodies ever catalogued. At the time of its 2018 announcement it ranked as the third most distant observed object, trailing only Eris and 2014 UZ224. By 2021, under its nominal orbit, just five other known minor planets sit farther out: Eris at 95.9 AU, 2020 FA31 at 97.2 AU, 2020 FY30 at 99.0 AU, 2018 VG18 at 123.5 AU, and 2018 AG37 at approximately 132 AU. The object's current distance of about 90 AU is roughly double the outer boundary of the torus-shaped Kuiper belt that encircles Neptune's orbit. Beyond that lies the vast, spherical Oort cloud, whose existence is inferred from the trajectories of long-period comets but whose individual members are far too small to detect with present-day instruments except during rare stellar occultations. As of March 2018, eight hundred and four known objects have aphelia exceeding 89 AU, and studying that outer population will likely require entirely new instruments, such as the proposed Whipple spacecraft mission designed to probe out to ten thousand AU.

The Leleākūhonua Mix-Up

In November 2015, a reporter from Nature attended a Division for Planetary Sciences meeting of the American Astronomical Society, where the Trujillo team presented their October discoveries. Shortly afterward, Nature published a press release announcing V774104—the internal designation for 2015 TH367—as the most distant Solar System object ever observed, at roughly 103 AU. The story attracted considerable media attention, yet it contained two significant errors. First, the object in question was not TH367 at all but 2015 TG387, the sednoid now known as 541132 Leleākūhonua, which had also been identified that same night. The two internal designations had been conflated in the reporting. Second, the stated distance was substantially greater than the true value for either object, though it remains unclear whether this was a misreporting or an inaccurate preliminary estimate derived from an extremely short observation arc. Compounding the confusion, the Trujillo team had not yet submitted the discovery to the Minor Planet Center nor issued their own press statement, leaving no independent public record to correct the errors before they spread.