New Object Found in Far Outer Solar System

Astronomers have discovered an object—2015 TG387—that could help in the hunt for a hypothesized Planet Nine in the distant reaches of the solar system.

2015 TG387, a.k.a. the “Goblin”, orbits in the inner Oort Cloud along with Sedna and 2014 VP113.
Roberto Candanosa / Scott Sheppard / Carnegie Institution for Science

A newly found body in the extreme backwaters of the solar system incrementally bolsters the case for an undiscovered planet lurking far from the Sun.

Officially dubbed 2015 TG387 and nicknamed “The Goblin”, the object resides in the inner Oort Cloud, a region beyond the Kuiper Belt that until now harbored only two other known bodies: the dwarf planet Sedna and the less well-known 2012 VP113.

The Goblin keeps its distance from the Sun, never coming closer than about 65 astronomical units (a.u.), or just under twice Pluto’s closest approach. However, that’s downright cozy compared to how far out it goes. The Goblin’s highly elongated orbit takes it to a whopping 2,300 a.u., much farther away than its two comrades in the inner Oort Cloud.

The discovery was announced October 2nd by the International Astronomical Union’s Minor Planet Center.

Scott Sheppard (Carnegie Institution for Science) and colleagues discovered 2015 TG387 in images acquired in late 2015 at the 8.2-meter Subaru Telescope in Hawai‘i. At the time, 2015 TG387 was about 80 a.u. from the Sun. Since then, the team has been using other telescopes to track the object and refine its orbit. They estimate that the body is roughly 300 kilometers wide, roughly the width of Saturn’s moon Hyperion.

Discovery images of 2015 TG387, taken 3 hours apart at the Subaru Telescope on October 13, 2018. 2015 TG387 is the dot moving near the center.
Scott Sheppard

Extrapolating from the three objects now known in the inner Oort Cloud, the team estimates that this largely unexplored region of space might harbor roughly 2 million bodies wider than 40 kilometers. That puts the mass of this ensemble on par with the Kuiper Belt.

The bodies in the inner Oort Cloud, along with a smattering of other objects well beyond Neptune, are so remote that they are immune to the gravitational machinations of the known planets. That makes them unique probes for unknown forces at the limits of the solar system. For example, their aligned orbits suggest that a ninth planet, several times as massive as Earth, awaits discovery out there. While 2015 TG387 doesn’t make or break that case, the team’s simulations indicate that its orbit is at least consistent with a planet’s presence.