3rd October 2018

New extremely distant object found beyond Pluto

Astronomers have announced the discovery of 2015 TG387 (also known as "The Goblin"), a trans-Neptunian object and sednoid in the outermost part of the Solar System, which may help explain some apparent effects of a hypothetical planet named Planet Nine (or Planet X).



Credit: Roberto Molar Candanosa and Scott Sheppard, courtesy of Carnegie Institution for Science

Researchers in the U.S. are once again redefining our Solar System's edge. They report the discovery of a new, extremely distant object far beyond Pluto with an orbit that supports the presence of an even-farther-out, Super-Earth or larger Planet X. The object, designated 2015 TG387, was announced yesterday by the International Astronomical Union's Minor Planet Center. A paper with full details has also been submitted to The Astronomical Journal.

2015 TG387 – nicknamed "The Goblin" – was discovered about 80 astronomical units (AU) from the Sun, a measurement defined as the distance between the Earth and Sun. For context, Pluto is around 34 AU, so the Goblin is about two and a half times further away from the Sun than Pluto is right now.

The new object is on a very elongated orbit and never comes closer to the Sun (a point called perihelion) than about 65 AU. Only 2012 VP113 and Sedna at 80 and 76 AU respectively have more distant perihelia than this. However, while the Goblin has the third-most-distant perihelion, its orbital semi-major axis is larger than both 2012 VP113 and Sedna's, meaning it travels much farther from the Sun than they do. At its furthest point, it reaches all the way out to 2,300 AU or about 1,150,000 light seconds (ls). This makes it one of the few known objects that never comes close enough to the giant planets Neptune and Jupiter to have significant gravitational interactions with them.

"These so-called Inner Oort Cloud objects like 2015 TG387, 2012 VP113, and Sedna are isolated from most of the Solar System's known mass, which makes them immensely interesting," explains Scott Sheppard, from the Carnegie Institution for Science. "They can be used as probes to understand what is happening at the edge of our Solar System."



Credit: Roberto Molar Candanosa and Scott Sheppard, courtesy of Carnegie Institution for Science

The object with the most distant orbit at perihelion, 2012 VP113, was also discovered by Sheppard and his colleague Chad Trujillo, who announced that find in March 2014. It led Sheppard and Trujillo to notice similarities in the orbits of several extremely distant objects, and they proposed the presence of an unknown planet several times larger than Earth – sometimes called Planet X or Planet 9 – orbiting the Sun well beyond Pluto at hundreds of AUs.

“We think there could be thousands of small bodies like 2015 TG387 out on the Solar System’s fringes, but their distance makes finding them very difficult,” said the University of Hawaii’s David Tholen, a co-author on the paper. “Currently, we would only detect 2015 TG387 when it is near its closest approach to the Sun. For some 99% of its 40,000-year orbit, it would be too faint to see.”

The new object was discovered as part of the team’s ongoing hunt for unknown dwarf planets and Planet X. It is the largest and deepest survey ever conducted for distant Solar System objects.

“These distant objects are like breadcrumbs leading us to Planet X," explains Sheppard. "The more of them we can find, the better we can understand the outer Solar System and the possible planet we think is shaping their orbits, a discovery that would redefine our knowledge of the Solar System's evolution.”



Artist’s conception of a distant Planet X, which could be shaping the orbits of smaller extremely distant outer Solar System objects.

It took the team a few years of observations to obtain a good orbit for 2015 TG387, because it moves so slowly and has such a long orbit. They first observed the object in October 2015 at the Japanese Subaru 8-metre telescope located atop Mauna Kea in Hawaii. Follow-up observations at the Magellan telescope at Carnegie’s Las Campanas Observatory in Chile and the Discovery Channel Telescope in Arizona were obtained in 2015, 2016, 2017 and 2018 to determine the correct orbit.

2015 TG387 is likely on the small end of being a dwarf planet, with a diameter of about 300 km (196 miles). For comparison, Pluto is 2,377 km (1,477 miles) and Sedna is 1,800 km (1,118 miles). The location in the sky where it reaches perihelion is similar to 2012 VP113, Sedna, and most other known extremely distant trans-Neptunian objects, suggesting that something is pushing them into similar types of orbits.

The team ran computer simulations for how different hypothetical Planet X orbits would affect the orbit of 2015 TG387. The simulations included a Super-Earth-mass planet at several hundred AU on an elongated orbit, as proposed by Caltech’s Konstantin Batygin and Michael Brown in 2016. Most of the simulations showed that not only was 2015 TG387’s orbit stable for the age of the Solar System, but it was actually shepherded by Planet X’s gravity, which keeps the smaller 2015 TG387 away from the massive planet. This gravitational shepherding could explain why the most-distant objects in our Solar System have similar orbits. These orbits keep them from ever approaching the proposed planet too closely, which is similar to how Pluto never gets too close to Neptune even though their orbits cross.

“What makes this result really interesting is that Planet X seems to affect 2015 TG387 the same way as all the other extremely distant Solar System objects. These simulations do not prove that there’s another massive planet in our Solar System – but they are further evidence that something big could be out there,” Trujillo concludes.



Two images of 2015 TG387, taken three hours apart.

Credit: Scott Sheppard

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