The Goblin is “about 300 kilometers in diameter, on the small end of a dwarf planet,” said Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington who discovered the object along with colleagues at Northern Arizona University, University of Hawaii and the University of Oklahoma. Dwarf planet Pluto, by comparison, is six times as wide.
Sheppard has embarked on an ongoing survey to find tiny planetoids on the solar system’s outer rim. He’s interested in the Goblin because it “always stays well beyond the giant planet region,” referring to the lineup of our solar system’s four biggest planets: Jupiter, Saturn, Uranus and Neptune. Because 2015 TG387 exists so far away, speaking in terms of miles becomes unwieldy. Instead, astronomers refer to its orbit in astronomical units, or AU, where 1 AU is the distance between the sun and Earth. Pluto sits at an average of 40 AU from the sun. The Goblin comes no closer than 65 AU.
Only a few known objects in our solar system have comparable orbits, such as dwarf planets 2012 VP113 (nickname: Biden) and Sedna. And 2015 TG387′s lopsided elliptical orbit takes it much farther away than those two remote objects — at its farthest, the Goblin reaches 2,300 AU, into a region of space called the Oort cloud. This also means the Goblin takes 40,000 years to complete one orbit of the sun. If we set our calendars by 2015 TG387, then one “Goblin year” ago the last of the Neanderthals walked the earth.
Confirming the orbit of 2015 TG387 required repeated observations, through May 2018, because the planet moves so slowly. The astronomers were lucky to catch the Goblin when they did. As it travels along 99 percent of its orbit, 2015 TG387 is too far and too faint to be detected. Sheppard said he predicts thousands of objects the same size as 2015 TG387 dot the rim of our solar system. But they are likewise too distant to be seen the vast majority of the time. He anticipates astronomers will be able to detect only another dozen or so objects in the next few years of the survey.
“Objects such as 2015 TG387 allow us to probe not only the makeup of the solar system itself but also the gravitational mechanisms that sculpt it,” said Konstantin Batygin, a planetary scientist at the California Institute of Technology who was not involved with the observation. “This is a great discovery indeed.”
The Goblin’s orbit is very skewed, and so is Sedna’s and Biden’s. Sheppard says a large and unknown planet could be “shepherding” these dwarf planets, directing them like a cosmic border collie around the solar system’s fringe.
Sheppard is not the only astronomer to propose that a putative planet, called Planet Nine or Planet X, lurks at the dark edge of the solar system. The planet, if it exists, would be bigger than the Goblin or Pluto. Batygin, in a 2016 paper in the Astronomical Journal, estimated Planet Nine would be up to 10 times as massive as Earth.
As such, Planet Nine would be a “massive perturber,” as Sheppard called it in a 2014 Nature article. Smaller objects like the Goblin have to dance around Planet Nine, or else they would collide with it or be ejected from their orbits. So far, all of the objects Sheppard has spotted appear to dance as predicted.
“This clustering can only be maintained if the solar system hosts an additional, yet unseen, super-Earth type planet,” Batygin said. He added, “I’m running code as we speak that evaluates how the inferred orbit and mass of Planet Nine are affected by this new object.” The Goblin sits right in the middle of the cluster of known objects, he said, and the astronomical search helps scientists home in on Planet Nine’s location.
In 2016, Sheppard told The Washington Post that he put the odds of Planet Nine’s existence at about 60 percent. Now he’s up to 80 percent, he said. “If the trends are true, then we don’t know of another explanation for why they would be grouped in an orbit like this,” Sheppard said. Though the trends have held, the astronomers admit they are dealing with a small number of known objects. For now, Sheppard is eager to find more.