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A dwarf planet hitherto unknown surrounds the confines of our solar system, announced last week the Minor Planet Center of the International Astronomical Union. Officially designated 2015 TG387, the small spherical object is probably an ice ball. Astronomers first observed the dwarf planet on October 13, 2015 from the Subaru telescope at the Mauna Kea observatories in Hawaii. Embracing the spirit of October near Halloween – and for lack of something pronounceable – its discoverers dubbed the 2015 TG387 "The Goblin".
The goblin is "about 300 kilometers in diameter, located at the end of a dwarf planet," said Scott Sheppard, astronomer of the Carnegie Institution for Science in Washington, who discovered the object with colleagues from the Northern Arizona University, University of Hawaii and University of Oklahoma. In comparison, the Pluto dwarf planet is six times larger.
Sheppard has embarked on a permanent investigation to find tiny planetoids on the outer edge of the solar system. He is interested in the goblin because he "always stays well beyond the giant planet", referring to the range of the four largest planets in our solar system: Jupiter, Saturn, Uranus and Neptune. Because 2015 TG387 exists so far, talking in terms of miles becomes difficult to handle. Instead, astronomers refer to its orbit as astronomical units, or AU, where 1 AU is the distance between the sun and the Earth. Pluto sits at an average of 40 AU from the sun. The goblin does not approach more than 65 AU.
Only a few known objects in our solar system have comparable orbits, such as the 2012 dwarf planets VP113 (nickname: Biden) and Sedna. And the TG387's unbalanced elliptical orbit of 2015 takes it much farther than these two distant objects – at its furthest, the Goblin reaches 2,300 AU, in a region of the space called cloud of Oort. It also means that the goblin takes 40,000 years to orbit the sun. If we set our calendars by 2015 TG387, there is a year "Goblin", the last of the Neanderthals walked on Earth.
To confirm the 2015 orbit, the TG387 required repeated observations until May 2018 because the planet is moving very slowly. Astronomers were lucky enough to catch the goblin when they did it. While it travels 99% of its orbit, the 2015 TG387 is too far away and too weak to be detected. Sheppard said that he predicted thousands of objects the same size as 2015 TG387 dotting the edge of our solar system. But they are also too far away to be seen most of the time. It predicts that astronomers will only be able to detect a few dozen other objects over the next few years.
"Objects such as 2015 TG387 allow us to probe not only the composition of the solar system, but also the gravitational mechanisms that shape it," said Konstantin Batygin, scientist in planetary science at the California Institute of Technology, who said not participated in the observation. "It's a great find indeed."
The goblin's orbit is very asymmetric, as are Sedna and Biden. Sheppard says that a large unknown planet could "guide" these dwarf planets, directing them like a cosmic border collie around the fringe of the solar system.
Sheppard is not the only astronomer to suggest that a putative planet, called Planet Nine or Planet X, is hiding 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 article in Astronomical Journal, estimated that Planet Nine would be ten times more massive than Earth.
As such, Planet Nine would be a "massive disruptor", as Sheppard called it in a 2014 article on Nature. Smaller objects, like the goblin, have to dance around the planet Nine, otherwise they could collide with it or be ejected from their orbit. Until now, all the objects Spotted by Sheppard seem to dance as expected.
"This grouping can only be maintained if the solar system hosts an extra, still invisible, super-Earth planet," said Batygin. He added, "I'm using a code that evaluates how this new object affects the alleged orbit and the mass of Planet Nine." The goblin sits in the middle of the bunch of known objects, Astronomical research is helping scientists to get to the Planet Nine site, he said.
In 2016, Sheppard told the Washington Post that he estimated that Planet Nine had about 60 percent chance of being. Now he is 80%, he says. "If the trends are true, we do not know any other explanation of why they would be grouped into an orbit like this," Sheppard said. Although trends have been maintained, astronomers admit that they deal with a small number of known objects. For now, Sheppard is eager to find more.
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