Somewhere beyond the orbit of Neptune might be a massive trans-Neptunian object (TNO) with 10 times the mass of Earth. This "invisible" and undiscovered planet's highly-elongated and unnatural orbit lasts an astounding 15,000 years.
Some astronomers are adamant this object -- popularly referred to as Planet Nine (also called Planet X) -- exists, and new evidence seems to prove it does. The evidence has to do with the movements of the dwarf planet 2015 TG387, or The Goblin. The object is called The Goblin because of the initials "TG," and because it was first observed before Halloween on Oct. 13, 2015.
What makes 2015 TG387 quite interesting is it doesn't interact with other planets in the Solar System. It should do this, according to the law of gravity, but that it doesn't means something big is preventing it from doing so.
2015 TG387 isn't close enough to Jupiter, Saturn, Uranus, or Neptune to be gravitationally influenced by these gas giants. Its orbit around the solar system, however, clearly illustrates its being influenced by something -- probably Planet Nine (also written as Planet 9).
Some astronomers contend the orbit of 2015 TG387 suggests -- but doesn't conclusively prove -- the existence of Planet Nine in the outer Solar System beyond the Oort Cloud.
2015 TG387 is 80 astronomical units (AU) from the Sun. It's some two-and-a-half times farther out than Pluto's current location. 2015 TG387 is the third sednoid to be discovered, following 90377 Sedna and 2012 VP113. A sednoid is a TNO with a perihelion greater than 50 AU and a semi-major axis greater than 150 AU.
2015 TG387 never interacts with anything that we know of in the solar system, said Scott Sheppard, the co-discoverer of this sednoid and an astronomer at the Carnegie Institution for Science. Shephard pointed out that 2015 TG387 somehow t had to get on its elongated orbit in the past. He said the biggest question is what did this sednoid interact with to get there?
Mathematical simulations show that 2015 TG387 was shifted into its highly-elongated orbit by interactions with a larger body. This large minor planet will likely be Planet Nine. Finding an incredibly faint and elusive planet over 30 AU from the Sun is taxing the limits of current technologies, however.
Estimates for the size of Planet Nine are astounding. This minor planet might de a super-Earth-sized planet. It might have an estimated mass equivalent to 10 Earths, a diameter up to four times that of the Earth. It might also have dim surface albedo, which means it's a black body that absorbs all incident radiation (including sunlight) and therefore appears invisible to our instruments.
The existence of Planet Nine was first posited in 2014 by Sheppard and Chad Trujillo. They believe there may be a "massive trans-Neptunian planet" on the outskirts of our Solar System due to "similarities in the orbits" of distant objects that orbit Neptune.
Konstantin Batygin and Mike Brown of Caltech in January 2016 announced they'd found signs of Planet Nine using modeling and computer simulations. Then came Swedish simulations arguing Planet Nine might have been formed around another star, and was later captured by our Sun.
Batygin and Brown in October 2016 also said the existence of Planet Nine adds "wobble" to our Solar System, tilting it in relation to the Sun.
