On January 20th, 2016, scientists announced the discovery of what could be a giant planet 10 times the mass of the Earth in the Kuiper Belt at the outer edge of our solar system. Since then, it's circumstances have remained vastly unknown, but we can say for sure that.. something's there. We can also now say that the unconfirmed planet is suspected to be a rogue planet, a planet that does not orbit a star but orbits the galaxy directly. They are considered "free floating planets" and sometimes even "orphan" or "wandering" planets.
This was first suspected when scientists detected an unknown gravitational influence in our outer solar system two years ago. The theory states that the planet was roaming in its galaxy when it came a bit too close to our solar system, and the gravitational pull of our sun was so strong that it pulled the planet into it's orbit. Space researchers James Vesper and Paul Mason from New Mexico State University have come to this conclusion with strong evidence through 156 different simulations that this theory is entirely plausible and are looking forward to officially confirming the existence of Planet Nine in the coming months. According to the researchers, if Planet Nine is out there, it is likely a former rogue.
Yirka, Bob, "Simulations suggest Planet Nine may have been a rogue", 12 Jan. 2017 https://phys.org/news/2017-01-simulations-planet-rogue.html
Wall, Mike, "Mysterious Planet Nine May Be a Captured 'Rogue' World", 10 Jan. 2017 http://www.space.com/35277-planet-nine-captured-rogue-exoplanet.html