PSO J318.5-22 is theorized to be a rogue planet.
A rogue planet, also known as an interstellar planet, nomad planet, free-floating planet, orphan planet, starless planet, or sunless planet is a type of planet that orbits a galactic center directly, and does not orbit any star or brown dwarf. These planets may have been ejected from their planetary system in which they formed through a number of different causes or they may have never been gravitationally bound to a star or brown dwarf in the first place. The Milky Way Galaxy alone may harbor billions of rogue planets.
Some planetary-mass objects may have formed in a similar way to stars, and the International Astronomical Union has proposed that those objects be called sub-brown dwarfs. A possible example is Cha 110913-773444, which might have been ejected and become a rogue planet, or otherwise formed on its own to become a sub-brown dwarf.