Although it is estimated that there are at least several billion rogue planets drifting between the stars of the Milky Way Galaxy, the difficulty of detecting them means that only a handful have so far been identified. PSO J318.5-22 is one of these, a brown dwarf floating independently of any star through the constellation of Capricornus.
Between twenty and twenty-five million years ago, a group of stars emerged from a common origin. Today these stars are spread across the sky, forming an association known as the Beta Pictoris Moving Group. Among these stars is an object of a different kind, PSO J318.5-22, a rogue planet that does not orbit a parent star of its own but floats freely in space some eighty light years from the Solar System.
PSO J318.5-22 is faintly detectable in this infrared image of a point in the southern parts of Capricornus. Imagery provided by Aladin sky atlas
The origin of this rogue planet is not known with certainty, but it seems most likely to have originally formed in orbit around one of the stars of the moving group, and then been flung out of that orbit by gravitational perturbations. PSO J318.5-22 is rather younger than the stars of the group (at about twelve million years) which further suggests that it formed later around one of the group's stars.
Physically, PSO J318.5-22 is extremely massive for a planetary body, with more than six times the mass of Jupiter (though it is only half as large again as that planet, indicating that it is rather denser). These characteristics classify the object as a brown dwarf, rather than a typical planet. Spectral analyses of the object imply that it possesses a violent and turbulent atmosphere in which elements including molten iron stir together at extreme temperatures.