Rogue Planets

This is an artist's depiction of a rogue planet. A rogue planet is a planet that is free floating in space unbounded to any orbit. Astronomers believe that some of these planets may have once been in orbit of a star as any other planet until they were ejected for any given reason. In order to find these rogue planets, astronomers must use different techniques from the ones they use to find exoplanets. This is due to dependency of the technique on periodic orbits of the exoplanets about stars. Two methods that astronomers use to find rogue planets are gravitational microlensing and direct imaging. 

Source: http://en.wikipedia.org/wiki/Rogue_planet