A rogue planet is a planetary-mass object that is not gravitationally bound to any star.
These worlds are also called free-floating planets or isolated planetary-mass objects. Some may have formed inside planetary systems and later been ejected by close gravitational encounters with larger planets or passing stars. Others may have formed more like very small stars, through the collapse of gas clouds without ever orbiting a star.
Rogue planets are difficult to find because they do not shine by reflected starlight from a nearby sun. Astronomers can sometimes image young, warm examples directly, while gravitational microlensing can reveal a planet when its gravity briefly magnifies the light of a background star. Microlensing is particularly useful for objects too faint or isolated to see directly.
“Rogue planet” is not the same as “brown dwarf.” Brown dwarfs are substellar objects defined mainly by their formation and mass-related inability to sustain ordinary hydrogen fusion; some isolated planetary-mass objects may blur that classification. The term describes a planet-like object’s starless status, not necessarily its origin.