A rogue planet — also known as an interstellar planet, nomad planet, free-floating planet or orphan planet — is a planetary-mass object which has either been ejected from its system or was never gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the galaxy directly. Astronomers agree that either way, the definition of planet should depend on its current observable state and not its origin.
Larger planetary-mass objects which were not ejected, but have always been free-floating, are thought to have formed in a similar way to stars, and the I...