Rogue Planet

A rogue planet, also know as a interstellar planet, nomad planet, free -floating planet, orphan planet (and several more terms) is a type of planet that does not orbit a stellar object, but instead orbits the galactic core directly Rogue planet include sub brown dwarfs, planets that form and behave like a Brown Dwarf. Others may formed by their Planetary System's Giant planets migrating inwards, using gravity to throw nearby planets out of the system. Even if it didn't, nearby stars passing by may cause the planet to be ejected. Although it is unlikely if would harbour life, life can form in subsurface oceans, rogue planets can also keep warm by tidal forces of it's moon or high atmospheric pressure.

Worldbuilding in Practice
TSD-194 is a rogue planet that orbits The Hathor Galaxy and has a mass of around 6 Earth masses. The planet was probably formed in a nearby Planetary System that ejected it away, we are not really sure how the planet gone rogue, but scientists assumed that it may got thrown away by a Gas Giant moving inwards. Although it's at a frigid -270 degree celsius. It may harbour life in its recent discovered subsurface ocean.