Some “rogue” planets float through space, untethered to a star, moving according to gravitational interactions and the galactic potential.
Studying rogue planets informs us about planetary formation and ejection.
They may host subsurface oceans heated by internal radioactive decay.
Some rogue planets are as large as Jupiter yet roam the galaxy alone.
[NASA Exoplanet Archive, exoplanetarchive.ipac.caltech.edu]