Planets can be thrown into interstellar space
Planet-formation models and microlensing observations indicate that free-floating worlds can be ejected from young planetary systems or possibly form without a star. NASA’s Roman mission is expected to expand the census.
Internal heat can last
Radioactive decay and leftover formation heat continue after a planet loses starlight. A thick hydrogen atmosphere or a deep ice shell can trap enough geothermal heat in some models to maintain liquid water.
Oceans could be hidden
A rogue world might have a dark surface at tens of kelvin while maintaining a subsurface ocean, analogous in broad principle to icy ocean moons. Chemistry near rock–water interfaces could provide local energy.
Finding one would be difficult
Without reflected starlight or periodic transits, rogue planets are hard to characterise. Microlensing can reveal mass and abundance, but determining atmosphere, ocean chemistry or biology would be far more demanding.
Research record
Stevenson (1999), Nature — life-sustaining planets in interstellar spacePeer-reviewed · open sourceNASA — Roman and rogue planetsNASA · open sourceNASA Science — Roman exoplanet scienceNASA · open sourceCould Life Exist on a Planet With No Star?
Direct answer: Life on a starless planet is physically conceivable if internal heat and insulation maintain liquid water and chemical energy. No inhabited rogue planet is known.
Plausible habitat class; entirely unconfirmed biologically.