Space works like a billiard table. Massive outer planets, like heavy balls, transfer their energy to light inner worlds. Such a gravitational kick can fling a small planet out of the stellar system. It turns into a free-floating wanderer — a rogue exoplanet without a star.
Detecting these invisible bodies is hard: they neither emit nor reflect light. Astronomers use photometry — catching the moment when a wandering planet's gravity briefly brightens a distant star (gravitational lensing). And planets around stars are found using the transit method — by tiny dips in brightness.
Gravitational collisions not only birth outcasts. They reshape orbits: super-Earths near the star can spiral even closer or fall into the fire. The fastest wanderers leave the galaxy forever, carrying possible life beneath an icy crust, warmed by internal heat.
Rogue planets can keep their moons and sustain subsurface oceans heated by radioactive decay — life could exist there without starlight.
Alan Dean Foster's novel 'The Wanderer' depicts a civilization that developed on a rogue planet without starlight.