Rogue Planets

There are billions of rogue planets in any given galaxy – planets which do not belong to a star system. Some may have been ejected from their system, doomed to wander the void eternally alone, while others may have formed independently. The closest rogue planet to Earth is PiSO (PSO J318.5-22) which is 80 light years away and has an immense network of geothermally heated caves.

Rogue planets drift in cold interstellar space; while most have no atmosphere (or a frozen atmosphere) some – for reasons of pressure or other factors – can maintain one. Indeed, some rogue planets are able to sustain liquid water due to geothermal heat, while those with satellites also benefit from tidal heating. Civilizations can evolve never having seen a local star or sun.

Rogue planets can also exist in between galaxies. For these planets, the night sky is devoid of much other than the occasional distant galaxy. Civilizations on such planets may not even be aware of the existence of stars.