In their seemingly hopeful quest to find reasonable alternatives to Earth, the ever-philosophical design house Kurzgesagt explains in a nutshell how life could possibly form, evolve and sustain itself in oceans far beneath the thick frozen surface of rogue planets, provided that the conditions are amenable to such life.
…So how could life sustain itself at the bottom of a completely dark cold ocean in complete darkness? In volcanically active areas there are hydrothermal vents… In the dark ocean of a rogue planet, similar events or volcanic activity could be the starting point and basis for complex ecosystems we can only imagine right now.
