  Exoplanets with lower equilibrium temperatures than Earth and primordial
hydrogen atmospheres that evaporate after formation should pass through
transient periods where oceans can form on their surfaces, as liquid water can
form below a few thousand bar pressure and H2-H2 collision-induced absorption
provides significant greenhouse warming. The duration of the transient period
depends on the planet size, starting H2 inventory and star type, with the
longest periods typically occurring for planets around M-class stars. As
pre-biotic compounds readily form in the reducing chemistry of hydrogen-rich
atmospheres, conditions on these planets could be favourable to the emergence
of life. The ultimate fate of any emergent organisms under such conditions
would depend on their ability to adapt to (or modify) their gradually cooling
environment.
