Planetary entropy production as a thermodynamic constraint for exoplanet habitability

Any biosphere emerges, lives, and grows producing entropy. Entropy production is a thermodynamic function crucial in the framework of non-equilibrium thermodynamics as it is directly related to the dynamical behaviour of far-from equilibrium systems. The extent of entropy production is proportional to the ability of such systems to dissipate free energy and thus to ‘live’, to evolve, to grow in complexity. Generally, a certain threshold of entropy production must be exceeded for the emergence of complex self-organizing structures. Thus, the entropy production can be considered as the thermodynamic thrust that drives life emergence and evolution. In this perspective, we propose that the value of the planetary entropy production (PEP) can provide a first order estimate of the thermodynamic potential of planetary environment to sustain a complex biosphere. Here we use a simplified approach to evaluate the upper limit to the PEP and to the corresponding free energy as function of stellar temperature and orbital parameters of the planet. We found that only Earth-like planets in the circumstellar habitable zone (CHZ) of G and F stars can have a PEP value higher than the Earth value. Further significant thermodynamic differences exist between the inner and outer edge of the CHZ, with the inner edge being thermodynamically more advantageous for the development of complex biospheres. Interestingly, among the recently proposed habitable exoplanets, the ones belonging to the Hycean planets appear the thermodynamically best candidates.