Modelling the long-term stability of high-metallicity atmospheres. Where we stand and what to do next.

The atmospheric characterization of sub-Neptune-size exoplanets has experienced a rapid development in recent years. This progress has been enabled by observations made with JWST and other telescopes on the ground. The near-future perspective is similarly optimistic with, for example, ARIEL and the E-ELT becoming active before the end of the decade. These observations are helping us determine which planets were able to retain an atmosphere, whether primary or secondary, and what they are actually composed of. Equally important, significant work is underway to trace the evolution of the planets under the evolving influence of their host stars. In this talk, I will review the current understanding of the processes that drive the mass loss from and compositional evolution of Sub-Neptune-size exoplanets. I will highlight recent advances in the chemical-collisional-radiative modelling of their atmospheres, and identify areas where additional work is needed to produce reliable predictions. Lastly, I will connect with observations when possible and emphasize the significance of using multiple diagnostics to test the available theories.