Estimate of Core-Powered Mass-Loss of Primary Atmosphere of the Mini-Neptune (Ocean Planet) HD 207496b

Abstract

The study presents modeling results of the primary atmosphere loss for the young mini-Neptune HD 207496b under the influence of thermal flux from its core, with a significant mass fraction of water in its composition. For exoplanet HD 207496b, (Barros et al., 2023) considered two internal structure scenarios: (1) a rocky (iron–silicate) core surrounded by a hydrogen–helium envelope, and (2) an ocean world with an iron core, silicate mantle, and extended water mantle. Both scenarios demonstrate potentially high efficiency of hydrogen–helium atmosphere loss through photoevaporation. Evdokimov and Shematovich (2025) evaluated the escape of the primary hydrogen–helium envelope via an alternative mechanism—thermal flux from the core. Their results showed that for a rocky core with a primordial hydrogen–helium atmosphere under the adopted model parameters, this mechanism proves insufficiently effective and should not significantly impact the gaseous envelope’s evolution. This work demonstrates that if HD 207496b currently represents a rocky core covered by a water mantle with a steam atmosphere (ocean world), the thermal flux from the planetary interior could have driven substantial loss of its primary hydrogen–helium atmosphere within the first few to tens of millions of years of evolution. The remaining primary gaseous envelope would subsequently undergo erosion via photoevaporation. Thus, HD 207496b’s evolutionary history may have included distinct phases dominated by different atmospheric loss mechanisms. The study reveals that atmospheric loss efficiency strongly depends on both the core radius and its internal energy. These parameters require refinement and are linked to interior structure models (including subsurface temperature profiles) as well as potential additional energy sources—core compression, radiogenic heating, and tidal heating.