Water- and metal-enriched atmospheres with CHAIN: the case of the young planets

The recent advancements in observational techniques and planet formation models have revealed that the atmospheres of sub-Neptune-mass planets may not be limited to mostly hydrogen-helium compositions as thought before. Instead, the observations suggest that the atmospheres of planets near the radius valley (hence, on the edge of evaporation according to the classic theory) may be strongly enriched by water or other heavy elements (e.g. Benneke et al., 2019, Brande et al., 2022, Piaulet et al., 2023, Holmberg&Madhusudhan 2024, Piaulet-Grohayeb et al., 2024). This possibility was already probed in the frame of some structure and formation models (e.g. Schlichting&Young 2022, Burn et al., 2024).

 

The effect that this variety in atmospheric compositions has on the atmospheric evolution on the population-wide level yet requires a thorough investigation. A few models have shown that the atmosphere's enrichment in water (e.g. Johnstone et al., 2020, Yoshida et al., 2022, 2025, Garcia Munoz 2023) or other heavy elements (e.g. Johnstone et al., 2018, 2021, Yoshida et al., 2024, Linssen et al., 2024) can lead to a significant reduction in atmospheric escape. However, as the known exoplanets span a wide range of parameters (both in terms of their mass, radius, and temperature and the high-energy environments set by the host star and the orbit), the validity of extrapolating these results, obtained for a few specific planets, to the whole planetary population is ambiguous. This task is further complicated by the constraint on the atmospheric composition for the majority of the known exoplanets being very loose.

 

In the present work, we make a tentative parameter space study for the enriched atmospheres of young mini-Neptunes. We employ the Cloudy e Hydro Ancora INsieme (CHAIN) model (Kubyshkina et al., 2024), which we already applied to study the escape of water- and metal-enriched atmospheres (see Egger et al., 2024, 2025, Piaulet-Grohayeb et al., 2024). In both cases, we adjust the elemental abundancies (O/H ratio in the former case, and Li/H-Zn/H ratios in the latter) in the atmospheres to match the desired compositions but do not enforce the abundance of any molecules; the stability of all molecules included in the model is evaluated by the chemical framework of the Cloudy models (Ferland et al. 2017, Chatzikos et al. 2023). As we are most interested in the early phases of atmospheric evolution (when most of the bulk atmospheric losses presumably occur), we focus on high extreme ultraviolet (EUV) and X-ray irradiation levels typical for young stars. We evaluate the dependence of the atmospheric dynamics on the water- and metal-enrichment level and how this dependence changes with planet's temperature, atmospheric mass fraction, and the spectral energy distribution of the host star.

 

We find that the reduction of the escape with increasing water/metal fraction does not work the same way for different young planets, and can vary between a factor of a few to a factor of a few tens for the mean molecular weight of ~8 m_H. The mechanism of reduction is also not the same, and if the chemical effects (including molecular cooling, metal line cooling and heating, and the effects connected to the excitation of hydrogen species) are most pronounced for the hot planets and planets with compact atmospheres, the atmospheres of cooler planets are more likely to switch from the hydrodynamic to the Jeans-like escape regime when the mean molecular weight of the atmosphere increases. Furthermore, for the same mean molecular weight, water enrichment appears generally more effective in the atmospheric mass loss rate reduction compared to the uniform enrichment in heavy elements over solar-like abundance. Finally, we find that besides the irradiation in the EUV range (10-91 nm), the atmospheric parameters (and in the extreme cases, escape) depend significantly on the stellar irradiation level in the far-ultraviolet (particularly 100-200 nm and specifically Ly-alpha line) and near-infrared (780-3000 nm).

 

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