Metal-enriched Atmospheres in Warm (Super- and Sub-) Neptunes Induced by Extreme Atmospheric Escape

Abstract Planet formation impacts exoplanet atmospheres by accreting metals in solid form, leading to atmospheric carbon-to-oxygen ratios (C/O) and sulfur-to-nitrogen ratios (S/N) that deviate from those of their host stars. Recent observations indicate differing metal abundances in planetary atmospheres compared to their stellar companions. However, these observations are biased toward mature planets, raising questions about whether these abundances result from formation or evolved over time. Another way to alter an atmosphere is through the escape of particles due to thermal heating. This study examines how billions of years of particle escape affect metal abundances. Using an adjusted stellar evolution code incorporating hydrodynamic escape, we model a warm ( T eq ≈ 1000 K) super-Neptune-type planet ( M ini = 26 M ⊕ ) orbiting a solar-type star. Our results show increased metal-to-hydrogen abundances of ∼50–70 × initial enrichment after 10 Gyr. We also see a 0.88 × decrease in C/O abundance and a 1.27 × increase in S/N abundance, which can affect the interpretation of planet formation parameters. We also simulate the evolving atmosphere using chemical kinetics and radiative transfer codes, finding substantial increases in SO 2 , CO 2 , and H 2 O abundances and a decrease in CH 4 abundance. These changes are easily observable in the IR wave band transmission spectrum. Our findings demonstrate that extreme escape of lighter particles significantly influences the evolution of warm Neptunes and complicates the interpretation of their observational data. This highlights the need to consider long-term atmospheric evolution in understanding exoplanet compositions.