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

TL;DR

This paper investigates how long-term atmospheric escape of lighter particles alters the metal abundances and spectral signatures of warm Neptunes, revealing significant evolutionary effects that impact interpretation of observational data.

Contribution

It introduces a model combining stellar evolution, hydrodynamic escape, and atmospheric chemistry to study long-term atmospheric evolution of warm Neptunes, highlighting the impact of particle escape on observable properties.

Findings

Metal-to-hydrogen ratios increase by 50-70x after 10 Gyr.

C/O ratio decreases by 0.88x, S/N ratio increases by 1.27x.

Observable spectral features include increased SO₂, CO₂, H₂O, and decreased CH₄.

Abstract

Planet formation impacts exoplanet atmospheres by accreting metals in solid form, leading to atmospheric C/O and S/N ratios that deviate from their host stars. Recent observations indicate differing metal abundances in planetary atmospheres compared to their stellar companions (e.g., Alderson et al. 2022; Bean et al. 2023). 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 qescape 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 ($T_{\mathrm{eq}} \approx 1000$ K) super-Neptune-type planet ($M_{\mathrm{ini}} = 26$ $M_{\oplus}$) orbiting a solar-type star. Our results show increased metal-to-hydrogen abundances of $\sim$50-70x initial enrichment after 10 Gyr. We also see a 0.88x decrease in C/O abundance and a 1.27x increase in S/N abundance, which can affect the interpretation of planet formation parameters. We also simulate the evolving atmosphere using chemical kinetics and radiative transfer codes, finding substantial increases in SO$_2$, CO$_2$, and H$_2$O abundances and a decrease in CH$_4$ abundance. These changes are easily observable in the IR waveband 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.