Astro-Seminar by Haitao Li: The long-term evolution of atmospheric loss and habitability of rocky exoplanets

 The long-term evolution of atmospheric loss plays a pivotal role in determining the habitability of rocky exoplanets, particularly those orbiting M-dwarf stars where intense stellar winds and extreme ultraviolet (XUV) radiation can drive significant ion escape. Motivated by the need to characterize the potential for sustained atmospheres and habitability, this study focuses on the Kepler-1649 system—a natural laboratory featuring two Earth-sized rocky planets (Kepler-1649 b and c) around an M5V star—as a case study for simulating atmospheric evolution over billions of years. The primary objective is to develop a comprehensive model of atmospheric ion escape over gigayear timescales, addressing the challenge of quantifying how evolving stellar activity influences the retention of planetary atmospheres and, consequently, the long-term habitability of rocky exoplanets. Employing a multispecies magnetohydrodynamic (MS-MHD) numerical simulation framework, we model the interaction between the planetary atmospheres and time-varying stellar winds, incorporating stellar spin-down and XUV flux evolution calibrated from observations of M-dwarfs. Non-parametric regression techniques, are used to analyze trends in escape rates from 0.8 to 4.0 Gyr. Our results reveal a systematic decline in ion escape rates by 2–3 orders of magnitude over this period, with oxygen ions (O⁺) dominating (98.3%–99.9% of total escape). For Kepler-1649 b (closer orbit), the O⁺ escape rate starts at 4.47×10²⁷ s⁻¹ at 0.8 Gyr but drops to 2.22×10²⁴ s⁻¹ by 4.0 Gyr, while for Kepler-1649 c, rates evolve from lower initial values but persist higher in later stages due to a transition to sub-magnetosonic stellar wind interactions, forming Alfvén wing structures that suppress escape. Cumulative atmospheric loss equates to approximately 0.32 bar (b) and 0.16 bar (c) of CO₂-equivalent over the simulated timeframe, indicating potential for atmosphere retention despite early erosion. In conclusion, both planets demonstrate the capacity to maintain thick atmospheres over billions of years as stellar activity wanes, highlighting a novel sub-magnetosonic escape regime that shifts traditional paradigms of atmospheric evolution in M-dwarf systems. This work provides the first quantitative gigayear-scale predictions of ion escape for an M-dwarf exoplanet system, offering critical insights for habitability assessments and testable hypotheses for future observations with instruments like the James Webb Space Telescope (JWST), while advancing modeling frameworks for upcoming exoplanet missions. Reference: ApJL, 994:L50, 2025 (arXiv:2504.12541v2).