Modeling Atmospheric Ion Escape from Kepler-1649 b and c over Time

Journal article

Rocky planets orbiting M dwarf stars are prime targets for atmospheric characterization, yet their long-term evolution under intense stellar winds and high-energy radiation remains poorly constrained. The Kepler-1649 system, hosting two terrestrial exoplanets orbiting an M5V star, provides a valuable laboratory for studying atmospheric evolution in the extreme environments typical of M dwarf systems. In this Letter, we show that both planets could have retained atmospheres over gigayear timescales. Using a multispecies magnetohydrodynamic model, we simulate atmospheric ion escape driven by stellar winds and extreme-ultraviolet radiation from 0.8 to 4.0 Gyr. The results reveal a clear decline in total ion escape rates with stellar age, as captured by a nonparametric LOWESS regression, with O+ comprising 98.3%–99.9% of the total loss. Escape rates at 4.0 Gyr are 2 to 3 orders of magnitude lower than during early epochs. At 0.8 Gyr, planet b exhibits 3.79× higher O+ escape rates than planet c, whereas by 4.0 Gyr its O+ escape rates becomes 39.5× lower. This reversal arises from a transition to sub-magnetosonic star–planet interactions, where the fast magnetosonic Mach number, Mf, falls below unity. Despite substantial early atmospheric erosion, both planets may have retained significant atmospheres, suggesting potential long-term habitability. These findings offer predictive insight into atmospheric retention in the Kepler-1649 system and inform future JWST observations of similar M dwarf terrestrial exoplanets aimed at refining habitability assessments.

Authors: Li, H; Dong, C; Xie, L; He, X; Chin, L

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal Letters
• Volume: 994
• Issue: 2
• Article number: L50
• Publication date: 2025-11-26
• Acceptance date: 2025-11-03
• DOI: 10.3847/2041-8213/ae1a65
• EISSN: 2041-8213
• ISSN: 2041-8205
• Language: English
• Keywords: Exoplanet atmospheres; Stellar winds; Magnetohydrodynamical simulations; Astrobiology; Habitable planets