Exoplanet atmospheres

Rocky planets orbiting M-dwarf stars are prime targets for characterizing terrestrial atmospheres, yet their long-term evolution under intense stellar winds and high-energy radiation remains poorly understood. The Kepler-1649 system, which hosts two terrestrial exoplanets orbiting an M5V star, presents a valuable opportunity to explore atmospheric evolution in the extreme environments characteristic of M-dwarf stellar systems. In this Letter we show that both planets could have retained atmospheres over gigayear timescales. Using a multi-species magnetohydrodynamic model, we simulate atmospheric ion escape driven by stellar winds and extreme ultraviolet radiation from 0.7 to 4.8 Gyrs. The results show that total ion escape rates follow a power-law decline (∝ τ ^−1.6 for Kepler-1649 b, ∝ τ ^−1.5 for Kepler-1649 c ), with O+ dominating atmospheric loss (76.8%–98.7%). The escape rates at 4.8 Gyrs are two orders of magnitude lower than those during the early epochs (1.9 × 1027 s^−1 at 0.7 Gyr vs. 3.0 × 1025 s^−1 at 4.8 Gyrs for planet b ), while planet b consistently exhibits 1.1–1.9× higher O+ escape rates than planet c due to its closer orbit (0.051 AU vs. 0.088 AU). Despite substantial early atmospheric erosion, both planets may still retain significant atmospheres, suggesting the potential for long-term habitability. These findings offer predictive insight into atmospheric retention in M-dwarf systems and inform future JWST observations aimed at refining habitability assessments.

The sub-Neptune planets have no solar system analogues, and their low bulk densities suggest thick atmospheres containing degenerate quantities of volatiles and H/He, surrounding cores of unknown sizes. Measurements of their atmospheric composition can help break these degeneracies, but many previous studies at low spectral resolution have largely been hindered by clouds or hazes, returning muted spectra. Here, we present the first comprehensive study of a short-period sub-Neptune using ground-based, high-resolution spectroscopy, which is sensitive to the cores of spectral lines that can extend above potential high altitude aerosol layers. We observe four CRIRES+ K-band transits of the warm sub-Neptune GJ 3090 b (T eq = 693 ± 18 K) which orbits an M2V host star. Despite the high quality data and sensitivity to CH4, H2O, NH3, and H2S, we detect no molecular species. Injection-recovery tests are consistent with two degenerate scenarios. First, GJ 3090 b may host a highly metal-enriched atmosphere with > 150 Z ⊙ and mean molecular weight > 7.1 g mol −1, representing a volatile dominated envelope with a H/He mass fraction xH/He<33 per cent, and an unconstrained aerosol layer. Second, the data are consistent with a high altitude cloud or haze layer at pressures < 3.3 ×10−5 bar, for any metallicity. GJ 3090 b joins the growing evidence to suggest that high metallicity atmospheres and high altitude aerosol layers are common within the warm (500 < Teq < 800 K) sub-Neptune population. We discuss the observational challenges posed by the M-dwarf host star, and suggest observing strategies for transmission spectroscopy of challenging targets around M-dwarfs for existing and ELT instrumentation.