Planetary Climate Dynamics

Abstract:

Recent James Webb Space Telescope observations of cool, rocky exoplanets reveal a probable lack of thick atmospheres, suggesting the prevalent escape of the “secondary” atmospheres formed after losing primordial hydrogen. Yet, simulations indicate that the hydrodynamic escape of secondary atmospheres, composed of nitrogen and carbon dioxide, requires intense fluxes of ionizing radiation (X-ray and extreme ultraviolet (XUV)) to overcome the effects of high molecular weight and efficient line cooling. This transonic outflow of hot, ionized metals (not hydrogen) presents a novel astrophysical regime ripe for exploration. We introduce an analytic framework to determine which planets retain or lose their atmospheres, positioning them on either side of the cosmic shoreline. We model the radial structure of escaping atmospheres as polytropic expansions—power-law relationships between density and temperature driven by local XUV heating. Our approach diagnoses line cooling with a three-level atom model and incorporates how ion–electron interactions reduce the mean molecular weight. Crucially, hydrodynamic escape onsets for a threshold XUV flux depend upon the atmosphere’s gravitational binding. The ensuing escape rates either scale linearly with XUV flux when weakly ionized (energy limited) or are controlled by a collisional–radiative thermostat when strongly ionized. Thus, airlessness is determined by whether the XUV flux surpasses the critical threshold during the star’s active periods, accounting for expendable primordial hydrogen and revival by volcanism. We explore atmospheric escape from the young Sun Mars and Earth, LHS 1140 b and c, and TRAPPIST-1 b. Our modeling characterizes the bottleneck of atmospheric loss on the occurrence of observable Earth-like habitats and offers analytic tools for future studies.

Abstract:

Abstract:

We present a new analysis of the spectroscopic variability of WISE J104915.57−531906.1AB (WISE 1049AB, L7.5+T0.5), observed using the NIRSpec instrument on board the James Webb Space Telescope (GO 2965; PI: Biller). We explore the variability of the dominant molecular bands present in their 0.6–5.3 μm spectra (H2O, CH4, and CO), finding that the B component exhibits a higher maximum deviation than the A component in all the wavelength ranges tested. The light curves reveal wavelength-dependent (atmospheric depth) and possibly chemistry-dependent variability. In particular, for the A component, the variability in the light curves at the wavelengths traced by the CH4 and CO molecular absorption features is higher than that for of H2O, even when both trace similar pressure levels. We conclude that clouds alone are unlikely to explain the increased variability of CO and CH4 with respect to H2O, suggesting that an additional physical mechanism is needed to explain the observed variability. This mechanism is probably due to thermochemical instabilities. Finally, we provide visual representations of the 3D atmospheric maps reconstructed for both components using the molecular band contributions at different pressure levels and the fits of planetary-scale waves.

Abstract:

Over the past two decades, increasingly precise observations have revealed that exoplanet atmospheres are chemically diverse and often far from equilibrium, with processes such as vertical mixing, photochemistry, and atmospheric circulation producing significant departures from thermochemical expectations. As large surveys across a wide range of planets begin to uncover population-level chemical trends, a coherent interpretation of these patterns remains elusive, particularly for gas giants and Neptune-like planets where disequilibrium processes likely dominate. The ESA Ariel mission will provide the first homogeneous, statistically significant atmospheric data set for nearly a thousand exoplanets, offering an unprecedented opportunity to investigate the origins of chemical diversity across planetary populations. This white paper highlights central scientific questions for understanding atmospheric chemistry and recognized as priorities for the community. These questions span the mechanisms driving disequilibrium chemistry, the role of sulfur- and phosphorus-bearing species, the influence of stellar activity, the formation of haze precursors, and the chemical evolution of atmospheres around stars of different ages and types. Because these topics connect chemistry, physics, and planetary evolution, they form the core focus of the Ariel Chemistry Working Group, which synthesizes current knowledge and identifies the diagnostics best addressed by Ariel’s spectral capabilities. For each theme, we outline observational strategies and representative targets already included in the Mission Candidate Sample, illustrating Ariel’s ability to address these questions. By linking large-scale observations to predictive atmospheric models, this work supports refinement of Ariel’s target selection and enables population-level studies that will transform our understanding of planetary atmospheres.