Planetary Climate Dynamics

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Novel Physics of Escaping Secondary Atmospheres May Shape the Cosmic Shoreline

2026 ApJ 998 236

Authors:

Richard D. Chatterjee, Raymond T. Pierrehumbert

Abstract:

Recent James Webb Space Telescope observations of cool, rocky exoplanets reveal a probable lack of thick atmospheres, suggesting prevalent escape of the secondary atmospheres formed after losing primordial hydrogen. Yet, simulations indicate that hydrodynamic escape of secondary atmospheres, composed of nitrogen and carbon dioxide, requires intense fluxes of ionizing radiation (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 mean molecular weight. Crucially, hydrodynamic escape onsets for a threshold XUV flux dependent upon the atmosphere's gravitational binding. 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 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.

The JWST Weather Report from the Nearest Brown Dwarfs. III. Heterogeneous Clouds and Thermochemical Instabilities as Possible Drivers of WISE 1049AB’s Spectroscopic Variability

Astrophysical Journal 997:2 (2026)

Authors:

N Oliveros-Gomez, E Manjavacas, T Karalidi, M Phillippe, B Campos Estrada, B Biller, JM Vos, J Faherty, X Chen, TJ Dupuy, T Henning, AM McCarthy, PS Muirhead, EKH Lee, P Tremblin, J Ramirez, G Suarez, BJ Sutlieff, X Tan, N Crouzet

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.

Irradiated Atmospheres. IV. Effect of Mixing Heat Flux on Chemistry

Astrophysical Journal 995:2 (2025)

Authors:

ZT Zhang, W Zhong, W Wang, J Guo, X Tan, B Ma, R Wei, C Yu

Abstract:

Vertical mixing disrupts the thermochemical equilibrium and introduces additional heat flux that alters exoplanetary atmospheric temperatures. We investigate how this mixing-induced heat flux affects atmospheric chemistry. Temperature increase in the lower atmosphere by the mixing-induced heat flux alters species abundances there and modifies those in the upper atmosphere through vertical transport. In the lower atmosphere, most species follow thermodynamic equilibrium with temperature changes. In the upper layers, species mixing ratios depend on the positions of quenching levels relative to the regions exhibiting significant mixing-induced temperature variations. When the quenching level resides within such a region (e.g., CO, CH4, and H2O with strong mixing), the mixing ratios in the upper atmosphere are modified due to changes in the quenched ratios affected by the temperature variation in the lower atmosphere. This alters the mixing ratio of other species (e.g., NO and CO2) through the chemical reaction network, whose quenching occurs in the region without much temperature change. The mixing ratios of CH4, H2O, and NH3 decrease in the lower atmosphere with increasing mixing heat flux, similarly reducing these ratios in the upper atmosphere. Conversely, the mixing ratios of CO, CO2, and NO rise in the lower atmosphere, with CO and CO2 also increasing in the upper levels, although NO decreases. Weaker host star irradiation lowers the overall temperature of the planet, allowing a smaller mixing to have a similar effect. We conclude that understanding the vertical mixing heat flux is essential for accurate atmospheric chemistry modeling and retrieval.

JWST NIRSpec finds no clear signs of an atmosphere on TOI-1685 b

(2025)

Authors:

Chloe E Fisher, Matthew J Hooton, Amà lie Gressier, Merlin Zgraggen, Meng Tian, Kevin Heng, Natalie H Allen, Richard D Chatterjee, Brett M Morris, Nicholas W Borsato, Nà stor Espinoza, Daniel Kitzmann, Tobias G Meier, Lars A Buchhave, Adam J Burgasser, Brice-Olivier Demory, Mark Fortune, H Jens Hoeijmakers, Raphael Luque, Erik A Meier Valdés, Joà OM Mendonça, Bibiana Prinoth, Alexander D Rathcke, Jake Taylor

Diversity in the haziness and chemistry of temperate sub-Neptunes

Nature Astronomy Springer Nature (2025) 1-14

Authors:

Pierre-Alexis Roy, Björn Benneke, Marylou Fournier-Tondreau, Louis-Philippe Coulombe, Caroline Piaulet-Ghorayeb, David Lafrenière, Romain Allart, Nicolas B Cowan, Lisa Dang, Doug Johnstone, Adam B Langeveld, Stefan Pelletier, Michael Radica, Jake Taylor, Loïc Albert, René Doyon, Laura Flagg, Ray Jayawardhana, Ryan J MacDonald, Jake D Turner

Abstract:

Recent transit observations of K2-18 b and TOI-270 d revealed strong molecular absorption signatures, lending credence to the idea that temperate sub-Neptunes (equilibrium temperature Teq = 250–400 K) have upper atmospheres mostly free of aerosols. These observations also indicated higher-than-expected CO2 abundances on both planets, implying bulk compositions with high water mass fractions. However, it remains unclear whether these findings hold true for all temperate sub-Neptunes. Here we present the JWST NIRSpec/PRISM 0.7–5.4-μm transmission spectrum of a third temperate sub-Neptune, the 2.4 R⊕ planet LP 791-18 c (Teq = 355 K), which is even more favourable for atmospheric characterization thanks to its small M6 host star. Intriguingly, despite the radius, mass and equilibrium temperature of LP 791-18 c being between those of K2-18 b and TOI-270 d, we find a drastically different transmission spectrum. Although we also detect methane on LP 791-18 c, its transit spectrum is dominated by strong haze scattering and there is no discernible CO2 absorption. Overall, we infer a deep metal-enriched atmosphere (246–415 times solar) for LP 791-18 c, with a CO2-to-CH4 ratio smaller than 0.07 (at 2σ), indicating less H2O in the deep envelope of LP 791-18 c and implying a relatively dry formation inside the water-ice line. These results show that sub-Neptunes that are near analogues in density and temperature can show drastically different aerosols and envelope chemistry and are intrinsically diverse beyond a simple temperature dependence.