Harrison Nicholls (he/him)

Abstract:

Abstract We demonstrate that the redox-sensitivity of mantle outgassing can trigger transient episodes of atmospheric reinflation in highly irradiated and geochemically reduced super-Earths, a mechanism we term “reflation.” Mantle redox governs the outgassing and speciation of CHONS volatiles, setting the background secondary atmospheric composition during extended photoevaporation at highly irradiated conditions. Using simulations of the coupled atmosphere–interior evolution of irradiated super-Earths, we illustrate that reduced mantles close to the iron–wüstite buffer initially produce CO-dominated atmospheres. Hydrodynamic escape continuously removes volatiles while outgassing from the melt replenishes the atmosphere with H 2 , converted from H 2 O dissolved in the underlying magma ocean. This leads to a late-stage transition from C- to H-dominated gas that transiently reinflates super-Earth atmospheres and decreases their bulk densities by up to ∼60% between several hundreds of megayears to a gigayear after their formation, prior to complete atmospheric erosion by photoevaporation. In contrast, oxidized mantles, closer to Earth-like geochemistry, strongly buffer their atmospheric composition while exposed to hydrodynamic escape, producing monotonic radius deflation. Reflation events are triggered by geochemically reduced mantles, intermediate escape efficiencies, high irradiation, and initial water inventories ≳ 5 Earth oceans. This redox-dependent evolutionary divergence hinges on the sensitive feedback between interior and atmospheric evolution serving as a potential tracer of historical geochemical state. Population-level reflation signatures of close-in super-Earths may thus serve as tracers of interior geochemistry and formation conditions.

Abstract:

Recent JWST observations have begun to constrain the atmospheres of highly irradiated super-Earth exoplanets. The dayside emission spectrum of the ultra-short-period super-Earth TOI-561 b is inconsistent with a bare-rock surface and instead favours a thick volatile envelope sustained over the system's roughly 10 Gyr lifetime (Teske et al. 2025). A volatile, likely CO/CO2-bearing secondary atmosphere has also been inferred for 55 Cancri e (Hu et al. 2024), with comparable indications now reported for additional ultra-short-period rocky planets (August et al. 2025; Monaghan et al. 2025; Park Coy et al. 2026). These detections challenge the canonical expectation that ultra-short-period rocky planets are stripped to bare rock by stellar irradiation, and motivate a coupled treatment of magma-ocean evolution and atmospheric loss to interpret them (Lichtenberg & Miguel 2025; Lichtenberg et al. 2025).We address this question with the open-source PROTEUS framework (Lichtenberg et al. 2021, Nicholls et al. 2024), which self-consistently advances the energy balance of a cooling rocky interior, the redox-controlled outgassing of CHONS volatiles between the magma ocean and the overlying atmosphere (Nicholls et al. 2024), the radiative-convective structure of the resulting secondary atmosphere (Nicholls et al. 2025), and energy-limited hydrodynamic escape driven by the host star's evolving XUV output. Starting from a fully molten state, we follow super-Earth analogues on ultra-short-period orbits through several Gyr while tracking the surface pressure, atmospheric composition, mean molecular weight, and observable bulk density as the interior and atmosphere co-evolve.We identify a new evolutionary climate pathway that we term reflation. In geochemically reduced super-Earths, with mantle oxygen fugacity near the iron-wuestite buffer, the initial outgassed atmosphere is dominated by CO while hydrogen remains stored as H2O dissolved in the underlying magma ocean. As hydrodynamic escape strips the CO-rich envelope, the dropping surface pressure releases this hydrogen reservoir, and equilibrium chemistry under reducing conditions converts much of it to H2. The transient transformation from a carbon-dominated to a hydrogen-dominated atmosphere lowers the mean molecular weight, expands the scale height, and reduces the planet's bulk density by up to roughly 60 percent over several hundred Myr to a few Gyr, before final atmospheric erosion. Oxidised, Earth-like mantles instead outgas heavy CO2- and SO2-rich envelopes that buffer the mean molecular weight throughout, producing monotonic deflation and substantially longer atmospheric survival.Mapping the parameter space of mantle oxygen fugacity, initial volatile inventory, escape efficiency, and orbital distance, we find that reflation events concentrate at oxygen fugacities within about two log units of the iron-wuestite buffer, intermediate escape efficiencies, semi-major axes inside roughly 0.05 au, and initial hydrogen inventories above five Earth-ocean equivalents. Reflation thus provides a direct, redox-sensitive link between deep interior geochemistry and a transient observable under-density on irradiated super-Earths, complementing emerging interpretations of individual systems such as L 98-59 d (Nicholls et al. 2026).We discuss the prospects for detecting reflated super-Earths in ongoing JWST programmes, including dedicated follow-up of TOI-561 b and other ultra-short-period super-Earths, and through population-level surveys with PLATO and Ariel, where joint constraints on age, radius, mass, and atmospheric composition may disentangle reflation from competing scenarios and open a direct observational window onto the mantle redox state of rocky exoplanets. References:August, P. C., Buchhave, L. A., Diamond-Lowe, H., et al. 2025, A&A, 695, A171.Cesario, L., Lichtenberg, T., Attia, M., Nicholls, H., Kisvardai, I., & Changeat, Q. 2026, submitted.Hu, R., Bello-Arufe, A., Zhang, M., et al. 2024, Nature, 630, 609.Lichtenberg, T., Bower, D. J., Hammond, M., et al. 2021, J. Geophys. Res. Planets, 126, e2020JE006711.Lichtenberg, T., & Miguel, Y. 2025, in Treatise on Geochemistry, vol. 7, Elsevier, 51-112.Lichtenberg, T., Shorttle, O., Teske, J. K., & Kempton, E. M.-R. 2025, Science, 390, eads3660.Monaghan, C., Roy, P.-A., Benneke, B., et al. 2025, AJ, 169, 239.Nicholls, H., Lichtenberg, T., Bower, D. J., & Pierrehumbert, R. T. 2024, J. Geophys. Res. Planets, 129, e2024JE008576.Nicholls, H., Pierrehumbert, R. T., Lichtenberg, T., Soucasse, L., & Smeets, S. 2025, MNRAS, 536, 2957.Nicholls, H., Lichtenberg, T., Chatterjee, R. D., Guimond, C. M., Postolec, E., & Pierrehumbert, R. T. 2026, Nature Astronomy.Park Coy, B., Xue, Q., Weiner Mansfield, M., et al. 2026, arXiv:2604.11911.Teske, J. K., Wallack, N. L., Piette, A. A. A., Dang, L., Lichtenberg, T., et al. 2025, ApJL, 995, L39.