Harrison Nicholls (he/him)

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.

Abstract:

Sub-Neptunes are the most common type of detected exoplanet, yet their observed masses and radii are degenerate with several interior structures. One possibility is that sub-Neptunes have silicate/iron interiors and H-dominated atmospheres ( < 3.8 g mol), i.e., they are ‘gas dwarfs’. If gas dwarfs have molten interiors, interactions between their magma oceans and atmospheres will produce distinct observational signatures. These signatures may break the degeneracy in interior structure, while providing insight into their interior processes, history, and population trends. We expect all such planets are born molten, but under what conditions do they remain molten today? We use the coupled interior-climate evolution model, proteus, to estimate the ‘solidification shoreline’: the instellation flux boundary (as a function of stellar ) that separates molten gas dwarfs from solidified ones. Our results show that 98 per cent of detected sub-Neptunes occupy a region of parameter space consistent with their having permanent magma oceans, if they are gas dwarfs. While mantle and bulk volatile C/H ratio both influence magma ocean cooling, planets with oxidizing mantles and carbon-rich atmospheres are likely to have high mean-molecular weight atmospheres ( > 3.8 g mol) and are thus outside the scope of this study. Therefore, most detected sub-Neptunes, if they are gas dwarfs, have permanent magma oceans. This result motivates further research into the interactions between molten interiors and overlying atmospheres, and campaigns to identify unambiguous signatures of these interactions.

Abstract:

Spectroscopic characterization of rocky exoplanets with the James Webb Space Telescope has brought the origin and evolution of their atmospheres into the focus of exoplanet science. Time-evolved models of the feedback between interior and atmosphere are critical to predict and interpret these observations and link them to the solar system terrestrial planets. However, models differ in methodologies and input data, which can lead to significant differences in interpretation. In this paper, we present the experimental protocol of the Coupled atmospHere Interior modeL Intercomparison (CHILI) project. CHILI is an (exo)planet model intercomparison project within the Climates Using Interactive Suites of Intercomparisons Nested for Exoplanet Studies (CUISINES) framework, which aims to support a diverse set of multimodel intercomparison projects in the exoplanet community. The present protocol includes the initial set of participating magma ocean models, divided into evolutionary and static models, and two types of test categories, one focused on solar system planets (Earth and Venus) and the other on exoplanets orbiting low-mass M dwarfs. Both test categories aim to quantify the evolution of key markers of the links between planetary atmospheres and interiors over geological timescales. The proposed tests would allow us to quantify and compare the differences between coupled atmosphere–interior models used by the exoplanet and planetary science communities. Results from the proposed tests will be published in dedicated follow-up papers. To encourage the community to join this comparison effort, and as an example, we present initial test results for the early Earth and TRAPPIST-1 b, conducted with models differing in the treatment of energy transport in the planetary interior and atmosphere, surface boundary layer, geochemistry, and in- and outgassing of volatile compounds.

Abstract:

ABSTRACT While Earth locks much of its carbon in its crust as carbonates, Venus retains a comparable carbon inventory almost entirely in its atmosphere as CO$_2$. On Earth, the geological carbon cycle that has produced this vast crustal carbonate inventory is regulated by biology, liquid water, and plate tectonics, which together have stabilized climate over geological time-scales. Venus presently lacks all these processes. We test whether Venus’s massive CO$_2$ atmosphere is diagnostic of a specific evolutionary pathway by quantifying three routes: primary magma-ocean outgassing, secondary volcanic degassing in a stagnant-lid regime, and remobilization of crustal carbonates after climate destabilization. Using a coupled climate–weathering framework, we find that a past habitable Venus could have stored $\sim$20 bar of CO$_2$ as crustal carbonates. Following the transition to runaway conditions, crustal heating releases this reservoir over tens of Myr. In stagnant-lid secondary-degassing models with a MORB-like mantle, outgassing reaches only $\sim$25 bar CO$_2$, limited by progressive mantle volatile depletion. However, Venus-like inventories can be achieved through: (i) magmatic carbon enrichment, (ii) increased magmatic delivery to the surface (high extrusion or melt production), and (iii) the recycling of undegassed carbon back into the planet’s interior. Primary magma-ocean outgassing can generate $\gt 10^2$ bar CO$_2$, but the retained fraction after early escape remains uncertain. Ultimately, a Venus-like massive CO$_2$ atmosphere is an equifinal outcome and does not uniquely diagnose a temperate past.

Abstract:

Abstract In the aftermath of the Moon-forming giant impact, the Hadean Earth’s mantle and surface crystallized from a global magma ocean blanketed by a dense volatile-rich atmosphere. While prior studies have explored the thermal evolution of such early-Earth scenarios under idealized, oxidizing conditions, the potential feedback between tidal heating driven by Earth–Moon orbital forcing and variable redox scenarios have not yet been explored in detail. We investigate whether tidal heating could have prolonged this early magma ocean phase and supported quasi-steady state epochs of global radiative equilibrium: periods of thermal balance between outgoing radiation and interior heat flux. Using the PROTEUS simulation framework, we simulate Earth’s early evolution under a range of plausible tidal power densities, oxygen fugacities, and volatile inventories. Our results suggest that feedback between tidal heating and atmospheric forcing can induce substantial variation in magma ocean lifetimes, from ∼30 Myr up to ∼500 Myr, sensitive to interior redox conditions. Global radiative equilibrium epochs commonly arise across this range, lasting from ∼2 to ∼320 Myr, and typically occur from 24 Myr after the Moon-forming impact. Under oxidizing conditions, late-stage H 2 O degassing promotes melt retention and sustained heating due to its significant contribution to greenhouse forcing. Weak tides increase the atmospheric abundance of H 2 S and NH 3 and deplete CO. Therefore, the feedback between tides and atmospheric forcing induces a disequilibrium signature in the magma ocean atmosphere.