Harrison Nicholls (he/him)

Abstract:

Abstract Static structure models, which map mass-radius constraints to bulk planet composition, are frequently used to categorise exoplanets due to their computational efficiency and the high-level insight they offer into planetary properties. However, static structure models typically have simplified atmospheric treatments, which may introduce systematic biases when interpreting the structures – and therefore the climates – of sub-Neptunes and super-Earths. We present a framework for recovering exoplanet properties using static structure models that accounts for necessary physical-chemical complexity in their atmospheres. We produce a comprehensive library of 504,000 exoplanet simulations that unify deep planetary interior structure with radiative-convective-chemical climate calculations. From these models we demonstrate that a planet’s envelope mass fraction – a critical parameter to infer – is frequently degenerate with its instellation flux and atmospheric metallicity, and sensitive to the treatment of gravitational acceleration at the mbar level. Such uncertainties have significant implications for inferring planetary processes, as our modelling shows that habitable-zone sub-Neptunes readily host supercritical surfaces or deep magma oceans, despite their temperate irradiation regime. To marginalise over these uncertainties, we introduce a Bayesian retrieval tool that uses our library of self-consistent models. By applying this Bayesian approach to case-studies of π Men  c and TOI-421 b, we show that robust physical interpretations are achievable through whole-planet mass-radius retrievals. While new data from JWST, Ariel, and PLATO will expand our observational horizon, physically-consistent modelling provides the means to transition from categorical interpretations toward a comprehensive picture of the exoplanet continuum.

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:

The geochemical evolution of long-lived magma oceans is strongly regulated by volatile exchange between the molten mantle and the atmosphere. For planets inside the runaway-greenhouse limit, this coupled evolution can persist for billions of years, governing bulk density, surface conditions, and long-term geodynamics. However, most existing studies assume Earth-like (oxidized) conditions and neglect the influence of redox state on melt thermodynamics and volatile release. We quantify how experimentally derived, oxygen-fugacity-dependent melting curves implemented within the coupled interior-atmosphere framework PROTEUS propagate into the thermal structure, melt fraction, and rheological evolution of rocky exoplanet interiors, applying this to the short-period super-Earth GJ 1132 b. We find strongly non-linear thermal responses to variations in melting curves. In volatile-poor systems, reduced meltingcurves ( f O2 ≤ IW) promote earlier deep-mantle crystallisation relative to oxidised (IW + 2.0) and Earth-like (IW+4.0) cases (range IW−4.0 to IW+4.0), favouring late-stage surface magma oceans sustained by greenhouse warming, while oxidized melting curves maintain higher melt fractions and a vertically extended magma ocean. Reduced mantles produce massive H2-CO-rich atmospheres; oxidized mantles favour thinner H2O-CO2 envelopes. In volatile-rich systems, the interior reaches radiative equilibrium at high melt fractions, sustaining a steady-state global magma ocean in which melting curve variations do not significantly influence solidification timing. This indicates a hierarchical control: volatile inventory and surface oxygen fugacity act as the primary regulators of thermal state, while oxygen-fugacity-dependent melting relations provide a secondary modulation. These contrasting regimes produce distinct atmospheric compositions and formation timescales, offering testable spectral predictions for close-in rocky exoplanets evaluable with forthcoming JWST observations.

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.