Planetary Climate Dynamics

Abstract:

We report the detection of photometric variability in the directly imaged super-Jupiter β Pictoris b (β Pic b). Using JWST NIRCam dual-band coronagraphic imaging, we conducted a 16 hr continuous photometric monitoring campaign in the F210M and F410M filters. We developed and validated a time-series photometry framework that combines point-spread function subtraction, principal component analysis for systematic noise removal, and injection-and-recovery tests to confirm signal fidelity. Both light curves show consistent sinusoidal variability at ∼5σ and ≫5σ significance in the F210M and F410M bands, respectively. A joint sinusoidal fit yields a rotation period of Prot = 9.00 ± 0.13 hr and variability amplitudes of 0.85% ± 0.07% and 0.89% ± 0.04% in F210M and F410M, respectively. The near-identical amplitudes and periods in both bands confirm a common astrophysical origin in a heterogeneous atmosphere. Combining Prot with the previously measured projected rotational velocity, we constrain the line-of-sight spin axis inclination of β Pic b. The result favors an equator-on viewing geometry, consistent with line-of-sight spin–orbit alignment: the planetary spin axis, orbital plane, debris disk, and stellar equator are all mutually aligned. This stands in sharp contrast to the large obliquities of wide-separation companions that are likely formed via gravitational fragmentation. Together with the system’s young age, this observation provides independent dynamical evidence that β Pic b formed via core accretion. This result constitutes the first detection of rotational modulation in a close-in, high-contrast exoplanet that likely formed via core accretion, demonstrating that time-series coronagraphic imaging with JWST opens a powerful new window onto the rotation, atmospheric dynamics, and spin–orbit architecture of this population.

Abstract:

ABSTRACT Static structure models, which map mass–radius constraints to bulk planet composition, are frequently used to categorize 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 marginalize 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 $\pi$ Men  c and TOI-421 b, we show that robust physical interpretations are achievable through whole-planet mass–radius retrievals. While new data from James Webb Space Telescope (JWST), Ariel, and PLAnetary Transits and Oscillations of stars (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:

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.

Abstract:

We report the direct imaging discovery of a third exoplanet in the β Pictoris (β Pic) system. We detect β Pictoris d (β Pic d) in noncoronagraphic observations obtained with the Very Large Telescope (VLT) Enhanced Resolution Imager and Spectrograph (ERIS), as well as multi-epoch archival datasets from the JWST Near Infrared Camera (NIRCam) and VLT/SPHERE. Astrometric measurements over an 11 yr baseline demonstrate that it is consistent with a gravitationally bound source with orbital motion. Joint multi-planet orbit fits of all three planets in the system yield a semimajor axis of 26.0−6.1+2.2 au and inclination 89.0−0.6+0.7 deg for planet d. β Pic d has a larger orbital semimajor axis than the other known planets in the system, but is coplanar with the inner two planets, and its orbit is consistent with sculpting the inner edge of the debris disk. β Pic d has a contrast of ΔL′=12.11±0.15 mag, with colors and luminosity that closely match those of 51 Eri b, another exoplanet in the β Pic moving group. Its VLT/ERIS and JWST/NIRCam colors are distinct from those of free-floating planetary-mass objects of a similar age and temperature. Its red F410M − F444W color indicates strong CO2 absorption in its atmosphere and suggests significant enhancement in metals compared to free-floating objects. From the ATMO hot-start evolutionary models, we estimate an effective temperature of 600−60+45 K and mass of 2.4 ± 0.6 MJup, which also closely matches similar estimates for 51 Eri b. β Pic d is among the lowest-mass exoplanets imaged from the ground. This discovery highlights the deep sensitivity achievable with ground-based imaging in the mid-infrared and the discovery potential of future high-contrast observations with the Extremely Large Telescope.