Simon Müller

Context . The radiative opacity plays a critical role in shaping both the thermal evolution and interior structure of giant planets. At temperatures of ∼2000 K, there is a hydrogen-transparency region that creates a window of reduced opacity, which can give rise to detached, deep radiative zones nestled between two convective zones. This local minimum in the opacity could be further deepened by the depletion of alkali metals. While the existence of such zones has been explored in the context of Jupiter and Saturn, their influence on the evolution and characterisation of cold to warm giant exoplanets remains unstudied. Aims . We investigate how opacity windows and the resulting deep radiative zones affect the cooling, radius evolution, and characterisation of the interiors and atmospheres of giant exoplanets. Methods . We computed thermal evolution models for cold to warm Jupiters spanning masses of 0.3–4.0 M _J , envelope metallicities from one to ten times solar, equilibrium temperatures of 100–800 K, and a parametrised reduction in the radiative opacity. Results . Detached deep radiative zones develop in moderately irradiated Jupiters older than a few gigayears even with unmodified opacities, and earlier and more extensively when the opacity is reduced. The age and equilibrium temperature at which a detached deep radiative zone appears depends on the planetary mass, envelope metallicity, and opacity, with metal-enrichment suppressing deep radiative zones at low equilibrium temperatures but promoting them at higher ones. A deep opacity window accelerates the planetary cooling, reducing the predicted radii by up to 5% and interior temperatures by several tens of percent. This translates to a ∼10 percentage point difference in the inferred bulk metallicity. Conclusions . Deep radiative zones are likely common in warm giant exoplanets and could decouple atmospheric composition from bulk interior composition, complicating the interpretation of atmospheric observations. We suggest that the opacity treatment introduces significant uncertainties in atmospheric and interior characterisation.

HATS-6 b is one of several recently discovered giant exoplanets orbiting M dwarf stars (GEMS) and is part of a JWST survey that aims to compare bulk and atmospheric properties of these rare planets against their FGK star counterparts. HATS-6 b is a warm ( T _eq ∼ 700 K), Saturn-mass ( M _p ∼ 0.3 M _Jup ), Jupiter-radius ( R _p ∼ 1 R _Jup ) planet that transits its star every ∼3 days. In this study, we present the transmission spectrum of HATS-6 b obtained with two transits using the PRISM mode of JWST Near Infrared Spectrograph (NIRSpec), spanning a wavelength range of 0.6−5.3 μ m. Analyzing these JWST observations using an iterative approach between forward modeling and free chemistry retrievals, we derive a low-metallicity ( log [ M / H ] = − 1.9 9 − 0.2 + 0.2 ) subsolar-C/O ( log [ C / O ] = − 0.4 6 − 0.2 + 0.2 ) atmosphere, and find strong evidence for H ₂ O, CH ₄ , and NH ₃ at volume mixing ratios (in log [ X ] ) of − 4.8 8 − 0.24 + 0.25 , − 5.3 8 − 0.19 + 0.18 , and − 6.0 3 − 0.19 + 0.18 , respectively. We consistently retrieve a significantly lower T _eq than predicted from the orbital configuration of HATS-6 b, which was impervious to any data reduction and retrieval choices, suggesting a nonzero Bond albedo. Our planetary interior models retrieve bulk metallicities 3 orders of magnitude larger than our retrieved atmospheric metallicity, also suggesting that the atmosphere is not well mixed. We find an excess feature around 3 μ m, and expand on possible explanations for this, such as the presence of HCN or hydrocarbons like C ₂ H ₄ . Yet, due to the degeneracies present for hydrocarbon features in this wavelength region, we do not draw any conclusions about the excess feature and instead encourage further observations and follow-up of this intriguing target.

The growing sample of giant exoplanets around M dwarf stars (GEMS) helps probe the extremes of giant planet formation. Comparing the properties of this sample with their FGK counterparts can help us understand how planet formation and migration depend on stellar mass. We initiated a large Cycle 2 JWST transmission spectroscopy survey of seven GEMS. Here we present the atmospheric characterization using two JWST transits of TOI-5293 A b, a 0.5 M _Jup planet orbiting an early M dwarf with a period of ∼3 days. The two NIRSpec/PRISM transits indicate the planet is eclipsing a rapidly changing (heterogeneous) stellar photosphere. We see that Visit 1 had heterogeneity crossings across the entire transit chord, rendering inferences from it to be unreliable. The Visit 1 spectrum exhibits a downward slope at μ m suggestive of stellar contamination from faculae. In contrast, for Visit 2 we are able to model the heterogeneity crossings and obtain a transmission spectrum free from stellar contamination. We therefore limit our conclusions to a detailed analysis of Visit 2, and using Bayesian free chemistry retrievals, we find a low atmospheric metallicity ( log [ M / H ] = − 1.0 3 − 0.44 + 0.53 × solar) and supersolar C/O ratio ( 1.2 3 − 0.75 + 2.94 ). The retrievals yield Bayes factors that indicate strong evidence for CH ₄ as well as low-significance detections of CO ₂ , H ₂ O, and NH ₃ . Finally, using thermal evolution models we find that the radius of TOI-5293 A b is inflated above theoretical expectations (∼1.07 R _Jup ), despite it having an temperature of ∼700 K, and hence we were unable to constrain its bulk composition.

Hot Jupiters (HJs) are commonly thought to host the strongest dynamo-generated magnetic fields among exoplanets, potentially exceeding Jupiter’s by an order of magnitude. Yet, despite extensive radio and spectroscopic searches for star–planet interactions (SPI), no unambiguous detections have been made. Here, we present new magnetic field estimates for individual HJs together with general magnetic field scaling laws. Assuming their dynamos operate in the fast-rotating regime (Ro < 0.1), we combine one-dimensional evolutionary models with integral magnetic field scaling laws under two heating scenarios: deep and shallow energy deposition within the convective zone. Using interpolators built from the simulation data, we derive optimistic (deep heating) and pessimistic (shallow heating) magnetic field strengths, and construct scaling relations based on observables, i.e., planetary mass, radius, and incident stellar flux. The deep-heating scenario maximizes convective heat flux in the dynamo and predicts strong magnetic fields of up to ~100 G, whereas shallow heating suppresses convection and yields much weaker fields of order ~1 G. These results challenge the standard assumption that inflated HJs necessarily host exceptionally strong magnetic fields because of their large radii and vigorous interiors. Instead, many HJs may generate fields too weak to produce detectable coherent radio emission or strong SPI signatures, naturally explaining the persistent non-detections in radio surveys. Future facilities may distinguish between these interior-heating scenarios through improved magnetic field constraints.

HATS-75 b is one of the recently discovered Giant Exoplanets Around M-dwarf Stars (GEMS) with a transmission spectrum shaped by both its atmosphere and the active stellar surface it transits. As part of a JWST program studying seven GEMS, we observed three transits of HATS-75 b with the NIRSpec PRISM instrument (0.6–5.3 μ m). The planet’s spectra exhibit a slightly larger transit depth at shorter wavelengths, indicative of hazes or stellar contamination due to stellar heterogeneities outside the transit chord, i.e., the transit light source (TLS) effect. While both a hazy atmospheric model or TLS model can replicate the transmission spectrum, independent evidence (e.g., stellar rotation, spot-crossing events) favors a model that includes contamination from unocculted starspots and faculae. Within this stellar heterogeneity/TLS-based framework, atmospheric retrievals yield remarkably low atmospheric metallicity ( log [ M / H ] = − 1.7 4 − 0.76 + 0.92 ) and supersolar carbon-to-oxygen ( C / O = 1.0 4 − 0.09 + 0.40 ), which paired with a best-fit interior model with bulk metallicity of Z _p = 0.20 ± 0.04 implies poor vertical mixing within the planet. Retrievals also detect robust absorption signatures of CH ₄ , CO, and CO ₂ . We obtain only an upper limit for H ₂ O, consistent with its atmospheric spectral features being masked by stellar contamination. These results underscore the importance of accounting for stellar heterogeneity when interpreting exoplanet transmission spectra and highlight HATS-75 b as a significant asset to our understanding of giant exoplanets around M dwarfs with JWST.

Recent discoveries of transiting giant exoplanets ( R _p ≳ 8 R _⊕ ) around M dwarfs present an opportunity to investigate their atmospheric compositions and explore how such massive planets form around low-mass stars contrary to the prediction from formation models. We present the first transmission spectra of TOI-5205b, a short-period ( P = 1.63 days) Jupiter-like planet ( M _p = 1.08 M _J and R _p = 0.94 R _J ) orbiting an M4 dwarf ( M _⋆ = 0.392 M _⊙ , R _⋆ = 0.394 R _⊙ ). We obtained three transits using the PRISM mode of the JWST Near Infrared Spectrograph spanning 0.6–5.3 μ m. The data reveal significant stellar contamination that is evident in the light curves as spot-crossing events and in the transmission spectra as a larger transit depth at bluer wavelengths. Atmospheric retrievals demonstrate that stellar contamination from unocculted starspots and faculae is the dominant component of the transmission spectrum at wavelengths λ ≲ 3.0 μ m, reducing the sensitivity to the presence of clouds or hazes in our models and preventing detection of H _2O . The wavelength coverage enabled a robust detection of CH ₄ and H ₂ S, which have detectable molecular features between 3.0 and 5.0 μ m. For both clear or cloudy atmospheres, Bayesian retrievals consistently favored an atmosphere with subsolar metallicity (3 σ upper limit of log [ M / H ] ≲ − 1.24 ) and supersolar C/O ratio (3 σ lower limit of log [ C / O ] ≳ 0.09 ), although this may partly be driven by the nondetection of water due to stellar contamination. Planetary interior models predict a bulk metallicity of 10%–20%, which is larger than the atmospheric metallicity and suggests that the interior of TOI-5205b is decoupled from its atmosphere.

The evolution of gaseous planets is a complex process influenced by various physical parameters and mechanisms. In this study, we present critical modifications to the Modules for Experiments in Stellar Astrophysics ( MESA ) code to enhance its applicability to giant planet modeling. We introduce an equation of state specifically tailored for materials at planetary conditions. The equation of state considers the thermodynamic properties of hydrogen-helium mixtures and heavy elements, improving the accuracy of internal structure calculations. We also present modifications to the radiative opacity to allow the modeling of grains, clouds and opacity windows. Furthermore, we refine the treatment of convective mixing processes in MESA to better replicate convective mixing with the presence of composition gradients. Finally, we add a treatment for helium rain and settling. These modifications aim to enhance the predictive capabilities of MESA for giant planet evolution and are publicly available. We hope that these improvements will lead to a deeper understanding of giant planet evolution in the Solar System and beyond.

It has been suggested that Jupiter’s fuzzy core could be a result of a giant impact. Here, we investigate the expected impact conditions from N-body simulations. We then use state-of-the-art smoothed particle hydrodynamics simulations to investigate the results of impacts with different conditions including various impactor masses and composition, different formation stages in Jupiter’s growth, and different resolutions. We next simulate the long-term thermal evolution of Jupiter postimpact. We find that 3D N-body simulations predict rather oblique impacts, and that head-on collisions are rare. Moreover, our results show that, even under a head-on collision, Jupiter’s fuzzy core cannot be formed. We next simulated Jupiter’s thermal evolution and showed that, unless postimpact temperatures are extremely low, a giant impact would not lead to an extended dilute core as inferred by interior models. We conclude that Jupiter’s fuzzy core is not caused by an impact and is likely to be an outcome of its formation process.

Introduction: The Juno mission has provided the community with accurate measurements of Jupiter’s gravitational field, providing tighter constraints for interior models. Structure models that fit Juno data find that the planet is inhomogeneous in composition and consists of a fuzzy core. Several scenarios for the formation of Jupiter’s fuzzy core were proposed, including the concurrent accretion of heavy elements and H-He gas, core erosion by large-scale or double-diffusive convection and a giant impact that mixed the primordial heavy-element core, distributing the heavy elements beyond the central region. While the possibility of forming Jupiter’s fuzzy core via a giant impact has been studied in prior work [1,2], further investigations of this scenario are required, as each study was limited in several aspects. In this study, we re-asses the giant impact scenario while considering the constraints imposed by using N-body formation simulations, smoothed particle hydrodynamics (SPH) impact simulations with state-of-the-art equations of state and high resolution, and post-impact thermal evolution simulations.Methods: To assess the expected impact conditions between proto-Jupiter and large embryos, we use an N-body code developed by [3] based on the IAS15 integrator. We simulate the evolution of 10 embryos of the local isolation mass, separated by 5 or 8 mutual Hill radii with different initial inclinations. The simulation uses the gaseous disk model and the growth model of proto-Jupiter from [4] as well as tidal damping of embryos.We simulate the impacts using an SPH code based on pkdgrav3 [5], derived from the Lagrangian including corrections for variable smoothing length with a kernel that can be sampled with 400 neighbors without suffering from the pairing instability. The code includes improvements for modeling giant impacts such as an interface/free-surface correction and a generalized EOS interface to use state-of-the-art equations of state. The code shows excellent scaling on HPC systems with either CPU based or hybrid CPU/GPU architecture, enabling the use of several billion particles in a simulation.To simulate Jupiter’s post-impact evolution, we use a modified version of the Modules for Experiments in Stellar Astrophysics code [6]. We use 1D heavy-element fraction profiles extracted from the SPH results and imprint hot, warm and cold temperature profiles. We then evolve all these models for 4.56 Gyr, predicting Jupiter’s current-day composition profiles.Results: We find that impacts of large embryos on growing Jupiter are unlikely and if an impact happens, it is very likely at an oblique angle, not head-on. This is in stark contrast to the findings of [1] who found that head-on impacts are rather likely to occur during Jupiter’s growth.We simulate this oblique impact, as well as a head-on and an intermediate impact with SPH to see the effect on the core-envelope structure. The oblique impact does not disrupt the core and even though some impactor material gets mixed into the envelope, most of the mass coalesces onto the core (see Figure 2). In the head-on (see Figure 3) and intermediate impact, the core is fully or partially disrupted, but, in all cases, the heavy elements quickly settle in a massive pure heavy-element core which confirms the findings of [2]. Parts of the heavy elements, especially those originating in the impactor are strongly shock- and compression heated, leading to a very hot core.The thermal evolution simulations show that when considering reasonable interior temperatures for Jupiter post-impact, giant impacts do not lead to an extended dilute core as inferred by interior models. Convective mixing tends to homogenize the planetary envelope, leaving a distinct compact core (see Figure 4).Conclusions: We explored the possibility that Jupiter’s fuzzy core is the result of a giant impact. We find that giant impacts on growing Jupiter are unlikely and rather oblique. Such an impact is not able to disturb Jupiter’s core enough to create a fuzzy core. Even in a head-on impact, no fuzzy core remains. If a fuzzy core were formed by a GI, the thermal evolution would quickly result in a homogeneous envelope with a compact core. Jupiter’s fuzzy core has thus not formed via a giant impact. The formation is likely related to Jupiter’s formation history, which also more naturally explains why Saturn also has a fuzzy core.Figure 1: Probability for a collision occurring during Jupiter’s growth. Figure 2: Density slices through the oblique collision with 10^9 particles with sidelength of 50 R⊕. Figure 3: Density slices through the head-on collision with 10^9 particles with sidelength of 50 R⊕. Figure 4: Heavy-element fraction after the impact and after the post-impact evolution.References:[1] Liu, S.-F. et al. Nature 572, 355–357 (2019).[2] Sandnes, T. D., Eke, V. R., Kegerreis, J. A., Massey, R. J. & Teodoro, L. F. A. https://doi.org/10.48550/arXiv.2412.06094 (2024).[3] Shibata, S. & Helled, R. A&A 689, A26 (2024).[4] Shibata, S., Helled, R. & Kobayashi, H. Monthly Notices of the Royal Astronomical Society 519, 1713–1731 (2023).[5] Potter, D., Stadel, J. & Teyssier, R. Comput. Astrophys. 4, 2 (2017).[6] Müller, S. & Helled, R. ApJ 967, 7 (2024).

Understanding the formation and bulk composition of Uranus and Neptune remains one of the outstanding challenges in planetary science. These ice giants likely formed in a region of the protosolar nebula where both rocky and icy solids were present, but their relative contributions to the planets' final compositions are still debated. The presence of 40Ar in Uranus’s envelope has been proposed as a promising diagnostic to distinguish between these contributions. 40Ar is a radiogenic isotope produced by the decay of 40K, which is hosted exclusively in silicate (rocky) material. In contrast, both the dominant isotope of argon (36Ar) and 40Ar are present in the protosolar nebula and get incorporated into solids primarily through condensation or clathration. Thus, to infer the amount of silicates incorporated into Uranus from either the absolute abundance of 40Ar in Uranus today or from the 40Ar/36Ar ratio, one must know both the primordial 40Ar/36Ar ratio and the absolute abundance of argon incorporated into Uranus. While uncertainties surrounding the primordial 40Ar/36Ar ratio have been previously discussed, our study focuses on the second key unknown: the absolute abundance of argon incorporated during the formation of the Ice Giant.A major complication arises from the fact that the abundance of argon in the nebula was not spatially uniform. Near the argon iceline—the location where argon transitions between gas and solid—its distribution was highly sensitive to the thermal and dynamical evolution of the disk. In addition, if clathrate hydrates played a significant role in noble gas trapping, then the location of the argon clathrate iceline also becomes critical. These factors can result in substantial radial gradients in the abundance of argon in both vapor and solid phases, thereby introducing ambiguity in the interpretation of any measured isotopic ratio in terms of planetary composition.To tackle this challenge, we model the radial distribution of primordial argon in the protosolar nebula before its incorporation into the building blocks of Uranus and Neptune. By utilizing plausible disk temperature profiles and conditions for condensation and clathration, we quantify the variability in the abundance of argon across the region where the ice giants likely formed. From this, we derive a range of expected 40Ar/36Ar ratios in Uranus and Neptune, reflecting formation conditions between two end-member scenarios: one in which the planets predominantly accreted icy material, and another in which rocky material was the primary contributor. Our findings demonstrate that the isotopic ratio serves as a valuable compositional tracer, provided it is interpreted within the framework of a physically consistent model of nebular evolution and volatile transport.These model predictions can be directly tested by future in situ measurements of noble gas abundances and isotopic ratios in the atmospheres of Uranus and Neptune. In particular, the proposed Uranus Orbiter and Probe mission, recommended by the U.S. Planetary Science and Astrobiology Decadal Survey 2023–2032, offers a unique opportunity to resolve the rocky-to-icy compositional dichotomy of the ice giants. By placing constraints on the formation pathways of Uranus and Neptune, such measurements will also provide critical insights into the long-term thermal evolution and internal heat transport of the ice giants. They will also help refine our broader understanding of planet formation processes in both our own solar system and extrasolar planetary systems.