EXOA16
The session also includes contributions that explore the evolution of rocky exoplanets from the perspective of changes to the composition of the Galaxy with time. For example, the inventories of the principal long-lived heat-producing elements, HPEs, 40-K, 235-238-U and 232-Th, are encoded in stellar metallicity, age, and nucleosynthetic history. Yet, the long-term thermal evolution of rocky exoplanets driven by different HPE inventories is often overlooked, even though this evolution is a critical factor in interpreting physical properties including retrieved data for secondary and hybrid atmospheres.
Key areas of interest include:
Atmospheric Characterization: New results on the composition, structure, and dynamics of the atmospheres of small exoplanets, utilizing facilities like JWST and focusing on the volatile content, clouds, and potential detection of biosignatures. This also includes work on secondary and hybrid atmosphere generation.
Mass-Radius Relationship and Interior Structure: Studies that probe the transition between rocky Earths/Super-Earths and volatile-dominated Mini-Neptunes, including interior modeling, precise density measurements, and the physical processes driving atmospheric loss, e.g., photoevaporation.
Formation and Evolution: Theoretical and observational work investigating the birth environments, migration, and long-term evolution of small planets, particularly concerning their initial volatile budget and the influence of the host star. This also includes the long-term thermal evolution of rocky exoplanets driven by different HPE inventories and the expression of different evolutionary tracks as a function of parameters such as system age.
Detection and Demographics: Advances in the discovery and precise characterization of small exoplanets from current and future surveys, e.g., TESS and PLATO, and studies of their occurrence rates and population demographics across different stellar types.
Galactic Chemical Evolution and Rocky Exoplanets: Contributions that report current understanding of how galactic chemical evolution, GCE, processes affect the abundances of the rock-forming elements, Mg, Si, O, Al, Fe, Ca, Na, K, on controls for the abundances of HPEs, and on plausible radiogenic heat budgets of rocky exoplanets across different stellar populations in the Milky Way.
The session encompasses work related to observational, theoretical and experimental studies of small exoplanets, including rocky exoplanets connecting GCE and nucleosynthesis with exoplanetary geodynamics, as well as how these affect magnetic dynamo generation, volcanic activity, and secondary/hybrid atmosphere generation.
The session aims to advance our understanding of the fundamental physics governing the formation and diversity of the most common planets in the Galaxy and their potential for hosting life.
Molten super-Earths are ideal candiates to probe the interior compositions of rocky exoplanets due to the possibility of direct measurements of the magma composition by probing the vapours that are outgassed. 55 Cnc e is one extensively observed planet falling into this category. The atmosphere of this hot super-Earth shows sub-weekly variability in emission (Demory et al. 2016, Meier-Valdez et al. 2023, Patel et al. 2024). Among the suggested reaons for this is an outgassing – cloud formation cycle (Loftus et al. 2025). We constuct a pipeline which combines radiative transfer with equilibrium chemistry, a cloud formation model, and outgassing of the magma to investigate whether variable cloud decks can form in atmospheres of lava worlds. We find that this can happen at timescales which are in agreement with the sub-weekly variability observed on 55 Cnce by Patel et al. 2024. A thick cloud deck is crucial to induce a change in outgassing and in emission in the wavelength range of JWST. We conclude that cloud-outgassing cycles could occur on molten super-Earths and that they can be observed by current and future instruments like JWST and Ariel. Further observations and modeling efforts are needed to confirm or falsify if such a cycle is the reason for the variabilitty observed on 55 Cnce.
In this talk I will focus on the conditions required for variable cloud formation and outgassing as well as on the variability in emission we expect in the wavelength range of JWST during multiple stages of the cycle and how this compares to observations of 55 Cnce.
How to cite: Janssen, L.: Thick cloud decks induce variability in atmospheres of molten super-Earths, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-5, https://doi.org/10.5194/epsc2026-5, 2026.
How to cite: Attia, M. and Lichtenberg, T.: PALEOS: Multiphase Equations of State for Rocky Planet Interiors, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-14, https://doi.org/10.5194/epsc2026-14, 2026.
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 melting
curves ( 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.
How to cite: Sastre, M., Lichtenberg, T., Soucasse, L., Bower, D., Nicholls, H., and Kamp, I.: Geophysical and atmospheric implications of fO2-dependent melting on rocky exoplanets, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-149, https://doi.org/10.5194/epsc2026-149, 2026.
The atmospheric characterisation of temperate exoplanets is now becoming accessible with JWST, providing a critical connection between Solar System planets and the more commonly observed hot-Jupiters. K2-18 b, a temperate sub-Neptune orbiting an M dwarf, has emerged as a benchmark case following extensive JWST observations and ongoing debate regarding its atmospheric composition [1-10].
We investigated the atmosphere of K2-18 b using a self-consistent forward model [11] in order to constrain its metallicity, composition, and thermal structure, with a particular emphasis on the role of disequilibrium chemistry, photochemical hazes, and clouds. For the first time in this context, we also assessed the impact of photoelectrons on the atmospheric chemistry of an exoplanet.
We employed a one-dimensional model that couples stellar energy deposition, disequilibrium gas-phase chemistry, and haze and cloud microphysics to generate physically consistent atmospheric scenarios. We explored a wide range of metallicities and intrinsic temperatures, evaluated haze and cloud formation, and compared the resulting transmission spectra with available JWST observations (NIRISS, NIRSpec, MIRI) reduced using multiple independent pipelines [1-5].
We demonstrate that a high metallicity (200-400×solar) H2-rich atmosphere consistently reproduces the observed transit spectra of K2-18 b, largely independent of the data reduction pipeline used. The atmospheric composition is strongly shaped by disequilibrium chemistry, with CH4 dominating the spectrum alongside significant contributions from CO2 and OCS, and a potential contribution from C2H4 at mid-infrared wavelengths. Photochemical hazes play a key role in shaping the thermal structure, producing a temperature minimum near the 10–100 mbar level that enables efficient condensation of H2O and suppresses its gaseous abundance in the region probed by transit observations. Photoelectrons enhance the production of several disequilibrium species, particularly nitrogen-bearing molecules, although their direct impact on the current transmission spectra remains limited. Under sufficiently strong haze cooling, condensation of NH4SH provides a natural explanation for the apparent absence of NH3 in the observed spectra.
In summary, our results indicate that the JWST observations of K2-18 b are best explained by a hazy, high-metallicity sub-Neptune atmosphere shaped by disequilibrium chemistry. The combined effects of photochemical hazes and cloud formation are essential for interpreting the current K2-18 b observations. While uncertainties remain regarding haze optical properties, no additional molecular species beyond those considered here are required to reproduce the observed spectra.
References
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6. Welbanks, L., Nixon, M. C., McGill, P., et al. 2026, Nat.Astron, 10, 234
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12. Lavvas, P., Liu, R., Tinetti, G., et al. 2026, A&A, in press.
How to cite: Lavvas, P., Liu, R., Tinetti, G., Paraskevaidou, S., Drossart, P., and Coustenis, A.: The atmosphere of K2-18 b: The role of hazes, clouds, and photoelectrons, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-256, https://doi.org/10.5194/epsc2026-256, 2026.
PLATO (PLAnetary Transits and Oscillations of stars) is ESA’s M3 mission and designed to detect and characterize extrasolar planets by high-precision, long-term photometric and asteroseismic monitoring of a large number of stars. PLATO will detect small planets around bright stars, including terrestrial planets in the habitable zone of solar-like stars. With asteroseismology of their host stars and complementary radial velocity observation from ground-based telescopes, planets will be characterized for their radius, mass, and age with high accuracy. PLATO will provide us the first large-scale catalogue of well-characterized small planets up to intermediate orbital periods, relevant for a meaningful comparison to planet formation theories and to better understand planet evolution. It will make possible comparative exoplanetology to place our solar system planets in a broader context. PLATO will study host stars using asteroseismology, allowing us to determine the stellar properties with high accuracy, substantially enhancing our knowledge of stellar structure and evolution.
PLATO is scheduled for a launch date in Q1 2027. At this point, spring 2026, the payload including 26 wide-angle cameras with 12cm aperture each is finalized and integrated on the satellite platform. Final spacecraft and ground-segment tests will be performed during 2026. This presentation will give an overview of the PLATO science goals, of its instrument and mission profile status as well as the planned data releases.
How to cite: Rauer, H., Aerts, C., Belkacem, K., Deleuil, M., Gizon, L., Günther, M., Heras, A., Mas-Hesse, M., Pagano, I., Piotto, G., Pollacco, D., Ragazzoni, R., Ramsay, G., and Udry, S.: Small exoplanets from PLATO, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-365, https://doi.org/10.5194/epsc2026-365, 2026.
The thermal evolution of rocky exoplanets is modulated by (i) left-over (gravitational) heat of accretion, and (ii) radiogenic heating from the decay of long-lived radioactive isotopes, particularly K, Th, U, and U. These heat-producing elements (HPEs) are synthesized through distinct nucleosynthetic pathways and evolved in both relative and absolute abundance over time as traced via galactic chemical evolution (GCE) – and where possible – direct or proxy spectroscopic observations. We synthesize current understanding of how GCE processes control the radiogenic heat budgets of rocky exoplanets by mapping HPEs across different stellar populations in the Milky Way. Combining GCE models with thermal evolution simulations reveals that planets forming around older, metal-poor stars possess significantly lower initial radiogenic heat inventories compared to younger, metal-rich systems. Depending on the HPE, initial radiogenic heat production in Earth-mass planets can vary by a factor of 2–10 between different stellar environments. We explore how this variation impacts planetary geodynamics, magnetic dynamo generation, volcanic activity and secondary atmosphere outgassing, as well as biocompatibility. We present theoretical models showing that stagnant-lid rocky exoplanets around old stars may exhaust their radiogenic heat budgets within 2–4 Gyr, potentially entering Venus-like states and losing their capacity to maintain temperate climates through carbon cycling. Conversely, planets with enhanced HPE abundances may experience excessive radiogenic heating, leading to prolonged magma ocean phases, hyper-volcanism or dynamo failure. We identify a "metallicity Goldilocks zone" near solar values where persistent magnetic dynamos can be sustained. Our analysis demonstrates that the galactic context of planetary formation - encoded in stellar metallicity, age, and nucleosynthetic history of the HPEs - governs both long-term thermal evolution and an exoplanet’s biocompatibility.
How to cite: Mojzsis, S. J., Pignatari, M., Trueman, T. C. L., Issa, J., Dennisenkov, P. A., Herwig, F., Perdikakis, G., and Kremer, B.: Galactic radioactive links to rocky worlds, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-337, https://doi.org/10.5194/epsc2026-337, 2026.
Motivation
The thermal structure, heating/cooling balance, and mass loss of exoplanet atmospheres are controlled by a multitude of factors, including the host star's properties, the planet's formation history and size, and the chemical composition of the atmosphere itself. In terms of chemistry, metallicity and the C/O ratio are commonly used to characterise exoplanet atmospheres. While these metrics work reasonably well for hydrogen-dominated gas giants, they cannot capture the atmospheric complexity of rocky exoplanets. Given the chemical diversity already detected in exoplanets, from H2O, CO2, CH4, to SO2 and potentially CS2, and knowing that nitrogen- and oxygen-rich atmospheres both exist in our own Solar System, the question of how much chemistry alone matters, and how to best describe it, comes naturally.
Model and grid
We constructed a grid of upper atmospheric models for an Earth- (1 au) and Venus-like (~0.7 au) planet by systematically varying the elemental ratios of hydrogen, carbon, nitrogen, oxygen, and sulfur while keeping all other parameters fixed. Each composition was modelled to steady state using The Kompot Code, a first-principles self-consistent 1D model that accounts for the mutual influence of atmospheric photochemistry with thermal structure, stellar irradiation, and vertical transport via eddy and molecular diffusion. The figure below illustrates the modelling workflow. The equilibrium chemistry model GGchem is used as an intermediary to produce molecular abundances which are used as a starting condition for Kompot.
Thermal structure
We find that chemistry has an important impact on both the atmospheric thermal structure and mass loss. While temperatures in the middle regions of the upper atmosphere (around 10-5 to 10-6 bar) remain broadly similar across compositions, exobase temperatures range from a few hundred to several thousand Kelvin depending on the elemental mixture. This is driven primarily by the heating/cooling balance and the presence of efficient cooling molecules such as CO2 and H2O. These elevated exobase temperatures directly correspond to significantly enhanced thermal mass loss.
Mass loss and irradiation dependence
For Earth-like irradiation conditions, around 70% of the compositions remain stable, with a mass loss rate < 1 kg/s. Isolating the cases with significant mass loss reveals clear correlations: escape rates increase with H/O and N/O, while higher O/S and C/N ratios act to stabilise the atmosphere. These elemental ratios control the abundances of different molecules in the atmosphere. Therefore, they trace the presence of efficient cooling molecules such as CO2, which directly reduce mass loss rates. The figure below shows the dependence of the mass loss rate on the H/O ratio, with the addition of Venus, Earth, and Mars for reference.
For a closer-in Venus-like planet, H/O remains a consistent driver of mass loss, while N/O becomes less significant. C/N also loses its significance in reducing the mass loss. More strikingly, water, which has no significance for mass loss at Earth-like distances, becomes strongly correlated with increased mass loss at Venus-like distances, suggesting a fundamental shift in the role of individual molecules with irradiation level. Similarly, CO2 is strongly anticorrelated with mass loss at 1 au, but this correlation all but disappears at 0.7 au. The figure below shows these correlation shifts, both for elemental ratios and molecules.
Implications
None of this behaviour can be captured by the C/O ratio alone. Rocky exoplanets therefore require a more complete elemental description. Our results offer new constraints for assessing atmospheric survival and have direct implications for how we interpret and prioritise observations of rocky- and sub-Neptune exoplanet atmospheres with the JWST and the upcoming ESA Ariel mission. These results also open the question of whether mass loss rates and thermal structure can be observationally traced to molecular abundances or elemental ratios, and if the opposite is feasible as well.
How to cite: Stanković, I., Güdel, M., Boro Saikia, S., Van Looveren, G., and Robeling, N.-M.: Does chemistry alone significantly shape rocky exoplanet atmospheres?, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-462, https://doi.org/10.5194/epsc2026-462, 2026.
The radius valley separating super-Earths from sub-Neptunes is a key diagnostic of small-planet formation and atmospheric evolution, yet its dependence on host-star properties remains debated. We address this question with a cross-matched database integrating most of the known small exoplanets and their host stars, built by combining published archives with a wide set of complementary stellar measurements. The dataset is going to be released through an interactive web application that supports flexible queries, downloads, and diagnostic plots, and is designed to provide a reproducible reference frame for population-level studies.
Using this resource, we carry out a population-level clustering analysis of the super-Earth and sub-Neptune distributions with respect to available stellar and planetary parameters, discriminating potential distinct sub-populations characterized by different evolutive histories.
We discuss how this empirical picture constrains current photoevaporation and core-powered mass-loss scenarios, and how the database can serve as a benchmark for population studies of small exoplanets ahead of PLATO.
How to cite: Greco, N., Lanzafame, A. C., Pagano, I., Drago, V., Scandariato, G., and Spada, F.: Patterns across the radius valley: stellar-property dependence of the small-exoplanet distribution from a multi-catalogue database, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-663, https://doi.org/10.5194/epsc2026-663, 2026.
Tidal torques influence the rotation of close-in exoplanets in a well-known way: they drive the rotation of the exoplanets to a synchronous stationary state where the planetary rotational and orbital periods are equal. This happens because of the proximity of these planets to the host star. So, not only are the tides more intense, but the stronger stellar radiation prevents the planets from maintaining an atmosphere, whether primary or secondary. However, if the planets are not too close to the host star, a dense secondary atmosphere can be formed, and thermal tides can deform this atmosphere in such a way that the gravitational attraction of the host star may create an additional torque as important as the torque due to the gravitational tide. This happens in Venus and may happen in any planet in the habitable zone of a star whose mass approaches the mass of the Sun. It is worth recalling that the study of the rotation of Venus with a new version of the creep tide theory (Folonier et al. 2025; Ferraz-Mello, 2026) shows that, in the absence of an atmospheric accelerating torque, the planet's current retrograde rotation is reversed, becoming direct in a few million years. This result indicates that the retrograde rotation of Venus cannot be simply due to a random catastrophic event, such as a collision with another body (e.g., a retrograde moon) or to eventual chaotic transitions associated with large obliquities.
What happens with Venus is simpler and may also be found among the internal exoplanets of other planetary systems. Our knowledge of Venus' rotation may be used to define what to expect for the rotation of Earth-like exoplanets in or near the habitable zone of stars with masses approaching the solar mass. Indeed, we know that the retrograde rotation of Venus is maintained by accelerating torques due to the atmospheric thermal tides, which offset the braking torques due to the gravitational tides (Ingersoll and Dobrovolskis, 1978).
The mathematical analysis of the equations describing the effects of the two torques shows that the phase space of these equations exhibits a feature common to many first-order differential equations: a pitchfork bifurcation (Ferraz-Mello, 2026). Without an atmosphere, the equation presents the well-known stable synchronous attractor: the gravitational tides tend to synchronize the planet’s rotational and orbital motions. With an atmosphere, an accelerating torque is added, and if the atmosphere is dense enough, the composition of the two torques may lead to different attractors. With the conditions observed on Venus, the synchronous stationary solution becomes unstable. The former stable attractor bifurcates into two asynchronous branches: one supersynchronous and one subsynchronous. Besides, the subsynchronous branch may reach negative rotation values.
Therefore, the slow formation of a dense atmosphere on an exo-Earth initially devoid of a significant one may lead its rotation to cross the bifurcation that makes possible the rotation to become retrograde. The planet must remain without a significant atmosphere for several million years to allow the rotation to become synchronized before the bifurcation is reached. Then, with almost equal probabilities, it evolves towards one of two asynchronous attractors. The evolution towards a retrograde rotation does not necessarily happen. It is necessary that, after the bifurcation, evolution goes along the subsynchronous branch and that the atmosphere becomes dense enough to transform the subsynchronous, but still direct, rotation into a retrograde one. The conclusion comes from a simplified model, but it is very robust and also present in studies with more complex models (Correia and Laskar, 2003). Its simplicity suggests that many Earth-like exoplanets in or near the habitable zone of solar-type stars may exhibit retrograde rotation. Like Venus!
References
Correia, A.C. and Laskar, J., Icarus, 163, 24-45 (2003)
Ferraz-Mello, S., Astron. J. 171: 206 (2026).
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Ingersoll, A.P. and Dobrovolskis, A.R., 1978, Nature, 275, 37-38 (1978).
Support: FAPESP, CNPq
How to cite: Ferraz-Mello, S.: Rotational synchronization and desynchronization of terrestrial planets in the habitable zone of solar-type stars., Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-678, https://doi.org/10.5194/epsc2026-678, 2026.
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.
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How to cite: Cesario, L., Lichtenberg, T., Attia, M., Nicholls, H., Kisvárdai, I., and Changeat, Q.: Reflation: redox-driven atmospheric inflation as a tracer of super-Earth geochemistry, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-824, https://doi.org/10.5194/epsc2026-824, 2026.
Although nearly half of the exoplanets in the galaxy are expected to be sub-Neptunes, their atmospheric characterisation efforts with the James Webb Space Telescope (JWST) are being limited by the understanding of interactions between multiple chemical reservoirs, including magma oceans, water oceans and clouds. In this work, we couple interior and atmospheric chemical reservoirs using multi-phase chemical networks, including the effects of equilibrium and disequilibrium chemistry, to account for the partitioning of volatiles between silicate melt, liquid water and condensates. These volatiles include H2O, C-bearing and S-bearing molecules detected in JWST transmission spectra of sub-Neptunes (e.g., K2-18 b, TOI-270 d, TOI-421 b), as well as species predicted by magma-gas, water-gas and condensate-gas reactions. Our results indicate strong degeneracies in atmospheric retrievals between sub-Neptunes hosting magma oceans and water oceans across the near- and mid-infrared, extending known degeneracies between molecular abundances and cloud opacity. Combined with interior constraints from mass-radius data, these degeneracies can be mitigated through simultaneous ultraviolet, visible, and infrared spectroscopy, which helps differentiate between competing compositional scenarios for sub-Neptunes.
How to cite: Hakim, K., Decin, L., Rivoldini, A., Namur, O., and Van Hoolst, T.: Degeneracies between signatures of magma oceans, water oceans and clouds in sub-Neptune atmospheres, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-894, https://doi.org/10.5194/epsc2026-894, 2026.
There are eight planets in the Solar System, each unique in many ways but all sharing one similarity: they are formed from the same material as the Sun they orbit. As such, the variation between the 6286 exoplanets (NASA, 2026) that have been discovered orbiting a variety of different stars cannot be examined by looking at our Solar System alone. However, it can give us some clues. The Earth is depleted in more volatile elements, elements with relatively low condensation temperatures, relative to Solar values (e.g. Yoshizaki and McDonough 2021; Palme and O’Neill 2014; Halliday and Porcelli 2001). Similarly, measurements of Martian material show that Mars is also depleted in these volatile elements, but to a lesser degree than the Earth is (e.g. Yoshizaki and McDonough 2020; Sossi and Fegley 2018). Compositional models of Venus and Mercury also suggest a volatile depletion, this time greater than the Earth’s. These volatile depletion trends can be modelled in a variety of different ways. We adopt the model of Wang et al. 2019, a slope in log space, gradient α, which describes volatile depletion relative to Solar abundances as a function of condensation temperature. We use this α to quantify the bulk abundance of elements in rocky exoplanets. Previously, Mojzsis et al. 2023 speculated that this α was related to the solar constant (the amount of energy received by an object at a given distance from its star, S<sub>0<sub> such that α = S<sub>0<sub><sup>1/ 3<sup> . We further support this relationship and apply it to exoplanet systems, as it has been suggested that this process of devolatilisation applies to the formation of all rocky planets (Wang et al. 2019). This allows us to explore the possible variation in exoplanet compositions around M-dwarf stars, a spectral class of star previously unexplored when modelling exoplanet compositions (e.g. Spaargaren et al. 2025, 2023). Additionally, we are able to predict the compositions of known exoplanets using this method, potentially aiding interpretations of atmospheric data and assessments of habitability.
The basis for the composition of exoplanets should first be the composition of the star that they orbit (Bonsor et al. 2021; Doyle et al. 2019). Spaargaren et al. 2025, 2023 used the composition of FGK stars in the Solar neighbourhood found in the Hypatia and GALAH catalogues to model the compositions of rocky exoplanets around their stars. We take this one step further, using the devolatilisation trends at different semi-major axes from different stellar abundances, as shown in Figure 1. For example, a planet at 1 AU around an F star should have a volatile depletion trends similar to those of Mercury: highly depleted in volatile elements and refractory rich. At 1 AU around an M-dwarf, a planet should exhibit depletion trends similar to CC meteorites relative to their stellar abundances. Such planets are likely volatile-rich, potentially retaining near-stellar abundances of the most volatile elements. Unlike previous studies, we are able to probe into the composition of rocky exoplanets around these M-dwarfs, a population of stars that make up the majority of the Solar neighbourhood and host a variety of small rocky planets. This has never been done before as elemental abundances are difficult to observe in M-dwarfs, unlike FGK stars. It is important to examine the exoplanets around M-dwarfs, however, as rocky exoplanets are easy to observe around these types of stars. In order to discern M-dwarf abundances, we use observations combined with the galactic chemical evolution (GCE) models of Trueman et al. 2026, which allow us to quantify the abundances of a variety of different elements found in M-dwarfs. Thus, we combine stellar abundance measurements with devolatilisation trends to predict bulk rocky planet compositions as a function of spectral class and orbital distance. This will aid us making and interpreting exoplanet observations as well as leading to insights in atmospheric formation, tectonic regimes, mineralogy and habitability.
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Figure 1: Adapted from Mojzsis et al. 2023, the relationship between α, the level of volatile depletion, and S<sub>0<sub>, the energy received by a planet at a given distance from the Sun. Top bars indicate equivalent orbital distances for planets receiving the same stellar flux around stars of different spectral classes.
How to cite: Dale, K. I., Mojzsis, S. J., Spaargaren, R. J., and Trueman, T. C. L.: Using Volatile Depletion Trends to Predict Exoplanet Compositions, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-932, https://doi.org/10.5194/epsc2026-932, 2026.
The long-lived radioisotope 40K (half-life of 1.248 Gyr) is a dominant heating source driving mantle convection and volcanic activity in rocky exoplanets. Whilst the initial radiogenic heat budget of 40K in the Solar System can be inferred from meteoritic chronology, it is unclear whether the 40K/K ratio is consistent with galactic chemical evolution (GCE) or if it comes mostly from a single local stellar ejecta event. In this work, we use the OMEGA+ GCE code to predict the 40K/K abundance ratio in the interstellar medium (ISM) as a function of Galactic age. Our GCE models assume that some fraction of massive stars undergo a C-O shell merger, which significantly boost the production of light odd-Z elements in the pre-explosive yields [1, 2, 3] and are necessary to reproduce non-local thermodynamic equilibrium spectroscopic abundances. We incorporate merger yields from the literature [4], spanning a range of initial masses and metallicities into two massive-star yield sets [5, 6] using a self-consistent approach without needing to modify global GCE parameters. Our results allow us to determine confidence intervals (CIs) for the 40K/K ratio in the ISM as a function of Galactic age; these CIs take into account the intrinsic metallicity scatter in the solar neighbourhood, different fractions of massive stars that undergo C-O mergers, stellar yield uncertainties, and observationally constrained initial GCE parametrisations. From our CIs, we quantify the likelihood that the Solar System's 40K budget was inherited from the Galactic background. More generally, our results can be applied to planet-hosting stars of known metallicity, however, the CIs become dominated by stellar age uncertainties.
References
[1] L. Roberti, M. Pignatari, A&A, 703, id.L15, 8 pp. (2025)
[2] J. Issa, F. Herwig, S. J. Mojzsis, et al., Apj, 997:314 (10pp), (2026)
[3] C. Ritter, R. Andrassy, B. Côté, MNRAS: Letters, 474, Issue 1, p.L1-L6, (2018)
[4] C. Ritter, F. Herwig, S. Jones, et al, MNRAS, 480, Issue 1, p.538-571, (2018)
[5] K. Nomoto, C. Kobayashi, N. Tominaga, ARA&A, 51, 457, (2013)
[6] M. Limongi, A. Chieffi, ApJ, 647, 483, (2018)
How to cite: Trueman, T. C. L., Mojzsis, S. J., and Pignatari, M.: Galactic Chemical Evolution of 40K as a heat source for rocky exoplanets, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1013, https://doi.org/10.5194/epsc2026-1013, 2026.
Atmospheric superrotation is commonly associated with the formation of strong eastward equatorial jets driven by wave and flow interactions. However, its connection with the global reorganisation of atmospheric energy transport remains less understood, particularly in Earth-like planetary atmospheres. In this work, we investigate superrotation from an energetic and transport-based perspective using a suite of three-dimensional global circulation model simulations of Earth-like planets performed with the THOR dynamical core.
We explore circulation behaviour across a broad range of planetary rotation rates and radiative timescales within a double-grey radiative transfer framework, focusing on the competition between meridional overturning transport and zonal redistribution of energy. By analysing global diagnostics of meridional and zonal transport, superrotation indices, and the relative contributions from mean circulation and eddies, we identify non-monotonic transitions between distinct transport regimes as rotational constraints become increasingly important.
Our results suggest that the atmospheric circulation initially increases its large-scale transport efficiency as the circulation becomes dynamically organised. However, the meridional overturning branch eventually reaches a maximum efficiency or loses effectiveness, after which the circulation increasingly reorganises toward zonal redistribution and strong equatorial jets. In this picture, superrotation emerges not simply as an isolated jet phenomenon, but as part of a broader atmospheric response to limitations in overturning-supported energy transport.
The rotation experiments reveal a clear transition between overturning-dominated and jet-supported transport regimes, with maximum zonal transport occurring at intermediate-to-slow rotation rates. In contrast, the radiative-adjustment scaling experiments do not exhibit a simple saturation of the equatorial jet, suggesting that the jet-supported transport branch can continue strengthening even after the overturning circulation becomes inefficient. Overall, our results support the idea that superrotation can be interpreted as a dynamically adaptive transport regime emerging from the reorganisation of atmospheric energy redistribution, while also highlighting the richer nonlinear behaviour present in fully three-dimensional radiative circulation models.
How to cite: Soriano Guerrero, C.: Energetic Control of Atmospheric Regime Transitions, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1076, https://doi.org/10.5194/epsc2026-1076, 2026.
Sub-neptunes are exoplanets that have a thick gaseous envelope dominated by hydrogen and helium, sometimes enriched with water, methane or other volatile compounds while super-Earths could be rocky planets or ocean worlds with a thinner atmosphere. Some of these planets are probably tidally-locked, resulting in a priori important day-night temperature contrast.
However, wind speeds remain poorly constrained and poorly understood. Several global climate models (GCMs) have been developed to determine the atmospheric circulation that might govern such planets.
Recent studies of the atmosphere of K2-18b show broadly consistent results regarding its general circulation, whilst highlighting the influence of several physical parameters. The zonal circulation of K2-18b is predominantly dominated by easterly winds and by the presence of one or more super-rotating equatorial jets in the upper atmosphere, as observed in several simulations. Both [1] and [2] identify this structure. The studies also converge on a global circulation organised between the day side and the night side, typical of a synchronously rotating planet. [3] and [1] describe a dominant day-night overturning circulation in the upper atmosphere. The simulations by [2] also indicate an equatorial updraft and polar subsidence, accompanied by strong vertical mixing. This work also highlights the importance of physical parameters on atmospheric dynamics. Stellar metallicity [3] and spin-orbit resonance significantly alter their atmospheric circulation [3,4].
In the case of sub-neptunes with high metallicity, condensable heavy compounds can have a key dynamic role . It has been shown that condensable heavy compounds can inhibit convection [5], [6]. By adding a convection scheme consistent with the presence of heavy compouds, [4] found a similar overturning circulation between dayside and nightside and pole to equator cells as on [2]. Secondly, the high abundance of these compounds can generate significant molar mass gradients during condensation, thereby altering the zonal and meridional circulation of these planets. However, to date, the effect of a variable molar mass due to the abundance of condensable heavy compounds has not yet been investigated in the primitive and thermodynamic equations.
To reproduce the thermal and dynamic structure of sub-neptunes, numerical simulations with a resolution of 2° were carried out between 1 bar and 10 mbar, using the DYNAMICO dynamic core [7] and coupling it to the Generic PCM. We decided to simulate a K2-18b-like planet using the same abundances and k-distribution model as [6]. The condensable tracer used is water, which is predicted to condense at the pressures and temperatures under consideration. Potential enthalpy was chosen as the conservative variable instead of potential temperature in order to account for the variation in molar mass within the dynamic core. The dry and wet convective adjustments were modified accordingly to allow for these molecular weight effects. The precipitation scheme takes into account condensation, re-evaporation and mass redistribution within the atmospheric layers.
Three configurations were tested: the first, in which the molar mass varies throughout the GCM; the second, in which it varies only in the dry and wet convection schemes, as in [4]; and the third, in which the molar mass is kept constant. In these three simulations, we investigated the zonal circulation and the resulting thermal structure. A dynamic analysis was carried out using the Transformed eulerian formalism to highlight the circulation cells and the transport of H2O within these cells.
References
[1] Innes & Pierrehumbert (2022), The Astrophysical Journal, 927(1):38
[2] Liu et al. (2025), Monthly Notices of the Royal Astronomical Society, 538(4):2463–2482, 541(4):2897–2916
[3] Charnay et al. (2021), Astronomy and Astrophysics, 646:A171
[4] Barrier & Madhusudhan (2025), Monthly Notices of the Royal Astronomical Society, 538(4):2463–2482
[5] Leconte et al. (2017), Astronomy and Astrophysics, 598:A98
[6] Leconte et al. (2024), Astronomy and Astrophysics, 686:A131
[7] Dubos et al. (2015), Geoscientific Model Development, 8(10):3131– 3150.
How to cite: Milcareck, G., Leconte, J., Guerlet, S., Spiga, A., Millour, E., and Clement, N.: GCM simulations of sub-neptunes with a condensable heavy compound, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1100, https://doi.org/10.5194/epsc2026-1100, 2026.
With the current and upcoming generations of space and ground based instruments the characterisation of atmospheres and surfaces of rocky exoplanets is going to be possible. Even though the interpretation will remain challenging and include many degeneracies, they will open a new window to the understanding of the diversity of rocky exoplanets throughout our galaxy. Our capabilities of constraining the surface conditions and compositions will depend on our understanding of their connected observables.
Based on atmospheric loss calculations, rocky planets on close in orbits are expected to be bare rocks, rather than remaining their atmosphere. While this is likely detrimental for the search for habitable planets around M dwarfs, it might allow direct constrains of their surface composition by investigations of emitted and reflected lights.
For planets, which retrain their atmosphere, the observation of the planetary surface is only possible through observations of their atmosphere. Modelling of the planetary atmospheres reveals the presence of distinctly different compositions. Besides atmospheres with H2 or O2 as characterising species, the atmospheric types include atmospheres with the coexistence of redox pairs (especially CH4 and CO2) in the atmosphere and ones with substantial SO2. These compositional differences can in principle also be observed spectroscopically.
Being able to constrain the atmospheric type for a given exoplanet does not only provide information on the atmospheric composition, but also the mineralogy of the surface in chemical contact with this atmosphere. Some minerals (CaSO4, FeS, and others) are indicative of the surface in contact with a given atmospheric type. Therefore the atmosphere can be used as a window to the composition of surfaces of rocky exoplanets.
How to cite: Herbort, O. and Sereinig, L.: Observations as a window to the diversity of rocky exoplanet surfaces, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1211, https://doi.org/10.5194/epsc2026-1211, 2026.
Observations of the ultrashort-period rocky exoplanet 55 Cancri e have revealed large and unexplained variability in both visible and infrared brightness. The planet’s mid-infrared brightness temperature has been observed to vary by approximately 1400 K, while the secondary eclipse depth in the visible band has been observed to vary by more than 100 ppm. We investigate whether self-sustained oscillations due to nonlinear feedbacks between magma ocean surface temperature and silicate cloud cover can explain these extreme variations.
We propose a magma temperature–cloud feedback mechanism illustrated in the figure below. Under cloud-free conditions, intense stellar irradiation heats the magma ocean, enhancing the evaporation of silicate vapor that subsequently condenses into reflective silicate clouds. Once formed, these clouds attenuate incoming stellar radiation, cooling the surface, reducing vapor supply, and suppressing further cloud formation. A time delay between surface heating and cloud formation, potentially caused by atmospheric transport and cloud microphysics, enables self-sustained oscillations in both surface temperature and cloudiness.
Using a simple theoretical model, we show that a broad range of planetary parameters can potentially reproduce the observed variability in infrared secondary eclipse depths. The model also predicts out-of-phase oscillations between visible and infrared brightness, consistent with recent JWST observations. In addition, time-varying and spatially nonuniform cloud cover can potentially produce variations in both the amplitude and phase offset of planetary phase curves.
These results suggest that observable weather cycles driven by magma ocean–cloud feedbacks may operate on lava planets with atmospheres. We further discuss observational tests of the proposed mechanism and future modeling efforts using more sophisticated models to further evaluate this hypothesis.
How to cite: Luo, Y., Loftus, K., Kite, E., and Fan, B.: Magma Temperature–Cloud Feedback as a Possible Explanation for Extreme Brightness Variability on the Lava Planet 55 Cancri e, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1213, https://doi.org/10.5194/epsc2026-1213, 2026.
K2-56 b is a long period (41.68d) sub-Neptune exoplanet that was previously reported as a high-density planet with interior composition lying in the transition region between pure rocky and a planet with a significant volatile envelope.
In this work, we report the follow-up CHEOPS photometry and HARPS-N RV observations to provide in-depth characterisation of K2–56 b. We found that the exoplanet has a radius of 2.72 +/- 0.10 R_e and a mass of 14.11 +/- 3.06 M_e, corresponding to a bulk density of 3.85+/-0.94 g cm^{-3}. Unlike previous work, our results showed that K2-56 b is instead a low-density sub-Neptune with composition consistent with a planet with a significant gas envelope or a water world. With the precisely derived planet parameters, we modelled the interior structure of K2-56 b. If the planet were formed inside the iceline, it could host a H/He envelope of up to 2.5% with a very low envelope water mass fraction. Whereas if the planet were formed outside the ice line, it could hold an envelope mass fraction of up to nearly 40% and an envelope water mass fraction of almost 99%.
K2-56b orbits a bright host star and the sub-Neptune is likely to host a significant fraction of the gas envelope, making this exoplanet an attractive target for future atmospheric characterisation.
How to cite: Lam, K. and Luque, R.: Is K2-56b a dense sub-Neptune?, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1229, https://doi.org/10.5194/epsc2026-1229, 2026.
Rocky exoplanets around M dwarfs have been investigated by many theoretical and observational studies in the recent years. In contrast to close-in, hot rocky exoplanets often termed "exo-Venuses", recent observational campaigns suggest that many of these planets may either not posses an atmosphere, or only a very thin atmosphere either full of hazes or not made of CO2, therefore not resembling Venus.
In this study we aim at predicting possible atmospheric evolutionary pathways of such planetary atmospheres starting from compositional and evolutionary constraints of the star-planet system (including redox profiles in the interior set by core formation). We then model the evolution of the atmospheres depending on outgassing from the interior, atmospheric chemical equilibrium, greenhouse effects, water condensation incl. the atmospheric cold trap, solubility of volatiles in the melt, and atmospheric escape.
We show that depending on various key factors such as planetary mass, initial bulk redox state, magma ocean evolution and atmospheric escape efficiency, different types of atmospheres are predicted, with CO2-dominated, Venus-like atmospheres being restricted to specific scenarios, only, while in other scenarios for example bare-rock atmospheres or thin N2-O2-dominated, abiotically generated atmospheres will be more likely, suggesting a wide diversity of "exo-Venus" atmospheres.
How to cite: Noack, L., Baumeister, P., and Innes, H.: Atmospheric evolution of rocky planets around M dwarfs is shaped by interior redox processes, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1272, https://doi.org/10.5194/epsc2026-1272, 2026.
Cool sub-Neptune planets within the habitable zone of M-dwarf stars have become a central focus of investigation in exoplanetary science. To date, K2-18b remains the only sub-Neptune with equilibrium temperature below 300 K in that star environment that has been observed with the James Webb Space Telescope. Although it shows an extensive transmission spectrum, both its atmospheric composition and internal structure remain debated. Since the atmospheric study alone cannot fully determine the nature of K2-18b, we present a comprehensive analysis that combines atmospheric retrievals with models of planetary formation and evolution.
Using updated stellar parameters, we perform a uniform reduction of all the JWST transmission datasets using the Eureka! package. We obtain a transmission spectrum spanning 0.6-12 micrometers, combining spectra observed with NIRISS/SOSS, NIRSpec/G235H, NIRSpec/G395H, and MIRI/LRS instrumentations. We perform several Bayesian free-chemistry retrieval models using the TauREx3 framework, exploring different molecules and temperature-pressure profiles. In addition, we test a power-law cloud parameterisation through a new TauRex3 plugin, specifically implemented for this project. Our favoured atmospheric model indicates a hydrogen-dominated atmosphere containing 1.6% CH4, 0.13% CO2 and a weak signature of C2H4, together with photochemical hazes.
We determine that the best-fitting atmospheric model for K2-18b has a low mean molecular weight of about 2.8 amu, mainly composed of light elements. We perform a detailed evolution model of the planetary envelope, finding that at early ages the envelope size was about two times larger than at present age, but its mass was just 10% larger. We find that hydrodynamic photoevaporation processes played a marginal role in shaping the atmospheric content, which instead are mostly driven by the natural gravitational shrinking of the planet. In this way, the current composition of K2-18b is likely primordial, supporting the atmospheric model output.
We complement free-chemistry retrieval models with chemical equilibrium computations to interpret the retrieved molecular abundances under different atmospheric scenarios. We test a well-mixed mini-Neptune atmosphere, a well-mixed Hycean world, and a vertically stratified atmosphere, establishing that the retrieved abundances of CH4, CO2 and C2H4 are best reproduced by the first scenario. This result is further supported by population synthesis simulations, finding that K2-18 b has formed at several astronomical units from its host star and migrated inward during the protoplanetary disk phase. Due to this, we predict a mainly rocky core, with a limited ice fraction of < 25%, which is in agreement with the mini-Neptune scenario.
Our findings indicate that the use of multiple types of analysis together leads to a deeper understanding of complex planets like K2-18 b. Furthermore, our results may be used to investigate how other sub-Neptunes evolve within the habitable zone of M-dwarf stars, in order to build a solid sample and finally understand the nature of these planets.
How to cite: Mascolo, Y.: Atmospheric and evolutionary characterisation of K2-18b, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-742, https://doi.org/10.5194/epsc2026-742, 2026.
The radius cliff is a prominent feature in the radius–occurrence distribution of close-in exoplanets, yet its physical origin remains uncertain. Understanding this feature is crucial for constraining the formation and evolutionary pathways of sub-Neptune planets, which constitute the majority of the observed exoplanet population. Proposed explanations include runaway gas accretion, atmospheric mass loss, and the dissolution of hydrogen into the planetary interior. However, none of these mechanisms has yet reproduced the observed radius cliff robustly. The hydrogen dissolution hypothesis is currently limited by uncertainties in the thermochemical properties of materials under the high-pressure and high-temperature conditions characteristic of sub-Neptune interiors.
We develop equilibrium chemistry models of sub-Neptunes that incorporate hydrogen dissolution into their magma mantles, informed by constraints from recent high-pressure, high-temperature laboratory experiments. The primary objective is to assess whether hydrogen partitioning between the interior and the atmosphere can account for the observed radius cliff and to quantify its role relative to competing formation and evolution scenarios.
Our results show that a large fraction of total hydrogen partitions into the interior, thereby increasing atmospheric metallicity, which can be constrained observationally. We also evaluate the effect of H2-H2O miscibility in the atmosphere on the partitioning of hydrogen. These results will improve the physical interpretation of exoplanet population statistics and contribute to a more unified theoretical framework for explaining the observed radius distribution.
How to cite: Vranckx, L., Hakim, K., Rivoldini, A., Van Hoolst, T., Bower, D., and Namur, O.: The role of interior-atmosphere partitioning of hydrogen in shaping the radius cliff in the exoplanet frequency-radius distribution, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-19, https://doi.org/10.5194/epsc2026-19, 2026.
The recently discovered four-planet system around LHS 1903 exhibits a remarkable diversity in inferred water mass fractions and bulk densities, suggesting strongly contrasting ice-to-rock ratios among its planets [1]. Interior structure models indicate that the innermost planet, LHS 1903 b, is predominantly rocky and water-poor, while planets c and d appear substantially volatile-rich and may contain up to ~25-33 wt% water. The outermost planet, LHS 1903 e, appears mostly rocky but may still contain a moderate water fraction (down to 7%). Such diversity raises important questions regarding the origin and transport of water-bearing material during the formation of the system.
Here we investigate whether the diversity of ice-to-rock ratios inferred in the LHS 1903 system can be explained using a formation framework analogous to those recently proposed for the Galilean moons and the TRAPPIST-1 system [2, 3]. In these scenarios, hydrated phyllosilicate-rich particles drift inward through the protoplanetary disk and dehydrate once they cross the phyllosilicate dehydration line (PDL), which is at 500-600 K in the protoplanetary disk. The released water vapor diffuses both inward and outward before recondensing beyond the snow line, thereby enriching solids in water ice.
We adapt this framework to the physical conditions expected in the LHS 1903 protoplanetary disk and investigate whether the inferred planetary compositions can be reproduced through a combination of thermal dehydration, vapor redistribution, ice recondensation, and planetary migration. Preliminary calculations suggest that the volatile-rich planets c and d accreted near the snow line from solids enriched by outwardly diffusing water vapor, whereas planet b likely formed interior to the PDL from dry or partially dehydrated material. In contrast, planet e may have formed beyond the snow line and accreted primarily from hydrated but only weakly processed minerals. This interpretation is consistent with its inferred water mass fraction (~7–28%), which overlaps the typical ∼5–20 wt% water contents expected for hydrated silicates and aqueously altered materials. The resulting formation sequence naturally explains the large diversity in water mass fractions observed within the compact architecture of the LHS 1903 system.
This work suggests that the diversity of ice-to-rock ratios observed in compact exoplanetary systems may arise from local thermochemical processing of hydrated rocks within protoplanetary disks rather than requiring the direct inward transport of primordial icy planetesimals from distant cold reservoirs. The LHS 1903 system may therefore represent another exoplanetary analog of the compositional gradients observed among the Galilean satellites and TRAPPIST-1 planets.
References
[1] Wilson, T.G., et al. 2026. Science, DOI: 10.1126/science.adl2348
[2] Mousis O., Schneeberger A., Lunine J.I., Glein C.R., Bouquet A., Vance S.D., 2023, ApJL, 944, L37. doi:10.3847/2041-8213/acb5a4
[3] Schneeberger A., Mousis O., Deleuil M., Lunine J.I., 2024, A&A, 682, L10. doi:10.1051/0004-6361/202348309
How to cite: Mousis, O., Bennacer, Y., Glein, C., Schneeberger, A., and Bottke, W.: A Thermochemical Origin for the Diversity of Ice-to-Rock Ratios in the LHS 1903 System, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-557, https://doi.org/10.5194/epsc2026-557, 2026.
Initial phases of planet formation are ruled by sticking between dust particles and aggregates thereof. Simple van der Waals forces are too small to allow growth into a size regime where hydrodynamic instabilities might take over. However, tribocharging during the bouncing phase can increase the stickiness. Based on the experiments presented here, we propose that these electrostatic sticking forces might be self-regulating. That is, bouncing collisions might amplify the charges to the point where particles stick together and no further bouncing and charging occurs. This way, bouncing phases might be turned into growth phases, just adapting to the need for a given parameter set (grain size, collision velocity).
This concept is suggested by our experimental investigation of charged particles under prolonged microgravity conditions. We placed an ensemble of monodisperse sub-millimeter basalt spheres in a collision chamber on a suborbital flight. The experimental procedure included agitating the chamber and hence the basalt spheres. Collisions generated charges on the surfaces of the involved particles via tribocharging. In addition to the beads, the glass windows of the chamber charged too sporadically and over time in certain spots. At the beginning, a gentle shake was enough to remove the particles from the charge spots. Over the course of the experiment, successive collisions led to an increase of charges on the charge spots. Particles became harder to remove until they finally couldn’t be removed at all, even at the maximum vibration amplitude. We estimated the charge evolution on the spots by evaluating the motion of grains interacting with the spots. This includes trajectories of unbound grains, the oscillations of captured particles and, finally, the analysis of rigid-like sticking. These localized charge spots could enhance preplanetary growth of agglomerates during bouncing. As collisional charging stops if particles stick together, the charging and electrostatic forces might be self-regulating to the point where they are just large enough for the given parameters to promote growth. Supposedly, this also works during fragmentation phases of pebbles, which would shift the fragmentation barrier and allow the growth of still larger pebbles.
How to cite: Penner, J., Teiser, J., Wenders, N., Jankowiak, J., Wieland, F., and Wurm, G.: Self-Regulation of Amplifying Sticking Forces During Collisional Growth of Preplanetary Pebbles, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-607, https://doi.org/10.5194/epsc2026-607, 2026.
We report the results of the atmospheric characterization and Bayesian retrieval analysis for two distinct exoplanetary classes: the mini-Neptune K2-18b and the Earth-sized terrestrial exoplanet LHS 475b. For K2-18b, our retrieval models successfully reproduce the spectral signatures of methane (CH4) and carbon dioxide (CO2) and effectively rule out the existence of high-altitude opaque clouds at the 10 mbar pressure level. For LHS 475b, the study focuses on evaluating the presence of a secondary atmosphere and establishing upper limits on the abundances of major volatile species.
The investigation utilizes high-precision transmission spectra retrieved from the NASA Exoplanet Archive (JWST observations). We integrate datasets from the NIRISS (0.8–2.8 μm) and NIRSpec (2.7–5.2 μm) instruments to ensure broad-band infrared coverage essential for identifying molecular absorption features. Forward modeling based on radiative transfer equations and Bayesian retrieval pipelines were implemented within the TauREx 3 framework. To maintain consistency with the observational resolving power, all theoretical spectra were convolved with a Gaussian filter (σ = 30).
Additionally, by applying comparative retrieval analysis, we quantify the impact of photochemical hazes and cloud deck muting on spectral interpretation. This approach allows for the mitigation of parameter degeneracies and the reconstruction of precise Temperature-Pressure (T-P) profiles. Our findings highlight the critical role of multi-instrument retrieval in verifying atmospheric conditions on sub-Neptunes and terrestrial planets.
How to cite: Lobodenko, M., Kozak, L., and Cockell, C.: Spectral Analysis and Atmospheric Retrieval of Terrestrial Exoplanets and Mini-Neptunes Using the TauREx 3 Framework, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-749, https://doi.org/10.5194/epsc2026-749, 2026.
How to cite: Shih, J., Heng, K., and Owen, J. E.: Coupling Surface-Atmosphere Feedbacks: A Scheme for Modelling Atmosphere Evolution of Rocky Exoplanets, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1299, https://doi.org/10.5194/epsc2026-1299, 2026.
