Interaction phenomena between atmospheric layers in planetary atmospheres give rise to highly complex exchanges influencing thermal structure, composition and dynamics. In particular, the question of hydrodynamic escape in exoplanet atmospheres is a key issue for quantifying the mass loss in these atmospheres. Short-period exoplanets subject to violent interactions with their host star are particularly sensitive to these effects [1,2]. The scientific program presented here called AETHER (Atmospheric Escape and Transfer phenomena in Heated Exoplanets by stellar Radiation) combining modelling and future observations is a continuation of a pioneer work connecting lower and upper atmospheric modelling in exoplanets [3].

• In order to address these questions, a model is presented combining energy deposition (itself constrained by XUV emissions from the parent star), the chemistry induced in an atmosphere of hydrogen, helium and heavier elements, and dynamical redistribution to go beyond one-dimensional models. The objective of the model is to characterize observables in exoplanets spectroscopy, in order to adapt the model to real observations, and to constrain in fine the atmospheric escape.
• Observables to characterize these phenomena include Ly-α lines (observable from space, but perturbed by reabsorption from the interstellar medium), metallic lines [4], and more recently the He-I [5,6,7] which coupled with observations of the H-α line has enabled advances on the models [8,9]. An atmospheric radiative transfer model incorporating the predictions of the energy deposition model [10] completes the atmospheric modeling towards the lower atmospheric levels.
• Models for the He I triplet at 1.083µm have been scaled to the well described planet HD 209458b. Assuming a He absorption taken from Carmenes analysis [11], line intensities can be predicted for observations by high resolution spectroscopy. Different planets have been studied for higher XUV fluxes and escape mass, like in HAT-P-11b [3, 6]. For still higher irradiation, like on HAT-P-32b [5,12] the detectability would be more easily ensured.
• In addition to these lines, the simulation has focused on the detectability of hydrogen quadrupole lines : H₂ lines in 1-0 and 2-0 bands at 2.3 and 1.15 µm respectively are modelled in the radiative transfer simulation. These transitions have been detected on Jupiter [13] in particular conditions, as in the auroral regions. Detecting direct H₂ transitions would not be only a textbook achievement in the study of exoplanets, but would pave the way for direct measurement of the atmospheric temperatures at their pressure level of formation (typically ~100 mbar in transit spectroscopy in H filter, for the 2-0 vibrational band of H₂). In addition to the normal atmosphere calculations, a probable upper atmosphere heating, would enhance the possibility to detect H₂ by populating the upper transitional levels by analogy with Jupiter [14].
• Targets for future observations of exoplanets are in the class of hot (super)Neptunes, with “puffy” atmospheres and large atmospheric scale heights. A good template for the simulations would be the planet WASP-193b [2]. This planet is particularly intriguing as a low-density Super-Neptune, described in the discovery paper as "an exquisite target for characterization by transmission spectroscopy." With a Transmission Spectroscopy Metric (TSM) of approximately 600, WASP-193b ranks as the fourth highest among all known exoplanets, making it a prime candidate for atmospheric studies. The full list of parameters of WASP-193b can be obtained from [2], the most important being a radius R~1.463 R_Jup, an equilibrium temperature T_eq=1252 K and a mass M_p=0.141 M_Jup, leading to a density of 0.059 g.cm^-3.
• The program AETHER will be developped in preparation for the Ariel mission - even if observations in this field are beyond the reach of its instruments, understanding and modeling them is a key element for a better knowledge of the chemistry of the lower atmosphere, the main target of Ariel observations.

Figure caption : a) He line prediction: (case 0) equilibrium temperature 1252 K at 1 micro bar and solar abundance modified by Wasp-193 [Fe/H]; (case 1) boundary condition set by the lower atmosphere chemistry model, using the same SED for both regions (self-consistent). b) H2-q contribution prediction

References

[1] García Muñoz, A.; Fossati, L.; Youngblood, A.; Nettelmann, N.; Gandolfi, D.; Cabrera, J.; Rauer, H. A Heavy Molecular Weight Atmosphere for the Super-Earth π Men c. 2021ApJ...907L..36G

[2] Barkaoui, Khalid et al. An extended low-density atmosphere around the Jupiter-sized planet WASP-193 b, (2024) NatAs, 8, 909B

[3] Ben-Jaffel, Lotfi; Ballester, Gilda E.; García Muñoz, Antonio;  Lavvas, Panayotis; Sing, David K.; Sanz-Forcada, Jorge;  et al. Signatures of strong magnetization and a metal-poor atmosphere for a Neptune-sized exoplanet. (2022) NatAs, 6, 141.

[4] Casasayas-Barris N. et al. Atmospheric characterization ofthe ultra-hot Jupiter MASCARA-2b/KELT-20b. Detection of CaII, FeII, NaI, and the Balmer series transit.  spectroscopy. (2019) A&A, 628A, 9

[5] Czesla, S. et al, Hα and He I absorption in HAT-P-32 b observed with CARMENES. Detection of Roche lobe overflow and mass loss (2022) A&A, 657A, 6

[6] Allart, R. et al.  Spectrally resolved helium absorption from the extended atmosphere of a warm Neptune-mass exoplanet (2018) Science, 362, 1384

[7] Masson A. et al. Probing atmospheric escape through metastable He I triplet lines in 15 exoplanets observed with SPIRou. (2024) Astronomy & Astrophysics, Volume 688, A179.

[8] Lampon, M.et al. 2023. Characterisation of the upper atmospheres of HAT-P-32b. A&A 673 A140

[9] Sanz-Forcada, et al. Connection between planetary He I λ10 830 Å absorption and extreme-ultraviolet emission of planet-host stars. 2025A&A. 693A.285S

[10] Arfaux, Anthony; Lavvas, Panayotis Coupling haze and cloud microphysics in WASP-39b's atmosphere based on JWST observations (2024) MNRAS, 530, 482A

[11] Lampon, M. et al. Modelling the He I triplet absorption at 10 830 Å in the atmosphere of HD 209458 b. (2020) A&A, 636A, 13L.

 [12] Yan, D. et al. A possibly solar metallicity atmosphere escaping from HAT-P-32b revealed by Hα and He absorption (2024) A&A, 686A, 208

[13] Kim, S.; Temperatures of the Jovian auroral zone inferred from 2-μm H₂ quadropole line observations. (1990) Icarus, 84, 54.

[14] Barthelemy, M. et al. H₂ vibrational temperatures in the upper atmosphere of Jupiter. (2005) A&A, 437, 329

How to cite: Drossart, P., Garcia-Munoz, A., Lavvas, P., Ben Jaffel, L., Beaulieu, J.-P., Sanz-Forcada, J., Orell-Miquel, J., Yan, D., Masson, A., Palle, E., and Vinatier, S.: From atmospheres to exospheres in exoplanets, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-30, https://doi.org/10.5194/epsc-dps2025-30, 2025.

How to cite: Masson, A., Vinatier, S., and Bézard, B. and the PLANETS - WP4: A Homogeneous Study of Exoplanetary Atmospheres Using High-Resolution Transit Spectroscopy, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-644, https://doi.org/10.5194/epsc-dps2025-644, 2025.

Introduction

     Lyman-alpha transmission spectroscopy has been a powerful tool for observing hydrogen escape from close-in exoplanets. For instance, Ly-α observations of GJ 436b showed a maximum transit depth of 56% – compared to a 0.69% transit depth in optical wavelengths – due to the hydrodynamic escape of hydrogen from the planet (1). Ly-α observations assist in understanding the evolution of such exoplanets, including characterization of the atmosphere.

     To date, the Ly-α transit of terrestrial-sized exoplanets has yielded only non-detections for exoplanets such as Trappist 1b/c (2) and 55 Cn e (3) using the Hubble Space Telescope. These non-detections possibly indicate that terrestrial exoplanets do not have enough hydrogen escape to be observed in Ly-α. Despite these non-detections, Earth’s hydrogen exosphere has been shown to extend out past 38 Earth-radii, and modelling has suggested that an exoplanet with an Earth-like hydrogen exosphere orbiting an M-dwarf star would be observable with future space telescopes (4).

     This work models the Ly-α transit of varying terrestrial exoplanets to diagnose atmospheric composition. For example, how does the Ly-α transit of a desiccated planet like Venus compare to a water-rich planet like Earth? Can we discern these differences from future space telescope observations? To test this, we model the thermal escape of hydrogen from terrestrial exoplanets and compute the associated Ly-α transit depth. Atmospheric parameters in the upper atmosphere, such as the mixing ratio of hydrogen, are varied to analyze the resulting Ly-α transit depth. From this, we examine trends in the transit depth to characterize the atmospheres of terrestrial exoplanets. In addition, we derive key attributes of exoplanets that would be detectable with future space telescopes.

Methods

     In this work, the hydrogen exosphere is modelled using the Chamberlain approach (5). Key input parameters needed are the exobase height, the number density of hydrogen, and the exobase temperature. The exobase height and hydrogen number density are calculated by setting up a 2-component atmosphere at the homopause (the altitude where different species can diffusively separate, situated at 100 km from the surface with a total species number density of 10¹⁹  molecules cm^-3), with a given hydrogen mixing ratio. The species diffuse upward from the homopause until they reach the exobase where the mean free path of the atmosphere is equal to the scale height. We also account for diffusion-limited escape. In a scenario where Jean’s escape is larger than the diffusion-limited escape, the number density of hydrogen is scaled to the diffusion-limited value. The exobase temperature is a free parameter in our model, though observations from the solar system indicate that a CO₂ dominated exobase is cooler than an atomic oxygen-dominated exobase such as Earth’s, due to infrared cooling.

     Once the hydrogen exosphere is modelled, the radiative transfer code from the open-source model Sunbather (6) is used to model the transit. By default, we consider the planet to be mid-transit with an impact factor of 0. Factors affecting the transit depth, such as thermal line broadening are included.

     We consider a wide parameter space of planets between 0.5 to 2 Earth radii, with atmospheric temperatures between 100 to 700 K, and hydrogen mixing ratios between 10^-10 and 1.  For each run of the model, the hydrogen exosphere is computed at several exospheric temperatures between 100 and 1000 K to span the range of exobase temperatures observed in the solar system.

Results

     An example of the number density of hydrogen in the exosphere and the resulting Ly-α transit in shown in Figure 1. In this case, we have modelled an Earth-sized planet with a hydrogen mixing ratio of 10^-6 at the homopause that diffuses through an upper atmosphere of atomic oxygen. The exobase is 228 km from the surface with a hydrogen number density of 3.4 x 10⁴, limited by the diffusion. The exobase is dominated by atomic oxygen, and modelled to be at 1000 K. The Ly-α transit depth around a sun-sized star is 275 ppm, much smaller than observed for close-in Neptunes, but possibly observable with future space telescope technology at a distance where geocoronal contamination is minimal.

   
Figure 1: (a) Hydrogen number density in the exosphere as a function of distance from the planet (b) The Ly-α transit in ppm.

     Figure 2 shows the transit depth of exoplanets with varying masses and hydrogen mixing ratios at the homopause. Here a fixed exobase temperature of 1000 K  was used though the simulations were also run with other exobase temperatures (not shown here, though the Ly-α transit typically increases with increasing exobase temperature due to the hydrogen atoms escaping the exobase more easily). The transit depth shows a strong dependence on hydrogen mixing ratio, with higher hydrogen mixing ratios resulting in a larger transit depth. The mass of the planet also affects the transit depth, with lower mass planets allowing for hydrogen escape more easily than more massive planets.

Figure 2: Ly-α transit depths for exoplanets with varying masses and hydrogen mixing ratios.

We will also present the effects of photoionization, making the hydrogen no longer observable in Ly-α. Lastly, we will test the ability to detect these Ly-α transits with space telescopes.

(1) Ehrenreich D, Bourrier V, Wheatley PJ, des Etangs AL, Hébrard G, Udry S, et al. A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b. Nature. 2015 Jun;522(7557):459–61.

(2) Bourrier V, Ehrenreich D, Wheatley PJ, Bolmont E, Gillon M, Wit J de, et al. Reconnaissance of the TRAPPIST-1 exoplanet system in the Lyman-α line. Astron Astrophys. 2017 Mar 1;599:L3.

(3) Salz M, Czesla S, Schneider PC, Schmitt JHMM. Simulating the escaping atmospheres of hot gas planets in the solar neighborhood. Astron Astrophys. 2016 Feb 1;586:A75.

(4) Santos LA dos, Bourrier V, Ehrenreich D, Kameda S. Observability of hydrogen-rich exospheres in Earth-like exoplanets. Astron Astrophys. 2019 Feb 1;622:A46.

(5) Chamberlain JW. Planetary coronae and atmospheric evaporation. Planet Space Sci. 1963 Aug 1;11(8):901–60.

(6) Linssen D, Shih J, MacLeod M, Oklopčić A. The open-source sunbather code: Modeling escaping planetary atmospheres and their transit spectra. Astron Astrophys. 2024 Aug 1;688:A43.

How to cite: Bischof, G., Wordsworth, R., and Moores, J. E.: Characterization of Terrestrial Exoplanet Atmospheres through Lyman-alpha Transit Observations, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-185, https://doi.org/10.5194/epsc-dps2025-185, 2025.

TOI-270 d, a sub-Neptune (R_p ≈ 2.2 R_⊕, T_eq ≈ 340 K) orbiting a nearby M3V star, offers an exceptional opportunity for atmospheric characterization of a temperate sub-Neptune with the James Webb Space Telescope (JWST). Recent analyses of transit data from NIRSpec and NIRISS have yielded conflicting interpretations of the planet’s atmosphere: a high-metallicity, miscible envelope (Benneke et al. 2024) versus a lower metallicity ”hycean” world (Holmberg &
Madhusudhan 2024). To resolve this discrepancy, we conducted an independent data reduction and retrieval analysis and find evidence for a more complex sulfur-chemistry in TOI-270 d’s atmosphere.

Using JWST’s NIRSpec/BOTS G395H and NIRISS/SOSS GR700XD modes (spanning 0.8–5.2 μm), we produce transit spectra at the native instrument resolution, preserving the maximal amount of information obtained from the observations. We perform atmospheric retrievals over a grid of binsizes, showing that retrievals at native spectral resolution and small binsizes (<16 pixels) are only reliable if the instrument-specific line-spread functions are accounted for.

Our fiducial retrieval results reveal a high-metallicity atmosphere (M/H ≈ 160× solar) with an elevated mean molecular weight (μ ≈ 5.6 amu), confidently detecting CH₄ and CO₂. But we also find that the Bayes Factor in favor of the inclusion of CS₂ and NH₃ are highly dependent on the deployed spectral resolution, varying between 3-78 and even reaching 30’000 in one case. We therefore demonstrate that coarse spectral binning introduces biases, particularly for molecules that do not dominate the shape of the spectrum.

Figure 1: The Bayes Factor for the favored model extensions relative to the fiducial model. All mentioned additions are favored with a Bayes Factor >600 indicating strong evidence for their preference over the fiducial model. 

Equipped with a solid baseline atmosphere model, we test for the potential presence of various other molecules. We identify multiple majorly favored configurations (Bayes Factor 600-16000, see FIgure 1). Most plausibly, we find a rich sulfur chemistry containing both H₂CS and CS at well restricted abundances, potentially providing chemical pathways that could explain the large abundance >0.1% of CS₂ found across all retrievals. Alternatively, the inclusion of CH₃Cl or CH₃F, retrieved at abundances of > 100 ppm, leads to similar improvements in the Bayesian evidence, as does any combination of these molecules and the aforementioned sulfur model. The data alone does not allow us to identify a preferred model, due to the fact that the opacities of CH₃Cl, CH₃F and H₂CS are almost identical in our probed spectral range (see Figure 2). 

Figure 2: The model spectra of the fiducial model as well as the favored model extensions. We show a binned down version of our transmission spectrum for visual clarity. The fit to the data is improved mainly around 3.4 μm and we can clearly see that the model extensions are almost identical across the spectrum. 

Our results highlight critical challenges in characterizing temperate sub-Neptunes. The degeneracy between biosignature gases (e.g., CH₃Cl and CH₃F) and abiotic sulfur compounds, like H₂CS, underscores the need for the detailed exploration of sulfur-chemistry in the temperature regime of planets like TOI-270 d. Furthermore, we demonstrate that large Bayes Factors can be achieved at higher spectral resolutions, even for species lacking isolated absorption features, emphasizing the importance of carefully selecting spectral resolution and accounting for its impact when analyzing transmission spectroscopy data.

How to cite: Felix, L., Kitzmann, D., Demory, B.-O., and Mordasini, C.: Evidence for sulfur-chemistry in the sub-Neptune TOI-270 d, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1750, https://doi.org/10.5194/epsc-dps2025-1750, 2025.

The James Webb Space Telescope (JWST) has been revolutionizing the field of exoplanets by delivering high-precision spectra that constrain atmospheric compositions with unprecedented detail. JWST is best at constraining the atmospheric compositions for warm-to-hot exoplanets with hydrogen-dominated atmospheres and large-scale heights. To date, JWST has already provided constraints on the abundances of dominant carbon and oxygen carrier gases in over a dozen such targets. Under standard assumptions, many of these exoplanet targets are expected to have abundant methane (CH₄) in their observed atmosphere. However, methane was only spotted on a few of these existing targets, giving rise to the so-called "missing methane" problem.

Here, we use published JWST results in combination with a simple, geochemistry-inspired model to explore whether elevated internal temperatures (T_int) can account for the CH₄ depletion in these exoplanet atmospheres. Instead of using computationally intensive forward grid models to search for the optimum parameters (in this case, T_int) to fit the observed spectra, we construct a fast analytical framework that focuses on two key chemical equilibria: CO-CH₄ and CO-CO₂. The equilibrium constants are calculated using Gibbs energies from the NIST-JANAF Thermochemical Tables (Chase, 1998), and the quench temperatures of these two chemical equilibria are linked by a relationship provided in Glein et al. (2025): T_CO-CO2 ≈ 0.8T_CO-CH4. Following a similar and revised methodology as Glein et al. (2025), we can translate JWST-constrained CHO species abundances into a pressure–temperature (P–T) space consistent with equilibrium chemistry.

We generate a suite of P-T relationships consistent with the observed bulk atmospheric composition and a second set of P-T profiles with varying internal temperatures (characterized by T_int). For each target, we identify the minimum T_int required for the P–T profile to intersect the constrained chemical equilibrium region, which corresponds to the condition under which the observed H₂O, CO₂, CH₄, and CO abundances can be reproduced. We first tested this approach against WASP-107 b, for which detailed forward modeling exists, and found our derived minimum T_int (minimum T_int = 500 K) to be consistent with results from the forward grid modeling approach (T_int = 460±40 K, Sing et al. 2024).

We apply this method to eleven warm-to-hot exoplanets with reliable JWST-retrieved constraints on the volume mixing ratios of H₂O, CO₂, CH₄, and CO. For six of these targets, including WASP-107 b, HD 189733 b, HIP 67522 b, WASP-69 b, HAT-P-18 b, and GJ 3470 b (data from Sing et al. 2024; Welbanks et al. 2024; Fu et al. 2022, 2024; Thao et al. 2024; Schlawin et al. 2024), we can derive minimum internal temperatures consistent with the observed CH₄ depletion. The inferred T_int values range from 70 K to 900 K for these targets (see Figure 1). We then compare these values to predictions from standard planetary evolution models, finding that several targets require additional internal heat sources (e.g., tidal dissipation and/or ohmic dissipation) to sustain such elevated T_int values at their current ages and masses.

In particular, for the five planets exhibiting strong CH₄ depletion (WASP-107 b, HD 189733 b, HIP 67522 b, WASP-69 b, and HAT-P-18 b), high T_int leads to deeper quench levels at higher temperatures where CO dominates over CH₄ (which also naturally requires rapid mixing) to explain the observed missing methane. In contrast, for targets like WASP-80 b, where CH₄ is detected, or for very hot planets where quenching occurs high up in the atmosphere in the radiative zone, internal temperature plays a less critical role and cannot be constrained with this method.

Our framework offers a computationally efficient and consistent method to quickly infer internal temperatures of exoplanets based on chemical equilibrium constraints. While complementary to the forward grid modeling approach, our approach provides intuitive visualizations that allow fundamental trends to be identified. Its simplicity and speed make it well-suited for analyzing a much broader sample as JWST and future space telescopes such as ARIEL continue to deliver high-quality atmospheric data across a growing and diverse target population.

Figure 1: Pressure–temperature (P–T) profiles for exoplanet targets where internal temperature (T_int) can be constrained. Solid colored curves indicate predicted profiles consistent with the retrieved CH₄-CO-CO₂-H₂O volume mixing ratios from JWST observations (the allowed parameter space is in gray). Dashed curves represent P–T profiles with lower T_int values that are incompatible with the observed atmospheric compositions.

References:

Beatty, T. G., et al. (2024). Sulfur dioxide and other molecular species in the atmosphere of the sub-Neptune GJ 3470 b. The Astrophysical Journal Letters, 970(1), L10.

Chase, M. W., Jr. 1998, NIST-JANAF Thermochemical Tables, Parts I and II (4th ed.; Woodbury, NY: American Institute of Physics)

Fu, G., et al. (2022). Water and an escaping helium tail detected in the hazy and methane-depleted atmosphere of HAT-P-18b from JWST NIRISS/SOSS. The Astrophysical Journal Letters, 940(2), L35.

Fu, G., et al. (2024). Hydrogen sulfide and metal-enriched atmosphere for a Jupiter-mass exoplanet. Nature, 632(8026), 752-756.

Glein, C. R., Yu, X., & Luu, C. N. (2025). Deciphering Sub-Neptune Atmospheres: New Insights from Geochemical Models of TOI-270 d, in press at ApJ.

Schlawin, E., et al. (2024). Multiple Clues for Dayside Aerosols and Temperature Gradients in WASP-69 b from a Panchromatic JWST Emission Spectrum. The Astronomical Journal, 168(3), 104.

Sing, D. K., et al. (2024). A warm Neptune’s methane reveals core mass and vigorous atmospheric mixing. Nature, 630(8018), 831-835.

Thao, P. C. et al. (2024). The Featherweight Giant: Unraveling the Atmosphere of a 17 Myr Planet with JWST. The Astronomical Journal, 168(6), 297.

Welbanks, L., et al. (2024). A high internal heat flux and large core in a warm Neptune exoplanet. Nature, 630(8018), 836-840.

How to cite: Yu, X. and Glein, C.: Unusually Hot Interiors Could Reconcile the Missing Methane Problem for Warm-to-Hot Exoplanets with Hydrogen Atmospheres, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1220, https://doi.org/10.5194/epsc-dps2025-1220, 2025.

Studying chemical composition is fundamental to model the formation history of planets and planetary systems. With the first JWST data and the upcoming Ariel satellite, we expect a leap forward in the exoplanet’s atmosphere field. Most current atmospheric retrieval methods assume thermochemical equilibrium or freely parameterized profiles. These approaches can miss important disequilibrium processes like photochemistry and vertical mixing, which significantly affect the observed spectra. This study explores how including disequilibrium chemistry affects the retrieval of key atmospheric parameters, such as metallicity and the carbon-to-oxygen (C/O) ratio. We focus on whether these effects are detectable using data from the Hubble Space Telescope and the James Webb Space Telescope. Our five targets are HAT-P-12b, HD 209458b, WASP-6b, WASP-17b, and WASP-39b, which have temperatures ranging from 1000K to 1700K and radii ranging from 0.9 to 1.9 Jupiter radius.

We used the TauREx 3.1 atmospheric retrieval framework (Al-Refaie et al. 2019), coupled with FRECKLL (Al-Refaie et al. 2024), a disequilibrium chemistry plugin that includes vertical mixing and photochemistry. FRECKLL computes steady-state chemical abundances under disequilibrium conditions and gives them to TauREx for radiative transfer and retrieval analysis. A Bayesian nested sampling algorithm is used for parameter estimation. The principle is to start from a transmission spectroscopy dataset and to look for the model that characterizes the atmosphere in the most probable way, in other words, the spectrum that best fits the data points. For very hot planets, thermochemical equilibrium may be close to reality, but for less hot planets, vertical mixing and photodissociation bring these planets out of equilibrium. This work is a follow-up study of Panek et al. (2023) and is complementary of Bardet et al. (2025) (in review), a study that was mainly looking at the same effect on emission spectra.

We will present one of the very rare retrieval analysis taking into accounts disequilibrium chemistry and we will evaluate how this new sophisticated method improve the analysis of observations. The detectability of these effects strongly depends on spectral resolution and wavelength coverage. We expect to see the biggest improvement with JWST data, thanks to its better spectral coverage. This study demonstrates the feasibility and importance of incorporating disequilibrium chemistry into retrieval models. Doing so can significantly improve our interpretation of exoplanetary atmospheres and help refine models of planet formation and evolution. These findings also highlight the need to revisit previous retrieval studies that used only equilibrium models, especially for cooler planets where disequilibrium chemistry is expected to play a major role.

How to cite: Panek, E., Bardet, D., Beaulieu, J.-P., Drossart, P., and Venot, O.: Analysis of disequilibrium chemistry in five exoplanets’ atmosphere, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1042, https://doi.org/10.5194/epsc-dps2025-1042, 2025.

The ESA M4 mission Ariel, the atmospheric remote-sensing infrared exoplanet large-survey, has been adopted within the Cosmic Vision science programme of ESA. The goal of Ariel is to investigate the atmospheres of planets orbiting distant stars in order to address the fundamental questions on how planetary systems form and evolve and to investigate the chemical composition of exoplanetary atmospheres. During its 4-year mission, Ariel will observe a diverse sample of up to 1000 exoplanets, i.a. covering  super-Earths and sub-Neptunes and building on (future) findings from space missions like PLATO, JWST, TESS, and CHEOPS - ranging from Jupiter- and Neptune-size down to super-Earth size, in a wide variety of environments, in the visible and the infrared. The main focus of the mission will be on warm and hot planets in orbits close to their star. Some of the planets may be in the habitable zones of their stars, however. The analysis of Ariel spectra and photometric data will allow to extract the chemical fingerprints of gases and condensates in the planets’ atmospheres, including the elemental composition for the most favourable targets. The Ariel mission has been developed by a consortium of more than 60 institutes from 16 ESA member state countries, including UK, France, Italy, Poland, Spain, the Netherlands, Belgium, Austria, Denmark, Ireland, Hungary, Sweden, Czech Republic, Germany, Portugal, with an additional contribution from NASA. 

How to cite: Lueftinger, T., Tinetti, G., Salvignol, J.-C., and Eccleston, P.: Ariel - The ESA M4 Space Mission to Focus on the Nature Of Exoplanets , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1807, https://doi.org/10.5194/epsc-dps2025-1807, 2025.

Understanding the diverse formation and migration pathways that shape exoplanetary systems requires characterizing both their atmospheric properties and their orbital dynamics. A key dynamical diagnostic is the projected spin-orbit angle, the alignment between the stellar spin and the planetary orbit which provides crucial tests for theoretical models. This angle can be determined using the Rossiter-McLaughlin effect. Although measurements exist for over 200 planets, the overall distribution of these angles remains incompletely understood, motivating further observations across the full parameter space. In this talk, we present measurements of spin-orbit angles for 25 systems identifying several system that have likely underwent disc-free migration. We highlight the synergy between the two approaches and comment on the current overlap between targets with spin-orbit angle measurements and those having measurements for atmospheric characterization. While we find no strong observational biases due to the spin-orbit angle, we note that the majority of planets, especially sub-Neptunes, with atmospheric data still lack spin-orbit measurements. This incompleteness of the dynamical information may limit the interpretation of upcoming atmospheric surveys.

How to cite: Zak, J., Bocchieri, A., and Boffin, H.: Planet migration in the era of JWST and Ariel, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-577, https://doi.org/10.5194/epsc-dps2025-577, 2025.

The characterization of super-Earth to Neptune-sized exoplanets depends critically on understanding their formation and thermal evolution. We investigate the evolutionary pathways of super-Earths formed with compositionally polluted envelopes, focusing on the role of silicate rainout and its impact on the observed mass-radius relation. Our results show that the energy released during silicate rainout inflates the planetary radius during early evolutionary stages, when stellar irradiation is most intense. This radius inflation enhances atmospheric mass loss by a factor of 2–8 compared to planets with initially clean envelopes. Moreover, neglecting this effect can lead to overestimates of hydrogen-helium content in young super-Earths. These findings highlight the need for accurate age measurements—such as those anticipated from the PLATO mission—to reliably interpret the observed properties of young exoplanets.

How to cite: Vazan, A. and Ormel, C.: Radius Inflation in Super-Earths Born with Polluted Envelopes, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-213, https://doi.org/10.5194/epsc-dps2025-213, 2025.

How to cite: Plotnykov, M. and Valencia, D.: Effects of observation uncertainty on interior parameters precision , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1755, https://doi.org/10.5194/epsc-dps2025-1755, 2025.

Large-scale exoplanet discovery via transit photometry (as exemplified by the Kepler Space Telescope) has recently enabled sophisticated population-level modeling of exoplanet parameters. Perhaps the three most relevant dimensions of exoplanet parameter space are radius, period, and mass; via just these parameters, trends in exoplanet densities, interiors, compositions and habitability can be better understood. For radius and period, the full Kepler catalog (i.e., DR25 from Lissauer et al. 2024) remains the largest, most accurate, completely homogeneous exoplanet radius-period dataset. Each data point was collected in the same way and processed through a consistent pipeline. Kepler’s well-understood biases further increase its utility for studying the true radius-period distribution of close-in exoplanets.

However, the Kepler catalog lacks direct mass measurements. Most Kepler planets are out of reach of radial velocity studies, and further analysis must be performed on Kepler light curves to glean mass information for Kepler planets. Planet–planet dynamical interactions, typically detected through Transit Timing Variations (TTVs), are easiest to interpret in systems with multiple transiting planets (“multis”). Most Kepler planets show no detectable planet-planet interactions, even in multi-planet systems and even when assuming unphysically high densities. But, in systems that do have planet-planet interactions, the best mass inference is obtained when the system is modeled “photodynamically”, i.e., leveraging information from blended light curve photometry/dynamical models. Combining the best mass inferences from photodynamical modeling with the best demographic analysis from Kepler has been limited by the fact that there are no homogeneous dynamical analyses of the entire Kepler multi population. 

 We present the Kepler Multis Dynamical Catalog (KMDC), a recently completed (by Jones et al. in preperation) photodynamical catalog containing model-fitting posteriors for ≳90% of all Kepler multis. The KMDC supports many future analyses in exoplanetary architectures, interiors, and dynamics with important implications for the formation and evolution of planetary systems. This work focuses on using the KMDC to model the true underlying mass-radius-period distribution of exoplanets. 

The KMDC enables a deeper and more accurate demographic analysis of planetary masses, radii, and densities as a function of period (and other parameters) for several reasons. First, the catalog contains mass posteriors for hundreds of previously undescribed systems. While most of the well-constrained masses are not new discoveries, previous mass measurements are a biased sample since they are often chosen by easily detectable TTVs. We remove that bias here since we modeled systems independent of mass detectability. Second, the catalog is homogenous, with data drawn only from one discovery source and run through a consistent photodynamical modeling process, reducing the biases that result from mixing heterogeneous survey, observation, and modeling methods. Finally, the catalog is nearly complete, allowing us to utilize well-understood biases of Kepler planet discovery. Even though most planets’ masses reflect our density prior (uniform from 0.01–30 g/cm3), we leverage all available Kepler data and have over 100 planets with masses measured with a precision of 25%  (see Figure 1). 

We demonstrate the ability of the KMDC to describe the population-level trends of  exoplanet characteristics. Following Foreman-Mackey et al. 2014, we are starting with a 3-dimensional mass-radius-period non-parametric grid model of planetary occurrence rates. This is effectively extending the previously-derived radius-period occurrence rate grids of Hsu et al. 2018 into the mass dimension.

Figure 1: A subsample of the KMDC mass-radius distribution for small planets with well-measured masses. The full KMDC consists of many more measurements. Some planets have unphysical densities attributable to overfitting. Using catalog–level analyses and/or physical models can improve the prior probability distribution to minimize the effect of outliers. 

We will also explore the value of parametric models, e.g., using populations of planets with different compositions, power–law occurrence rates, etc. Previous mass-radius-period parametric models have used much less data; we hope to follow these studies' work (e.g., Neil & Rogers 2020) and improve on their results with the KMDC.

How to cite: Blodgett, S., Ragozzine, D., and Jones, D.: Modeling the True Underlying Mass-Radius-Period Distribution of Exoplanets Using a Homogeneous, Kepler-Derived, Photodynamical Catalog, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1206, https://doi.org/10.5194/epsc-dps2025-1206, 2025.

Rocky exoplanets are expected to show a large diversity of planetary interiors, surface conditions, and atmospheric compositions. While all of these are connected through geological processes, direct observations are challenging even with current and upcoming ground and space based instruments, showing the importance of models combining planetary atmospheres, surfaces, and interiors.

Our modelling approach for rocky exoplanets provides a bottom-to-top surface-atmosphere model. The crust-atmosphere interaction layer is the basis of the 1D atmospheric model which includes the effects of element depletion due to cloud formation. The atmospheric and cloud composition are therefore a result of the surface composition and the pressure-temperature structure.

Modelling the variety of different gas-phase compositions reveals the existence of three distinct atmospheric types (H₂ rich, O₂ rich, or the coexistence of CH₄ and CO₂), defined by their atmospheric composition. In addition to these distinct atmospheric types, the presence of some cloud condensates provides constraints on the planetary surface temperature and pressure. Furthermore, the composition of the planetary surface mineralogy in contact with the atmosphere provides distinct transitions linked to the atmospheric types.

Investigations of various elemental compositions based on different rock compositions reveal links of surface minerals to the atmospheric types. In particular, the sulphur chemistry can be constrained. While the sulphur cloud condensates of H₂S and H₂SO₄ only form for planets with high surface pressures and/or temperatures, the sulphur-bearing condensates at the planetary surface (including especially FeS, FeS₂, and CaSO₄) are directly linked to the atmospheric type. 

This work shows that in principle, spectroscopic investigations of rocky exoplanet atmospheres can constrain the atmospheric composition to a specific atmospheric type and therefore put some constraints on the expected surface mineralogy. 

How to cite: Herbort, O. and Sereinig, L.: Using atmospheric types of rocky exoplanets to constrain planetary surfaces, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-648, https://doi.org/10.5194/epsc-dps2025-648, 2025.

The low bulk density of sub-Nepuntes suggest that these planets contain a significant amount of volatile elements, yet their precise composition remains uncertain. The two leading composition models are the gas dwarf scenario and the water world scenario. In the first scenario, sub-Neptunes are Earth-like planets surrounded by an H₂/He-dominated atmosphere. In the second scenario, sub-Neptunes form beyond the ice line and accrete several tens of percents of water as well as a few weight percent of H₂/He (Burn et al. 2024). Current evolution models predict that the evolution of the planet radius with time differs significantly between the gas dwarfs and water worlds, making it possible to distinguish between the two composition types (Aguichine et al. 2024, Rogers 2025).  However, these models assume that all water is confined to the atmosphere of the planet, overlooking the chemical exchange between the atmosphere and the molten interior.

We present a novel evolution framework that incorporates global chemical equilibrium calculations and fractionated mass loss to quantify the effects of the atmosphere-interior coupling on the radius evolution. Recent works by Werlen et al. (in prep) demonstrate that the chemical coupling between the atmosphere and interior modifies the atmospheric water mass fraction. Figure 1 below shows the water mass fraction in the envelope as a function of the total accreted water mass for planets with masses between 2 and 15 Earth masses, based on the results from Werlen et al. (in prep). The red line denotes the solar water mass fraction of 0.02. The colors represent the molar bulk C/O ratio, which can be seen as an indicator for the formation location of the planet. For water worlds that form outside the ice line as indicated by a high C/O ration and M_H2O/M_tot (accreted) > 10%, the bulk of the accreted water is sequestered into the interior of the planet. The water mass fraction in the atmosphere is thus limited to ~10%. Planets formed inside the ice line on the other hand can have high water mass fractions in the atmosphere due to the endogenic production of water.

Motivated by the results of Werlen et al. (in prep), we integrate global chemical equilibrium calculations into our planetary structure models to track the distribution of volatiles between atmosphere and interior. The updated structure models are then embedded in a thermal evolution framework that accounts for the cooling by radiation and the radiogenic heating. Figure 2 below compares the evolution of the atmosphere thickness, defined as R_transit - R_solid, between a gas dwarf with a pure H₂/He atmosphere and a water world with a realistic water distribution. In both cases, the total planet mass is 5 Earth masses, the envelope mass is 0.05 Earth masses and the equilibrium temperature is 880 K. As expected, the atmosphere thickness of the water world is smaller than the one of the gas dwarf. Nevertheless, both scenarios display similar temporal evolution trends. An important implication of our findings is that it will be challenging to distinguish between sub-Neptunes that formed inside or outside the ice line using the population of planets with different ages observed by TESS. 

Our evolution framework also incorporates fractionated mass loss driven by photoevaporation, following the approach of Valatsou et al. (in prep). The mass loss model self-consistently calculates the EUV and X-ray absorption depth (R_XUV), the sound speed at the R_XUV, and the resulting mass loss rate for atmospheres containing a mix of H₂, He, and H₂O. More importantly, the model includes the dissociation of the H₂ and H₂O into atomic hydrogen and oxygen. Due to the mass difference, hydrogen escapes more efficiently, while the oxygen escape rate is regulated by the balance between the gravitational settling and the diffusive drag from the escaping hydrogen. Valatsou et al. (in prep) find that this progress leads to significant atmospheric fractionation: oxygen is largely retained in the atmosphere, while hydrogen escapes efficiently. As a result, the atmospheres of sub-Neptunes become enriched in oxygen over time. 

Our evolution framework offers a more complete picture of the evolution of sub-Neptune by including both the chemical coupling of the atmosphere and interior, as well as the fractionated atmospheric escape. The predicted similarity in the radius evolution of gas dwarfs and water worlds, suggests that the radius-age relation alone may not be enough to distinguish between the two composition types. Instead, our framework lays the groundwork for understanding the bulk composition and formation location of sub-Neptunes based on their radius, age, and atmospheric composition.

References:

• Aguichine, A., Batalha, N., Fortney, J. J., et al. 2024, arXiv e-prints, arXiv:2412.17945
• Burn, R., Mordasini, C., Mishra, L., et al. 2024b, Nature Astronomy, 8, 463
• Rogers, J. G. 2025, MNRAS, 539, 2230
• Valatsou, M., Owen, J., Dorn, C., in prep
• Werlen, A., Dorn, C., Burn, R., Schlichting, H., Grimm, S., Young, E., in prep

How to cite: Steinmeyer, M.-L., Werlen, A., Valatsou, M., Grimm, S., Marty, P., and Dorn, C.: Water worlds or Gas Dwarfs? Coupled Interior-Atmosphere Models Blur the Lines in Sub-Neptune Radius Evolution, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1670, https://doi.org/10.5194/epsc-dps2025-1670, 2025.

Recent studies suggest that most exoplanets—or their progenitors—begin life enveloped in hydrogen.  This primordial atmosphere interacts with the planet’s interior over timescales of millions to billions of years, making atmosphere–interior coupling essential for understanding planetary formation and evolution.  Yet these processes remain poorly constrained because they occur under extreme pressures and temperatures.  To probe them, we performed computational experiments using DFT-based molecular dynamics across a vast P-T regime typical of super-Earths and sub-Neptunes. We mapped out the critical curve which demarcates regimes where a single, well-mixed hydrogen-water fluid is stable and where it splits into distinct hydrogen-rich and water-rich phases. Our critical curve agrees well with existing experimental data and shows the influence of a change in fluid structure from molecular to atomic near 30-100 GPa and 3000-4000 K.

These results not only have far-reaching consequences for water-rich planets with hydrogen atmospheres like Uranus, Neptune, K2-18 b, and TOI-270 d but also bring into question the deeply ingrained notion in our community of a sharp boundary between the interior and an overlying atmosphere. Hot and young planets should have envelopes where hydrogen and water are entirely mixed, i.e., exist as a single homogeneous phase. However, as the planet cools, its deep interior should experience phase separation of hydrogen and water: leading to a "rainfall" or rainout of water towards the deeper interior, a consequent increase in internal luminosity, and the emergence of inner and outer envelopes that are hydrogen- and water-rich, respectively, and whose compositions or metallicities should depend on the planets age and instellation. Our results thus help improve our constraints on planets that are likely to have water oceans, which future surveys could leverage. Furthermore, our findings have implications for atmosphere loss and magnetic field generation. Our work thus demonstrates the importance of better understanding atmosphere-interior interactions, especially as we enter the era of James Webb Space Telescope, PLATO, the proposed Uranian Orbiter and Probe, and other next-generation observatories.

How to cite: Gupta, A., Stixrude, L., and Schlichting, H.: The story of hydrogen and water: new insights into the interaction of planet atmospheres and interiors, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1285, https://doi.org/10.5194/epsc-dps2025-1285, 2025.

Thanks to recent advances in infrared spectroscopy, we have entered a new era of detailed atmospheric characterization of sub-Neptunes, potentially providing constraints on their hidden interiors. One possible structure of sub-Neptunes is a rocky core surrounded by a thick hydrogen-dominated atmosphere. The strong blanketing effect of these thick hydrogen-dominated atmospheres can keep the underlying rocky surfaces hot enough to melt and vaporize, leading to gas-magma interactions that may alter the atmospheric composition. Detecting such magma-derived atmospheric species could therefore provide evidence for a rocky interior. Motivated by this, atmospheric models have been developed to explore chemical interactions between hydrogen-dominated atmospheres and underlying magma oceans with various redox states. Recent models have predicted monosilane, SiH₄, as a potential atmospheric species derived from magma oceans in sub-Neptunes, but have suggested that it is highly depleted in the observable atmospheric layers. 

Here, we propose that SiH₄ can persist throughout the atmospheres of sub-Neptunes with FeO-free reduced magma oceans by considering the dissolution of H₂O into the magma oceans, a factor not accounted for in previous models. In this study, we investigate the atmospheric composition of sub-Neptunes with reduced FeO-free magma oceans using a one-dimensional atmospheric model based on the chemical equilibrium of H-/O-/Si-bearing species. This model incorporates the vaporization of SiO₂, silicate condensation, and the dissolution of H₂O into the magma ocean. Our results demonstrate that SiH₄ is produced through the reaction between SiO vaporized from magma and H₂, and that the reduction of H₂O due to dissolution shifts the equilibrium further toward SiH₄ production. We find that the effect of water dissolution enhances the atmospheric SiH₄ molar fraction to 0.1-10%, preventing its reversion to silicates in the upper atmospheric layers. Finally, we present the conditions under which large amounts of SiH₄ are produced and discuss the atmospheric spectra of sub-Neptune in such cases. Our results suggest that the detection of SiH₄ in future observations of sub-Neptunes would provide compelling evidence for the presence of a rocky core with a reduced magma ocean.

How to cite: Ito, Y., Kimura, T., Ohno, K., Fujii, Y., and Ikoma, M.: Monosilane Worlds: Sub-Neptunes with Atmospheres Shaped by Reduced Magma Oceans, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-87, https://doi.org/10.5194/epsc-dps2025-87, 2025.

In the past five years, there has been increasing interest in sub-Neptune exoplanets in the habitable zone of their stars. These planets are abundant and easier to observe via transmission spectroscopy owing to their hydrogen-rich atmospheres.  Claimed biosignature detections on the sub-Neptune K2-18 b also motivate further studies into whether their climates are suitable for life. Whether it is possible for sub-Neptunes in the traditional Earth-like habitable zone to have liquid water oceans (and be a “Hycean” planet) is debated. One possible limiting factor is the inhibition of convection in which radiative layers forming in condensing regions (due to mean molecular weight gradients) make planetary interiors too hot for oceans to condense. In this work we aim to determine the conditions under which ocean formation on a sub-Neptune can occur, assuming that the hydrogen-water envelope evolves from a hot, well-mixed initial state.

To determine the conditions under which oceans will form on a sub-Neptune, we calculate the atmospheric structure at which an interior adiabat passes through the critical point of water (for a given deep water composition). Temperature-pressure profiles warmer than this “critical profile” will form water clouds in the upper atmosphere; profiles colder than this will bubble out hydrogen from a water rich ocean state. To calculate the critical profile, we use a non-ideal Peng-Robinson equation of state to calculate the moist adiabatic gradient in adiabatic regions, and Rosseland mean opacities to calculate the temperature gradient in the deeper regions where convection is inhibited. We explore the parameter space of sub-Neptunes by varying the temperature at 1 bar, the internal heat flux and the background composition. 

We find that the critical ocean-forming state must have an extremely cold upper atmosphere if there are convectively inhibited layers. At 1 bar, the temperature in most simulations is <100 K (Figure 1), which would correspond with orbital distances well beyond the traditional habitable zone. At the upper end of this temperature range, a high internal water content approaching 100% is required for ocean formation. With reduced internal heat fluxes (<0.01 W/m^2), the presence of radiative layers depends on the composition of the atmosphere, with the inclusion of high-opacity greenhouse gases such as CH4 and CO2 sustaining high temperature gradients at lower internal heat fluxes. Our results suggest that the current population of detected sub-Neptunes are unlikely to host liquid water oceans.

Figure 1: A range of temperature-pressure profiles which pass through the critical point of water for a stated internal water content (annotated on lines). For the given internal water content, this profile represents the state in which any colder profile can form a liquid water ocean. The critical curve of water is shown in black, and regions with convective inhibition are highlighted with thicker lines. For all deep internal water contents, the atmosphere at 1 bar must be extremely cold for liquid water oceans to form.

How to cite: Innes, H. and Cano Amoros, M.: Hycean worlds can only form under extremely cold and water-rich conditions, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1585, https://doi.org/10.5194/epsc-dps2025-1585, 2025.

Atmospheres of hot sub-Neptunes are being characterised by the James Webb Space Telescope (JWST). Recent modelling efforts suggest that the magma-atmosphere coupling of hot sub-Neptunes strongly impacts their atmospheric chemistry, enriching the atmosphere with gases formed from magma-bearing elements, e.g., SiO, SiH4. We implement an equilibrium chemistry reaction network at the magma-atmosphere boundary with a new Python-based code, Atmodeller, to simulate gas-gas reactions, magma-atmosphere reactions, gas solubility in magma and real gas behaviour. We demonstrate significant effects of ideal gas versus real gas assumptions and gas solubility in magma for sub-Neptunes. We also advocate for more laboratory experiments to be conducted to understand the chemical composition of sub-Neptunes. These populations should exhibit distinct atmospheric signatures that are potentially detectable with JWST now and with ARIEL in the future. 

How to cite: Hakim, K., Bower, D. J., Sossi, P., and Seidler, F.: Influence of Magma on the Atmospheric Chemical Abundances of Hot Sub-Neptunes, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1598, https://doi.org/10.5194/epsc-dps2025-1598, 2025.

The discovery of terrestrial exoplanets within the Earth-to-super-Earth mass domain led to the question of whether these bodies developed atmospheres and what kind of atmospheres surround them. Observations at some planets within the above-mentioned mass domain revealed the existence of a large population of planets with less than three Earth masses that possess huge hydrogen-dominated primordial atmospheres, which must be remnants from the protoplanetary gas disk. These discoveries stimulate questions about whether planets within the Earth-mass domain could be within the habitable zone of solar-like stars that could have accreted and kept hydrogen- and/or helium-dominated primordial atmospheres. Since the gas disk consists mainly of these elements, we study the possible enrichment of the primordial helium fraction in hydrodynamically escaping initially hydrogen-dominated atmospheres of terrestrial planets between 0.75 to 3.0 Earth masses inside the habitable zones of Sun-like G-type host stars, considering different stellar evolutionary tracks of their high-energy emission. Depending on the planet’s accreted mass during the gas disk phase and the stellar radiation, we show that through hydrodynamic escape, Earth-mass planets inside the HZ of solar-like stars with masses between about 0.95 and 1.25 Earth masses can end with helium-dominated primordial atmospheres.
Our finding has important implications for the evolution of "Earth-like Habitats", as these thick helium-enriched primordial atmospheres can inhibit the habitability of terrestrial planets. From our results we conclude that thoroughly understanding the complex interplay between a rocky planet's accretion speed and the lifetime of the gas disk, the accumulation of primordial atmospheres, and the evolution of the extreme ultraviolet radiation of a planet's host star is key to understanding how terrestrial planets can develop later into "Earth-like Habitats" where nitrogen-oxygen-dominated secondary atmospheres can evolve. The upcoming generation of giant telescopes, such as the Extremely Large Telescope, may enable us to observe and explore these atmospheres.

How to cite: Lammer, H., Scherf, M., Erkaev, N. V., Kubyshkina, D., Gorbunova, K. D., Fossati, L., and Woitke, P.: Earth-mass planets with primordial hydrogen and helium atmospheres in the habitable zone of solar-like stars, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-505, https://doi.org/10.5194/epsc-dps2025-505, 2025.