Europlanet Science Congress 2021
Virtual meeting
13 – 24 September 2021
Europlanet Science Congress 2021
Virtual meeting
13 September – 24 September 2021
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Exoplanet observations, modelling and experiments: Characterization of their atmospheres

The field of extrasolar planets is one of the most rapidly changing areas of astrophysics and planetary science. Ground-based surveys and dedicated space missions have already discovered more than 4000 planets with many more detections expected in the near future. A key challenge is now the characterisation of their atmospheres in order to answer to the questions: what are these worlds actually like and what processes govern their formation and evolution?

To answer these questions, a broad range of skills and expertise are required, stretching from Solar System science to statistical astrophysics, from ground-based observations to spacecraft measurements, and atmospheric/interior/orbital modelling. The numerous studies conducted in the past twenty years have unveiled a large diversity of atmospheres, from ultra-hot Jupiters to temperate super-Earths. The next generation of space and ground based facilities, including current high-resolution instruments (e.g. ESPRESSO, Spirou, CHEOPS, IGRINS, E-ELT, JWST, and Ariel) will characterise this multifarious population in stunning detail and challenge our current understanding. Both theoretical works and experimental measurements are required to prepare for such a change of scale.

This session will focus on the atmospheric characterisation of exoplanets and the conveners welcome any abstract related to this subject.

This session will also address results on all aspects of plasma physics and interactions of solar and stellar wind interactions with planets and exoplanets.

Convener: Olivia Venot | Co-conveners: Monika Lendl, Giuseppe Morello, Vivien Parmentier, Ingo Waldmann, Corentin Louis, Nicolas André

Wed, 22 Sep, 10:40–11:25

Artyom Aguichine et al.

Water-rich planets should be ubiquitous in the universe. Among the current exoplanet populations, many of those worlds are subject to important irradiation from their host star. As a consequence, water-rich worlds display supercritical water layer surrounded by an extended steam atmosphere making them good candidates for matching the observed mass-radius distribution of sub-Neptunes [1]. Here we describe a model that computes a realistic structure for water-rich planets by combining an interior model with an updated equation of state (EoS) for water, and an atmospheric model that takes into account radiative transfer. Our model has been applied to the GJ 9827 system as a test case and indicates Earth- or Venus-like interiors for planets b and c, respectively. Planet d could be an irradiated ocean planet with a water mass fraction of ∼20 ± 10%. We also provide mass-radius relationships for water-rich planets and their analytical expression. This allows one to directly retrieve a wide range of planetary compositions, without the requirement to run the model. The possible existence of such planets is discussed in light of atmospheric loss processes, suggesting that some sub-Neptunes are the outcome of planets that lost their H/He reservoirs.

Figure 1. Mass-radius relationships produced by our model (green, yellow and red thick lines) [2], compared to mass-radius relationships of planets with only condensed phases and no atmosphere (black, grey and light blue thin lines) [3,4]. A few planets of the solar system, the GJ-9827 system and the TOI-178 system are shown as well [5,6].

[1] Mousis, O., Deleuil, M., Aguichine, A., et al. 2020, ApJL, 896, L22.
[2] Aguichine, A., Mousis, O., Deleuil, M., et al. 2021, accepted in ApJ.
[3] Zeng, L., Sasselov, D. D., & Jacobsen, S. B. 2016, ApJ, 819, 127.
[4] Brugger, B., Mousis, O., Deleuil, M., et al. 2017, ApJ, 850, 93.
[5] Kosiarek, M. R., Berardo, D. A., Crossfield, I. J. M., et al. 2021, AJ, 161, 47.
[6] Leleu, A., Alibert, Y., Hara, N. C., et al. 2021, A&A, 649, A26.

How to cite: Aguichine, A., Mousis, O., Deleuil, M., and Marcq, E.: Modeling the structure of irradiated ocean planets - implications for mass-radius relationships, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-27,, 2021.

Lorena Acuña et al.

The increasing number of well characterised low-mass planets, combined with the valuable informations from stellar and planetary spectroscopy, opens the way to the modeling of planetary structures and compositions, which can be obtained with theoretical and numerical works. This approach gives a valuable insight to understand the formation of planetary systems in the low-mass range. We present a 1D planetary model where the interior is coupled with the atmosphere in radiative-convective equilibirum within a Bayesian retrieval scheme. In addition to a Fe core and a silicate mantle, we take into account water in all its possible phases, including steam and supercritical phases, which is necessary for systems with a wide range of stellar irradiations. 

Our interior-atmosphere model calculates the compositional and atmospheric parameters, such as Fe and water content, surface pressures, scale heights and albedos. We analyse the multiplanetary systems K2-138 and TRAPPIST-1, which present six low-mass planets with different densities and irradiations. From the individual composition of their planets, we derive a similar trend for both systems: a global increase on the water content with increasing distance from the star in the inner region of the systems, while the planets in the outer region present a constant water mass fraction. This trend reveals the possible effects of migration, formation location and atmospheric mass loss during their formation history.

How to cite: Acuña, L., Deleuil, M., Mousis, O., López, T. A., Morel, T., Santerne, A., and Marcq, E.: Characterising the interior structures and atmospheres of multiplanetary systems., Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-44,, 2021.

Tim Lichtenberg

Internal redox reactions may irreversibly alter the mantle composition and volatile inventory of terrestrial and super-Earth exoplanets and affect the prospects for atmospheric observations. The global efficacy of these mechanisms, however, hinges on the transfer of reduced iron from the molten silicate mantle to the metal core. Scaling analysis indicates that turbulent diffusion in the internal magma oceans of sub- Neptunes can kinetically entrain liquid iron droplets and quench core formation. This suggests that the chemical equilibration between core, mantle, and atmosphere may be energetically limited by convective overturn in the magma flow. Hence, molten super-Earths possibly retain a compositional memory of their accretion path. Redox control by magma ocean circulation is positively correlated with planetary heat flow, internal gravity, and planet size. The presence and speciation of remanent atmospheres, surface mineralogy, and core mass fraction of atmosphere-stripped exoplanets may thus constrain magma ocean dynamics.

How to cite: Lichtenberg, T.: Redox hysteresis of super-Earth exoplanets from magma ocean circulation , Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-131,, 2021.

Philipp Baumeister et al.

Liquid water is generally assumed to be the most important factor for the emergence of life, and so a major goal in exoplanet science is the search for planets with water oceans. On terrestrial planets, the silicate mantle is a large source of water, which can be outgassed into the atmosphere via volcanism. Outgassing is subject to a series of feedback processes between atmosphere and interior, which continually shape both atmospheric composition, pressure, and temperature, as well as interior dynamics. For example, water has a high solubility in surface lava, which can strongly limit its outgassing into the atmosphere even at low atmospheric pressures. In contrast, CO2 can be easily outgassed. This drives up the surface pressure and temperature, potentially preventing further water outgassing [1].

We present the results of an extensive parameter study, where we use a newly developed 1D numerical model to simulate the coupled evolution of the atmosphere and interior of terrestrial exoplanets up to 5 Earth masses around Sun-like stars, with internal structures ranging from Moon- to Mercury-like. The model accounts for the main mechanisms controlling the global-scale, long-term evolution of stagnant-lid rocky planets (i.e. bodies without plate tectonics), and it includes a large number of atmosphere-interior feedback processes, such as a CO2 weathering cycle, volcanic outgassing based on the pressure-dependent solubility of volatiles in surface lava, a water cycle between ocean and atmosphere, greenhouse heating, as well as the influence of a primordial H2 atmosphere, which can be lost through escape processes. While many atmosphere-interior feedback processes have been studied before in detail (e.g. [2, 3]), we present here a comprehensive model combining the important planetary processes across a wide range of terrestrial planets.

We find that a significant majority of high-density exoplanets (i.e. Mercury-like planets with large cores) are able to outgas and sustain water on their surface. In contrast, most planets with intermediate, Earth-like densities either transition into a runaway greenhouse regime due to strong CO2 outgassing, or retain part of their primordial atmosphere, which prevents water from being outgassed. This suggests that high-density planets could be the most promising targets when searching for suitable candidates for hosting liquid water. Furthermore, the degeneracy of the interior structures of high-density planets is limited compared to that of planets with Earth-like density, which further facilitates the characterization of these bodies, and our results predict largely uniform atmospheric compositions across the range of high-density planets, which could be verified by future spectroscopic measurements.



[1] Tosi, N. et al. The habitability of a stagnant-lid earth. A&A 605, A71 (2017).

[2] Noack, L., Rivoldini, A. & Van Hoolst, T. Volcanism and outgassing of stagnant-lid planets: Implications for the habitable zone. Physics of the Earth and Planetary Interiors 269, 40–57 (2017).

[3] Foley, B. J. & Smye, A. J. Carbon Cycling and Habitability of Earth-Sized Stagnant Lid Planets. Astrobiology 18, 873–896 (2018).

How to cite: Baumeister, P., Tosi, N., MacKenzie, J., and Grenfell, J. L.: Abundance of water oceans on high-density exoplanets from coupled interior-atmosphere modeling, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-270,, 2021.

Tobias G. Meier et al.

The vigour and style of mantle convection in tidally-locked super-Earths may be substantially different from Earth's regime where the surface temperature is spatially uniform and sufficiently cold to drive downwellings into the mantle.
The thermal phase curve for super-Earth LHS 3844b suggests a solid surface and lack of a substantial atmosphere. The dayside temperature is around 1040 K and the nightside temperature is around 0 K, which is unlike any temperature contrast observed at present day for planets in the Solar System. On the other hand, the thermal phase curve of super-Earth 55 Cnc e suggests much hotter temperatures with a nightside temperature around 1380 K and a substellar point temperature around 2700 K. The substellar point is also substantially shifted eastwards, which requires efficient energy circulation in the atmosphere.
Here, we use constraints from thermal phase curve observations to model the interior mantle flow. To constrain the surface temperature of  55 Cnc e, we use the results from general circulation models varying the atmospheric composition and optical depth.
Depending on how strong the surface temperature contrast is, this can lead to hemispheric tectonic regimes. Such a regime could influence a planet's atmosphere through interior-exterior coupling mechanisms (e.g. volcanic outgassing).  

We run geodynamic simulations of the interior mantle flow using the mantle convection code StagYY. The models are fully compressible with an Arrhenius-type viscosity law where the mantle is modelled with an upper mantle, a perovskite-layer and a post-perovskite layer. The lithospheric strength is modelled through a plastic yielding criteria and the heating mode is either basal heating only or mixed heating (basal and internal heating). 
For LHS 3844b we find that the surface temperature dichotomy can lead to a hemispheric tectonic regime depending on the strength of the lithosphere and the heating mode in the mantle. In a hemispheric tectonic regime, downwellings occur preferentially on one side and upwellings rise on the other side (Fig. A). We compare these results to the case of 55 Cnc e, where large parts of the surface could be molten. At first order we expect that a magma ocean could homogenise the temperatures at the interface between the magma ocean and the underlying solid mantle and therefore reduce the likelihood of hemispheric tectonics operating on 55 Cnc e (Fig. B).

For LHS 3844b, the contribution of the interior flux to the thermal phase curve is on the order of 15-30 K, and therefore below the detecting capabilities of current and near-future observations. However, for hemispheric tectonics, upwellings might lead to preferential melt generation and outgassing on one hemisphere that could manifest as a secondary signal in phase curve observations. Such signals could also be produced on hotter planets such as 55 Cnc e where parts of the surface are hot enough to melt. 

How to cite: Meier, T. G., Bower, D. J., Lichtenberg, T., Tackley, P. J., and Demory, B.-O.: Exploring the convection in super-Earths: Comparing LHS 3844b with 55 Cnc e, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-500,, 2021.

Vivian Adhiambo et al.

Earth has been the only known habitable world and thus used as a reference to understand habitability. The origin of life on Earth is not yet clearly understood, but known traces are up-to the Archean (∼ 3.5Ga, Ga-billion years). Earth had water and continents from the Hadean Earth (> 4.0Ga), which had different atmospheric conditions compared to the Archean Earth. Similarly, the current state and composition of atmosphere does not represent its future state. Climate changes are partly attributed to feedback mechanism between the internal processes and the atmosphere. And as such, each atmospheric state is depictive of an instance a long a trajectory path of a coupled evolution of Earth system. Venus was thought to be habitable until into the 1960s, when its surface was observed to be oven-hot with surface pressure a hundred times that of Earth. Why and when the evolutionary paths of Venus and Earth, which are similarly sized and should have similar internal compositions, started to diverge? Moreover, known exoplanets, planets and moons have very different geophysical characteristic from Earth. This implies exotic life might vary substantially from what we know. As a result, understanding evolution of rocky planets, that is their interior structure, atmospheres and climate regardless of their habitability is of great importance. In this work we study the relation between a rocky planet’s internal properties and its observable surface and atmosphere properties over time.   We explore the different convection regimes (stagnant lid, episodic-lid and tectonic), studying the relation between a planet’s viscous state, its interior composition and structure. Focusing on the effects of mantle convection on volatile recycling processes such as CO2 outgassing that influence the atmospheric state and climatic conditions over time. The computed models are then used to compute observables, that ultimately can be tested with observations.

How to cite: Adhiambo, V., Root, B., and Desert, J.-M.: The interior-atmosphere coupling of rocky worlds, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-664,, 2021.

Hugo Vivien et al.

This work aims to determine the mass-radius rela-tionships of highly irradiated (500< Tirr<2000K)small planets (0.2<M<2.3M⊕) with water con-tents up to 5%. To do so, we coupled an internalmodel of small terrestrial planets (Brugger et al.,2017) to the atmosphere model elaborated by Marcqet al. (2017, 2019), following the approach depictedin Aguichine et al. (2021) and Mousis et al. (2020).

We show that these planets, even with smallwater contents, can become strongly inflated andproduce large radii for small masses.We alsoshow that strongly irradiated small planets cannotsustain their atmospheres due to the lack of hy-drostatic stability, implying they cannot preserveany hydrosphere. The temperature and the watermass fraction are the key parameters controllingthe extent of inflation and the thickness of thesupercritical layer. An important amount of wateralso leads to the contraction of the rocky interior.However, the composition of the rocky interioronly has a limited impact on the final mass-radiusrelationship, and barely impacts the behavior of thehydrosphere.

How to cite: Vivien, H., Aguichine, A., Mousis, O., Deleuil, M., and Marcq, E.: Mass-Radius relationships of small, highly irradiated exoplanets with small water mass fractions, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-249,, 2021.

Lorenzo Pino et al.

Ultra-hot Jupiters (UHJs; Teq ≥ 2500 K) are the hottest gaseous giants known. They emerged as ideal laboratories to test theories of atmospheric structure and its link to planet formation. Indeed, because of their high temperatures, (1) they likely host atmospheres in chemical equilibrium and (2) clouds do not form in their day-side. Their continuum, which can be measured with space-facilities, can be mostly attributed to H- opacity, an indicator of metallicity. From the ground, the high spectral resolution emission spectra of UHJs contains thousands of lines of refractory (Fe, Ti, TiO, …) and volatile species (OH, CO, …), whose combined atmospheric abundances could track planet formation history in a unique way. In this talk, we take a deeper look to the optical emission spectrum of KELT-9b covering planetary phases 0.25 - 0.75 (i.e. between secondary eclipse and quadrature), and search for the effect of atmospheric dynamics and three-dimensionality of the planet atmosphere on the resolved line profiles, in the context of a consolidated statistical framework. We discuss the suitability of the traditionally adopted 1D models to interprete phase-resolved observations of ultra-hot Jupiters, and the potential of this kind of observations to probe their 3D atmospheric structure and dynamics. Ultimately, understanding which factors affect the line-shape in UHJs will also lead to more accurate and more precise abundance measurements, opening a new window on exoplanet formation and evolution.

How to cite: Pino, L., Brogi, M., Désert, J.-M., and Rauscher, E.: Searching for 3D effects in the optical, high spectral resolution emission spectrum of KELT-9b, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-308,, 2021.

Monika Stangret et al.

Ultra-hot Jupiters are defined as giant planets with equilibrium temperatures larger than 2000 K. Most of them are found orbiting bright A-F stars, making them extremely suitable object to study their atmospheres using high-resolution spectroscopy.

TOI-1431b, also known as MASCARA-5b, a newly discovered planet with the temperature of 2375 K is a prefect example of ultra-hot Jupiter. We studied this object using three transit observations obtained with high-resolution spectrographs HARPS-N and EXPRES. Analysis of Rossiter-McLaughlin effect shows that the planet is in the polar orbit, which speaks about an interesting dynamical history, and perhaps indicating the presence of more than one planet in the early history of this system. Applying the cross-correlation and transmission spectroscopy method, we find no evidence of atoms and molecules in this planet. There results are at odds with the other studies of similar UHJs orbiting bright stars, where various species have been found.

How to cite: Stangret, M., Palle, E., Casasayas-Barris, N., and Oshagh, M.: Studies of the atmosphere of ultra-hot Jupiter TOI-1431b/MASCARA-5b, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-511,, 2021.

Rico Landman et al.

Ultra-hot Jupiters have dayside temperatures similar to those of M-dwarfs. While molecular absorption from the hydroxyl radical (OH) is easily observed in near-infrared spectra of M-dwarfs, it is often not considered when studying the atmospheres of (ultra-)hot Jupiters. We use high-resolution spectroscopic near-infrared observations of a transit of WASP-76b obtained using CARMENES to assess the presence of OH. After validating the OH line list, we generate model transit spectra of WASP-76b with petitRADTRANS. The data are corrected for telluric contamination and cross-correlated with the model spectra. After combining all cross-correlation functions from the transit, a detection map is constructed. OH is detected in the atmosphere of WASP-76b with a signal-to-noise ratio of 6.1. From a Markov Chain Monte Carlo retrieval we obtain Kp=234 km/s and a blueshift of 13.9 km/s. Considering the fast spin-rotation of the planet, the OH signal is best explained with the signal mainly originating from the evening terminator and the presence of a strong day- to nightside wind. The signal appears to be broad, with a full width at half maximum of 16.2 km/s. The retrieval results in a weak constraint on the temperature of 2420-3150 K at the pressure of the OH signal. Our results demonstrate that OH is readily observable in the transit spectra of ultra-hot Jupiters. Studying this molecule can give new insights in the molecular dissociation processes in the atmospheres of such planets.

How to cite: Landman, R., Sánchez-López, A., Mollière, P., Kesseli, A., Louca, A., and Snellen, I.: Detection of OH at the evening terminator of the ultra-hot Jupiter WASP-76b, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-222,, 2021.

Alejandro Sánchez López et al.

Extreme temperature contrasts between the day and nightside of ultra-hot Jupiters (UHJ) result in significantly asymmetric atmospheres, with a region of extreme atmospheric expansion appearing over a small range of latitudes around the terminator. Over the course of a transit, WASP-76 b rotates by about 30° and hence temporal variations of the observable atmosphere could significantly affect the detectability of its constituents. Specifically, the trailing limb of this planet allows us to probe a significant portion of the inflated dayside, resulting in a higher atmospheric detectability. This geometric effect could mimic the observed time-variability of absorption signals due to condensation in the nightside of these planets, which has been recently reported for neutral iron in WASP-76 b. By studying molecules that are not expected to condense in the nightside of UHJs (~1000K), we can isolate the possible effect of different day and nightside scale heights. Here, we will analyze a stronger water vapor signal during the egress of the planet than at ingress, which cannot be explained by condensation and suggests that the extreme geometry of UHJ manifests itself as time-dependent absorption signals. Additionally, we report a redshifted HCN signature arising from the leading limb (i.e., observable in the first half of the transit and absent from the second half) and a weak evidence of ammonia using high-resolution observations of WASP-76 b with CARMENES.

How to cite: Sánchez López, A., Landman, R., Casasayas Barris, N., Kesseli, A., and Snellen, I.: Spatial characterization of the trailing and leading limbs of WASP-76b: Detection of H2O and HCN at high-resolution, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-563,, 2021.

Núria Casasayas-Barris et al.

Currently, one of the most used techniques to study the atmosphere of the exoplanets is transmission spectroscopy by means of high-resolution facilities (R > 105). This methodology has led to the detection of several species in the atmosphere of exoplanets, showing that ultra-hot Jupiters (Teq > 2000 K) are one of the most intriguing exoplanets, possessing the richest atmospheres measured to date. Here, using two transit observations with the high-resolution spectrograph CARMENES, we study the atmosphere of one of the most famous ultra-hot Jupiters: WASP-76b. We take advantage of the redder wavelength coverage of CARMENES, in comparison with the facilities used in previous studies of this same planet, and focus our analysis on the CaII IRT triplet at 850nm and the metastable HeI triplet at 1083nm. In line with recent studies, we detect ionised calcium in the atmosphere of WASP-76b and, additionally, find possible evidence of HeI. We contextualise our findings with previous atmospheric studies of other ultra-hot Jupiters and, in particular, with those showing the presence of CaII and HeI absorption in their transmission spectrum. We show that this planet is a potential candidate for further follow up studies of the HeI lines using high-resolution spectrographs located at larger telescopes, such as CRIRES+.

How to cite: Casasayas-Barris, N., Orell-Miquel, J., Stangret, M., Nortmann, L., Yan, F., Palle, E., Snellen, I., Oshagh, M., and Sánchez-Lopez, A.: The atmosphere of WASP-76b seen with CARMENES: looking for CaII IRT and HeI, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-622,, 2021.

Bob Jacobs et al.
The interpretation of general trends in exoplanet atmospheres is challenging because they exhibit a wide range of diverse properties in terms of composition, structure, and overall atmospheric physics. In this context exoplanet’s in extreme regimes can help to understand global planetary properties.
In this project, we focus on a few exoplanets that are outliers in their atmospheric properties and discuss what we can learn about the overall population of hot-jupiters from these peculiar objects.  We present studies of exo-atmospheric processes in extreme regimes of temperature, of entropy, of radiative and advective timescales, and of formation stages. This project combines these various physical properties in a unique and innovative manner to understand the most crucial properties of hot-Jupiters.

Practically, we leverage the unique capabilities of Hubble Space Telescope Wide Field Camera 3 together with novel data analysis techniques to understand the nature of a set of exoplanets that reside under these extreme conditions. Ultimately, this project enable us to improve our understanding of exo-atmospheric processes and planet formation that ultimately shape the atmospheres of hot Jupiters that are observed today. 

How to cite: Jacobs, B., Désert, J.-M., Barat, S., Line, M., and Pino, L.: Extreme exoplanets as a tool to understand trends in exoplanets atmospheres., Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-775,, 2021.