EXOA2
This session aims to: (1) share recent results from the latest JWST observations on exoplanetary atmospheres and future observational strategies, (2) present results from the latest state-of-the-art atmospheric models and retrievals, and discuss future development needed, (3) highlight experimental results on atmospheric studies and their complementarity to models and observations, and (4) improve synergies between these different approaches.
The James Webb Space Telescope’s unprecedented precision and wavelength coverage have already led to many new discoveries in the field of exoplanets. However, this increased precision means contamination of the data by the host star's stellar activity is also observed more clearly. This phenomenon is highly variable and can introduce false positives of molecular features, such as those of water, which is also found in sunspots. This makes optical transmission spectra more important than ever, as their coverage of bluer wavelengths can not only give a measure of the amount of activity going on, but also distinguish between different types of activity such as spots and faculae, which are degenerate in photometric monitoring. In this talk I will present an optical transmission spectrum of the hot Jupiter WASP-69 b taken with the FORS2 optical spectrograph on the VLT. I will identify the signatures of spots and faculae, which would not be distinguishable with JWST data alone, but are nonetheless needed to distinguish between molecule detections in the planet atmosphere and on the stellar surface.
How to cite: Petit dit de la Roche, D. and Lendl, M.: The importance of ground-based transmission spectra in characterising stellar activity for JWST, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-35, https://doi.org/10.5194/epsc2024-35, 2024.
The extra-solar planet WASP-18 b is placed on a very-short period (< 1 day) orbit around a F6V-type star (Teff = 6300 K), which results in a planetary dayside heated up to 3000 K. Planets with such extreme thermal conditions are called ultra-hot Jupiters (UHJs) and exhibit strong day-to-night temperature contrasts with atmospheric compositions dominated by dissociated molecules and ions. These properties make UHJs such as WASP-18 b particularly interesting for atmospheric characterisation all along their orbital phase angles through observations known as phase curves.
We present our work based on the analysis of phase-curve and occultation observations with the space telescopes CHEOPS, TESS and Spitzer, including unpublished data from all three instruments. The data sets span a temporal range of 15 years and a spectral range from the visible to the mid-infrared. We model and constrain the ellipsoidal variations and Doppler boosting, obtain unprecedented precision on the ephemerides, and derive an absolute planetary radius with a precision of 0.65%, or 550 km. We include the recently published JWST data (Coulombe et al. 2023) in our atmospheric retrieval, from which we derive constraints on the structure and composition of the atmosphere of WASP-18 b. We find a thermally inverted profile with a steep temperature gradient to explain the brightness temperature in Spitzer/IRAC/Channel 2 passband. We also detect a flux excess in the CHEOPS passband (visible) that allows us to assess the presence of reflected light and compute the first constained value of the geometric albedo for this planet.
How to cite: Deline, A. and the other co-authors (incl. L. Kreidberg and the CHEOPS consortium): Reflected light and dayside emission from the phase curves of the ultra-hot Jupiter WASP-18 b , Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-217, https://doi.org/10.5194/epsc2024-217, 2024.
WASP-121b has been established as a benchmark Ultra-Hot Jupiter (UHJ), serving as a laboratory for the atmospheric chemistry and dynamics of strongly irradiated extrasolar gas giants. Here, we present and analyze WASP-121b's transmission spectrum observed with NIRSpec G395H onboard the James Webb Space Telescope (JWST) and find evidence for the thermal dissociation of H2O and H2 on the planet's permanent dayside. This finding demonstrates the importance of the thermo-chemical heterogeneity within the planet's atmosphere for the observed transmission spectrum which simultaneously probes the planet's dayside and nightside hemispheres. Additionally, we detect SiO in agreement with chemical equilibrium. Constraining the abundance of SiO and abundance ratios between Si and other atoms in WASP-121b could help discriminate between possible formation scenarios for the planet. The three-dimensional nature of thermal dissociation in WASP-121b, however, poses a challenge to constrain molecular abundances and abundance ratios from the transmission spectrum. Atmospheric models capable of grasping the thermo-chemical heterogeneity in WASP-121b's atmosphere driven by thermal dissociation on the dayside and recombination on the nightside are thus needed to explore the planet's chemical inventory.
Fig. 1: WASP-121b's transmission spectrum observed using JWST/NIRSpec G395H and the model transmission spectra presented by Pluriel et al. (2020). Upper panel: The grey dots show the original data and the black dots show the data binned in wavelength. The model spectra presented by Pluriel et al. (2020) with varying degrees of molecular dissociation are plotted using lines. A wavelength-independent offset was added to the model spectra to fit the mean of the data between 2.9 µm and 3.7 µm. Lower panel: The residuals between the model spectra with added constant and the data are depicted using colored dots with the errorbars of the data propagated onto the residuals. A grey, dashed, horizontal line at 0 was added to guide the eye.
Reference: Pluriel, W., Zingales, T., Leconte, J., & Parmentier, V. 2020, A&A, 636, A66, doi: 10.1051/0004-6361/202037678
How to cite: Gapp, C., Evans-Soma, T. M., Barstow, J. K., Lothringer, J. D., Sing, D. K., Christie, D., Kreidberg, L., and Mayne, N. J.: WASP-121b's transmission spectrum observed with JWST/NIRSpec G395H reveals thermal dissociation and SiO in the atmosphere, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-191, https://doi.org/10.5194/epsc2024-191, 2024.
GJ-1214b, discovered by Charbonneau et al. (2009), is a transiting super-Earth/mini-Neptune with an M4.5-type host star at 14.6pc from the Sun. GJ-1214b is notable for its featureless transmission spectrum (Gillon et al., 2014; Kreidberg et al., 2014), which has been interpreted as evidence for the presence of clouds (ZnS, KCL) or photochemical hazes (Morley et al. 2013, 2015; Charnay et al. 2015; Gao and Benneke 2018; Ohno & Okuzumi 2018, Adam et al. 2019; Kawasima et al. 2019; Lavvas et al. 2019; Ohno et al. 2020; Christie et al. 2022). Latest observations of GJ 1214 b with JWST reveal a high-albedo atmosphere (Kempton et al. 2023), possibly rich in photochemical hazes (Gao et al. 2023).
We use a self-consistent model coupling among photochemistry, radiative transfer, thermal structure and haze & cloud microphysics to characterise the atmosphere of GJ1214b. We explore different metallicity cases to constrain the atmospheric composition and evaluate different scenarios for the photochemical haze mass fluxes and the eddy mixing efficient. We particularly focus on the interplay between photochemical hazes and clouds, considering the formation of ZnS, KCL and NaCL clouds on hazes.
We demonstrate that the novel JWST thermal emission observations provide much better constraints on the atmospheric metallicity compared to transit spectra alone, limiting the retrieved metallicity to values between ~1000x-3000x solar. Our results demonstrate that the atmospheric albedo is in the order of 10-20%, considering both the contributions of hazes and clouds. Transit spectra are mostly affected by hazes rather than clouds, but the implications of the clouds on the atmospheric thermal structure provide an indirect effect to affect the transit signature. We further discuss the role of sulfur chemistry in the formation of the haze and its potential signature in the transit spectra.
How to cite: Lavvas, P., Paraskevaidou, S., and Arfaux, A.: Characterising the atmosphere of GJ1214 b with JWST observations., Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-574, https://doi.org/10.5194/epsc2024-574, 2024.
Almost all of the detected hot Jupiters present clouds and hazes in their atmospheres. These ensembles of aerosols play an important role during primary transits blocking the stellar flux transmitted through the outer atmosphere of exoplanets, thus having an important influence on transmitted planetary spectra.
Transmission spectra are very sensitive to the presence of clouds in the exoplanet atmospheres (Fortney, 2005). Depending on their size, aerosols can leave smooth effects on the observed spectra. Sometimes, cloud opacity can hide the spectral signature of different molecules that would be otherwise imprinted in the signal. When clouds become optically thick, they block all the flux that would cross the levels below them, similarly to a solid surface (Benneke & Seager, 2012). Thus, a good understanding of the properties of the aerosols in exoplanets is essential to understand their gaseous envelopes.
Prior to James Webb Space Telescope (JWST) observations, spectral range was relatively limited and only combining different data sources, such as Hubble and Spitzer, it was possible to cover a big enough spectral range to reveal the smooth aerosol spectral signatures. For simplicity, most of the previous studies assumed flat or Rayleigh-like cloud opacities. Figure 1 allows checking that these simplifications were valid enough for the spectral ranges covered by the Hubble Space Telescope (HST). Nevertheless, recent JWST/NIRSpec data with its wider spectral range are a game changer allowing disentanglement of gaseous absorptions and cloud extinctions.
Figure 1: Simulated extinction for spherical aerosols of different radius distributions all over the complete spectral range of JWST/NIRSpec-PRISM. HST/STIS, HST/WFC3 and Spitzer/IRAC spectral ranges are delimited by grey background for comparisons. Note how some combinations of sizes and instruments would be indistinguishable from a flat extinction curve.
In this work, we have studied the cloud properties of planet WASP-39b using the spectra of the primary transit obtained with JWST/NIRSpec (Rustamkulov et al., 2023). To do so, we have implemented a nested-sampling Bayesian approach, which has become popular for the atmospheric retrievals during the last decade (Benneke & Seager, 2012; Fisher & Heng, 2018). This framework does not just allow fitting the parameters of a certain model, but it also allows selecting which is the best model for fitting the data observed among a set, each one with a variety of atmospheric parameters and assumptions through Bayesian evidence.
We tested a number of vertical distribution parameterisations found in the literature (Barstow, 2020). Following the Occam’s razor reasoning, for Bayesian evidences being equal, we favor models with the lesser number of free parameters. In our study, we find that a model with a semi-infinite bottom cloud and a clear atmosphere above is the best option, while there are many similarities between the retrieved parameters for all cases (e.g., similar cloud top pressures).
Then, we have studied the effect that the aerosol extinction shape has on the observed spectra. We again tested a number of extinction models and compared their Bayesian evidences. In this case, in spite of all of them achieving good fits (see figure 2), realistic simulations of a complex cloud extinction dependence with wavelength are preferred versus models assuming simpler wavelength dependencies. Extinction produced by big sized aerosols with an opacity growing with wavelength fits better the observed data and it seems to be a robust conclusion from different competing models.
Figure 2: Best fitted spectra for some cloud extinction models. Observational data are shown as black/grey dots with error bars.
These results also have an impact on the retrieved molecular abundances (see figure 3) suggesting possible overestimations of some chemical species or even calling into question the detection of some of them, as proposed in Lueber et al., 2024.
Figure 3: Values retrieved for some atmospheric parameters when using different models relative to a flat model retrieval.
References
Barstow, J. K. 2020, MNRAS, 497, 4183
Benneke, B. & Seager, S. 2012, ApJ, 753, 100
Fisher, C. & Heng, K. 2018, Monthly Notices of the Royal Astronomical Society, 481, 4698
Fortney, J. J. 2005, Monthly Notices of the Royal Astronomical Society, 364, 649
Lueber, A., Novais, A., Fisher, C. and Heng, K., arXiv:2405.02656.
Rustamkulov, Z., Sing, D. K., Mukherjee, S., et al. 2023, Nature, 614, 659
How to cite: Roy Perez, J., Pérez Hoyos, S., Barrado Izagirre, N., and Chen Chen, H.: WASP-39b cloud properties from JWST transit spectroscopy: a Bayesian analysis., Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-652, https://doi.org/10.5194/epsc2024-652, 2024.
With the James Webb Space Telescope (JWST) offering higher resolution data in space-based transmission spectroscopy, understanding the capabilities of our current atmospheric retrieval pipelines is essential. These new data cover wider wavelength ranges and at much higher spectral resolution than previous instruments have been able to offer. Therefore, it is often appealing to bin spectra to fewer points, better constrained in their transit depth, before using them as inputs for atmospheric retrievals. However, little quantitative analysis of the trade-off between spectral resolution and signal-to-noise ratio has been conducted thus far.
As such, we produce a simulation to mimic the observations of WASP-39b by the NIRSpec PRISM instrument on board JWST and assess the accuracy and consistency of retrievals while varying resolution and the average photometric error in these spectra. We repeat this analysis on three different simulation setups where each includes an opaque cloud layer at a different height in the atmosphere, varying the scale height of features seen in the resulting spectra. See figure 1 for examples of these spectra.
FIGURE 1: The simulated spectra used in the investigation. Panel a demonstrates the spectra with the three different cloud decks used in the investigation while panel b shows the extreme cases of binning considered in our resolution-error grid.
In general, we see the expected trend in our sensitivity maps; as resolution increases and the photometric error decreases, the accuracy of our retrievals improves. Clear boundaries between well and poorly retrieved regions are evident in these maps but the position of this boundary in resolution-error space and the difference in accuracy between these regions is heavily dependent on the cloud deck. See figure 2 for the example of retrievals run on the low cloud case.
We find that a much greater resolution is needed in the case of a high cloud deck since features are already heavily muted by the presence of the clouds. In the other two cases, there are large ‘safe’ zones (darker regions) in the parameter space and the average resolution and error of the NIRSpec PRISM observations of WASP-39b fall well within these regions.
FIGURE 2: The sensitivity maps for the retrieval grid with the low cloud spectrum. Blue crosses mark the positions of retrievals in the grid, the white marker indicates the position of the JWST NIRSpec PRISM observations of WASP-39b and the curved dashed lines indicate approximate ‘binning paths’. In the lower right panel we also present a collapsed version of the map for the log-abundance of water. In this case, the x-axis is unchanged but the y-axis marks the actual retrieved value for this parameter and the coloured lines show the trends for different sizes of error bars.
While this probes a specific case we also plot ‘binning paths’ in the resulting sensitivity maps to demonstrate the best attainable atmospheric parameter estimations starting from the position of the real JWST Early Release Science observation. If these maps can be generalised, binning paths could help to guide and inform future observations.
We also compare the retrievals in this grid (which use a constant resolution and have equal error bars on all points across the wavelength range of the spectrum) with a more realistic case by binning down from the ERS data. We find these results to be in good agreement with the trends presented in our sensitivity maps.
Additionally, we consider the correlation between parameters in these retrievals. It is expected that some parameters will show a strong correlation due to the similar effects that they cause in the spectrum but we focus on cases where we see a changing level of correlation as resolution increases. In the case of a low cloud deck, we see that increasing the resolution causes a decreasing correlation between the cloud pressure and several other parameters (specifically, planet radius, temperature and log-abundance of H2O). However, in the case of high clouds it is the log-abundance of CO which shows an increasing correlation with the CO2 and H2O log-abundances as resolution increases. Using this method we are able to determine the spectral resolution necessary to resolve these correlations in our atmospheric spectra.
How to cite: Davey, J., Waldmann, I., Yip, K. H., and Al-Refaie, A.: The Effects of Spectroscopic Binning on Atmospheric Retrievals, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-592, https://doi.org/10.5194/epsc2024-592, 2024.
Transmission spectroscopy of exoplanet atmospheres enables us to infer the characteristics of planets in foreign stellar systems. Terrestrial planets are of special interest as they increase our understanding of their origin and evolution preparing us to characterize a true Earth-analog planet in the future. Recent discoveries confirmed the presence of rocky planets in very short orbits, resulting in surface temperatures above the critical value of 2000K, where the surface rocks start to melt and release silicates into the atmosphere. Such worlds are denoted as Lava worlds and show different molecules with a large number of absorption lines.
Such molecules allow us to characterize lava world atmospheres even with ground-based high-resolution transit observations applying the cross-correlation method. More specifically, by applying injection recovery, in which template spectra are injected and recovered from the observations, it is possible to infer the lower limit of the atmospheric mean-molecular weight value, giving key insights into the atmospheric redox-state, recycling efficiency of atmophile elements between the melt-pool and solid-interior and atmospheric composition of lava worlds.
Recent JWST observations showed the likely presence of a CO/CO2 dominated atmosphere on the rocky planet 55 CNC e inferred from eclipse observations. This hints at the absence of a tenuous vaporized rock atmosphere, and the presence of a volatile dominated atmosphere. A volatile-rich atmosphere would result in a low mean-molecular weight atmosphere, making it possible to be inferred by applying ground-based observations.
We present several recent ground-based high-spectral resolution (R = 50 000) observations of 55 CNC e acquired at the Large-Binocular- telescope with the PEPSI instrument and show preliminary results on the inferred mean-molecular weight value. We compare this to the recent JWST results and discuss the implication of the results concerning the atmospheric redox state and the recycling efficiency of atmophile elements between the melt pool and solid interior.
How to cite: Keles, E.: Towards HADES: Deeply probing of Lava World Atmospheres, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-475, https://doi.org/10.5194/epsc2024-475, 2024.
This contribution provides valuable insights into two critical aspects essential for advancing our comprehension of (exo)planetary atmospheres. Firstly, it focuses on radiative transfer (RT) and inversion codes as powerful tools for characterizing planetary atmospheres. We present a comprehensive overview of various available codes, offering researchers a valuable resource for selecting and applying the most suitable code for their planetary atmospheric studies.
Secondly, the presentation delves into the domain of atomic and molecular (A&M) databases within the (exo)planetary community. A&M data form the bedrock of understanding physical and chemical processes within planetary atmospheres occurring within planetary atmospheres. By spotlighting existing A&M databases, we address crucial aspects such as accessibility, organization, limitations, and infrastructures, thereby illuminating their pivotal role in (exo)planetary research.
How to cite: Rengel, M. and Adamczewski, J.: Characterizing (exo)planetary atmospheres: An overview on radiative transfer, inversion codes, and atomic-molecular databases, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-857, https://doi.org/10.5194/epsc2024-857, 2024.
Habitable planets around M dwarfs are remarkable targets as they are among the least difficult terrestrial-size planets to detect and characterize. However, the climate characteristics on these planets are thought to be different from solar system terrestrial planets in particular because these planets are expected to be in a tidally-locked state, with a permanent irradiated hemisphere (dayside) and opposite one (nightside). If a planet orbits far from its host star, nightside temperatures can become extremely low, which leads volatile species such as CO2 and CH4 in the atmosphere to condense onto the surface. This phenomenon, known as atmospheric collapse, is thought to prevent habitable environment since removal from the atmosphere (especially CO2) decreases the stabilizing greenhouse effect.
We investigated the relationship between atmospheric collapse and habitability using the Generic Planetary Climate Model (Generic PCM), a 3-D global climate model historically developed at the Laboratoire de Météorologie Dynamique (LMD), by changing stellar insolation and CO2 partial pressure. The onset of atmospheric collapse for each case is decided by the surface amount of condensed CO2. As a result, we found cases where locally habitable environment on the dayside remains during/after the atmospheric collapse event in spite of the decrease in greenhouse effect. This is because the decrease in atmospheric mass and thus in atmospheric pressure makes day-night atmospheric heat transport less efficient, resulting in less of energy by insolation on the dayside. In addition, lower background gas cases were more habitable (i.e., more likely to have surface liquid water) over a wide range of CO2 partial pressure. While high background gas pressure is usually considered to enhance the greenhouse effect due to pressure broadening, on tidally-locked planets less background gas contributes to an increase in dayside temperatures.
To conclude, our results provide a new picture of the relationship between atmospheric collapse and habitability on tidally-locked, cool planets.
How to cite: Taniguchi, K., Kodama, T., Turbet, M., Chaverot, G., and Genda, H.: Atmospheric collapse and maintenance of habitable environment on tidally-locked exoplanets using a 3D-GCM, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-525, https://doi.org/10.5194/epsc2024-525, 2024.
In recent years, the study of exoplanetary atmospheres has increasingly relied on detailed disequilibrium chemistry models to understand atmospheric composition and dynamics. These models are computationally intensive, often requiring significant time and resources. We introduce CHEXANET, a novel U-Net-based neural network architecture designed to efficiently simulate disequilibrium chemistry in exoplanetary atmospheres, with network design driven by data exploration.
We have developed a machine learning framework that uses neural networks to predict the steady-state abundances of key chemical species in exoplanetary atmospheres. Our method involves training the neural networks on a comprehensive dataset generated from traditional disequilibrium chemistry models. Once trained, these networks can rapidly approximate the outcomes of the full models, substantially reducing computation time.
Our method was validated across a range of exoplanetary atmospheres, ranging from warm Neptunes to hot-Jupiters and an expansive range of chemistries. Our results demonstrate that the neural network models can achieve high accuracy in reproducing the steady-state solutions of complex chemical systems. Specifically, it significantly enhances computational efficiency, reducing the prediction time for atmospheric disequilibrium states to just one second per atmosphere on a standard personal computer—over a hundred times faster than traditional methods. Our approach accelerates the simulation process and makes it feasible to conduct extensive parameter studies and real-time atmospheric analysis.
A noteworthy part of the project is involving data-driven decisions in neural network design. We incorporated knowledge gained from preliminary data analysis, which included statistical analysis, principal component analysis, random forest analysis, and feature importance analysis. The examination stated above shows that involving initial parameters such as C/O ratio, temperature, metallicity, planet mass, and radius should significantly boost the predictive capabilities of simple U-net architecture. Figure 1 and 2 show the Mean Absolute Error of network prediction in correlation with initial parameters for two different networks, Model A, a simple U-net model, and Model E, a network incorporating initial parameters. A simple data-driven decision substantially reduced a bias observed in Model A. The future work will include a deeper description of data analysis and its influence on network behaviour to explain the Model's internal processes.
Our contribution highlights the potential of artificial intelligence in enhancing the capabilities of planetary science, providing a robust tool for future research in the characterization of exoplanets.
Figure 1: Pair plots illustrating the distribution of initial exoplanetary parameters: C/O ratio, Temperature (K), Metallicity, and Planet Mass (MJ), colour-coded by the Mean Absolute Error (MAE) for Model A. Notable is the cluster formation and a peak in MAE around a C/O ratio of 1.0 in the Temperature vs. C/O ratio plot, indicating a possible region of increased predictive difficulty. Moreover, the error for the higher C/O ratio decreases, as seen in the upper right corner of the plot. Temperature vs. Planet Mass plot reveals a non-linear pattern.
Figure 2: Pair plots illustrating the distribution of initial exoplanetary parameters: C/O ratio, Temperature (K), Metallicity, and Planet Mass (MJ), colour-coded by the Mean Absolute Error (MAE) for Model E. Although the prediction performance has improved compared to Figure 1, it is still noticeable that there is a cluster formation and a peak in MAE around a C/O ratio of 1.0 in the Temperature vs. C/O ratio plot, which suggests a possible area of increased predictive difficulty.
How to cite: Vojteková, A., Waldmann, I., Yip, K. H., Merín, B., and Al-Refaie, A. F.: A Novel Approach to Fast-Tracking Disequilibrium Chemistry Calculations for Exoplanets Using Neural Networks, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-440, https://doi.org/10.5194/epsc2024-440, 2024.
Introduction
Warm rocky exoplanets with instellations varying between those of Earth and Venus are key emerging objects of interest for current and future missions. They are favoured targets which will likely be found and characterized before their cooler, more Earth-like counterparts. Theory indicates these planets could be wet at formation and remain habitable long enough for life to develop. However, it is currently unclear for how long and to what extent an early ocean on such worlds could persist and influence climate and photochemical evolution, including the response of potential atmospheric biosignatures. In this work we test the climate-chemistry response, maintenance, and detectability of biosignatures in warm, water-rich atmospheres with Earth biomass fluxes within the framework of the planned LIFE mission.
Methodology
We use the global-mean, stationary, coupled climate-chemistry column model, 1D-TERRA to simulate the climate and chemistry of planetary atmospheres at different distances from the Sun, assuming Earth's planetary parameters and evolution. We run six biotic scenarios: starting with the modern Earth we increase the incoming instellation by up to fifty percent in steps of 10 percent. This corresponds to rocky exoplanets orbiting from 1.00 to 0.82 AU, approximately half the distance from Earth to Venus. To asses the effect of Earth's biomass fluxes, we repeat the six biotic scenarios without Earthlike biomass and fluxes. For all twelve simulations, we then calculate theoretical emission spectra using the GARLIC model based on the resulting atmospheric temperature and composition profiles. In addition, we use LIFEsim to add noise to and simulate observations of these spectra to assess how biotic and abiotic atmospheres of Earth-like planets can be distinguished.
Results
Our models show moderate ocean evaporation with increasing instellation (S), which results in water-rich atmospheres of roughly 0.01 bar (modern Earth instellation, S=1) up to 0.6 bar (S=1.5). Ozone, a key atmospheric biosignature, survives in the middle atmosphere in all scenarios as hydrogen oxide abundances remain stable since they react with nitrogen oxides instead. Methane is strongly removed for instellations that are 20% bigger than that of the Earth as rising water abundances increase hydroxyl (OH) via UV photolysis. Nitrous oxide (N2O) generally survives, mainly due to trade-off effects where enhanced photolytic loss in the upper layers due to higher instellation is counterbalanced by the stronger absorption of photons in the lower layers due to increased water abundance. Using LIFEsim, we find that O3 signatures at 9.6 μm reliably point to Earth-like biosphere surface fluxes of O2 for systems within 20 parsecs for integration times of 10 days. The differences in atmospheric temperature structures produced by differing H2O profiles between abiotic and biotic scenarios enable observations at 15.0 μm to reliably identify planets with a CH4 surface flux equal to that of Earth's biosphere for stars within 25 parsecs.
How to cite: Taysum, B., van Zelst, I., Lee Grenfell, J., Schreier, F., Cabrera, J., and Rauer, H.: Detectability of biosignatures in warm, water-rich atmospheres, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-831, https://doi.org/10.5194/epsc2024-831, 2024.
1) Introduction
With current observational constraints, a large fraction of observed exoplanets whose atmospheres can be studied with spectroscopy orbits closely from their host stars, receiving a high stellar flux and having elevated equilibrium temperature (Teq > 500 K). In particular, the high flux received in the ultraviolet (UV) can have important implications on the atmospheric composition of these planets. Indeed, UV photons can photodissociate atmospheric constituents and thus initiate photochemical reactions [1]. This impact of photochemistry on the composition of exoplanet atmospheres have been recently confirmed by observations of the warm gas giants WASP-39b and WASP-107b by JWST [2, 3]. Thus, the understanding of exoplanet atmospheres and the interpretation of their observations require the use of kinetic models that include photochemistry.
To implement photochemistry, 1D thermo-photochemical models use the UV absorption cross sections of molecules present in the atmospheres to calculate their photodissociation rates as a function of the altitude as well as the penetration depth of the UV photons through the atmosphere. To obtain accurate results, these models require data suitable for the conditions encounters in these environments often without equivalent in the solar system. However, the thermal dependency of the UV absorption cross sections of molecules is poorly known in the range of temperature observed in exoplanet atmospheres, which leads to uncertainties in the molecular abundances predicted by atmospheric models [4-6]. Here we present an experimental study of the thermal dependency of the UV absorption cross section of acetylene (C2H2) and ammonia (NH3) from 296 to 793 K and for a large spectral domain ranging from 115 to 230 nm.
2) Material and Methods
The absorption spectra of gaseous acetylene were measured at high temperature using a new custom-made UV spectroscopy platform developed at the Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA, France). Briefly, the setup consists in a high-temperature absorption cell composed of a quartz tube closed at each extremity by MgF2 windows mounted on stainless-steel flanges. The optical pathlength of the cell was 165 cm. The cell was installed in a furnace that can warm the cell up to 1373 K. For this study, spectra were measured at ambient temperature (296), 373, 473, 573, 673, and 773 K. We limited our measurements to a maximum temperature of 773 K because, at higher temperatures, we observed a fast thermal decomposition of C2H2 and NH3.
UV spectra of C2H2 and NH3 were measured from 115 to 230 nm using a McPherson 225 UHV monochromator. The absolute absorption cross sections of the molecules were calculated from the measured spectra using the Beer-Lambert law:
σ(λ, T) = (1/nL) × ln (I0/I)
where σ(λ, T) is the absorption cross section (cm2) at a given wavelength λ and temperature T, L is the optical pathlength (cm), n the volume density of the gas in the cell (cm-3), I0 the intensity of the light transmitted through an empty cell and I the intensity of the light transmitted through the cell containing a density n of gas. Considering the gas as a perfect gas, the density of the gas n in the cell was calculated using the equation n = P / kb T where P is the pressure of the gas inside the cell (Pa), T the temperature (K), and kb the Boltzmann constant (J K-1).
3) Results and Discussions
We found that the absolute absorption cross sections of C2H2 and NH3 increase with the temperature. We observed an increase in the intensity of the continuum and a decrease in the intensity of the absorption bands as the gas temperature increased, which result in an overall increase of the absolute absorption cross section of these molecules with temperature.
To go further, we quantified the impact of these new data on the prediction of exoplanet atmospheres composition using the 1D thermo-photochemical model FRECKLL [7, 8]. After modelling a hypothetical exoplanet atmosphere, we found that the abundance profile of C2H2 is slightly modified when the absorption cross section of C2H2 measured at 773 K is used instead of the one at 296 K. This small variation agrees with the fact that the absorption cross section of C2H2 increases at the maximum by a factor 20 at 773 K compared to 296 K. In addition, these changes in the absorption cross sections affect the penetration of the actinic flux through the atmosphere from 150 to 230 nm, resulting in an attenuation of the flux at higher altitudes when using the cross section measured at 773 K instead of that measured at 296 K. Such changes in the penetration of the actinic flux could have consequences on the abundance of other absorbing species.
Acknowledgements
This work is supported by the ANR project ‘EXACT’ (ANR-21-CE49-0008-01), the Centre National d'Etudes Spatiales (CNES), and the CNRS/INSU Programme National de Planétologie (PNP).
References
1. Moses, J.I., Chemical kinetics on extrasolar planets. Philos Trans A Math Phys Eng Sci, 2014. 372(2014): p. 20130073.
2. Tsai, S.-M., et al., Photochemically produced SO2 in the atmosphere of WASP-39b. Nature, 2023. 617(7961): p. 483-487.
3. Dyrek, A., et al., SO2, silicate clouds, but no CH4 detected in a warm Neptune. Nature, 2024. 625(7993): p. 51-54.
4. Venot, O., et al., VUV-absorption cross section of carbon dioxide from 150 to 800 K and applications to warm exoplanetary atmospheres⋆. A&A, 2018. 609: p. A34.
5. Ranjan, S., et al., Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H2O Cross Sections. The Astrophysical Journal, 2020. 896(2): p. 148.
6. Venot, O., et al., High-temperature measurements of VUV-absorption cross sections of CO2 and their application to exoplanets. A&A, 2013. 551: p. A131.
7. Veillet, R., et al., An extensively validated C/H/O/N chemical network for hot exoplanet disequilibrium chemistry. A&A, 2024. 682: p. A52.
8. Al-Refaie, et al., FRECKLL: Full and Reduced Exoplanet Chemical Kinetics distiLLed, AJ, In Press
How to cite: Fleury, B., Benilan, Y., Venot, O., and Veillet, R.: High-temperature measurements of VUV absorption cross sections with application to warm exoplanet atmospheres, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-187, https://doi.org/10.5194/epsc2024-187, 2024.
Ongoing mission such as TESS [1], CHEOPS [2] and JWST [3] as well as forthcoming missions such as PLATO [4] and ARIEL [5] have remarkably expanded our capacity to investigate planetary objects beyond our solar system, revealing new classes of planets, including hot-Jupiters [6, 7, 8, 9], mini-Neptunes [10, 11, 12] and super-Earths. Excitingly, among these several thousands of detected exoplanets, there are some rocky planets orbiting their star within the habitable zone. As planetary atmospheres likely play a pivotal role in shaping habitability, significant efforts have been made to characterize the physical properties and the complex chemistry occurring in rocky exoplanet atmospheres. As we discover a growing list of Earth-like exoplanets orbiting prevalent later star types, such as K and M dwarfs, discussions about the habitability of exoplanets around these longer-lived stars have been initiated [13]. There is an ongoing debate about the significance of potential biosignature compounds, like e.g. oxygen or ozone under various atmospheric conditions. With most of the oxygen in modern Earth’s atmosphere thought to be biotically produced through photosynthesis, oxygen and ozone are considered to be possible biomarker compounds [14]. Regarding ongoing discussions about habitability of early-Earth, early life probably evolved in an anoxic, high-UV environment [15]. Under such conditions, it can be shown that molecular oxygen also forms through photolysis of CO2 and subsequent recombination of O atoms [16]. The question to what extent these potential false positive biosignatures can be produced abiotically is therefore of great potential relevance for the interpretation of data from future missions. Further, biosignature abundances in exoplanet atmospheres may be greatly influenced by high energy particles (HEPs) emitted from stellar flares [17, 18]. Correct interpretation of spectral measurements therefore requires a new generation of photochemical-climate models that consider important factors such as the incoming stellar radiation the atmospheric mass and composition as well as the role of clouds and surface properties. However, as exoplanets often have no counterpart in our solar system, nor can in-situ data be acquired, model parameters are often inferred from limited data sets.
Consequently, careful comparisons with laboratory experiments plays a pivotal role in advancing our understanding of atmospheric processes [7]. Atmospheric simulation chambers are controlled laboratory environments that allow researchers to simulate and study complex phenomena that occur in the Earth's atmosphere and beyond. We present the Berlin Atmospheric Simulation Experimental Chamber (BASE) which is a versatile platform designed to replicate various atmospheric conditions representative for Earth-like exoplanet atmospheres. The BASE chamber is capable of simulating a range of atmospheric pressures ranging from 1 bar (e.g. modern Earth surface) down to a few mbar (e.g. Mars surface) and temperatures (293 K - 373K). It is equipped with a sophisticated gas mixing system that allows for precise control of atmospheric composition, including the introduction of trace gases and water vapor. Possible scenarios include primary, steam early-Earth-like, thin, cool Mars-like and thick, hot Venus-like atmospheres. In contrast to many atmospheric simulation chambers that lack simultaneous photon and electron irradiation capabilities, studies using BASE can examine photochemical processes driven by UV and Lyman alpha radiation, and electrons radiation. Experiments conducted at BASE focus on potential alterations of gaseous biomarkers and their distinction from potential abiotic sources [19] in various conditions. The chamber allows continuous spectroscopic real-time monitoring of gas samples across a wide wavelength range, covering VUV/VIS/NIR and simultaneous mass spectrometric analysis, allowing for precise measurements of gas composition, chemical reactions, and optical properties. We present first results on the photochemical formation and destruction processes of ozone in Earth’s stratosphere and extended experiments under enhanced UV irradiation. As has been recognized very early on, the lifetime of ozone in given atmospheric conditions is very sensitive to minor changes, with ozone production being a highly non-linear process. Therefore, future experiments at BASE aim to investigate ozone chemistry at elevated temperatures, radiation levels and increased amounts of H2O and NOx species in different atmospheric compositions. In combination with atmospheric modelling we envision deeper insight on the role of oxygen and ozone as gaseous biomarkers and the potential formation of ozone layers under atmospheric conditions of rocky exoplanets orbiting distant stars.
Acknowledgements
This work was supported by Volkswagen Foundation and its Freigeist Program (F.H., A.E.). A.E. also acknowledges support from the Ministry of Economics and Energy (Projekträger Deutsches Zentrum für Luft- und Raumfahrt, grants 50WB1623 and 50WB2023. Furthermore, funding and support from the Forschungskomission (via TEAMS funding to H.R. and A.E.) of Freie Universitaet Berlin is gratefully acknowledged.
References
- R. Ricker et al, AAS/Division for Extreme Solar Systems Abstracts 47 (2015) 503–01.
- Benz et al, Experimental Astronomy 51 (2021) 109–151.
- A. Sabelhaus et al, 5487 (2004) 550–563.
- Rauer et alAstronomische Nachrichten 337 (8-9) (2016) 961–963. doi: https://doi.org/10.1002/asna.201612408.
- Pascale et al, 5th IEEE International Workshop on Metrology for AeroSpace, IEEE, 2018, pp. 31–34.
- Barkaoui et al, The Astronomical Journal 157 (2) (2019) 43. doi:10.3847/1538- 3881/aaf422.
- J. Fortney et al, 2016, arXiv:1602.06305
- M G Soto et al, Monthly Notices of the Royal Astronomical Society, Volume 478, Issue 4, August 2018, Pages 5356–5365, https://doi.org/10.1093/mnras/sty1334
- K. Sing et al, The Astrophysical Journal 686 (1) (2008) 658. doi:10.1086/590075.
- Osborn, H. et al, AA 604 (2017) A19. doi:10.1051/0004-6361/201628932.
- D. Nielsen et al, Monthly Notices of the Royal Astronomical Society 492 (4) (2020) 5399–5412. arXiv: https://academic.oup.com/mnras/article-pdf/492/4/5399/32393017/staa197.pdf, doi:10.1093/mnras/staa197.
- Barraga´n et al, Monthly Notices of the Royal Astronomical Society 514 (2) (2022) 1606–1627. arXiv: https://academic.oup.com/mnras/article-pdf/514/2/1606/44022402/stac638.pdf, doi:10.1093/mnras/stac638.
- Gao et al, The Astrophysical Journal 806 (2) (2015). doi: https://dx.doi.org/10.1088/0004-637X/806/2/249
- Leger et al, Astronomy & Astrophysics, 1993, https://ui.adsabs.harvard.edu/abs/1993A&A...277..309L
- M. Canuto et al, Elsevier, Studies in Environmental Science, Volume 26, (1986), 51-102, https://doi.org/10.1016/S0166-1116(08)71794-1.
- Cockell CS et al, Photochem Photobiol. 2001;73(4):447-451. doi:10.1562/0031-8655(2001)073<0447:thotur>2.0.co;2
- L. Grenfell et al, Astrobiology 12 (12) (2012) 1109–1122.
- Herbst et al, Astronomy & Astrophysics 631 (2019) A101.
- W. Schwieterman et al, Astrobiology 18 (6) (2018) 663–708.
How to cite: Hofmann, F., Rauer, H., Grenfell, J. L., Taysum, B., and Elsaesser, A.: First insights from the Berlin Atmospheric Simulation Experimental Chamber (BASE), Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-946, https://doi.org/10.5194/epsc2024-946, 2024.
Context: Characterizing the atmospheres of small, temperate exoplanets addresses a major scientific challenge. The James Webb Space Telescope offers us the opportunity to study the atmospheric composition of temperate exoplanets as small as mini-Neptunes. Recent observations of K2-18b (Madhusudhan et al. 2023) and TOI-270d (Holmberg & Madhusudhan 2024; Benneke et al. 2024) have revealed atmospheres rich in hydrogen and carbon compounds. Methane and carbon dioxide have been detected in significant amounts. Moreover, the presence of haze is suggested by these observations (Madhusudhan et al. 2023), indicating that complex chemistry may be taking place in these atmospheres. However, our understanding of the chemistry governing these atmospheres remains largely unconstrained. Therefore, modeling and laboratory experiments are necessary to better understand these observations. Developing our knowledge of these atmospheres will help characterize their habitability.
1. Methods
Experimental Simulations: In this context, we carried out experimental simulations to reproduce the out-of-equilibrium chemistry that prevails in the upper layers of temperate exoplanets. We use a cold plasma reactor called PAMPRE (for Production d’Aérosols en Microgravité par Plasma REactif (Szopa et al. 2006)), a schematic diagram of which is shown in Figure 1. Our experimental procedure involves introducing into a reactive chamber selected gas mixtures at low pressure, analogous to the upper layers of the exoplanetary atmospheres of interest. The device then applies a radiofrequency discharge to the gas mixture, producing a plasma that triggers energetic processes. This experimental setup thus provokes an out-of-equilibrium chemistry similar to photodissociation intiated in the upper atmospheric layers exposed to energetic stellar particles and UV radiations. To track the chemical evolution in our experimental setup, we use mass spectrometry and infrared spectroscopy, enabling us to identify the formation of various chemical species.
Numerical Simulations: In parallel with our experimental work, we are carrying out numerical simulations using the ReactorUI code (Pernot 2023; Peng et al. 2014). ReactorUI was originally developed to simulate chemical reactions taking place in the reactor chamber in Titan’s anoxic N2/CH4 atmosphere. It has been updated for oxygenated reactions based on the chemical network of VULCAN, a 1D atmospheric simulation model (Tsai et al. 2017). This computational tool enables us to identify the chemical species produced in our experiments and elucidate the main chemical pathways leading to their formation. In addition, we compare VULCAN simulations to relate the results to expected atmospheric chemistry. This multi-faceted approach enables us to refine our understanding of the complex chemistry that occurs in the atmospheres of temperate exoplanets.
2. Results
Our experimentation highlights the production of complex organic compounds, as well as carbon monoxide and water vapour. We show that organic growth is favored in less oxidized environments, leading to the formation of long carbon chains, up to 6 carbons. Conversely, in more oxidized atmospheres, our initial results suggest the formation of oxidized organic compounds. Using infrared spectroscopy, we have identified a signature characteristic of carbonyls (C=O functional group) (Mobaraki & Hemmateenejad 2011), not associated with the signatures of CO2 or CO, as shown on Figure 2. This indicates a gas-phase oxidation reactions.
The first numerical simulations outputs are fairly consistent with the experimental ones. While VULCAN predicts for K2-18b atmosphere the presence of methanal (H2CO) and methanol (CH3OH) at 1mbar, with transport from the inner atmosphere making a significant contribution of thermodynamic equilibrium, ReactorUI confirms also the photochemical origin of these carbonyls and highlight the efficient formation of the simplest aldehyde, methanal, as shown on Figure 3.
Importantly, the volatility of organic compounds is reduced by these oxidation processes (Kroll & Seinfeld 2008), which may lead to their condensation in the atmospheric conditions prevailing on temperate exoplanets. These condensate species could serve as nucleation nuclei and contribute to the formation of haze or clouds (Zha et al. 2023).
Acknowledgements: N.C. thanks the European Research Council for funding via the ERC OxyPlanets projects (grant agreement No. 101053033)
Figure 1: Schematic diagram of the PAMPRE setup. Selected gas mixtures are injected into a reactive chamber at 1 mbar and 300 K and ionized in a plasma.
Figure 2: Infrared spectra of gaseous mixtures of 98% H2 + 1% CH4 + 1% CO (left) and 90% H2 + 5% CH4 + 5% CO (right). In the most oxidized mixture (5% CO, right), the characteristic signature of carbonyl compounds (C=O) appears around 1700 cm−1.
Figure 3: Proportion of several species predicted by the ReactorUI code, for different gas mixtures. H2C=O formation is predicted with a relatively high abundance, in the ppm range (10−6).
References
Benneke, B., et al. 2024, JWST Reveals CH4, CO2, and H2O in a Metal-rich Miscible Atmosphere on a Two-Earth-Radius Exoplanet
Holmberg, M., et al. 2024, Possible Hycean conditions in the sub-Neptune TOI-270 d
Kroll, J. H., et al. 2008, Atmospheric Environment, 42, 3593, Chemistry of secondary organic aerosol: Formation and evolution of low-volatility organics in the atmosphere
Madhusudhan, N., et al. 2023, Carbon-bearing Molecules in a Possible Hycean Atmosphere
Mobaraki, N., et al. 2011, Chemometrics and Intelligent Laboratory Systems, 109, 171
Peng, Z., et al. 2014, GeoResJ, 1-2, 33, Modeling of synchrotron-based laboratory simulations of Titan’s ionospheric photochemistry
Pernot, P. 2023, ppernot/ReactorUI: Ready for sulfur
Szopa, C., et al. 2006, Planetary and Space Science, 54, 394, PAMPRE: A dusty plasma experiment for Titan's tholins production and study
Tsai, S.-M., et al. 2017, ApJ, 228, 2, VULCAN: An Open-source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres
Zha, Q., et al. 2023, Oxidized organic molecules in the tropical free troposphere over Amazonia
How to cite: Sohier, O., Jaziri, Y., Vettier, L., Carrasco, N., Chatain, A., and Maratrat, L.: Understanding the chemistry of temperate exoplanets atmospheres: A study of oxidized organic compounds as precursors of photochemical condensates, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-218, https://doi.org/10.5194/epsc2024-218, 2024.
Introduction
With the new generation of space telescopes such as the James Webb Space Telescope (JWST), it is now becoming possible to characterize the asymmetries of the atmospheres of exoplanets. The atmospheres of Hot and Ultra-Hot Jupiters are highly heterogeneous and asymmetrical as temperatures of the day-side of tidally-locked planets can be drastically hotter than their respective night-side. The difference between the temperatures on the day-side and the night-side is especially extreme in the case of Ultra-Hot Jupiters and can reach 2000K. It has been shown that these changes may alter analyses and introduce biases into the unraveling of the molecular abundances of species in these atmospheres [1].
Methods
The Pytmosph3R framework [2] is able to generate synthetic observations (transmission or emission spectra), using 1D, 2D or 3D atmospheric models such as Global Circulation Models (GCM). We have extended this tool to introduce time-variability in the model [3], thus enabling us to reproduce light or phase-curves. The model includes thus the rotation of the planet as well as the calculation of the partial coverage of the atmosphere and the planet over the star during the ingress and egress.
Results
We find that the tidally-locked rotation of a Ultra-Hot Jupiter during a transit induces a non-negligible variation of the flux. This variation is a source of information on the chemical and thermal distribution of the atmosphere. We find the day-night thermal gradient present on Ultra-Hot Jupiters has an effect which could be mistaken with stellar limb-darkening: limb-darkening induces a reduction of the intensity of the flux covered by the planet on the edges of the stellar disc compared to the center, while the rotation of the planet and its atmosphere induces a reduction of the projected area of the atmosphere, due to the hotter (and larger) day side being shifted to either the east or the west during the early and late stages of the transit. This is shown in the figure below (the scale of the atmosphere has been exaggerated by a factor of 10 for visual purposes).
At the same time, this signal seems also to produce a signal opposite to tidal deformation. Indeed, the elongated form of the "rugby ball" of a tidally deformed planet covers more area during the early and late stage of the transit.
We also confirm the impact of the atmospheric and chemical distribution on variations of the central transit time, though the variations found are smaller than that of available observational data, which could indicate that the east-west asymmetries are underestimated, due to the chemistry or clouds. The east-west asymmetries being more pronounced in hot Jupiters rather than Ultra-Hot Jupiters, central transit time variations are better observed in lightcurves from Hot Jupiters. We illustrate this with simulations of Wasp-39b (Hot Jupiter) and Wasp-121b (Ultra-Hot Jupiter), such as shown below.
References
[1] W. Pluriel, J. Leconte, V. Parmentier, T. Zingales, A. Falco, F. Selsis, and P. Bordé. Toward a multidimensional analysis of transmission spectroscopy. II. Day-night-induced biases in retrievals from hot to ultrahot Jupiters. Astronomy & Astrophysics, 658:A42, Feb. 2022.
[2] A. Falco, T. Zingales, W. Pluriel, and J. Leconte. Toward a multidimensional analysis of transmission spectroscopy. I. Computation of transmission spectra using a 1D, 2D, or 3D atmosphere structure. Astronomy & Astrophysics, 658:A41, Feb. 2022.
[3] A. Falco, J. Leconte, A. Mechineau, and W. Pluriel. Signature of the atmospheric asymmetries of hot and ultra-hot Jupiters in lightcurves. Astronomy & Astrophysics, arXiv e-prints, arXiv:2402.12355, 2024
How to cite: Falco, A., Leconte, J., Pluriel, W., and Mechineau, A.: Signature of the atmospheric asymmetries of hot and ultra-hot Jupiters in light curves, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-258, https://doi.org/10.5194/epsc2024-258, 2024.
The abundant class of sub-Neptunes is a bridge between the well-studied gas giants and the mostly unexplored Earth-size planets. The Solar System lacks such a planet, reinforcing the importance of examining sub-Neptunes around other stars. To date, only a few sub-Neptune atmospheres have been successfully characterized, many of which have muted spectral features. Whether this is caused by high mean molecular weight atmospheres, high altitude aerosols, or a different mechanism is still unknown.
The Sub-neptune Planetary Atmosphere Characterization Experiment (SPACE) is a Hubble Space Telescope (HST) Multi-Cycle Treasury Program that aims to explore the physics and chemistry causing these muted features. It combines WFC3 near-infrared transmission spectroscopy and STIS ultraviolet stellar characterization for eight sub-Neptune systems. The targets span a physically motivated grid of planet radius (2 – 3.5 R⊕) and equilibrium temperature (400-1300 K) designed to reveal how sub-Neptune atmospheres are shaped by metal enrichment, disequilibrium chemistry, and aerosols (see Figure 1).
Figure 1: SPACE target systems superimposed on the grayscale density distribution of sub-Neptune exoplanets.
Transit spectroscopy of HD 86226c
Here, we present the first results for the hot sub-Neptune HD 86226c, a 2.2 R⊕ planet on a four-day orbit around its G-type host star. Its proximity to the star heats the planet to high equilibrium temperatures of 1300 K, making it an interesting target that is likely to be free of methane-based photochemical hazes.
Transit spectroscopy takes time-series observations of the star-planet system while the planet passes in front of its host star, blocking a wavelength-dependent fraction of the starlight. To date, seven transits of HD 86226c have been observed with WFC3, and two more are scheduled for observation in May 2024.
For the seven observed transits, we extracted the spectrum with the dedicated data reduction pipeline PACMAN (Zieba & Kreidberg 2022). The light curve from the time series observations was fitted to a transit model, where we corrected the observed flux for systematic deviations caused by the telescope. In addition to the usual telescope systematics, we find that decorrelating the flux from the spectrum position on the detector significantly improves the precision of the light curve. As shown in Figure 2, we divided the spectra into 11 wavelength bins for which the transit models were fitted individually. For these bins, the average root mean square flux deviation from the model is 110 parts per million (ppm).
Figure 2: Spectral light curves of HD 86226c obtained by combining the seven observed transits. The orbital phase is set such that it is zero during transit mid-time. The flux is measured relative to the out-of-transit stellar flux. Individual light curves are displayed offset by 0.002. The central wavelengths of the respective bins are shown by colored numbers in micrometers. Different markers represent data from different transits.
Spectrum analysis
The measured transit depth (Rplanet/Rstar)2 is shown as a function of wavelength in Figure 3. The spectrum agrees well with a constant transit depth of 405 ppm and does not show prominent spectral features. The spectrum was compared to forward models from the radiative transfer code petitRADTRANS (Mollière et al. 2019) for planetary atmospheres. Based on the observed spectrum, a cloud-free solar composition atmosphere is ruled out. The data are consistent with a hydrogen-dominated atmosphere with a cloud layer above 10-3 bar, or a cloud-free atmosphere with a metallicity well above 100 times the solar value. A pure water atmosphere is also consistent with the spectrum.
Figure 3: Transmission spectrum of HD 86226c obtained with the HST WFC3 instrument. The colored spectra show forward models of the planet's atmosphere from petitRADTRANS.
Implications
The featureless near-infrared transmission spectrum of HD 86226c implies that the hot sub-Neptune does not follow the trend of Neptune-sized objects identified by Brande et al. (2024), who predict an increasing feature size for planets with equilibrium temperatures over 700K (see Figure 4). While the high temperatures on this planet disfavor the formation of methane-based hazes, it is possible that the presence of silicate clouds mutes the spectral features. Alternatively, the flat spectrum can also be explained by a possible high metallicity of the atmosphere of the planet.
We demonstrate that hot sub-Neptunes do not guarantee the presence of large features in their spectra. To understand the composition of their atmospheres, it will be important to observe these targets with high-sensitivity instruments such as the James Webb Space Telescope. Following up with observations of a larger sample of sub-Neptunes will be a key strategy for characterizing the physical processes in the atmospheres of these small planets.
Figure 4: Spectral feature amplitude, AH, as a function of equilibrium Temperature, Teq, for the (sub-) Neptune sample analyzed by Brande et al. (2024, black) and of HD 86226c (red). Colored shaded regions show the feature amplitudes predicted by the models of Morley et al. (2015) for different cloud parametrizations.
References:
- Zieba & Kreidberg 2022, JOSS, 7, 4838
- Mollière, et al. 2019, A&A, 627, A67
- Brande, et al. 2024, ApJ, 961, L23
- Morley, C. V., et al. 2015, ApJ, 815, 110
How to cite: Kahle, K. A., Kreidberg, L., Mollière, P., and Brande, J. and the SPACE team: First results of the SPACE Program: The surprisingly featureless spectrum of hot sub-Neptune HD 86226c, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-627, https://doi.org/10.5194/epsc2024-627, 2024.
For several years, the metastable helium triplet line has been successfully used as a tracer to
probe atmospheric escape in transiting exoplanets. This absorption in the near-infrared (1083.3 nm) can be observed from the ground using high-resolution spectroscopy, providing new constraints on the mass loss rate and the temperature characterizing the upper atmosphere of close-in exoplanets. This work aims to search for the He triplet signature in fifteen transiting exoplanets, ranging from super-Earths to ultra-hot Jupiters, and observed with SPIRou, a high-resolution (R ∼70 000) near- infrared spectropolarimeter at the CFHT, to bring new constraints or to improve the existing ones regarding atmospheric escape through a homogeneous study. We developed a full data processing and analysis pipeline to correct for the residual telluric and stellar contributions. We then used two different 1D models based on the Parker-wind equations and NLTE radiative transfer to interpret the observational results. We confirm published He triplet detections for HAT-P-11 b, HD 189733 b, and WASP-69 b. We tentatively detect the signature of escaping He in HD 209458 b, GJ 3470 b, and WASP-76 b. We report new constraints on the mass loss rate and temperature for our three detections and set upper limits for the tentative and non-detections. We notably report improved constraints on the mass loss rate and temperature of the escaping gas for TOI-1807 b, and report a non-detection for the debated atmospheric escape in GJ 1214 b. We also conducted the first search for the He signature in GJ 486 b since its discovery and report a non-detection of the He triplet. Finally, we studied the impact of important model assumptions on our retrieved parameters, notably the limitations of 1D models and the influence of the H/He ratio on the derived constraints.
How to cite: Masson, A., Vinatier, S., Bézard, B., López-Puertas, M., Lampón González-Albo, M., Debras, F., Carmona, A., Klein, B., Artigau, E., Dethier, W., Pelletier, S., Hood, T., Allart, R., Bourrier, V., Cadieux, C., Charnay, B., Cowan, N., Delfosse, X., Donati, J.-F., and Gu, P.-G. and the additional authors in description: Probing atmospheric escape through metastable He I triplet lines in 15 exoplanets observed with SPIRou, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-924, https://doi.org/10.5194/epsc2024-924, 2024.
The advent of more accurate atmospheric abundance measurements opens a new window into giant planet characterization. Linking atmospheric measurements to the bulk planetary composition and the planetary origin is a key objective in planetary science. Typically, the observed atmospheric abundances are interpreted using rather simplified evolution and internal structure models. These models assume either a core+envelope or a homogeneous interior. However, we now know from the Solar System that the internal structure of giant planets is more complex. In addition, formation models clearly indicate that composition gradients are expected to form in the deep interior.
In this talk, we will present results from evolution simulations that account for composition gradients and convective mixing during the planetary evolution. For the planetary evolution, we use the Modules for Experiments in Stellar Astrophysics (MESA) code with state-of-the-art equations of state (EoS) for hydrogen, helium, water, and rock. Based on mixing length theory, we improved the treatment of convective boundaries in MESA to allow a comprehensive investigation of the long-term evolution of convective mixing in giant planets.
We consider a range of planetary masses (0.3-2 MJ), initial entropies (8-11 kb/mu) and heavy-element profiles. We will highlight trends of convection and dependencies between different planetary parameters, such as primordial entropy, composition profile and planetary mass. In addition, we test the influence of using different EoSs and the consideration of stellar irradiation. We find that convective mixing is most efficient at early times (the first 107 years of evolution) and that primordial composition gradients can be eroded.
The efficiency of convection is primarily driven by the underlying entropy profile. If the primordial entropy is sufficiently low, convective mixing can be inhibited and composition gradients can persist over evolutionary timescales. Moreover, since the primordial entropy increases with planetary mass, more massive planets will mix more effectively. Furthermore, we show that the EoS used plays a crucial role for the long-term evolution with primordial composition gradients: heavier elements are harder to mix and therefore lead to more stable configurations, also differences in the EoS of hydrogen and helium can alter the outcome, underlining the importance of using accurate EoSs.
Finally, we present a new analytical model that predicts convective mixing under the existence of composition (and entropy) gradients. We show that in most cases our analytical model reproduces well the results from the numerical simulations. It disentangles the key factors for convective mixing and allows to estimate the fraction of atmospheric-to-bulk metallicity without needing to model the evolution numerically.
Overall, we show that in several cases, the atmospheric composition can differ widely from the planetary bulk composition, with the exact outcome depending on the details. Our findings are critical for the interpretation of atmospheric abundance measurements and linking them to the planetary bulk composition and formation history.
How to cite: Knierim, H. and Helled, R.: Convective mixing in giant planets: When does the atmospheric composition represent the planetary bulk composition?, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1102, https://doi.org/10.5194/epsc2024-1102, 2024.
