Amélie Gressier

Observations et modélisations de l’atmosphère des exoplanètes : de la haute atmosphère des Jupiter-chauds à la surface des planètes terrestres tempérées, étude de la transition des Super-Terre aux Sub-Neptune avec un relevé Hubble: En août 2022, the JWST Transiting Exoplanet Community Early Release Science Team a révélé la détection de dioxyde de carbone dans l'atmosphère de la géante gazeuse WASP-39 b, démontrant les capacités exceptionnelles du Télescope spatial James Webb (JWST) dans la caractérisation de l'atmosphère des planètes extra-solaires. Alors que de nombreuses nouvelles données seront disponibles dans les années à venir, cette thèse propose d'utiliser les observations du télescope spatial Hubble (HST) en spectroscopie de transit dans le but de mieux préparer les futures analyses mais aussi de passer de l'étude d'une seule cible à l'étude statistique d'une population de planètes. Dans un premier temps, nous analysons la haute atmosphère des Jupiters-chauds avec les observations HST dans le proche ultra-violet (NUV). Nous présentons un outil pour corriger et ajuster les courbes de lumière en transits observés avec le Space Telescope Imaging Spectrograph (STIS). Nous confirmons l'évaporation de l'atmosphère de WASP-121 b et présentons de nouvelles observations de WASP-79 b. L'analyse de deux transits de cette planète montre une augmentation du rayon de la planète entre la mesure optique et NUV de 4.5sigma. Nous étudions, ensuite, les couches plus profondes de l'atmosphère de planètes plus petites avec les observations en proche infrarouge du Wide Field Camera 3 (WFC3). Les Sub-Neptunes ont une taille entre celle de la Terre et de Neptune (1-4REarth) et sont le type de planètes le plus commun dans notre Galaxie. Malgré leur omniprésence, elles n'ont pas d'équivalent dans notre système solaire et restent difficiles à caractériser. Leur distribution en taille et nombre, est bimodale, avec une diminution dans le nombre de planètes autour de 1.7REarth. Les planètes d'un rayon inférieur à 1.7REarth sont supposées rocheuses et sont appelées Super-Terre, tandis que les planètes au-dessus de ce seuil sont très probablement gazeuses et communément appelées Sub-Neptunes. Vingt-six spectres en transmission sont analysés de manière cohérente avec un code d'analyse Bayésienne, Tau-REX 3 afin d'explorer la transition entre Super-Terre et Sub-Neptune. Nous confirmons la présence d'une atmosphère pour 13 planètes dont le rayon est supérieur à 1.7REarth, la limite fixée pour distinguer les planètes rocheuses des planètes gazeuses. Nous fournissons le premier relevé des détections moléculaires pour les Super-Terre et les Sous-Neptunes en utilisant HST/WFC3 G141 (1.1-1.65 microns)et confirmons la détection d'eau pour neuf planètes. Cependant, de nombreuses dégénérescences subsistent dans l'analyse de ces spectres et il reste difficile de différencier une atmosphère primaire nuageuse d'une atmosphère de type terrestre et l'absorption de l'eau et du méthane autour de 1.4microns. Suite à ce résultat, nous concentrons notre modélisation sur deux points: l'impact de la sédimentation des nuages sur le spectre en transmission et la nature des espèces carbonées en utilisant Exo-REM, un modèle radiatif-convectif. Nous présentons des simulations 1D et 2D d'atmosphères chaudes de Sub-Neptune et construisons une grille de modèles en température et en métallicité. Nous affinons la nature des nuages dans l'atmosphère de Sub-Neptunes et leur impact sur la taille de la signature spectrale de l'eau. En modélisant les atmosphères chaudes de Sub-Neptunes, nous mettons en évidence les divergences entre la non-détection du méthane et sa prédiction. Nous proposons et comparons les stratégies d'observation JWST et ARIEL pour résoudre les dégénérescences et en particulier, différencier l'absorption de l'eau de celle du méthane dans les spectres en transmission. Les Sub-Neptunes tempérées à chaudes, avec une atmosphère étendue orbitant autour d'étoiles brillantes proches, sont d'excellentes cibles pour la spectroscopie de transmission avec le JWST combinant NIRISS et NIRSpec.

JWST is transforming our ability to characterize small exoplanets, from sub-Neptunes to rocky worlds. A key open question is whether highly irradiated rocky planets can retain atmospheres or are stripped bare by stellar irradiation—a boundary that remains to be mapped observationally. Here we present the first JWST secondary eclipse observations of the rocky exoplanet GJ 3473 b, obtained with MIRI F1500W photometry. Using four visits, we confidently detect the eclipse at an average depth of 186 ± 45 ppm, somewhat lower than expected for a blackbody. We test a wide range of data reduction and analysis assumptions and provide new insights into MIRI detector settling behavior that will benefit future observations. We model a suite of airless surfaces with varied compositions, textures, and degrees of space weathering, as well as idealized atmospheric scenarios including the possibility of atmospheric collapse. Both atmospheric and bare-rock interpretations remain consistent with the data, but we exclude thick CO ₂ atmospheres, placing a 95% credible upper limit of 1.2–6.5 bar on the surface pressure. We also find tentative evidence for visit-to-visit variability in eclipse depth (33–371 ppm), though additional data are required to confirm this. Our results highlight the challenges and intrinsic degeneracies in interpreting MIRI F1500W eclipse measurements of rocky exoplanets, indicating that such observations alone may not uniquely distinguish between bare-rock and atmospheric scenarios. Future spectroscopic or phase-curve observations will be required to determine whether or not GJ 3473 b hosts a substantial atmosphere.

One of the forefront goals in the field of exoplanets is the detection of an atmosphere on a temperate terrestrial exoplanet, and among the best suited systems to do so is TRAPPIST-1. However, JWST transit observations of the TRAPPIST-1 planets show significant contamination from stellar surface features that we are unable to confidently model. Here, we present the motivation and first observations of our JWST multicycle program of TRAPPIST-1 e, which utilize close transits of the airless TRAPPIST-1 b to model-independently correct for stellar contamination, with the goal of determining whether TRAPPIST-1 e has an Earth-like mean molecular weight atmosphere containing CO ₂ . We present our simulations, which show that with 15 close transit observations, we will be able to detect this atmosphere on TRAPPIST-1 e at Δ ln Z = 5 or greater confidence assuming we are able to correct for stellar contamination using the close transit observations. We also show the first three observations of our program. We find that our ability to correct for stellar contamination can be inhibited when strong stellar flares are present, as flares can break the assumption that the star does not change meaningfully between planetary transits. The cleanest observation demonstrates the removal of stellar contamination contribution through an increased preference for a flat line over the original TRAPPIST-1 e spectrum, but highlights how minor data analysis assumptions can propagate significantly when searching for small atmospheric signals. This is amplified when using the signals from multiple planets, which is important to consider as we continue our atmospheric search.

We report a non-detection of nightside emission from the warm super-Neptune HAT-P-26 b. Following a similar approach to that presented by J. Lustig-Yaeger et al. for WASP-17 b, we attempted to extract the nightside emission spectrum using the in-eclipse spectrum removal method to detect planetary infrared excess (PIE). However, our methods did not yield a reliable nightside spectrum for HAT-P-26 b, and instead the results appear consistent with contamination due to small changes in the star between transit and eclipse visits. This null result helps to illuminate astrophysical challenges that complicate applications of the PIE technique and highlight the need for close-in-time transit and eclipse observations for this analysis method.

Theoretical studies have suggested using planetary infrared excess (PIE) to detect and characterize the thermal emission of transiting and nontransiting exoplanets, although the PIE technique requires empirical validation. Here, we apply the PIE technique to a combination of JWST NIRSpec G395H transit and eclipse measurements of WASP-17b, a hot Jupiter orbiting an F-type star, obtained consecutively (one-half of the orbital period or 1.8 days apart) as part of the JWST-TST program to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy. Using the in-eclipse measured stellar spectrum to circumvent the need for ultraprecise stellar models, we extract the first JWST nightside emission spectrum of WASP-17b using only transit and eclipse data, thereby performing a controlled test of the PIE technique. From the WASP-17b nightside spectrum, we measure a nightside equilibrium temperature of 1006 ± 276 K and find tentative evidence for nightside SO ₂ absorption ( ln B = 1.4 ). In context with the dayside, the temperature of the nightside is consistent with: (1) previous eclipse mapping findings that suggest relatively inefficient day–night heat transport; and (2) a nonzero bond albedo of 0.4 2 − 0.10 + 0.06 . SO ₂ on the nightside, if confirmed, would represent the first direct evidence for transport-induced chemistry, matching previous model predictions and opening a new door onto the 3D nature of giant exoplanets. Our results suggest that PIE is feasible with JWST/NIRSpec for two epochs separated in time by significantly less than the rotation period of the host star.

We present the James Webb Space Telescope (JWST) transmission spectrum of the exoplanet HAT-P-26 b (18.6 M _⊕ , 6.33 R _⊕ ), based on a single transit observed with the JWST NIRSpec G395H grating. We detect water vapor ( ln B = 4.1), carbon dioxide ( ln B = 85.6), and sulfur dioxide ( ln B = 13.5) with high confidence, along with marginal indications for hydrogen sulfide and carbon monoxide ( ln B < 0.5). The detection of SO ₂ in a warm super-Neptune-sized exoplanet ( R _P ∼ 6 R _⊕ ) bridges the gap between previous detections in hot Jupiters and sub-Neptunes, highlighting the role of disequilibrium photochemistry across a broad range of exoplanet atmospheres, including those cooler than 1000 K. Our precise measurements of carbon, oxygen, and sulfur indicate an atmospheric metallicity of ∼10× solar and a subsolar C/O ratio. Retrieved molecular abundances are consistent within 2 σ with predictions from self-consistent models including photochemistry. The elevated CO ₂ abundance and possible H ₂ S signal may also reflect sensitivities to the thermal structure, cloud properties, or additional disequilibrium processes such as vertical mixing. We compare the SO ₂ abundance in HAT-P-26 b with that of 10 other JWST-observed giant exoplanets, and find a correlation with atmospheric metallicity. The trend is consistent with the prediction from I. J. M. Crossfield, showing a steep rise in SO ₂ abundance at low metallicities, and a more gradual increase beyond 30× solar. This work is part of a series of studies by our JWST Telescope Scientist Team (JWST-TST), in which we use Guaranteed Time Observations to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS).

It is an open question whether small planets around M dwarfs are able to maintain atmospheres. The Hot Rocks Survey aims to address this question by observing nine rocky exoplanets orbiting M dwarfs with MIRI emission photometry to constrain the onset of atmospheres. In this paper, we present two MIRI F1500W (15 μm) eclipses of LTT 3780 b, an ultrashort-period super-Earth (P = 0.768 days, R = 1.325 R _⊕, M = 2.46 M _⊕) that receives 111× Earth’s instellation, the highest in the survey. We find a combined eclipse depth of 312 ± 38 ppm, which is consistent between different data reduction and analysis assumptions, bolstering our confidence in the eclipse detection. This eclipse depth is consistent with the thermal emission from a bare rock surface, with a dayside temperature of T day = 114 3 − 99 + 104 K, 98% ± 9% of the maximum temperature predicted for a zero-albedo, zero heat redistribution blackbody. We are able to confidently rule out CO₂-based atmospheres down to a 0.01 bar surface pressure to greater than 3σ (ruling out an approximately Mars-like atmosphere). We are unable to rule out a pure H₂O 1 bar atmosphere, though we argue that this composition is unlikely on such a highly irradiated planet, nor O₂ atmospheres due to the lack of features in the bandpass, though we can put constraints on CO₂-mixture atmospheres. As a potential bare rock, we consider a variety of surface composition models, but are unable to distinguish between them. However, LTT 3780 b is an excellent target for follow-up JWST observations to determine its surface composition and rule out additional atmospheric compositions.

The TRAPPIST-1 system offers one of the best opportunities to characterize temperate terrestrial planets beyond our own solar system. Within the TRAPPIST-1 system, planet e stands out as highly likely to sustain surface liquid water if it possesses an atmosphere. Recently, we reported the first JWST/NIRSpec PRISM transmission spectra of TRAPPIST-1 e, revealing significant stellar contamination, which varied between the four visits. Here, we assess the range of planetary atmospheres consistent with our transmission spectrum. We explore a wide range of atmospheric scenarios via a hierarchy of forward modeling and retrievals. We do not obtain strong evidence for or against an atmosphere. Our results weakly disfavor CO₂-rich atmospheres for pressures corresponding to the surface of Venus and Mars and the cloud tops of Venus at 2σ. We exclude H₂-rich atmospheres containing CO₂ and CH₄ in agreement with past work but find that higher mean molecular weight, N₂-rich atmospheres with trace CO₂ and CH₄ are permitted by the data. Both a bare rock and N₂-rich atmospheric scenario provide adequate fits to the data but do not fully explain all features, which may be due to either uncorrected stellar contamination or atmospheric signals. Ongoing JWST observations of TRAPPIST-1 e, exploiting consecutive transits with TRAPPIST-1 b, will offer stronger constraints via a more effective stellar contamination correction. The present work is part of the JWST Telescope Scientist Team Guaranteed Time Observations, which is performing a Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS).

TRAPPIST-1 e is one of the very few rocky exoplanets that is both amenable to atmospheric characterization and resides in the habitable zone of its star—located at a distance from its star such that it might, with the right atmosphere, sustain liquid water on its surface. Here, we present a set of four JWST/NIRSpec PRISM transmission spectra of TRAPPIST-1 e obtained in mid-to-late 2023. Our transmission spectra exhibit similar levels of stellar contamination as observed in prior works for other planets in the TRAPPIST-1 system but over a wider wavelength range, showcasing the challenge of characterizing the TRAPPIST-1 planets even at relatively long wavelengths (3–5 μm). While we show that current stellar modeling frameworks are unable to explain the stellar contamination features in our spectra, we demonstrate that we can marginalize over those features instead using Gaussian processes, which enables us to perform novel exoplanet atmospheric inferences with our transmission spectra. In particular, we are able to rule out cloudy, primary H₂-dominated (≳80% by volume) atmospheres at better than a 3σ level. Constraints on possible secondary atmospheres on TRAPPIST-1 e are presented in a companion paper. Our work showcases how JWST is breaking ground in the precision needed to constrain the atmospheric composition of habitable-zone rocky exoplanets.

TRAPPIST-1 e is one of the very few rocky exoplanets that is both amenable to atmospheric characterization and that resides in the habitable zone of its star -- located at a distance from its star such that it might, with the right atmosphere, sustain liquid water on its surface. Here, we present a set of 4 JWST/NIRSpec PRISM transmission spectra of TRAPPIST-1 e obtained from mid to late 2023. Our transmission spectra exhibit similar levels of stellar contamination as observed in prior works for other planets in the TRAPPIST-1 system (Lim et al, 2023; Radica et al., 2024), but over a wider wavelength range, showcasing the challenge of characterizing the TRAPPIST-1 planets even at relatively long wavelengths (3-5 um). While we show that current stellar modeling frameworks are unable to explain the stellar contamination features in our spectra, we demonstrate that we can marginalize over those features instead using Gaussian Processes, which enables us to perform novel exoplanet atmospheric inferences with our transmission spectra. In particular, we are able to rule out cloudy, primary H$_2$-dominated ($\gtrsim$ 80$\%$ by volume) atmospheres at better than a 3$σ$ level. Constraints on possible secondary atmospheres on TRAPPIST-1 e are presented in a companion paper (Glidden et al., 2025). Our work showcases how JWST is breaking ground into the precisions needed to constrain the atmospheric composition of habitable-zone rocky exoplanets.