This project investigates the charge exchange process between the atmospheric escape of a hot jupiter and the wind of its host star as a possible explanation for the Lyman alpha high-velocity absorption of the stellar spectrum observed during transits. We use 2D hydrodynamical simulation to follow the dynamic of the two fluids (shocks, compression layers...) and we implement a chemistry module to compute the quantity of neutral hydrogen produced by charge exchange. The simulations show that Kelvin Helmholtz instabilities develop at the interface between the two flows leading to a mixing that gives enough energetic neutral hydrogen to explain the observed absorption. However works still need to be done, especially the implementation of coriolis force due to the orbital motion of the planet, to achieve a steady state and get the correct geometry for the interface.

Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability1. However, no unambiguous photochemical products have been detected in exoplanet atmospheres so far. Recent observations from the JWST Transiting Exoplanet Community Early Release Science Program2,3 found a spectral absorption feature at 4.05 μm arising from sulfur dioxide (SO2) in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 MJ) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of around 1,100 K (ref. 4). The most plausible way of generating SO2 in such an atmosphere is through photochemical processes5,6. Here we show that the SO2 distribution computed by a suite of photochemical models robustly explains the 4.05-μm spectral feature identified by JWST transmission observations7 with NIRSpec PRISM (2.7σ)8 and G395H (4.5σ)9. SO2 is produced by successive oxidation of sulfur radicals freed when hydrogen sulfide (H2S) is destroyed. The sensitivity of the SO2 feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of about 10× solar. We further point out that SO2 also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations.

Carbon dioxide (CO₂) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO₂ is an indicator of the metal enrichment (that is, elements heavier than helium, also called 'metallicity')^1-3, and thus the formation processes of the primary atmospheres of hot gas giants^4-6. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets^7-9. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO₂, but have not yielded definitive detections owing to the lack of unambiguous spectroscopic identification^10-12. Here we present the detection of CO₂ in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science programme^13,14. The data used in this study span 3.0-5.5 micrometres in wavelength and show a prominent CO₂ absorption feature at 4.3 micrometres (26-sigma significance). The overall spectrum is well matched by one-dimensional, ten-times solar metallicity models that assume radiative-convective-thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide and hydrogen sulfide in addition to CO₂, but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 micrometres that is not reproduced by these models.