A Homogeneous Study of Exoplanetary Atmospheres Using High-Resolution Transit Spectroscopy

Thousands of exoplanets have been confirmed in the last two decades, and yet observational constraints on their compositions have only been obtained for a few hundred of them so far. Current detection methods only give access to the radius and mass of a planet and therefore to its bulk density, which generally induces large degeneracies in terms of composition and structure. Characterization of exoplanet atmospheres has therefore emerged as a challenge for the exoplanet scientific community. Constraining the composition, dynamics, and overall structure of an exoplanet atmosphere allows us to infer its formation and evolution history and to put our Solar system in a broader context through comparative planetary science. Transmission spectroscopy has emerged as a powerful method to characterize exoplanetary atmospheres: when an exoplanet transits its host star, part of the stellar flux passes through the exoplanet’s atmosphere, imprinting the signature of its molecular components on the observed flux. Observing these transmission spectra with ground-based high-resolution spectrometers allows us to detect the species present in the atmosphere and probe the dynamics of the atmosphere by resolving the individual lines, all while being less sensitive to clouds and hazes' opacities than space-based observations at medium resolution. However, the analysis of ground-based observations requires state-of-the-art data reduction and processing methods to correct for the Earth's atmosphere and the background host star contributions and to extract the faint planetary signal whose individual line amplitudes are orders of magnitude weaker than the noise.

I will present the work, codes, and methodology developed during my PhD to characterize the atmosphere of fifteen exoplanets observed with the SPIRou instrument, a near-infrared spectropolarimeter at the Canada-France-Hawaii Telescope. This presentation will focus on our search for the metastable He triplet signature at 1083.3 nm (in vacuum), a near-infrared probe for atmospheric escape, and for molecular signatures to constrain the atmospheric composition and structure of 15 targets ranging from super-Earth and sub-Neptunes to hot Jupiters (Fig. 1.). I will present our results in terms of mass loss rate and escape temperature constraints obtained with Parker-wind modelisation of atmospheric escape (Fig. 2.). I will then discuss our results regarding the presence and abundances of molecules such as H₂O, CO, and CH₄, obtained with Cross Correlation Function and Nested Sampling methods coupled with a 1D radiative-convective equilibrium code and a high-resolution radiative transfer model (Fig. 3.). Applying the same reduction pipeline on a set of targets observed with the same instrument further allowed us to provide homogeneously retrieved constraints on these targets, paving the way toward a statistical understanding of exoplanets in terms of atmospheric composition and structure.

 

Fig. 1. List of the fifteen targets studied in this work

 

Fig. 2. Detection of the metastable He triplet lines (black) in HAT-P-11 b and fitting with a Parker wind escape model (red)

 

Fig. 3. Cross Correlation Function map in velocity space showing the detection of H₂O in WASP-127 b