Atmospheric characterisation of ultra-hot gas giant exoplanets

Thesis defense of Vincent Yariv - Friday, November 13th at 2.00 pm - Manuel Forestini room IPAG

Ultra-hot Jupiters (UHJs) are highly irradiated gas-giant exoplanets with atmospheric temperatures of several thousand kelvin, providing extreme laboratories for studying the physics and chemistry of planetary atmospheres. Their intense irradiation drives strong temperature contrasts between their permanent day and night sides and is expected to generate powerful atmospheric circulation, including winds that redistribute heat around the planet. As a result, their atmospheres are intrinsically three-dimensional, with chemical composition, temperature and velocity varying across the planetary disk. Characterising these variations is essential for understanding how energy and momentum are transported through planetary atmospheres, but remains challenging because conventional observations generally provide only disk-integrated measurements.

My thesis aims to obtain observational constraints on the atmospheres of UHJs using high-resolution emission spectroscopy, with a particular focus on developing techniques that can probe their three-dimensional structure and atmospheric circulation. I pursue two complementary approaches. First, I use high-resolution spectroscopy to constrain the chemical composition and temperature structure of the dayside atmosphere, while exploiting the Doppler shifts and line profiles of individual atomic and molecular species to investigate the effects of atmospheric dynamics. Second, I investigate Doppler eclipse mapping as a means of spatially resolving the thermal emission from an exoplanet during secondary eclipse. By measuring how the planet’s rotationally broadened spectral lines change as different regions of its dayside disappear behind the star, this technique can potentially recover information on the spatial distribution of atmospheric emission and, ultimately, atmospheric circulation.

I first analyse a large set of emission observations of the UHJ TOI-2109 b obtained with the Near-InfraRed Planet Searcher (NIRPS). I use the ATMOSPHERIX high-resolution spectroscopy pipeline to retrieve the atmospheric composition and temperature structure, finding evidence consistent with strong water dissociation and a possible depletion of titanium, potentially linked to cold-trapping on the planet’s cooler nightside. I then develop and test the application of Doppler eclipse mapping to high-resolution spectroscopy, using observations of WASP-33 b obtained with the Spectro-Polarimètre Infra-Rouge (SPIRou). I present the first observational attempt to spatially resolve an exoplanet atmosphere using this technique and assess its prospects for future high-resolution spectrographs, complemented by initial results from an ongoing follow-up study with the Immersion GRating INfrared Spectrometer-2 (IGRINS-2).

Together, these studies demonstrate how high-resolution emission spectroscopy can move beyond detecting atmospheric species in an unresolved atmosphere towards probing where those species are located and how the atmosphere moves. They provide observational steps towards a more complete, three-dimensional view of UHJ atmospheres, in which chemical composition, thermal structure and atmospheric circulation can be studied as interconnected components of the same planetary climate system.

Thesis Direction

• Xavier Bonfils & Thierry Forveille

Examination commitee

• Ignas Snellen, Professor / Scientific Director, U. Leiden, Reviewer
• Giovanna Tinetti, Professor / Vice Dean (research), King’s College London, Reviewer
• Nadège Meunier, Astronomer, IPAG Université Grenoble Alpes, Examiner
• Amaury Triaud, Professor, U. Birmingham, Examiner
• Matteo Brogi, Associate Professor, U. Turin, Examiner
• Nicolas Cowan, Professor, U. McGill, Guest