A story of bubbles and streams Morphologies and tracers of atmospheric escape in exoplanets

Atmospheric escape, the loss of atmospheric gas to space, plays a critical role in shaping planetary climates, compositions, and long-term habitability. This thesis examines the morphologies and observational signatures of atmospheric escape in exoplanets, focusing on two primary outflow types: bubble-shaped outflows driven by hot planetary winds and stream-like structures arising from cooler winds and tidal forces. Using three-dimensional hydrodynamic simulations combined with radiative transfer modeling, we study how interactions between stellar and planetary winds, as well as orbital dynamics, shape the helium 1083 nm triplet observed in transmission spectroscopy. We first present a theoretical framework for bubble-shaped outflows in tidally locked gas giants with strong day-to-night temperature contrasts. A parametric study reveals how varying wind conditions and atmospheric anisotropy influence helium absorption profiles. Notably, we find a clear correlation between increasing day-night anisotropy and the blueshift of the helium line. We then interpret asymmetric and widely extended helium absorption phase curves of HAT-P-67 b and HAT-P-32 b as signatures of leading, stream-like outflows from the day side. Our models reproduce the observed pre-transit absorption and provide constraints on mass-loss rates and outflow temperatures. This analysis is extended to the planets WASP-52 b and WASP-121 b. The helium observations and models of WASP-52 b suggest an intermediate outflow regime between a bubble and a stream, while the results for WASP-121 b suggest a pronounced leading stream of atmospheric escape. Together, these studies highlight the diagnostic power of three-dimensional models in interpreting helium observations and advancing our understanding of atmospheric escape and its role in exoplanetary evolution.