Exoplanets through time Atmospheric characterization of young transiting planets to trace signatures of early evolution

Demographic studies exoplanets have revealed population level trends, such as the exoplanet ‘radius valley’. A widely accepted hypothesis to explain these demographic features is that exoplanets on compact orbits experience significant atmospheric mass loss right after the end of their formation phase. This early evolution can shape the demographic properties of the mature exoplanet population.
However, it is not known how the initial stages of an exoplanet’s life looked like: what was their initial nature and composition? Are young exoplanets a homogeneous population or they exhibit diversity right after formation? How do they compare with their mature counterparts?
To address these questions, in this thesis we present the first observations of the atmospheres of young transiting planets with the Hubble Space Telescope and the James Webb Space Telescope. We characterize the atmospheres of two young planets (20-30 million years old) in the same system, V1298 Tau to measure their mass, atmospheric composition and internal entropy using transmission spectroscopy. We compare them with each other as well as with the mature exoplanet population to understand the impact of early evolutionary mechanisms on their atmosphere. Our findings challenge predictions about the atmospheric composition and internal entropy from the standard core-accretion planet formation and thermal evolution models. We explore new ideas about the internal structure of these planets which could potentially reconcile them with their mature counterparts.