Windows into Planetary Evolution: the Detection and Characterization of Young Worlds

Young exoplanets -- those less than a billion years old -- offer a unique window into the early stages of planetary formation and evolution. Gaining insight into how these worlds acquire and lose their atmospheres, migrate within their systems, and evolve into the diverse populations we observe today is essential for refining models of planet formation. However, this phase remains poorly understood due to the rarity of known young transiting systems and observational challenges such as stellar activity. In this thesis, I investigate the atmospheres of several young exoplanets using a combination of ground- and space-based observations, including data from the Spitzer, Hubble, and James Webb Space Telescopes. Through transmission spectroscopy across optical and infrared wavelengths, I probe their atmospheric compositions, characterize aerosol properties and stellar spot contributions, and infer planetary masses using atmospheric scale height measurements. The findings of this work include the discovery of two transiting sub-Neptune-sized exoplanets orbiting the young M dwarf, TOI-1224, adding to the small, but growing sample of young multiplanet systems. I also present the first atmospheric analysis of K2-25b, K2-33b, and HIP 67522 b, which reveals significant diversity in their observed properties. These findings provide new constraints on atmospheric evolution in young systems and help connect early formation processes to the characteristics of mature exoplanets.