Lessons Learnt on the Formation and Evolution of Young Planetary Systems from Ultraviolet Spectroscopy

Abstract Ultraviolet (UV) spectroscopy has played a central role in advancing our understanding of young planetary systems and their circumstellar environments. The UV domain uniquely probes hot plasmas, accretion shocks, magnetospheric accretion, stellar atmospheres, and outflows through spectral tracers that are otherwise inaccessible at optical or infrared wavelengths. Over the past several decades, space-based observatories have enabled high-resolution studies of resonance lines such as C IV, Si IV, N V, and Mg II, revealing the complex interplay between magnetospheric accretion, stellar activity, and mass outflows in pre-main-sequence stars. This review synthesizes observational results from major UV missions and examines how UV diagnostics constrain physical conditions in young stars, including temperature structure, mass accretion rates, wind kinematics, and magnetic topology. We discuss the evolution of theoretical models in light of UV data, identify persistent uncertainties, and highlight emerging directions for future instrumentation. UV spectroscopy remains indispensable for understanding stellar and planetary system formation.