First comparative exoplanetology between planets within the same system, and between young and mature sub-Neptunes

We present results from the first comparative exoplanetology program between sister planets in the same young (20-30Myr old) system, V1298 Tau, and with more mature planets. A powerful combination of HST and JWST observations for this system provide the first constraints on atmospheric composition and elemental abundance ratios of two young transiting sub-Neptune/super-Earth progenitors with unpreceedented precision. This enables us to test the impact of stellar UV flux (orbital distance) on the atmospheric chemistry, haze properties by comparing their relative compositions and constrain late formation and early evolutionary scenarios using these constraints.  We reveal key differences between the nature and composition of the newborn sub-Neptunes/super-Earths with their mature counterparts, which we interpret as the first observational evidence for evolution of atmospheric composition, which is likely to have important implications for the overall thermal and atmospheric evolution of such planets.