Abstract DGP2026-88

The Galaxy’s most common planetary systems consist of several Earth- to Neptune-sized planets on compact orbits. Progenitors to those can be observed today transting young (10–100 Myr) stars that host unusual compact multi-planet systems of large planets (≈5–10 Earth radii) arranged in a chain of near-mean-motion resonances. Such systems provide rare opportunities to jointly investigate early dynamical, interior, and atmospheric evolution. We present a portfolio of observations of such young transiting exoplanets, delivering high-precision radius and mass measurements that significantly improve constraints on their bulk densities and internal structures. We test whether such system formed in a resonant chain and subsequently evolved through tidal migration. Complementary Hubble and JWST observations reveal extended hydrogen–helium atmospheres, including unexpectedly low metallicities and lack of methane, pointing to unusual chemistry and strong atmospheric mixing. Evolution models suggest substantial atmospheric loss over the next gigayear, potentially transforming the planet into a Neptune/sub—Neptune sized world.  Ultimately, we predict the planets will contract to Earth-to-Neptune-radii and join the population of small planets that nature produces in abundance. These systems offer a benchmark for linking dynamical history with atmospheric and interior evolution, a synergy that will be greatly expanded by the large sample of young planets expected from ESA’s forthcoming PLATO mission.