Tracing the dynamical, interior, and atmospheric evolution of the young V1298 Tau planetary system

The Galaxy’s most common planetary systems consist of several Earth- to Neptune-sized planets on compact orbits, yet the young (10–30 Myr) star V1298 Tau hosts an unusual compact system of four large planets (≈5–10 Earth radii) arranged in a chain of near-mean-motion resonances that generate transit-timing variations of several hours. This system provides a rare opportunity to jointly investigate early dynamical, interior, and atmospheric evolution. We present new CHEOPS observations of the three innermost planets, delivering high-precision radius measurements that significantly improve constraints on their bulk densities and internal structures. Combined with recent mass estimates from transit-timing variations, we test whether the system formed in a resonant chain and subsequently evolved through tidal migration. We find that the planets are currently too far from exact commensurabilities for tidal dissipation to have driven them out of resonance, disfavouring a primordial full resonant chain. Accounting for post-formation planetary contraction further modifies the rate and direction of tidal migration, reducing the likelihood of resonance capture and suggesting formation with period ratios already below resonance. We also present complementary Hubble and JWST observations that reveal an extended hydrogen–rich atmosphere, with unexpectedly low metallicity for V1298 Tau b and a lack of methane, pointing to strong atmospheric mixing. Evolution models suggest substantial atmospheric loss over the next gigayear, potentially transforming the planet into a Neptune-sized world. V1298 Tau thus offers 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.