CHEOPS observations of V1298 Tau updated planetary densities and implications on the early evolution of the young system

The young (10–30 Myr) multiplanet system V1298 Tau presents a unique opportunity to probe the early formation and evolution of young systems. We present new CHEOPS observations of the three innermost planets, yielding high-precision planetary radii ((Formula presented) 5–11 R(Formula presented) ) and improving the radius ratios ((Formula presented) ) by 30-71 per cent compared to previous multiple TESS observations. Combined with refined period and mass determinations from transit-timing variation measurements, we derive revised bulk densities (0.06–0.23 (Formula presented) ) for these planets. We find that the innermost planet c is denser compared to the outermost planet at the 3.4(Formula presented) level, while the bulk densities of the three outermost planets are consistent within the reported uncertainties. These bulk densities suggest differing envelope mass fraction across the system, indicating differential atmospheric evolution in the young system. We further assess the early dynamical state of the V1298 Tau system and find that within the range of simulations performed we find no evidence for present-day mean-motion resonance trapping. As an independent diagnostic, we compute the forced eccentricities and low normalized angular momentum deficit exhibited by the system. Our simulations suggest that no past dynamical excitation is required to explain the present orbital architecture.