Low densities, eccentricities, and entropies in a young, compact multi-planet system

Close-in planetary systems are ubiquitous in the Galaxy, with constituent planet sizes that rarely exceed four times the size of Earth. The discovery of four large (5–10 Earth radii) planets transiting the young star V1298 Tau hinted that close-in, gaseous planets start off larger and with lower densities than the typical systems observed at mature stages. This interpretation is supported by planet formation models which predict larger sizes and lower densities for planets at young ages. It was surprising, then, when high planetary masses and densities were inferred for the two outer planets in the V1298 Tau system from radial velocity measurements. Here we report the results of a campaign to measure planet masses in the V1298 Tau system through transit-timing variations. We measure masses in the range of 3-21 Earth masses, corresponding to densities of 0.02-0.26 g/cm³. Dynamical simulations confirm our preferred orbital solutions are stable on timescales of 10⁶ yr and allow us to reject alternative solutions with higher planet masses. These low planetary densities are compatible with the substantial contraction and/or loss of H/He envelopes over timescales of at least tens of millions of years, and suggest the occurrence of rapid mass-loss during disk dispersal and a corresponding decrease in entropy known as “boil-off”.