Constraining the entropy of formation from young transiting planet

Abstract

Recently, K2 and TESS have discovered transiting planets with radii between ∼5 and 10 R⊕ around stars with ages <100 Myr. These young planets are likely to be the progenitors of the ubiquitous super-Earths/sub-Neptunes, which are well studied around stars with ages ≳1 Gyr. The formation and early evolution of super-Earths/sub-Neptunes are poorly understood. Various planetary origin scenarios predict a wide range of possible formation entropies. We show how the formation entropies of young (∼20–60 Myr), highly irradiated planets can be constrained if their mass, radius, and age are measured. This method works by determining how low-mass an H/He envelope a planet can retain against mass-loss, this lower bound on the H/He envelope mass can then be converted into an upper bound on the entropy. If planet mass measurements with errors ≲20 per cent can be achieved for the discovered young planets around DS Tuc A and V1298 Tau, then insights into their origins can be obtained. For these planets, higher measured planet masses would be consistent with the standard core-accretion theory. In contrast, lower planet masses (≲6–7 M⊕) would require a ‘boil-off’ phase during protoplanetary disc dispersal to explain.