On the road to the radius valley: distinguishing between gas dwarfs and water worlds with young transiting exoplanets

ABSTRACT The detection of young transiting exoplanets represents a new frontier in our understanding of planet formation and evolution. For the population of observed close-in sub-Neptunes, two proposed formation pathways can reproduce their observed masses and radii at ${{\sim}}$Gyr ages: the ‘gas dwarf’ hypothesis and the ‘water world’ hypothesis. We show that a sub-Neptune’s size at early ages ${\lesssim} 100$ Myr is strongly dependent on the bulk mean molecular weight within its envelope. As a result, gas dwarfs and water worlds should diverge in size at early ages since the mean molecular weight of gas dwarf envelopes is predicted to be smaller than that of water worlds. We construct population models under both scenarios that reproduce Kepler demographics in the age range ${\sim} 1\!-\!10$ Gyr. We find tentative evidence that the gas dwarf model is more consistent with the small population of young exoplanets ${<} 40$ Myr from the Transiting Exoplanet Survey Satellite. We show that planet radius is relatively insensitive to planet mass for young, puffy sub-Neptunes, meaning that well-characterized masses are not necessarily required to exploit the effects of mean molecular weight at the population level. We confirm the predicted difference in planet size between the models is also true under mixed-envelope scenarios, in which envelopes consist of mixtures of hydrogen and steam. We highlight that transit surveys of young exoplanets should target the youngest observable stellar clusters to exploit the effects of mean molecular weight.