Do Super-Puffs Defy Core Accretion? Population-Wide Interior Structure Constraints

Abstract

Sub-Saturn mass planets with extremely low bulk densities $(ρ\lesssim0.3)\mathrm{g/cm^3}$, or ``super-puffs'', are one of the most interesting and least understood populations of exoplanets. While many short-period super-puffs can be attributed to the effects of high irradiation and star-planet interactions, cold super-puffs appear to challenge the expectations of core accretion theory. We constrain the possible properties of 34 cold super-puffs by computing hydrostatic interior structures using PlanetSolver. We find that 28 planets in our sample can be reproduced by models consistent with core accretion based on their observed masses and radii and adjusting for planet age. We identify HIP 41378 f, Kepler-30 d, Kepler-51 d, Kepler-177 c, TOI-1420 b, and WASP-107 b as planets inconsistent with core accretion theory which necessitate a non-standard explanation (e.g. exo-rings). With the exception of TOI-1420 b, core accretion-compatible solutions are possible for these planets if an additional heat source is present. We modify planetary evolution models to determine whether enhanced radiogenic heating or late impacts with sub-planetary mass objects can plausibly inflate sub-Neptunes enough to achieve super-puff densities. We find that the effects of radiogenic heating are insufficient to produce super-puff densities, but that impacts can in many cases produce the necessary inflation for upwards of 1Gyr. We also compile and present here an index of all currently known super-puffs.