A Multiplanet System’s Sole Super-puff: Exploring Allowable Physical Parameters for the Cold Super-puff HIP 41378 f

Abstract

The census of known exoplanets exhibits a variety of physical parameters, including densities that are measured to span the range from less dense than Styrofoam to more dense than iron. These densities represent a large diversity of interior structures. Despite this staggering diversity, recent analyses have shown that the densities of planets that orbit a common star exhibit remarkable uniformity. A fascinating exception to this is the system HIP 41378 (also known as K2-93), which contains a super-puff planet, HIP 41378 f, as well as several planets with more typical bulk densities. The range of densities in this system begs the question of what physical processes are responsible for the disparate planetary structures in this system. In this paper, we consider how the densities of the planets in the HIP 41378 system would have changed over time as the host star evolved and the planets’ atmospheres were subsequently affected by the evolving insolation level. We also present a range of allowable core masses for HIP 41378 f based on the measured planet parameters, and we comment on the feasibility of the proposed existence of planetary rings around HIP 41378 f as an explanation for its current low density.