2025AJ....169..225Y

The “super-puffs” are a population of planets that have masses comparable to that of Neptune but radii similar to Jupiter, leading to extremely low bulk densities (ρ_p ≲ 0.2 g cm^–3) that are not easily explained by standard core accretion models. Interestingly, several of these super-puffs are found in orbits significantly misaligned with their host stars' spin axes, indicating past dynamical excitation that may be connected to their low densities. Here, we present new Magellan/Planet Finder Spectrograph radial velocity measurements of WASP-193, a late F star hosting one of the least dense transiting planets known to date (${M}_{p}=0.11{2}_{-0.034}^{+0.029}\,{M}_{{\rm{J}}}$, ${R}_{p}=1.31{9}_{-0.048}^{+0.056}\,{R}_{{\rm{J}}}$, ρ_p = 0.060 ± 0.019 g cm^–3). We refine the bulk properties of WASP-193 b and use interior structure models to determine that the planet can be explained if it consists of roughly equal amounts of metals and H/He, with a metal fraction of Z = 0.42. The planet is likely substantially reinflated due to its host star's evolution, and expected to be actively undergoing mass loss. We also measure the projected stellar obliquity using the Rossiter-McLaughlin effect, finding that WASP-193 b is on an orbit well aligned with the stellar equator, with $\lambda =1{6}_{-15}^{+16}$ degrees. WASP-193 b is the first Jupiter-sized super-puff on a relatively well-aligned orbit, suggesting a diversity of formation pathways for this population of planets.