The Super-Puff WASP-193b is On A Well-Aligned Orbit

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 (ρₚ 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/PFS RV measurements of WASP-193, a late F star hosting one of the least dense transiting planets known to date (Mₚ = 0.112+0.029-0.034\,MJ, Rₚ = 1.319+0.056-0.048\,RJ, ρₚ = 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 re-inflated 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 λ = 17⁺¹⁷₋₁₆ 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.