The Super-puff WASP-193 b 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/Planet Finder Spectrograph radial velocity 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  M_J , R_p = 1.319^+0.056_−0.048 R_J , 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 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 λ = 16⁺¹⁶₋₁₅ 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.