Journal ArticleThe Super-puff WASP-193 b is on a Well-aligned Orbit*Yee, Samuel W (ORCID:0000000179613907); Stefánsson, Gudmundur (ORCID:0000000174095688); Thorngren, Daniel (ORCID:0000000251138558); Monson, Andy (ORCID:0000000200482586); Hartman, Joel D (ORCID:0000000187326166); Charbonneau, David B (ORCID:000000029003484X); Teske, Johanna K (ORCID:0009000828015040); Butler, R Paul (ORCID:0000000313053761); Crane, Jeffrey D (ORCID:000000025226787X); Osip, David; Shectman, Stephen A (ORCID:0000000286816136)<title>Abstract</title> <p>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 (<italic>ρ</italic>_{<italic>p</italic>} ≲ 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 (<inline-formula><tex-math><CDATA/></tex-math><math overflow='scroll'><msub><mrow><mi>M</mi></mrow><mrow><mi>p</mi></mrow></msub><mo>=</mo><mn>0.11</mn><msubsup><mrow><mn>2</mn></mrow><mrow><mo>−</mo><mn>0.034</mn></mrow><mrow><mo>+</mo><mn>0.029</mn></mrow></msubsup><mspace width='0.25em'/><msub><mrow><mi>M</mi></mrow><mrow><mi mathvariant='normal'>J</mi></mrow></msub></math></inline-formula>,<inline-formula><tex-math><CDATA/></tex-math><math overflow='scroll'><msub><mrow><mi>R</mi></mrow><mrow><mi>p</mi></mrow></msub><mo>=</mo><mn>1.31</mn><msubsup><mrow><mn>9</mn></mrow><mrow><mo>−</mo><mn>0.048</mn></mrow><mrow><mo>+</mo><mn>0.056</mn></mrow></msubsup><mspace width='0.25em'/><msub><mrow><mi>R</mi></mrow><mrow><mi mathvariant='normal'>J</mi></mrow></msub></math></inline-formula>,<italic>ρ</italic>_{<italic>p</italic>} = 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<italic>Z</italic>= 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<inline-formula><tex-math><CDATA/></tex-math><math overflow='scroll'><mi>λ</mi><mo>=</mo><mn>1</mn><msubsup><mrow><mn>6</mn></mrow><mrow><mo>−</mo><mn>15</mn></mrow><mrow><mo>+</mo><mn>16</mn></mrow></msubsup></math></inline-formula>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.</p>American Astronomical Society Journals2025-03-2510654771The Astronomical Journal16942250004-6256https://doi.org/10.3847/1538-3881/adba5f2538457; 2108616National Science Foundation