A Comprehensive Analysis of the Panchromatic Transmission Spectrum of the Hot-Saturn WASP-96 b: Nondetection of Haze, Possible Sodium Limb Asymmetry, Stellar Characterization, and Formation History

Abstract We conduct a reanalysis of the JWST NIRISS/SOSS observation of the hot-Saturn WASP-96 b. Initial analysis of this data revealed an enhanced Rayleigh scattering slope at the blue end of the transmission spectrum, suggesting the presence of hazes at high altitudes. In this work, we report nondetection of this slope, confirming an atmosphere clear of high-altitude aerosols consistent with the pre-JWST results. Also contrary to the initial result, our results indicate the presence of gray cloud deck, although at relatively low altitudes/high pressures. We further combined the NIRISS/SOSS spectrum with the Very Large Telescope, Hubble Space Telescope, and Spitzer to produce a transmission spectrum from 0.35–5 μ m. We constrain the mass fraction of multiple chemical species, including: H 2 O = − 2.6 2 − 0.42 + 0.43 , K = − 5.7 6 − 1.13 + 1.05 , and Na = − 3.4 0 − 0.92 + 0.90 . C/O ratio and metallicity are tentatively constrained at substellar values ( C/O planet = 0.5 7 − 0.12 + 0.07 and Fe/H planet = 0.0 1 − 0.52 + 0.46 compared to C/O star = 0.92 ± 0.25 and Fe/H star = 0.24 ± 0.05). Inputting these composition constraints to interior structure models, we constrain a core mass of 4 3 − 15 + 8 M ⊕ . This, in addition to our inferred super-stellar refractory-to-oxygen ratio ( Δ log 10 ( R / O ) = 1.4 8 − 0.62 + 0.57 ) and substellar C/O ratio, suggests that the core of WASP-96 b likely formed outside of the water ice line, underwent disk-driven migration, and accreted its atmosphere inside the carbon soot line. We find evidence of atmospheric leading-trailing terminator asymmetries in the broadened sodium absorption feature with a transit time offset of 50 s, while the water features appear symmetric. CH 4 , CO, and CO 2 remain unconstrai