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

TL;DR

This study reanalyzed JWST data of WASP-96 b, confirming a clear atmosphere without hazes, constraining its atmospheric composition, and providing insights into its formation history and atmospheric asymmetries.

Contribution

It presents a comprehensive multi-instrument transmission spectrum analysis, refines atmospheric composition constraints, and offers new insights into the planet's formation and atmospheric asymmetries.

Findings

No high-altitude hazes detected, confirming a clear atmosphere.

Constraints on atmospheric chemical abundances and C/O ratio.

Evidence of sodium limb asymmetry with a 50-second transit offset.

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 non-detection 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 VLT, HST, and Spitzer to produce a transmission spectrum from 0.35 $\mu$m to 5 $\mu$m. We constrain the mass fraction of multiple chemical species, including: H$_2$O$=-2.62^{+0.43}_{-0.42}$, K$=-5.76^{+1.05}_{-1.13}$, and Na$=-3.40^{+0.90}_{-0.92}$. C/O ratio and metallicity are tentatively constrained at substellar values (C/O$_{planet}=0.57^{+0.07}_{-0.12}$ and [Fe/H]$_{planet}=0.01^{+0.46}_{-0.52}$ compared to C/O$_{star}=0.92\pm0.25$ and [Fe/H]$_{star}=0.24\pm0.05$). Inputing these composition constraints to interior models, we constrain a core mass of $43^{+8}_{-15}$ M$_\oplus$. This, in addition to our inferred super-stellar refractory-to-oxygen ratio ($\Delta\log_{10}(R/O)=1.48^{+0.57}_{-0.62}$) and substellar C/O ratio, suggests that the core of WASP-96 b likely formed outside of water iceline, 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 seconds, while the water features appear symmetric. CH$_4$, CO, and CO$_2$ remain unconstrained due to spectral coverage limits. Upcoming JWST NIRSpec/G395H observations (ID 4082, PI: M. Radica) will be crucial for constraining these carbon-bearing species.