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

We reanalyze WASP-96 b by combining JWST NIRISS/SOSS data with VLT/HST/Spitzer spectra to produce a continuous 0.35–5 micron transmission spectrum and perform joint atmospheric retrievals. The blue Rayleigh slope is not detected; the spectrum favors a gray cloud deck and provides robust constraints on Na, K, and H2O features, with no significant inter-instrument offsets. Retrieval favors C/O_planet ≈ 0.57 and near-solar metallicity, a core mass ≈ 43 Earth masses, and an enhanced refractory-to-oxygen ratio, consistent with formation outside the water iceline and disk-driven migration with atmospheric accretion inside the carbon soot line. A tentative sodium limb asymmetry is detected with a ~50 s mid-transit offset, while water features remain symmetric; future JWST NIRSpec/G395H data are needed to constrain carbon-bearing species.

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 $μ$m to 5 $μ$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 ($Δ\log_{10}(R/O)=1.48^{+0.57}_{-0.62}$) 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.

Figures (7)

• Figure 1: Comparison between the extracted order 1 contaminant spectrum and the best fitting PHOENIX stellar model spectrum corrected for SOSS throughput. The PHOENIX stellar model has $T_{eff} = 3900~K$, log g $= 4.5$, and [Fe/H] = 0.5. The large spike at $\sim$ 1.5 $\mu m$ is due to a zeroth order contaminant overlapping with the first order contaminant.
• Figure 2: Extraction of 1D spectrum for order 1 and order 2 spectral traces with simple box extraction. The box widths are optimized so that the scatter for the out-of-transit light curves is minimized.
• Figure 3: Best fitting white light curves with residuals. Top: the best-fit white light curves for SOSS order 1 and order 2 overplotted with the data. The reduced chi-square for order 1 is 0.91 and that for order 2 is 0.94. Bottom: The residuals for the white light curve fits. The scatter for order 1 is 157 ppm, and that for order 2 is 278 ppm. The red and blue dashed lines show median scatter of order 1 and order 2 residuals, respectively.
• Figure 4: The relationship between bin size and residual scatter for white light curve of order 1 and the white light curve of order 2. The blue and green dashed lines are the theoretical decrease for white noise (proportional to $1/\sqrt{N}$) for order 1 and order 2, respectively, and the horizontal dashed lines are estimates of the red noise. The maximum bin size is chosen such that there are at least 10 data points to measure the standard deviations reliably. The residuals generally trace the photon noise. The red noise level for order 2 is $\sim$ 0 and is therefore off the scale of this plot.
• Figure 5: SOSS transmission spectrum extracted with FIREFLy combined with VLT 2018Natur.557..526N, Hubble 2022MNRAS.515.3037N, and Spitzer 2022MNRAS.515.3037N spectrum. At overlapping wavelengths, the VLT and Hubble spectra are consistent with the SOSS spectrum.