Super-Solar Metallicity and Tentative Evidence for Photochemistry on WASP-96b from JWST and Ground-Based VLT Transmission Spectroscopy

TL;DR

This study combines JWST and ground-based spectra to analyze WASP-96b's atmosphere, revealing super-solar metallicity, potential photochemical SO2, and possible limb asymmetry, advancing understanding of exoplanet atmospheres.

Contribution

First combined JWST and ground-based transmission spectra analysis of WASP-96b, revealing super-stellar metallicity, photochemical signatures, and exploring atmospheric asymmetries.

Findings

WASP-96b has a super-stellar metallicity of 2-6 times stellar.

Detected SO2 consistent with photochemical models.

Optical slope suggests scattering aerosols or hazes.

Abstract

With its expanded wavelength coverage and increased precision compared to previous space-based observatories, JWST provides the opportunity to revisit benchmark planets and view them in a new light. Here, we conduct an in-depth study of the atmosphere of the hot-Saturn WASP-96b combining a new JWST NIRSpec/G395H transit with archival NIRISS/SOSS and VLT/FORS2 transmission spectra. The combined spectrum shows clearly-visible features from H2O, CO2, and Na. CO, though, remains unconstrained, precluding a firm metallicity derivation from free retrievals alone. However, self-consistent grids yield a broadly super-stellar atmospheric metallicity of 2-6x stellar. When combined with a roughly stellar C/O ratio ($0.41^{+0.10}_{-0.09}$ from self-consistent grids), we find that WASP-96b potentially formed via core-accretion beyond the H2O snowline and subsequently accreted volatile-rich material. Free retrievals also find a moderate preference (ln B=2.69) for models with SO2 versus without. WASP-96b falls directly on the proposed "SO2 shoreline" and the retrieved SO2 abundance is well-matched to predictions from photochemical models. Our combined spectrum displays an optical slope, which our models fit with opacity from scattering aerosols -- either small-particle condensate clouds or photochemical hazes -- though we cannot completely rule out the broad wings of Na or the effects of stellar contamination. Future observations are necessary to disentangle these effects. Finally, we explore the possibility for limb asymmetry in WASP-96b's transmission spectrum and provide several tests to identify asymmetries in our data. We encourage the community to prioritize the development of a robust pathway to quantify the presence of limb asymmetry -- particularly for low signal-to-noise cases.