Supersolar Metallicity and Tentative Evidence for Photochemistry on WASP-96 b from JWST and Ground-based VLT Transmission Spectroscopy

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-96 b combining a new JWST NIRSpec/G395H transit with archival NIRISS/SOSS and Very Large Telescope/FORS2 transmission spectra. The combined spectrum shows clearly visible features from H 2 O, CO 2 , and Na. CO, though, remains unconstrained, precluding a firm metallicity derivation from free retrievals alone. However, self-consistent grids yield a broadly superstellar atmospheric metallicity of 2–6× stellar. When combined with a roughly stellar C/O ratio ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0.4</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.09</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.10</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> from self-consistent grids), we find that WASP-96 b potentially formed via core-accretion beyond the H 2 O snowline and subsequently accreted volatile-rich material. Free retrievals also find a moderate preference ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ln</mml:mi> <mml:mi>B</mml:mi> </mml:math> = 2.69) for models with SO 2 versus without. WASP-96 b falls directly on the proposed “SO 2 shoreline” and the retrieved SO 2 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-96 b’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.