GEMS JWST: HATS-75 b -- A giant planet with a sub-solar metallicity atmosphere orbiting an M-dwarf

Abstract

HATS-75 b is one of the recently discovered Giant Exoplanets orbiting M-dwarf Stars (GEMS) with a transmission spectrum shaped by both its atmosphere and the active stellar surface it transits. As part of a JWST program studying 7 GEMS, we observed three transits of HATS-75 b with the NIRSpec PRISM instrument (0.6-5.3 um). The planet's spectra exhibit a slightly larger transit depth at shorter wavelengths, indicative of hazes or stellar contamination due to stellar heterogeneities outside the transit chord, i.e., the transit light source (TLS) effect. While both a hazy atmospheric model or TLS model can replicate the transmission spectrum, independent evidence (.e.g, stellar rotation, spot-crossing events) favors a model that includes contamination from unocculted starspots and faculae. Within this stellar heterogeneity / TLS-based framework, atmospheric retrievals yield remarkably low atmospheric metallicity (log[M/H]=-1.74^{+0.92}_{-0.76}) and super-solar carbon-to-oxygen (C/O=1.04^{+0.40}_{-0.09}), which paired with a best-fit interior model with bulk metallicity of Z_p=0.20+/-0.04, implies poor vertical mixing within the planet. Retrievals also detect robust absorption signatures of CH4, CO, and CO2. We obtain only an upper limit for H2O, consistent with its atmospheric spectral features being masked by stellar contamination. These results underscore the importance of accounting for stellar heterogeneity when interpreting exoplanet transmission spectra and highlight HATS-75 b as a significant asset to our understanding of giant exoplanets around M-dwarfs with JWST.