GEMS JWST: HATS-75 b A Giant Planet with a Subsolar Metallicity Atmosphere Orbiting an M Dwarf

HATS-75 b is one of the recently discovered Giant Exoplanets Around 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 seven GEMS, we observed three transits of HATS-75 b with the NIRSpec PRISM instrument (0.6–5.3 μm). 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_−0.76^+0.92) and supersolar carbon-to-oxygen (C/O=1.04_−0.09^+0.40), 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.