GEMS JWST A Subsolar Metallicity Atmosphere for Giant Planet TOI-5293 A b Orbiting a Rapidly Changing M Dwarf

The growing sample of giant exoplanets around M dwarf stars (GEMS) helps probe the extremes of giant planet formation. Comparing the properties of this sample with their FGK counterparts can help us understand how planet formation and migration depend on stellar mass. We initiated a large Cycle 2 JWST transmission spectroscopy survey of seven GEMS. Here we present the atmospheric characterization using two JWST transits of TOI-5293 A b, a 0.5 M_Jup planet orbiting an early M dwarf with a period of ∼3 days. The two NIRSpec/PRISM transits indicate the planet is eclipsing a rapidly changing (heterogeneous) stellar photosphere. We see that Visit 1 had heterogeneity crossings across the entire transit chord, rendering inferences from it to be unreliable. The Visit 1 spectrum exhibits a downward slope at <1 μm suggestive of stellar contamination from faculae. In contrast, for Visit 2 we are able to model the heterogeneity crossings and obtain a transmission spectrum free from stellar contamination. We therefore limit our conclusions to a detailed analysis of Visit 2, and using Bayesian free chemistry retrievals, we find a low atmospheric metallicity (log[M/H] = -1.03^+0.53_-0.44 × solar) and supersolar C/O ratio (1.23^+2.94_-0.75). The retrievals yield Bayes factors that indicate strong evidence for CH4 as well as low-significance detections of CO₂, H₂O, and NH₃. Finally, using thermal evolution models we find that the radius of TOI-5293 A b is inflated above theoretical expectations (∼1.07 R_Jup), despite it having an temperature of ∼700 K, and hence we were unable to constrain its bulk composition.