Seasonal Changes in the Atmosphere of HD 80606b Observed with JWST's NIRSpec/G395H

High-eccentricity gas giant planets serve as unique laboratories for studying the thermal and chemical properties of H/He-dominated atmospheres. One of the most extreme cases is HD 80606b – a hot Jupiter orbiting a sun-like star with an eccentricity of 0.93 – which experiences an increase in incident flux of nearly three orders of magnitude as the star-planet separation decreases from 0.88 au at apoastron to 0.03 au at periastron. We observed the planet's periastron passage using JWST's NIRSpec/G395H instrument (2.8-5.2 μ m) during a 21 hr window centered on the eclipse. We find that, as the planet passes through periastron, its emission spectrum transitions from a featureless blackbody to one in which CO and CH_4 absorption features are visible. We obtain significant detections of CH_4 during post-periapse phases at 3.7-4.8σ depending on the phase. Following periapse, CO and H_2O are also detected at 3.4σ and 3.1σ, respectively. Furthermore, we rule out the presence of a strong temperature inversion near the IR photosphere – predicted by GCMs to form temporarily during periapse passage – based on the lack of obvious emission features throughout the observing window. Our study demonstrates the feasibility of studying hot Jupiter atmospheres using partial phase curves obtained with NIRSpec/G395H.