October
2026
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2026AJ....172..234P
Authors
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Pan, Yanbo
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MacDonald, Ryan J.
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Espinoza, Néstor
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Prinoth, Bibiana
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Rustamkulov, Zafar
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Welbanks, Luis
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Kitzmann, Daniel
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Crouzet, Nicolas
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Walde, Anna
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Powell, Diana
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Ahrer, Eva-Maria
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Burgasser, Adam J.
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Christie, Duncan A.
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Cukier, Wolf
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Gascón, Carlos
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Holmberg, Måns
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Kennedy, Thomas
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López-Morales, Mercedes
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Mayne, N. J.
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Samra, Dominic
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Savel, Arjun B.
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Steinrueck, Maria E.
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Wirth, Christopher P.
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JWST GO 3969 Team
Abstract
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Exoplanets orbiting active stars offer distinct challenges for atmospheric characterization with the JWST. Among such worlds, the hot Saturn WASP-52b (Teq ~ 1300 K) orbits an active K-dwarf that complicates transmission spectroscopy via stellar contamination. Here, we present the first JWST NIRSpec G395H (2.8--5.2 μm) limb-averaged transmission spectrum of WASP-52b obtained through the JWST Morning/Evening Program (GO-3969), which aims to measure limb asymmetries across a sample of hot giant exoplanets. Our spectrum reveals the first detection of CO2 in WASP-52b's atmosphere ( lnB=54 / >10σ), with a retrieved abundance of log CO2=-5.09-1.14+1.09 , and confirmation of H2O ( lnB=5.9 / ~3σ--4σ), with an abundance of log H2O=-3.41-1.04+1.04 . We find a steep near-infrared slope that cannot be explained solely by H2O absorption, necessitating unocculted starspots ( lnB=5.7 / ~3σ--4σ), though with properties that vary across our data reductions. The shape and amplitude of the molecular bands additionally suggest high-altitude (<10 mbar) inhomogeneous clouds covering ≈40% ± 20% of the terminator ( lnB=1.9 / ~2.5σ), which provides evidence for the morning-evening terminator differences that are consensus predictions of general circulation models. WASP-52b's atmospheric metallicity can be subsolar, solar, or supersolar, due to order-of-magnitude uncertainties in the molecular abundances caused by degeneracies between compositions, clouds, and starspots. However, chemical equilibrium retrievals provide a similar statistical fit while favoring a tighter constraint with a super-solar metallicity (M/H =14.8-5.9+12.1× solar). These results highlight that chemical detections and aerosol properties may still be recovered for planets orbiting active stars via transmission spectroscopy.
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