JWST-TST DREAMS: A Supersolar Metallicity in WASP-17 b’s Dayside Atmosphere from NIRISS SOSS Eclipse Spectroscopy

We present the first emission spectrum of the hot Jupiter WASP-17 b using one eclipse observation from the JWST Near Infrared Imager and Slitless Spectrograph (NIRISS) Single Object Slitless Spectroscopy (SOSS) mode. Covering a wavelength range of 0.6–2.8 μm, our retrieval analysis reveals a strong detection of H2O in WASP-17 b’s dayside atmosphere (6.4σ). Our retrievals consistently favor a supersolar dayside H2O abundance and a noninverted temperature–pressure profile over a large pressure range. Additionally, our examination of the brightness temperature reveals excess emission below 1 μm, suggesting the possibility of a high internal temperature (600–700 K) and/or contributions from reflected light. We highlight that JWST emission spectroscopy retrieval results can be sensitive to whether negative eclipse depths are allowed at optical wavelengths during light-curve fitting. Our findings deepen our understanding of WASP-17 b’s atmospheric composition while also highlighting the sensitivity of our results to pressure–temperature profile parameterizations. This work is part of a series of studies by our JWST Telescope Scientist Team (TST), in which we will use Guaranteed Time Observations to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS).