JWST-TST DREAMS: A Supersolar Metallicity in WASP-17 b’s Dayside Atmosphere from NIRISS SOSS Eclipse Spectroscopy
A. Gressier, R. J. MacDonald, N. Espinoza, H. R. Wakeford, N. K. Lewis, J. Goyal, D. R. Louie, M. Radica, N. E. Batalha, D. Long, E. M. May, E. Mullens, S. Seager, K. B. Stevenson, J. A. Valenti, L. Alderson, N. H. Allen, C. I. Cañas, R. C. Challener, K. Colón, A. Glidden, D. Grant, J. Huang, Z. Lin, D. Valentine, C. M. Mountain, L. Pueyo, M. D. Perrin & R. P. Van Der Marel
Published in The Astronomical Journal, 2025
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 \(\mu\)m, our retrieval analysis reveals a strong detection of H2O in WASP-17 b’s dayside atmosphere (\(6.4\sigma\)). 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 \(\mu\)m, 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).
Recommended citation: Gressier, Amélie et al. (2025). 'JWST-TST DREAMS: A Supersolar Metallicity in WASP-17 b's Dayside Atmosphere from NIRISS SOSS Eclipse Spectroscopy.' The Astronomical Journal.
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