A Nightside Emission Spectrum of WASP-17b using the Planetary Infrared Excess Technique

To date, characterization of exoplanet atmospheres has relied largely on time-series observations of transiting exoplanets and high-contrast direct imaging. While these methods have yielded exciting scientific results, to truly explore the distribution of planet types and solar system structures, we will need to characterize both transiting *and* non-transiting planets. Previous studies (Stevenson 2020; Lustig-Yaeger et al. 2021) have demonstrated the theoretical utility of the planetary infrared excess (PIE) technique using simulated exoplanet observation data, but PIE requires validation via empirical measurements. Here we apply the PIE technique (XRP-80NSSC23K0373, PI Stevenson, Science PI Sotzen) to the combination of WASP-17b NIRSpec G395H transmission and emission spectra obtained as part of the JWST-TST program (GTO-1353, PI Lewis) to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS). We applied the PIE technique to these spectra in conjunction with a calibrated stellar spectrum in order to extract the first JWST nightside emission spectrum using only transit and eclipse data. We then performed atmospheric retrievals for composition and thermal structure. We will present the nightside emission spectrum as well as the nightside retrieval results. This groundbreaking study highlights our ability to constrain an exoplanet’s nightside emission with JWST, thus paving the way for further application of the PIE technique.