2025ApJ...994L...4L

Theoretical studies have suggested using planetary infrared excess (PIE) to detect and characterize the thermal emission of transiting and nontransiting exoplanets, although the PIE technique requires empirical validation. Here, we apply the PIE technique to a combination of JWST NIRSpec G395H transit and eclipse measurements of WASP-17b, a hot Jupiter orbiting an F-type star, obtained consecutively (one-half of the orbital period or 1.8 days apart) as part of the JWST-TST program to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy. Using the in-eclipse measured stellar spectrum to circumvent the need for ultraprecise stellar models, we extract the first JWST nightside emission spectrum of WASP-17b using only transit and eclipse data, thereby performing a controlled test of the PIE technique. From the WASP-17b nightside spectrum, we measure a nightside equilibrium temperature of 1006 ± 276 K and find tentative evidence for nightside SO₂ absorption ($\mathrm{ln}B=1.4$). In context with the dayside, the temperature of the nightside is consistent with: (1) previous eclipse mapping findings that suggest relatively inefficient day-night heat transport; and (2) a nonzero bond albedo of $0.4{2}_{-0.10}^{+0.06}$. SO₂ on the nightside, if confirmed, would represent the first direct evidence for transport-induced chemistry, matching previous model predictions and opening a new door onto the 3D nature of giant exoplanets. Our results suggest that PIE is feasible with JWST/NIRSpec for two epochs separated in time by significantly less than the rotation period of the host star.