JWST-TST DREAMS: The Nightside Emission and Chemistry of WASP-17b

TL;DR

This study validates the Planetary Infrared Excess (PIE) approach for JWST by extracting WASP-17b’s nightside emission through a model-independent iESR technique applied to two closely spaced epochs. Using JWST/NIRSpec G395H transit and eclipse data, the authors obtain nightside emission spectra and derive a nightside brightness temperature near $1005\pm256$ K, along with tentative evidence for nightside SO2, and they infer a nonzero Bond albedo of $0.42^{+0.06}_{-0.10}$. A two-step modeling framework—POSEIDON-based atmospheric retrievals plus photochemical-transport modeling—yields constraints on the nightside temperature-pressure structure and molecular abundances, linking the nightside chemistry to dayside conditions and indicating potential transport-induced chemistry. The results demonstrate PIE feasibility with JWST for two epochs and highlight the method’s potential to probe 3D exoplanet atmospheres, including chemistry driven by day-night transport, while outlining instrument-systematics challenges for extending PIE to other JWST instruments.

Abstract

Theoretical studies have suggested using planetary infrared excess (PIE) to detect and characterize the thermal emission of transiting and non-transiting exoplanets, however 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 (0.5 phase or 1.8 days apart) as part of the JWST-TST program to perform Deep Reconnaissance of Exoplanet Atmospheres through Multi-instrument Spectroscopy (DREAMS). Using the in-eclipse measured stellar spectrum to circumvent the need for ultra-precise 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 $1005 \pm 256$ K and find tentative evidence for nightside SO2 absorption ($\ln B = 1.45$, $2.3σ$). 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 non-zero bond albedo of $0.42^{+0.06}_{-0.10}$. 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 into 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.

Figures (3)

• Figure 1: PHOENIX stellar model fit (blue points) to the calibrated stellar spectrum of WASP-17A observed with NIRSpec G395H using the NRS1 detector (black points). The relative residuals between data and model are scattered around zero with a standard deviation of ${\sim$∼$}1.3\%$. Thermal emission from the planet must exceed this data-model noise floor to extract the PIE signal. In this case, WASP-17b's average dayside planet-to-star flux ratio in NRS1 is measured at 0.133% (indicated in lower panel), approximately $10\times$ below our model-derived limit.
• Figure 2: Planetary infrared excess spectra from the day (orange and red points) and nightsides (blue and cyan points) of WASP-17b. Dayside (nightside) 1 and 2 represent the pre-ingress and post-egress baseline spectra, respectively. The secondary eclipse dayside spectrum extracted using traditional methods is shown in black. Simple blackbody curves show the expected planet flux near the equilibrium temperature of WASP-17b (1700 K; pink line) and at 1000 K (navy blue line) for reference.
• Figure 3: Summary of retrieval results for the day (orange) and night (purple) side emission spectra of WASP-17b for retrievals using error inflation. Top Left: Emission spectra measurements fitted with the median retrieved model spectrum with envelopes for the 1 and 2 sigma posterior credible intervals. Error bars shown are inflated using \ref{['eqn:b_error']} with the median retrieved value of $b$. Top Right: Retrieved PT profile constraints. Bottom Row: Histograms showing the one-dimensional posterior probability density for a subset of retrieved parameters. The subplot titles list the 1 sigma posterior constraints.