From the Shadows: The Impact of Nightside Thermal Emission on Ultrahot Jupiter Transmission Spectrum Retrievals

Transmission spectroscopy is the most widely used technique for studying exoplanet atmospheres. Since the planetary nightside faces the observer during a transit, highly irradiated giant exoplanets with warm nightsides emit thermal radiation that can contaminate transmission spectra. Observations of ultrahot Jupiters in the near- and mid-infrared with JWST are especially susceptible to nightside contamination. However, nightside thermal emission is generally not considered in atmospheric retrievals of exoplanet transmission spectra. Here, we quantify the potential biases from neglecting nightside thermal emission in multidimensional atmospheric retrievals of an ultrahot Jupiter. Using simulated JWST transmission spectra of the ultrahot Jupiter WASP-33b (0.8–12 μ m), we find that transmission spectrum retrievals without nightside emission can overestimate molecular abundances by almost an order of magnitude and underestimate the dayside temperature by ≳400 K. We show that a modified retrieval prescription, including both transmitted light and nightside thermal emission, correctly recovers the atmospheric properties and is favored by Bayesian model comparisons. Nightside thermal contamination can be readily implemented in retrieval models via a first-order approximation, and we provide formulae to estimate whether this effect is likely to be significant for a given planet. We recommend that nightside emission should be included as standard practice when interpreting ultrahot Jupiter transmission spectra with JWST.