A New Method for Constraining 3D Exoplanet Atmospheres: Application to WASP-94A b’s Panchromatic Spectrum

Abstract A growing number of transiting exoplanets exhibit spectroscopic evidence that their leading and trailing limbs differ. Interpreting these observations is now the limiting step. The standard approach of fitting limb-resolved transit depths results in highly anticorrelated transit depths, and treating them as independent measurements can result in strongly biased (3 σ ) retrieved molecular abundances. We present an inference framework that avoids these correlations by using the temporal signature of limb asymmetry instead. This wavelength-dependent shift in mid-transit time, or “time-shift spectrum,” carries the limb-asymmetry information, is measurable from existing light curves with a spherically symmetric transit model, and is uncorrelated with the spherically averaged transit depth. Simultaneously fitting the resulting transmission and time-shift spectrum with an independent atmospheric column for each limb enables the retrieval of limb-resolved properties from two independent measurements. We demonstrate that this approach reduces the mean squared error on the retrieved molecular abundances by factors of ∼30–50 relative to the next best of the alternative approaches currently adopted in the field, and improves retrieved abundance precision by ∼3–6× . We apply the framework to WASP-94A b and present its panchromatic limb-resolved spectrum, spanning 0.8–5 μ m and combining NIRISS/SOSS and NIRSpec/G395H observations. The planet’s morning limb remains aerosol-obscured across this entire range, while its evening limb constrains the abundances of H 2 O, CO 2 , CO, and H 2 S. We measure a limb-to-limb temperature difference of 415 ± 58 K at 1 mbar. This limb-resolved analysis finds [M/H] = +0.32 − 0.11 + 0.11 and C/O = 0.724 − 0.013 + 0.013 , consistent with the host star.