Longitudinally Resolved Spectral Retrieval (ReSpect) of WASP-43b

Abstract

Thermal phase variations of short period planets indicate that they are not spherical cows: day-to-night temperature contrasts range from hundreds to thousands of degrees, rivaling their vertical temperature contrasts. Nonetheless, the emergent spectra of short-period planets have typically been fit using one-dimensional (1D) spectral retrieval codes that only account for vertical temperature gradients. The popularity of 1D spectral retrieval codes is easy to understand: they are robust and have a rich legacy in Solar System atmospheric studies. Exoplanet researchers have recently introduced multi-dimensional retrieval schemes for interpreting the spectra of short-period planets, but these codes are necessarily more complex and computationally expensive than their 1D counterparts. In this paper we present an alternative: phase-dependent spectral observations are inverted to produce longitudinally resolved spectra that can then be fitted using standard 1D spectral retrieval codes. We test this scheme on the iconic phase-resolved spectra of WASP-43b and on simulated JWST observations using the open-source pyratbay 1D spectral retrieval framework. Notably, we take the model complexity of the simulations one step further over previous studies by allowing for longitudinal variations in composition in addition to temperature. We show that performing 1D spectral retrieval on longitudinally resolved spectra is more accurate than applying 1D spectral retrieval codes to disk-integrated emission spectra, despite being identical in terms of computational load. We find that for the extant Hubble and Spitzer observations of WASP-43b the difference between the two approaches is negligible but that JWST phase measurements should be treated with longitudinally \textbf{re}solved \textbf{spect}ral retrieval (ReSpect).