Constraining mornings & evenings on distant worlds: a new semi-analytical approach and prospects with transmission spectroscopy

The technique of transmission spectroscopy -- the variation of the planetary radius with wavelength due to opacity sources in the planet's terminator region -- has been to date one of the most successful in the characterization of exoplanet atmospheres, providing key insights into the composition and structure of these distant worlds. A common assumption made when using this technique, however, is that the variations are the same in the entire terminator region. In reality, the morning and evening terminators might have distinct temperature, pressure and thus compositional profiles due to the inherent 3-D nature of the planet which would, in turn, give rise to different spectra on each side of it. Constraining those might be fundamental for our understanding of not only the weather patterns in these distant worlds, but also of planetary formation signatures which might only be possible to extract once these features are well understood. Motivated by this physical picture, in this work we perform a detailed study on the observational prospects of detecting this effect. We present an open-source semi-analytical framework with which this information can be extracted directly from transit lightcurves, and perform a detailed study on the prospects of detecting the effect with current missions such as TESS and upcoming ones such as JWST. Our results show that these missions show great promise for the detection of this effect. Transmission spectroscopy studies with JWST, in particular, could provide spectra of each of the limbs allowing us to convey 3-D information previously accessible only via phase-curves.