Preprint models impact parameter sweet spot for TRAPPIST-1 transmission spectra

The authors built a fiducial stellar activity model with two latitudinal spot regimes driven by Coriolis versus buoyancy forces. They simulated transmission spectra for each TRAPPIST-1 planet across a range of impact parameters, sampling active-region evolution over multiple visits. Moderate-b planets showed both lower mean TLS contamination and higher visit-to-visit variance once spots were concentrated at higher latitudes. This pattern held regardless of whether spots were cooler or hotter than the photosphere. The work synthesizes prior TLS theory from Rackham et al. (2018) and TRAPPIST-1 spot-mapping results from Morris et al. (2018) to predict that variance in correlated residuals across planets can constrain filling factors and spot sizes. A key limitation is that the model assumes fixed latitudinal distributions without magnetic-cycle evolution or differential rotation; longer baseline JWST observations of multiple planets would test whether observed scatter matches the predicted frequency and amplitude thresholds.