Towards Mitigating the Impact of Stellar Photospheric Heterogeneity on Precise Exoplanet Transmission Spectra

Exoplanet transmission spectroscopy, the study of spectroscopic transit depths, provides our best opportunity to characterize the atmospheres of temperate, Earth-sized exoplanets in the next two decades. However, this technique is subject to spurious signals introduced by the photospheric heterogeneity of exoplanet host stars, which may mimic or mask real exoplanetary signals. As the late-type hosts that provide the most favorable planet-to-star radius ratios for studying small exoplanets also tend to be more photospherically heterogeneous, our ability to disentangle stellar and planetary signals represents a possible limitation for precise transmission spectroscopy. Here we present our recent work to understand the scale of TLS signals with forward models and to characterize the photospheres of important exoplanet host stars through (1) transit crossings of magnetic active regions,(2) joint retrievals of stellar and exoplanetary properties from transmission spectra, and (3) stellar photospheric decomposition with moderate-resolution visual and near-infrared spectra. Constraining the photospheric heterogeneity of exciting exoplanet host stars with these approaches will be key to realizing the opportunity to characterize Earth-sized exoplanets via transits.