Towards robust corrections for stellar contamination in transmission spectra using HST, JWST, and TESS: first results from two Legacy programs

The first JWST transmission spectra have revealed exoplanet atmospheres in exquisite detail but also a stellar-contamination characterization bottleneck, particularly for M-dwarf planets. Here we present the first results from a large effort that leverages HST and JWST stellar spectra and TESS photometry to constrain stellar contamination in more than 30 exoplanet systems. We first explore the reliability of stellar models for deriving stellar-contamination corrections and find the limited fidelity of stellar models can dominate the noise budget of precise transmission spectra, especially for active or late-type stars. We then present a case study of HD 189733, an active K dwarf hosting a well-studied hot Jupiter. Using three archival HST/WFC3 datasets, we infer the spot temperature and covering fraction from the out-of-transit stellar spectra. We find the data are best explained by a model that includes a persistently high (~40%) covering fraction of ~3200 K spots that, given the target’s TESS variability, must be distributed primarily axisymmetrically. These results highlight (1) the need for better model-based and empirical constraints on host-star photospheres and (2) the unique role that TESS data play in this effort.