The Fault in our Stars: Towards Constraining Stellar Contamination in Exoplanet Transmission Spectra

Thursday 5 July, 11:50

Transmission spectroscopy provides a powerful probe of exoplanet atmospheres and the exciting possibility of studying even terrestrial exoplanet atmospheres in the near-term future. Increasingly precise observations, however, are revealing that transmission spectra are subject to astrophysical signals introduced by active regions in the host star photospheres, i.e, the transit light source effect. Here we provide an overview of stellar contamination in transmission spectra, drawing from the latest modeling as well as observations collected from both ground-based and space-based facilities. We summarize our work on constraining the impact of spots and faculae on transmission spectra using rotating photosphere models for F to M spectral types, detailing implications for searches for planetary atmospheric features and inferences of volatile budgets. We compare model predictions to state-of-the-art observational data from the G-dwarf hot Jupiter host WASP-19 to the ultracool-dwarf terrestrial planet host TRAPPIST-1, among others. For M dwarfs, including TRAPPIST-1, molecular absorption features in spots can imprint apparent features in transmission spectra comparable to or an order of magnitude larger than those of atmospheric features from rocky exoplanets, while contamination from unocculted faculae can mask real exoplanetary features. An emerging picture suggests that our understanding of stellar contamination will be a limiting factor for detecting atmospheric features in transmission spectra of low-mass exoplanets around late-type stars, including potential biosignatures from habitable zone exoplanets. We discuss how TESS and JWST results are impacted by stellar contamination and review the current ideas and proposals for correcting for the transit light source effect.

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