High-resolution transmission spectroscopy enables the characterization of exoplanetary atmospheres through individual atomic and molecular absorption lines. However, stellar surface inhomogeneities, such as spots and faculae, imprint spectroscopic signatures that can mimic or obscure planetary atmospheric signals, posing a major challenge for the detection and characterization of Earth-analogue atmospheres.
The Sun provides a unique opportunity to investigate these effects, as it is the only star whose surface can be spatially resolved. The Paranal Solar ESPRESSO Telescope (PoET), which feeds the ESPRESSO spectrograph at the VLT, delivers ultra-high-resolution (R > 200,000) observations over the optical wavelength range (380–780 nm), including both disk-resolved spectra of active regions and simultaneous disk-integrated ("Sun-as-a-star") observations. These observations provide a unique benchmark for testing and refining theoretical models of stellar activity relevant to high-resolution transmission spectroscopy.
As a first step toward this goal, we investigate how stellar spots affect the detectability of molecular features in high-resolution transmission spectroscopy using synthetic stellar models. By simulating transits of an atmosphere-less Earth analogue orbiting a Sun-like star, we isolate stellar-induced signals and explore how spot properties and their spatial distribution influence the detectability of molecular species commonly targeted in atmospheric studies. These simulations establish the theoretical framework that will be tested and refined using observations from PoET, ultimately improving the interpretation of high-resolution transmission spectra of Earth analogues with next-generation facilities such as ANDES at the ELT.
