Earth Analogs in Reflected Light: Insights from Early Spectral Characterization in Unconstrained Orbits

Abstract: A next generation of space-based observatories aims to detect and characterize potentially Earth-like exoplanets around Sun-like stars using reflected light spectroscopy. However, it remains unclear how such direct imaging observations$-$limited in spectral coverage and signal-to-noise ratio (S/N)$-$translate into constraints on atmospheric composition and habitability. Coronagraphs used for high-contrast imaging typically operate over narrow bandpasses, and exposure time limits can restrict data quality. To optimize observing strategies and instrument design, we use our atmospheric retrieval tool, $\texttt{rfast}$, to assess the performance of a $\mathit{Habitable\ Worlds\ Observatory}$-type mission across different spectral bandpasses ("Red", "Blue", "Visible", "NIR", and their combination) and S/N levels (10, 15, and 20; from moderate to moderate-high observation quality) in retrieving a wide range of 17 atmospheric, surface, bulk, and orbital parameters of a habitable Earth analog. We outline the observation requirements for each parameter and the detection capabilities of each case, within a novel scenario where spectral data are taken "early", prior to achieving orbit constraints (which may require repeat visits to a system). For coronagraph-restricted and NIR-only bandpasses, most of the limited retrievable information is already captured at S/N = 10, with little improvement at higher S/N. For broader spectral coverage, the quality and quantity of retrieved information improve with increasing S/N, but combining visible and NIR ranges provides the most comprehensive characterization, even at moderate S/N. To maximize returns, wider spectral coverage should be prioritized over improving S/N when spectral access is limited.