Retrieving the Red Edge on Earth-like Planets with Heterogeneous Clouds and Surfaces

Abstract The detection and characterization of potentially habitable exoplanets is one of the chief goals of astrophysics for the coming decades. Imaging in reflected light is well suited for characterizing Earth-like planets, because much can be learned about these planets in this wavelength range (i.e., ∼0.3–2 μ m). Several studies have been conducted to determine the abilities and limitations of reflectance spectroscopy, but most previous studies assumed a homogeneous atmospheric and surface composition. Here we investigate how heterogeneities in the atmosphere and surface of an Earth-like planet impact retrieval results. We extend the ExoReL ℜ retrieval framework to include a step function for retrieving wavelength-varying surface albedo. We then use it to retrieve on visible to near-infrared spectra of realistic 3D Earth models with different surface features in view and varying cloud types/distributions synthesized with the Planetary Spectrum Generator. Including the ability to fit for wavelength-dependent albedo mitigates degeneracies that arise when using 1D models to analyze 3D planets, and we recover an Earth-like planet in all cases. We detect surface albedo steps at ∼0.7 and ∼1.1 μ m despite clouds, both when significant lands are in view and when the spectra are averaged to account for a longer integration time. Our findings support the application of the vegetation red edge as a biosignature in the context of the Habitable Worlds Observatory. This study highlights the importance of considering a range of—particularly wavelength-dependent—surface albedos when using reflectance spectroscopy to characterize Earth-like exoplanets.