In Search of the Edge: A Bayesian Exploration of the Detectability of Red Edges in Exoplanet Reflection Spectra

TL;DR

This paper introduces a new method for detecting surface spectral features like the Earth's red edge in exoplanet reflection spectra, demonstrating improved retrieval accuracy and potential for identifying biosignatures.

Contribution

The study proposes a novel wavelength-dependent surface albedo parameterization that enhances the detection of spectral edges and improves atmospheric and surface property retrievals in exoplanet spectra.

Findings

Wavelength-dependent models outperform uniform albedo models in spectral retrievals.

The Earth's red edge can be precisely located within 70 nm at S/N=10.

Surface spectral features can be inferred from space-based direct imaging data.

Abstract

Reflection spectroscopy holds great promise for characterizing the atmospheres and surfaces of potentially habitable terrestrial exoplanets. The surface of the modern Earth exhibits a sharp albedo change near 750 nm caused by vegetation - the red edge - which would leave a strong spectral signature if present on an exoplanet. However, the retrieval of wavelength-dependent surface properties from reflection spectra has seen relatively little study. Here, we propose a new surface albedo parameterization capable of retrieving the wavelength location of a priori unknown 'edge-like' features. We demonstrate that a wavelength-dependent surface albedo model achieves higher accuracy in retrieving atmospheric composition. Wavelength-dependent surfaces are also generally preferred over a uniform albedo model when retrieving simulated reflection spectra for a modern Earth analog, even for moderate signal-to-noise ratios (S/N = 10) and Earth-like clouds. Further, the location of the modern Earth's red edge can be robustly and precisely constrained (within 70 nm for S/N = 10). Our results suggest that future space-based direct imaging missions have the potential to infer surface compositions for rocky exoplanets, including spectral edges similar to those caused by life on the modern Earth.