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

TL;DR

This study evaluates how different spectral bandpasses and S/N levels affect the ability of space-based observatories to characterize Earth-like exoplanets using reflected light spectroscopy, emphasizing the importance of spectral coverage.

Contribution

It introduces a framework for assessing the spectral and S/N requirements for retrieving planetary parameters from early spectral data of Earth analogs.

Findings

Broader spectral coverage enhances parameter retrieval.

Limited spectral bands can still provide significant information at moderate S/N.

Combining visible and NIR ranges yields the most comprehensive characterization.

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.