TY - JOUR
T1 - Detecting Oceans on Exoplanets with Phase-dependent Spectral Principal Component Analysis
AU - Ryan, Dominick J.
AU - Robinson, Tyler D.
N1 - Funding Information:
We thank N. Cowan for comments on an early draft of this work and for insightful discussions on various approaches to principal component analysis. J. Lustig-Yaeger kindly provided feedback on comparisons between work presented here and previous results in the literature. Also, we thank M. Marley for encouraging our writing on the utility of apparent albedo. Finally, we thank two anonymous reviewers for providing thoughtful and constructive feedback on drafts of this manuscript. D.J.R. acknowledges support from the Hooper Undergraduate Research Award program at Northern Arizona University. T.D.R. gratefully acknowledges support from NASA’s Exoplanets Research Program (No. 80NSSC18K0349), Exobiology Program (No. 80NSSC19K0473), and Habitable Worlds Program (No. 80NSSC20K0226), as well as the Nexus for Exoplanet System Science and NASA Astrobiology Institute Virtual Planetary Laboratory (No. 80NSSC18K0829). Some results in this paper have been derived using the HEALPix package (Górski et al. 2005).
Publisher Copyright:
© 2022. The Author(s). Published by the American Astronomical Society.
PY - 2022/2/1
Y1 - 2022/2/1
N2 - Stable surface liquid water is a key indicator of exoplanet habitability. However, few approaches exist for directly detecting oceans on potentially Earth-like exoplanets. In most cases, specular reflection of host starlight from surface bodies of water—referred to as ocean glint—proves to be an important aspect of liquids that can enable detection of habitable conditions. Here, we propose that spectral principal component analysis (PCA) applied to orbital phase-dependent observations of Earth-like exoplanets can provide a straightforward means of detecting ocean glint and thus habitability. Using high-fidelity, orbit-resolved spectral models of Earth, and for instrument capabilities applicable to proposed exo-Earth direct imaging concept missions, the extreme reddening effect of crescent-phase ocean glint is demonstrated as the primary spectral component that explains phase-dependent variability for orbital inclinations spanning 60°–90°. At smaller orbital inclinations where more-extreme crescent phases cannot be accessed, glint can still significantly increase planetary brightness but reddening effects are less pronounced, and as a result, glint is not plainly indicated by phase-dependent spectral PCA. Using instrument models for future exoplanet direct imaging mission concepts, we show that brightness enhancements due to glint could be detected across a wide range of orbital inclinations with typical exposure times measured in hours to weeks, depending on system distance and mission architecture. Thus, brightness increases due to glint are potentially detectable for Earth-like exoplanets for most system inclinations, and phase-dependent spectral PCA could indicate reddening due to glint for a subset of these inclinations.
AB - Stable surface liquid water is a key indicator of exoplanet habitability. However, few approaches exist for directly detecting oceans on potentially Earth-like exoplanets. In most cases, specular reflection of host starlight from surface bodies of water—referred to as ocean glint—proves to be an important aspect of liquids that can enable detection of habitable conditions. Here, we propose that spectral principal component analysis (PCA) applied to orbital phase-dependent observations of Earth-like exoplanets can provide a straightforward means of detecting ocean glint and thus habitability. Using high-fidelity, orbit-resolved spectral models of Earth, and for instrument capabilities applicable to proposed exo-Earth direct imaging concept missions, the extreme reddening effect of crescent-phase ocean glint is demonstrated as the primary spectral component that explains phase-dependent variability for orbital inclinations spanning 60°–90°. At smaller orbital inclinations where more-extreme crescent phases cannot be accessed, glint can still significantly increase planetary brightness but reddening effects are less pronounced, and as a result, glint is not plainly indicated by phase-dependent spectral PCA. Using instrument models for future exoplanet direct imaging mission concepts, we show that brightness enhancements due to glint could be detected across a wide range of orbital inclinations with typical exposure times measured in hours to weeks, depending on system distance and mission architecture. Thus, brightness increases due to glint are potentially detectable for Earth-like exoplanets for most system inclinations, and phase-dependent spectral PCA could indicate reddening due to glint for a subset of these inclinations.
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U2 - 10.3847/PSJ/ac4af3
DO - 10.3847/PSJ/ac4af3
M3 - Article
AN - SCOPUS:85130536274
SN - 2632-3338
VL - 3
JO - Planetary Science Journal
JF - Planetary Science Journal
IS - 2
M1 - 33
ER -