Astronomy & Telescopes

Habitable Worlds Observatory Living Worlds Working Group: Surface Biosignatures on Potentially Habitable Exoplanets

By Keith Cowing

Status Report

astro-ph.IM

January 15, 2026

Filed under Archean, astro-ph.IM, astronomy, astrophyisics, bacteriochlorophyll, Biosignatures, carotenoids, flagship mission, habitability, Habitable Exoplanet, Habitable Worlds Observatory, Habitable Zone, imaging, Living Worlds Community Working Group, Nonphotosynthetic Pigments, oxygenic photosynthesis, Proterozoic, purple non-sulfur bacteria, rocky exoplanet, Spectroscopy, Stellar Cartography

Habitable Worlds Observatory Living Worlds Working Group: Surface Biosignatures on Potentially Habitable Exoplanets — Left: Reflectance spectra of photosynthetic organisms on rock, mineral, and snow/ice abiotic surfaces with Archean, Proterozoic, and Modern atmospheric compositions (planetary spectra provided by Anna Grace Ulses, Univ. of Washington). The thick angled lines show the pigment ”edge” features in the spectrum. Right: Top: purple anoxygenic phototrophs with bacteriochlorophyll pigments absorbance and reflectance features at 805-880 nm at 15% planet coverage under an Archean atmosphere with 50% cloud cover. Middle: cyanobacterial microbial mat (oxygenic phototroph) with chlorophyll a absorption at 660-680 nm at 15% coverage under a Proterozoic atmosphere with 50% cloud cover. Note that the 3-D structure of the mat is effective at scattering light to create a ”red edge” feature similar to the vegetation red edge (VRE) shown in the bottom panel. Top two spectra provided by N. Parenteau, unpublished data. — astro-ph.IM

The Habitable Worlds Observatory (HWO) is the first NASA Astrophysics flagship mission with a key science goal of searching for signs of life on rocky habitable exoplanets beyond our solar system.

The Living Worlds Community Working Group was charged with investigating how HWO could characterize planets orbiting stars in the solar neighborhood, search for signs of life, and interpret potential biosignatures within a false positive and false negative framework.

The Surface Biosignatures Task assessed the measurement requirements and instrument needs to detect these biosignatures under an ‘Earth through time’ scenario. Surface biosignatures are planetary-scale spectral features resulting from absorption and/or scattering of radiation by organisms containing photosynthetic and non-photosynthetic pigments. This secondary class of biosignature can be used to corroborate atmospheric biosignatures by providing multiple lines of evidence to aid in assessing their biogenicity.

Furthermore, surface biopigments are the only way to detect more primitive forms of anoxygenic photosynthesis if oxygenic photosynthesis never evolved.

Key Findings: To detect biopigments on the surface of planets under Archean, Proterozoic, and Modern atmospheric compositions (15 percent coverage, 50 percent cloud cover), an SNR of 20-40 would be needed over 500-1100 nm.

However, there may be some cases in which lower SNR is required; studies are ongoing. Coronagraph requirements: (1) The detection of surface biosignatures would be greatly enhanced by having as many parallel coronagraph channels as possible across the entire wavelength range with no or minimal gaps between channels. (2) Retrieval studies revealed that restricted wavelength ranges (e.g., 0.4 – 0.7 microns), such as may be used during initial survey strategies, are not sufficient to deconvolve the biopigment features from the abiotic background.

Reflectance spectra of bacteriochlorophylls a and b (red), as well as carotenoids (yellow) from purple non-sulfur bacteria. Note absorption near-infrared features from Bacteriochlorophylls span between 800-1010 nm. Data from Coelho et al., 2024. — astro-ph.IM

Niki Parenteau, Anna Grace Ulses, Connor Metz, Nancy Y. Kiang, Ligia F. Coelho, Edward Schwieterman, Jonathan Grone, Giulia Roccetti, Svetlana Berdyugina, Eleonora Alei, Lucas Patty, Emilie Lafleche, Taro Matsuo, Dawn Cardace, Schuyler Borges, Avi Mandel, Kenneth Gordon, Joshua Krissansen-Totton, Giada Arney

Comments: This Surface Biosignatures Science Case Development Document (SCDD) was created during the Habitable Worlds Observatory Science, Technology, and Architecture Review Team (START) Community Working Group activities. 15 pages, 11 figures
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2601.08883 [astro-ph.IM] (or arXiv:2601.08883v1 [astro-ph.IM] for this version)
https://doi.org/10.48550/arXiv.2601.08883
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Submission history
From: Niki Parenteau
[v1] Mon, 12 Jan 2026 20:34:11 UTC (4,656 KB)
https://arxiv.org/abs/2601.08883

Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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