Habitable Worlds Observatory (HWO): Living Worlds Community Working Group: The Search for Life on Potentially Habitable Exoplanets

TL;DR

The paper addresses the challenge of detecting signs of life on potentially habitable exoplanets with the Habitable Worlds Observatory. It proposes a comprehensive, Earth-history–driven framework for identifying biosignatures across oxidized and anoxic atmospheres, incorporating a rigorous false-positive/false-negative mindset and corroborating habitability via surface and atmospheric context. The approach combines broad spectral coverage (NUV to NIR), robust interpretation strategies, and a quantified target sample plan to constrain the prevalence of observable biospheres in the galaxy. Its significance lies in guiding the design requirements of a flagship telescope (aperture, wavelength range, and instrument suite) to maximize the chances of detecting life beyond Earth while providing a principled basis for evaluating non-detections. The work emphasizes readiness for unexpected planetary architectures and the need to adapt biosignature interpretation to diverse stellar environments.

Abstract

The discovery of a biosphere on another planet would transform how we view ourselves, and our planet Earth, in relation to the rest of the cosmos. We now know Earth is one planet among eight circling our sun; our sun is part of a swirling galaxy of over one hundred billion other suns; and our galaxy is one of untold billions in the universe. While we do not yet know how many, if any, other biospheres exist on the countless worlds orbiting countless other suns, we stand at the precipice of a new era of discovery, enabled by powerful new facilities able to peer across the light years into the atmospheres of planets similar to our own. This article is an adaptation of a science case document (SCDD) developed for the NASA Astrophysics Flagship mission the Habitable Worlds Observatory (HWO) Science, Technology, and Architecture Review Team (START) Living Worlds Community Working Group.

Figures (20)

• Figure 2: Biosignatures and habitability indicators of Earth through time reflect how life on our planet has co-evolved with its environment. The biosignatures of Earth through time are a useful minimum set of spectral features to seek on exoplanets with HWO.
• Figure 3: Biosignatures must be considered in the context of their environments to rule out false positives. Possible false positive planets are shown, with relevant spectral features labeled. False positives relevant to oxygenated planets are on the bottom (Phanerozoic, or modern, Earth and Proterozoic Earth); those relevant to anoxic planets (Archean Earth) are on the top. Figure by Samantha Gilbert-Janizek (UW).
• Figure 4: The dominant biosignatures of Earth, and their detectability, have varied over our planet’s geological history. Different photochemistries around different stars can also alter the relative detectability of biosignatures. A wavelength range from the NUV to the NIR ensures that the dominant biosignatures over Earth history can be detected on exoplanets.
• Figure 5: Spectral features of Modern Earth at quadrature phase (half illumination). This figure is reproduced with minor modifications from Schwieterman et al. (2018a) under Creative Commons Attribution License CC-BY 4.0. Figure modifications include labels of additional gas features, an inset of a simulated Earth at half illumination, and an inset color bar indicating the visible wavelength range.
• Figure 6: Types of biosignatures and false positives for different types of environments.