Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life

Abstract

In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets—Biosignatures—Habitability markers—Photosynthesis—Planetary surfaces—Atmospheres—Spectroscopy—Cryptic biospheres—False positives. Astrobiology 18, 663–708.

Table of Contents

• 1. Introduction

•     1.1. Requirements for life

•     1.2. Exoplanet biosignature definitions

•     1.3. Biosignature categories

• 2. Evaluating Planetary Habitability

• 3. Overview of Terrestrial Exoplanet Modeling Studies

•     3.1. Observations of earth

•     3.2. Spectral models

•     3.3. Photochemical studies of terrestrial atmospheres

•     3.4. Earth through time

• 4. Gaseous Biosignatures

•     4.1. Gaseous biosignature overview

•     4.2. Earth-like atmospheres

•        4.2.1. Oxygen (O ₂)

•        4.2.2. Ozone (O ₃)

•        4.2.3. Methane (CH ₄)

•        4.2.4. Nitrous oxide (N ₂O)

•        4.2.5. Sulfur gases (DMS, DMDS, CH ₃SH) and relation to detectable C ₂H ₆

•        4.2.6. Methyl chloride (CH ₃Cl)

•        4.2.7. Haze as a biosignature

•        4.2.8. Other gases

•     4.3. “False positives” for biotic O ₂/O ₃ and possible spectral discriminators

•     4.4. Biosignatures in other types of atmospheres

•     4.5. Effects of the host star spectrum on photochemistry

•     4.6. Impacts of flares and particle events on biosignature gases

• 5. Surface Biosignatures

•     5.1. Photosynthesis

•        5.1.1. Principles of photosynthesis

•           5.1.1.1. Relationship between band gap wavelength and reductant in the generation of biogenic gases and pigment color

•           5.1.1.2. Uniqueness of OP

•        5.1.2. Photosynthetic pigments and the color of phototrophs

•           5.1.2.1. Structure

•           5.1.2.2. Light absorption

•        5.1.3. The vegetation “red edge”

•        5.1.4. Speculation about photosynthesis and pigment signatures on exoplanets

•     5.2. Retinal pigments

•     5.3. Alternative surface biosignatures: nonphotosynthetic pigments and reflectance features

•     5.4. False positive surface biosignatures

•     5.5. Chiral and polarization biosignatures

•     5.6. Fluorescence and bioluminescence

• 6. Temporal Biosignatures

•     6.1. Oscillations of atmospheric gases

•     6.2. Oscillations in surface signatures

• 7. Assessing Biosignature Plausibility

•     7.1. Chemical disequilibrium

•     7.2. Biomass estimation

•     7.3. Applications of network theory to biosignatures

• 8. Cryptic Biospheres: “False Negatives” for Life?

• 9. Prospects for Detecting Exoplanet Biosignatures

• 10. Summary