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Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life

Edward W. Schwieterman, Nancy Y. Kiang, Mary N. Parenteau, Chester E. Harman, Shiladitya DasSarma, Theresa M. Fisher, Giada N. Arney, Hilairy E. Hartnett, Christopher T. Reinhard, Stephanie L. Olson, Victoria S. Meadows, Charles S. Cockell, Sara I. Walker, John Lee Grenfell, Siddharth Hegde, Sarah Rugheimer, Renyu Hu, Timothy W. Lyons

arXiv:1705.05791v3astro-ph.EP

TL;DR

As exoplanet observations advance toward atmospheric and surface characterization, researchers need an updated synthesis of remotely detectable biosignatures and ways to assess their plausibility. This review surveys gaseous, surface, and temporal signatures and concludes that interpreting them requires planetary context, with Earth’s vegetation reflectance analogue requiring 1% spectrophotometric precision and at least 10% cloud-free exo-vegetation coverage.

  • Problem

    An updated synthesis is needed to assess exoplanet biosignatures and interpret potential detections amid false positives and incomplete contextual evidence.

  • Method

    The paper reviews literature on gaseous, surface, and temporal biosignatures, using spectral models and plausibility-assessment approaches to connect detectability with planetary context.

  • Results

    1% spectrophotometric precision and 10% or more cloud-free surface coverage of exo-vegetation are likely required to detect an Earth-analogue VRE signature.

  • Takeaways & Limitations

    Potential biosignature detections should be evaluated with environmental context because single gases or gas combinations may not provide robust evidence of life alone.

  • Takeaways & Limitations

    The VRE remains an Earth-based biosignature whose universality is still an open question.

Abstract

from arXiv · show

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 state-of-the-art 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 for a given atmospheric signature. We focus particularly on advances made since the seminal review by Des Marais et al. (2002). The purpose of this work is not to propose new biosignatures strategies, a goal left to companion papers in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward.

1. Introduction T

The paper reviews remotely detectable exoplanet biosignatures as observations advance from detecting planets to characterizing potentially habitable atmospheres and surfaces. It organizes potential biosignatures into gaseous, surface, and temporal categories while emphasizing uncertainty, abiotic alternatives, and connections across existing literature.

  • Motivation: Exoplanet surveys have confirmed thousands of planets, creating opportunities to characterize nearby potentially habitable worlds and search their atmospheres and surfaces for biosignatures.Transit and radial-velocity surveys established the population, while upcoming transmission and direct-imaging spectroscopy enable characterization.
  • Contribution: The review updates earlier biosignature work by connecting existing studies, incorporating specialist literature, and surveying gaseous, surface, and temporal biosignatures.It is intended as a starting point rather than a complete recapitulation, with future recommendations reserved for companion articles.
  • Foundations: The search is guided by Earth’s biosphere and the universality of physical and chemical laws, while recognizing that defining life remains constrained by terrestrial understanding.The paper treats evolutionary capacity as a defining aspect of life and discusses energy sources and likely liquid solvents in the broader conceptual background.
  • Biosignature definitions: A biosignature is an object, substance, or pattern whose origin specifically requires biology, but an exoplanet signal generally remains a potential biosignature with alternative explanations.The signal may be spectroscopic and can imply a gas or surface feature, while measurement uncertainty and abiotic false positives limit certainty.
  • Biosignature categories: Because no universally accepted classification exists, the review groups potential exoplanet biosignatures into gaseous, surface, and temporal categories.Gaseous signatures derive directly or indirectly from metabolism, whereas surface signatures involve reflected or scattered radiation altered by organisms.

2. Evaluating Planetary Habitability

Planetary habitability is rapidly screened using the Habitable Zone, but confirming habitability requires evaluating atmospheric, stellar, and surface conditions beyond orbital location. Liquid water and sufficient greenhouse warming are minimum requirements, while direct water detection would provide stronger evidence.

  • 2. Evaluating Planetary Habitability: The Habitable Zone is the stellar-distance range where a planet with a given atmosphere could maintain surface liquid water.HZ estimates can be rapidly assessed from semimajor axis and stellar luminosity when those observables are adequately constrained.
  • 2. Evaluating Planetary Habitability: HZ boundaries vary with stellar type because stellar spectra produce different planetary albedos even for atmospheres of constant composition.HZ definitions also depend on factors such as planetary gravity.
  • 2. Evaluating Planetary Habitability: A planet within the HZ is necessary but not sufficient for habitability, which requires liquid water and at least one noncondensable greenhouse gas sufficient to warm the surface.Outer-HZ limit cycles can alternate between globally glaciated and climatically warm states, motivating consideration of restrictive 1D and 3D model limits.
  • 2. Evaluating Planetary Habitability: Direct detection of surface liquid water, potentially through glint, is the most straightforward way to determine planetary habitability.Spectral indicators can also include H2O absorption bands alongside appropriate temperature, pressure, and gases such as N2 or O2.

3. Overview of Terrestrial Exoplanet Modeling Studies

Terrestrial exoplanet modeling studies combine observations, radiative-transfer and photochemical models, and Earth-evolution studies to assess the remote detectability of biosignatures. These models support simulations across observing modes and planetary conditions, but unconstrained atmospheric results and weakly expressed early life require cautious interpretation.

  • Modeling approaches: System-level studies use data–model comparisons, photochemical and spectral models, and Earth-evolution models to investigate exoplanet biosignatures.These approaches provide frameworks and foundational concepts for interpreting potential biosignatures.
  • Spectral modeling: Radiative-transfer models calculate atmospheric, aquatic, and canopy scattering and absorption to generate synthetic spectra and estimate biosignature detectability.Model selection depends on the observing mode, while aerosols, clouds, and surface reflectance define important spectral inputs.
  • Spectral modeling: Validated Earth spectral models can be modified across viewing geometries, cloud conditions, atmospheric compositions, and surface features to assess biosignature detectability.The VPL 3D spectral Earth model incorporates gaseous absorption, Rayleigh scattering, modern continental and surface distributions, and realistic cloud cover.
  • Photochemical modeling: Photochemical models couple stellar radiation, chemical reactions, atmospheric mixing, and deposition to solve for self-consistent atmospheric compositions.Their boundary conditions include atmospheric mass fluxes and stellar spectral energy flux, which can fundamentally alter atmospheric composition.
  • Model limitations: Unconstrained photochemical simulations represent plausible sustainable atmospheric states rather than necessarily current compositions, and multiple states or boundary conditions may be degenerate.Potential observations also require caution because emerging or weakly atmospheric life may be difficult to detect, including uncertain early O2 production by cyanobacteria.

4. Gaseous Biosignatures · 4.1. Gaseous biosignature overview

Gaseous biosignatures may arise directly from biological production or indirectly through environmental processing of biogenic products. Their interpretation requires spectral discrimination among gases and careful consideration of environmental context because many biogenic gases are not uniquely biological.

  • 4. Gaseous Biosignatures: Gaseous biosignatures can originate from direct biological production or secondary compounds formed by environmental processing of biogenic products.O2 from photosynthesis and stratospheric O3 formed through photochemical reactions involving O2 illustrate these pathways.
  • 4. Gaseous Biosignatures: O2 produced by photosynthesis and O3 formed photochemically from O2 are examples of gaseous biosignature pathways.
  • 4. Gaseous Biosignatures: Many biogenic gases are not uniquely biological, so identifying them as life signs depends strongly on environmental context.
  • 4.1. Gaseous biosignature overview: Atmospheric gases become spectrally detectable when they interact with photons through dissociation, electronic, or vibrorotational transitions.
  • 4.1. Gaseous biosignature overview: Spectral range and/or resolution must distinguish gases sharing wavelengths to identify their presence or absence in an exoplanet atmosphere.
  • 4.1. Gaseous biosignature overview: Figure 6 presents line absorption intensities or absorption cross sections for the biosignature gases discussed in the section.The reference spectra draw from the HITRAN 2012 and PNNL spectral databases.

4.2. Earth-like atmospheres

Earth-like atmospheric biosignatures center on gases linked to biological metabolism, especially photosynthetic O2 and its photochemical product O3, while CH4 and N2O require careful abiotic-source assessment. Spectral overlap, stellar environment, and geochemical processes complicate interpretation, so observations at multiple wavelengths are important.

  • Oxygen (O2): Modern atmospheric O2 is a leading biosignature candidate because pO2 = 0.21, it has potentially detectable spectral signatures, and Earth’s O2 is sourced from photosynthesis.Oxygenic photosynthesis uses H2O and CO2 with abundant photons, producing organic matter and O2 as a waste product.
  • Oxygen (O2): Earth’s atmospheric oxygen varied substantially, with Archean pO2 < 10-7 and CH4 at 100–1000 ppm before the GOE increased pO2 by several orders of magnitude.The GOE occurred ca. 2.4 Ga near the beginning of the Proterozoic eon and altered atmospheric chemistry.
  • Ozone (O3): O3 can proxy photosynthetically generated O2, reaches peak concentrations of up to 10 ppm on Earth, and has absorption features spanning UV-VIS-NIR-MIR wavelengths.The 9.65 mm band is a prime infrared target, but overlaps with CO2 and sulfur-bearing gases, requiring spectral information at other wavelengths.
  • Methane (CH4): CH4 is produced by methanogenesis and is Earth’s dominant nonanthropogenic atmospheric source, but abiotic CH4 can be abundant in reducing planet-building environments.Titan’s atmosphere contains 5% CH4 by volume, and CH4 has strongest absorption bands at 1.65, 2.4, 3.3, and 7–8 mm.
  • Nitrous oxide (N2O): N2O is a strong biosignature candidate because modern abiotic sources are generally small and its preindustrial atmospheric concentration was *270 ppb, but stellar context remains essential.Lightning contributed 0.002% of total atmospheric N2O on Earth, while young or magnetically active stars may enable enhanced abiotic production.

4.3. ‘‘False positives’’ for biotic O2/O3 and possible spectral discriminators

Understanding has shifted from viewing abiotic O2 as largely confined outside the habitable zone to recognizing plausible in-zone mechanisms, including CO2 photolysis and extreme hydrogen escape. These mechanisms can produce spectral discriminators that inform biosignature-observing strategies.

  • Historical perspective: Earlier consensus limited abiotic O2 on terrestrial exoplanets to worlds outside the habitable zone.Tectonic activity, abundant water, and hydrological cycling were thought to remove abiotic O2 through geochemical and weathering reactions.
  • Abiotic mechanisms: Plausible mechanisms now generate abiotic O2 on planets within the habitable zone, including robust CO2 photolysis and extreme hydrogen escape followed by O2 buildup.
  • Spectral discriminators: Abiotic processes can leave diagnostic spectra, including simultaneous CO and O2 from CO2 photolysis or absent N2 indicated by missing (N2)2 absorption.Atmospheric mass may also be constrained through Rayleigh scattering.
  • Observational implications: Recognizing plausible abiotic O2 mechanisms motivates instrument and observing-strategy decisions for characterizing potential biosignatures.The topic is developed further in Meadows (2017) and a companion article by Meadows et al. (2018).

4.4. Biosignatures in other types of atmospheres

H2-dominated atmospheres could support habitable rocky planets beyond the traditional habitable-zone outer edge, but their biosignatures remain difficult to establish because comparable Solar System scenarios are lacking. Proposed gases include NH3, CH3Cl, DMS, and N2O, while broader searches may assess all small molecules for atmospheric buildup and spectral detectability.

  • H2-dominated atmospheres: H2-dominated atmospheres could be habitable with little CO2 at instellations below predictions for the traditional habitable-zone outer edge.Kepler demographics also indicate that intermediate-radius planets between Earth and Neptune are the most common type, although that conclusion has limitations.
  • H2-dominated atmospheres: The absence of Solar System examples of abiotic or biological rocky planets with significant H2 fractions complicates biosignature establishment.Early Mars and early Earth may nevertheless have had climatically significant H2 components requiring further study.
  • Candidate gases: Candidate biosignature gases in H2-dominated atmospheres include NH3, CH3Cl, DMS, and N2O, but NH3 can also arise from abiotic outgassing.NH3 production from N2 and H2 is exothermic but kinetically inhibited at habitable temperatures.
  • Candidate gases: N2O has no abiotic sources on an H2-dominated world but is energetically unfavorable as an energy-yielding product, allowing only niche incidental production.Additional gases remain highly speculative; one proposed strategy starts with all small molecules and filters them by atmospheric buildup and spectral detectability.

4.5. Effects of the host star spectrum on photochemistry

The host star’s spectrum, especially its UV output, can dramatically change planetary atmospheric composition by altering photochemistry. Low-UV environments permit accumulation of UV-sensitive gases, whereas high UV and luminosity can drive water-vapor loss.

  • Effects of the host star spectrum on photochemistry: Host-star spectral changes can dramatically alter steady-state atmospheric composition by changing the wavelengths of light reaching a planet.This effect is described for atmospheres with fixed gas supplies, such as specified volcanic outgassing.
  • Effects of the host star spectrum on photochemistry: Lower UV can allow gases normally destroyed by photolysis to reach high, potentially unphysical concentrations when other sinks are omitted.UV photons drive most atmospheric photochemistry, so reduced UV weakens an important destruction pathway.
  • Effects of the host star spectrum on photochemistry: High UV radiation combined with higher stellar luminosity can photolyze and ultimately remove atmospheric water vapor.This provides a contrasting high-UV pathway in which photochemistry drives water loss rather than gas accumulation.
  • Effects of the host star spectrum on photochemistry: CH4 and N2O show runaway accumulation around UV-inactive late M stars, persisting even after large increases in UV flux.These results suggest higher concentrations may be expected for planets orbiting typically UV-quiet stars.

4.6. Impacts of flares and particle events on biosignature gases

Stellar flares, charged particle events, and coronal mass ejections vary with stellar size and age and can affect planetary atmospheres. On an Earthlike planet, charged particles from a strong flare cause a larger, slower O3 decrease than enhanced UV irradiation alone.

  • Stellar activity: Flares, charged particle events, and coronal mass ejections vary in strength and frequency with stellar size and age.The young Sun was likely more active than today, while M4-and-later stars can remain active for up to 8 Gyr.
  • Atmospheric effects: A single strong stellar flare initially causes only a small decrease in O3 from enhanced UV irradiation on an Earthlike planet.The passage contrasts this initial UV-driven effect with the larger impact from charged particles.
  • Atmospheric effects: Charged particles cause a much larger O3 decrease over weeks to months following a strong stellar flare.Subsequent flares can occur within the flare recovery window, potentially extending the disturbance.

5.2. Retinal pigments

Retinal pigments, especially bacteriorhodopsin in haloarchaea, can produce remotely detectable purple surface coloration and may represent a biosignature linked to early life. Their interpretation is constrained because known haloarchaea are aerobic and restricted to hypersaline evaporitic environments, although anaerobic diversity broadens the possibilities.

  • Retinal pigments: Bacteriorhodopsin gives halophilic Archaea cultures a bright purple color observable through remote sensing.Retinal-based pigments support light-driven proton pumping for ATP synthesis, but this process is not connected to carbon fixation and is therefore not considered photosynthesis.
  • Retinal pigments: Retinal’s central role in lipid metabolism and bioenergetics, together with its widespread distribution, supports its possible importance in early terrestrial and extraterrestrial life.DasSarma proposed the purple retinal pigment of haloarchaeal bacteriorhodopsin as a surface biosignature of early Earth.
  • Retinal pigments: Known bacteriorhodopsin-containing haloarchaea are aerobic heterotrophs, implying evolution after cyanobacterial oxygen production.Their exclusive growth in hypersaline evaporitic settings may also limit their relevance because such environments may have been spatially restricted on early Earth.
  • Retinal pigments: The discovery of an obligate anaerobic haloarchaeal genus expands the diversity relevant to retinal-pigment biosignature interpretations, although its lineage remains unresolved.The relationship of this anaerobic lineage to aerobic haloarchaea has not yet been established.
  • Retinal pigments: Haloarchaea-dominated salt ponds and hypersaline lakes can display pink, red, or orange coloration produced by carotenoid and rhodopsin pigments.Examples include environments containing Halobacterium salinarum and Salinibacter ruber, where salt-tolerant microorganisms dominate pond coloration.

5.4. False positive surface biosignatures

Surface spectral edges from pigment-bearing organisms can be compelling biosignatures, but edge features generally are not uniquely biological. Mineral semiconductors and other Solar System surfaces can produce similar spectral slopes or transitions, especially in low-resolution measurements.

  • False positive surface biosignatures: The VRE has no exact spectral mimic among common abiotic materials, but edge features in general are not uniquely biological.This distinction limits the reliability of interpreting spectral edges as definitive surface biosignatures.
  • False positive surface biosignatures: Mineral semiconductors such as cinnabar and sulfur can produce abrupt albedo increases at 0.6 and 0.45mm, respectively, mimicking blue-shifted VRE analogues.Their electronic band-gap energies generate the false-positive transitions.
  • False positive surface biosignatures: False positives are particularly acute for low-resolution spectrophotometric measurements that quantify spectral transitions similarly to the NDVI.Many Solar System surfaces also show red-to-infrared increases, though more gently sloped than the VRE.

5.5. Chiral and polarization biosignatures

Chiral molecules are produced as racemic mixtures abiotically but show enantiomeric preferences in all known organisms, making chirality a potentially generic biosignature. Linear and circular polarization spectroscopy could remotely detect these signatures, although Earthshine observations detected only a linear-polarization signal.

  • Abiotic synthesis produces racemic mixtures, whereas all known organisms preferentially use one enantiomeric form in larger molecules.Examples include amino acids, sugars, and nucleic acids; the passage identifies this preference across bacteria, archaea, eukaryotes, and viruses.
  • Because chirality could be generic to all life, it may reveal extant organisms even when they differ substantially from terrestrial life.
  • <10% enantiomeric excess has been detected for a few amino acids in some carbonaceous meteorites, while these compounds are rare or undetected in Earth’s biosphere.An enantiomeric excess of sugar acids was also found in a variety of carbonaceous meteorites.
  • Linear and circular polarization spectroscopy can in principle remotely detect planetary-scale chirality through molecular orientation and pigment–protein interactions.Linear polarization probes pigment orientation, while circular polarization examines excitonic coupling in pigment–protein complexes; chiral compounds also generate circular-polarization features.
  • 10–15% noncloud covered vegetation provided the best fit for one Earthshine observation window, but circular polarization was not detected.Dusty lunar-surface reflection may have interfered with the polarization signal, leaving the diagnostic value unresolved.

5.6. Fluorescence and bioluminescence

Fluorescence and bioluminescence are potential surface biosignatures involving organism-derived photons. Fluorescence reprocesses absorbed photons, whereas bioluminescence directly produces photons through luciferin oxidation.

  • Fluorescence and bioluminescence: Organism-derived photons represent another category of potential surface biosignature.
  • Fluorescence and bioluminescence: Chlorophyll autofluorescence reprocesses absorbed higher energy photons into emitted lower energy photons and has been observed by low Earth-orbiting satellites.It has been used to characterize plant health and primary productivity.
  • Fluorescence and bioluminescence: Bioluminescence directly produces photons through oxidation of a luciferin molecule across independently evolved lineages including bacteria, fish, plankton, and insects.Luciferin is a general category of light-emitting molecules.

6. Temporal Biosignatures

Temporal biosignatures are time-dependent changes in atmospheric gases, surface reflectance, or direct biological light emission linked to biological activity. Earth examples include seasonal gas oscillations and vegetation-driven spectral changes, but detecting comparable signals and excluding abiotic causes is difficult.

  • Temporal Biosignatures: Temporal biosignatures are measurable time-dependent modulations in gas concentrations, surface spectral albedo, or organism-produced light linked to biological action.Examples include oscillations in atmospheric gases, changing surface spectra, and bioluminescence when the detectable change is directly connected to biology.
  • Atmospheric Gas Oscillations: Earth’s biosphere seasonally modulates CO2, O2, O3, and CH4, with vegetation-driven CO2 oscillation being the best-known example.CO2 decreases during spring growth as vegetation fixes it into organic matter, then rises in fall and winter as consumption slows.
  • Atmospheric Gas Oscillations: 50 ppm is the approximate midlatitude seasonal O2 amplitude, while CH4 variability is dominated by OH interactions and only partly biogenic.CH4 reaches its annual minimum in northern summer, with a more muted winter minimum.
  • Atmospheric Gas Oscillations: 1–3% is the seasonal variation order for CO2 and CH4, whereas O2 changes by ~0.02%, making Earth-like gas oscillations challenging for next-generation observatories.CH4’s variation is only partly biogenic, and saturated absorption bands can produce even smaller measurable spectral changes.
  • Atmospheric Gas Oscillations: Seasonal CO2-ice sublimation is a potential false positive that must be ruled out when interpreting temporal gas oscillations.Ideal signals occur at detectable, nonsaturated background abundances that produce measurable spectral variability.
  • Surface Reflectance Changes: Seasonal growth and senescence of continental green plants can produce temporal VRE changes, and coincident phase changes may strengthen a biological interpretation.Changes in surface albedo or reflected-light spectra provide another form of temporal biosignature.

7. Assessing Biosignature Plausibility

Assessing biosignature plausibility requires planetary-context analysis because individual gases or gas combinations may have false-positive explanations. The section reviews chemical disequilibrium, biomass estimation, and chemical-reaction-network topology as complementary approaches, while noting important detection and validation limitations.

  • Motivation: Single-gas or gas-combination detections require planetary-context assessment because potential false positives can prevent them from constituting robust evidence of life alone.Interpretation depends on evaluating whether measured spectral properties are likely biological in context.
  • Chemical disequilibrium: Chemical disequilibrium uses atmospheric gas combinations such as O2–CH4, whose persistent coexistence requires continual resupply, to guide searches for life.Abiotic and biotic inputs must be disentangled, and CH4 is less abundant and less spectrally detectable than O2 in modern Earth’s atmosphere.
  • Chemical disequilibrium: The largest terrestrial free-energy disequilibrium may be substantial atmospheric O2 and N2 coexisting with a liquid-water ocean, requiring detection of all three components.In equilibrium, lightning-driven chemistry would convert N2 and O2 into oxidized nitrogen compounds that are rapidly washed into the ocean.
  • Biomass estimation: Thermodynamic biomass models estimate whether metabolism could generate detectable biogenic gases for a specific planet–star combination without assuming terrestrial fluxes.They provide first-order plausibility estimates based on the biomass necessary to produce a detectable signature and primarily target gases produced by metabolisms exploiting chemical-potential gradients.
  • Network theory: Chemical-reaction-network topology offers a potential biosignature because Earth’s atmospheric network is hierarchical, modular, and more similar to metabolic networks than other planetary atmospheres.Preliminary comparisons found other planetary atmospheric networks to be more random, but small datasets may partly explain the observed differences.
  • Network theory: Network-topology methods require further validation against nonbiological systems and sensitivity assessment before their promise for evaluating habitability and global biosphere influence can be established.Hot Jupiters are specifically identified as an important comparison class for testing the technique.

8. Cryptic Biospheres: ‘‘False Negatives’’ for Life?

Remote biosignatures can produce false negatives because some inhabited worlds may have low-productivity, chemically cryptic, or subsurface biospheres. Earth’s history shows that even a persistently inhabited planet may lack detectable O2 or muted surface signatures.

  • 8. Cryptic Biospheres: ‘‘False Negatives’’ for Life?: Chemosynthetic biospheres may have orders of magnitude lower productivity than photosynthetic ones, reducing atmospheric chemical disequilibrium and detectability.Endolithic communities can remain hidden beneath rock surfaces, while subsurface oceans may host remotely cryptic microbial biospheres.
  • 8. Cryptic Biospheres: ‘‘False Negatives’’ for Life?: Archean Earth contained no detectable O2 or O3 despite being persistently habitable and inhabited, cautioning against relying on the O2-CH4 biosignature couple.Its atmosphere was significantly more reducing than today’s and likely contained significant detectable amounts of CH4.
  • 8. Cryptic Biospheres: ‘‘False Negatives’’ for Life?: Surface signatures on early Earth may have been muted because vascular land plants have only existed for the last *470 million years.Microbial mats inhabited land surfaces over 1 billion years ago, but would have needed significant areal extent comparable to, or larger than, vegetation today to strengthen detection.

9. Prospects for Detecting Exoplanet Biosignatures

Detecting exoplanet biosignatures will depend on observatory, target-system, planetary, and spectral-range factors. ELTs, LUVOIR/HabEx, JWST, and WFIRST offer differing characterization prospects, while transmission spectroscopy has important biosignature-coverage limits.

  • Observational prospects: Biosignature detectability depends on telescope architecture, target-system distance and properties, planetary parameters, and accessible spectral range.Relevant factors include aperture, coronagraph throughput, instrument sensitivity, spectral type, exozodiacal light, planetary size, albedo, and composition.
  • Observational prospects: ELTs and LUVOIR/HabEx could independently identify initial candidates and characterize them for biosignatures.The passage contrasts these capabilities with JWST, which is likely to characterize only a handful of potentially habitable planets with the required fidelity.
  • Observational prospects: Transmission spectroscopy probes atmospheric habitability markers and biosignatures at a single phase, excluding all potential surface biosignatures and most proposed temporal biosignatures.This limitation means transmission observations cannot access the full range of biosignature categories discussed in the review.
  • Observational prospects: WFIRST could potentially characterize Earth-sized planets in the habitable zones of Alpha Centauri A and B after its planned 2025 launch.The passage also states that WFIRST might characterize a handful of other targets, but the supplied text is truncated.
  • Related work: Companion articles examine upcoming missions, observatory capabilities, expected biosignature-detection timelines, and frameworks for evaluating biosignature detections.Fujii et al. (2018) addresses missions and capabilities, while Catling et al. (2018) and Walker et al. (2018) provide evaluation frameworks.

10. Summary

The review surveys gaseous, surface, and temporal exoplanet biosignatures, emphasizing photosynthetic biospheres, environmental context, and the need to distinguish false positives. It also highlights observational and theoretical advances that could improve biosignature characterization, while noting that some inhabited planets may remain remotely undetectable.

  • Gaseous biosignatures: Photosynthetic biospheres are likely to produce the most detectable signs of life, but individual gases in Earth-like atmospheres cannot confirm life because false positives are possible.Candidate gases include O2, O3, CH4, C2H6, N2O, CH3Cl, CH3SH, DMS, and DMDS.
  • Environmental context: Environmental context is essential for interpreting gaseous biosignatures, including surface liquid water for the N2-O2-ocean disequilibrium signature.Liquid water might be detected through glint.
  • Surface biosignatures: 1% spectrophotometric precision and 10% or more cloud-free exo-vegetation coverage may be required to detect an Earth-like disk-averaged VRE.The VRE remains the most well-studied surface biosignature.
  • Temporal biosignatures: Temporal biosignatures may include seasonal gas modulation, changing surface signatures, and organismal light emission, but they are less studied than gaseous or surface biosignatures.Examples include CO2 or O2 modulation, VRE analogues, bioluminescence, and fluorescence.
  • Detection prospects and frameworks: Earth history indicates that inhabited planets with global biospheres may lack remotely detectable life signs, while broad spectral capabilities and high resolving power could improve success.New or evolving frameworks include disequilibrium updates, biomass models, photosystem discoveries in OP, and network theory applied to atmospheric biosignatures.
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