Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life

Astrobiology, Год журнала: 2018, Номер 18(6), С. 663 - 708

Опубликована: Май 4, 2018

In the coming years and decades, advanced space- ground-based observatories will allow an unprecedented opportunity to probe atmospheres surfaces of potentially habitable exoplanets for signatures life. Life on Earth, through its gaseous products reflectance scattering properties, has left fingerprint spectrum our planet. Aided by universality laws physics chemistry, we turn Earth's biosphere, both in present geologic time, analog that aid search life elsewhere. Considering insights gained from modern ancient broader array hypothetical exoplanet possibilities, have compiled a comprehensive overview current understanding potential biosignatures, including gaseous, surface, temporal biosignatures. We additionally survey biogenic spectral features are well known specialist literature but not yet been robustly vetted context briefly review advances assessing biosignature plausibility, novel methods determining chemical disequilibrium remotely obtainable data assessment tools minimum biomass required maintain short-lived gases as atmospheric signatures. focus particularly made since seminal Des Marais et al. The purpose this work is propose new strategies, goal companion articles series, literature, draw meaningful connections between seemingly disparate areas, clear way path forward. Key Words: Exoplanets—Biosignatures—Habitability markers—Photosynthesis—Planetary surfaces—Atmospheres—Spectroscopy—Cryptic biospheres—False positives. Astrobiology 18, 663–708. 1. Introduction 1.1. Requirements 1.2. Exoplanet definitions 1.3. Biosignature categories 2. Evaluating Planetary Habitability 3. Overview Terrestrial Modeling Studies 3.1. Observations earth 3.2. Spectral models 3.3. Photochemical studies terrestrial 3.4. Earth time 4. Gaseous Biosignatures 4.1. 4.2. Earth-like 4.2.1. Oxygen (O2) 4.2.2. Ozone (O3) 4.2.3. Methane (CH4) 4.2.4. Nitrous oxide (N2O) 4.2.5. Sulfur (DMS, DMDS, CH3SH) relation detectable C2H6 4.2.6. Methyl chloride (CH3Cl) 4.2.7. Haze 4.2.8. Other 4.3. “False positives” biotic O2/O3 possible discriminators 4.4. other types 4.5. Effects host star photochemistry 4.6. Impacts flares particle events 5. Surface 5.1. Photosynthesis 5.1.1. Principles photosynthesis 5.1.1.1. Relationship band gap wavelength reductant generation pigment color 5.1.1.2. Uniqueness OP 5.1.2. Photosynthetic pigments phototrophs 5.1.2.1. Structure 5.1.2.2. Light absorption 5.1.3. vegetation “red edge” 5.1.4. Speculation about 5.2. Retinal 5.3. Alternative surface biosignatures: nonphotosynthetic 5.4. False positive biosignatures 5.5. Chiral polarization 5.6. Fluorescence bioluminescence 6. Temporal 6.1. Oscillations 6.2. 7. Assessing Plausibility 7.1. Chemical 7.2. Biomass estimation 7.3. Applications network theory 8. Cryptic Biospheres: Negatives” Life? 9. Prospects Detecting 10. Summary

Язык: Английский

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Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment

Astrobiology, Год журнала: 2018, Номер 18(6), С. 630 - 662

Опубликована: Май 10, 2018

We describe how environmental context can help determine whether oxygen (O2) detected in extrasolar planetary observations is more likely to have a biological source. Here we provide an in-depth, interdisciplinary example of O2 biosignature identification and observation, which serves as the prototype for development general framework assessment. Photosynthetically generated potentially strong biosignature, at high abundance, it was originally thought be unambiguous indicator life. However, faces two major challenges: (1) only present abundance relatively short period Earth's history (2) now know several potential mechanisms that generate abundant without life being present. Consequently, our ability interpret both presence absence exoplanetary spectrum relies on understanding context. examine coevolution with early environment identify interplay sources sinks may suppressed release into atmosphere billion years, producing false negative biologically O2. These studies suggest characteristics enhance negatives should considered when selecting targets searches. review most recent knowledge positives O2, processes atmospheric biosphere. examples future photometric, spectroscopic, time-dependent other aspects used rule out thereby increase confidence any observed indeed biosignature. insights will guide inform exoplanet characterization missions. Key Words: Biosignatures—Oxygenic photosynthesis—Exoplanets—Planetary atmospheres. Astrobiology 18, 630–662.

Язык: Английский

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Identifying molecules as biosignatures with assembly theory and mass spectrometry

Nature Communications, Год журнала: 2021, Номер 12(1)

Опубликована: Май 24, 2021

Abstract The search for alien life is hard because we do not know what signatures are unique to life. We show why complex molecules found in high abundance universal biosignatures and demonstrate the first intrinsic experimentally tractable measure of molecular complexity, called assembly index (MA). To this calculate complexity several million validate that their can be determined by mass spectrometry. This approach allows us identify from a set diverse samples around world, outer space, laboratory, demonstrating it possible build detection experiment based on MA could deployed extraterrestrial locations, used as scale quantify constraints needed direct prebiotically plausible processes laboratory. Such an vital finding elsewhere universe or creating de-novo lab.

Язык: Английский

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Exoplanet Biosignatures: Observational Prospects

Astrobiology, Год журнала: 2018, Номер 18(6), С. 739 - 778

Опубликована: Июнь 1, 2018

Exoplanet hunting efforts have revealed the prevalence of exotic worlds with diverse properties, including Earth-sized bodies, which has fueled our endeavor to search for life beyond Solar System. Accumulating experiences in astrophysical, chemical, and climatological characterization uninhabitable planets are paving way potentially habitable planets. In this paper, we review possibilities limitations characterizing temperate terrestrial future observational capabilities through 2030s beyond, as a basis broad range discussions on how advance "astrobiology" exoplanets. We discuss observability not only proposed biosignature candidates themselves, but also more general planetary properties that provide circumstantial evidence, since evaluation any candidate relies their context. Characterization Earth-size coming years will focus those around nearby late-type stars. JWST later 30 meter-class ground-based telescopes empower chemical investigations. Spectroscopic studies solar-type stars likely require designated spacecraft mission direct imaging, leveraging technologies already being developed tested part WFIRST mission. Successful initial few targets be an important touchstone toward detailed scrutiny larger survey envisioned 2030. The outlook paper presents may help develop new techniques detect relevant features well frameworks diagnose based observables.

Язык: Английский

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Evolved Climates and Observational Discriminants for the TRAPPIST-1 Planetary System

The Astrophysical Journal, Год журнала: 2018, Номер 867(1), С. 76 - 76

Опубликована: Ноя. 1, 2018

The TRAPPIST-1 planetary system provides an unprecedented opportunity to study terrestrial exoplanet evolution with the James Webb Space Telescope (JWST) and ground-based observatories. Since M dwarf planets likely experience extreme volatile loss, may have highly-evolved, possibly uninhabitable atmospheres. We used a versatile, 1D terrestrial-planet climate model line-by-line radiative transfer mixing length convection (VPL Climate) coupled photochemistry simulate environmental states for planets. present equilibrium climates self-consistent atmospheric compositions, observational discriminants of post-runaway, desiccated, 10-100 bar O2- CO2-dominated atmospheres, including interior outgassing, as well water-rich compositions. Our simulations show range surface temperatures, most which are not habitable, although aqua-planet e could maintain temperate given Earth-like geological outgassing CO2. find that desiccated h produce habitable temperatures beyond maximum greenhouse distance. Potential these atmospheres in transmission emission spectra influenced by photochemical processes aerosol formation, include collision-induced oxygen absorption (O2-O2), O3, CO, SO2, H2O, CH4 features, transit signals up 200 ppm. simulated consistent K2, HST, Spitzer observations For several we b is unlikely aerosols. These results can inform JWST observation planning data interpretation other

Язык: Английский

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Deciphering Biosignatures in Planetary Contexts

Astrobiology, Год журнала: 2019, Номер 19(9), С. 1075 - 1102

Опубликована: Июль 23, 2019

Microbial life permeates Earth's critical zone and has likely inhabited nearly all our planet's surface near subsurface since before the beginning of sedimentary rock record. Given vast time that Earth been teeming with life, do astrobiologists truly understand what geological features untouched by biological processes would look like? In search for extraterrestrial in Universe, it is to determine constitutes a biosignature across multiple scales, how this compares "abiosignatures" formed nonliving processes. Developing standards abiotic biotic characteristics provide quantitative metrics comparison different data types observational frames. The evidence detection falls into three categories biosignatures: (1) substances, such as elemental abundances, isotopes, molecules, allotropes, enantiomers, minerals, their associated properties; (2) objects are physical mats, fossils including trace-fossils microbialites (stromatolites), concretions; (3) patterns, three-dimensional or conceptual

Язык: Английский

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Phosphine on Venus Cannot Be Explained by Conventional Processes

Astrobiology, Год журнала: 2021, Номер 21(10), С. 1277 - 1304

Опубликована: Июль 20, 2021

The recent candidate detection of ~1 ppb phosphine in the middle atmosphere Venus is so unexpected that it requires an exhaustive search for explanations its origin. Phosphorus-containing species have not been modelled Venus' before and our work represents first attempt to model phosphorus Venusian atmosphere. We thoroughly explore potential pathways formation a environment, including planet's atmosphere, cloud haze layers, surface, subsurface. investigate gas reactions, geochemical photochemistry, other non-equilibrium processes. None these production are sufficient explain presence levels on Venus. If PH3's confirmed, therefore highly likely be result process previously considered plausible conditions. could unknown geochemistry, or even aerial microbial life, given Earth exclusively associated with anthropogenic biological sources. adds complexity chemical processes environment motivates situ follow up sampling missions Our analysis provides template investigation as biosignature worlds.

Язык: Английский

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Intelligence as a planetary scale process

International Journal of Astrobiology, Год журнала: 2022, Номер 21(2), С. 47 - 61

Опубликована: Фев. 7, 2022

Abstract Conventionally, intelligence is seen as a property of individuals. However, it also known to be collectives. Here, we broaden the idea collective and extend planetary scale. We consider ways in which appearance technological may represent kind scale transition, thus might not something happens on planet but planet, much some models propose origin life itself was phenomenon. Our approach follows recognition among researchers that correct understand key aspects its evolution planetary, opposed more traditional focus individual species. explore concept prove useful for three distinct domains: Earth Systems Exoplanet studies; Anthropocene Sustainability study Technosignatures Search Extraterrestrial Intelligence (SETI). argue explorations intelligence, defined acquisition application knowledge operating at integrated into function coupled systems, can framework understanding possible paths long-term inhabited planets including future trajectories predicting features intelligentially steered other worlds.

Язык: Английский

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Bioverse: A Comprehensive Assessment of the Capabilities of Extremely Large Telescopes to Probe Earth-like O₂ Levels in Nearby Transiting Habitable-zone Exoplanets

The Astronomical Journal, Год журнала: 2023, Номер 165(6), С. 267 - 267

Опубликована: Июнь 1, 2023

Abstract Molecular oxygen is a strong indicator of life on Earth and may indicate biological processes exoplanets too. Recent studies proposed that Earth-like O 2 levels might be detectable nearby using high-resolution spectrographs future extremely large telescopes (ELTs). However, these did not consider constraints like relative velocities, planet occurrence rates, target observability. We expanded past by creating homogeneous catalog 286,391 main-sequence stars within 120 pc Gaia DR3 used the Bioverse framework to simulate likelihood finding transiting analogs. also simulated survey M dwarfs 20 accounting for η ⊕ estimates, transit probabilities, observability determine how long ELTs theoretical 50–100 m ground-based need observe probe with an R = 100,000 spectrograph. This would only possible 50 yr up ∼21% M-dwarf systems if suitable habitable-zone analog was discovered, assuming signals from every observable partial each ELT can combined. If so, could TRAPPIST-1 d–g 16–55 yr, respectively, about half time 500,000 These results have important implications whether analogs via transmission spectroscopy. Our work provides most comprehensive assessment date capabilities search beyond solar system.

Язык: Английский

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Exoplanet Biosignatures: A Framework for Their Assessment

Astrobiology, Год журнала: 2018, Номер 18(6), С. 709 - 738

Опубликована: Апрель 20, 2018

Finding life on exoplanets from telescopic observations is an ultimate goal of exoplanet science. Life produces gases and other substances, such as pigments, which can have distinct spectral or photometric signatures. Whether not found with future data must be expressed probabilities, requiring a framework biosignature assessment. We present in we advocate using biogeochemical "Exo-Earth System" models to simulate potential biosignatures spectra photometry. Given actual observations, simulations are used find the Bayesian likelihoods those occurring for scenarios without life. The latter includes "false positives" where abiotic sources mimic biosignatures. Prior knowledge factors influencing planetary inhabitation, including previous combined give posterior probability existing given exoplanet. Four components observation analysis necessary. 1) Characterization stellar (e.g., age spectrum) exoplanetary system properties, "external" parameters mass radius) determine exoplanet's suitability 2) "internal" climate) evaluate habitability. 3) Assessment within environmental context (components 1-2) any corroborating evidence. 4) Exclusion false positives. resulting probabilities life's existence map five confidence levels, ranging "very likely" (90-100%) unlikely" ($\le$10%) inhabited.

Язык: Английский

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