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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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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The Surface UV Environment on Planets Orbiting M Dwarfs: Implications for Prebiotic Chemistry and the Need for Experimental Follow-up

The Astrophysical Journal, Год журнала: 2017, Номер 843(2), С. 110 - 110

Опубликована: Июль 10, 2017

Abstract Potentially habitable planets orbiting M dwarfs are of intense astrobiological interest because they the only rocky worlds accessible to biosignature search over next 10+ years a confluence observational effects. Simultaneously, recent experimental and theoretical work suggests that UV light may have played key role in origin life on Earth, especially RNA. Characterizing environment M-dwarf is important for understanding whether as we know it could emerge such worlds. In this work, couple radiative transfer models observed spectra determine prebiotic Earth-analog dwarfs. We calculate dose rates quantify impact different host stars prebiotically photoprocesses. find access 100–1000 times less bioactive fluence than young Earth. It unclear UV-sensitive chemistry been abiogenesis, known plausible pathways pyrimidine ribonucleotide synthesis, function planets. This uncertainty affects objects like recently discovered habitable-zone Proxima Centauri, TRAPPIST-1, LHS 1140. Laboratory studies sensitivity putative irradiation level required resolve uncertainty. If steady-state output insufficient power these pathways, transient elevated due flares suffice; laboratory can constrain possibility well.

Язык: Английский

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Exoplanet Biosignatures: Future Directions

Astrobiology, Год журнала: 2018, Номер 18(6), С. 779 - 824

Опубликована: Июнь 1, 2018

We introduce a Bayesian method for guiding future directions detection of life on exoplanets. describe empirical and theoretical work necessary to place constraints the relevant likelihoods, including those emerging from better understanding stellar environment, planetary climate geophysics, geochemical cycling, universalities physics chemistry, contingencies evolutionary history, properties as an emergent complex system, mechanisms driving emergence life. provide examples how formalism could guide search strategies, determining observations prioritize or deciding between targeted searches larger lower resolution surveys generate ensemble statistics address methodology constrain prior probability with without positive detection. Key Words: Exoplanets—Biosignatures—Life detection—Bayesian analysis. Astrobiology 18, 779–824. 1. Introduction 2. Setting Stage: What Is Life? Biosignature? 3. Detecting Unknown Biology Worlds: A Framework 3.1. Habitability in framework biosignatures 4. P(data|abiotic) 4.1. Stellar environment 4.2. Climate geophysics 4.2.1. Coupled tectonic–climate models 4.2.2. Community GCM projects generating P(data|life) 4.3. Geochemical 4.3.1. Anticipating unexpected: statistical approaches characterizing atmospheres non-Earth-like worlds 5. 5.1. Black-box living processes 5.1.1. Type classification Seager et al. (2013a) 5.1.1.1. Energy capture (type I) 5.1.1.2. Biomass II) 5.1.1.3. Other uses III) 5.1.1.4. Products modification gases IV) 5.1.2. Alternatives type 5.1.2.1. I, energy 5.1.2.2. II, biomass 5.1.2.3. III, "other uses" 5.1.2.4. IV 5.1.3. When is it appropriate deconstruct black box? 5.2. Life improbable chemistry 5.3. process 5.3.1. coevolution its planet: Earth example 5.3.2. Calculating conditional probabilities biological evolution past biogeochemical states 5.4. Insights universal biology 5.4.1. Network 5.4.2. Universal scaling laws, applicable other worlds? 6. P(life) 6.1. P(emerge): constraining origins 6.2. Biological innovations 7. Example: Atmospheric Oxygen 8. Tuning Search Strategies Based 9. Conclusions Acknowledgments Author Disclosure Statement References Abbreviations Used

Язык: Английский

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Habitability and Biosignatures of Hycean Worlds

The Astrophysical Journal, Год журнала: 2021, Номер 918(1), С. 1 - 1

Опубликована: Авг. 26, 2021

We investigate a new class of habitable planets composed water-rich interiors with massive oceans underlying H2-rich atmospheres, referred to here as Hycean worlds. With densities between those rocky super-Earths and more extended mini-Neptunes, can be optimal candidates in the search for exoplanetary habitability may abundant exoplanet population. bulk properties (masses, radii, temperatures), potential habitability, observable biosignatures planets. show that significantly larger compared previous considerations planets, radii large 2.6 Earth (2.3 radii) mass 10 masses (5 masses). construct zone (HZ), considering stellar hosts from late M sun-like stars, find it wider than terrestrial-like HZ. While inner boundary HZ corresponds equilibrium temperatures high ~500 K dwarfs, outer is unrestricted arbitrarily orbital separations. Our investigations include tidally locked `Dark Hycean' worlds permit conditions only on their permanent nightsides `Cold see negligible irradiation. Finally, we observability possible atmospheres. number trace terrestrial biomarkers which expected present atmospheres would readily detectable using modest observing time James Webb Space Telescope (JWST). identify sizable sample nearby ideal targets such observations biosignatures.

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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Hidden Water in Magma Ocean Exoplanets

The Astrophysical Journal Letters, Год журнала: 2021, Номер 922(1), С. L4 - L4

Опубликована: Ноя. 1, 2021

We demonstrate that the deep volatile storage capacity of magma oceans has significant implications for bulk composition, interior and climate state inferred from exoplanet mass radius data. Experimental petrology provides fundamental properties on ability water melt to mix. So far, these data have been largely neglected mass-radius modeling. Here, we present an advanced model water-rich rocky exoplanets. The new allows us test effects rock melting redistribution between ocean atmosphere calculated planet radii. Models with without partitioning lead deviations in up 16% a fixed composition mass. This is within current accuracy limits individual systems statistically testable population level. Unrecognized mantle retrievals may thus underestimate planetary content by one order magnitude.

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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