ABIOTIC O2LEVELS ON PLANETS AROUND F, G, K, AND M STARS: POSSIBLE FALSE POSITIVES FOR LIFE?

Eta-Earth Revisited II: Deriving a Maximum Number of Earth-Like Habitats in the Galactic Disk

In Lammer et al. (2024), we defined Earth-like habitats (EHs) as rocky exoplanets within the habitable zone of complex life (HZCL) on which Earth-like N2-O2-dominated atmospheres with minor amounts of CO2 can exist, and derived a formulation for estimating the maximum number of EHs in the galaxy given realistic probabilistic requirements that have to be met for an EH to evolve. In this study, we apply this formulation to the galactic disk by considering only requirements that are already scientifically quantifiable. By implementing literature models for star formation rate, initial mass function, and the mass distribution of the Milky Way, we calculate the spatial distribution of disk stars as functions of stellar mass and birth age. For the stellar part of our formulation, we apply existing models for the galactic habitable zone and evaluate the thermal stability of nitrogen-dominated atmospheres with different CO2 mixing ratios inside the HZCL by implementing the newest stellar evolution and upper atmosphere models. For the planetary part, we include the frequency of rocky exoplanets, the availability of surface water and subaerial land, and the potential requirement of hosting a large moon by evaluating their importance and implementing these criteria from minima to maxima values as found in the scientific literature. We also discuss further factors that are not yet scientifically quantifiable but may be requirements for EHs to evolve. Based on such an approach, we find that EHs are relatively rare by obtaining plausible maximum numbers of 2.5 - 2.4 + 71.6 × 10 5 and 0.6 - 0.59 + 27.1 × 10 5 planets that can potentially host N2-O2-dominated atmospheres with maximum CO2 mixing ratios of 10% and 1%, respectively, implying that, on average, a minimum of ∼ 10 3 - 10 6 rocky exoplanets in the HZCL are needed for 1 EH to evolve. The actual number of EHs, however, may be substantially lower than our maximum ranges since several requirements with unknown occurrence rates are not included in our model (e.g., the origin of life, working carbon-silicate and nitrogen cycles); this also implies extraterrestrial intelligence (ETI) to be significantly rarer still. Our results illustrate that not every star can host EHs nor can each rocky exoplanet within the HZCL evolve such that it might be able to host complex animal-like life or even ETIs. The Copernican Principle of Mediocrity therefore cannot be applied to infer that such life will be common in the galaxy.

Chapter 4: A Geological and Chemical Context for the Origins of Life on Early Earth

Within the first billion years of Earth's history, the planet transformed from a hot, barren, and inhospitable landscape to an environment conducive to the emergence and persistence of life. This chapter will review the state of knowledge concerning early Earth's (Hadean/Eoarchean) geochemical environment, including the origin and composition of the planet's moon, crust, oceans, atmosphere, and organic content. It will also discuss abiotic geochemical cycling of the CHONPS elements and how these species could have been converted to biologically relevant building blocks, polymers, and chemical networks. Proposed environments for abiogenesis events are also described and evaluated. An understanding of the geochemical processes under which life may have emerged can better inform our assessment of the habitability of other worlds, the potential complexity that abiotic chemistry can achieve (which has implications for putative biosignatures), and the possibility for biochemistries that are vastly different from those on Earth.

Variations in climate habitability parameters and their effect on Earth's biosphere during the Phanerozoic Eon

Essential insights on the characterization and quality of a detectable biosphere are gained by analyzing the effects of its environmental parameters. We compiled environmental and biological properties of the Phanerozoic Eon from various published data sets and conducted a correlation analysis to assess variations in parameters relevant to the habitability of Earth's biosphere. We showed that environmental parameters such as oxygen, global average surface temperatures, runoff rates and carbon dioxide are interrelated and play a key role in the changes of biomass and biodiversity. We showed that there were several periods with a highly thriving biosphere, with one even surpassing present day biodiversity and biomass. Those periods were characterized by increased oxygen levels and global runoff rates, as well as moderate global average surface temperatures, as long as no large or rapid positive and/or negative temperature excursions occurred. High oxygen contents are diagnostic of biomass production by continental plant life. We find that exceptionally high oxygen levels can at least in one instance compensate for decreased relative humidities, providing an even more habitable environment compared to today. Beyond Earth, these results will help us to understand how environmental parameters affect biospheres on extrasolar planets and guide us in our search for extraterrestrial life.

Metal-rich stars are less suitable for the evolution of life on their planets

Atmospheric ozone and oxygen protect the terrestrial biosphere against harmful ultraviolet (UV) radiation. Here, we model atmospheres of Earth-like planets hosted by stars with near-solar effective temperatures (5300 to 6300 K) and a broad range of metallicities covering known exoplanet host stars. We show that paradoxically, although metal-rich stars emit substantially less ultraviolet radiation than metal-poor stars, the surface of their planets is exposed to more intense ultraviolet radiation. For the stellar types considered, metallicity has a larger impact than stellar temperature. During the evolution of the universe, newly formed stars have progressively become more metal-rich, exposing organisms to increasingly intense ultraviolet radiation. Our findings imply that planets hosted by stars with low metallicity are the best targets to search for complex life on land.

Distinguishing Biotic vs. Abiotic Origins of ‘Bio’signatures: Clues from Messy Prebiotic Chemistry for Detection of Life in the Universe

It is not a stretch to say that the search for extraterrestrial life is possibly the biggest of the cosmic endeavors that humankind has embarked upon. With the continued discovery of several Earth-like exoplanets, the hope of detecting potential biosignatures is multiplying amongst researchers in the astrobiology community. However, to be able to discern these signatures as being truly of biological origin, we also need to consider their probable abiotic origin. The field of prebiotic chemistry, which is aimed at understanding enzyme-free chemical syntheses of biologically relevant molecules, could particularly aid in this regard. Specifically, certain peculiar characteristics of prebiotically pertinent messy chemical reactions, including diverse and racemic product yields and lower synthesis efficiencies, can be utilized in analyzing whether a perceived ‘signature of life’ could possibly have chemical origins. The knowledge gathered from understanding the transition from chemistry to biology during the origin of life could be used for creating a library of abiotically synthesized biologically relevant organic molecules. This can then be employed in designing, standardizing, and testing mission-specific instruments/analysis systems, while also enabling the effective targeting of exoplanets with potentially ‘ongoing’ molecular evolutionary processes for robust detection of life in future explorative endeavors.

Oxygenic photosynthetic responses of cyanobacteria exposed under an M-dwarf starlight simulator: Implications for exoplanet's habitability

INTRODUCTION

The search for life on distant exoplanets is expected to rely on atmospheric biosignatures detection, such as oxygen of biological origin. However, it is not demonstrated how much oxygenic photosynthesis, which on Earth depends on visible light, could work under spectral conditions simulating exoplanets orbiting the Habitable Zone of M-dwarf stars, which have low light emission in the visible and high light emission in the far-red/near-infrared. By utilizing cyanobacteria, the first organisms to evolve oxygenic photosynthesis on our planet, and a starlight simulator capable of accurately reproducing the emission spectrum of an M-dwarf in the range 350-900 nm, we could answer this question.

METHODS

We performed experiments with the cyanobacterium Chlorogloeopsis fritschii PCC6912, capable of Far-Red Light Photoacclimation (FaRLiP), which allows the strain to harvest far-red in addition to visible light for photosynthesis, and Synechocystis sp. PCC6803, a species unable to perform this photoacclimation, comparing their responses when exposed to three simulated light spectra: M-dwarf, solar and far-red. We analysed growth and photosynthetic acclimation features in terms of pigment composition and photosystems organization. Finally, we determined the oxygen production of the strains directly exposed to the different spectra.

RESULTS

Both cyanobacteria were shown to grow and photosynthesize similarly under M-dwarf and solar light conditions: Synechocystis sp. by utilizing the few photons in the visible, C. fritschii by harvesting both visible and far-red light, activating the FaRLiP response.

DISCUSSION

Our results experimentally show that an M-dwarf light spectrum could support a biological oxygen production similar to that in solar light at the tested light intensities, suggesting the possibility to discover such atmospheric biosignatures on those exoplanets if other boundary conditions are met.

Rapid timescale for an oxic transition during the Great Oxidation Event and the instability of low atmospheric O2

Understanding the rise of atmospheric oxygen on Earth is important for assessing precursors to complex life and for evaluating potential future detections of oxygen on exoplanets as signs of extraterrestrial biospheres. However, it is unclear whether Earth’s initial rise of O₂ was monotonic or oscillatory, and geologic evidence poorly constrains O₂ afterward, during the mid-Proterozoic (1.8 billion to 0.8 billion years ago). Here, we used a time-dependent photochemical model to simulate oxygen’s rise and the stability of subsequent O₂ levels to perturbations in supply and loss. Results show that large oxygen fluctuations are possible during the initial rise of O₂ and that Mesoproterozoic O₂ had to exceed 0.01% volume concentration for atmospheric stability.

The Great Oxidation Event (GOE), arguably the most important event to occur on Earth since the origin of life, marks the time when an oxygen-rich atmosphere first appeared. However, it is not known whether the change was abrupt and permanent or fitful and drawn out over tens or hundreds of millions of years. Here, we developed a one-dimensional time-dependent photochemical model to resolve time-dependent behavior of the chemically unstable transitional atmosphere as it responded to changes in biogenic forcing. When forced with step-wise changes in biogenic fluxes, transitions between anoxic and oxic atmospheres take between only 10² and 10⁵ y. Results also suggest that O₂ between ~10−8 and ~10−4 mixing ratio is unstable to plausible atmospheric perturbations. For example, when atmospheres with these O₂ concentrations experience fractional variations in the surface CH₄ flux comparable to those caused by modern Milankovich cycling, oxygen fluctuates between anoxic (~10−8) and oxic (~10−4) mixing ratios. Overall, our simulations are consistent with possible geologic evidence of unstable atmospheric O₂, after initial oxygenation, which could occasionally collapse from changes in biospheric or volcanic fluxes. Additionally, modeling favors mid-Proterozoic O₂ exceeding 10−4 to 10−3 mixing ratio; otherwise, O₂ would periodically fall below 10−7 mixing ratio, which would be inconsistent with post-GOE absence of sulfur isotope mass-independent fractionation.

Planetary Minerals Catalyze Conversion of a Polycyclic Aromatic Hydrocarbon to a Prebiotic Quinone: Implications for Origins of Life

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in astrochemical environments and are disbursed into planetary environments via meteorites and extraterrestrial infall where they may interact with mineral phases to produce quinones important for origins of life. In this study, we assessed the potential of the phyllosilicates montmorillonite (MONT) and kaolinite (KAO), and the enhanced Mojave Mars Simulant (MMS) to convert the PAH anthracene (ANTH) to the biologically important 9,10-anthraquinone (ANTHQ). All studied mineral substrates mediate conversion over the temperature range assessed (25-500°C). Apparent rate curves for conversion were sigmoidal for MONT and KAO, but quadratic for MMS. Conversion efficiency maxima for ANTHQ were 3.06% ± 0.42%, 1.15% ± 0.13%, and 0.56% ± 0.039% for MONT, KAO, and MMS, respectively. We hypothesized that differential substrate binding and compound loss account for the apparent conversion kinetics observed. Apparent loss rate curves for ANTH and ANTHQ were exponential for all substrates, suggesting a pathway for wide distribution of both compounds in warmer prebiotic environments. These findings improve upon our previously reported ANTHQ conversion efficiency on MONT and provide support for a plausible scenario in which PAH-mineral interactions could have produced prebiotically relevant quinones in early Earth environments.

The role of the three body photodissociation channel of water in the evolution of dioxygen in astrophysical applications

A recent experiment at the Dalian Coherent Light Source (DCLS) has provided measurements of the partial cross sections for the photodissociation of water vapor over an unprecedented range of wavelengths in the vacuum ultraviolet (VUV) region. It was found that the three body dissociation channel, H + H + O(3P/1D), becomes prominent at wavelengths shorter than the Lyman α-line at 121.6 nm. The present work explores the kinetic consequences of this discovery for several astrophysically motivated examples. The irradiation of a dilute low-temperature gas by unscreened solar radiation, similar to early stage photochemical processing in a comet coma, shows significant increase in the production of O2-molecules at shorter times, <1 day, that might physically correspond to the photochemical reaction zone of the coma. Several examples of planetary atmospheres show increased O-atom production at high altitudes but relatively little modification of the equilibrium O2 concentrations predicted by conventional models.

Super-Earths, M Dwarfs, and Photosynthetic Organisms: Habitability in the Lab

In a few years, space telescopes will investigate our Galaxy to detect evidence of life, mainly by observing rocky planets. In the last decade, the observation of exoplanet atmospheres and the theoretical works on biosignature gasses have experienced a considerable acceleration. The most attractive feature of the realm of exoplanets is that 40% of M dwarfs host super-Earths with a minimum mass between 1 and 30 Earth masses, orbital periods shorter than 50 days, and radii between those of the Earth and Neptune (1–3.8 R⊕). Moreover, the recent finding of cyanobacteria able to use far-red (FR) light for oxygenic photosynthesis due to the synthesis of chlorophylls d and f, extending in vivo light absorption up to 750 nm, suggests the possibility of exotic photosynthesis in planets around M dwarfs. Using innovative laboratory instrumentation, we exposed different cyanobacteria to an M dwarf star simulated irradiation, comparing their responses to those under solar and FR simulated lights. As expected, in FR light, only the cyanobacteria able to synthesize chlorophyll d and f could grow. Surprisingly, all strains, both able or unable to use FR light, grew and photosynthesized under the M dwarf generated spectrum in a similar way to the solar light and much more efficiently than under the FR one. Our findings highlight the importance of simulating both the visible and FR light components of an M dwarf spectrum to correctly evaluate the photosynthetic performances of oxygenic organisms exposed under such an exotic light condition.

Phosphine as a Biosignature Gas in Exoplanet Atmospheres

A long-term goal of exoplanet studies is the identification and detection of biosignature gases. Beyond the most discussed biosignature gas O₂, only a handful of gases have been considered in detail. In this study, we evaluate phosphine (PH₃). On Earth, PH₃ is associated with anaerobic ecosystems, and as such, it is a potential biosignature gas in anoxic exoplanets. We simulate the atmospheres of habitable terrestrial planets with CO₂- and H₂-dominated atmospheres and find that PH₃ can accumulate to detectable concentrations on planets with surface production fluxes of 10¹⁰ to 10¹⁴ cm^-2 s^-1 (corresponding to surface concentrations of 10s of ppb to 100s of ppm), depending on atmospheric composition and ultraviolet (UV) irradiation. While high, the surface flux values are comparable to the global terrestrial production rate of methane or CH₄ (10¹¹ cm^-2 s^-1) and below the maximum local terrestrial PH₃ production rate (10¹⁴ cm^-2 s^-1). As with other gases, PH₃ can more readily accumulate on low-UV planets, for example, planets orbiting quiet M dwarfs or with a photochemically generated UV shield. PH₃ has three strong spectral features such that in any atmosphere scenario one of the three will be unique compared with other dominant spectroscopic molecules. Phosphine's weakness as a biosignature gas is its high reactivity, requiring high outgassing rates for detectability. We calculate that tens of hours of JWST (James Webb Space Telescope) time are required for a potential detection of PH₃. Yet, because PH₃ is spectrally active in the same wavelength regions as other atmospherically important molecules (such as H₂O and CH₄), searches for PH₃ can be carried out at no additional observational cost to searches for other molecular species relevant to characterizing exoplanet habitability. Phosphine is a promising biosignature gas, as it has no known abiotic false positives on terrestrial planets from any source that could generate the high fluxes required for detection.

Oxidized micrometeorites suggest either high pCO2 or low pN2 during the Neoarchean

Tomkins et al. [A. G. Tomkins et al., Nature 533, 235-238 (2016)] suggested that iron oxides contained in 2.7-Ga iron micrometeorites can be used to determine the concentration of O₂ in the Archean upper atmosphere. Specifically, they argued that the presence of magnetite in these objects implies that O₂ must have been near present-day levels (∼21%) within the altitude range where the micrometeorites were melted during entry. Here, we reevaluate their data using a 1D photochemical model. We find that atomic oxygen, O, is the most abundant strong oxidant in the upper atmosphere, rather than O₂ But data from shock tube experiments suggest that CO₂ itself may also serve as the oxidant, in which case micrometeorite oxidation really constrains the CO₂/N₂ ratio, not the total oxidant abundance. For an atmosphere containing 0.8 bar of N₂, like today, the lower limit on the CO₂ mixing ratio is ∼0.23. This would produce a mean surface temperature of ∼300 K at 2.7 Ga, which may be too high, given evidence for glaciation at roughly this time. If pN₂ was half the present value, and warming by other greenhouse gases like methane was not a major factor, the mean surface temperature would drop to ∼291 K, consistent with glaciation. This suggests that surface pressure in the Neoarchean may need to have been lower-closer to 0.6 bar-for CO₂ to have oxidized the micrometeorites. Ultimately, iron micrometeorites may be an indicator for ancient atmospheric CO₂ and surface pressure; and could help resolve discrepancies between climate models and existing CO₂ proxies such as paleosols.

Atmospheric CO2 levels from 2.7 billion years ago inferred from micrometeorite oxidation

Earth's atmospheric composition during the Archean eon of 4 to 2.5 billion years ago has few constraints. However, the geochemistry of recently discovered iron-rich micrometeorites from 2.7 billion-year-old limestones could serve as a proxy for ancient gas concentrations. When micrometeorites entered the atmosphere, they melted and preserved a record of atmospheric interaction. We model the motion, evaporation, and kinetic oxidation by CO₂ of micrometeorites entering a CO₂-rich atmosphere. We consider a CO₂-rich rather than an O₂-rich atmosphere, as considered previously, because this better represents likely atmospheric conditions in the anoxic Archean. Our model reproduces the observed oxidation state of micrometeorites at 2.7 Ga for an estimated atmospheric CO₂ concentration of >70% by volume. Even if the early atmosphere was thinner than today, the elevated CO₂ level indicated by our model result would help resolve how the Late Archean Earth remained warm when the young Sun was ~20% fainter.

Circular Spectropolarimetric Sensing of Vegetation in the Field: Possibilities for the Remote Detection of Extraterrestrial Life

Homochirality is a generic and unique property of all biochemical life, and the fractional circular polarization of light it induces therefore constitutes a potentially unambiguous biosignature. However, while high-quality circular polarimetric spectra can be easily and quickly obtained in the laboratory, accurate measurements in the field are much more challenging due to large changes in illumination and target movement. In this study, we measured various targets in the field, up to distances of a few kilometers, using the dedicated circular spectropolarimeter TreePol. We show how photosynthetic life can readily be distinguished from abiotic matter. We underline the potential of circular polarization signals as a remotely accessible means to characterize and monitor terrestrial vegetation, for example, for agriculture and forestry. In addition, we discuss the potential of circular polarization for the remote detection of extraterrestrial life.

How to define obligatory anaerobiosis? An evolutionary view on the antioxidant response system and the early stages of the evolution of life on Earth

One of the former definitions of "obligate anaerobiosis" was based on three main criteria: 1) it occurs in organisms, so-called obligate anaerobes, which live in environments without oxygen (O₂), 2) O₂-dependent (aerobic) respiration, and 3) antioxidant enzymes are absent in obligate anaerobes. In contrast, aerobes need O₂ in order to grow and develop properly. Obligate (or strict) anaerobes belong to prokaryotic microorganisms from two domains, Bacteria and Archaea. A closer look at anaerobiosis covers a wide range of microorganisms that permanently or in a time-dependent manner tolerate different concentrations of O₂ in their habitats. On this basis they can be classified as obligate/facultative anaerobes, microaerophiles and nanaerobes. Paradoxically, O₂ tolerance in strict anaerobes is usually, as in aerobes, associated with the activity of the antioxidant response system, which involves different antioxidant enzymes responsible for removing excess reactive oxygen species (ROS). In our opinion, the traditional definition of "obligate anaerobiosis" loses its original sense. Strict anaerobiosis should only be restricted to the occurrence of O₂-independent pathways involved in energy generation. For that reason, a term better than "obligate anaerobes" would be O₂/ROS tolerant anaerobes, where the role of the O₂/ROS detoxification system is separated from O₂-independent metabolic pathways that supply energy. Ubiquitous key antioxidant enzymes like superoxide dismutase (SOD) and superoxide reductase (SOR) in contemporary obligate anaerobes might suggest that their origin is ancient, maybe even the beginning of the evolution of life on Earth. It cannot be ruled out that c. 3.5 Gyr ago, local microquantities of O₂/ROS played a role in the evolution of the last universal common ancestor (LUCA) of all modern organisms. On the basis of data in the literature, the hypothesis that LUCA could be an O₂/ROS tolerant anaerobe is discussed together with the question of the abiotic sources of O₂/ROS and/or the early evolution of cyanobacteria that perform oxygenic photosynthesis.

The Role of N2 as a Geo-Biosignature for the Detection and Characterization of Earth-like Habitats

Since the Archean, N₂ has been a major atmospheric constituent in Earth's atmosphere. Nitrogen is an essential element in the building blocks of life; therefore, the geobiological nitrogen cycle is a fundamental factor in the long-term evolution of both Earth and Earth-like exoplanets. We discuss the development of Earth's N₂ atmosphere since the planet's formation and its relation with the geobiological cycle. Then we suggest atmospheric evolution scenarios and their possible interaction with life-forms: first for a stagnant-lid anoxic world, second for a tectonically active anoxic world, and third for an oxidized tectonically active world. Furthermore, we discuss a possible demise of present Earth's biosphere and its effects on the atmosphere. Since life-forms are the most efficient means for recycling deposited nitrogen back into the atmosphere at present, they sustain its surface partial pressure at high levels. Also, the simultaneous presence of significant N₂ and O₂ is chemically incompatible in an atmosphere over geological timescales. Thus, we argue that an N₂-dominated atmosphere in combination with O₂ on Earth-like planets within circumstellar habitable zones can be considered as a geo-biosignature. Terrestrial planets with such atmospheres will have an operating tectonic regime connected with an aerobic biosphere, whereas other scenarios in most cases end up with a CO₂-dominated atmosphere. We conclude with implications for the search for life on Earth-like exoplanets inside the habitable zones of M to K stars.

Follow the Oxygen: Comparative Histories of Planetary Oxygenation and Opportunities for Aerobic Life

Aerobic respiration-the reduction of molecular oxygen (O₂) coupled to the oxidation of reduced compounds such as organic carbon, ferrous iron, reduced sulfur compounds, or molecular hydrogen while conserving energy to drive cellular processes-is the most widespread and bioenergetically favorable metabolism on Earth today. Aerobic respiration is essential for the development of complex multicellular life; thus the presence of abundant O₂ is an important metric for planetary habitability. O₂ on Earth is supplied by oxygenic photosynthesis, but it is becoming more widely understood that abiotic processes may supply meaningful amounts of O₂ on other worlds. The modern atmosphere and rock record of Mars suggest a history of relatively high O₂ as a result of photochemical processes, potentially overlapping with the range of O₂ concentrations used by biology. Europa may have accumulated high O₂ concentrations in its subsurface ocean due to the radiolysis of water ice at its surface. Recent modeling efforts suggest that coexisting water and O₂ may be common on exoplanets, with confirmation from measurements of exoplanet atmospheres potentially coming soon. In all these cases, O₂ accumulates through abiotic processes-independent of water-oxidizing photosynthesis. We hypothesize that abiogenic O₂ may enhance the habitability of some planetary environments, allowing highly energetic aerobic respiration and potentially even the development of complex multicellular life which depends on it, without the need to first evolve oxygenic photosynthesis. This hypothesis is testable with further exploration and life-detection efforts on O₂-rich worlds such as Mars and Europa, and comparison to O₂-poor worlds such as Enceladus. This hypothesis further suggests a new dimension to planetary habitability: "Follow the Oxygen," in which environments with opportunities for energy-rich metabolisms such as aerobic respiration are preferentially targeted for investigation and life detection.

Modeling Repeated M Dwarf Flaring at an Earth-like Planet in the Habitable Zone: Atmospheric Effects for an Unmagnetized Planet

Understanding the impact of active M dwarf stars on the atmospheric equilibrium and surface conditions of a habitable zone Earth-like planet is key to assessing M dwarf planet habitability. Previous modeling of the impact of electromagnetic (EM) radiation and protons from a single large flare on an Earth-like atmosphere indicated that significant and long-term reductions in ozone were possible, but the atmosphere recovered. However, these stars more realistically exhibit frequent flaring with a distribution of different total energies and cadences. Here, we use a coupled 1D photochemical and radiative-convective model to investigate the effects of repeated flaring on the photochemistry and surface UV of an Earth-like planet unprotected by an intrinsic magnetic field. As input, we use time-resolved flare spectra obtained for the dM3 star AD Leonis, combined with flare occurrence frequencies and total energies (typically 10^30.5 to 10³⁴ erg) from the 4-year Kepler light curve for the dM4 flare star GJ1243, with varied proton event impact frequency. Our model results show that repeated EM-only flares have little effect on the ozone column depth but that multiple proton events can rapidly destroy the ozone column. Combining the realistic flare and proton event frequencies with nominal CME/SEP geometries, we find the ozone column for an Earth-like planet can be depleted by 94% in 10 years, with a downward trend that makes recovery unlikely and suggests further destruction. For more extreme stellar inputs, O₃ depletion allows a constant ∼0.1-1 W m^-2 of UVC at the planet's surface, which is likely detrimental to organic complexity. Our results suggest that active M dwarf hosts may comprehensively destroy ozone shields and subject the surface of magnetically unprotected Earth-like planets to long-term radiation that can damage complex organic structures. However, this does not preclude habitability, as a safe haven for life could still exist below an ocean surface.

The Ladder of Life Detection

We describe the history and features of the Ladder of Life Detection, a tool intended to guide the design of investigations to detect microbial life within the practical constraints of robotic space missions. To build the Ladder, we have drawn from lessons learned from previous attempts at detecting life and derived criteria for a measurement (or suite of measurements) to constitute convincing evidence for indigenous life. We summarize features of life as we know it, how specific they are to life, and how they can be measured, and sort these features in a general sense based on their likelihood of indicating life. Because indigenous life is the hypothesis of last resort in interpreting life-detection measurements, we propose a small but expandable set of decision rules determining whether the abiotic hypothesis is disproved. In light of these rules, we evaluate past and upcoming attempts at life detection. The Ladder of Life Detection is not intended to endorse specific biosignatures or instruments for life-detection measurements, and is by no means a definitive, final product. It is intended as a starting point to stimulate discussion, debate, and further research on the characteristics of life, what constitutes a biosignature, and the means to measure them.

A More Comprehensive Habitable Zone for Finding Life on Other Planets

The habitable zone (HZ) is the circular region around a star(s) where standing bodies of water could exist on the surface of a rocky planet. Space missions employ the HZ to select promising targets for follow-up habitability assessment. The classical HZ definition assumes that the most important greenhouse gases for habitable planets orbiting main-sequence stars are CO2 and H2O. Although the classical HZ is an effective navigational tool, recent HZ formulations demonstrate that it cannot thoroughly capture the diversity of habitable exoplanets. Here, I review the planetary and stellar processes considered in both classical and newer HZ formulations. Supplementing the classical HZ with additional considerations from these newer formulations improves our capability to filter out worlds that are unlikely to host life. Such improved HZ tools will be necessary for current and upcoming missions aiming to detect and characterize potentially habitable exoplanets.

Evolution of Earth-like Planetary Atmospheres around M Dwarf Stars: Assessing the Atmospheres and Biospheres with a Coupled Atmosphere Biogeochemical Model

Earth-like planets orbiting M dwarfs are prominent targets when searching for life outside the Solar System. We apply our Coupled Atmosphere Biogeochemical model to investigate the coupling between the biosphere, geosphere, and atmosphere in order to gain insight into the atmospheric evolution of Earth-like planets orbiting M dwarfs and to understand the processes affecting biosignatures and climate on such worlds. This is the first study applying an automated chemical pathway analysis quantifying the production and destruction pathways of molecular oxygen (O₂) for an Earth-like planet with an Archean O₂ concentration orbiting in the habitable zone of the M dwarf star AD Leonis, which we take as a type-case of an active M dwarf. The main production arises in the upper atmosphere from carbon dioxide photolysis followed by catalytic hydrogen oxide radical (HO_x) reactions. The strongest destruction does not take place in the troposphere, as was the case in Gebauer et al. ( 2017 ) for an early Earth analog planet around the Sun, but instead in the middle atmosphere where water photolysis is the strongest. Results further suggest that these atmospheres are in absolute terms less destructive for O₂ than for early Earth analog planets around the Sun despite higher concentrations of reduced gases such as molecular hydrogen, methane, and carbon monoxide. Hence smaller amounts of net primary productivity are required to oxygenate the atmosphere due to a change in the atmospheric oxidative capacity, driven by the input stellar spectrum resulting in shifts in the intrafamily HO_x partitioning. Under the assumption that an atmosphere of an Earth-like planet survived and evolved during the early high-activity phase of an M dwarf to an Archean-type composition, a possible "Great Oxidation Event," analogous to that on Early Earth, would have occurred earlier in time after the atmospheric composition was reached, assuming the same atmospheric O₂ sources and sinks as on early Earth. Key Words: Earth-like-Oxygen-M dwarf stars-Atmosphere-Biogeochemistry-Photochemistry-Biosignatures-Earth-like planets. Astrobiology 18, 856-872.

Exoplanet Biosignatures: Future Directions

We introduce a Bayesian method for guiding future directions for detection of life on exoplanets. We describe empirical and theoretical work necessary to place constraints on the relevant likelihoods, including those emerging from better understanding stellar environment, planetary climate and geophysics, geochemical cycling, the universalities of physics and chemistry, the contingencies of evolutionary history, the properties of life as an emergent complex system, and the mechanisms driving the emergence of life. We provide examples for how the Bayesian formalism could guide future search strategies, including determining observations to prioritize or deciding between targeted searches or larger lower resolution surveys to generate ensemble statistics and address how a Bayesian methodology could constrain the prior probability of life with or without a positive detection. Key Words: Exoplanets-Biosignatures-Life detection-Bayesian analysis. Astrobiology 18, 779-824.

Exoplanet Biosignatures: A Framework for Their Assessment

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 or not life is found with future data must be expressed with probabilities, requiring a framework of biosignature assessment. We present a framework in which we advocate using biogeochemical "Exo-Earth System" models to simulate potential biosignatures in spectra or photometry. Given actual observations, simulations are used to find the Bayesian likelihoods of those data occurring for scenarios with and without life. The latter includes "false positives" wherein abiotic sources mimic biosignatures. Prior knowledge of factors influencing planetary inhabitation, including previous observations, is combined with the likelihoods to give the Bayesian posterior probability of life existing on a given exoplanet. Four components of observation and analysis are necessary. (1) Characterization of stellar (e.g., age and spectrum) and exoplanetary system properties, including "external" exoplanet parameters (e.g., mass and radius), to determine an exoplanet's suitability for life. (2) Characterization of "internal" exoplanet parameters (e.g., climate) to evaluate habitability. (3) Assessment of potential biosignatures within the environmental context (components 1-2), including corroborating evidence. (4) Exclusion of false positives. We propose that resulting posterior Bayesian probabilities of life's existence map to five confidence levels, ranging from "very likely" (90-100%) to "very unlikely" (<10%) inhabited. Key Words: Bayesian statistics-Biosignatures-Drake equation-Exoplanets-Habitability-Planetary science. Astrobiology 18, 709-738.

Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life

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.

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

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

Exoplanet Biosignatures: At the Dawn of a New Era of Planetary Observations

The rapid rate of discoveries of exoplanets has expanded the scope of the science possible for the remote detection of life beyond Earth. The Exoplanet Biosignatures Workshop Without Walls (EBWWW) held in 2016 engaged the international scientific community across diverse scientific disciplines, to assess the state of the science and technology in the search for life on exoplanets, and to identify paths for progress. The workshop activities resulted in five major review papers, which provide (1) an encyclopedic review of known and proposed biosignatures and models used to ascertain them (Schwieterman et al., 2018 in this issue); (2) an in-depth review of O2 as a biosignature, rigorously examining the nuances of false positives and false negatives for evidence of life (Meadows et al., 2018 in this issue); (3) a Bayesian framework to comprehensively organize current understanding to quantify confidence in biosignature assessments (Catling et al., 2018 in this issue); (4) an extension of that Bayesian framework in anticipation of increasing planetary data and novel concepts of biosignatures (Walker et al., 2018 in this issue); and (5) a review of the upcoming telescope capabilities to characterize exoplanets and their environment (Fujii et al., 2018 in this issue). Because of the immense content of these review papers, this summary provides a guide to their complementary scope and highlights salient features. Strong themes that emerged from the workshop were that biosignatures must be interpreted in the context of their environment, and that frameworks must be developed to link diverse forms of scientific understanding of that context to quantify the likelihood that a biosignature has been observed. Models are needed to explore the parameter space where measurements will be widespread but sparse in detail. Given the technological prospects for large ground-based telescopes and space-based observatories, the detection of atmospheric signatures of a few potentially habitable planets may come before 2030. Key Words: Exoplanets-Biosignatures-Remote observation-Spectral imaging-Bayesian analysis. Astrobiology 18, 619-626.

Organic Haze as a Biosignature in Anoxic Earth-like Atmospheres

Early Earth may have hosted a biologically mediated global organic haze during the Archean eon (3.8-2.5 billion years ago). This haze would have significantly impacted multiple aspects of our planet, including its potential for habitability and its spectral appearance. Here, we model worlds with Archean-like levels of carbon dioxide orbiting the ancient Sun and an M4V dwarf (GJ 876) and show that organic haze formation requires methane fluxes consistent with estimated Earth-like biological production rates. On planets with high fluxes of biogenic organic sulfur gases (CS2, OCS, CH3SH, and CH3SCH3), photochemistry involving these gases can drive haze formation at lower CH4/CO2 ratios than methane photochemistry alone. For a planet orbiting the Sun, at 30× the modern organic sulfur gas flux, haze forms at a CH4/CO2 ratio 20% lower than at 1× the modern organic sulfur flux. For a planet orbiting the M4V star, the impact of organic sulfur gases is more pronounced: at 1× the modern Earth organic sulfur flux, a substantial haze forms at CH4/CO2 ∼ 0.2, but at 30× the organic sulfur flux, the CH4/CO2 ratio needed to form haze decreases by a full order of magnitude. Detection of haze at an anomalously low CH4/CO2 ratio could suggest the influence of these biogenic sulfur gases and therefore imply biological activity on an exoplanet. When these organic sulfur gases are not readily detectable in the spectrum of an Earth-like exoplanet, the thick organic haze they can help produce creates a very strong absorption feature at UV-blue wavelengths detectable in reflected light at a spectral resolution as low as 10. In direct imaging, constraining CH4 and CO2 concentrations will require higher spectral resolution, and R > 170 is needed to accurately resolve the structure of the CO2 feature at 1.57 μm, likely the most accessible CO2 feature on an Archean-like exoplanet. Key Words: Organic haze-Organic sulfur gases-Biosignatures-Archean Earth. Astrobiology 18, 311-329.

The Habitability of Proxima Centauri b: Environmental States and Observational Discriminants

Proxima Centauri b provides an unprecedented opportunity to understand the evolution and nature of terrestrial planets orbiting M dwarfs. Although Proxima Cen b orbits within its star's habitable zone, multiple plausible evolutionary paths could have generated different environments that may or may not be habitable. Here, we use 1-D coupled climate-photochemical models to generate self-consistent atmospheres for several evolutionary scenarios, including high-O2, high-CO2, and more Earth-like atmospheres, with both oxic and anoxic compositions. We show that these modeled environments can be habitable or uninhabitable at Proxima Cen b's position in the habitable zone. We use radiative transfer models to generate synthetic spectra and thermal phase curves for these simulated environments, and use instrument models to explore our ability to discriminate between possible planetary states. These results are applicable not only to Proxima Cen b but to other terrestrial planets orbiting M dwarfs. Thermal phase curves may provide the first constraint on the existence of an atmosphere. We find that James Webb Space Telescope (JWST) observations longward of 10 μm could characterize atmospheric heat transport and molecular composition. Detection of ocean glint is unlikely with JWST but may be within the reach of larger-aperture telescopes. Direct imaging spectra may detect O4 absorption, which is diagnostic of massive water loss and O2 retention, rather than a photosynthetic biosphere. Similarly, strong CO2 and CO bands at wavelengths shortward of 2.5 μm would indicate a CO2-dominated atmosphere. If the planet is habitable and volatile-rich, direct imaging will be the best means of detecting habitability. Earth-like planets with microbial biospheres may be identified by the presence of CH4-which has a longer atmospheric lifetime under Proxima Centauri's incident UV-and either photosynthetically produced O2 or a hydrocarbon haze layer. Key Words: Planetary habitability and biosignatures-Planetary atmospheres-Exoplanets-Spectroscopic biosignatures-Planetary science-Proxima Centauri b. Astrobiology 18, 133-189.

Reflections on O2 as a Biosignature in Exoplanetary Atmospheres

Oxygenic photosynthesis is Earth's dominant metabolism, having evolved to harvest the largest expected energy source at the surface of most terrestrial habitable zone planets. Using CO₂ and H₂O-molecules that are expected to be abundant and widespread on habitable terrestrial planets-oxygenic photosynthesis is plausible as a significant planetary process with a global impact. Photosynthetic O₂ has long been considered particularly robust as a sign of life on a habitable exoplanet, due to the lack of known "false positives"-geological or photochemical processes that could also produce large quantities of stable O₂. O₂ has other advantages as a biosignature, including its high abundance and uniform distribution throughout the atmospheric column and its distinct, strong absorption in the visible and near-infrared. However, recent modeling work has shown that false positives for abundant oxygen or ozone could be produced by abiotic mechanisms, including photochemistry and atmospheric escape. Environmental factors for abiotic O₂ have been identified and will improve our ability to choose optimal targets and measurements to guard against false positives. Most of these false-positive mechanisms are dependent on properties of the host star and are often strongest for planets orbiting M dwarfs. In particular, selecting planets found within the conservative habitable zone and those orbiting host stars more massive than 0.4 M_⊙ (M3V and earlier) may help avoid planets with abundant abiotic O₂ generated by water loss. Searching for O₄ or CO in the planetary spectrum, or the lack of H₂O or CH₄, could help discriminate between abiotic and biological sources of O₂ or O₃. In advance of the next generation of telescopes, thorough evaluation of potential biosignatures-including likely environmental context and factors that could produce false positives-ultimately works to increase our confidence in life detection. Key Words: Biosignatures-Exoplanets-Oxygen-Photosynthesis-Planetary spectra. Astrobiology 17, 1022-1052.

Natural and artificial spectral edges in exoplanets

Technological civilizations may rely upon large-scale photovoltaic arrays to harness energy from their host star. Photovoltaic materials, such as silicon, possess distinctive spectral features, including an ‘artificial edge' that is characteristically shifted in wavelength shortwards of the ‘red edge' of vegetation. Future observations of reflected light from exoplanets would be able to detect both natural and artificial edges photometrically, if a significant fraction of the planet's surface is covered by vegetation or photovoltaic arrays, respectively. The stellar energy thus tapped can be utilized for terraforming activities by transferring heat and light from the day side to the night side on tidally locked exoplanets, thereby producing detectable artefacts.

False Negatives for Remote Life Detection on Ocean-Bearing Planets: Lessons from the Early Earth

Ocean-atmosphere chemistry on Earth has undergone dramatic evolutionary changes throughout its long history, with potentially significant ramifications for the emergence and long-term stability of atmospheric biosignatures. Though a great deal of work has centered on refining our understanding of false positives for remote life detection, much less attention has been paid to the possibility of false negatives, that is, cryptic biospheres that are widespread and active on a planet's surface but are ultimately undetectable or difficult to detect in the composition of a planet's atmosphere. Here, we summarize recent developments from geochemical proxy records and Earth system models that provide insight into the long-term evolution of the most readily detectable potential biosignature gases on Earth-oxygen (O2), ozone (O3), and methane (CH4). We suggest that the canonical O2-CH4 disequilibrium biosignature would perhaps have been challenging to detect remotely during Earth's ∼4.5-billion-year history and that in general atmospheric O2/O3 levels have been a poor proxy for the presence of Earth's biosphere for all but the last ∼500 million years. We further suggest that detecting atmospheric CH4 would have been problematic for most of the last ∼2.5 billion years of Earth's history. More broadly, we stress that internal oceanic recycling of biosignature gases will often render surface biospheres on ocean-bearing silicate worlds cryptic, with the implication that the planets most conducive to the development and maintenance of a pervasive biosphere will often be challenging to characterize via conventional atmospheric biosignatures. Key Words: Biosignatures-Oxygen-Methane-Ozone-Exoplanets-Planetary habitability. Astrobiology 17, 287-297.

Remote Sensing of Potential Biosignatures from Rocky, Liquid, or Icy (Exo)Planetary Surfaces

To detect signs of life by remote sensing on objects of our Solar System and on exoplanets, the characterization of light scattered by surface life material could complement possible clues given by the atmospheric composition. We reviewed the reflectance spectra of a broad selection of major biomolecules that constitute terrestrial carbon-based life from 0.4 to 2.4 μm, and we discuss their detectability through atmospheric spectral windows. Biomolecule features in the near-infrared (0.8-2.4 μm) will likely be obscured by water spectral features and some atmospheric gases. The visible range (0.4-0.8 μm), including the strong spectral features of pigments, is the most favorable. We investigated the detectability of a pigmented microorganism (Deinococcus radiodurans) when mixed with silica sand, liquid water, and water-ice particles representative of diverse surfaces of potentially habitable worlds. We measured the visible to near-infrared reflectance spectra (0.4-2.4 μm) and the visible phase curves (at 0.45 and 0.75 μm) of the mixtures to assess how the surface medium and the viewing geometry affect the detectability of the microorganisms. The results show that ice appears to be the most favorable medium for the detection of pigments. Water ice is bright and featureless from 0.4 to 0.8 μm, allowing the absorption of any pigment present in the ice to be well noticeable. We found that the visible phase curve of water ice is the most strongly affected by the presence of pigments, with variations of the spectral slope by more than a factor of 3 with phase angles. Finally, we show that the sublimation of the ice results in the concentration of the biological material onto the surface and the consequent increase of its signal. These results have applications to the search for life on icy worlds, such as Europa or Enceladus. Key Words: Remote sensing-Biosignatures-Reflectance spectroscopy-Exoplanets-Spectroscopic biosignatures-Pigments. Astrobiology 17, 231-252.