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Cited by 225Space Science and Extraterrestrial LifeStellar, planetary, and galactic studiesPlanetary Science and Exploration

Exoplanet Biosignatures: A Framework for Their Assessment

Open access
Catling, David C., Krissansen-Totton, Joshua, Kiang, Nancy Y., Crisp, David, Robinson, Tyler D., DasSarma, Shiladitya, Rushby, Andrew J., Del Genio, Anthony +2 more·2018·Astrobiology
DOI 10.1089/ast.2017.1737PMC PMC6049621
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Abstract

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.

AI snapshot

Written from the paper’s full text.

TL;DR

The paper proposes a Bayesian framework for assessing exoplanet biosignatures, combining prior knowledge about habitability with likelihoods of spectral or photometric data under scenarios with and without life. It outlines four observational/analytical components and maps posterior probabilities of life to five qualitative confidence levels.

Key findings

  • The framework’s central quantity is \(P(\text{life} \mid \text{data, context})\), calculated from priors \(P(\text{life} \mid C)\) and likelihoods \(P(D \mid C, \text{life})\) and \(P(D \mid C, \text{no life})\), where the latter includes abiotic false positives.
  • In an illustrative example with prior \(P(\text{life} \mid C)=0.5\), \(P(D \mid C,\text{life})=0.80\), and \(P(D \mid C,\text{no life})=0.25\), the posterior is \(0.76 \equiv 76\%\).
  • The authors propose five confidence levels for life detection: Level 1 “very likely inhabited” 90–100%, Level 2 “likely inhabited” 66–100%, Level 3 “about as likely as not inhabited” 33–66%, Level 4 “likely uninhabited” 0–33%, and Level 5 “very likely uninhabited” 0–10%.
  • The four required components are: (1) stellar and external exoplanetary system properties, (2) internal exoplanet atmosphere/climate/surface properties, (3) search for potential biosignatures in data, and (4) exclusion of false positives.

Methods

  • Conceptual and statistical framework paper, not an empirical exoplanet observation study; it develops Bayes’ theorem for biosignature assessment and discusses required observations and models.
  • It advocates using biogeochemical “Exo-Earth System” models to simulate potential biosignatures in spectra or photometry and to estimate Bayesian likelihoods with and without life.
  • It reviews desirable stellar, planetary, atmospheric, and surface parameters; candidate gaseous and surface biosignatures; and false-positive scenarios, with illustrative examples rather than new data analysis.

Limitations

  • The paper states that current knowledge of priors such as \(P(\text{life} \mid \text{Earth-like})\) is highly uncertain, ranging from life being very common to very rare, and that extraterrestrial life has not yet been detected.
  • It states that many false-positive scenarios and surface biosignature assessments remain unresolved; for example, “a quantitative framework for evaluating surface biosignatures remains unclear,” and the confidence-level examples are “purely illustrative.”
  • Evident from the text: the framework is not tested on real exoplanet data, relies on future models and observations, and its posterior probabilities cannot yet be quantified for specific exoplanets without detailed calculations.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1089/ast.2017.1737

Cites

263 of 263 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 200 cited sources checked · 0 supported · 0 flagged

  • Interpretationcitation

    The distinction between inhabitation and habitability can become blurred.

    “Consequently, the distinction between inhabitation and habitability can become blurred (Goldblatt, 2016).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Goldblatt, 2016

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  • Premisecitation

    The probability P(life|context) for a particular exoplanet under observation is related to the term fl in the Drake equation, which applies to an ensemble of exoplanets.

    “The probability P(life | context) shown in Figure 1 refers to a particular exoplanet under observation but is related to the term fl in the Drake equation that applies to an ensemble of exoplanets (Des Marais, 2015).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Des Marais, 2015

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  • Premisecitation

    The habitable zone concept assumes that P(life|C) is higher for exoplanets orbiting within a certain range of star-planet distances.

    “Regarding habitability, the ubiquitous concept of the HZ is built upon the assumption that P(life | C) is higher where the context is an exoplanet orbiting within a certain range of star–planet distances (Kasting et al., 1993; Kasting, 1997; Kopparapu et al., 2013).”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Kasting et al., 1993

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    • cites Kasting, 1997

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    • cites Kopparapu et al., 2013

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  • Premisecitation

    On Earth, the biosphere modulates all major atmospheric gases, such as O2 and N2, and most trace gases with the exception of noble gases.

    “On Earth, the biosphere modulates all the major atmospheric gases, such as O2 and N2, and most trace gases with the exception of noble gases (e.g., Catling and Kasting, 2017).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Catling and Kasting, 2017

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  • Interpretationreasoning

    Exoplanet data might be overoptimistically interpreted to indicate the presence of life if life is actually very rare in the galaxy.

    “This Bayesian insight cautions us that exoplanet data might be overoptimistically interpreted to indicate the presence of life, if life is actually very rare in the galaxy.”
    Quote in the paperSubstantial claim
  • Classified as probably not a substantial claim. 98
  • May combine two claims. 68

  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing).

    Interpretationcitation

    Given the present stage of understanding and lack of data, the possibilities for P(life | Earth-like) range from life being very common to very rare.

    “Nonetheless, given the present stage of understanding and lack of data, the possibilities for P(life | Earth-like) range from life being very common to very rare, as has been much discussed in the literature”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Carter, 1983

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    • cites Spiegel and Turner, 2012

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    • cites Scharf and Cronin, 2016

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  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Thick H2-rich atmospheres can provide warming far beyond the conventional habitable zone, even in interstellar space.

    “This assumption has been questioned because thick H2-rich atmospheres can provide warming far beyond the conventional HZ, even in interstellar space (Stevenson, 1999; Pierrehumbert and Gaidos, 2011; Seager, 2013).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Stevenson, 1999

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    • cites Pierrehumbert and Gaidos, 2011

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    • cites Seager, 2013

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  • Probably not a substantial claim: may not pass S4 (based).

    Interpretationreasoning

    Bayes' theorem reveals current ignorance about life on exoplanets.

    “As we shall see, Bayes' theorem quickly reveals our current ignorance about life on exoplanets.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    The base rate frequency of occurrence of inhabited exoplanets around all main sequence stars in the galaxy can be derived from the formalism and would require multiple nondetections of life or detections of multiple biospheres to be quantified, with each observation adjusting priors.

    “A related quantity can be derived from our formalism, which is the “base rate” frequency of occurrence of inhabited exoplanets around all main sequence stars in the galaxy”
    • Also stated in: § 2. A Bayesian Framework for Biosignature Assessment and Life Detection · hedged
      “Rather than a single exoplanet, this derived quantity would require multiple nondetections of life or even detections of multiple biospheres to be quantified, each observation causing an adjustment of priors.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S2 (checkable), S4 (based). May combine two claims.

    Interpretationreasoning

    Hidden or cryptic biospheres are not practical candidates for life detection on exoplanets, so their consideration is not relevant to the empirical perspective of this article.

    “Hidden or cryptic biospheres are not practical candidates for life detection on exoplanets, so their consideration is not relevant to the empirical perspective of this article.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    In the Drake equation, fl is defined as the fraction of habitable planets on which life actually develops.

    “Specifically, in the Drake equation, fl is defined as “the fraction of… [habitable] planets on which life actually develops” (Drake, 1965).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Drake, 1965

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  • May combine two claims.

    Premisecitation

    Remote observations of exoplanets for biosignatures will always be limited, and in situ observations of even the nearest exoplanets are very far in the future.

    “In assessing biosignatures, we face the problem of assigning a probability to whether an exoplanet is inhabited based on remote observations that will always be limited, especially given that in situ observations of even the nearest exoplanets are very far in the future (e.g., Lubin, 2016; Heller and Hippke, 2017; Manchester and Loeb, 2017).”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Lubin, 2016

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    • cites Heller and Hippke, 2017

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    • cites Manchester and Loeb, 2017

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  • May combine two claims.

    Interpretationreasoning

    The conventional habitable zone provides a guide based on real prior knowledge about CO2-H2O-rich planets from Earth, whereas habitable H2-rich worlds remain hypothetical speculations.

    “However, the conventional HZ provides a guide based on real prior knowledge about CO2–H2O-rich planets from a real world—the Earth—whereas habitable H2-rich worlds remain hypothetical speculations.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    The estimated base-rate frequency of life on C-type exoplanets, P(life|C), would be significantly improved if life were detected elsewhere or if environments on exoplanets were found to support the HZ concept.

    “Thus, this estimate would be significantly improved if we actually detect any life elsewhere or found environments on exoplanets to support the HZ concept.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

    Interpretationreasoning

    When calculating the posterior by integration over all possible variables weighted by their probability of occurrence, a reduced set of marginal variables will be important, whereas nuisance variables can be marginalized out.

    “we expect that a reduced set of those variables—the so-called marginal variables in Bayesian statistics—will be important, whereas nuisance variables can be discarded, that is, “marginalized out.””
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    The Earth is inhabited, and consequently P(life | Earth-like) > 0.

    “The Earth is inhabited. Consequently, P(life | Earth-like) > 0, which is an aspect of empirical astronomy to which we now turn.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    The prior conditional probabilities P(life|M-stars), etc. are currently unknown, except P(life|G-stars) is nonzero because life exists on Earth.

    “The prior conditional probabilities P(life | M-stars), etc. are currently unknown, except we know that P(life | G-stars) is nonzero because life exists on Earth.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisereasoning

    Because extraterrestrial life has not yet been detected, the present state of scientific knowledge about the a priori probability of life occurring in the C-type that is an Earth-like, habitable exoplanet is based on two broad approaches: consideration of the origin and persistence of life from laboratory and theoretical studies, and the fact that detectable life exists on Earth, a habitableplanet

    “Because extraterrestrial life has not yet been detected, the present state of scientific knowledge about the a priori probability of life occurring in the C-type that is an Earth-like, habitable exoplanet is based on two broad approaches: (1) consideration of the origin and persistence of life on a planet from laboratory and theoretical studies and (2) the fact that detectable life exists on Earth”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationnone given

    The probabilities in the Bayesian framework will not be single numbers but functions of numerous parameters.

    “In reality, of course, the probabilities shown in Figure 1 will not be single numbers but functions of numerous parameters.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationcitation

    Measurements such as the surface temperature or the detection of liquid water on the surface would provide direct confirmation that an exoplanet is habitable, after the conventional definition of a habitable planet as one with a solid surface that has an extensive covering of liquid water.

    “Measurements such as the surface temperature or the detection of liquid water on the surface would provide direct confirmation that an exoplanet is habitable”
    • Also stated in: § 3.2. Component 2: the “internal” properties of the exoplanet atmosphere, climate, and surface · plain
      “after the conventional definition of a habitable planet as one with a solid surface that has an extensive covering of liquid water”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Robinson, 2018

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    • cites Kasting et al., 1993

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  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationreasoning

    Other data given in Table 2 would increase confidence that the exoplanet is Earth-like in detail rather than just similar in size and mass, like Venus.

    “Other data given in Table 2 would increase confidence that the exoplanet is Earth-like in detail rather than just similar in size and mass, like Venus.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S4 (based). May combine two claims.

    Premisereasoning

    A potentially inhabited exoplanet is embedded within the environment of its host star, so knowledge of the properties of the host star is critical for understanding exoplanet habitability.

    “A potentially inhabited exoplanet is embedded within the environment of its host star, so knowledge of the properties of the host star is critical for understanding exoplanet habitability.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Interpretationreasoning

    Exoplanet mass and radius provide the mean density of the exoplanet and the gravitational acceleration at the surface, g, which are very useful contextual parameters for the likelihoods and priors shown in Figure 1.

    “Among basic physical properties, the exoplanet mass and radius provide the mean density of the exoplanet and the gravitational acceleration at the surface, g, which are very useful contextual parameters for the likelihoods and priors shown in Figure 1.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given

    The prior P(life | C) would increase if we knew that an exoplanet surface has an ocean.

    “the prior, P (life | C), would increase if we knew that an exoplanet surface has an ocean.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationcitation

    Knowledge of the stellar rotation rate would help characterize the shortwave emission of a host star, particularly the large Lyman-alpha output at 121 nm, which produces photons that drive photolysis reactions in planetary atmospheres and feeds into estimated data likelihoods.

    “so knowledge of the stellar rotation rate would help characterize the shortwave emission of a host star, particularly the large Lyman-α output at 121 nm”
    • Also stated in: § 3.1. Component 1: stellar properties and “external” properties of the exoplanetary system · plain
      “which produces photons that drive photolysis reactions in planetary atmospheres, and feeds into estimated data likelihoods shown in Figure 1.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Linsky et al., 2013

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  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisenone given

    The exoplanet effective blackbody temperature Teff can be calculated if the Bond albedo and incoming stellar flux are known.

    “Clearly, the exoplanet effective blackbody temperature Teff can be calculated if the Bond albedo and incoming stellar flux are known.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Interpretationcitation

    Knowledge of the rotation rate of an exoplanet is highly desirable for a basic model of climate and habitability, and is context that would affect both likelihoods and priors shown in Figure 1.

    “Knowledge of the rotation rate of an exoplanet is highly desirable for a basic model of climate and habitability, and is context that would affect both likelihoods and priors in shown Figure 1.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    On Earth, deep-sea hyperthermophilic microbes can grow at 121°C and survive up to 130°C.

    “On Earth, deep-sea hyperthermophilic microbes can grow at 121°C and survive up to 130°C (Kashefi and Lovley, 2003).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Kashefi and Lovley, 2003

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  • May combine two claims.

    Interpretationreasoning

    Knowing about other planets and their properties in an extrasolar system could be important for deducing the formation history of the star-planet system, the possible orbital evolutionary path of the exoplanet of interest, and providing information that bears on climate stability.

    “Knowing about other planets and their properties in an extrasolar system could be important for deducing the formation history of the star–planet system, the possible orbital evolutionary path of the exoplanet of interest, and providing information that bears on climate stability.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    Stellar and exoplanetary system external properties provide context for understanding an exoplanet's habitability, but direct measurements concerning an exoplanet's internal properties are also needed to assess habitability, including bulk atmospheric composition and structure, climate, and surface state.

    “Stellar and exoplanetary system external properties described for “component 1” provide context for understanding an exoplanet's habitability, but direct measurements concerning an exoplanet's “internal” properties are also needed to assess habitability, including bulk atmospheric composition and structure, climate, and surface state”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S2 (checkable).

    Interpretationreasoning

    It is critical to evaluate the plausibility of a false positive detection of life.

    “This Bayesian insight reveals how critical it is to evaluate the plausibility of a false positive detection of life.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    On modern Earth, S* = 1361 W/m2, AB = 0.3, Teff = 255 K, Tsurfe = 288 K, and ΔTgreenhouse = 33 K.

    “On the modern Earth, values of parameters in Eq. 13 are S* = 1361 W/m2 (Kopp and Lean, 2011), AB = 0.3 (Palle et al., 2003; Stephens et al., 2015), Teff = 255 K, Tsurfe = 288 K, and ΔTgreenhouse = 33 K (Schmidt et al., 2010).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Kopp and Lean, 2011

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    • cites Palle et al., 2003

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    • cites Stephens et al., 2015

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  • May combine two claims.

    Premisereasoning

    In the carbonate-silicate hypothesis, HZ planets near the inner radius should have relatively low pCO2, whereas HZ planets near the outer radius should have higher pCO2.

    “In this carbonate–silicate hypothesis, a trend is predicted wherein HZ planets near the inner radius of the HZ should have relatively low pCO2, whereas HZ planets near the outer radius of the HZ should have higher pCO2.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    The extremes of Bond albedo in the solar system are comets at ∼0.04 and Enceladus at 0.89.

    “Moreover, the extremes of Bond albedo in the solar system are comets at ∼0.04 and Enceladus at 0.89 (Howett et al., 2010; Encrenaz, 2014).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Howett et al., 2010

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    • cites Encrenaz, 2014

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  • May combine two claims.

    Interpretationcitation

    Bulk atmospheric composition could affect thermal phase curves, and thus likelihoods of photometric data with and without life.

    “Hence, the bulk atmospheric composition could affect thermal phase curves, and thus likelihoods of photometric data with and without life (Fig. 1).”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Premisenone given

    Bulk composition fixes the specific heat capacity at constant pressure cp, which sets the dry adiabatic lapse rate Γad = g/cp.

    “Furthermore, bulk composition fixes the specific heat capacity at constant pressure cp, which sets the dry adiabatic lapse rate Γad (decline of temperature with altitude in the convective part of a dry troposphere), given by Γad = g/cp.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Premisereasoning

    The bulk atmospheric composition must be known to apply an equation of state.

    “The bulk atmospheric composition must be known to apply an equation of state.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    On a tidally locked body, atmospheres with bigger molar mass tend to have larger day-night temperature differences, narrower super-rotating jets, and smaller zonal wind speeds with more latitudinal variation.

    “On a tidally locked body, atmospheres with bigger molar mass tend to have larger day–night temperature differences, narrower super-rotating jets, and smaller zonal wind speeds with more latitudinal variation (Zhang and Showman, 2017).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Zhang and Showman, 2017

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    Premisecitation

    Carbon planets are predicted to be water-poor and uninhabitable to life as we know it.

    ““Carbon planets,” which are hypothesized to form in C-rich and O-poor protoplanetary disks, would also have solid surfaces but are predicted to be water-poor and uninhabitable to life as we know it (Bond et al., 2010; Carter-Bond et al., 2012; Elser et al., 2012; Moriarty et al., 2014).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Bond et al., 2010

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    • cites Carter-Bond et al., 2012

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    • cites Elser et al., 2012

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  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisenone given

    Planet formation models suggest that high C/O ratio disks could produce planets with iron cores, mantles of SiC or TiC, and diamond or graphite crusts.

    “Planet formation models suggest that high C/O ratio disks could produce planets with iron cores, mantles of SiC or TiC, and diamond or graphite crusts.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisenone given

    Along with gravity, bulk composition determines the atmospheric scale height.

    “Along with gravity, the bulk composition also determines the atmospheric scale height”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    On Earth, environmental feedbacks on CO2 maintain long-term habitability over ∼1 m.y. timescales through the carbonate-silicate geochemical cycle.

    “On Earth, environmental feedbacks on CO2 maintain long-term habitability over ∼1 m.y. timescales through the carbonate–silicate geochemical cycle (Walker et al., 1981; Kasting and Catling, 2003; Krissansen-Totton and Catling, 2017; Krissansen-Totton et al., 2018a).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Walker et al., 1981

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    • cites Kasting and Catling, 2003

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    • cites Krissansen-Totton and Catling, 2017

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  • May combine two claims.

    Interpretationreasoning

    If carbon planets exist, reactions with carbon of any water delivered during accretion could preclude the presence of liquid water, although liquid hydrocarbons on the surface might be possible.

    “If such planets exist, reactions with carbon of any water delivered to the planet during accretion could preclude the presence of liquid water, although liquid hydrocarbons on the surface might be possible.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    Glint alone just indicates a liquid, rather than water per se; hydrocarbon lakes on Titan produce glint.

    “However, glint alone just indicates a liquid, rather than water per se. For example, hydrocarbon lakes on Titan produce glint (Stephan et al., 2010; Soderblom et al., 2012).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Stephan et al., 2010

      Not checked in the free run.

    • cites Soderblom et al., 2012

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  • May combine two claims.

    Premisecitation

    With some knowledge of composition, the strength of weak bands can be used to estimate pressure; the CO2 content from the weak 1.6 μm band was used to estimate surface pressure on Mars.

    “With some knowledge of composition, the strength of weak bands can also be used to estimate pressure, for example, in early remote sensing of the solar system, the CO2 content found from the weak 1.6 μm band of CO2 was used to estimate the surface pressure on Mars (Owen and Kuiper, 1964).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Owen and Kuiper, 1964

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  • May combine two claims.

    Interpretationnone given

    A 1/λ4 slope of the continuum in the UV-visible part of the spectrum could indicate Rayleigh scattering of particular molecular or aerosol species and possible transparency of an atmosphere caused by haze-free chemistry of an oxidizing atmosphere.

    “A 1/λ4 slope (where λ is wavelength) of the continuum in the UV-visible part of the spectrum could indicate Rayleigh scattering of particular molecular or aerosol species and possible transparency of an atmosphere (i.e., high single scattering albedo) caused by the haze-free chemistry of an oxidizing atmosphere.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisereasoning

    Clouds indicate the presence of a volatile phase, which could be water if spectra are consistent with liquid water drops or water ice particles.

    “Clouds indicate the presence of a volatile phase, which could be water if spectra are consistent with liquid water drops or water ice particles.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationcitation

    Hazes could be the photochemical products of volcanic or biogenic gases, which in certain chemical cases could indicate a volcanically active body or a potentially inhabited early Earth-like body.

    “Hazes could be the photochemical products of volcanic or biogenic gases, which in certain chemical cases could indicate a volcanically active body (Kaltenegger et al., 2010; Misra et al., 2015) or a potentially inhabited early Earth-like body (Arney et al., 2016), respectively.”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Kaltenegger et al., 2010

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    • cites Misra et al., 2015

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    • cites Arney et al., 2016

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  • May combine two claims.

    Premisecitation

    The coexistence of abundant H2 and CO2 is another potential antibiosignature; these gases are a redox couple for microbial CH4 production.

    “Another potential antibiosignature is the coexistence of abundant H2 and CO2. These gases are a redox couple for microbial CH4 production, which molecular phylogenetic studies suggest may be one of the most primitive metabolisms on Earth (Weiss et al., 2016; Wolfe and Fournier, 2018).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Weiss et al., 2016

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    • cites Wolfe and Fournier, 2018

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  • May combine two claims.

    Premisereasoning

    Surface barometric pressure is an indicator of habitability, as sufficient pressure is required to prevent the rapid sublimation of any surface liquid water.

    “Surface barometric pressure is an indicator of habitability (as sufficient pressure is required to prevent the rapid sublimation of any surface liquid water), and, when combined with g, gives the total atmospheric mass.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    Carbon monoxide (CO) is an antibiosignature candidate, with absorptions at 4.67 (strong), 2.34 (weak), and 1.58 μm (very weak).

    “One antibiosignature candidate is carbon monoxide (CO) (Zahnle et al., 2008; Gao et al., 2015), which has absorptions at 4.67 (strong), 2.34 (weak), and 1.58 μm (very weak) (Wang et al., 2016).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Zahnle et al., 2008

      Not checked in the free run.

    • cites Gao et al., 2015

      Not checked in the free run.

    • cites Wang et al., 2016

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    The width of pressure-broadened molecular absorption bands can help constrain atmospheric pressure, but is degenerate with composition.

    “In general, the width of pressure-broadened molecular absorption bands can help to constrain atmospheric pressure, but is degenerate with composition, which also affects broadening (Chamberlain and Hunten, 1987, pp 175–176).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Chamberlain and Hunten, 1987

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    We should expect a habitable exoplanet with surficial liquid water to have substantial gas-phase H2O in its atmosphere as a result of evaporation, and numerous H2O vapor absorption bands should be present.

    “We should expect a habitable exoplanet with surficial liquid water to have substantial gas-phase H2O in its atmosphere as a result of evaporation and so numerous H2O vapor absorption bands should be present.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisenone given

    Clouds and hazes are observed on all planets in the solar system with thick atmospheres, although planets such as Earth are much less cloudy than giant planets.

    “Clouds and hazes are observed on all planets in the solar system with thick atmospheres, although planets such as Earth are much less cloudy than giant planets.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisereasoning

    Oxidizing atmospheres can contain widespread H2SO4 aerosols but not Titan-like global organic hazes.

    “Oxidizing atmospheres can contain widespread H2SO4 aerosols but not Titan-like global organic hazes.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    For tidally locked rocky exoplanets, the day-night temperature gradient depends on surface pressure, equilibrium temperature, and composition, so thermal phase curves can be used to infer surface pressure.

    “Finally, for the specific case of tidally locked rocky exoplanets, the day–night temperature gradient depends on surface pressure, equilibrium temperature, and composition”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Koll and Abbot, 2016

      Not checked in the free run.

  • May combine two claims.

    Interpretationnone given

    The vegetation red edge remains the only well-accepted surface biosignature and is caused by red absorbance by chlorophyll a in plant leaves and scattering in the NIR where the pigment does not absorb.

    “The vegetation red edge remains the only well-accepted surface biosignature and is caused by red absorbance by chlorophyll a in plant leaves and scattering in the NIR where the pigment does not absorb.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The most popular metric for the red edge is the normalized difference vegetation index (NDVI) = (RNIR − RRED)/(RNIR + RRED).

    “The most popular metric is the normalized difference vegetation index (NDVI) = (RNIR − RRED)/(RNIR + RRED) (Huete et al., 1994; Myneni et al., 1995; Myneni et al., 1997; Tucker et al., 2005), where RRED is the surface reflectance in the red band and RNIR is the reflectance in the NIR band.”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Huete et al., 1994

      Not checked in the free run.

    • cites Myneni et al., 1995

      Not checked in the free run.

    • cites Tucker et al., 2005

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisenone given

    MODIS and Landsat TM use a 630-690 nm red band.

    “The Earth-observing satellites Moderate Resolution Imaging Spectrometer (MODIS) and the Landsat Thematic Mapper (TM), which were designed to map vegetation, use a 630–690 nm red band.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Similar retrievals are being considered for the search for biosignatures on Earth-like worlds.

    “Similar retrievals are being considered for the search for biosignatures on Earth-like worlds (von Paris et al., 2013; Barstow and Irwin, 2016; Barstow et al., 2016).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites von Paris et al., 2013

      Not checked in the free run.

    • cites Barstow and Irwin, 2016

      Not checked in the free run.

    • cites Barstow et al., 2016

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S4 (based).

    Interpretationnone given

    A redox reaction as a source of metabolic energy, whether derived from environmental chemical gradients or from excitation of electrons by photons, is a universal aspect of chemistry that should apply to life anywhere.

    “However, in general, a redox reaction as a source of metabolic energy, whether derived from environmental chemical gradients or from excitation of electrons by photons, is a universal aspect of chemistry that should apply to life anywhere.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Premisenone given

    The abundance of a potentially biogenic gas can be determined most simply in the UV, visible or NIR where reflected light dominates over thermal emission from a planetary atmosphere and surface, so that thermal IR can be ignored.

    “The abundance of a potentially biogenic gas can be determined most simply in the UV, visible or NIR where reflected light dominates over thermal emission from a planetary atmosphere and surface, so that thermal IR can be ignored.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisenone given

    For the NIR band, plant leaves have an NIR reflectance plateau across ∼760-1100 nm; however, this range is also impacted by water absorbance bands.

    “For the NIR band, plant leaves have an NIR reflectance plateau across ∼760–1100 nm; however, this range is also impacted by water absorbance bands.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationreasoning

    Iterative procedures would fit a spectrum to a line or multiple lines to estimate the gas abundance(s) if the spectral resolution and signal-to-noise are sufficient.

    “Thus, iterative procedures would fit a spectrum to a line or multiple lines to estimate the gas abundance(s) if the spectral resolution and signal-to-noise are sufficient.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Premisenone given

    Most currently accepted biosignatures consist of spectral evidence of a biogenic gas molecule.

    “Most currently accepted biosignatures consist of spectral evidence of a biogenic gas molecule.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: Earth's biosphere and its biosignatures are suitable analogs for interpreting potential exoplanet biosignatures.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisereasoning

    For the red, the band is best centered around the peak pigment absorbance at 650-680 nm.

    “For the red, the band is best centered around the peak pigment absorbance at 650–680 nm.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisereasoning

    MODIS and Landsat TM precisely target a 780-900 nm NIR band to avoid overlap with the red edge and with water.

    “MODIS and Landsat TM precisely target a 780–900 nm NIR band to avoid overlap with the red edge and with water.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Premisenone given

    If unsaturated and saturated lines of a constituent are measured, curve of growth laboratory data can be used to find the column abundance of the gas, N0.

    “If unsaturated and saturated lines of a constituent are measured, “curve of growth” laboratory data (of the increasing absorption with mass path of absorber) can be used to find the column abundance of the gas, N0.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationnone given

    Among phototrophs, the red edge is not unique in expressing these functions but it is widespread over Earth's surface and lacks any strong false positives for life.

    “Among phototrophs, the red edge is not unique in expressing these functions but it is widespread over Earth's surface and lacks any strong false positives for life.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Premisenone given

    In the general case, an effective airmass will depend on an exoplanet's phase and the way brightness varies across the disk.

    “In the general case, an effective airmass will depend on an exoplanet's phase (e.g., full or quarter disk illumination) and the way brightness varies across the disk.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisenone given

    Earth-observing satellites quantify the red edge by using the contrast between visible and NIR spectral regions.

    “Earth-observing satellites quantify the red edge by using the contrast between visible and NIR spectral regions.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S4 (based). May combine two claims.

    Interpretationreasoning

    For gases, a distinct spectral feature or number of features would identify the presence of a potential biogenic gas and further analysis would deduce a concentration or column abundance of the molecule in an exoplanet atmosphere.

    “For gases, a distinct spectral feature or number of features would identify the presence of a potential biogenic gas and further analysis would deduce a concentration or column abundance of the molecule in an exoplanet atmosphere.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The lack of SO2 and HCl spectral absorption has been used as evidence for absence of volcanic activity on Mars.

    “The lack of SO2 and HCl spectral absorption has been used as evidence for absence of volcanic activity on Mars (Hartogh et al., 2010; Krasnopolsky, 2012; Khayat et al., 2017).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Hartogh et al., 2010

      Not checked in the free run.

    • cites Krasnopolsky, 2012

      Not checked in the free run.

    • cites Khayat et al., 2017

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisereasoning

    In principle, if the line width ΔνL comes from pressure broadening, then, assuming temperature T is known or can be estimated, one can calculate a total pressure p and a volume mixing ratio of a biosignature gas.

    “In principle, if the line width ΔνL comes from pressure broadening, then, assuming temperature T is known or can be estimated, one can calculate a total pressure p and a volume mixing ratio of a biosignature gas, ci = pi/p, where pi is the partial pressure of the absorbing gas, defined by the ideal gas law and Dalton's law:”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Premisenone given

    In a more modern treatment, we know enough about the spectral structure of bands to account for line saturation and overlap explicitly to determine gas abundances from the spectrum of multiple lines.

    “In a more modern treatment, we know enough about the spectral structure of bands to account for line saturation and overlap explicitly to determine gas abundances from the spectrum of multiple lines.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given

    The colors of other pigments may be fortuitous without a clear function with respect to light interactions.

    “The colors of other pigments may be fortuitous without a clear function with respect to light interactions.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S4 (based).

    Interpretationnone given

    A quantitative framework for evaluating surface biosignatures remains unclear.

    “A quantitative framework for evaluating surface biosignatures remains unclear.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given

    The prominent wavelength bands will rely on molecular energy transduction constraints and the efficiencies, which result from contingent evolutionary pathways, including competitive survival strategies.

    “However, the prominent wavelength bands will rely on molecular energy transduction constraints and the efficiencies, which result from contingent evolutionary pathways, including competitive survival strategies.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing), S4 (based).

    Interpretationnone given

    If detected, seasonal variations would be especially informative.

    “If detected, seasonal variations would be especially informative.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S4 (based).

    Interpretationnone given

    Pigments whose role is light harvesting and screening of ionizing radiation or excess light must have a spectral absorbance that depends on the available light.

    “Clearly, pigments whose role is light harvesting and screening of ionizing radiation or excess light must have a spectral absorbance that depends on the available light.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Similar arguments about CH4:H2 ratios have also been applied to life within the solar system, on both Enceladus and Mars.

    “Similar arguments about CH4:H2 ratios have also been applied to life within the solar system, on both Enceladus (Waite et al., 2017) and Mars (Sholes et al., 2017).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Waite et al., 2017

      Not checked in the free run.

    • cites Sholes et al., 2017

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S4 (based). May combine two claims.

    Interpretationnone given

    Except for chirality and the terrestrial vegetation red edge, the uniqueness of the other surface spectral signatures and potential for false positives have yet to be scrutinized.

    “Except for chirality and the terrestrial vegetation red edge, the uniqueness of the other surface spectral signatures and potential for false positives have yet to be scrutinized.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Some pigments adapt colors in real time to fluctuating colors of light.

    “Also, some pigments adapt colors in real time to fluctuating colors of light (Kehoe and Gutu, 2006).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Kehoe and Gutu, 2006

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Biogenic surface spectral features are also the result of ecological signaling, community signatures, chiral molecules, and the bidirectional reflectance distribution function that accounts for angle-dependent reflectivity.

    “Biogenic surface spectral features are also the result of ecological signaling (Schwieterman et al., 2015a), community signatures (Parenteau et al., 2015), chiral molecules (Sparks et al., 2009; Sparks et al., 2012), and the bidirectional reflectance distribution function that accounts for angle-dependent reflectivity (Doughty and Wolf, 2010).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Schwieterman et al., 2015a

      Not checked in the free run.

    • cites Parenteau et al., 2015

      Not checked in the free run.

    • cites Sparks et al., 2009

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Chlorophyll pigments have recently been discovered in cyanobacteria, tuned to absorb in the far red/NIR as long ∼0.77 μm.

    “Moreover, chlorophyll pigments have recently been discovered in cyanobacteria, tuned to absorb in the far red/NIR as long ∼0.77 μm (Chen, 2014; Li and Chen, 2015).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Chen, 2014

      Not checked in the free run.

    • cites Li and Chen, 2015

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

    Interpretationnone given

    Color variations on a rotating planet might provide circumstantial support for life and would be additional photometric data.

    “Color variations on a rotating planet might provide circumstantial support for life and would be additional photometric “data” shown in Figure 1.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Arnold (2008) found that the red edge detectability ranged from 1% to 12% in observations and models.

    “They found that the red edge detectability ranged from 1% to 12% in observations and models.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Arnold (2008)

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    Pigments that absorb in the visible include cytochromes (∼0.40 μm), carotenoids (0.28-0.55 μm), cyanobacterial and chloroplast accessory pigments (0.40-0.74 μm), bacteriochlorophyll pigments (0.74-1.03 μm), and retinal pigments (0.53-0.63 μm).

    “Pigments that absorb in the visible include, for example, cytochromes (∼0.40 μm), carotenoids (0.28–0.55 μm), cyanobacterial and chloroplast accessory pigments (0.40–0.74 μm), bacteriochlorophyll pigments (0.74–1.03 μm), and retinal pigments (0.53–0.63 μm) (DasSarma, 2006; Hegde et al., 2015).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites DasSarma, 2006

      Not checked in the free run.

    • cites Hegde et al., 2015

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

    Interpretationnone given

    Biological denitrification produces N2O, but on Earth at least it also produces even more N2, so that the presence of the two gases together might support the idea that N2O is biological.

    “Biological denitrification produces N2O, but (on Earth, at least) it also produces even more N2, so that the presence of the two gases together might support the idea that N2O is biological.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationnone given

    Some pigments may be stratified and tuned depending on the environmental conditions.

    “Moreover, some pigments may be stratified and tuned depending on the environmental conditions.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Premisenone given

    On the current Earth, the ratio CO2:CH4:H2 is ∼400:1.8:0.5, where the numbers are in ppmv.

    “On the current Earth, the ratio CO2:CH4:H2 is ∼400:1.8:0.5, where the numbers are in ppmv.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    If CH4:CO2 ratios exceed ∼0.1, and there is adequate UV flux from the star, an organic haze is produced, which may decrease biological productivity through an antigreenhouse effect on the climate.

    “In addition, if CH4:CO2 ratios exceed ∼0.1, and there is adequate UV flux from the star, an organic haze is produced (Trainer et al., 2006; Haqq-Misra et al., 2008; Arney et al., 2016), which may decrease biological productivity through an antigreenhouse effect on the climate (Haqq-Misra et al., 2008).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Trainer et al., 2006

      Not checked in the free run.

    • cites Haqq-Misra et al., 2008

      Not checked in the free run.

    • cites Arney et al., 2016

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    A methodology to quantify thermodynamic chemical disequilibrium has been established and could be applied.

    “A methodology to quantify thermodynamic chemical disequilibrium has been established and could be applied (Krissansen-Totton et al., 2016a, 2018b).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Krissansen-Totton et al., 2016a

      Not checked in the free run.

    • cites Krissansen-Totton et al., 2018b

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

    Interpretationnone given

    The inference of surficial liquid water could come from abundant H2O vapor lines in the spectrum, evidence of glint, or polarized reflected light.

    “Alternatively (or additionally), the inference of surficial liquid water could come from abundant H2O vapor lines in the spectrum, evidence of glint, or polarized reflected light (discussed in section 3.3.2).”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    On an oxidized world, photosynthesis requires CO2 as a chemical substrate, so detection of CO2 would support the possibility that O2 is a biosignature.

    “On an oxidized world, photosynthesis requires CO2 as a chemical substrate, so detection of CO2 would support the possibility that O2 is a biosignature.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S4 (based). May combine two claims.

    Interpretationnone given

    The most fundamental evidence supporting inhabitation would be a surface emission temperature, Tsurfe, that allows stable liquid water on an exoplanet, which would increase the prior P(life | C) and improve modeling of the data likelihoods.

    “The most fundamental evidence supporting inhabitation would be a surface emission temperature, Tsurfe, that allows stable liquid water on an exoplanet, which would increase the prior P(life | C) and improve modeling of the data likelihoods shown in Figure 1.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: The Bayesian framework with priors and likelihoods can be meaningfully applied to exoplanet biosignature assessment.

  • May combine two claims.

    Interpretationcitation

    A small amount of CH4 in a hydrogen-rich atmosphere (a high H2:CH4 ratio) could result from abiotic chemistry, as found today on the giant planets of the solar system, and the interpretation of CH4 as a biosignature would be doubtful because such atmospheres could be primordial.

    “In contrast, a small amount of CH4 in a hydrogen-rich atmosphere (a high H2:CH4 ratio) could result from abiotic chemistry, as found today on the giant planets of the solar system (Irwin, 2009), and the interpretation of CH4 as a biosignature would be doubtful because such atmospheres could be primordial.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Irwin, 2009

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    To calculate chemical disequilibrium properly for an Exo-Earth, one must consider all fluid phases because the largest source of disequilibrium in Earth's atmosphere-ocean system is attributable to the coexistence of N2, O2, and liquid water.

    “To calculate chemical disequilibrium properly for an Exo-Earth, one must consider all fluid phases because the largest source of disequilibrium in Earth's atmosphere–ocean system is attributable to the coexistence of N2, O2, and liquid water, as quantified by Krissansen-Totton et al. (2016a) and qualitatively noted by, for example, Lewis and Randall (1923, pp 567–568) and Emerson and Hedges (2008,”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Krissansen-Totton et al., 2016a

      Not checked in the free run.

    • cites Lewis and Randall (1923)

      Not checked in the free run.

    • cites Emerson and Hedges (2008)

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    There is not yet a consensus on why the vegetation red edge is red, although some argue that chlorophyll a evolved to exploit the large number of photons in the red part of the Sun's spectrum convolved with the free energy that each converted photon contributes and transmission of Earth's atmosphere.

    “There is not yet a consensus on why the vegetation red edge is red, although some argue that chlorophyll a evolved to exploit the large number of photons in the red part of the Sun's spectrum convolved with the free energy that each converted photon contributes and transmission of Earth's atmosphere (Bjorn, 1976; Milo, 2009; Marosvolgyi and van Gorkom, 2010; Björn and Ghiradella, 2015).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Bjorn, 1976

      Not checked in the free run.

    • cites Milo, 2009

      Not checked in the free run.

    • cites Marosvolgyi and van Gorkom, 2010

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Interpretationnone given

    Considerable work still needs to be done to understand the coevolution of biogeochemical cycles and atmospheric composition on Earth, which is being addressed by using Earth System models.

    “Of course, considerable work still needs to be done to understand the coevolution of biogeochemical cycles and atmospheric composition on Earth, which is being addressed by using “Earth System” models.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Such temporal variations occur on Earth.

    “Such temporal variations occur on Earth (e.g., Schlesinger and Bernhardt, 2013, Chap. 11).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Schlesinger and Bernhardt, 2013

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing). May combine two claims.

    Interpretationreasoning

    Since organisms can have more than one strategy for survival, and we cannot measure environmental variables like pH remotely, constraining the suitable environments for these alternative biosignatures is a matter of more research.

    “Since organisms can have more than one strategy for survival, and we cannot measure environmental variables like pH remotely, constraining the suitable environments for these alternative biosignatures is a matter of more research.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Interpretationnone given

    A general procedure for excluding false positives of biogenic processes might include identifying a plausible abiotic source and then detecting or supporting the abiotic source by corroborating evidence.

    “A general procedue for excluding false positives of biogenic processes might include the following steps. First, it is necessary to identify a plausible abiotic source. Second, the abiotic source must be detected or supported by corroborating evidence.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing), S4 (based).

    Interpretationnone given

    Other pigments may be related to nano- or microstructure and be purely evolutionary contingent.

    “Other pigments may be related to nano- or microstructure and be purely evolutionary contingent.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Zahnle and Catling (2017) note the tendency of oxygen to photochemically buffer H:O loss at 2:1 ratio at high escape rates.

    “Zahnle and Catling (2017) note the tendency of oxygen to photochemically buffer H:O loss at 2:1 ratio at high escape rates.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Zahnle and Catling (2017)

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Considerable research has been devoted to identifying abiotic sources of O2 and its photochemical product O3.

    “Considerable research has been devoted to identifying abiotic sources of O2 and its photochemical product O3 (Hu et al., 2012b; Tian et al., 2014; Domagal-Goldman et al., 2014; Wordsworth and Pierrehumbert, 2014; Gao et al., 2015; Harman et al. 2015).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Hu et al., 2012b

      Not checked in the free run.

    • cites Tian et al., 2014

      Not checked in the free run.

    • cites Domagal-Goldman et al., 2014

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Interpretationreasoning

    A better constraint of one particular parameter could remove degeneracies and produce a better model fit to the spectrum.

    “For example, a better constraint of one particular parameter could remove degeneracies (i.e., where two or more parameters have similar overall effects) and produce a better model fit to the spectrum.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S4 (based).

    Premisenone given

    ETI machines can exist on an airless moon or planet.

    “The latter is because ETI machines can exist on an airless moon or planet.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Interpretationreasoning

    False positives need to be evaluated and quantified by a likelihood P(D | C, no life) in our framework.

    “False positives need to be evaluated, as noted in Figure 2, and quantified by a likelihood P(D | C, no life) in our framework (Fig. 1).”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The Bayesian framework with priors and likelihoods can be meaningfully applied to exoplanet biosignature assessment.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Interpretationcitation

    Bacteria such as the radiotolerant Deinococcus radiodurans manufacture antioxidant carotenoids, such as deinoxanthin, in response to UV, and extraterrestrial microbe-like life may have similar physiological response.

    “Bacteria such as the radiotolerant Deinococcus radiodurans manufacture antioxidant carotenoids, such as deinoxanthin, in response to UV, and extraterrestrial microbe-like life may have similar physiological response (Tian and Hua, 2010).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Tian and Hua, 2010

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

    Premisenone given

    A short lifetime (∼10 years) and an inferred large flux into the atmosphere are the case for CH4 in Earth's current O2-rich atmosphere.

    “A short lifetime (∼10 years) and an inferred large flux into the atmosphere are the case for CH4 in Earth's current O2-rich atmosphere.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S1 (asserted), S3 (load-bearing), S4 (based).

    Interpretationnone given

    In the future, an Exo-Earth System model could be configured in the form of an inverse fit to spectra or photometry data from exoplanets.

    “In the future, an Exo-Earth System model could be configured in the form of an inverse fit to spectra or photometry data from exoplanets (Fig. 1).”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

    Interpretationreasoning

    The latter would necessarily use radiative transfer; models could also include atmospheric and ocean dynamics.

    “The latter would necessarily use radiative transfer; models could also include atmospheric and ocean dynamics.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationreasoning

    An investigation of covariance of uncertain parameters in such a fit would show which parameters we need to know better from future observations to improve confidence that the spectrum really represents biology.

    “An investigation of covariance of uncertain parameters in such a fit would show which parameters we need to know better from future observations to improve confidence that the spectrum really represents biology.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S4 (based).

    Interpretationreasoning

    Ultimately, such forward models would be used to generate a synthetic reflectance or transmission spectrum and Bayesian likelihoods, P(D | C, life) and P(D | C, no life).

    “Ultimately, such forward models would be used to generate a synthetic reflectance or transmission spectrum and Bayesian likelihoods, P(D | C, life) and P(D | C, no life) (Fig. 1).”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: The Bayesian framework with priors and likelihoods can be meaningfully applied to exoplanet biosignature assessment.

  • May combine two claims.

    Interpretationnone given

    If just gaseous components are known for an exoplanet, it may be possible to estimate their kinetic instability, that is, residence time against photochemical destruction, and deduce whether a large, potentially biogenic source is needed.

    “If just gaseous components are known for an exoplanet, it may be possible to estimate their kinetic instability, that is, residence time against photochemical destruction, and deduce whether a large, potentially biogenic source is needed.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    Neutral O2 is undetectable on Venus, the one case of a postrunaway greenhouse in our solar system, where oxygen must have been either lost efficiently or taken up by a magma ocean, which may also occur on exoplanets.

    “It is notable that neutral O2 is undetectable on Venus, the one case of a postrunaway greenhouse in our solar system, where oxygen must have been either lost efficiently (Zahnle and Kasting, 1986; Zahnle et al., 1988) or taken up by a magma ocean (Hamano et al., 2013), which may also occur on exoplanets (Schaefer et al., 2016).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Zahnle and Kasting, 1986

      Not checked in the free run.

    • cites Zahnle et al., 1988

      Not checked in the free run.

    • cites Hamano et al., 2013

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S1 (asserted), S3 (load-bearing).

    Premisecitation

    In one such case, oceans are hypothesized to vaporize and be lost from HZ planets in the luminous premain sequence phases of M dwarfs, leaving behind very O2-rich atmospheres.

    “In one such case, oceans are hypothesized to vaporize and be lost from HZ planets in the luminous premain sequence phases of M dwarfs, leaving behind very O2-rich atmospheres (Luger and Barnes, 2015).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Luger and Barnes, 2015

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given

    Relatively little attention has been given to abiotic scenarios where other candidate biosignature gases could be significant.

    “However, relatively little attention has been given to abiotic scenarios where other candidate biosignature gases could be significant.”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Interpretationreasoning

    The discussion of non-ETI biosignatures indicates the need to understand the biogeochemistry of exoplanets to interpret biosignatures through calculations of data likelihoods and interpret the context for a Bayesian prior, P(life | C), needed for the overall biosignature framework.

    “The aforementioned discussion of non-ETI biosignatures indicates the obvious need to understand the biogeochemistry of exoplanets to interpret biosignatures through calculations of data likelihoods and interpret the context for a Bayesian prior, P(life | C), needed for the overall biosignature framework (Fig. 1).”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The Bayesian framework with priors and likelihoods can be meaningfully applied to exoplanet biosignature assessment.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationreasoning

    With data of sufficient fidelity, a Bayesian fit would allow for quantitative constraints on possible gas fluxes that are needed to support a potential biosignature gas.

    “With data of sufficient fidelity, a Bayesian fit would allow for quantitative constraints on possible gas fluxes that are needed to support a potential biosignature gas.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: Future exoplanet observations can achieve sufficient spectral resolution and signal-to-noise to detect and quantify biosignature gases and surface features.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationreasoning

    The amount of CH4 that can be produced abiotically on an exoplanet as a false positive remains a matter of research.

    “Thus, how much CH4 can be produced abiotically on an exoplanet as a false positive remains a matter of research.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: The geological process of serpentinization produces H2 when liquid H2O oxidizes iron in rocks.
    • Rests on an assumption: More than 90% of CH4 on Earth has a direct biogenic source or comes indirectly from biology through thermal breakdown of old organic matter.

  • Classified as probably not a substantial claim.

    Premisecitation

    Several theoretical studies have suggested that abiotic, bulk O2 could be present under certain hypothesized circumstances.

    “For example, as reviewed by Meadows et al. (2018, in this issue), several theoretical studies have suggested that abiotic, bulk O2 could be present under certain hypothesized circumstances.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Meadows et al. (2018)

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing). May combine two claims.

    Interpretationreasoning

    A positive SETI signal provides certainty about the presence of ETI, by definition, but does not necessarily indicate that a biosphere or that biosignature gases are present.

    “A positive SETI signal provides certainty about the presence of ETI, by definition, but does not necessarily indicate that a biosphere or that biosignature gases are present.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The CH4 in earlier serpentinization experiments may have arisen from organic contamination.

    “suggesting that CH4 may have arisen in earlier experiments from organic contamination (McCollom, 2016).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites McCollom, 2016

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    O2 could build up abiotically on highly irradiated planets as a result of escape of hydrogen from water photolysis.

    “It has also been proposed that O2 could build up abiotically on highly irradiated planets as a result of escape of hydrogen from water photolysis (Luger and Barnes, 2015).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Luger and Barnes, 2015

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing). May combine two claims.

    Interpretationreasoning

    The extent to which purely hypothetical ideas about O2 buildup are realized remains to be determined, but their possibility demands caution.

    “The extent to which purely hypothetical ideas about O2 buildup are realized remains to be determined, but their possibility demands caution.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationreasoning

    A null SETI signal detected at 100 Ma from Earth by an ETI civilization observing our planet remotely should not have affected their priors for the existence of non-ETI life on Earth at all because a null SETI signal excludes ETI only.

    “For example, a null SETI signal detected at 100 Ma from Earth by an ETI civilization observing our planet remotely should not have affected their priors for the existence of non-ETI life on Earth at all because a null SETI signal excludes ETI only.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing), S4 (based).

    Interpretationnone given

    A model could incorporate how the interior thermal evolution of a planet couples to mantle processes and outgassing of volcanic and metamorphic gases.

    “A model could incorporate how the interior thermal evolution of a planet couples to mantle processes and outgassing of volcanic and metamorphic gases.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S4 (based).

    Interpretationnone given

    For exoplanets, we conceive of Exo-Earth System models that would couple a surface biosphere to atmospheric chemistry and climate.

    “For exoplanets, we conceive of “Exo-Earth System” models, as mentioned earlier, that would couple a surface biosphere to atmospheric chemistry and climate.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    Exoplanet priors for ETI are potentially completely different to the habitable requirements for a non-ETI biosphere that produces gas or pigment biosignatures; ETI priors are also subject to considerably more speculation, given unknown evolutionary and sociological factors.

    “Hence, exoplanet priors for ETI are potentially completely different to the habitable requirements for a non-ETI biosphere that produces gas or pigment biosignatures; ETI priors are also subject to considerably more speculation, given unknown evolutionary and sociological factors.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Initial reports of ammonia and methane in the NIR reflection spectrum of the polar caps of Mars observed by the Mariner 7 flyby were subsequently shown to be due to solid CO2 at the surface.

    “Similarly, initial reports of ammonia and methane in the NIR reflection spectrum of the polar caps of Mars observed by the Mariner 7 flyby were subsequently shown to be due to solid CO2 at the surface (Herr and Pimentel, 1969).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Herr and Pimentel, 1969

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S4 (based).

    Premisenone given

    Pigments and surface biosignatures represent potentially confirmatory biosignatures.

    “Pigments and surface biosignatures (Table 5) represent potentially confirmatory biosignatures.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    The IPCC's five basic probability levels are very likely 90-100%, likely 66-100%, about as likely as not 33-66%, unlikely 0-33%, and very unlikely 0-10%.

    “Their set of five basic probability levels are very likely 90–100%, likely 66–100%, about as likely as not 33–66%, unlikely 0–33%, and very unlikely 0–10% (Stocker et al., 2013, their Box TS.1).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Stocker et al., 2013

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Hypothetical planets that lack noncondensable N2 and build up O2 from water vapor destruction and loss would lack N4 dimer signatures and may have strong O4.

    “Hypothetical planets that lack noncondensable N2 (if that is cosmochemically plausible) and build up O2 from water vapor destruction and loss (Wordsworth and Pierrehumbert, 2014) would lack the N4 dimer signatures and may have strong O4.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Wordsworth and Pierrehumbert, 2014

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The Sinton bands were subsequently shown to be due to terrestrial water with deuterium (HDO).

    “Subsequent work showed that the Sinton bands were due to terrestrial water with deuterium, HDO (Rea et al., 1965).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Rea et al., 1965

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing), S4 (based).

    Premisenone given

    Covariances of parameters identified in inverse modeling with a chemical-radiative atmospheric forward model would help prioritize which parameters are most important for follow-up observations.

    “Covariances of parameters identified in inverse modeling with a chemical-radiative atmospheric forward model would help prioritize which parameters are most important for follow-up observations.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    When an exoplanet hosts a moon with its own atmosphere, a single spectrum including both moon and exoplanet could resemble a mixture of both atmospheres and could generate chemical disequilibrium false positives.

    “When an exoplanet hosts a moon with its own atmosphere, a single spectrum that includes both moon and exoplanet could end up resembling a mixture of both atmospheres and perhaps could generate chemical disequilibrium false positives (Rein et al., 2014).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Rein et al., 2014

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisenone given

    The biogenic nature of biosignature molecules must be assessed through environmental context and corroborating evidence such as other atmospheric composition information.

    “The biogenic nature of such molecules must be assessed through the environmental context deduced through components 1 and 2 and also any corroborating evidence such as other information on atmospheric composition.”
    Quote in the paperSubstantial claim

  • Classified as probably not a substantial claim.

    Premisenone given

    Thermal emittance and Bond albedo are needed to estimate surface temperature and whether liquid water is stable on the surface, as opposed to merely being in the habitable zone.

    “These include the thermal emittance and Bond albedo, which are needed to estimate the surface temperature (i.e., whether liquid water is stable on the surface) as opposed to merely being in a HZ.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    O2 in the coma of comet 67P/Churyumov-Gerasimenko may be formed abiotically by collision of energetic water ions with oxygen-containing minerals on the comet's surface.

    “Here, O2 may be formed abiotically by collision of energetic water ions with oxygen-containing minerals on the comet's surface (Yao and Giapis, 2017).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Yao and Giapis, 2017

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Clathrates in cold regions could melt and input methane into an exoplanet atmosphere.

    “For example, clathrates in cold regions could melt and input methane into an atmosphere (Levi et al., 2014).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Levi et al., 2014

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisereasoning

    Models of atmospheric composition and climate are an important part of assessing the biogenic nature of biosignatures.

    “Models of atmospheric composition and climate are an important part of such an assessment.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Premisenone given

    Time-variable photometry may also be useful for detecting biosignatures.

    “but time-variable photometry may also be useful (Fig. 1).”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisenone given

    If high accuracy is known at the upper and lower bounds of probabilities, the IPCC also recommends "virtually certain" for 99-100%, "extremely likely" for 95-100%, "extremely unlikely" for 0-5%, and "exceptionally unlikely" for 0-1%.

    “If high accuracy is known at the upper and lower bounds of probabilities, the IPCC also recommends “virtually certain” for 99–100%, “extremely likely” for 95–100%, “extremely unlikely” for 0–5% probability, and “exceptionally unlikely” for 0–1%.”
    Quote in the paperNot a substantial claim
140 of 140

  • The paper relies on this without support.

    Life produces gases and other substances, such as pigments, that can have distinct spectral or photometric signatures.

    “Life produces gases and other substances, such as pigments, which can have distinct spectral or photometric signatures.”

  • The authors state this as an assumption.

    For the framework, the presence of life is treated as a binary variable: a remotely detectable surface biosphere is either present or absent.

    “We take the presence of life to mean the existence of a surface biosphere that is remotely detectable, which we treat as a binary variable, that is, either an exoplanet biosphere is present or it is not.”
    “Because we assume that a biosphere is either present on an exoplanet or absent, it follows that these two are priors are related by”

  • The authors state this as an assumption.

    P(life|O-stars) is negligible.

    “Most people would also consider it reasonable to assume that P(life | O-stars) is negligible because O-stars have main sequence lifetimes that range from less than one to a few million years (Weidner and Vink, 2010).”

  • The authors state this as an assumption.

    In the absence of other information, a prior of P(life | Earth-like) = 0.5 is a practical starting point for an Earth-like planet in the HZ of a Sun-like star.

    “in the absence of any other information, if we found an Earth-like planet in the HZ of a Sun-like star, a starting point—based on empirical data of our current solar system—might be to assume a prior of P(life | Earth-like) = 0.5”

  • The paper relies on this without support.

    Earth's biosphere and its biosignatures are suitable analogs for interpreting potential exoplanet biosignatures.

    “Most currently accepted biosignatures consist of spectral evidence of a biogenic gas molecule.”

  • The paper relies on this without support.

    The Bayesian framework with priors and likelihoods can be meaningfully applied to exoplanet biosignature assessment.

    “Environmental information could be used to improve estimates of data likelihoods or alter the priors shown in Figure 1, whereas new biosignature data would add to the “data” box of Figure 1.”

  • The paper relies on this without support.

    Future exoplanet observations can achieve sufficient spectral resolution and signal-to-noise to detect and quantify biosignature gases and surface features.

    “With data of sufficient fidelity, a Bayesian fit would allow for quantitative constraints on possible gas fluxes that are needed to support a potential biosignature gas.”

  • The paper relies on this without support.

    The geological process of serpentinization produces H2 when liquid H2O oxidizes iron in rocks.

    “The geological process of serpentinization produces H2 when liquid H2O oxidizes iron in rocks.”

  • The paper relies on this without support.

    More than 90% of CH4 on Earth has a direct biogenic source or comes indirectly from biology through thermal breakdown of old organic matter.

    “More than 90% of CH4 on Earth has a direct biogenic source or comes indirectly from biology through thermal breakdown of old organic matter”
9 of 9

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Organic-haze methane threshold differs by a factor of 100

    Major

    The table says organic haze indicates CO2/CH4 < 0.1, equivalent to CH4/CO2 > 10. The text says haze is produced when CH4/CO2 exceeds approximately 0.1, giving a threshold 100 times lower.

1 of 1

Cites

Flagged first, then by citations

Research works printed but not linked: 25

  1. Beichman C.A., 2007, Protostars and Planets V, 915
  2. Bjorn L.O., 1976, Photosynthetica, 10, 121
  3. Chamberlain J.W., 1987, Theory of Planetary Atmospheres
  4. DasSarma S., 2006, Microbe, 1, 120
  5. Drossart P., 1993, Planet Space Sci, 41, 551
  6. Koonin E.V., 2012, The Logic of Chance: The Nature and Origin of Biological Evolution
  7. Kump L.R., 2010, The Earth System
  8. Ledrew G., 2001, J Roy Astron Soc Can, 95, 32
  9. Lewis G.N., 1923, Thermodynamics and the Free energy of Chemical Substances
  10. Lubin P., 2016, J Br Interplanet Soc, 69, 40
  11. Mackenzie F.T., 2006, Carbon in the Geobiosphere: Earth's Outer Shell
  12. Monod J., 1971, Chance and Necessity: An Essay on the Natural Philosophy of Modern Biology
  13. Owen T.C., 1964, Commun Lunar Planet Lab, 32, 113
  14. Parenteau M.N., 2015, Astrobiology Science Conference
  15. Qiu J., 2003, J Geophys Res Atmos, 108
  16. Sagan C., 1994, Pale Blue Dot: A Vision of the Human Future in Space
  17. Shklovskii I.S., 1966, Intelligent Life in the Universe
  18. Slonczewski J.L., 2010, Microbe, 5, 110
  19. Stocker T.F., 2013, Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change
  20. Stone J.V., 2013, Bayes' Rule: A Tutorial Introduction to Bayesian Analysis
  21. Twomey S., 1996, Introduction to the Mathematics of Inversion in Remote Sensing and Indirect Measurements
  22. Wang J., 2015, Astron J, 149
  23. Williams D.M., 1996, Circumstellar Habitable Zones, 43
  24. Wozniak B., 2007, Light Absorption in Sea Water
  25. Zahnle K., 2010, CSH Perspect Biol, 2, a004895

Cited by

Sorted by citations

Related

Authors
Catling, D. C., Krissansen-Totton, J., Kiang, N. Y., Crisp, D., Robinson, T. D., DasSarma, S., Rushby, A. J., Del Genio, A., Bains, W., & Domagal-Goldman, S.
Affiliations
  • University of Washington (US)
  • Astrobiology Program, Department of Earth and Space Sciences, University of Washington, Seattle, Washington.
  • Goddard Institute for Space Studies (US)
  • California Institute of Technology (US)
  • University of California, Santa Cruz (US)
  • University of Maryland, Baltimore (US)
  • Ames Research Center (US)
  • NASA Goddard Institute for Space Studies, New York, New York.
  • Massachusetts Institute of Technology (US)
  • Goddard Space Flight Center (US)
Corresponding author
Catling, David C.
ORCID
Venue
Astrobiology
Publisher
SAGE Publications
Published
2018-06
Volume
18
Issue
6
Pages
709-738
ISSN
  • 1531-1074 (ISSN-L)
  • 1557-8070
DOI
10.1089/ast.2017.1737
PMC
PMC6049621
OpenAlex
W2949593113
Cited by
225
References
258
Citations by year
2026: 17 · 2025: 30 · 2024: 21 · 2023: 30 · 2022: 29 · 2021: 28 · 2020: 16 · 2019: 27 · 2018: 25 · 2017: 1
FWCI
12.86
Type
journal article
Language
en
Open access
Licence
cc-by-nc
Locations
  • Astrobiology (publishedVersion, cc-by-nc)
  • Europe PMC (PubMed Central) (publishedVersion, other-oa)
  • PubMed Central (publishedVersion, cc-by-nc)
  • PubMed (publishedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • arXiv (Cornell University)
Funders
  • National Aeronautics and Space Administration (NNX15AR63H, NNX15AM07G, Exobiology grant NNX15AM07G)
  • Universities Space Research Association
  • University of Oxford
  • NASA Astrobiology Institute (NNX15AM07G)
  • Ames Research Center
Topics
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Space Science and Extraterrestrial Life
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Stellar, planetary, and galactic studies
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Planetary Science and Exploration
Keywords
exoplanet biosignatures, habitability, false positives, Drake equation, exoplanet characterization, planetary climate
MeSH terms
Origin of Life, Bayes Theorem, Extraterrestrial Environment, Planets, Exobiology

About these checks

Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

Claims read by deepseek-v4.1-flash under definition version 3 (extract-v3) on 2026-10-10.

Classified by jev-1.13-20260917 (gold-guideline-v0.2) on 2026-10-10.

Statistics and arithmetic are recomputed, not judged.

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