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Cited by 173Space Science and Extraterrestrial LifeStellar, planetary, and galactic studiesPlanetary Science and Exploration

Reflections on O 2 as a Biosignature in Exoplanetary Atmospheres

Open access
Meadows, Victoria S.·2017·Astrobiology
DOI 10.1089/ast.2016.1578PMC PMC5655594
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Abstract

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

AI snapshot

Written from the paper’s full text.

TL;DR

O₂ remains a highly detectable biosignature, but this review argues it is no longer uniquely robust because several abiotic mechanisms can produce abundant O₂ or O₃ on habitable-zone planets, especially around M dwarfs. It synthesizes false-positive mechanisms and recommends target selection plus spectral discriminants such as O₄, CO, CO₂, CH₄, H₂O, and N₄ to distinguish abiotic from biological oxygen.

Key findings

  • O₂ has strong advantages as a biosignature: it is 21% of Earth’s atmosphere, uniformly mixed through the atmospheric column, and has strong visible/NIR absorption; its photochemical by-product O₃ has strong UV, visible, and 9.6 μm MIR bands, and O₄ collision pairs have a density-squared dependence useful for identifying dense O₂ atmospheres.
  • Multiple abiotic O₂/O₃ false positives are now known: pre-main-sequence M dwarf water loss can produce hundreds to thousands of bar of O₂ (photolysis of an Earth ocean can yield ∼240 bar O₂); low noncondensable gas inventories can yield ∼15% O₂; desiccated CO₂-rich M dwarf atmospheres can equilibrate at 50% CO₂, 30% CO, and 15% O₂; CO₂ photolysis can yield <0.02% O₂ for F/G-star planets, <6% for M dwarf planets with O₂-saturated oceans, and 0.2% for M dwarfs with high FUV/NUV ratios.
  • Abiotic O₃ can reach 0.1 times Earth’s ozone column for M dwarf planets in some models, and Domagal-Goldman et al. (2014) generated abiotic O₃ only a factor of 10 less than Earth’s current ozone abundance, with small abiotic O₂ around 40 ppm.
  • Discriminants include O₄ features at 0.4–1.0 μm for direct imaging and 0.6–1.3 μm for transmission, CO and CO₂ in transmission from 0.6–2.5 μm, N₄ near 4.1 μm, and absence of H₂O or CH₄; target selection may favor host stars more massive than 0.4 M⊙ (M3V and earlier) and planets within the conservative habitable zone.

Methods

  • This is a review/synthesis paper, not a primary observational or modeling study; it evaluates published photochemical, climate-photochemistry, atmospheric-escape, and spectral-detectability literature on O₂ and O₃ as exoplanet biosignatures.
  • The paper compares abiotic O₂/O₃ generation mechanisms, including water loss, low noncondensable gas inventories, CO₂ photolysis, and stellar UV-driven photochemistry, and summarizes them in Table 1 with associated targets, potential O₂/O₃ amounts, and spectral discriminants.
  • It also assesses observational strategies for transmission and direct imaging, including wavelength ranges needed to detect O₂, O₃, O₄, CO, CO₂, CH₄, H₂O, and N₄, and discusses target selection for JWST, WFIRST, HabEx/LUVOIR, and ground-based telescopes.

Limitations

  • The paper states that key boundary conditions are poorly understood, especially aqueous CO + O₂ reaction rates, surface weathering, mantle sequestration, and catalytic CO₂ recombination on Venus; these are not generally included in terrestrial exoplanet photochemical models.
  • The paper states that detectability of many discriminants is challenging: N₄ at 4.1 μm is difficult, O₄ and O₂ signals can be below instrument systematics for JWST, and M dwarf habitable-zone planets are hard to observe with direct imaging.
  • Evident from the text: because this is a synthesis of models with differing assumptions, the reported abiotic O₂/O₃ abundances vary widely and depend strongly on assumed sinks, stellar UV spectra, and planetary properties; the paper does not present new data to resolve those uncertainties.

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

Integrity signals

Registry

Crossref · 10.1089/ast.2016.1578

Cites

264 of 264 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 316 cited sources checked · 0 supported · 0 flagged

  • Findingcitation

    Multiple geological and geochemical lines of evidence suggest that anoxygenic photosynthesizers were present on Earth by at least 3.3 Ga and possibly as far back as 3.5 Ga or 3.7 Ga.

    “Multiple geological and geochemical lines of evidence suggest that anoxygenic photosynthesizers were present on Earth by at least 3.3 Ga (Westall et al., 2011) and possibly as far back as 3.5 Ga (Buick et al., 1981) or 3.7 Ga (Nutman et al., 2016).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Westall et al., 2011

      Not checked in the free run.

    • cites Buick et al., 1981

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    • cites Nutman et al., 2016

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  • Findingreasoning

    Environmental factors for abiotic O2 have been identified.

    “Environmental factors for abiotic O2 have been identified and will improve our ability to choose optimal targets and measurements to guard against false positives.”
    Quote in the paperSubstantial claim
  • Findingcitation

    O2 has distinct, strong absorption in the visible and near-infrared.

    “O2 has strong features at UV wavelengths <0.2 μm and in the visible/NIR.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Rothman et al., 2013

      Not checked in the free run.

  • Findingcitation

    O2 is spectrally active at visible and near-IR wavelengths.

    “O2 is also spectrally active at visible and near-IR (NIR) wavelengths”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Des Marais et al., 2002

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    • cites Brandt and Spiegel, 2014

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    • cites Misra et al., 2014a

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  • Findingcitation

    The vegetation red edge produces a 2% variation in Earth's disk-averaged spectrum as vegetated hemispheres replace ocean-dominated views.

    “This effect produces a 2% variation in Earth's disk-averaged spectrum, as vegetated hemispheres of Earth replace ocean-dominated views”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Arnold et al., 2002

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    • cites Montañés-Rodríguez et al., 2006

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    • cites Tinetti et al., 2006b

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  • Classified as probably not a substantial claim. 51
  • May combine two claims. 118

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

    Findingcitation

    The annual cycle of global atmospheric CO2 abundance produces a 1-2% change in CO2 abundance over the year.

    “This cycle produces a 1–2% change in the CO2 abundance over the year”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Keeling et al., 1976, 1996

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

    • cites Bacastow et al., 1985 source

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  • May combine two claims.

    Findingcitation

    Phototrophs can produce surface reflectance and seasonally dependent biosignatures.

    “Phototrophs can also produce surface reflectance and seasonally dependent biosignatures”
    Quote in the paperNot a substantial claim

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

    Findingcitation

    The vegetation red edge is currently Earth's dominant surface biosignature, producing a sharp rise in reflectivity longward of chlorophyll absorption at 688 nm.

    “The vegetation red edge is currently Earth's dominant surface biosignature, producing a sharp rise in reflectivity longward of chlorophyll absorption at 688 nm”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Gates et al., 1965

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

    Findingnone given

    O2 has high abundance and uniform distribution throughout the atmospheric column.

    “O2 has other advantages as a biosignature, including its high abundance and uniform distribution throughout the atmospheric column and its distinct, strong absorption in the visible and near-infrared.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: O2 is evenly mixed throughout Earth's atmospheric column and composes 21% of the atmosphere.

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

    Findingnone given

    O2 is evenly mixed throughout Earth's atmospheric column and composes 21% of the atmosphere.

    “which is evenly mixed throughout the atmospheric column and composes 21% of our atmosphere”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: O2 is evenly mixed throughout Earth's atmospheric column and composes 21% of the atmosphere.

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

    Findingcitation

    Photosynthetic organisms, including cyanobacteria and vegetation, have left a global mark on Earth's current environment, significantly modifying its atmosphere, surface, geochemical cycling, ecological structure, and seasonal appearance.

    “Photosynthetic organisms, including cyanobacteria and vegetation, have left a global mark on Earth's current environment—significantly modifying its atmosphere, surface, geochemical cycling, ecological structure, and seasonal appearance”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Meadows, 2008

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    • cites Meadows and Seager, 2010

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

    Findingcitation

    Recent sulfur isotope studies place the irreversible global accumulation of O2 at 2.33 Ga.

    “with recent sulfur isotope studies (Luo et al., 2016) placing it at 2.33 Ga”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Luo et al., 2016

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  • May combine two claims.

    Findingcitation

    O4 collisional pairs form preferentially in high-density, high-O2 atmospheres, although O4 can also be seen in Earth's disk-averaged and transmission spectra.

    “O2 collisional pairs (O4) form preferentially in high-density, high-O2 atmospheres, although O4 can also be seen in Earth's disk-averaged and transmission spectra”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Tinetti et al., 2006a

      Not checked in the free run.

    • cites Pallé et al., 2009

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  • May combine two claims.

    Findingcitation

    O2 has strong features at UV wavelengths <0.2 μm and in the visible/NIR, including the γ band at 0.628 μm, B-band at 0.688 μm, A-band at 0.762 μm, and the a 1Δg band at 1.269 μm; the A-band is the strongest.

    “O2 has strong features at UV wavelengths <0.2 μm and in the visible/NIR. The latter include the γ band at 0.628 μm, B-band at 0.688 μm, A-band at 0.762 μm, and the a 1Δg band at 1.269 μm; the A-band is the strongest of these”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Rothman et al., 2013

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

    Findingcitation

    Biomarkers in fluid inclusions suggest that oxygenic photosynthesis was active at 2.45 Ga.

    “Biomarkers in fluid inclusions also suggest that oxygenic photosynthesis was active at 2.45 Ga”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Dutkiewicz et al., 2006

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    • cites George et al., 2008, 2009

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

    Findingnone given

    Over time O2 has risen to be the second most abundant gas in Earth's atmosphere, after N2.

    “Over time O2 has risen to be the second most abundant gas in our atmosphere, after the dominant N2”
    Quote in the paperNot a substantial claim

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

    Findingcitation

    Evidence for early O2 production was subsequently confirmed by nitrogen and selenium isotope studies.

    “This evidence for early O2 production was subsequently confirmed by nitrogen (Garvin et al., 2009) and selenium (Stüeken et al., 2015a) isotope studies.”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Garvin et al., 2009

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    • cites Stüeken et al., 2015a

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  • May combine two claims.

    Findingcitation

    Several abiotic sources of N2O have been identified, including chemodenitrification via brine/rock reactions in hypersaline environments, photolysis of nitrate ions in soils, and photochemical oxidation of atmospheric NH3.

    “These include chemodenitrification via brine/rock reactions in hypersaline environments, as observed in the Antarctic Dry Valleys (Samarkin et al., 2010; Peters et al., 2014); photolysis of nitrate ions in soils, an abiotic effect that may also show a seasonal signal (Rubasinghege et al., 2011); or photochemical oxidation of atmospheric NH3 (Stein and Yung, 2003).”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Samarkin et al., 2010

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    • cites Peters et al., 2014

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    • cites Rubasinghege et al., 2011

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

    Findingnone given

    For modern Earth, N2O and CH4 abundances are low, at 0.3 and 1.8 ppm, respectively.

    “For modern Earth, both the N2O and CH4 abundances are low, at 0.3 and 1.8 ppm, respectively.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingcitation

    Hu et al. (2012) explored abiotic generation of O2 on potentially habitable planets assuming no volcanic outgassing, but with rainout of oxidized species included, and achieved an abundance of only 5 × 10−3 PAL (0.1% abundance), which is also highly unlikely to be detectable.

    “Later, Hu et al. (2012) explored abiotic generation of O2 on potentially habitable planets assuming no volcanic outgassing, but with rainout of oxidized species included, and achieved an abundance of only 5 × 10−3 PAL (0.1% abundance), which is also highly unlikely to be detectable.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Hu et al., 2012

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  • Classified as probably not a substantial claim.

    Findingcitation

    Segura et al. (2007) revisited the possible photolytic production of abiotic O2 in CO2-rich atmospheres with the three O2 sinks included.

    “Segura et al. (2007) revisited the possible photolytic production of abiotic O2 in CO2-rich atmospheres with the three O2 sinks discussed above included.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Segura et al., 2007

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  • Classified as probably not a substantial claim.

    Findingcitation

    They explored whether a CO2-rich (up to 2 bar) atmosphere irradiated with strong incident UV from a young Sun-like star (EK Dra; G1.5V) could generate detectable levels of O2.

    “They explored whether a CO2-rich (up to 2 bar) atmosphere irradiated with strong incident UV from a young Sun-like star (EK Dra; G1.5V)—a best-case scenario for CO2 photolysis—could generate detectable levels of O2.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Segura et al., 2007

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

    Findingnone given

    Initial modeling results attempting to identify false-positive mechanisms concentrated on photolysis of CO2 as a possible source of abiotic O2.

    “Initial modeling results attempting to identify false-positive mechanisms concentrated on photolysis of CO2 as a possible source of abiotic O2.”
    Quote in the paperNot a substantial claim

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

    Findingcitation

    A subsequent study argued that abiotic photogeneration of O2 from water-splitting may be possible if significantly large areas of the planet supported a catalyst.

    “although a subsequent study argued that this may be possible, if significantly large areas of the planet supported a catalyst”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Narita et al., 2015

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  • May combine two claims.

    Findingcitation

    The martian atmosphere today contains 0.1% O2, and Mars may have built up more O2 if it had been more massive and therefore resistant to O2's atmospheric loss.

    “The martian atmosphere today contains 0.1% O2, and Mars may have built up more O2 if it had been more massive and therefore resistant to O2's atmospheric loss”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites McElroy, 1972

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

    Findingcitation

    Léger et al. (2011) discussed the efficacy of known catalysts for abiotic photogeneration of O2 from water-splitting on a planetary surface but concluded that it was highly unlikely to occur on a habitable planet.

    “As another possible abiotic mechanism for O2 buildup, Léger et al. (2011) discussed the efficacy of known catalysts for abiotic photogeneration of O2 from water-splitting on a planetary surface but concluded that it was highly unlikely to occur on a habitable planet”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Léger et al., 2011

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  • May combine two claims.

    Findingcitation

    These hot sub-Neptunes would also display very high fractions of water vapor (from 1% to 60%) that would persist to high altitude and potentially be visible to transit spectroscopy.

    “However, these hot sub-Neptunes would also display very high fractions of water vapor (from 1% to 60%) that would persist to high altitude (Hu and Seager, 2014) and potentially be visible to transit spectroscopy.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Hu and Seager, 2014

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  • May combine two claims.

    Findingcitation

    These simulations show that planets with hydrogen fractions below about 50%, and with C/O ratios <0.1, form O2-rich atmospheres of at least 20% O2.

    “Specifically, these simulations show that planets with hydrogen fractions below about 50%, and with C/O ratios <0.1, form O2-rich atmospheres of at least 20% O2”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Hu and Seager, 2014

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  • May combine two claims.

    Interpretationcitation

    Oxygenic photosynthesis would have a significant evolutionary advantage, allowing it to dominate its planetary surface environment, which in turn would make it more detectable on a planetary scale.

    “oxygenic photosynthesis would have a significant evolutionary advantage, allowing it to dominate its planetary surface environment (Kiang et al., 2007b; Léger et al., 2011; Kiang, 2014), which in turn would make it more detectable on a planetary scale”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Kiang et al., 2007b

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    • cites Léger et al., 2011

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    • cites Kiang, 2014 source

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

    Interpretationnone given

    Earth's photosynthesis is an ancient metabolic process that developed relatively rapidly, appearing within a billion years after the Moon-forming impact.

    “Earth's photosynthesis is also an ancient metabolic process that, despite its apparent complexity, developed relatively rapidly—appearing within a billion years after the Moon-forming impact.”
    Quote in the paperNot a substantial claim

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

    Interpretationnone given

    The amplitude of the annual CO2 cycle in the global average is a function of land mass distribution and obliquity.

    “and its amplitude in the global average is a function of our land mass distribution and obliquity”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationnone given

    Searching for O4 or CO in the planetary spectrum, or the lack of H2O or CH4, could help discriminate between abiotic and biological sources of O2 or O3.

    “Searching for O4 or CO in the planetary spectrum, or the lack of H2O or CH4, could help discriminate between abiotic and biological sources of O2 or O3.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    Anoxygenic photosynthesizers would have been challenging to detect remotely in the global average, as they were spatially restricted to sources of strong reductants and are not known to produce gaseous by-products.

    “Consequently, anoxygenic photosynthesizers would have been challenging to detect remotely in the global average, as they were spatially restricted to these sources of strong reductants and are not known to produce gaseous by-products.”
    Quote in the paperNot a substantial claim

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

    Interpretationnone given

    Oxygenic photosynthesis evolved to harvest the largest expected energy source at the surface of most terrestrial habitable zone planets.

    “Oxygenic photosynthesis is Earth's dominant metabolism, having evolved to harvest the largest expected energy source at the surface of most terrestrial habitable zone planets.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: Oxygenic photosynthesis is Earth's dominant metabolism.

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

    Interpretationcitation

    N2 may also be biologically mediated through biological fixation, mineral absorption, and sequestration into the mantle.

    “which may also be biologically mediated through biological fixation, mineral absorption, and sequestration into the mantle”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Catling and Kasting, 2007

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    • cites Johnson and Goldblatt, 2015

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  • May combine two claims.

    Interpretationreasoning

    The near-constant O2 mixing ratio is due to relatively low volcanic outgassing rates, a lack of strong surface sinks from the oxidized crust, and O2's relative robustness to photolytic destruction in the stratosphere.

    “This is due to relatively low volcanic outgassing rates, a lack of strong surface sinks from our oxidized crust, and O2's relative robustness to photolytic destruction in the stratosphere.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Transit spectroscopy cannot probe as deeply into an atmosphere as direct imaging due to high clouds or hazes or effects of refraction, so it is generally most sensitive to upper tropospheres and stratospheres of terrestrial planets.

    “As transit spectroscopy cannot probe as deeply into an atmosphere as direct imaging due to either the presence of high clouds or hazes (Kreidberg et al., 2014; Charnay et al., 2015; Arney et al., 2016, 2017) or the effects of refraction (García Muñoz et al., 2012; Bétrémieux and Kaltenegger, 2013, 2014; Misra et al., 2014b), this technique is generally most sensitive to the upper tropospheres and”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Kreidberg et al., 2014

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    • cites Charnay et al., 2015

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    • cites Arney et al., 2016, 2017

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  • May combine two claims.

    Interpretationcitation

    Both ethane and ammonia have their strongest features in the MIR near 13 and 9 μm, respectively, and sufficiently sensitive spectra of terrestrial planet atmospheres may be difficult to obtain at these wavelengths using transmission with JWST.

    “Both ethane and ammonia have their strongest features in the MIR (near 13 and 9 μm, respectively; Rothman et al., 2013), and sufficiently sensitive spectra of terrestrial planet atmospheres may be difficult to obtain at these wavelengths using transmission with the James Webb Space Telescope (JWST; Arney et al., 2017).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Rothman et al., 2013

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    • cites Arney et al., 2017

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  • May combine two claims.

    Premisenone given

    The timing of the origin of oxygenic photosynthesis is not known but can be constrained by its subsequent impact on the environment.

    “The timing of the origin of oxygenic photosynthesis is not known but can be constrained by its subsequent impact on the environment.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    N2 is a homonuclear molecule with no permanent dipole and absorbs strongly only in the ultraviolet (80-100 nm).

    “N2 is a homonuclear molecule with no permanent dipole, and it absorbs strongly only in the ultraviolet (80–100 nm)”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Rothman et al., 2013

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

    Premisenone given

    O3 can also be produced by the photolysis of other oxygen-bearing compounds, such as CO2 and H2O.

    “However, O3 can also be produced by the photolysis of other oxygen-bearing compounds, such as CO2 and H2O.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationcitation

    O2 has an advantage over other proposed biosignature molecules, including methanethiol and dimethyl disulfide, which are more susceptible to photolysis by UV radiation and confined largely to the lower troposphere.

    “This gives O2 an advantage over other proposed, often more complex biosignature molecules, including methanethiol, dimethyl disulfide (e.g., Pilcher, 2003; Domagal-Goldman et al., 2011; Seager and Bains, 2015), which are more susceptible to photolysis by UV radiation and so are confined largely to the lower troposphere”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Pilcher, 2003

      Not checked in the free run.

    • cites Domagal-Goldman et al., 2011

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    • cites Seager and Bains, 2015

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  • May combine two claims.

    Interpretationcitation

    The O4 band at 1.269 μm coincides with the O2 a 1Δg band and O2-N2 collisionally induced absorption, which can complicate its detailed interpretation, although all these features clearly indicate the presence of O2.

    “This latter O4 band coincides with the O2 a 1Δg band and O2-N2 collisionally induced absorption (Smith and Newnham, 2000; Pallé et al., 2009), which can complicate its detailed interpretation, although all these features clearly indicate the presence of O2.”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Smith and Newnham, 2000

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    • cites Pallé et al., 2009

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

    Premisereasoning

    The spectral region below 100 nm is not accessible by ground-based telescopes due to strong absorption by N2 in Earth's atmosphere.

    “The spectral region below 100 nm is also not accessible by ground-based telescopes due to strong absorption by N2 in Earth's atmosphere.”
    Quote in the paperNot a substantial claim

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

    Interpretationcitation

    The limited environments suitable for early anoxygenic photosynthesizers provided evolutionary pressure for the development of a second photosystem that could use water and produce oxygen as a waste product.

    “The limited environments suitable for early anoxygenic photosynthesizers provided evolutionary pressure for the development of a second photosystem that, when linked to the other, could use weaker—but abundant and globally widespread—reductants such as water, and produce oxygen as a waste product”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Blankenship, 2010

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  • May combine two claims.

    Interpretationcitation

    These disequilibria may have been more detectable during Earth's Proterozoic period, when lower O2 abundance could still generate potentially detectable O3 yet also allow CH4 to exist at higher atmospheric abundances than currently possible.

    “Consequently, these disequilibria may have been more detectable during Earth's Proterozoic period, when the lower O2 abundance could still generate potentially detectable O3 yet also allow CH4 to exist at higher atmospheric abundances than is currently possible”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Segura et al., 2003

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    • cites Meadows, 2005

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    • cites Kaltenegger et al., 2007

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  • May combine two claims.

    Premisenone given

    These sinks include volcanic outgassing of reduced compounds, redox balance at the surface, and rainout of oxidized species, like H2O2.

    “These sinks include volcanic outgassing of reduced compounds, redox balance at the surface, and rainout of oxidized species, like H2O2.”
    Quote in the paperSubstantial claim

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

    Interpretationreasoning

    JWST has less sensitivity for exoplanet transmission spectra at longer wavelengths, due to increasing thermal background and the drop in stellar brightness.

    “Similarly, JWST has less sensitivity for exoplanet transmission spectra at these longer wavelengths, due to increasing thermal background and the drop in stellar brightness.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationcitation

    An N2O-rich atmosphere may have formed during the Proterozoic due to the biological unavailability of Cu in a strongly sulfidic ocean; the dearth of Cu would have inhibited the final step of denitrification and preferentially released N2O rather than N2 into the atmosphere.

    “Similarly, an N2O-rich atmosphere may have formed during the Proterozoic due to the biological unavailability of Cu in a strongly sulfidic ocean. The dearth of Cu would have inhibited the final step of the denitrification process and preferentially released N2O—rather than N2—into the planet's atmosphere”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Buick, 2007

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    N2O has strong features in the MIR, so the O3 (9.6 μm)/N2O (8.5 μm) pair would be the most diagnostic, although N2O also absorbs, albeit more weakly, in the NIR at 2.11 and 2.25 μm.

    “N2O has strong features in the MIR, so the O3 (9.6 μm)/N2O (8.5 μm) pair would be the most diagnostic, although N2O also absorbs, albeit more weakly, in the NIR at 2.11 and 2.25 μm”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Rothman et al., 2013

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    For the O2/N2/ocean disequilibrium biosignature, a surface ocean might be sought by using reflective glint from the ocean in the planet's phase-dependent photometry at visible to NIR wavelengths, while oxygen could be sought as either O2 or O4 in the visible or NIR.

    “For the O2/N2/ocean disequilibrium biosignature, a surface ocean might be sought by using reflective “glint” from the ocean in the planet's phase-dependent photometry at visible to NIR wavelengths (Williams and Gaidos, 2008; Robinson et al., 2010, 2014; Zugger et al., 2011; Cowan et al., 2012) while the oxygen could be sought as either O2 or O4 in the visible or NIR”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Williams and Gaidos, 2008

      Not checked in the free run.

    • cites Robinson et al., 2010, 2014

      Not checked in the free run.

    • cites Zugger et al., 2011

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    Atmospheric N2 may be sought near 4.1 μm via N4 collisional absorption from pairs of N2 molecules, which overlaps but can be distinguished from the 4.2 μm CO2 absorption band in Earth's atmosphere.

    “Alternatively, atmospheric N2 may be sought near 4.1 μm via N4 (also written as N2-N2) collisional absorption from pairs of N2 molecules (Lafferty et al., 1996), which overlap, but can be distinguished from, the 4.2 μm CO2 absorption band in Earth's atmosphere”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Lafferty et al., 1996

      Not checked in the free run.

    • cites Schwieterman et al., 2015b

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    The disequilibrium of O2 and CH4 implies active sources of both gases, which on Earth include photosynthetically generated O2 and CH4 from methanogenic bacteria and archaea, which outproduce geologically generated methane from serpentinization and other water/rock reactions by at least a factor of 25.

    “Their disequilibrium implies active sources of both gases, which on Earth include photosynthetically generated O2, and CH4 from methanogenic bacteria and archaea, which outproduce geologically generated methane from serpentinization (Kelley et al., 2005; Guzmán-Marmolejo et al., 2013) and other water/rock reactions by at least a factor of 25”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Kasting and Catling, 2003

      Not checked in the free run.

    • cites Kharecha et al., 2005

      Not checked in the free run.

    • cites Etiope and Sherwood Lollar, 2013

      Not checked in the free run.

  • May combine two claims.

    Interpretationnone given

    These disequilibrium biosignatures are potentially observable, although the O2/N2/ocean is perhaps the most challenging.

    “These disequilibrium biosignatures are potentially observable, although the O2/N2/ocean is perhaps the most challenging.”
    Quote in the paperSubstantial claim

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

    Premisecitation

    Another proposed disequilibrium biosignature is the combination of O2 from photosynthesis and N2O from bacterial denitrification.

    “Another proposed disequilibrium biosignature is the combination of O2 from photosynthesis and N2O from bacterial denitrification”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Lovelock, 1975

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    N2 could be searched for in the 80-100 nm region of the UV, or via N4 absorption near 4.1 μm.

    “N2 could be searched for in the 80–100 nm region of the UV, or via N4 absorption near 4.1 μm”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Schwieterman et al., 2015b

      Not checked in the free run.

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

    Interpretationcitation

    Coronal mass ejections from the early Sun may have allowed energetic particles to drive the formation of N2O on the early Earth via destruction of N2 and CO2 in the upper atmosphere.

    “It has also recently been postulated that coronal mass ejections from the early Sun may have allowed energetic particles to drive the formation of N2O on the early Earth via destruction of N2 and CO2 in the upper atmosphere”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Airapetian et al., 2016

      Not checked in the free run.

  • May combine two claims.

    Interpretationnone given

    O2 and CH4 may be detectable in a visible to NIR transmission or direct imaging spectrum of an exoplanet's atmosphere, or as the O3 (9.6 μm)/CH4 (7.7 μm) pair in the MIR.

    “O2 and CH4 may be detectable in a visible to NIR transmission or direct imaging spectrum of an exoplanet's atmosphere, or as the O3 (9.6 μm)/CH4 (7.7 μm) pair in the MIR.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    The MIR wavelength region is not currently under consideration for future spaceborne exoplanet direct imaging telescopes, although it may be accessible to ground-based telescopes for very nearby planetary systems.

    “The MIR wavelength region is also not currently under consideration for future spaceborne exoplanet direct imaging telescopes (Dalcanton et al., 2015), although it may be accessible to ground-based telescopes for very nearby planetary systems”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Dalcanton et al., 2015

      Not checked in the free run.

    • cites Males et al., 2014

      Not checked in the free run.

    • cites Snellen et al., 2015

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    If the low-noncondensable-gas process persists for 4 billion years around a Sun-like star and the resultant O2 is absorbed by the planet's surface and interior instead of building up and shutting off water loss, then as much as a third of an Earth ocean could ultimately be lost.

    “If this process persists for 4 billion years around a Sun-like star—and the resultant O2 is absorbed by the planet's surface and interior, instead of building up in the atmosphere and shutting off the water loss—then as much as a third of an Earth ocean could ultimately be lost from the planetary environment.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Wordsworth and Pierrehumbert, 2014

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    This would help discriminate them from habitable Earth-like planets, which have water in the troposphere but relatively dry stratospheres, that would produce weak or no water vapor features in transmission.

    “This would help discriminate them from habitable Earth-like planets, which have water in the troposphere but relatively dry stratospheres, that would produce weak or no water vapor features in transmission.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Paleobarometry measurements of 2.7 Ga fossilized raindrop imprints indicate that Earth's atmospheric pressure was no more than twice the modern value but potentially as low as 0.5 bar.

    “Paleobarometry measurements of 2.7 Ga fossilized raindrop imprints indicate that Earth's atmospheric pressure was no more than twice the modern day's but potentially as low as 0.5 bar (Som et al., 2012).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Som et al., 2012

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Recent evidence from preserved micrometeorites indicates that the Archean stratosphere contained oxygen in abundances similar to modern Earth's, possibly formed by a water-rich stratosphere undergoing photolysis.

    “Interestingly, even though the Archean Earth's troposphere likely contained very little oxygen, there is recent evidence from preserved micrometeorites that the stratosphere of the Archean contained oxygen in abundances similar to modern Earth's, as might possibly have been formed by a water-rich stratosphere that was undergoing photolysis (Tomkins et al., 2016).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Tomkins et al., 2016

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    A terrestrial planet undergoing a runaway greenhouse or a frozen Mars-like planet beyond the outer edge of the habitable zone could both build up large amounts of O2, and possibly O3, in their atmospheres.

    “However, a terrestrial planet undergoing a runaway greenhouse or a frozen Mars-like planet beyond the outer edge of the habitable zone could both build up large amounts of O2, and possibly O3, in their atmospheres”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Schindler and Kasting, 2000

      Not checked in the free run.

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

    Interpretationnone given

    Photochemistry plays a significant role in all of these mechanisms and can leave telltale signs of its role in forming abiotic oxygen or ozone in the planetary atmosphere.

    “Photochemistry plays a significant role in all of these mechanisms and can leave telltale signs of its role in forming abiotic oxygen or ozone in the planetary atmosphere.”
    Quote in the paperNot a substantial claim

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

    Premisecitation

    Venus' water loss and subsequent oxidation of the mantle potentially enhanced the outgassing of N2.

    “with Venus' water loss and subsequent oxidation of the mantle potentially enhancing the outgassing of N2 (Wordsworth, 2016).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Wordsworth, 2016

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    In the low-noncondensable-gas mechanism, the cold trap is ineffective when atmospheric temperature is high or the inventory of noncondensable gases is low, allowing water to rise into the stratosphere, be photolyzed, and leave oxygen to build up abiotically after hydrogen escape.

    “This can occur either when the atmospheric temperature is high or when the inventory of noncondensable gases—for example, N2 or Ar—is low. In these cases, water rises into the stratosphere, where it is more vulnerable to photolysis by incident stellar UV radiation. The resultant H atoms escape, leaving oxygen to build up abiotically in the atmosphere.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Wordsworth and Pierrehumbert, 2014

      Not checked in the free run.

  • May combine two claims.

    Interpretationnone given

    Surface ice would preclude reactions with a reduced surface, and the lack of reduced gases from volcanic outgassing would allow O2 from photolysis of H2O or CO2 to accumulate in the planetary atmosphere.

    “Surface ice would preclude reactions with a reduced surface, and the lack of reduced gases from volcanic outgassing would allow O2 from photolysis of H2O or CO2 to accumulate in the planetary atmosphere.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationcitation

    Such a planet will be outside or very close to the conservative habitable zone boundaries for its mass and atmospheric characteristics and, if showing the abundant O2 of a recent runaway or a hot Neptune, will also exhibit strong stratospheric H2O, but no O3 in either transmission or direct detection spectra.

    “Such a planet will be outside or very close to the conservative habitable zone boundaries for its mass and atmospheric characteristics (Kopparapu et al., 2013, 2014) and, if showing the abundant O2 of a recent runaway or a hot Neptune, will also exhibit strong stratospheric H2O, but no O3 in either transmission or direct detection spectra.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Kopparapu et al., 2013, 2014

      Not checked in the free run.

  • Classified as probably not a substantial claim. May combine two claims.

    Interpretationcitation

    Although buildup of significant amounts of abiotic O2 or O3 on habitable planets was initially precluded, it was thought more likely to occur on some uninhabitable planets.

    “Although buildup of significant amounts of abiotic O2 or O3 on habitable planets was initially precluded, it was thought more likely to occur on some uninhabitable planets.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationcitation

    The runaway greenhouse state would evaporate the planetary ocean and modify the atmospheric temperature structure to allow water to permeate the planet's stratosphere, above any UV-protective ozone layer.

    “The runaway greenhouse state would evaporate the planetary ocean and modify the atmospheric temperature structure to allow water to permeate the planet's stratosphere, above any UV-protective ozone layer.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Schindler and Kasting, 2000

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Gas vesicle size distributions in 2.7 Ga volcanic rocks derived an atmospheric pressure of 0.23 ± 0.23 bar, and combined with previous studies this suggested 0.5 bar was a likely upper limit to Archean atmospheric pressure.

    “Som et al. (2016) used the distribution of sizes in gas vesicles in 2.7 Ga volcanic rocks to derive an atmospheric pressure of 0.23 ± 0.23 bar, which when combined with the previous studies suggested that 0.5 bar was a likely upper limit to the atmospheric pressure in Earth's Archean.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Som et al., 2016

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Photolysis of water and subsequent hydrogen loss from terrestrial atmospheres depleted in noncondensable gases can generate abiotic O2, and this mechanism may be the most difficult to recognize.

    “The possible abiotic O2 generation mechanism that will likely be the most difficult to recognize involves photolysis of water—and subsequent hydrogen loss—from terrestrial atmospheres that are depleted in noncondensable gases (Wordsworth and Pierrehumbert, 2014).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Wordsworth and Pierrehumbert, 2014

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    In the case of the planet beyond the outer edge of the habitable zone with abundant O2, and for an older runaway greenhouse atmosphere close to the star that also exhibits both O2 and a large O3 component, water vapor will instead be weak or nonexistent in transmission or direct imaging spectra.

    “In the case of the planet beyond the outer edge of the habitable zone with abundant O2, and for an older runaway greenhouse atmosphere close to the star that also exhibits both O2 and a large O3 component, water vapor will instead be weak or nonexistent in transmission or direct imaging spectra”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Schindler and Kasting, 2000

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    Several of these false-positive mechanisms are more likely on planets orbiting in the habitable zones of M dwarf stars, but at least one mechanism could operate for planets orbiting in the habitable zones of stars of any spectral type, including G dwarfs.

    “Several of these false-positive mechanisms are more likely on planets orbiting in the habitable zones of M dwarf stars (Domagal-Goldman and Meadows, 2010; Domagal-Goldman et al., 2014; Tian et al., 2014; Gao et al., 2015; Harman et al., 2015; Luger and Barnes, 2015), but at least one mechanism could operate for planets orbiting in the habitable zones of stars of any spectral type, including G dwar”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Domagal-Goldman and Meadows, 2010

      Not checked in the free run.

    • cites Domagal-Goldman et al., 2014

      Not checked in the free run.

    • cites Tian et al., 2014

      Not checked in the free run.

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

    Premisecitation

    Mars and Venus have smaller (2 × 10−4) and larger (3.3 times) atmospheric nitrogen abundances than Earth.

    “In our own solar system, Mars and Venus have smaller (2 × 10−4) and larger (3.3 times) atmospheric nitrogen abundances than Earth (Mancinelli and Banin, 2003; Hunten, 1993), with Venus' water loss and subsequent oxidation of the mantle potentially enhancing the outgassing of N2 (Wordsworth, 2016).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Mancinelli and Banin, 2003

      Not checked in the free run.

    • cites Hunten, 1993

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    In aggregate, the research indicates that there are several mechanisms that could produce abiotic O2 and O3 in a planet's atmosphere, with each presenting a potential false positive to different degrees.

    “In aggregate, this research indicates that there are several mechanisms that could produce abiotic O2 and O3 in a planet's atmosphere, with each presenting a potential false positive to different degrees.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Planets in the outer regions of the habitable zone for M0-M3V stars are the least likely to generate significant amounts of abiotic O2, along with M1-M3V dwarfs with lower stellar XUV, because slower water loss may preclude O2 generation and buildup.

    “Planets in the outer regions of the habitable zone for M0–M3V stars are the least likely to generate significant amounts of abiotic O2 (Luger and Barnes, 2015), along with M1–M3V dwarfs with lower stellar XUV, as slower water loss may preclude O2 generation and buildup (Tian, 2015).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Luger and Barnes, 2015

      Not checked in the free run.

    • cites Tian, 2015

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    This catalytic cycle feedback places an upper limit of ∼50% on the CO2 that can be destroyed via photolysis, but this is still enough to generate Earth-like abundances of abiotic O2 and O3.

    “This catalytic cycle feedback places an upper limit of ∼50% on the CO2 that can be destroyed via photolysis, but this is still enough to generate Earth-like abundances of abiotic O2 and O3.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Gao et al., 2015

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Once they enter the habitable zone, M dwarf planets with O2-rich atmospheres may be completely desiccated, or could still support a surface ocean, especially if the superluminous phase ends before the runaway process is complete.

    “Once they enter the habitable zone, these M dwarf planets with O2-rich atmospheres may be completely desiccated, or could still support a surface ocean, especially if the superluminous phase ends before the runaway process is complete (Luger and Barnes, 2015; Barnes et al., 2016).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Luger and Barnes, 2015

      Not checked in the free run.

    • cites Barnes et al., 2016

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Water entering the stratosphere is enabled either by loss of an ocean in a runaway greenhouse process, most effective for late-type M dwarfs, or via lack of noncondensable gases, which could affect planets orbiting stars of any spectral type.

    “Water entering the stratosphere is either enabled by loss of an ocean in a runaway greenhouse process (Luger and Barnes, 2015)—a mechanism that is most effective for late-type (i.e., less massive) M dwarfs—or via lack of noncondensable gases in the planetary atmosphere, which could affect planets orbiting stars of any spectral type (Wordsworth and Pierrehumbert, 2014).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Luger and Barnes, 2015

      Not checked in the free run.

    • cites Wordsworth and Pierrehumbert, 2014

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Before settling onto the main sequence, young stars can be significantly more luminous due to extended contraction, so planets forming in the future main sequence habitable zone are subjected to very high radiation early.

    “Before settling in to their main sequence hydrogen-burning phase, young stars can be significantly more luminous due to additional energy production from their extended contraction phase (e.g., Baraffe et al., 1998), so planets that form in what will become the main sequence habitable zone are subjected to very high levels of radiation early on (Lissauer, 2007).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Baraffe et al., 1998

      Not checked in the free run.

    • cites Lissauer, 2007

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisecitation

    For hot Earths, hundreds to thousands of bar of O2 could persist only for planets with very large initial water abundances greater than 5% wt %, which over 5 billion years would never be sufficiently depleted in water to reach the O2 escape runaway scenario.

    “For these “hot Earths,” they concluded that hundreds to thousands of bar of O2 could persist only for planets with very large initial water abundances of >5% wt %, which over 5 billion years would never be sufficiently depleted in water to get to the O2 escape “runaway” scenario.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Schaefer et al., 2016

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    If desiccated, CO2-rich planets are common in the habitable zones of M dwarfs, then the abiotic conditions that lead to such high oxygen levels could be common also.

    “If desiccated, CO2-rich planets are common in the habitable zones of M dwarfs, then the abiotic conditions that lead to such high oxygen levels could be common also.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    The runaway mechanism could produce an O2-dominated atmosphere of hundreds of bar, and the lack of noncondensable gases could potentially result in atmospheres that are ∼15% O2.

    “The runaway mechanism could produce an O2-dominated atmosphere of hundreds of bar, and the lack of noncondensable gases could potentially result in atmospheres that are ∼15% O2.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Luger and Barnes, 2015

      Not checked in the free run.

    • cites Wordsworth and Pierrehumbert, 2014

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    A planet that enters the habitable zone prior to 1 billion years can shut down hydrodynamic escape and avoid massive O2 loss, and can do so with water abundances significantly lower than 5% wt %.

    “However, as Luger and Barnes (2015) and Barnes et al. (2016) show, a planet that enters the habitable zone prior to 1 billion years can also shut down hydrodynamic escape and avoid massive O2 loss and could do so with water abundances significantly lower than 5% wt %.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Luger and Barnes, 2015

      Not checked in the free run.

    • cites Barnes et al., 2016

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Modeling suggests that during the superluminous phase a terrestrial planet forming within the future main sequence habitable zone around an M dwarf may lose up to several Earth ocean equivalents of water due to evaporation and hydrodynamic escape, leading to generation of large amounts of abiotic O2.

    “Modeling suggests that during this superluminous phase a terrestrial planet that forms within what will become the main sequence habitable zone around an M dwarf star may lose up to several Earth ocean equivalents of water due to evaporation and hydrodynamic escape, and this can lead to generation of large amounts of abiotic O2 (Luger and Barnes, 2015; Tian, 2015).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Luger and Barnes, 2015

      Not checked in the free run.

    • cites Tian, 2015

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    The increased O2 abundance led to high O3 concentrations, and O3 photolytic products form another CO2-regenerating catalytic cycle.

    “the increased O2 abundance also led to high O3 concentrations, and O3 photolytic products form another CO2-regenerating catalytic cycle.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Gao et al., 2015

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    CO2 is likely a common atmospheric gas on terrestrial planets; in the solar system CO2 dominates the atmospheres of Mars and Venus and was likely a significant component of early Earth's atmosphere.

    “CO2 in particular is likely a common atmospheric gas on terrestrial planets. In our own solar system, CO2 dominates the atmospheric composition on Mars and Venus and was likely a significant component of the early Earth's atmosphere as well (Kasting, 1993; Sheldon, 2006; Sleep, 2010; Driese et al., 2011).”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Kasting, 1993

      Not checked in the free run.

    • cites Sheldon, 2006

      Not checked in the free run.

    • cites Sleep, 2010

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    The pre-main sequence superluminous phase is most pronounced and of longer duration for smaller-mass M dwarfs than for any other stellar spectral type, and can extend for up to 1 billion years.

    “This superluminous phase is most pronounced—and of longer duration—for smaller-mass M dwarfs than for any other stellar spectral type, where it can extend for up to 1 billion years (Baraffe et al., 1998).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Baraffe et al., 1998

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    An ocean of water may have remained in Earth's mantle after the initial volatile loss and is being more slowly outgassed.

    “An ocean of water may have remained in the mantle after this initial loss (Albarède, 2009; Sleep et al., 2012), and it is being more slowly outgassed.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Albarède, 2009

      Not checked in the free run.

    • cites Sleep et al., 2012

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Gao et al.'s mechanism relies on the planet being desiccated, so that water is not available to catalyze the recombination of CO2 and the balance between CO2 photolysis and recombination would instead be shifted toward production of large amounts of atmospheric O2.

    “Their mechanism relies on the planet being desiccated, so that water is not available to catalyze the recombination of CO2 and the balance between CO2 photolysis and recombination would instead be shifted toward production of large amounts of atmospheric O2.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Gao et al., 2015

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    F dwarf planets may still be affected by high levels of abiotic O3 generation from CO2 photolysis, and planets orbiting F-K dwarfs may also be susceptible to abiotic O2 generation if their atmospheres have a lower abundance of noncondensable gases.

    “However, F dwarf planets may still be affected by high levels of abiotic O3 generation from CO2 photolysis (Domagal-Goldman et al., 2014), and planets orbiting F-K dwarfs may also be susceptible to abiotic O2 generation if their atmospheres have a lower abundance of noncondensable gases (Wordsworth and Pierrehumbert, 2014).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Domagal-Goldman et al., 2014

      Not checked in the free run.

    • cites Wordsworth and Pierrehumbert, 2014

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    They are less affected by refraction in the planetary atmosphere, and the favorable star/planet size ratio produces deeper spectral features for atmospheric gases.

    “They are less affected by refraction in the planetary atmosphere, and the favorable star/planet size ratio produces deeper spectral features for atmospheric gases.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    For the FUV/NUV mechanism, O2 abundances as high as 0.2% to 6% are predicted, with higher values corresponding to little or no O2 sinks in the planetary environment.

    “For the cases considered for this mechanism, O2 abundances as high as 0.2% to 6% are predicted, with higher values corresponding to little or no O2 sinks in the planetary environment.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Tian et al., 2014

      Not checked in the free run.

    • cites Harman et al., 2015

      Not checked in the free run.

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

    Interpretationreasoning

    Direct imaging observations can obtain a disk-averaged photometric or spectroscopic measurement that captures light from the top of the planetary atmosphere down to the emitting or reflecting layer-which could be a haze, cloud, a level of the atmosphere, or the actual planetary surface.

    “Direct imaging observations can be used to obtain a disk-averaged photometric or spectroscopic measurement that captures light from the top of the planetary atmosphere down to the emitting or reflecting layer—which could be a haze, cloud, a level of the atmosphere, or the actual planetary surface.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    This is important for biosignature detection and false-positive assessment, as the planet's troposphere usually contains a significant fraction of atmospheric mass (80% for Earth), and the near-surface environment is most likely to record surface and interior processes such as life, volcanism, and a hydrological cycle.

    “This is important for biosignature detection and false-positive assessment, as the planet's troposphere usually contains a significant fraction of the atmospheric mass (80% in the case of Earth), and the near-surface environment is most likely to record the presence of surface and interior processes such as life, volcanism, and a hydrological cycle.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisereasoning

    FUV radiation can favor the generation of ozone via photolysis of CO2 (and O2), whereas MUV or NUV radiation photolytically destroys O3.

    “FUV radiation can favor the generation of ozone via photolysis of CO2 (and O2), whereas MUV or NUV radiation photolytically destroys O3.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Since these false positives are most likely to affect planets orbiting M dwarfs, they will impact searches for biosignatures using space-based and ground-based transmission spectroscopy, as well as ground-based direct imaging observations.

    “Consequently, since these false positives are most likely to affect planets orbiting M dwarfs, they will impact searches for biosignatures using both space-based (Deming and Seager, 2009; Hedelt et al., 2013; Misra et al., 2014b) and ground-based transmission spectroscopy (Snellen et al., 2013; Rodler and López-Morales, 2014), as well as ground-based direct imaging observations”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Deming and Seager, 2009

      Not checked in the free run.

    • cites Hedelt et al., 2013

      Not checked in the free run.

    • cites Misra et al., 2014b

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Venus likely underwent a catastrophic runaway greenhouse, and the observed enhancement in its atmospheric D/H ratio suggests that Venus lost a global ocean of at least 3 m in depth.

    “Venus may have been wetter in the past but likely underwent a catastrophic runaway greenhouse, and the observed enhancement in its atmospheric D/H ratio suggests that Venus lost a global ocean of at least 3 m in depth (De Bergh et al., 1991).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites De Bergh et al., 1991

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Ground-state O2 has never been detected on Venus, and observational upper limits suggest uniform mixing ratios of <2 ppm above 60 km.

    “However, ground-state O2 has never been detected on Venus, and observational upper limits suggest uniform mixing ratios of <2 ppm above 60 km (Trauger and Lunine, 1983; Mills, 1999).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Trauger and Lunine, 1983

      Not checked in the free run.

    • cites Mills, 1999

      Not checked in the free run.

  • Classified as probably not a substantial claim. May combine two claims.

    Premisecitation

    To avoid planets most susceptible to generating large amounts of false O2, Luger and Barnes (2015) suggest targeting planets orbiting stars more massive than 0.4 M⊙ (M3V and earlier) and especially favoring planets toward the outer edge of the habitable zone.

    “To avoid those planets most susceptible to generating large amounts of false O2, the work of Luger and Barnes (2015, their Fig. 13) suggests that we should target planets orbiting stars more massive than 0.4 M⊙ (M3V and earlier) and especially favor those planets found toward the outer edge of the habitable zone around these stars.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Luger and Barnes, 2015

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    The modern venusian atmosphere is extremely hydrogen poor, with about 30 ppm of water vapor below its cloud deck, about 2-5 ppm of H2O above it, and HF and HCl at abundances much less than 1 ppm.

    “The modern venusian atmosphere is extremely hydrogen poor, with only about 30 ppm of water vapor below its cloud deck (Meadows and Crisp 1996; Arney et al., 2014), and about 2–5 ppm of H2O above it (Cottini et al., 2012), with the other principal H-bearing gases, HF and HCl, found at abundances much less than 1 ppm (Krasnopolsky, 2010).”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Meadows and Crisp 1996

      Not checked in the free run.

    • cites Arney et al., 2014

      Not checked in the free run.

    • cites Cottini et al., 2012

      Not checked in the free run.

  • May combine two claims.

    Interpretationnone given

    A 10 km altitude is within the troposphere for an Earth-like planet but well above the majority of tropospheric water vapor, and Earth's clouds.

    “A 10 km altitude is within the troposphere for an Earth-like planet but well above the majority of tropospheric water vapor, and Earth's clouds.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    There may be fewer of these targets available, as earlier-type M dwarfs are less common, and their habitable zone planets orbit farther from their star and would be less likely to be found in transit.

    “However, there may be fewer of these targets available, as these earlier-type M dwarfs are less common, and their habitable zone planets, which orbit farther from their star, would be less likely to be found in transit.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationnone given

    In practice, wavelength-dependent opacity of a terrestrial planetary atmosphere will limit how deep a transmission spectrum can probe; opacity is a function of composition and mass, including Rayleigh scattering.

    “However, in practice, the wavelength-dependent opacity of a terrestrial planetary atmosphere will limit how deep in the atmosphere a transmission spectrum is able to probe. This opacity will be a function of the atmosphere's composition and mass, and includes the effects of Rayleigh scattering.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationnone given

    Little is known about catalytic CO2 recombination processes; consequently, these mechanisms are not generally included in photochemical codes for terrestrial planets, including the models used in the photochemical false-positive studies mentioned above.

    “Little is actually known about these catalytic CO2 recombination processes; consequently, these mechanisms are not generally included in photochemical codes for terrestrial planets, including the models that were used in the photochemical false-positive studies mentioned above (Hu et al., 2012; Domagal-Goldman et al., 2014; Tian et al., 2014; Gao et al., 2015; Harman et al., 2015).”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Hu et al., 2012

      Not checked in the free run.

    • cites Domagal-Goldman et al., 2014

      Not checked in the free run.

    • cites Tian et al., 2014

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    Exploring and understanding the possible mechanisms for abiotic O2/O3 production enables identification of planets that are less susceptible to abiotic generation and allows recognition of planetary and stellar characteristics that help discriminate between abiotic and biological sources.

    “Exploring and understanding the possible mechanisms for abiotic O2/O3 production enable identification of planets that are less susceptible to abiotic generation and allow us to recognize planetary and stellar characteristics that will help discriminate between abiotic and biological sources.”
    Quote in the paperSubstantial claim

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

    Interpretationnone given

    The degree to which a planet has an atmosphere rich in CO2, or a noncondensable atmospheric component, is dependent primarily on planetary and planetary system properties, including planet formation and evolution, and delivery of volatiles from the protoplanetary disk.

    “The degree to which a planet has an atmosphere rich in CO2, or a noncondensable atmospheric component, is dependent primarily on planetary and planetary system properties, including planet formation and evolution, and the delivery of volatiles from the protoplanetary disk.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    O3 was produced at 10% of Earth's current O3 column abundance in the simulations of Domagal-Goldman et al. (2014) for M dwarf planets, without appreciable buildup of abiotic O2.

    “O3 was produced at 10% of Earth's current O3 column abundance in the simulations of Domagal-Goldman et al. (2014) for M dwarf planets, without appreciable buildup of abiotic O2.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Domagal-Goldman et al., 2014

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    This implies that shortly after its formation, O2 is efficiently scrubbed from Venus's atmosphere, possibly by a chlorine-containing catalyst that accelerates the recombination of O with CO to reform CO2.

    “This implies that shortly after its formation, the O2 is efficiently scrubbed from the atmosphere, possibly by a chlorine-containing catalyst that accelerates the recombination of O with CO to reform CO2 (Yung and DeMore, 1982).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Yung and DeMore, 1982

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    Venus appears to stand as a sobering counterexample for abiotic O2 production, as its postrunaway, mostly desiccated, high-CO2 atmosphere is currently undergoing rapid photolysis-yet Venus does not exhibit a high atmospheric abundance of abiotic O2 or O3.

    “Venus, for example, appears to stand as a sobering counter example for abiotic O2 production, as its postrunaway, mostly desiccated, high-CO2 atmosphere is currently undergoing rapid photolysis—yet Venus does not exhibit a high atmospheric abundance of abiotic O2 or O3.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Planetary atmospheres that favor abiotic O2 or O3 generation are therefore a fundamental planetary property and largely independent of characteristics of the host star, which are more readily observable.

    “Planetary atmospheres that favor abiotic O2 or O3 generation are therefore a fundamental planetary property and largely independent of the characteristics of the host star, which are more readily observable.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Due to more favorable geometry, refraction is not the limiting factor in probing deeper altitudes for planets orbiting later-type M dwarfs, and accessing the lowest atmospheric layer is theoretically possible.

    “However, due to a more favorable geometry, refraction is not the limiting factor in probing deeper altitudes for planets orbiting later-type M dwarfs, and accessing the lowest atmospheric layer is theoretically possible (Bétrémieux and Kaltenegger, 2014; Misra et al., 2014b).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Bétrémieux and Kaltenegger, 2014

      Not checked in the free run.

    • cites Misra et al., 2014b

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    The existing studies suggest that false positives for O2 and O3 in transit spectroscopy can be mitigated by selecting planets in the habitable zones of earlier-type M dwarfs, although this is at odds with other target selection criteria.

    “The existing studies suggest that false positives for O2 and O3 in transit spectroscopy can be mitigated by selecting planets in the habitable zones of earlier-type M dwarfs—although this is clearly at odds with the other target selection criteria described above.”
    Quote in the paperSubstantial claim

  • Classified as probably not a substantial claim.

    Premisecitation

    Recombination of photolyzed CO2 can be slowed by a parent star with a higher FUV to MUV and NUV ratio compared to the Sun, because higher FUV photolyzes CO2 but lower MUV-NUV radiation inhibits the photolysis of water and other HOx chemistry that would drive recombination.

    “It has also been proposed that recombination of photolyzed CO2 can be slowed by a parent star with a higher FUV (λ < 200 nm) to MUV and NUV (200 nm < λ < 440 nm) ratio when compared to the Sun, as the higher FUV photolyzes CO2, but the lower MUV-NUV radiation inhibits the photolysis of water and other HOx chemistry that would drive recombination (Tian et al., 2014; Harman et al., 2015).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Tian et al., 2014

      Not checked in the free run.

    • cites Harman 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

    M dwarf habitable zones are challenging to observe with direct imaging, and first-generation direct imagers such as WFIRST will struggle to spectrally characterize habitable zone super-Earths.

    “However, as discussed, M dwarf habitable zones are challenging to observe with direct imaging, and the first generation of direct imagers (e.g., WFIRST) will struggle to spectrally characterize habitable zone super-Earths.”
    Quote in the paperNot a substantial claim

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

    Interpretationnone given

    The best targets for direct imaging will be orbiting hotter F and G dwarf stars, where catastrophic loss of H2O in the main sequence habitable zone and the corresponding O2 false positive are less likely.

    “The best targets will also be orbiting hotter F and G dwarf stars, where catastrophic loss of H2O in the main sequence habitable zone, and the corresponding O2 false positive, is less likely.”
    Quote in the paperNot a substantial claim

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

    Interpretationreasoning

    To discriminate the low noncondensable gas abiotic O2 mechanism with least ambiguity, observations of O2 and N2 collisional pairs over 0.6 to 4.5 μm for transmission and 0.4 to 4.5 μm for direct imaging would be desirable.

    “Consequently, to discriminate this abiotic O2 generation mechanism with the least ambiguity, observations of O2 and N2 collisional pairs, over a wavelength range from 0.6 to 4.5 μm for transmission and 0.4 to 4.5 μm for direct imaging, would be desirable.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    The lower 0.1 PAL O3 abundance from this abiotic mechanism becomes apparent in other visible O3 features, which are much weaker than Earth's, and could help quantify total O3 column abundance.

    “However, the lower, 0.1 PAL O3 abundance from this abiotic mechanism becomes apparent when looking at other O3 features in the visible—which are much weaker than Earth's—and could be used to help to quantify the total O3 column abundance (Domagal-Goldman et al., 2014).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Domagal-Goldman et al., 2014

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    In the CO2 photolysis mechanism, both CO2 and CO would constitute large fractions of the atmosphere and would be more detectable in transit spectra than the abiotic O2.

    “However, in this mechanism, both CO2 and CO would constitute large fractions of the atmosphere and would be more detectable in transit spectra than the abiotic O2 (Schwieterman et al., 2016a)”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Schwieterman et al., 2016a

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Abiotic O2 buildup was most abundant for planets orbiting M dwarfs, which will be difficult to observe with direct imaging.

    “In addition, abiotic O2 buildup was again most abundant for planets orbiting M dwarfs, which will be difficult to observe with direct imaging.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Tian et al., 2014

      Not checked in the free run.

    • cites Harman et al., 2015

      Not checked in the free run.

  • May combine two claims.

    Interpretationnone given

    In atmospheres where O2 is formed from water photolysis via weak cold-trapping, the atmospheres will not necessarily be depleted in water vapor, so detection of water vapor would not conclusively rule out abiotic O2 production.

    “Although the O2 is formed from water photolysis, these atmospheres will not necessarily be depleted in water vapor, so detection of water vapor would not conclusively rule out abiotic O2 production in this case.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    For the AD Leo case, the false positive is confined to UV wavelengths, which will not be observable for habitable zone exoplanets with JWST, WFIRST, or ground-based extremely large telescopes and must wait for HDST-class telescopes in the 2030+ time frame.

    “In this case then, the false positive is confined to observations at UV wavelengths, which will not be observable for habitable zone exoplanets with JWST, WFIRST, or ground-based extremely large telescopes and must wait for HDST-class telescopes in the 2030+ time frame.”
    Quote in the paperSubstantial claim

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

    Premisenone given

    For Venus, high sulfuric acid hazes truncate the observable atmosphere at 30 mbar at visible wavelengths.

    “This is the case for Venus, where high sulfuric acid hazes truncate the observable atmosphere at 30 mbar at visible wavelengths.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationnone given

    Detection and quantification of pressure-dependent O4 and N4 collisionally induced absorption would be the most unambiguous discriminant, but potentially the most challenging to observe.

    “The most unambiguous discriminant, but potentially the most challenging to observe, would be detection and quantification of pressure-dependent O4 and N4 collisionally induced absorption.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationcitation

    For N2- and H2O-rich habitable zone planets, abiotic generation of O2 or O3 by photolysis is still possible, but models predict relatively small amounts, and O2 will be extremely challenging to detect with next-generation telescopes.

    “Finally, for N2- and H2O-rich habitable zone planets, abiotic generation of O2 or O3 by photolysis is still possible, but in these cases the models predict relatively small amounts, and for the O2 in particular, these will be extremely challenging to detect with next-generation telescopes.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    For N2, absorption from collisional pairs (N4) could be sought near 4.1 μm, and this absorption is broader at high abundance and could help constrain N2 fraction.

    “For N2, absorption from collisional pairs (N4) could be sought near 4.1 μm. This absorption is broader at high abundance and could help constrain N2 fraction (Schwieterman et al., 2015b).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Schwieterman et al., 2015b

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Even at 1 bar of O2, corresponding O4 features in the visible from 0.34 to 0.65 μm become significant, in addition to NIR features at 1.06 and 1.27 μm.

    “even at 1 bar of O2 the corresponding O4 features in the visible from 0.34 to 0.65 μm become significant (see Fig. 1 in Schwieterman et al., 2016b)—in addition to the NIR features at 1.06 and 1.27 μm.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Schwieterman et al., 2016b

      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

    O4 collisionally induced absorption is a broad continuum absorption, and high-resolution ground-based spectroscopy will find it extremely challenging to separate the planet's O4 absorption from that in Earth's atmosphere.

    “It is also important to note that O4 collisionally induced absorption is a broad continuum absorption, and high-resolution ground-based spectroscopy will find it extremely challenging to separate the planet's O4 absorption from that in Earth's atmosphere.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    TRAPPIST-1b and TRAPPIST-1c receive insolation 4.3 and 2.3 times Earth's, respectively, and orbit an M8V dwarf believed older than 500 million years.

    “Similarly, the recently discovered TRAPPIST-1b and TRAPPIST-1c planets (Gillon et al., 2016, 2017), which orbit an M8V dwarf believed to be older than 500 million years, both receive insolation in excess of Venus', at 4.3 and 2.3 times Earth's insolation”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Gillon et al., 2016

      Not checked in the free run.

    • cites Gillon et al., 2017

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    GJ 1132b receives 19 times Earth's insolation and orbits an M3.5V star at least 5 billion years old.

    “A potential candidate for these observational tests includes GJ 1132b (Berta-Thompson et al., 2015) which receives 19 times Earth's insolation (∼3 times Mercury's insolation) and orbits an M3.5V, at least 5-billion-year-old star.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Berta-Thompson et al., 2015

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    The high-FUV M3.5 dwarf AD Leo produced a relatively large absorption feature at UV wavelengths but negligible O2 or O3 absorption features elsewhere in the spectrum.

    “The high-FUV M3.5 dwarf AD Leo also produced a relatively large absorption feature at UV wavelengths but with negligible O2 or O3 absorption features elsewhere in the spectrum.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Domagal-Goldman et al., 2014

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    O2 absorption is directly proportional to the number density of the gas, whereas collisionally induced absorption of O4 has a density-squared dependence, so O4 increases in strength relative to O2 as gas number density increases.

    “In contrast to O2 absorption, which is directly proportional to the number density of the gas, the collisionally induced absorption of O4 has a density-squared dependence, so it increases in strength—relative to O2—as the number density of the gas increases (e.g., Misra et al., 2014a).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Misra et al., 2014a

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    If more desiccated, CO2-rich, O2-poor Venus-like atmospheres are detected, especially for older stars, this may indicate that close-in terrestrial planets can effectively lose or internally sequester O2 from a lost ocean, or that these planets form dry.

    “Conversely, if more desiccated, CO2-rich, O2-poor Venus-like atmospheres are detected, especially for older stars, this may indicate that close-in terrestrial planets are able to effectively lose, or internally sequester, O2 from a lost ocean (e.g., Schaefer et al., 2016), or that these planets form dry.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Schaefer et al., 2016

      Not checked in the free run.

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

    Premisenone given

    Hot Earths orbiting M dwarfs would have too high an insolation level to be habitable.

    “Hot Earths orbiting M dwarfs would have too high an insolation level to be habitable”
    Quote in the paperNot a substantial claim

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

    Interpretationcitation

    Larger-aperture, more sensitive telescopes may be able to measure the altitude-dependent disparity in abiotic O2 abundance if they can take sensitive, time-dependent observations of a planet as it transits.

    “Looking to the future, larger-aperture, more sensitive telescopes may be able to measure this altitude-dependent disparity in abiotic O2 abundance, if they are able to take sensitive, time-dependent observations of a planet as it transits.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    At non-UV wavelengths, N2 is only accessible through its collisional pair at 4.1 μm, and this is a challenging observation to make.

    “While searching for N2 in the planet's spectrum is desirable, at non-UV wavelengths N2 is only accessible through its collisional pair at 4.1 μm, and this is a challenging observation to make (Schwieterman et al., 2015b).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Schwieterman et al., 2015b

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    In Domagal-Goldman et al. (2011), the resulting biosignature was ethane, produced as a by-product of the photolytic destruction of biogenic sulfur gases, rather than a biogenic sulfur gas such as dimethyl sulfide.

    “The resulting biosignature was not, as expected, a biogenic sulfur gas such as dimethyl sulfide, but rather ethane, which was produced as a by-product of the photolytic destruction of the biogenic sulfur gases.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Domagal-Goldman et al. (2011)

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    The presence of strong CO and CO2 in transmission spectra would point to an abiotic origin from CO2 photolysis for observed O2 and would likely be more detectable than O2 itself.

    “The presence of strong CO and CO2 in transmission spectra would also point to an abiotic origin from CO2 photolysis for observed O2 (Wang et al., 2016) and would also likely be more detectable than the O2 itself (Schwieterman et al., 2016a,b).”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Wang et al., 2016

      Not checked in the free run.

    • cites Schwieterman et al., 2016a

      Not checked in the free run.

    • cites Schwieterman et al., 2016b

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    The recently identified abiotic sources of large atmospheric fractions (>6%) of O2 for habitable zone planets are early stellar evolution and planetary atmospheric hydrogen loss, lack of noncondensable species, photochemical production in atmospheres with no aqueous CO sinks, and desiccated CO2-dominated atmospheres.

    “The recently identified abiotic sources of large atmospheric fractions (>6%) of O2 for habitable zone planets are as follows: early stellar evolution and planetary atmospheric hydrogen loss (Luger and Barnes, 2015; Tian, 2015), lack of noncondensable species (Wordsworth and Pierrehumbert, 2014), photochemical production in atmospheres with no aqueous CO sinks (Harman et al., 2015), and desiccated,”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Luger and Barnes, 2015

      Not checked in the free run.

    • cites Tian, 2015

      Not checked in the free run.

    • cites Wordsworth and Pierrehumbert, 2014

      Not checked in the free run.

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

    Interpretationnone given

    Searching for the signs of strong O4 or CO in the planetary spectrum, or the lack of H2O or CH4, could help discriminate between biological and abiotic sources of O2 or O3.

    “Additionally, searching for the signs of strong O4 or CO in the planetary spectrum, or the lack of H2O or CH4, could help discriminate between biological and abiotic sources of O2 or O3.”
    Quote in the paperNot a substantial claim

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

    Interpretationreasoning

    In these cases, observations of O3 and O4 at visible wavelengths and quantification of O4 or N4 at NIR wavelengths could help rule out these scenarios.

    “In these cases, observations of O3 and O4 at visible wavelengths and quantification of O4 or N4 at NIR wavelengths could help rule out these scenarios.”
    Quote in the paperNot a substantial claim

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

    Interpretationreasoning

    To search for life on a distant world, it is not enough to just detect O2 or O3 in the planetary atmosphere; other molecules, including O4, CO, CO2, CH4, H2O, and N4, should also be sought.

    “The recent detailed study of O2 as a biosignature has shown us that to search for life on a distant world it is not enough to just detect O2 or O3 in the planetary atmosphere—other molecules, including O4, CO, CO2, CH4, H2O, and N4 should also be sought, encouraging us to obtain the broadest wavelength range possible when characterizing potentially habitable extrasolar planets.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: Oxygenic photosynthesis on Earth produces atmospheric O2 and has significantly modified Earth's atmosphere.

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

    Interpretationreasoning

    Observations out to 4.5 μm will be desirable to identify abiotic O2 generated by water loss from a low inventory of noncondensable gases and to also pick up stronger CO2 and CO bands to strengthen interpretation of some of the other false-positive mechanisms.

    “whereas observations out to 4.5 μm will be desirable to identify abiotic O2 generated by water loss from a low inventory of noncondensable gases and to also pick up stronger CO2 and CO bands to strengthen interpretation of some of the other false-positive mechanisms.”
    Quote in the paperNot a substantial claim

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

    Interpretationreasoning

    Target selection of host stars more massive than 0.4 M⊙ (M3V and earlier) and planets within the conservative habitable zone may help avoid planets with abundant abiotic O2 due to water loss.

    “Target selection of host stars more massive than 0.4 M⊙ (M3V and earlier) and planets within the conservative habitable zone may help avoid planets with abundant abiotic O2 due to water loss.”
    Quote in the paperNot a substantial claim

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

    Interpretationnone given

    The atmospheric oxygen that photosynthesizers produce has many advantages as a biosignature for first-generation terrestrial planet characterization missions, including its relative abundance and uniform distribution throughout the atmospheric column.

    “The atmospheric oxygen that photosynthesizers produce has many advantages as a biosignature for first-generation terrestrial planet characterization missions, including its relative abundance and uniform distribution throughout the atmospheric column.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: Oxygenic photosynthesis on Earth produces atmospheric O2 and has significantly modified Earth's atmosphere.

  • May combine two claims.

    Interpretationcitation

    For JWST, these false positives will either be too small to be detected with transmission spectroscopy, or telltale signs such as abundant CO and CO2 will likely be seen before the false-positive O2 signal.

    “For the upcoming JWST mission, these false positives will either be too small to be detected with transmission spectroscopy, or telltale signs, such as abundant CO and CO2, will likely be seen in the spectrum of the planet before the false-positive O2 signal is detected (Schwieterman et al., 2016a).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Schwieterman et al., 2016a

      Not checked in the free run.

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

    Interpretationreasoning

    If visible measurements of O3 and O2 are attempted, it may be possible to quantify abiotically generated O3 and discriminate it from the larger fraction expected from a productive photosynthetic biosphere.

    “but if visible measurements of O3 and O2 are also attempted, it may be possible to quantify the abiotically generated O3 and discriminate it from the larger fraction expected from a productive photosynthetic biosphere.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    Biological O2 fluxes are likely concentrated near the surface, resulting in higher near-surface abundances, or if O2 is abundant, an evenly mixed profile throughout the atmospheric column.

    “In contrast, biological O2 fluxes are likely concentrated near the surface, resulting in higher near-surface abundances, or if the O2 is abundant, an O2 profile that is evenly mixed throughout the atmospheric column.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Recent research has improved understanding of potential false positives for O2 and shown that each false-positive mechanism may reveal itself via observable properties of the planet or star.

    “In summary, recent research has improved our understanding of potential false positives for O2 and shown that each false-positive mechanism may reveal itself via observable properties of the planet or star.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationcitation

    The amount of atmospheric CH4, or its absence, will help identify these false positives but will likely be more challenging to detect or set limits on, especially in direct imaging.

    “The amount of atmospheric CH4, or its absence, will also help identify these false positives (Domagal-Goldman et al., 2014) but will likely be more challenging to detect or set limits on, especially in direct imaging.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Domagal-Goldman et al., 2014

      Not checked in the free run.

  • May combine two claims.

    Premiseown data

    Even with 6% O2, the signal is at the 1 ppm level in the simulated JWST transmission spectrum, which is likely not achievable.

    “Even with 6% O2, the signal is at the 1 ppm level in the simulated JWST transmission spectrum (Fig. 2), which is likely not achievable.”
    Quote in the paperSubstantial claim

  • Classified as probably not a substantial claim.

    Interpretationreasoning

    To search for CH4, CO, and CO2, which will help discriminate several other possible abiotic mechanisms, observations out to 2.5 μm will be required.

    “To search for the CH4, CO, and CO2, which will help discriminate several other possible abiotic mechanisms, observations out to 2.5 μm will be required”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Observation of O4 collisional pairs could discriminate O2 generated by atmospheric loss, and O4 may be more detectable than O2 in both direct imaging and transit spectra.

    “Observation of collisional pairs of O2 molecules, O4, could discriminate O2 generated by atmospheric loss, and the O4 molecule may be more detectable than the O2 molecule in both direct imaging and transit spectra (Misra et al., 2014a; Schwieterman et al., 2016a, 2016b).”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Misra et al., 2014a

      Not checked in the free run.

    • cites Schwieterman et al., 2016a

      Not checked in the free run.

    • cites Schwieterman et al., 2016b

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    These mechanisms work predominantly for planets orbiting M dwarf stars, which are more amenable to transmission spectroscopy and could be observed using current and future ground-based telescopes with direct imaging and/or high-resolution spectroscopy.

    “These mechanisms work predominantly for planets orbiting M dwarf stars, which are more amenable to transmission spectroscopy and which could also be observed using current and future ground-based telescopes with direct imaging and/or high-resolution spectroscopy capabilities (e.g., Snellen et al., 2013; Males et al., 2014; Rodler and López-Morales, 2014; Quanz et al., 2015; Crossfield, 2016; Lovis”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Snellen et al., 2013

      Not checked in the free run.

    • cites Males et al., 2014

      Not checked in the free run.

    • cites Rodler and López-Morales, 2014

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    For direct imaging observations, photochemical production of ozone for planets orbiting F dwarf stars, atmospheric loss near the inner edge of the habitable zone, or production of O2 via photolysis in planetary atmospheres with low noncondensable gas fractions could produce the strongest false positives.

    “For direct imaging observations, photochemical production of ozone for planets orbiting F dwarf stars, atmospheric loss near the inner edge of the habitable zone, or production of O2 via photolysis in planetary atmospheres with low noncondensable gas fractions could produce the strongest false positives.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    O2's relative abundance and uniform distribution throughout the atmospheric column allow it to be visible in the presence of planetwide clouds and to be searched for in transmission observations that, due to refraction, may be limited to probing planetary stratospheres for signs of life.

    “This allows O2 to be visible in the presence of planetwide clouds and to be searched for in transmission observations that, due to refraction, may be limited to probing planetary stratospheres for signs of life.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: Oxygenic photosynthesis on Earth produces atmospheric O2 and has significantly modified Earth's atmosphere.

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

    Interpretationreasoning

    An advanced telescope well beyond JWST's capabilities would be needed to make such a measurement.

    “An advanced telescope—well beyond JWST's capabilities—would be needed to make such a measurement.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    The highest abiotic O2 abundance of 6% for a planet orbiting GJ 876 assumed no sinks for O2, and this amount dropped by a factor of 1000 when more realistic boundary conditions were used.

    “The highest abiotic O2 abundance of 6% for a planet orbiting GJ 876 assumed no sinks for O2 in the planetary atmosphere, and this amount dropped by a factor of 1000 when more realistic boundary conditions were used (Harman et al., 2015).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Harman et al., 2015

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Refraction effects on transiting planets allow refracted light prior to ingress and post egress to probe deep into a planet's atmosphere, whereas observations on the star's disk probe higher in the atmosphere.

    “This may be possible using the effects of refraction on observations of transiting planets, as prior to ingress (and post egress), when the planet is not yet on the stellar disk, refracted light can probe deep into a planet's atmosphere, whereas observations of the planet on the star's disk probe higher in the atmosphere (Misra et al., 2014b).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Misra et al., 2014b

      Not checked in the free run.

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

    Interpretationnone given

    Discriminating the false positive due to water loss and the formation of massive O2 atmospheres can be done with observations of O4 absorption in the visible and NIR, requiring wavelength ranges from 0.4 to 1.0 μm for direct imaging and 0.6 to 1.3 μm for transmission.

    “Discriminating the false positive due to water loss and the formation of massive O2 atmospheres can be done with observations of O4 absorption in the visible and NIR, requiring wavelength ranges from 0.4 to 1.0 μm for direct imaging and 0.6 to 1.3 μm for transmission.”
    Quote in the paperNot a substantial claim
157 of 157

  • The paper relies on this without support.

    Oxygenic photosynthesis is Earth's dominant metabolism.

    “Oxygenic photosynthesis is Earth's dominant metabolism”

  • The paper relies on this without support.

    Photosynthetic O2 has long been considered particularly robust as a sign of life on a habitable exoplanet due to the lack of known false positives.

    “Photosynthetic O2 has long been considered particularly robust as a sign of life on a habitable exoplanet, due to the lack of known “false positives””

  • The paper relies on this without support.

    The strongest and most widely recognized sign of life on Earth is photosynthetically generated O2.

    “The strongest and most widely recognized sign of life on our planet is photosynthetically generated O2”

  • The paper relies on this without support.

    O2 is evenly mixed throughout Earth's atmospheric column and composes 21% of the atmosphere.

    “which is evenly mixed throughout the atmospheric column and composes 21% of our atmosphere”

  • The authors state this as an assumption.

    Redox balance holds for an Earth-like 288 K planet, with loss of hydrogen to space balanced by the surface removal rate of oxidizing material.

    “Under the assumption of redox balance—with loss of hydrogen to space balanced by the surface removal rate of oxidizing material for an Earth-like (288 K surface temperature) planet—the resultant atmospheric fraction of O2 was found to be 0.15 bar (Wordsworth and Pierrehumbert, 2014).”

  • The authors state this as an assumption.

    For Proxima Centauri b, the host star's XUV flux was constant for the first 3 billion years.

    “estimates for maximum ocean loss during the pre-main sequence phase span less than one ocean, assuming a constant XUV flux for the host star for the first 3 billion years (Ribas et al., 2016)”

  • The authors state this as an assumption.

    For Proxima Centauri b, the XUV flux scales with the decreasing bolometric luminosity.

    “to 3–10 oceans lost, assuming that the XUV scales with the decreasing bolometric luminosity (Barnes et al., 2016).”

  • The authors state this as an assumption.

    In the Hu et al. (2012) scenario, there was no volcanic outgassing.

    “Earlier work by Hu et al. (2012) had also been able to generate high levels of O3 in CO2-dominated atmospheres orbiting Sun-like stars using a similar mechanism, but only if the assumption of no volcanic outgassing was used.”

  • The authors state this as an assumption.

    In the Tian et al. (2014) model cases with the highest O2 and O3 values, CO and O2 cannot recombine in seawater, implying no aqueous sink for CO.

    “Additionally, the highest O2 and O3 values modeled were associated with an assumption that CO and O2 cannot recombine in seawater, which therefore assumes no aqueous sink for CO, as well.”

  • The paper relies on this without support.

    Terrestrial exoplanets with different N2 inventories are entirely plausible.

    “Terrestrial exoplanets with different N2 inventories are entirely plausible, and the delivery and loss of volatiles from a terrestrial planet are dependent on a number of planetary and exogenous processes that will be difficult to know a priori.”

  • The authors state this as an assumption.

    A redox imbalance at the atmosphere-ocean interface would increase the ocean concentration of the species contributing to the redox imbalance.

    “They assumed that a redox imbalance at the atmosphere-ocean interface would work to increase the ocean concentration of the species contributing to the redox imbalance.”

  • The paper relies on this without support.

    Oxygenic photosynthesis on Earth produces atmospheric O2 and has significantly modified Earth's atmosphere.

    “Phototrophs have developed a metabolism that can capitalize on plentiful sunlight, abundant water, and the carbon dioxide in our atmosphere to dominate our planet's surface biosphere and significantly modify our planet's atmosphere.”

  • The authors state this as an assumption.

    The existence of abiotic sources is likely true, or should at least be assumed, for all proposed biosignatures, given the complexity and probable diversity of planetary environments.

    “However, the existence of abiotic sources is likely true, or should at least be assumed, for all proposed biosignatures, given the complexity and probable diversity of planetary environments.”
13 of 13

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.

  • 0.2% O2 is not 10−3 of the stated present atmospheric level

    Major

    The paper states that Earth's present O2 abundance is 21%. One-thousandth of 21% is 0.021%, not 0.2%; the stated equivalence overstates the modeled abundance by about a factor of ten.

    Reported 0.2 · recomputed 0.021

1 of 1

Cites

Flagged first, then by citations

Research works printed but not linked: 5

  1. Baraffe I., 1998, Astron Astrophys, 337, 403
  2. Henry T.J., 2016, AAS Meeting Abstracts
  3. Meadows V., 2010, Exoplanets, 441
  4. Spergel D., 2015, arXiv preprint
  5. Walker J.C., 1977, Evolution of the Atmosphere

Cited by

Sorted by citations

Related

Authors
Meadows, V. S.
Affiliations
NASA Astrobiology Institute (US)
Corresponding author
Meadows, Victoria S.
ORCID
Meadows, Victoria S.
Venue
Astrobiology
Publisher
SAGE Publications
Published
2017-10
Volume
17
Issue
10
Pages
1022-1052
ISSN
  • 1531-1074 (ISSN-L)
  • 1557-8070
DOI
10.1089/ast.2016.1578
PMC
PMC5655594
OpenAlex
W2610516798
Cited by
173
References
264
Citations by year
2026: 10 · 2025: 20 · 2024: 12 · 2023: 19 · 2022: 17 · 2021: 19 · 2020: 16 · 2019: 22 · 2018: 32 · 2017: 4
FWCI
11.07
Type
journal article
Language
en
Open access
Licence
www.liebertpub.com/nv/resources-tools/text-and-data-mining-policy/121/ (tdm)
Locations
  • Astrobiology (publishedVersion)
  • Europe PMC (PubMed Central) (publishedVersion, cc-by-nc)
  • PubMed Central (publishedVersion, cc-by-nc)
  • PubMed (publishedVersion)
Funders
  • National Aeronautics and Space Administration (NNA13AA93A, NNH12ZDA002C)
  • NASA Astrobiology Institute (NNH12ZDA002C, NNA13AA93A)
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 atmospheres, oxygenic photosynthesis, false positives, ozone, M dwarf planets, habitable zone, atmospheric escape, planetary spectra, life detection, water loss
MeSH terms
Atmosphere, Extraterrestrial Environment, Oxygen, Water, Planets, Exobiology, Life, Photochemical Processes, Telescopes

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.

Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.

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