ABIOTIC O<sub>2</sub>LEVELS ON PLANETS AROUND F, G, K, AND M STARS: POSSIBLE FALSE POSITIVES FOR LIFE?

Harman, Chester E.; Schwieterman, Edward W.; Schottelkotte, J. C.; Kasting, James F.

doi:10.1088/0004-637x/812/2/137

Cited by 204 publications

(317 citation statements)

References 146 publications

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“…Tian et al (2014) calculated that under these UV conditions, O 2 and O 3 could be 2-3 orders of magnitude greater than in the atmospheres of HZ planets around Sun-like stars producing a potential false-positive detection of a biosignature (Harman et al 2015). In a few cases, likely during a flare, we see significant deviation of the GALEX [f F U V /f N U V ] G ratio from the norm for a given star reaching levels >1.…”

Section: Discussionmentioning

confidence: 85%

HAZMAT. II. Ultraviolet Variability of Low-mass Stars in the GALEX Archive

Miles

Shkolnik

2017

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The ultraviolet (UV) light from a host star influences a planet's atmospheric photochemistry and will affect interpretations of exoplanetary spectra from future missions like the James Webb Space Telescope. These effects will be particularly critical in the study of planetary atmospheres around M dwarfs, including Earth-sized planets in the habitable zone. Given the higher activity levels of M dwarfs compared to Sun-like stars, time resolved UV data are needed for more accurate input conditions for exoplanet atmospheric modeling. The Galaxy Evolution Explorer (GALEX ) provides multi-epoch photometric observations in two UV bands: near-ultraviolet (NUV; 1771 -2831Å) and far-ultraviolet (FUV; 1344 -1786Å). Within 30 pc of Earth, there are 357 and 303 M dwarfs in the NUV and FUV bands, respectively, with multiple GALEX observations. Simultaneous NUV and FUV detections exist for 145 stars in both GALEX bands. Our analyses of these data show that low-mass stars are typically more variable in the FUV than the NUV. Median variability increases with later spectral types in the NUV with no clear trend in the FUV. We find evidence that flares increase the FUV flux density far more than the NUV flux density, leading to variable FUV to NUV flux density ratios in the GALEX bandpasses.The ratio of FUV to NUV flux is important for interpreting the presence of atmospheric molecules in planetary atmospheres such as oxygen and methane as a high FUV to NUV ratio may cause false-positive biosignature detections. This ratio of flux density in the GALEX bands spans three orders of magnitude in our sample, from 0.008 to 4.6, and is 1 to 2 orders of magnitude higher than for G dwarfs like the Sun. These results characterize the UV behavior for the largest set of low-mass stars to date.

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Section: Discussionmentioning

confidence: 85%

HAZMAT. II. Ultraviolet Variability of Low-mass Stars in the GALEX Archive

Miles

Shkolnik

2017

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“…Then, the difficulty will be to discard falsepositivedetections of O 2 . For instance, dioxygen may not necessarily be a biosignature (Domagal-Goldman et al 2014;Wordsworth & Pierrehumbert 2014;Harman et al 2015;Schwieterman et al 2016aSchwieterman et al , 2016b. Lifeless, terrestrial planets in the habitable zone of any type of star can develop oxygendominated atmospheres through photolysis of H 2 O (e.g., Wordsworth & Pierrehumbert 2014) or CO 2 (e.g., Harman et al 2015).…”

Section: Discussionmentioning

confidence: 99%

“…However, O 2 due to CO 2 photolysis would not be detectable with current and planned space-and ground-based instruments on planets around F-and G-type stars. Planets around Kstarsand especially Mstarsmay produce detectable abiotic O 2 because of the low-UVflux from their parent stars (Harman et al 2015). Also, strong O 4 features that could be visible in transmitted spectra at 1.06 and 1.27 μm or in UV/VIS/NIR reflected light spectra by a next-generation directimaging telescopesuch as LUVOIR/HDST or HabEx could be a sign of an abiotic dioxygen-dominated atmosphere that suffered massive Hescape (see Schwieterman et al 2016aSchwieterman et al , 2016b.…”

Section: Discussionmentioning

confidence: 99%

The O₂ A-Band in the Fluxes and Polarization of Starlight Reflected by Earth-Like Exoplanets

Fauchez

Rossi

Stam

2017

ApJ

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Earth-like, potentially habitable exoplanets are prime targets in the search for extraterrestrial life. Information about their atmospheres and surfaces can be derived by analyzing thelight of the parent star reflected by the planet. We investigate the influence of the surface albedo A s , the optical thickness b cloud ,thealtitude of water clouds, and the mixing ratio of biosignature O 2 on the strength of the O 2 A-band (around 760 nm) in theflux and polarization spectra of starlight reflected by Earth-like exoplanets. Our computations for horizontally homogeneous planets show that small mixing ratios (η<0.4) will yield moderately deep bands in flux and moderate-to-small band strengths in polarization, and that clouds will usually decrease the band depth in flux and the band strength in polarization. However, cloud influence will be strongly dependent onproperties such as optical thickness, top altitude, particle phase, coverage fraction, andhorizontal distribution. Depending on the surface albedoand cloud properties, different O 2 mixing ratios η can give similar absorption-band depths in flux and band strengths in polarization, especially if the clouds have moderate-to-high optical thicknesses. Measuring both the flux and the polarization is essential to reduce the degeneracies, although it will not solve them,especiallynot for horizontally inhomogeneous planets. Observations at a wide range of phase angles and with a high temporal resolution could help to derive cloud properties and, once those are known, the mixing ratio of O 2 or any other absorbing gas.

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“…High-energy stellar flux heats upper planetary atmospheres and initiates photochemistry (e.g., Lammer et al 2007;Miguel et al 2015;Rugheimer et al 2015;Arney et al 2017). UVdriven photochemistry can produce and destroy potential biosignatures (O 2 , O 3 , and CH 4 ) and habitability indicators (H 2 O and CO 2 ) in exoplanet atmospheres (Hu et al 2012;Domagal-Goldman et al 2014;Tian et al 2014;Wordsworth & Pierrehumbert 2014;Gao et al 2015;Harman et al 2015;Luger & Barnes 2015). In particular, the ratio of far-to near-UV flux determines which photochemical reactions will dominate and thus the resultant planetary atmosphere.…”

Section: Uv and Ca Iikemission From M Dwarfsmentioning

confidence: 99%

The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs

Youngblood

Loyd

Brown

et al. 2017

ApJ

104

113

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Characterizing the UV spectral energy distribution (SED) of an exoplanet host star is critically important for assessing its planet's potential habitability, particularly for M dwarfs, as they are prime targets for current and nearterm exoplanet characterization efforts and atmospheric models predict that their UV radiation can produce photochemistry on habitable zone planets different from that on Earth. To derive ground-based proxies for UV emission for use when Hubble Space Telescope (HST) observations are unavailable, we have assembled a sample of 15 early to mid-M dwarfs observed by HST and compared their nonsimultaneous UV and optical spectra. We find that the equivalent width of the chromospheric Ca IIKline at 3933 Å, when corrected for spectral type, can be used to estimate the stellar surface flux in ultraviolet emission lines, including H ILyα. In addition, we address another potential driver of habitability: energetic particle fluxes associated with flares. We present a new technique for estimating soft X-ray and >10 MeV proton flux during far-UV emission line flares (Si IV and He II) by assuming solar-like energy partitions. We analyze several flares from the M4 dwarf GJ 876 observed with HST and Chandra as part of the MUSCLES Treasury Survey and find that habitable zone planets orbiting GJ 876 are impacted by large Carrington-like flares with peak soft X-ray fluxes 10 −3 W m −2 and possible proton fluxes ∼10 2 -10 3 pfu, approximately four orders of magnitude more frequently than modern-day Earth.

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

Cited by 204 publications

References 146 publications

HAZMAT. II. Ultraviolet Variability of Low-mass Stars in the GALEX Archive

HAZMAT. II. Ultraviolet Variability of Low-mass Stars in the GALEX Archive

The O₂ A-Band in the Fluxes and Polarization of Starlight Reflected by Earth-Like Exoplanets

The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs

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

Cited by 204 publications

References 146 publications

HAZMAT. II. Ultraviolet Variability of Low-mass Stars in the GALEX Archive

HAZMAT. II. Ultraviolet Variability of Low-mass Stars in the GALEX Archive

The O2 A-Band in the Fluxes and Polarization of Starlight Reflected by Earth-Like Exoplanets

The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs

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

The O₂ A-Band in the Fluxes and Polarization of Starlight Reflected by Earth-Like Exoplanets