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

doi:10.1089/ast.2016.1589

. 2018 Feb;18(2):133-189.

doi: 10.1089/ast.2016.1589. Epub 2018 Feb 12.

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

Victoria S Meadows^{1

2}, Giada N Arney^{1

2

3}, Edward W Schwieterman^{1

2

4

5}, Jacob Lustig-Yaeger^{1

2}, Andrew P Lincowski^{1

2}, Tyler Robinson^{2

6}, Shawn D Domagal-Goldman^{2

7}, Russell Deitrick^{1

2}, Rory K Barnes^{1

2}, David P Fleming^{1

2}, Rodrigo Luger^{1

2}, Peter E Driscoll^{2

8}, Thomas R Quinn^{1

2}, David Crisp^{2

9}

Affiliations

¹ 1 Astronomy Department, University of Washington , Seattle, Washington.
² 2 NASA Astrobiology Institute-Virtual Planetary Laboratory Lead Team , USA .
³ 3 Planetary Systems Laboratory, NASA Goddard Space Flight Center , Greenbelt, Maryland.
⁴ 4 NASA Postdoctoral Program, Universities Space Research Association , Columbia, Maryland.
⁵ 5 Department of Earth Sciences, University of California at Riverside , Riverside, California.
⁶ 6 Department of Astronomy and Astrophysics, University of California , Santa Cruz, California.
⁷ 7 Planetary Environments Laboratory, NASA Goddard Space Flight Center , Greenbelt, Maryland.
⁸ 8 Department of Terrestrial Magnetism, Carnegie Institution for Science , Washington, DC.
⁹ 9 Jet Propulsion Laboratory, California Institute of Technology , Pasadena, California.

PMID: 29431479
PMCID: PMC5820795
DOI: 10.1089/ast.2016.1589

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

Victoria S Meadows et al. Astrobiology. 2018 Feb.

. 2018 Feb;18(2):133-189.

doi: 10.1089/ast.2016.1589. Epub 2018 Feb 12.

Authors

Affiliations

¹ 1 Astronomy Department, University of Washington , Seattle, Washington.
² 2 NASA Astrobiology Institute-Virtual Planetary Laboratory Lead Team , USA .
³ 3 Planetary Systems Laboratory, NASA Goddard Space Flight Center , Greenbelt, Maryland.
⁴ 4 NASA Postdoctoral Program, Universities Space Research Association , Columbia, Maryland.
⁵ 5 Department of Earth Sciences, University of California at Riverside , Riverside, California.
⁶ 6 Department of Astronomy and Astrophysics, University of California , Santa Cruz, California.
⁷ 7 Planetary Environments Laboratory, NASA Goddard Space Flight Center , Greenbelt, Maryland.
⁸ 8 Department of Terrestrial Magnetism, Carnegie Institution for Science , Washington, DC.
⁹ 9 Jet Propulsion Laboratory, California Institute of Technology , Pasadena, California.

PMID: 29431479
PMCID: PMC5820795
DOI: 10.1089/ast.2016.1589

Abstract

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

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Conflict of interest statement

No competing financial interests exist.

Figures

<b>FIG. 1.</b> — **FIG. 1.**
Earth's atmospheric transmittance calculated by SMART and used for simulations of ground-based observations.

<b>FIG. 2.</b> — **FIG. 2.**
The spectrum of Proxima Centauri (at the distance of planet b) compared to the solar spectrum. Proxima Centauri b receives about 0.66 times the insolation Earth receives at 1 AU from the Sun.

<b>FIG. 3.</b> — **FIG. 3.**
Input spectral surface albedos for modeled planetary scenarios. Composite 1 is a weighted average of 65.6% seawater, 13.6% grassland/brush, 4% conifer forest, 5.5% soil/desert (kaolinite), and 11.3% snow/ice. Composite 2 is 65.6% seawater, 23.1% soil/desert, and 11.3% snow/ice. All surface spectral albedos are sourced from the USGS spectral library (Clark *et al.,* 2007), except for the conifer forest, which is from the ASTER spectral library (Baldridge *et al.,* 2009).

<b>FIG. 4.</b> — **FIG. 4.**
Temperature (top x axes; black dashed) and gas mixing ratio profiles (bottom x axes) for the self-consistent high-O₂ (95%), post-runaway atmospheres with a surface ocean remaining (left) and completely desiccated (right). Differences in temperature and O₃ profiles are primarily driven by the presence or absence of water vapor.

<b>FIG. 5.</b> — **FIG. 5.**
Temperature (top axes; black dashed) and gas mixing ratio profiles (bottom axes) for a 10 bar (left) and 90 bar (right) O₂-rich and CO₂-rich atmosphere with 45% CO₂, 45% O₂, and ∼10% N₂.

<b>FIG. 6.</b> — **FIG. 6.**
Temperature (top x axes; black dashed) and gas mixing ratio profiles (bottom x axes) for a 10 bar (left) and a 90 bar (right) Venus-like, CO₂ atmosphere. The H₂SO₄ saturation mixing ratio profile is shown as the black dotted curve, which traces the true H₂SO₄ mixing ratio for much of the atmosphere, indicating that H₂SO₄ is fully saturated and clouds could condense. The differences in gas mixing ratio profiles between these two atmospheres are due to adjustments made to coincide with the regions of cloud formation. Like Venus, H₂SO₄ aerosols are present down to approximately 10 bar in both cases. The temperature profiles are similar above 0.1 bar, where atmospheres are generally optically thin, but the 90 bar atmosphere reaches much hotter temperatures at the surface due to the longer dry adiabat.

<b>FIG. 7.</b> — **FIG. 7.**
Temperature (top x axis; black dashed) and gas mixing ratios (bottom x axis) for a desiccated CO₂/O₂/CO atmosphere. CO₂, O₂, and CO are the most abundant gases and result from photochemistry of outgassed CO₂. Only major and spectrally observable gases are shown. The temperature profile is the result of VPL Climate using the gas mixing ratio values of Gao *et al.* (; case 4).

<b>FIG. 8.</b> — **FIG. 8.**
Temperature (top x axes; black dashed) and gas mixing ratio profiles (bottom x axes) for the preindustrial modern Earth orbiting the Sun (left) and a photochemically self-consistent Earth-like atmosphere for Proxima Centauri b (right). The mixing ratios of trace gas species CH₄, N₂O, and CO for Proxima Centauri b are determined from the flux required to produce their preindustrial concentration in Earth's atmosphere (see Section 4.2.3.1). Given the same fluxes as Earth, CH₄, CO, and N₂O would exist in Proxima Centauri b's atmosphere in much greater abundance. Note that a CO₂ abundance of 5% was used for the Earth-like Proxima Centauri b planet, as that abundance is required to raise the globally averaged surface temperature to 273 K.

<b>FIG. 9.</b> — **FIG. 9.**
Temperature (top x axes; black dashed) and gas mixing ratio profiles (bottom x axes) for Archean analog planets orbiting Proxima Cen b. The moderate temperature inversion seen in the right (1.5% CH₄) plot is due to UV absorption by the haze. UV shielding by the haze in the 1.5% CH₄ plot (right) prevents photolysis of gases such as methane and ethane at higher altitudes than in the 1% CH₄ plot (left). Note the temperature profile becomes an isoprofile above the top of the climate model grid when passed into the photochemical model.

<b>FIG. 10.</b> — **FIG. 10.**
Archean analog haze particle number density (bottom x axis; solid line) and particle radius (top x axis; dashed line) for a hazy planet orbiting Proxima Centauri b. Note the sharp decrease in particle number density (and corresponding increase in particle size) at about 60 km in altitude where fractal particle formation begins to occur.

<b>FIG. 11.</b> — **FIG. 11.**
Synthetic thermal phase curve variation spectrum of the photochemically self-consistent Earth-like atmospheric case with 50% cloud coverage (top panel), the hazy Archean-like atmospheric case with 50% cloud coverage (second panel), the desiccated O₂-rich atmospheric case (third panel), and the 90 bar cloudy Venus-like atmospheric case (bottom panel). The solid and gray lines show the emitted flux from the dayside of the planet (observed at 0°) minus the emitted flux from the nightside of the planet (observed at 180°), divided by the average flux received from the star plus planet. The solid line depicts the maximum possible signal strength for a phase curve spectrum of the planet in the case that there is no flux emitted from the nightside, while the gray line shows the signal strength for a case with a nightside 20 K cooler than the dayside. The colored horizontal lines show the high-resolution model spectrum convolved with the 7 JWST/MIRI filter bands longward of where Proxima Centauri exceeds the instrument brightness limit. Each upper-left inset shows the phase-dependent planet-to-star flux contrast ratios in the MIRI filter bands for the cases with no flux from the nightside (solid lines) and with a 20 K cooler nightside (dotted lines).

<b>FIG. 12.</b> — **FIG. 12.**
Reflected light spectra of the 10 bar, high-O₂ (95%) atmospheres (cloud free) with a surface ocean remaining (top) and completely desiccated (bottom). We define the reflectivity (π*I/F*) of the planet as the outgoing top of atmosphere flux (πI), where I is the radiance, divided by the stellar flux incident at the top of the atmosphere (F). Note the strong O₄ bands present in the UV/VIS/NIR. Both atmospheres were simulated with 0.5% CO₂.

<b>FIG. 13.</b> — **FIG. 13.**
Similar to the reflected light spectra of Fig. 12, but for the clear O₂-CO₂ (45% O₂, 45% CO₂, 10% N₂, 20 ppm H₂O) atmospheres with surface pressures of 10 bar (top) and 90 bar (bottom) for comparison with O₂-only and Venus cases.

<b>FIG. 14.</b> — **FIG. 14.**
Similar to the reflected light spectra of Fig. 12, but for the Venus-like worlds with 10 bar (top) and 90 bar (bottom) CO₂ atmospheres. Strong CO₂ absorption features are present at several wavelengths. The two spectra are similar because the cloud decks in both atmospheres occur at roughly the same pressure, so about the same amount of total atmosphere is sensed in each spectrum.

<b>FIG. 15.</b> — **FIG. 15.**
Similar to the reflected light spectra of Fig. 12, but for the 1 bar desiccated CO₂/O₂/CO atmosphere with a wavelength-dependent Mars average surface albedo (top panel), preindustrial Earth (middle panel), and Archean Earth (bottom panel). The top panel shows a desiccated planet with an outgassed CO₂ atmosphere, which can support a stable CO₂/O₂/CO/O₃ atmosphere without life (Gao *et al.,* 2015). The lack of H₂O is an indicator of the abiotic nature of the atmospheric O₂. The middle panel shows Proxima Centauri b simulated as an Earth-like planet with 21% O₂ and 5% CO₂, and 50% cloud cover. The UV to NIR spectrum contains features from Rayleigh scattering, O₃, O₂, H₂O, CO₂, and CH₄. The bottom panel shows Archean Earth-like planets with a photochemically self-consistent organic haze (orange) and without a haze (purple). Note the overlap of some H₂O and CH₄ absorption bands and the strong haze absorption feature at short wavelengths. The surface albedo assumes the composite albedo described in Section 3.7.5 with no vegetation.

<b>FIG. 16.</b> — **FIG. 16.**
The effect of the host star's SED on the planet's composition and spectrum. For comparison, the Earth orbiting the Sun (orange) and the photochemically self-consistent Earth orbiting Proxima Centauri (purple) are shown in reflected light (top) and in transmission (bottom). For the transmission spectra, the Earth-Sun case is adjusted so that its maximum tangent pressure matches that expected for an Earth-like planet in orbit around Proxima Centauri at Proxima Centauri b's orbital position. The surface fluxes were kept constant in both cases. These plots illustrate the strong impact of the star's spectrum on atmospheric photochemistry and composition, and that Earth itself cannot be used to represent a planet with Earth-like surface fluxes orbiting an M dwarf. The primary differences in the spectra are in the NIR and are due to differences in total methane concentration, which is much higher for Proxima Centauri b. Absorption from O₃ is also sensitive to the host star spectrum, and this is most prominent in the transmission spectrum. Note that differences in the stellar SEDs have been divided out. A cloud cover fraction of 50% is simulated for both cases.

<b>FIG. 17.</b> — **FIG. 17.**
Transmission spectra of the 10 bar, 95% O₂ atmospheres with a surface water ocean (purple) and with complete desiccation (orange). The effective radius of the atmosphere (in km) is shown on the left y axis to emphasize the vertical extent of the absorption features, while the transit depth is shown on the right y axis to emphasize the strength and detectability of spectral features if Proxima Cen b were observed to transit. The contrasts in O₃ absorption stem from the extent to which O₃ remains abundant deep in the atmosphere, which is ultimately due to the presence or absence of water vapor.

<b>FIG. 18.</b> — **FIG. 18.**
Similar to the transmission spectra of Fig. 17, but for the O₂-CO₂ atmospheres with surface pressures of 10 bar (purple) and 90 bar (orange). The two spectra are almost identical except that the pressure at which the atmosphere becomes optically thick, while the same for both planets, occurs at a higher altitude in the 90 bar case.

<b>FIG. 19.</b> — **FIG. 19.**
Similar to the transmission spectra of Fig. 17, but for the Venus-like worlds with 90 bar (orange) and 10 bar (purple) CO₂ atmospheres. H₂SO₄ absorption features can be seen at IR wavelengths, and the spectrum is flat at wavelengths shorter than 1 μm.

<b>FIG. 20.</b> — **FIG. 20.**
Similar to the transmission spectra of Fig. 17, but the top panel is for the heavily H-depleted CO₂/O₂/CO atmosphere in photochemical equilibrium from Section 4.2.2.3 (also see Gao *et al.,* 2015). Note the strong CO bands at 2.35 and 4.6 μm compared to other cases. The middle panel shows the photochemically self-consistent modern Earth (black) with preindustrial biological fluxes of CH₄, CO, and N₂O orbiting Proxima Centauri b. It also shows Earth's spectrum with CH₄ (orange) and H₂O (blue) removed, to show the relative contributions of each to the spectrum. The weak H₂O features result from transmission probing higher, drier altitudes down to about 10 km, but water being mostly concentrated in the lower portion of the troposphere, below ∼12 km. In addition to their intrinsic weakness, the H₂O features are also swamped by CH₄ absorption, which also raises the effective tangent height to drier altitudes. The bottom panel shows the Archean Earth-like planets with (orange) and without (purple) organic haze orbiting Proxima Centauri. A haze absorption feature is present near 6 μm, and haze produces the spectral slope continuing into the NIR for the hazy spectrum.

<b>FIG. 21.</b> — **FIG. 21.**
Simulated coronagraph spectrum (left column) and the SNR in each spectral element (right column) for the 10 bar O₂-rich planet with an ocean surface using three different future telescope concepts: a 6.5 m HabEx (top row), a 16 m LUVOIR (middle row), and a 30 m ground-based telescope (bottom row). The simulated observations, showing 1σ errors, and the SNR calculations assume an integration time of 10 h per nulling bandwidth. Dashed vertical lines are placed at IWA = 1λ/D (red), 2λ/D (green), and 3λ/D (blue) to show the long-wavelength limit for the given telescope diameter and planet-star angular separation. All spectra are shown for a spectral resolution of R = 70.

<b>FIG. 22.</b> — **FIG. 22.**
Similar to the coronagraph simulations of Fig. 21, but for the desiccated 10 bar O₂-rich planet.

<b>FIG. 23.</b> — **FIG. 23.**
Similar to the coronagraph simulations of Fig. 21, but for the 90 bar cloud-covered Venus-like planet.

<b>FIG. 24.</b> — **FIG. 24.**
Similar to the coronagraph simulations of Fig. 21, but for the desiccated CO₂/O₂/CO planet.

<b>FIG. 25.</b> — **FIG. 25.**
Similar to the coronagraph simulations of Fig. 21, but for the photochemically self-consistent modern Earth-like planet.

<b>FIG. 26.</b> — **FIG. 26.**
Similar to the coronagraph simulations of Fig. 21, but for the hazy Archean Earth-like planet.

<b>FIG. 27.</b> — **FIG. 27.**
Optical spectra and the corresponding colors that the human eye would perceive for Proxima Centauri and five of our simulated planetary states. From upper left to lower right: the M5.5V red dwarf star Proxima Centauri, Earth-like with no clouds, hazy Archean with no clouds, O₂-dominated with an ocean, desiccated O₂-dominated, and cloudy Venus-like. The background of each plot is colored as it would appear to the human eye. The shaded area under the curve represents the individual color of each wavelength at the simulated spectral resolution. Each planetary spectrum represents a convolution of the stellar spectrum with the planetary albedo, which is dictated by the planet's surface and atmospheric composition. Furthermore, each perceived color represents a convolution of the planetary spectrum with the human eye.

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References

1. Airapetian V.S., Glocer A., Khazanov G.V., Loyd R.O.P., France K., Sojka J., Danchi W.C., and Liemohn M.W. (2017) How hospitable are space weather affected habitable zones? The role of ion escape. Astrophys J 836:L3
1. Albarède F. (2009) Volatile accretion history of the terrestrial planets and dynamic implications. Nature 461:1227–1233 - PubMed
1. Anglada-Escudé G., Amado P.J., Barnes J., Berdiñas Z.M., Butler R.P., Coleman G.A., de la Cueva I., Dreizler S., Endl M., Giesers B. J.effers S.V., Jenkins J.S., Jones H.R., Kiraga M., Kürster M., López-González M.J., Marvin C.J., Morales N., Morin J., Nelson R.P., Oritz J.L., Ofir A., Paardekooper S.-J., Reiners A., Rodríguez E., Rodríguez-López C., Sarmiento L.F., Strachan J.P., Tsapras Y., Tuomi M., and Zechmeister M. (2016) A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature 536:437–440 - PubMed
1. Armstrong J.C., Barnes R., Domagal-Goldman S., Breiner J., Quinn T.R., and Meadows V.S. (2014) Effects of extreme obliquity variations on the habitability of exoplanets. Astrobiology 14:277–291 - PMC - PubMed
1. Arney G., Meadows V., Crisp D., Schmidt S.J., Bailey J., and Robinson T. (2014) Spatially resolved measurements of H2O, HCl, CO, OCS, SO2, cloud opacity, and acid concentration in the Venus near-infrared spectral windows. J Geophys Res: Planets 119:1860–1891

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[1] Airapetian V.S., Glocer A., Khazanov G.V., Loyd R.O.P., France K., Sojka J., Danchi W.C., and Liemohn M.W. (2017) How hospitable are space weather affected habitable zones? The role of ion escape. Astrophys J 836:L3

[2] Airapetian V.S., Glocer A., Khazanov G.V., Loyd R.O.P., France K., Sojka J., Danchi W.C., and Liemohn M.W. (2017) How hospitable are space weather affected habitable zones? The role of ion escape. Astrophys J 836:L3

[3] Albarède F. (2009) Volatile accretion history of the terrestrial planets and dynamic implications. Nature 461:1227–1233 - PubMed

[4] Albarède F. (2009) Volatile accretion history of the terrestrial planets and dynamic implications. Nature 461:1227–1233 - PubMed

[5] Anglada-Escudé G., Amado P.J., Barnes J., Berdiñas Z.M., Butler R.P., Coleman G.A., de la Cueva I., Dreizler S., Endl M., Giesers B. J.effers S.V., Jenkins J.S., Jones H.R., Kiraga M., Kürster M., López-González M.J., Marvin C.J., Morales N., Morin J., Nelson R.P., Oritz J.L., Ofir A., Paardekooper S.-J., Reiners A., Rodríguez E., Rodríguez-López C., Sarmiento L.F., Strachan J.P., Tsapras Y., Tuomi M., and Zechmeister M. (2016) A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature 536:437–440 - PubMed

[6] Anglada-Escudé G., Amado P.J., Barnes J., Berdiñas Z.M., Butler R.P., Coleman G.A., de la Cueva I., Dreizler S., Endl M., Giesers B. J.effers S.V., Jenkins J.S., Jones H.R., Kiraga M., Kürster M., López-González M.J., Marvin C.J., Morales N., Morin J., Nelson R.P., Oritz J.L., Ofir A., Paardekooper S.-J., Reiners A., Rodríguez E., Rodríguez-López C., Sarmiento L.F., Strachan J.P., Tsapras Y., Tuomi M., and Zechmeister M. (2016) A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature 536:437–440 - PubMed

[7] Armstrong J.C., Barnes R., Domagal-Goldman S., Breiner J., Quinn T.R., and Meadows V.S. (2014) Effects of extreme obliquity variations on the habitability of exoplanets. Astrobiology 14:277–291 - PMC - PubMed

[8] Armstrong J.C., Barnes R., Domagal-Goldman S., Breiner J., Quinn T.R., and Meadows V.S. (2014) Effects of extreme obliquity variations on the habitability of exoplanets. Astrobiology 14:277–291 - PMC - PubMed

[9] Arney G., Meadows V., Crisp D., Schmidt S.J., Bailey J., and Robinson T. (2014) Spatially resolved measurements of H2O, HCl, CO, OCS, SO2, cloud opacity, and acid concentration in the Venus near-infrared spectral windows. J Geophys Res: Planets 119:1860–1891

[10] Arney G., Meadows V., Crisp D., Schmidt S.J., Bailey J., and Robinson T. (2014) Spatially resolved measurements of H2O, HCl, CO, OCS, SO2, cloud opacity, and acid concentration in the Venus near-infrared spectral windows. J Geophys Res: Planets 119:1860–1891

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

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

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