Efforts to spectrally characterize the atmospheric compositions of temperate terrestrial exoplanets orbiting M dwarf stars with JWST are now underway. Key molecular targets of such searches include ...O2 and CO, which are potential indicators of life. Recently, it was proposed that CO2 photolysis generates abundant (≳0.1 bar) abiotic O2 and CO in the atmospheres of habitable M dwarf planets with CO2-rich atmospheres, constituting a strong false positive for O2 as a biosignature and further complicating efforts to use CO as a diagnostic of surface biology. Importantly, this implied that TRAPPIST-1e and TRAPPIST-1f, now under observation with JWST, would abiotically accumulate abundant O2 and CO, if habitable. Here, we use a multi-model approach to reexamine photochemical O2 and CO accumulation on planets orbiting M dwarf stars. We show that photochemical O2 remains a trace gas on habitable CO2-rich M dwarf planets, with earlier predictions of abundant O2 and CO due to an atmospheric model top that was too low to accurately resolve the unusually high CO2 photolysis peak on such worlds. Our work strengthens the case for O2 as a biosignature gas, and affirms the importance of CO as a diagnostic of photochemical O2 production. However, observationally relevant false-positive potential remains, especially for O2's photochemical product O3, and further work is required to confidently understand O2 and O3 as biosignature gases on M dwarf planets.
Abstract This study aims to identify exemplary science cases for observing N 2 O, CH 3 Cl, and CH 3 Br in exoplanet atmospheres at abundances consistent with biogenic production using a space-based ...mid-infrared nulling interferometric observatory, such as the Large Interferometer For Exoplanets (LIFE) mission concept. We use a set of scenarios derived from chemical kinetics models that simulate the atmospheric response of varied levels of biogenic production of N 2 O, CH 3 Cl, and CH 3 Br in O 2 -rich terrestrial planet atmospheres to produce forward models for our LIFE sim observation simulator software. In addition, we demonstrate the connection to retrievals for selected cases. We use the results to derive observation times needed for the detection of these scenarios and apply them to define science requirements for the mission. Our analysis shows that in order to detect relevant abundances with a mission like LIFE in its current baseline setup, we require: (i) only a few days of observation time for certain very nearby “golden target” scenarios, which also motivate future studies of “spectral-temporal” observations (ii) ∼10 days in certain standard scenarios such as temperate, terrestrial planets around M star hosts at 5 pc, (iii) ∼50–100 days in the most challenging but still feasible cases, such as an Earth twin at 5 pc. A few cases with very low fluxes around specific host stars are not detectable. In summary, the abundances of these capstone biosignatures are detectable at plausible biological production fluxes for most cases examined and for a significant number of potential targets.
Abstract Atmospheric pollutants such as chlorofluorocarbons and NO 2 have been proposed as potential remotely detectable atmospheric technosignature gases. Here we investigate the potential for ...artificial greenhouse gases including CF 4 , C 2 F 6 , C 3 F 8 , SF 6 , and NF 3 to generate detectable atmospheric signatures. In contrast to passive incidental by-products of industrial processes, artificial greenhouse gases would represent an intentional effort to change the climate of a planet with long-lived, low-toxicity gases and would possess low false positive potential. An extraterrestrial civilization may be motivated to undertake such an effort to arrest a predicted snowball state on their home world or to terraform an otherwise uninhabitable terrestrial planet within their system. Because artificial greenhouse gases strongly absorb in the thermal mid-infrared window of temperate atmospheres, a terraformed planet will logically possess strong absorption features from these gases at mid-infrared wavelengths (∼8–12 μ m), possibly accompanied by diagnostic features in the near-infrared. As a proof of concept, we calculate the needed observation time to detect 1 10(100) ppm of C 2 F 6 /C 3 F 8 /SF 6 on TRAPPIST-1 f with JWST MIRI’s Low Resolution Spectrometer (LRS) and NIRSpec. We find that a combination of 110(100) ppm each of C 2 F 6 , C 3 F 8 , and SF 6 can be detected with a signal-to-noise ratio ≧ 5 in as few as 2510(5) transits with MIRI/LRS. We further explore mid-infrared direct-imaging scenarios with the Large Interferometer for Exoplanets mission concept and find these gases are more detectable than standard biosignatures at these concentrations. Consequently, artificial greenhouse gases can be readily detected (or excluded) during normal planetary characterization observations with no additional overhead.
Abstract Radiative transfer (RT) models are critical in the interpretation of exoplanetary spectra, in simulating exoplanet climates, and when designing the specifications of future flagship ...observatories. However, most models differ in methodologies and input data, which can lead to significantly different spectra. In this paper, we present the experimental protocol of the Modeling Atmospheric Lines By the Exoplanet Community (MALBEC) project. MALBEC is an exoplanet model intercomparison project that belongs to the Climates Using Interactive Suites of Intercomparisons Nested for Exoplanet Studies framework, which aims to provide the exoplanet community with a large and diverse set of comparison and validation of models. The proposed protocol tests include a large set of initial participating RT models, a broad range of atmospheres (from hot Jupiters to temperate terrestrials), and several observation geometries, which would allow us to quantify and compare the differences between different RT models used by the exoplanetary community. Two types of tests are proposed: transit spectroscopy and direct imaging modeling, with results from the proposed tests to be published in dedicated follow-up papers. To encourage the community to join this comparison effort and as an example, we present simulation results for one specific transit case (GJ-1214 b), in which we find notable differences in how the various codes handle the discretization of the atmospheres (e.g., sub-layering), the treatment of molecular opacities (e.g., correlated- k , line-by-line) and the default spectroscopic repositories generally used by each model (e.g., HITRAN, HITEMP, ExoMol).
Abstract
The first potential exoplanetary biosignature detections are likely to be ambiguous due to the potential for false positives: abiotic planetary processes that produce observables similar to ...those anticipated from a global biosphere. Here we propose a class of methylated gases as corroborative “capstone” biosignatures. Capstone biosignatures are metabolic products that may be less immediately detectable, but have substantially lower false-positive potential, and can thus serve as confirmation for a primary biosignature such as O
2
. CH
3
Cl has previously been established as a biosignature candidate, and other halomethane gases such as CH
3
Br and CH
3
I have similar potential. These gases absorb in the mid-infrared at wavelengths that are likely to be captured while observing primary biosignatures such as O
3
or CH
4
. We quantitatively explore CH
3
Br as a new capstone biosignature through photochemical and spectral modeling of Earthlike planets orbiting FGKM stellar hosts. We also reexamine the biosignature potential of CH
3
Cl over the same set of parameters using our updated model. We show that CH
3
Cl and CH
3
Br can build up to relatively high levels in M dwarf environments and analyze synthetic spectra of TRAPPIST-1e. Our results suggest that there is a coadditive spectral effect from multiple CH
3
X gases in an atmosphere, leading to an increased signal-to-noise and greater ability to detect a methylated gas feature. These capstone biosignatures are plausibly detectable in exoplanetary atmospheres, have low false-positive potential, and would provide strong evidence for life in conjunction with other well-established biosignature candidates.
Abstract
Efforts to spectrally characterize the atmospheric compositions of temperate terrestrial exoplanets orbiting M dwarf stars with JWST are now underway. Key molecular targets of such searches ...include O
2
and CO, which are potential indicators of life. Recently, it was proposed that CO
2
photolysis generates abundant (≳0.1 bar) abiotic O
2
and CO in the atmospheres of habitable M dwarf planets with CO
2
-rich atmospheres, constituting a strong false positive for O
2
as a biosignature and further complicating efforts to use CO as a diagnostic of surface biology. Importantly, this implied that TRAPPIST-1e and TRAPPIST-1f, now under observation with JWST, would abiotically accumulate abundant O
2
and CO, if habitable. Here, we use a multi-model approach to reexamine photochemical O
2
and CO accumulation on planets orbiting M dwarf stars. We show that photochemical O
2
remains a trace gas on habitable CO
2
-rich M dwarf planets, with earlier predictions of abundant O
2
and CO due to an atmospheric model top that was too low to accurately resolve the unusually high CO
2
photolysis peak on such worlds. Our work strengthens the case for O
2
as a biosignature gas, and affirms the importance of CO as a diagnostic of photochemical O
2
production. However, observationally relevant false-positive potential remains, especially for O
2
's photochemical product O
3
, and further work is required to confidently understand O
2
and O
3
as biosignature gases on M dwarf planets.
In the near future, extremely large ground-based telescopes may conduct some of the first searches for life beyond the solar system. High spectral resolution observations of reflected light from ...nearby exoplanetary atmospheres could be used to search for the biosignature oxygen. However, while Earth's abundant O2 is photosynthetic, early ocean loss may also produce high atmospheric O2 via water vapor photolysis and subsequent hydrogen escape. To explore how to use spectra to discriminate between these two oxygen sources, we generate high-resolution line-by-line synthetic spectra of both a habitable Earth-like and post-ocean-loss Proxima Centauri b. We examine the strength and profile of four bands of O2 from 0.63 to 1.27 m, and quantify their relative detectability. We find that 10 bar O2 post-ocean-loss atmospheres have strong suppression of oxygen bands, and especially the 1.27 um band. This suppression is due to additional strong, broad O2-O2 collisionally induced absorption (CIA) generated in these more massive O2 atmospheres, which is not present for the smaller amounts of oxygen generated by photosynthesis. Consequently, any detection of the 1.27 m band in reflected light indicates lower Earth-like O2 levels, which suggests a likely photosynthetic origin. However, the 0.69 m O2 band is relatively unaffected by O2-O2 CIA, and the presence of an ocean-loss high-O2 atmosphere could be inferred via detection of a strong 0.69 m O2 band, and a weaker or undetected 1.27 m band. These results provide a strategy for observing and interpreting O2 in exoplanet atmospheres, that could be considered by future ground-based telescopes.
Abstract
Accurately measuring and modeling the Ly
α
(Ly
α
;
λ
1215.67 Å) emission line from low-mass stars is vital for our ability to build predictive high energy stellar spectra, yet interstellar ...medium (ISM) absorption of this line typically prevents model-measurement comparisons. Ly
α
also controls the photodissociation of important molecules, like water and methane, in exoplanet atmospheres such that any photochemical models assessing potential biosignatures or atmospheric abundances require accurate Ly
α
host star flux estimates. Recent observations of three early M and K stars (K3, M0, M1) with exceptionally high radial velocities (>100 km s
−1
) reveal the intrinsic profiles of these types of stars as most of their Ly
α
flux is shifted away from the geocoronal line core and contamination from the ISM. These observations indicate that previous stellar spectra computed with the
PHOENIX
atmosphere code have underpredicted the core of Ly
α
in these types of stars. With these observations, we have been able to better understand the microphysics in the upper atmosphere and improve the predictive capabilities of the
PHOENIX
atmosphere code. Since these wavelengths drive the photolysis of key molecular species, we also present results analyzing the impact of the resulting changes to the synthetic stellar spectra on observable chemistry in terrestrial planet atmospheres.
This chapter reviews proposed exoplanet biosignatures, including their biological origins, observable features, atmospheric sinks, and potentially confounding abiotic sources. Emphasis is placed on ...material published since past comprehensive reviews while providing a foundational understanding of each named biosignature. Topics include possible gaseous biosignatures (e.g., O\(_2\), O\(_3\), CH\(_4\), N\(_2\)O, DMS, CH\(_3\)Cl, C\(_5\)H\(_8\), NH\(_3\), PH\(_3\)), surface biosignatures (e.g., vegetation red edge, other pigment features, polarization signatures), and temporal biosignatures (e.g., atmospheric seasonality). Potential frameworks for assessing remote biosignatures are described. Text and table summaries provide references to relevant original research articles.
Efforts to spectrally characterize the atmospheric compositions of temperate
terrestrial exoplanets orbiting M-dwarf stars with the James Webb Space
Telescope (JWST) are now underway. Key molecular ...targets of such searches
include O$_2$ and CO, which are potential indicators of life. Recently, it was
proposed that CO$_2$ photolysis generates abundant ($\gtrsim0.1$ bar) abiotic
O$_2$ and CO in the atmospheres of habitable M-dwarf planets with CO$_2$-rich
atmospheres, constituting a strong false positive for O$_2$ as a biosignature
and further complicating efforts to use CO as a diagnostic of surface biology.
Importantly, this implied that TRAPPIST-1e and TRAPPIST-1f, now under
observation with JWST, would abiotically accumulate abundant O$_2$ and CO, if
habitable. Here, we use a multi-model approach to re-examine photochemical
O$_2$ and CO accumulation on planets orbiting M-dwarf stars. We show that
photochemical O$_2$ remains a trace gas on habitable CO$_2$-rich M-dwarf
planets, with earlier predictions of abundant O$_2$ and CO due to an
atmospheric model top that was too low to accurately resolve the unusually-high
CO$_2$ photolysis peak on such worlds. Our work strengthens the case for O$_2$
as a biosignature gas, and affirms the importance of CO as a diagnostic of
photochemical O$_2$ production. However, observationally relevant false
positive potential remains, especially for O$_2$'s photochemical product O$_3$,
and further work is required to confidently understand O$_2$ and O$_3$ as
biosignature gases on M-dwarf planets.
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