Abiotic O2_{2} Levels on Planets around F, G, K, and M Stars: Possible False Positives for Life?

In the search for life on Earth-like planets around other stars, the first (and likely only) information will come from the spectroscopic characterization of the planet's atmosphere. Of the countless number of chemical species terrestrial life produces, only a few have the distinct spectral features and the necessary atmospheric abundance to be detectable. The easiest of these species to observe in Earth's atmosphere is O$_{2}$ (and its photochemical byproduct, O$_{3}$). But O$_{2}$ can also be produced abiotically by photolysis of CO$_{2}$, followed by recombination of O atoms with each other. CO is produced in stoichiometric proportions. Whether O$_{2}$ and CO can accumulate to appreciable concentrations depends on the ratio of far-UV to near-UV radiation coming from the planet's parent star and on what happens to these gases when they dissolve in a planet's oceans. Using a one-dimensional photochemical model, we demonstrate that O$_{2}$ derived from CO$_{2}$ photolysis should not accumulate to measurable concentrations on planets around F- and G-type stars. K-star, and especially M-star planets, however, may build up O$_{2}$ because of the low near-UV flux from their parent stars, in agreement with some previous studies. On such planets, a 'false positive' for life is possible if recombination of dissolved CO and O$_{2}$ in the oceans is slow and if other O$_{2}$ sinks (e.g., reduced volcanic gases or dissolved ferrous iron) are small. O$_{3}$, on the other hand, could be detectable at UV wavelengths ($\lambda$ < 300 nm) for a much broader range of boundary conditions and stellar types.Comment: 20 pages text, 9 figure

Modeling pN2 through Geological Time: Implications for Planetary Climates and Atmospheric Biosignatures

Nitrogen is a major nutrient for all life on Earth and could plausibly play a similar role in extraterrestrial biospheres. The major reservoir of nitrogen at Earth's surface is atmospheric N2, but recent studies have proposed that the size of this reservoir may have fluctuated significantly over the course of Earth's history with particularly low levels in the Neoarchean - presumably as a result of biological activity. We used a biogeochemical box model to test which conditions are necessary to cause large swings in atmospheric N2 pressure. Parameters for our model are constrained by observations of modern Earth and reconstructions of biomass burial and oxidative weathering in deep time. A 1-D climate model was used to model potential effects on atmospheric climate. In a second set of tests, we perturbed our box model to investigate which parameters have the greatest impact on the evolution of atmospheric pN2 and consider possible implications for nitrogen cycling on other planets. Our results suggest that (a) a high rate of biomass burial would have been needed in the Archean to draw down atmospheric pN2 to less than half modern levels, (b) the resulting effect on temperature could probably have been compensated by increasing solar luminosity and a mild increase in pCO2, and (c) atmospheric oxygenation could have initiated a stepwise pN2 rebound through oxidative weathering. In general, life appears to be necessary for significant atmospheric pN2 swings on Earth-like planets. Our results further support the idea that an exoplanetary atmosphere rich in both N2 and O2 is a signature of an oxygen-producing biosphere.Comment: 33 pages, 11 figures, 2 tables (includes appendix), published in Astrobiolog

A Limited Habitable Zone for Complex Life

The habitable zone (HZ) is commonly defined as the range of distances from a host star within which liquid water, a key requirement for life, may exist on a planet's surface. Substantially more CO2 than present in Earth's modern atmosphere is required to maintain clement temperatures for most of the HZ, with several bars required at the outer edge. However, most complex aerobic life on Earth is limited by CO2 concentrations of just fractions of a bar. At the same time, most exoplanets in the traditional HZ reside in proximity to M dwarfs, which are more numerous than Sun-like G dwarfs but are predicted to promote greater abundances of gases that can be toxic in the atmospheres of orbiting planets, such as carbon monoxide (CO). Here we show that the HZ for complex aerobic life is likely limited relative to that for microbial life. We use a 1D radiative-convective climate and photochemical models to circumscribe a Habitable Zone for Complex Life (HZCL) based on known toxicity limits for a range of organisms as a proof of concept. We find that for CO2 tolerances of 0.01, 0.1, and 1 bar, the HZCL is only 21%, 32%, and 50% as wide as the conventional HZ for a Sun-like star, and that CO concentrations may limit some complex life throughout the entire HZ of the coolest M dwarfs. These results cast new light on the likely distribution of complex life in the universe and have important ramifications for the search for exoplanet biosignatures and technosignatures.Comment: Revised including additional discussion. Published Gold OA in ApJ. 9 pages, 5 figures, 5 table

Identifying Planetary Biosignature Impostors: Spectral Features of CO and O4 Resulting from Abiotic O2/O3 Production

O2 and O3 have been long considered the most robust individual biosignature gases in a planetary atmosphere, yet multiple mechanisms that may produce them in the absence of life have been described. However, these abiotic planetary mechanisms modify the environment in potentially identifiable ways. Here we briefly discuss two of the most detectable spectral discriminants for abiotic O2/O3: CO and O4. We produce the first explicit self-consistent simulations of these spectral discriminants as they may be seen by JWST. If JWST-NIRISS and/or NIRSpec observe CO (2.35, 4.6 um) in conjunction with CO2 (1.6, 2.0, 4.3 um) in the transmission spectrum of a terrestrial planet it could indicate robust CO2 photolysis and suggest that a future detection of O2 or O3 might not be biogenic. Strong O4 bands seen in transmission at 1.06 and 1.27 um could be diagnostic of a post-runaway O2-dominated atmosphere from massive H-escape. We find that for these false positive scenarios, CO at 2.35 um, CO2 at 2.0 and 4.3 um, and O4 at 1.27 um are all stronger features in transmission than O2/O3 and could be detected with SNRs $\gtrsim$ 3 for an Earth-size planet orbiting a nearby M dwarf star with as few as 10 transits, assuming photon-limited noise. O4 bands could also be sought in UV/VIS/NIR reflected light (at 0.345, 0.36, 0.38, 0.445, 0.475, 0.53, 0.57, 0.63, 1.06, and 1.27 um) by a next generation direct-imaging telescope such as LUVOIR/HDST or HabEx and would indicate an oxygen atmosphere too massive to be biologically produced.Comment: 7 pages, 4 figures, accepted to the Astrophysical Journal Letter

A Re-Appraisal of CO/O2_2 Runaway on Habitable Planets Orbiting Low-Mass Stars

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. Significantly, 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.Comment: Submitted to AAS Journals; comments and criticism solicited at [email protected]. 3 Figures, 1 Table in main text; 3Figures, 5 Tables in S

Rethinking CO Antibiosignatures in the Search for Life Beyond the Solar System

Some atmospheric gases have been proposed as counter indicators to the presence of life on an exoplanet if remotely detectable at sufficient abundance (i.e., antibiosignatures), informing the search for biosignatures and potentially fingerprinting uninhabited habitats. However, the quantitative extent to which putative antibiosignatures could exist in the atmospheres of inhabited planets is not well understood. The most commonly referenced potential antibiosignature is CO, because it represents a source of free energy and reduced carbon that is readily exploited by life on Earth and is thus often assumed to accumulate only in the absence of life. Yet, biospheres actively produce CO through biomass burning, photooxidation processes, and release of gases that are photochemically converted into CO in the atmosphere. We demonstrate with a 1D ecosphere-atmosphere model that reducing biospheres can maintain CO levels of approximately 100 ppmv (parts per million by volume) even at low H2 fluxes due to the impact of hybrid photosynthetic ecosystems. Additionally, we show that photochemistry around M dwarf stars is particularly favorable for the buildup of CO, with plausible concentrations for inhabited, oxygen-rich planets extending from hundreds of ppm to several percent. Since CH4 buildup is also favored on these worlds, and because O2 and O3 are likely not detectable with the James Webb Space Telescope, the presence of high CO (greater than 100 ppmv) may discriminate between oxygen-rich and reducing biospheres with near-future transmission observations. These results suggest that spectroscopic detection of CO can be compatible with the presence of life and that a comprehensive contextual assessment is required to validate the significance of potential antibiosignatures

Surface and Temporal Biosignatures

Recent discoveries of potentially habitable exoplanets have ignited the prospect of spectroscopic investigations of exoplanet surfaces and atmospheres for signs of life. This chapter provides an overview of potential surface and temporal exoplanet biosignatures, reviewing Earth analogues and proposed applications based on observations and models. The vegetation red-edge (VRE) remains the most well-studied surface biosignature. Extensions of the VRE, spectral "edges" produced in part by photosynthetic or nonphotosynthetic pigments, may likewise present potential evidence of life. Polarization signatures have the capacity to discriminate between biotic and abiotic "edge" features in the face of false positives from band-gap generating material. Temporal biosignatures -- modulations in measurable quantities such as gas abundances (e.g., CO2), surface features, or emission of light (e.g., fluorescence, bioluminescence) that can be directly linked to the actions of a biosphere -- are in general less well studied than surface or gaseous biosignatures. However, remote observations of Earth's biosphere nonetheless provide proofs of concept for these techniques and are reviewed here. Surface and temporal biosignatures provide complementary information to gaseous biosignatures, and while likely more challenging to observe, would contribute information inaccessible from study of the time-averaged atmospheric composition alone.Comment: 26 pages, 9 figures, review to appear in Handbook of Exoplanets. Fixed figure conversion error

Giant Outer Transiting Exoplanet Mass (GOT 'EM) Survey. I. Confirmation of an Eccentric, Cool Jupiter With an Interior Earth-sized Planet Orbiting Kepler-1514*

Despite the severe bias of the transit method of exoplanet discovery toward short orbital periods, a modest sample of transiting exoplanets with orbital periods greater than 100 days is known. Long-term radial velocity (RV) surveys are pivotal to confirming these signals and generating a set of planetary masses and densities for planets receiving moderate to low irradiation from their host stars. Here, we conduct RV observations of Kepler-1514 from the Keck I telescope using the High Resolution Echelle Spectrometer. From these data, we measure the mass of the statistically validated giant ($1.108\pm0.023$ $R_{\rm J}$) exoplanet Kepler-1514 b with a 218 day orbital period as $5.28\pm0.22$ $M_{\rm J}$. The bulk density of this cool ($\sim$390 K) giant planet is $4.82^{+0.26}_{-0.25}$ g cm$^{-3}$, consistent with a core supported by electron degeneracy pressure. We also infer an orbital eccentricity of $0.401^{+0.013}_{-0.014}$ from the RV and transit observations, which is consistent with planet-planet scattering and disk cavity migration models. The Kepler-1514 system contains an Earth-size, Kepler Object of Interest on a 10.5 day orbit that we statistically validate against false positive scenarios, including those involving a neighboring star. The combination of the brightness ($V$=11.8) of the host star and the long period, low irradiation, and high density of Kepler-1514 b places this system among a rare group of known exoplanetary systems and one that is amenable to continued study.Comment: 18 pages, 9 figures, accepted for publication in the Astronomical Journa

The C. elegans ephrin EFN-4 functions non-cell autonomously with heparan sulfate proteoglycans to promote axon outgrowth and branching

The Eph receptors and their cognate ephrin ligands play key roles in many aspects of nervous system development. These interactions typically occur within an individual tissue type, serving either to guide axons to their terminal targets or to define boundaries between the rhombomeres of the hindbrain. We have identified a novel role for the Caenorhabditis elegans ephrin EFN-4 in promoting primary neurite outgrowth in AIY interneurons and D-class motor neurons. Rescue experiments reveal that EFN-4 functions non-cell autonomously in the epidermis to promote primary neurite outgrowth. We also find that EFN-4 plays a role in promoting ectopic axon branching in a C. elegans model of X-linked Kallmann syndrome. In this context, EFN-4 functions non-cell autonomously in the body wall muscle, and in parallel with HS biosynthesis genes and HSPG core proteins, which function cell autonomously in the AIY neurons. This is the first report of an epidermal ephrin providing a developmental cue to the nervous system