94 research outputs found

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

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    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 O2_{2} (and its photochemical byproduct, O3_{3}). But O2_{2} can also be produced abiotically by photolysis of CO2_{2}, followed by recombination of O atoms with each other. CO is produced in stoichiometric proportions. Whether O2_{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 O2_{2} derived from CO2_{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 O2_{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 O2_{2} in the oceans is slow and if other O2_{2} sinks (e.g., reduced volcanic gases or dissolved ferrous iron) are small. O3_{3}, on the other hand, could be detectable at UV wavelengths (λ\lambda < 300 nm) for a much broader range of boundary conditions and stellar types.Comment: 20 pages text, 9 figure

    A Limited Habitable Zone for Complex Life

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    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

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

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

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    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 O2_2 and CO, which are potential indicators of life. Recently, it was proposed that CO2_2 photolysis generates abundant (≳0.1\gtrsim0.1 bar) abiotic O2_2 and CO in the atmospheres of habitable M-dwarf planets with CO2_2-rich atmospheres, constituting a strong false positive for O2_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 O2_2 and CO, if habitable. Here, we use a multi-model approach to re-examine photochemical O2_2 and CO accumulation on planets orbiting M-dwarf stars. We show that photochemical O2_2 remains a trace gas on habitable CO2_2-rich M-dwarf planets, with earlier predictions of abundant O2_2 and CO due to an atmospheric model top that was too low to accurately resolve the unusually-high CO2_2 photolysis peak on such worlds. Our work strengthens the case for O2_2 as a biosignature gas, and affirms the importance of CO as a diagnostic of photochemical O2_2 production. However, observationally relevant false positive potential remains, especially for O2_2's photochemical product O3_3, and further work is required to confidently understand O2_2 and O3_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

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

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    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

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

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    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

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

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    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±0.0231.108\pm0.023 RJR_{\rm J}) exoplanet Kepler-1514 b with a 218 day orbital period as 5.28±0.225.28\pm0.22 MJM_{\rm J}. The bulk density of this cool (∼\sim390 K) giant planet is 4.82−0.25+0.264.82^{+0.26}_{-0.25} g cm−3^{-3}, consistent with a core supported by electron degeneracy pressure. We also infer an orbital eccentricity of 0.401−0.014+0.0130.401^{+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 (VV=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

    Exoplanet Diversity in the Era of Space-based Direct Imaging Missions

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    This whitepaper discusses the diversity of exoplanets that could be detected by future observations, so that comparative exoplanetology can be performed in the upcoming era of large space-based flagship missions. The primary focus will be on characterizing Earth-like worlds around Sun-like stars. However, we will also be able to characterize companion planets in the system simultaneously. This will not only provide a contextual picture with regards to our Solar system, but also presents a unique opportunity to observe size dependent planetary atmospheres at different orbital distances. We propose a preliminary scheme based on chemical behavior of gases and condensates in a planet's atmosphere that classifies them with respect to planetary radius and incident stellar flux.Comment: A white paper submitted to the National Academy of Sciences Exoplanet Science Strateg

    An ideal journey: Making bus travel desirable

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    © 2016 Informa UK Limited, trading as Taylor & Francis Group. This paper explores the ways in which people use their travel-time on local buses, and explains how this knowledge can assist with efforts in many ‘auto-centric’ societies to make bus travel more attractive and encourage a shift away from excessive private car use. Framing the discussion around the concept of an ‘ideal bus journey’, this paper examines whether travel-time activities on-board the bus give subjective value to the journey experience. Particular attention is given to emergent mobile Information and Communications Technologies, which are rapidly reconfiguring the ways in which we can inhabit and use mobile spaces such as the bus. This paper reports a novel mixed-methodology, creating a synthesised analysis of online discussions, focus groups, and a large-scale questionnaire survey of 840 bus users in Bristol, UK. The findings demonstrate that the bus is a very active space, with high levels of travel-time activity. The most popular activities on the bus are those related to relaxation and personal benefit, such as reading, listening to music, and browsing the internet. It is the passengers themselves that are largely in control of their in-vehicle experience, being able to craft a range of different positive journey experiences through travel-time activity. However, negative experiences are very common, and there is a need to challenge unfavourable public perception and media representations of bus travel to create a more positive cultural construction of the bus which would allow for the concept of an ‘ideal journey’ to be more easily realised. Passengers are the main creators of their travel-time experience, however there is much that can be done by bus operators to facilitate different types of activity and encourage a desirable public space. The overarching message is that there is a distinct opportunity to unlock travel-time activity as a ‘Unique Selling Point’ of the bus. Creating a perception of the bus journey as a desirable piece of time will allow local bus services to compete with the car on their own terms, and assist with international efforts to encourage people out of their cars and onto public transport for some trips
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