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Scharf C, Witkowski O. Rebuilding the Habitable Zone from the Bottom up with Computational Zones. ASTROBIOLOGY 2024; 24:613-627. [PMID: 38853680 DOI: 10.1089/ast.2023.0035] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2024]
Abstract
Computation, if treated as a set of physical processes that act on information represented by states of matter, encompasses biological systems, digital systems, and other constructs and may be a fundamental measure of living systems. The opportunity for biological computation, represented in the propagation and selection-driven evolution of information-carrying organic molecular structures, has been partially characterized in terms of planetary habitable zones (HZs) based on primary conditions such as temperature and the presence of liquid water. A generalization of this concept to computational zones (CZs) is proposed, with constraints set by three principal characteristics: capacity (including computation rates), energy, and instantiation (or substrate, including spatial extent). CZs naturally combine traditional habitability factors, including those associated with biological function that incorporate the chemical milieu, constraints on nutrients and free energy, as well as element availability. Two example applications are presented by examining the fundamental thermodynamic work efficiency and Landauer limit of photon-driven biological computation on planetary surfaces and of generalized computation in stellar energy capture structures (a.k.a. Dyson structures). It is suggested that CZs that involve nested structures or substellar objects could manifest unique observational signatures as cool far-infrared emitters. While these latter scenarios are entirely hypothetical, they offer a useful, complementary introduction to the potential universality of CZs.
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Affiliation(s)
- Caleb Scharf
- NASA Ames Research Center, Moffett Field, California, USA
| | - Olaf Witkowski
- Cross Labs, Cross Compass Ltd., Kyoto, Japan
- College of Arts and Sciences, University of Tokyo, Tokyo, Japan
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2
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Méndez A, Rivera-Valentín EG, Schulze-Makuch D, Filiberto J, Ramírez RM, Wood TE, Dávila A, McKay C, Ceballos KNO, Jusino-Maldonado M, Torres-Santiago NJ, Nery G, Heller R, Byrne PK, Malaska MJ, Nathan E, Simões MF, Antunes A, Martínez-Frías J, Carone L, Izenberg NR, Atri D, Chitty HIC, Nowajewski-Barra P, Rivera-Hernández F, Brown CY, Lynch KL, Catling D, Zuluaga JI, Salazar JF, Chen H, González G, Jagadeesh MK, Haqq-Misra J. Habitability Models for Astrobiology. ASTROBIOLOGY 2021; 21:1017-1027. [PMID: 34382857 DOI: 10.1089/ast.2020.2342] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
Habitability has been generally defined as the capability of an environment to support life. Ecologists have been using Habitat Suitability Models (HSMs) for more than four decades to study the habitability of Earth from local to global scales. Astrobiologists have been proposing different habitability models for some time, with little integration and consistency among them, being different in function to those used by ecologists. Habitability models are not only used to determine whether environments are habitable, but they also are used to characterize what key factors are responsible for the gradual transition from low to high habitability states. Here we review and compare some of the different models used by ecologists and astrobiologists and suggest how they could be integrated into new habitability standards. Such standards will help improve the comparison and characterization of potentially habitable environments, prioritize target selections, and study correlations between habitability and biosignatures. Habitability models are the foundation of planetary habitability science, and the synergy between ecologists and astrobiologists is necessary to expand our understanding of the habitability of Earth, the Solar System, and extrasolar planets.
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Affiliation(s)
- Abel Méndez
- Planetary Habitability Laboratory, University of Puerto Rico at Arecibo, Puerto Rico, USA
| | | | - Dirk Schulze-Makuch
- Center for Astronomy and Astrophysics, Technische Universität Berlin, Berlin, Germany; German Research Centre for Geosciences, Section Geomicrobiology, Potsdam, Germany; Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Stechlin, Germany
| | | | - Ramses M Ramírez
- University of Central Florida, Department of Physics, Orlando, Florida, USA; Space Science Institute, Boulder, Colorado, USA
| | - Tana E Wood
- USDA Forest Service International Institute of Tropical Forestry, San Juan, Puerto Rico, USA
| | - Alfonso Dávila
- NASA Ames Research Center, Moffett Field, California, USA
| | - Chris McKay
- NASA Ames Research Center, Moffett Field, California, USA
| | - Kevin N Ortiz Ceballos
- Planetary Habitability Laboratory, University of Puerto Rico at Arecibo, Puerto Rico, USA
| | | | | | | | - René Heller
- Max Planck Institute for Solar System Research; Institute for Astrophysics, University of Göttingen, Germany
| | - Paul K Byrne
- North Carolina State University, Raleigh, North Carolina, USA
| | - Michael J Malaska
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
| | - Erica Nathan
- Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, USA
| | - Marta Filipa Simões
- State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Taipa, Macau SAR, China
| | - André Antunes
- State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Taipa, Macau SAR, China
| | | | | | - Noam R Izenberg
- Johns Hopkins Applied Physics Laboratory, Laurel, Maryland, USA
| | - Dimitra Atri
- Center for Space Science, New York University Abu Dhabi, United Arab Emirates
| | | | | | | | | | - Kennda L Lynch
- Lunar and Planetary Institute, USRA, Houston, Texas, USA
| | | | - Jorge I Zuluaga
- Institute of Physics / FCEN - Universidad de Antioquia, Medellín, Colombia
| | - Juan F Salazar
- GIGA, Escuela Ambiental, Facultad de Ingeniería, Universidad de Antioquia, Medellín, Colombia
| | - Howard Chen
- Northwestern University, Evanston, Illinois, USA
| | - Grizelle González
- USDA Forest Service International Institute of Tropical Forestry, San Juan, Puerto Rico, USA
| | | | - Jacob Haqq-Misra
- Blue Marble Space Institute of Science, Seattle, Washington, USA
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Abstract
Most stars in the Universe are red dwarfs. They outnumber stars like our Sun by a factor of 5 and outlive them by another factor of 20 (population-weighted mean). When combined with recent observations uncovering an abundance of temperate, rocky planets around these diminutive stars, we are faced with an apparent logical contradiction-Why do we not see a red dwarf in our sky? To address this "red sky paradox," we formulate a Bayesian probability function concerning the odds of finding oneself around an F/G/K-spectral type (Sun-like) star. If the development of intelligent life from prebiotic chemistry is a universally rapid and ensured process, the temporal advantage of red dwarfs dissolves, softening the red sky paradox, but exacerbating the classic Fermi paradox. Otherwise, we find that humanity appears to be a 1-in-100 outlier. While this could be random chance (resolution I), we outline three other nonmutually exclusive resolutions (II to IV) that broadly act as filters to attenuate the suitability of red dwarfs for complex life. Future observations may be able to provide support for some of these. Notably, if surveys reveal a paucity of temperate rocky planets around the smallest (and most numerous) red dwarfs, then this would support resolution II. As another example, if future characterization efforts were to find that red dwarf worlds have limited windows for complex life due to stellar evolution, this would support resolution III. Solving this paradox would reveal guidance for the targeting of future remote life sensing experiments and the limits of life in the cosmos.
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Turbet M, Bolmont E, Bourrier V, Demory BO, Leconte J, Owen J, Wolf ET. A Review of Possible Planetary Atmospheres in the TRAPPIST-1 System. SPACE SCIENCE REVIEWS 2020; 216:100. [PMID: 32764836 PMCID: PMC7378127 DOI: 10.1007/s11214-020-00719-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 01/14/2020] [Accepted: 07/04/2020] [Indexed: 06/11/2023]
Abstract
TRAPPIST-1 is a fantastic nearby (∼39.14 light years) planetary system made of at least seven transiting terrestrial-size, terrestrial-mass planets all receiving a moderate amount of irradiation. To date, this is the most observationally favourable system of potentially habitable planets known to exist. Since the announcement of the discovery of the TRAPPIST-1 planetary system in 2016, a growing number of techniques and approaches have been used and proposed to characterize its true nature. Here we have compiled a state-of-the-art overview of all the observational and theoretical constraints that have been obtained so far using these techniques and approaches. The goal is to get a better understanding of whether or not TRAPPIST-1 planets can have atmospheres, and if so, what they are made of. For this, we surveyed the literature on TRAPPIST-1 about topics as broad as irradiation environment, planet formation and migration, orbital stability, effects of tides and Transit Timing Variations, transit observations, stellar contamination, density measurements, and numerical climate and escape models. Each of these topics adds a brick to our understanding of the likely-or on the contrary unlikely-atmospheres of the seven known planets of the system. We show that (i) Hubble Space Telescope transit observations, (ii) bulk density measurements comparison with H2-rich planets mass-radius relationships, (iii) atmospheric escape modelling, and (iv) gas accretion modelling altogether offer solid evidence against the presence of hydrogen-dominated-cloud-free and cloudy-atmospheres around TRAPPIST-1 planets. This means that the planets are likely to have either (i) a high molecular weight atmosphere or (ii) no atmosphere at all. There are several key challenges ahead to characterize the bulk composition(s) of the atmospheres (if present) of TRAPPIST-1 planets. The main one so far is characterizing and correcting for the effects of stellar contamination. Fortunately, a new wave of observations with the James Webb Space Telescope and near-infrared high-resolution ground-based spectrographs on existing very large and forthcoming extremely large telescopes will bring significant advances in the coming decade.
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Affiliation(s)
- Martin Turbet
- Observatoire Astronomique de l'Université de Genève, 51 chemin de Pégase, 1290 Sauverny, Switzerland
| | - Emeline Bolmont
- Observatoire Astronomique de l'Université de Genève, 51 chemin de Pégase, 1290 Sauverny, Switzerland
| | - Vincent Bourrier
- Observatoire Astronomique de l'Université de Genève, 51 chemin de Pégase, 1290 Sauverny, Switzerland
| | - Brice-Olivier Demory
- Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
| | - Jérémy Leconte
- Laboratoire d'astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
| | - James Owen
- Astrophysics Group, Department of Physics, Imperial College London, Prince Consort Rd, London, SW7 2AZ UK
| | - Eric T Wolf
- Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309 USA
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5
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Testing Earthlike Atmospheric Evolution on Exo-Earths through Oxygen Absorption: Required Sample Sizes and the Advantage of Age-based Target Selection. ACTA ACUST UNITED AC 2020. [DOI: 10.3847/1538-4357/ab8fad] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
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6
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7
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Ramirez RM. A Complex Life Habitable Zone Based On Lipid Solubility Theory. Sci Rep 2020; 10:7432. [PMID: 32366889 PMCID: PMC7198600 DOI: 10.1038/s41598-020-64436-z] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2019] [Accepted: 04/13/2020] [Indexed: 11/09/2022] Open
Abstract
To find potentially habitable exoplanets, space missions employ the habitable zone (HZ), which is the region around a star (or multiple stars) where standing bodies of water could exist on the surface of a rocky planet. Follow-up atmospheric characterization could yield biosignatures signifying life. Although most iterations of the HZ are agnostic regarding the nature of such life, a recent study argues that a complex life HZ would be considerably smaller than that used in classical definitions. Here, I use an advanced energy balance model to show that such an HZ would be considerably wider than originally predicted given revised CO2 limits and (for the first time) N2 respiration limits for complex life. The width of this complex life HZ (CLHZ) increases by ~35% from ~0.95–1.2 AU to 0.95–1.31 AU in our solar system. Similar extensions are shown for stars with stellar effective temperatures between 2,600–9,000 K. I define this CLHZ using lipid solubility theory, diving data, and results from animal laboratory experiments. I also discuss implications for biosignatures and technosignatures. Finally, I discuss the applicability of the CLHZ and other HZ variants to the search for both simple and complex life.
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Affiliation(s)
- Ramses M Ramirez
- Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo, Japan. .,Space Science Institute, Boulder, Co, USA.
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8
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Longo A, Damer B. Factoring Origin of Life Hypotheses into the Search for Life in the Solar System and Beyond. Life (Basel) 2020; 10:E52. [PMID: 32349245 PMCID: PMC7281141 DOI: 10.3390/life10050052] [Citation(s) in RCA: 13] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2020] [Revised: 04/14/2020] [Accepted: 04/22/2020] [Indexed: 01/13/2023] Open
Abstract
Two widely-cited alternative hypotheses propose geological localities and biochemical mechanisms for life's origins. The first states that chemical energy available in submarine hydrothermal vents supported the formation of organic compounds and initiated primitive metabolic pathways which became incorporated in the earliest cells; the second proposes that protocells self-assembled from exogenous and geothermally-delivered monomers in freshwater hot springs. These alternative hypotheses are relevant to the fossil record of early life on Earth, and can be factored into the search for life elsewhere in the Solar System. This review summarizes the evidence supporting and challenging these hypotheses, and considers their implications for the search for life on various habitable worlds. It will discuss the relative probability that life could have emerged in environments on early Mars, on the icy moons of Jupiter and Saturn, and also the degree to which prebiotic chemistry could have advanced on Titan. These environments will be compared to ancient and modern terrestrial analogs to assess their habitability and biopreservation potential. Origins of life approaches can guide the biosignature detection strategies of the next generation of planetary science missions, which could in turn advance one or both of the leading alternative abiogenesis hypotheses.
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Affiliation(s)
- Alex Longo
- National Aeronautics and Space Administration Headquarters, Washington, DC 20546, USA
- Department of Geology, The University of North Carolina, Chapel Hill, NC 27599, USA
| | - Bruce Damer
- Department of Biomolecular Engineering, University of California, Santa Cruz, CA 95064, USA or
- Digital Space Research, Boulder Creek, CA 95006, USA
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9
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Clouds will Likely Prevent the Detection of Water Vapor in JWST Transmission Spectra of Terrestrial Exoplanets. ACTA ACUST UNITED AC 2020. [DOI: 10.3847/2041-8213/ab6200] [Citation(s) in RCA: 33] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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10
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11
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Theoretical Reflectance Spectra of Earth-like Planets through Their Evolutions: Impact of Clouds on the Detectability of Oxygen, Water, and Methane with Future Direct Imaging Missions. ACTA ACUST UNITED AC 2019. [DOI: 10.3847/1538-3881/ab14e3] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
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12
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13
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Climate Sensitivity to Carbon Dioxide and the Moist Greenhouse Threshold of Earth-like Planets under an Increasing Solar Forcing. ACTA ACUST UNITED AC 2018. [DOI: 10.3847/1538-4357/aaea5f] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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14
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Way M, Del Genio AD, Aleinov I, Clune TL, Kelley M, Kiang NY. Climates of Warm Earth-like Planets I: 3-D Model Simulations. THE ASTROPHYSICAL JOURNAL. SUPPLEMENT SERIES 2018; 239:24. [PMID: 30948861 PMCID: PMC6443379 DOI: 10.3847/1538-4365/aae9e1] [Citation(s) in RCA: 47] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
We present a large ensemble of simulations of an Earth-like world with increasing insolation and rotation rate. Unlike previous work utilizing idealized aquaplanet configurations we focus our simulations on modern Earth-like topography. The orbital period is the same as modern Earth, but with zero obliquity and eccentricity. The atmosphere is 1 bar N2-dominated with CO2=400 ppmv and CH4=1 ppmv. The simulations include two types of oceans; one without ocean heat transport (OHT) between grid cells as has been commonly used in the exoplanet literature, while the other is a fully coupled dynamic bathtub type ocean. The dynamical regime transitions that occur as day length increases induce climate feedbacks producing cooler temperatures, first via the reduction of water vapor with increasing rotation period despite decreasing shortwave cooling by clouds, and then via decreasing water vapor and increasing shortwave cloud cooling, except at the highest insolations. Simulations without OHT are more sensitive to insolation changes for fast rotations while slower rotations are relatively insensitive to ocean choice. OHT runs with faster rotations tend to be similar with gyres transporting heat poleward making them warmer than those without OHT. For slower rotations OHT is directed equator-ward and no high latitude gyres are apparent. Uncertainties in cloud parameterization preclude a precise determination of habitability but do not affect robust aspects of exoplanet climate sensitivity. This is the first paper in a series that will investigate aspects of habitability in the simulations presented herein. The datasets from this study are opensource and publicly available.
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Affiliation(s)
- M.J. Way
- NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY, 10025, USA
- Department of Physics and Astronomy, Uppsala University, Uppsala, 75120, Sweden
| | - Anthony D. Del Genio
- NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY, 10025, USA
| | - Igor Aleinov
- NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY, 10025, USA
- Center for Climate Systems Research, Columbia University, New York, NY 10025, USA
| | - Thomas L. Clune
- Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, USA
| | - Maxwell Kelley
- NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY, 10025, USA
| | - Nancy Y. Kiang
- NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY, 10025, USA
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15
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Abiotic O2 Levels on Planets around F, G, K, and M Stars: Effects of Lightning-produced Catalysts in Eliminating Oxygen False Positives. ACTA ACUST UNITED AC 2018. [DOI: 10.3847/1538-4357/aadd9b] [Citation(s) in RCA: 31] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
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16
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17
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Abstract
The habitable zone (HZ) is the circular region around a star(s) where standing bodies of water could exist on the surface of a rocky planet. Space missions employ the HZ to select promising targets for follow-up habitability assessment. The classical HZ definition assumes that the most important greenhouse gases for habitable planets orbiting main-sequence stars are CO2 and H2O. Although the classical HZ is an effective navigational tool, recent HZ formulations demonstrate that it cannot thoroughly capture the diversity of habitable exoplanets. Here, I review the planetary and stellar processes considered in both classical and newer HZ formulations. Supplementing the classical HZ with additional considerations from these newer formulations improves our capability to filter out worlds that are unlikely to host life. Such improved HZ tools will be necessary for current and upcoming missions aiming to detect and characterize potentially habitable exoplanets.
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18
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UV Surface Environments and Atmospheres of Earth-like Planets Orbiting White Dwarfs. ACTA ACUST UNITED AC 2018. [DOI: 10.3847/1538-4357/aacbc7] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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19
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Fujii Y, Angerhausen D, Deitrick R, Domagal-Goldman S, Grenfell JL, Hori Y, Kane SR, Pallé E, Rauer H, Siegler N, Stapelfeldt K, Stevenson KB. Exoplanet Biosignatures: Observational Prospects. ASTROBIOLOGY 2018; 18:739-778. [PMID: 29938537 PMCID: PMC6016572 DOI: 10.1089/ast.2017.1733] [Citation(s) in RCA: 20] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/03/2017] [Accepted: 03/13/2018] [Indexed: 05/04/2023]
Abstract
Exoplanet hunting efforts have revealed the prevalence of exotic worlds with diverse properties, including Earth-sized bodies, which has fueled our endeavor to search for life beyond the Solar System. Accumulating experiences in astrophysical, chemical, and climatological characterization of uninhabitable planets are paving the way to characterization of potentially habitable planets. In this paper, we review our possibilities and limitations in characterizing temperate terrestrial planets with future observational capabilities through the 2030s and beyond, as a basis of a broad range of discussions on how to advance "astrobiology" with exoplanets. We discuss the observability of not only the proposed biosignature candidates themselves but also of more general planetary properties that provide circumstantial evidence, since the evaluation of any biosignature candidate relies on its context. Characterization of temperate Earth-sized planets in the coming years will focus on those around nearby late-type stars. The James Webb Space Telescope (JWST) and later 30-meter-class ground-based telescopes will empower their chemical investigations. Spectroscopic studies of potentially habitable planets around solar-type stars will likely require a designated spacecraft mission for direct imaging, leveraging technologies that are already being developed and tested as part of the Wide Field InfraRed Survey Telescope (WFIRST) mission. Successful initial characterization of a few nearby targets will be an important touchstone toward a more detailed scrutiny and a larger survey that are envisioned beyond 2030. The broad outlook this paper presents may help develop new observational techniques to detect relevant features as well as frameworks to diagnose planets based on the observables. Key Words: Exoplanets-Biosignatures-Characterization-Planetary atmospheres-Planetary surfaces. Astrobiology 18, 739-778.
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Affiliation(s)
- Yuka Fujii
- NASA Goddard Institute for Space Studies, New York, New York, USA
- Earth-Life Science Institute, Tokyo Institute of Technology, Ookayama, Meguro, Tokyo, Japan
| | - Daniel Angerhausen
- CSH Fellow for Exoplanetary Astronomy, Center for Space and Habitability (CSH), Universität Bern, Bern, Switzerland
- Blue Marble Space Institute of Science, Seattle, Washington, USA
| | - Russell Deitrick
- Department of Astronomy, University of Washington, Seattle, Washington, USA
- NASA Astrobiology Institute's Virtual Planetary Laboratory
| | - Shawn Domagal-Goldman
- NASA Astrobiology Institute's Virtual Planetary Laboratory
- NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
| | - John Lee Grenfell
- Department of Extrasolar Planets and Atmospheres (EPA), Institute of Planetary Research, German Aerospace Centre (DLR), Berlin, Germany
| | - Yasunori Hori
- Astrobiology Center, National Institutes of Natural Sciences (NINS), Mitaka, Tokyo, Japan
| | - Stephen R. Kane
- Department of Earth Sciences, University of California, Riverside, California, USA
| | - Enric Pallé
- Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain
- Departamento de Astrofísica, Universidad de La Laguna, Tenerife, Spain
| | - Heike Rauer
- Department of Extrasolar Planets and Atmospheres (EPA), Institute of Planetary Research, German Aerospace Centre (DLR), Berlin, Germany
- Center for Astronomy and Astrophysics, Berlin Institute of Technology, Berlin, Germany
| | - Nicholas Siegler
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
- NASA Exoplanet Exploration Office
| | - Karl Stapelfeldt
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
- NASA Exoplanet Exploration Office
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20
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Schwieterman EW, Kiang NY, Parenteau MN, Harman CE, DasSarma S, Fisher TM, Arney GN, Hartnett HE, Reinhard CT, Olson SL, Meadows VS, Cockell CS, Walker SI, Grenfell JL, Hegde S, Rugheimer S, Hu R, Lyons TW. Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life. ASTROBIOLOGY 2018; 18:663-708. [PMID: 29727196 PMCID: PMC6016574 DOI: 10.1089/ast.2017.1729] [Citation(s) in RCA: 110] [Impact Index Per Article: 18.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/27/2017] [Accepted: 12/10/2017] [Indexed: 05/04/2023]
Abstract
In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets-Biosignatures-Habitability markers-Photosynthesis-Planetary surfaces-Atmospheres-Spectroscopy-Cryptic biospheres-False positives. Astrobiology 18, 663-708.
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Affiliation(s)
- Edward W. Schwieterman
- Department of Earth Sciences, University of California, Riverside, California
- NASA Postdoctoral Program, Universities Space Research Association, Columbia, Maryland
- NASA Astrobiology Institute, Virtual Planetary Laboratory Team, Seattle, Washington
- NASA Astrobiology Institute, Alternative Earths Team, Riverside, California
- Blue Marble Space Institute of Science, Seattle, Washington
| | - Nancy Y. Kiang
- NASA Astrobiology Institute, Virtual Planetary Laboratory Team, Seattle, Washington
- NASA Goddard Institute for Space Studies, New York, New York
| | - Mary N. Parenteau
- NASA Astrobiology Institute, Virtual Planetary Laboratory Team, Seattle, Washington
- NASA Ames Research Center, Exobiology Branch, Mountain View, California
| | - Chester E. Harman
- NASA Astrobiology Institute, Virtual Planetary Laboratory Team, Seattle, Washington
- NASA Goddard Institute for Space Studies, New York, New York
- Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York
| | - Shiladitya DasSarma
- Department of Microbiology and Immunology, University of Maryland School of Medicine, Baltimore, Maryland
- Institute of Marine and Environmental Technology, University System of Maryland, Baltimore, Maryland
| | - Theresa M. Fisher
- School of Earth and Space Exploration, Arizona State University, Tempe, Arizona
| | - Giada N. Arney
- NASA Astrobiology Institute, Virtual Planetary Laboratory Team, Seattle, Washington
- Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland
| | - Hilairy E. Hartnett
- School of Earth and Space Exploration, Arizona State University, Tempe, Arizona
- School of Molecular Sciences, Arizona State University, Tempe, Arizona
| | - Christopher T. Reinhard
- NASA Astrobiology Institute, Alternative Earths Team, Riverside, California
- School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia
| | - Stephanie L. Olson
- Department of Earth Sciences, University of California, Riverside, California
- NASA Astrobiology Institute, Alternative Earths Team, Riverside, California
| | - Victoria S. Meadows
- NASA Astrobiology Institute, Virtual Planetary Laboratory Team, Seattle, Washington
- Astronomy Department, University of Washington, Seattle, Washington
| | - Charles S. Cockell
- University of Edinburgh School of Physics and Astronomy, Edinburgh, United Kingdom
- UK Centre for Astrobiology, Edinburgh, United Kingdom
| | - Sara I. Walker
- Blue Marble Space Institute of Science, Seattle, Washington
- School of Earth and Space Exploration, Arizona State University, Tempe, Arizona
- Beyond Center for Fundamental Concepts in Science, Arizona State University, Tempe, Arizona
- ASU-Santa Fe Institute Center for Biosocial Complex Systems, Arizona State University, Tempe, Arizona
| | - John Lee Grenfell
- Institut für Planetenforschung (PF), Deutsches Zentrum für Luft und Raumfahrt (DLR), Berlin, Germany
| | - Siddharth Hegde
- Carl Sagan Institute, Cornell University, Ithaca, New York
- Cornell Center for Astrophysics and Planetary Science, Cornell University, Ithaca, New York
| | - Sarah Rugheimer
- Department of Earth and Environmental Sciences, University of St. Andrews, St. Andrews, United Kingdom
| | - Renyu Hu
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California
- Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California
| | - Timothy W. Lyons
- Department of Earth Sciences, University of California, Riverside, California
- NASA Astrobiology Institute, Alternative Earths Team, Riverside, California
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Redox Evolution via Gravitational Differentiation on Low-mass Planets: Implications for Abiotic Oxygen, Water Loss, and Habitability. ACTA ACUST UNITED AC 2018. [DOI: 10.3847/1538-3881/aab608] [Citation(s) in RCA: 40] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
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Balbi A, Tombesi F. The habitability of the Milky Way during the active phase of its central supermassive black hole. Sci Rep 2017; 7:16626. [PMID: 29192275 PMCID: PMC5709466 DOI: 10.1038/s41598-017-16110-0] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2017] [Accepted: 11/03/2017] [Indexed: 12/05/2022] Open
Abstract
During the peak of their accretion phase, supermassive black holes in galactic cores are known to emit very high levels of ionizing radiation, becoming visible over intergalactic distances as quasars or active galactic nuclei (AGN). Here, we quantify the extent to which the activity of the supermassive black hole at the center of the Milky Way, known as Sagittarius A* (Sgr A*), may have affected the habitability of Earth-like planets in our Galaxy. We focus on the amount of atmospheric loss and on the possible biological damage suffered by planets exposed to X-ray and extreme ultraviolet (XUV) radiation produced during the peak of the active phase of Sgr A*. We find that terrestrial planets could lose a total atmospheric mass comparable to that of present day Earth even at large distances (~1 kiloparsec) from the galactic center. Furthermore, we find that the direct biological damage caused by Sgr A* to surface life on planets not properly screened by an atmosphere was probably significant during the AGN phase, possibly hindering the development of complex life within a few kiloparsecs from the galactic center.
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Affiliation(s)
- Amedeo Balbi
- Dipartimento di Fisica, Università degli Studi di Roma "Tor Vergata", Via della Ricerca Scientifica, 00133, Roma, Italy.
| | - Francesco Tombesi
- Dipartimento di Fisica, Università degli Studi di Roma "Tor Vergata", Via della Ricerca Scientifica, 00133, Roma, Italy
- Department of Astronomy, University of Maryland, College Park, MD 20742, USA
- NASA/Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA
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The Surface UV Environment on Planets Orbiting M Dwarfs: Implications for Prebiotic Chemistry and the Need for Experimental Follow-up. ACTA ACUST UNITED AC 2017. [DOI: 10.3847/1538-4357/aa773e] [Citation(s) in RCA: 74] [Impact Index Per Article: 10.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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Komacek TD, Abbot DS. EFFECT OF SURFACE-MANTLE WATER EXCHANGE PARAMETERIZATIONS ON EXOPLANET OCEAN DEPTHS. THE ASTROPHYSICAL JOURNAL 2016; 832:54. [PMID: 30705445 PMCID: PMC6350802 DOI: 10.3847/0004-637x/832/1/54] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Terrestrial exoplanets in the canonical habitable zone may have a variety of initial water fractions due to random volatile delivery by planetesimals. If the total planetary water complement is high, the entire surface may be covered in water, forming a "waterworld." On a planet with active tectonics, competing mechanisms act to regulate the abundance of water on the surface by determining the partitioning of water between interior and surface. Here we explore how the incorporation of different mechanisms for the degassing and regassing of water changes the volatile evolution of a planet. For all of the models considered, volatile cycling reaches an approximate steady state after ~2 Gyr. Using these steady states, we find that if volatile cycling is either solely dependent on temperature or seafloor pressure, exoplanets require a high abundance (≳0.3% of total mass) of water to have fully inundated surfaces. However, if degassing is more dependent on seafloor pressure and regassing mainly dependent on mantle temperature, the degassing rate is relatively large at late times and a steady state between degassing and regassing is reached with a substantial surface water fraction. If this hybrid model is physical, super-Earths with a total water fraction similar to that of the Earth can become waterworlds. As a result, further understanding of the processes that drive volatile cycling on terrestrial planets is needed to determine the water fraction at which they are likely to become waterworlds.
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Affiliation(s)
- Thaddeus D Komacek
- Lunar and Planetary Laboratory, Department of Planetary Sciences, University of Arizona, Tucson, AZ, USA
| | - Dorian S Abbot
- Department of the Geophysical Sciences, University of Chicago, Chicago, IL, USA
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Haqq-Misra J, Kopparapu RK, Batalha NE, Harman CE, Kasting JF. LIMIT CYCLES CAN REDUCE THE WIDTH OF THE HABITABLE ZONE. THE ASTROPHYSICAL JOURNAL 2016; 827:120. [PMID: 30116072 PMCID: PMC6090540 DOI: 10.3847/0004-637x/827/2/120] [Citation(s) in RCA: 84] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
The liquid water habitable zone (HZ) describes the orbital distance at which a terrestrial planet can maintain above-freezing conditions through regulation by the carbonate-silicate cycle. Recent calculations have suggested that planets in the outer regions of the HZ cannot maintain stable, warm climates, but rather should oscillate between long, globally glaciated states and shorter periods of climatic warmth. Such conditions, similar to "Snowball Earth" episodes experienced on Earth, would be inimical to the development of complex land life, including intelligent life. Here, we build on previous studies with an updated energy balance climate model to calculate this "limit cycle" region of the HZ where such cycling would occur. We argue that an abiotic Earth would have a greater CO2 partial pressure than today because plants and other biota help to enhance the storage of CO2 in soil. When we tune our abiotic model accordingly, we find that limit cycles can occur but that previous calculations have overestimated their importance. For G stars like the Sun, limit cycles occur only for planets with CO2 outgassing rates less than that on modern Earth. For K- and M-star planets, limit cycles should not occur; however, M-star planets may be inhospitable to life for other reasons. Planets orbiting late G-type and early K-type stars retain the greatest potential for maintaining warm, stable conditions. Our results suggest that host star type, planetary volcanic activity, and seafloor weathering are all important factors in determining whether planets will be prone to limit cycling.
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Affiliation(s)
- Jacob Haqq-Misra
- Blue Marble Space Institute of Science, 1001 4th Avenue, Suite 3201, Seattle, WA 98154, USA
- NASA Astrobiology Institute's Virtual Planetary Laboratory, P.O. Box 351580, Seattle, WA 98195, USA
| | - Ravi Kumar Kopparapu
- Blue Marble Space Institute of Science, 1001 4th Avenue, Suite 3201, Seattle, WA 98154, USA
- NASA Astrobiology Institute's Virtual Planetary Laboratory, P.O. Box 351580, Seattle, WA 98195, USA
- NASA Goddard Space Flight Center, 8800 Greenbelt Road, Mail Stop 699.0, Building 34, Greenbelt, MD 20771, USA
- Department of Astronomy, University of Maryland, College Park, MD 20742, USA
| | - Natasha E Batalha
- NASA Astrobiology Institute's Virtual Planetary Laboratory, P.O. Box 351580, Seattle, WA 98195, USA
- Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
- Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, University Park, PA 16802, USA
| | - Chester E Harman
- NASA Astrobiology Institute's Virtual Planetary Laboratory, P.O. Box 351580, Seattle, WA 98195, USA
- Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, University Park, PA 16802, USA
- Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, USA
| | - James F Kasting
- NASA Astrobiology Institute's Virtual Planetary Laboratory, P.O. Box 351580, Seattle, WA 98195, USA
- Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, University Park, PA 16802, USA
- Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, USA
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Heller R, Pudritz RE. The Search for Extraterrestrial Intelligence in Earth's Solar Transit Zone. ASTROBIOLOGY 2016; 16:259-270. [PMID: 26967201 DOI: 10.1089/ast.2015.1358] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Over the past few years, astronomers have detected thousands of planets and candidate planets by observing their periodic transits in front of their host stars. A related method, called transit spectroscopy, might soon allow studies of the chemical imprints of life in extrasolar planetary atmospheres. Here, we address the reciprocal question, namely, from where is Earth detectable by extrasolar observers using similar methods. We explore Earth's transit zone (ETZ), the projection of a band around Earth's ecliptic onto the celestial plane, where observers can detect Earth transits across the Sun. ETZ is between 0.520° and 0.537° wide due to the noncircular Earth orbit. The restricted Earth transit zone (rETZ), where Earth transits the Sun less than 0.5 solar radii from its center, is about 0.262° wide. We first compile a target list of 45 K and 37 G dwarf stars inside the rETZ and within 1 kpc (about 3260 light-years) using the Hipparcos catalogue. We then greatly enlarge the number of potential targets by constructing an analytic galactic disk model and find that about 10(5) K and G dwarf stars should reside within the rETZ. The ongoing Gaia space mission can potentially discover all G dwarfs among them (several 10(4)) within the next 5 years. Many more potentially habitable planets orbit dim, unknown M stars in ETZ and other stars that traversed ETZ thousands of years ago. If any of these planets host intelligent observers, they could have identified Earth as a habitable, or even as a living, world long ago, and we could be receiving their broadcasts today. The K2 mission, the Allen Telescope Array, the upcoming Square Kilometer Array, or the Green Bank Telescope might detect such deliberate extraterrestrial messages. Ultimately, ETZ would be an ideal region to be monitored by the Breakthrough Listen Initiatives, an upcoming survey that will constitute the most comprehensive search for extraterrestrial intelligence so far.
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Affiliation(s)
- René Heller
- 1 Max Planck Institute for Solar System Research , Göttingen, Germany
| | - Ralph E Pudritz
- 2 Origins Institute , McMaster University , Hamilton, Canada
- 3 Department of Physics and Astronomy, McMaster University , Hamilton, Canada
- 4 Max Planck Institute for Astronomy , Heidelberg, Germany
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Cockell CS, Bush T, Bryce C, Direito S, Fox-Powell M, Harrison JP, Lammer H, Landenmark H, Martin-Torres J, Nicholson N, Noack L, O'Malley-James J, Payler SJ, Rushby A, Samuels T, Schwendner P, Wadsworth J, Zorzano MP. Habitability: A Review. ASTROBIOLOGY 2016; 16:89-117. [PMID: 26741054 DOI: 10.1089/ast.2015.1295] [Citation(s) in RCA: 94] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/25/2023]
Abstract
Habitability is a widely used word in the geoscience, planetary science, and astrobiology literature, but what does it mean? In this review on habitability, we define it as the ability of an environment to support the activity of at least one known organism. We adopt a binary definition of "habitability" and a "habitable environment." An environment either can or cannot sustain a given organism. However, environments such as entire planets might be capable of supporting more or less species diversity or biomass compared with that of Earth. A clarity in understanding habitability can be obtained by defining instantaneous habitability as the conditions at any given time in a given environment required to sustain the activity of at least one known organism, and continuous planetary habitability as the capacity of a planetary body to sustain habitable conditions on some areas of its surface or within its interior over geological timescales. We also distinguish between surface liquid water worlds (such as Earth) that can sustain liquid water on their surfaces and interior liquid water worlds, such as icy moons and terrestrial-type rocky planets with liquid water only in their interiors. This distinction is important since, while the former can potentially sustain habitable conditions for oxygenic photosynthesis that leads to the rise of atmospheric oxygen and potentially complex multicellularity and intelligence over geological timescales, the latter are unlikely to. Habitable environments do not need to contain life. Although the decoupling of habitability and the presence of life may be rare on Earth, it may be important for understanding the habitability of other planetary bodies.
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Affiliation(s)
- C S Cockell
- 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK
| | - T Bush
- 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK
| | - C Bryce
- 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK
| | - S Direito
- 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK
| | - M Fox-Powell
- 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK
| | - J P Harrison
- 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK
| | - H Lammer
- 2 Austrian Academy of Sciences, Space Research Institute , Graz, Austria
| | - H Landenmark
- 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK
| | - J Martin-Torres
- 3 Division of Space Technology, Department of Computer Science, Electrical and Space Engineering, Luleå University of Technology , Kiruna, Sweden; and Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Armilla, Granada, Spain
| | - N Nicholson
- 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK
| | - L Noack
- 4 Department of Reference Systems and Planetology, Royal Observatory of Belgium , Brussels, Belgium
| | - J O'Malley-James
- 5 School of Physics and Astronomy, University of St Andrews , St Andrews, UK; now at the Carl Sagan Institute, Cornell University, Ithaca, NY, USA
| | - S J Payler
- 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK
| | - A Rushby
- 6 Centre for Ocean and Atmospheric Science (COAS), School of Environmental Sciences, University of East Anglia , Norwich, UK
| | - T Samuels
- 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK
| | - P Schwendner
- 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK
| | - J Wadsworth
- 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK
| | - M P Zorzano
- 3 Division of Space Technology, Department of Computer Science, Electrical and Space Engineering, Luleå University of Technology , Kiruna, Sweden; and Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Armilla, Granada, Spain
- 7 Centro de Astrobiología (CSIC-INTA) , Torrejón de Ardoz, Madrid, Spain
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Harman CE, Schwieterman EW, Schottelkotte JC, Kasting JF. ABIOTIC O2LEVELS ON PLANETS AROUND F, G, K, AND M STARS: POSSIBLE FALSE POSITIVES FOR LIFE? ACTA ACUST UNITED AC 2015. [DOI: 10.1088/0004-637x/812/2/137] [Citation(s) in RCA: 150] [Impact Index Per Article: 16.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Mulders GD, Ciesla FJ, Min M, Pascucci I. THE SNOW LINE IN VISCOUS DISKS AROUND LOW-MASS STARS: IMPLICATIONS FOR WATER DELIVERY TO TERRESTRIAL PLANETS IN THE HABITABLE ZONE. ACTA ACUST UNITED AC 2015. [DOI: 10.1088/0004-637x/807/1/9] [Citation(s) in RCA: 75] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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