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Vilović I, Schulze-Makuch D, Heller R. Variations in climate habitability parameters and their effect on Earth's biosphere during the Phanerozoic Eon. Sci Rep 2023; 13:12663. [PMID: 37542097 PMCID: PMC10403619 DOI: 10.1038/s41598-023-39716-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2023] [Accepted: 07/29/2023] [Indexed: 08/06/2023] Open
Abstract
Essential insights on the characterization and quality of a detectable biosphere are gained by analyzing the effects of its environmental parameters. We compiled environmental and biological properties of the Phanerozoic Eon from various published data sets and conducted a correlation analysis to assess variations in parameters relevant to the habitability of Earth's biosphere. We showed that environmental parameters such as oxygen, global average surface temperatures, runoff rates and carbon dioxide are interrelated and play a key role in the changes of biomass and biodiversity. We showed that there were several periods with a highly thriving biosphere, with one even surpassing present day biodiversity and biomass. Those periods were characterized by increased oxygen levels and global runoff rates, as well as moderate global average surface temperatures, as long as no large or rapid positive and/or negative temperature excursions occurred. High oxygen contents are diagnostic of biomass production by continental plant life. We find that exceptionally high oxygen levels can at least in one instance compensate for decreased relative humidities, providing an even more habitable environment compared to today. Beyond Earth, these results will help us to understand how environmental parameters affect biospheres on extrasolar planets and guide us in our search for extraterrestrial life.
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Affiliation(s)
- Iva Vilović
- Astrobiology Research Group, Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, 10623, Berlin, Germany.
| | - Dirk Schulze-Makuch
- Astrobiology Research Group, Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, 10623, Berlin, Germany
- GFZ German Research Center for Geosciences, Section Geomicrobiology, 14473, Potsdam, Germany
- Department of Experimental Limnology, Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), 16775, Stechlin, Germany
- School of the Environment, Washington State University, Pullman, Washington, USA
| | - René Heller
- Max-Planck-Institut für Sonnensystemforschung, 37077, Göttingen, Germany
- Institut für Astrophysik, Georg-August-Universität Göttingen, 37077, Göttingen, Germany
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2
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Höning D, Spohn T. Land Fraction Diversity on Earth-like Planets and Implications for Their Habitability. ASTROBIOLOGY 2023; 23:372-394. [PMID: 36848252 DOI: 10.1089/ast.2022.0070] [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: 06/18/2023]
Abstract
A balanced ratio of ocean to land is believed to be essential for an Earth-like biosphere, and one may conjecture that plate-tectonics planets should be similar in geological properties. After all, the volume of continental crust evolves toward an equilibrium between production and erosion. If the interior thermal states of Earth-sized exoplanets are similar to those of Earth-a straightforward assumption due to the temperature dependence of mantle viscosity-one might expect a similar equilibrium between continental production and erosion to establish, and hence a similar land fraction. We show that this conjecture is not likely to be true. Positive feedback associated with the coupled mantle water-continental crust cycle may rather lead to a manifold of three possible planets, depending on their early history: a land planet, an ocean planet, and a balanced Earth-like planet. In addition, thermal blanketing of the interior by the continents enhances the sensitivity of continental growth to its history and, eventually, to initial conditions. Much of the blanketing effect is, however, compensated by mantle depletion in radioactive elements. A model of the long-term carbonate-silicate cycle shows the land and the ocean planets to differ by about 5 K in average surface temperature. A larger continental surface fraction results both in higher weathering rates and enhanced outgassing, partly compensating each other. Still, the land planet is expected to have a substantially dryer, colder, and harsher climate possibly with extended cold deserts in comparison with the ocean planet and with the present-day Earth. Using a model of balancing water availability and nutrients from continental crust weathering, we find the bioproductivity and the biomass of both the land and ocean planets to be reduced by a third to half of those of Earth. The biosphere on these planets might not be substantial enough to produce a supply of free oxygen.
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Affiliation(s)
- Dennis Höning
- Potsdam-Institute for Climate Impact Research, Potsdam, Germany
| | - Tilman Spohn
- International Space Science Institute, Bern, Switzerland
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3
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Graham RJ. High pCO 2 Reduces Sensitivity to CO 2 Perturbations on Temperate, Earth-like Planets Throughout Most of Habitable Zone. ASTROBIOLOGY 2021; 21:1406-1420. [PMID: 34375145 DOI: 10.1089/ast.2020.2411] [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: 06/13/2023]
Abstract
The nearly logarithmic radiative impact of CO2 means that planets near the outer edge of the liquid water habitable zone (HZ) require ∼106 × more CO2 to maintain temperatures that are conducive to standing liquid water on the planetary surface than their counterparts near the inner edge. This logarithmic radiative response also means that atmospheric CO2 changes of a given mass will have smaller temperature effects on higher pCO2 planets. Ocean pH is linked to atmospheric pCO2 through seawater carbonate speciation and calcium carbonate dissolution/precipitation, and the response of pH to changes in pCO2 also decreases at higher initial pCO2. Here, we use idealized climate and ocean chemistry models to demonstrate that CO2 perturbations large enough to cause catastrophic changes to surface temperature and ocean pH on temperate, low-pCO2 planets in the innermost region of the HZ are likely to have much smaller effects on planets with higher pCO2, as may be the case for terrestrial planets with active carbonate-silicate cycles receiving less instellation than the Earth. Major bouts of extraterrestrial fossil fuel combustion or volcanic CO2 outgassing on high-pCO2 planets in the mid-to-outer HZ should have mild or negligible impacts on surface temperature and ocean pH. Owing to low pCO2, Phanerozoic Earth's surface environment may be unusually volatile compared with similar planets receiving lower instellation.
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Affiliation(s)
- Robert J Graham
- Atmospheric, Oceanic, and Planetary Physics, Clarendon Laboratory, Department of Physics, University of Oxford, Oxford, United Kingdom
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4
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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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5
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Schulze-Makuch D, Heller R, Guinan E. In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World. ASTROBIOLOGY 2020; 20:1394-1404. [PMID: 32955925 PMCID: PMC7757576 DOI: 10.1089/ast.2019.2161] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Accepted: 08/07/2020] [Indexed: 06/11/2023]
Abstract
The fact that Earth is teeming with life makes it appear odd to ask whether there could be other planets in our galaxy that may be even more suitable for life. Neglecting this possible class of "superhabitable" planets, however, could be considered anthropocentric and geocentric biases. Most important from the perspective of an observer searching for extrasolar life is that such a search might be executed most effectively with a focus on superhabitable planets instead of Earth-like planets. We argue that there could be regions of astrophysical parameter space of star-planet systems that could allow for planets to be even better for life than our Earth. We aim to identify those parameters and their optimal ranges, some of which are astrophysically motivated, whereas others are based on the varying habitability of the natural history of our planet. Some of these conditions are far from being observationally testable on planets outside the solar system. Still, we can distill a short list of 24 top contenders among the >4000 exoplanets known today that could be candidates for a superhabitable planet. In fact, we argue that, with regard to the search for extrasolar life, potentially superhabitable planets may deserve higher priority for follow-up observations than most Earth-like planets.
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Affiliation(s)
- Dirk Schulze-Makuch
- Astrobiology Group, Center for Astronomy and Astrophysics, Technische Universität Berlin, Berlin, Germany
- GFZ German Research Center for Geosciences, Section Geomicrobiology, Potsdam, Germany
- Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Department of Experimental Limnology, Stechlin, Germany
- School of the Environment, Washington State University, Pullman, Washington, USA
| | - René Heller
- Max Planck Institute for Solar System Research, Göttingen, Germany
- Institute for Astrophysics, Georg-August University, Göttingen, Germany
| | - Edward Guinan
- Department of Astrophysics and Planetary Science, Villanova University, Villanova, Pennsylvania, USA
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6
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Saha S, Nagaraj N, Mathur A, Yedida R, H R S. Evolution of novel activation functions in neural network training for astronomy data: habitability classification of exoplanets. THE EUROPEAN PHYSICAL JOURNAL. SPECIAL TOPICS 2020; 229:2629-2738. [PMID: 33194093 PMCID: PMC7651829 DOI: 10.1140/epjst/e2020-000098-9] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/08/2020] [Accepted: 09/02/2020] [Indexed: 06/11/2023]
Abstract
Quantification of habitability is a complex task. Previous attempts at measuring habitability are well documented. Classification of exoplanets, on the other hand, is a different approach and depends on quality of training data available in habitable exoplanet catalogs. Classification is the task of predicting labels of newly discovered planets based on available class labels in the catalog. We present analytical exploration of novel activation functions as consequence of integration of several ideas leading to implementation and subsequent use in habitability classification of exoplanets. Neural networks, although a powerful engine in supervised methods, often require expensive tuning efforts for optimized performance. Habitability classes are hard to discriminate, especially when attributes used as hard markers of separation are removed from the data set. The solution is approached from the point of investigating analytical properties of the proposed activation functions. The theory of ordinary differential equations and fixed point are exploited to justify the "lack of tuning efforts" to achieve optimal performance compared to traditional activation functions. Additionally, the relationship between the proposed activation functions and the more popular ones is established through extensive analytical and empirical evidence. Finally, the activation functions have been implemented in plain vanilla feed-forward neural network to classify exoplanets. The mathematical exercise supplements the grand idea of classifying exoplanets, computing habitability scores/indices and automatic grouping of the exoplanets converging at some level.
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Affiliation(s)
- Snehanshu Saha
- CSIS and APPCAIR, BITS Pilani K K Birla, Goa Campus, Sancoale, India
| | - Nithin Nagaraj
- Consciousness Studies Programme, National Institute of Advanced Studies, Bengaluru, India
| | - Archana Mathur
- Nitte Meenakshi Institute of Technology, Bengaluru, India
| | | | - Sneha H R
- Nitte Meenakshi Institute of Technology, Bengaluru, India
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7
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Abstract
Most definitions of life assume that, at a minimum, life is a physical form of matter distinct from its environment at a lower state of entropy than its surroundings, using energy from the environment for internal maintenance and activity, and capable of autonomous reproduction. These assumptions cover all of life as we know it, though more exotic entities can be envisioned, including organic forms with novel biochemistries, dynamic inorganic matter, and self-replicating machines. The probability that any particular form of life will be found on another planetary body depends on the nature and history of that alien world. So the biospheres would likely be very different on a rocky planet with an ice-covered global ocean, a barren planet devoid of surface liquid, a frigid world with abundant liquid hydrocarbons, on a rogue planet independent of a host star, on a tidally locked planet, on super-Earths, or in long-lived clouds in dense atmospheres. While life at least in microbial form is probably pervasive if rare throughout the Universe, and technologically advanced life is likely much rarer, the chance that an alternative form of life, though not intelligent life, could exist and be detected within our Solar System is a distinct possibility.
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9
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On A Hypothetical Mechanism of Interstellar Life Transfer Trough Nomadic Objects. ORIGINS LIFE EVOL B 2020; 50:87-96. [PMID: 32034615 DOI: 10.1007/s11084-020-09591-z] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2019] [Accepted: 01/03/2020] [Indexed: 10/25/2022]
Abstract
Lethal radiation, low vacuum pressure and low temperatures - this is how space welcomes organisms. Crossing of immense interstellar distances inflates the exposure time of biological material to harmful space conditions. This paper discusses the intriguing possibility of a life-bearing exoplanet being ejected from its planetary system and carrying life across interstellar distances (nomadic = free floating = rogue planet). The proposed interstellar panspermia mechanism reduces the exposure time to space conditions and provides multiple chances for interactions between microbes-bearing rock debris and exoplanets within system the nomadic object encountered on its way. The testing strategy is outlined and discussed in the paper, including testable predictions the proposed hypothesis makes.
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10
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Lenardic A, Seales J. Different Is More: The Value of Finding an Inhabited Planet That Is Far from Earth2.0. ASTROBIOLOGY 2019; 19:1398-1409. [PMID: 31411492 DOI: 10.1089/ast.2018.1833] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
The search for an inhabited planet, beyond our own, is a driver of planetary exploration in our solar system and beyond. Using information from our own planet to inform search strategies allows for a targeted search. It is, however, worth considering some span in the strategy and in a priori expectation. An inhabited, Earth-like planet is one that would be similar to Earth in ways that extend beyond having biota. To facilitate a comparative cost/risk/benefit analysis of different potential search strategies, we use a metric akin to the Earth-similarity index. The metric extends from zero, for an inhabited planet that is like Earth in all other regards (i.e., zero differences), toward end-member values for planets that differ from Earth but maintain life potential. The analysis shows how finding inhabited planets that do not share other Earth characteristics could improve our ability to assess galactic life potential without a large increase in time-commitment costs. Search strategies that acknowledge the possibility of such planets can minimize the potential of exploration losses (e.g., searching for long durations to reach conclusions that are biased). Discovering such planets could additionally provide a test of the Gaia hypothesis-a test that has proven difficult when using only Earth as a laboratory. Finally, we discuss how an Earth2.0 narrative that has been presented to the public as a search strategy comes with nostalgia-laden baggage that does not best serve exploration.
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Affiliation(s)
- Adrian Lenardic
- Department of Earth, Environmental and Planetary Science, Rice University, Houston, Texas
| | - Johnny Seales
- Department of Earth, Environmental and Planetary Science, Rice University, Houston, Texas
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11
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Abstract
In a multiverse context, determining the probability of being in our particular universe depends on estimating its overall habitability compared to other universes with different values of the fundamental constants. One of the most important factors in determining this is the fraction of planets that actually develop life, and how this depends on planetary conditions. Many proposed possibilities for this are incompatible with the multiverse: if the emergence of life depends on the lifetime of its host star, the size of the habitable planet, or the amount of material processed, the chances of being in our universe would be very low. If the emergence of life depends on the entropy absorbed by the planet, however, our position in this universe is very natural. Several proposed models for the subsequent development of life, including the hard step model and several planetary oxygenation models, are also shown to be incompatible with the multiverse. If any of these are observed to play a large role in determining the distribution of life throughout our universe, the multiverse hypothesis will be ruled out to high significance.
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13
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Veras D, Armstrong DJ, Blake JA, Gutiérrez-Marcos JF, Jackson AP, Schäefer H. Dynamical and Biological Panspermia Constraints Within Multi-planet Exosystems. ASTROBIOLOGY 2018; 18:1106-1122. [PMID: 30095987 DOI: 10.1089/ast.2017.1786] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
As discoveries of multiple planets in the habitable zone of their parent star mount, developing analytical techniques to quantify extrasolar intra-system panspermia will become increasingly important. Here, we provide user-friendly prescriptions that describe the asteroid impact characteristics which would be necessary to transport life both inwards and outwards within these systems within a single framework. Our focus is on projectile generation and delivery and our expressions are algebraic, eliminating the need for the solution of differential equations. We derive a probability distribution function for life-bearing debris to reach a planetary orbit, and describe the survival of micro-organisms during planetary ejection, their journey through interplanetary space, and atmospheric entry.
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Affiliation(s)
- Dimitri Veras
- 1 Centre for Exoplanets and Habitability, University of Warwick , Coventry, United Kingdom
- 2 Department of Physics, University of Warwick , Coventry, United Kingdom
| | - David J Armstrong
- 1 Centre for Exoplanets and Habitability, University of Warwick , Coventry, United Kingdom
- 2 Department of Physics, University of Warwick , Coventry, United Kingdom
| | - James A Blake
- 1 Centre for Exoplanets and Habitability, University of Warwick , Coventry, United Kingdom
- 2 Department of Physics, University of Warwick , Coventry, United Kingdom
| | - Jose F Gutiérrez-Marcos
- 1 Centre for Exoplanets and Habitability, University of Warwick , Coventry, United Kingdom
- 3 School of Life Sciences, University of Warwick , Coventry, United Kingdom
| | - Alan P Jackson
- 4 Centre for Planetary Sciences, University of Toronto at Scarborough , Toronto, Canada
- 5 School of Earth and Space Exploration, Arizona State University , Tempe, Arizona
| | - Hendrik Schäefer
- 1 Centre for Exoplanets and Habitability, University of Warwick , Coventry, United Kingdom
- 3 School of Life Sciences, University of Warwick , Coventry, United Kingdom
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14
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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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Walker SI, Bains W, Cronin L, DasSarma S, Danielache S, Domagal-Goldman S, Kacar B, Kiang NY, Lenardic A, Reinhard CT, Moore W, Schwieterman EW, Shkolnik EL, Smith HB. Exoplanet Biosignatures: Future Directions. ASTROBIOLOGY 2018; 18:779-824. [PMID: 29938538 PMCID: PMC6016573 DOI: 10.1089/ast.2017.1738] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/08/2017] [Accepted: 03/13/2018] [Indexed: 05/08/2023]
Abstract
We introduce a Bayesian method for guiding future directions for detection of life on exoplanets. We describe empirical and theoretical work necessary to place constraints on the relevant likelihoods, including those emerging from better understanding stellar environment, planetary climate and geophysics, geochemical cycling, the universalities of physics and chemistry, the contingencies of evolutionary history, the properties of life as an emergent complex system, and the mechanisms driving the emergence of life. We provide examples for how the Bayesian formalism could guide future search strategies, including determining observations to prioritize or deciding between targeted searches or larger lower resolution surveys to generate ensemble statistics and address how a Bayesian methodology could constrain the prior probability of life with or without a positive detection. Key Words: Exoplanets-Biosignatures-Life detection-Bayesian analysis. Astrobiology 18, 779-824.
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Affiliation(s)
- Sara I. Walker
- 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
- Blue Marble Space Institute of Science, Seattle, Washington
| | - William Bains
- EAPS (Earth, Atmospheric and Planetary Science), MIT, Cambridge, Massachusetts
- Rufus Scientific Ltd., Royston, United Kingdom
| | - Leroy Cronin
- School of Chemistry, University of Glasgow, Glasgow, United Kingdom
| | - Shiladitya DasSarma
- Department of Microbiology and Immunology, Institute of Marine and Environmental Technology, University of Maryland School of Medicine, Baltimore, Maryland
| | - Sebastian Danielache
- Department of Materials and Life Science, Faculty of Science and Technology, Sophia University, Tokyo, Japan
- Earth Life Institute, Tokyo Institute of Technology, Tokyo, Japan
| | - Shawn Domagal-Goldman
- NASA Goddard Space Flight Center, Greenbelt, Maryland
- NASA Astrobiology Institute, Virtual Planetary Laboratory Team, University of Washington, Seattle, Washington
| | - Betul Kacar
- Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts
- NASA Astrobiology Institute, Reliving the Past Team, University of Montana, Missoula, Montana
- Department of Molecular and Cell Biology, University of Arizona, Tucson, Arizona
- Department of Astronomy and Steward Observatory, University of Arizona, Tucson, Arizona
| | - Nancy Y. Kiang
- NASA Goddard Institute for Space Studies, New York, New York
| | - Adrian Lenardic
- Department of Earth Science, Rice University, Houston, Texas
| | - Christopher T. Reinhard
- School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia
- NASA Astrobiology Institute, Alternative Earths Team, University of California, Riverside, California
| | - William Moore
- Department of Atmospheric and Planetary Sciences, Hampton University, Hampton, Virginia
- National Institute of Aerospace, Hampton, Virginia
| | - Edward W. Schwieterman
- Blue Marble Space Institute of Science, Seattle, Washington
- NASA Astrobiology Institute, Virtual Planetary Laboratory Team, University of Washington, Seattle, Washington
- NASA Astrobiology Institute, Alternative Earths Team, University of California, Riverside, California
- Department of Earth Sciences, University of California, Riverside, California
- NASA Postdoctoral Program, Universities Space Research Association, Columbia, Maryland
| | - Evgenya L. Shkolnik
- School of Earth and Space Exploration, Arizona State University, Tempe, Arizona
| | - Harrison B. Smith
- School of Earth and Space Exploration, Arizona State University, Tempe, Arizona
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17
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18
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Cataldi G, Brandeker A, Thébault P, Singer K, Ahmed E, de Vries BL, Neubeck A, Olofsson G. Searching for Biosignatures in Exoplanetary Impact Ejecta. ASTROBIOLOGY 2017; 17:721-746. [PMID: 28692303 DOI: 10.1089/ast.2015.1437] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
With the number of confirmed rocky exoplanets increasing steadily, their characterization and the search for exoplanetary biospheres are becoming increasingly urgent issues in astrobiology. To date, most efforts have concentrated on the study of exoplanetary atmospheres. Instead, we aim to investigate the possibility of characterizing an exoplanet (in terms of habitability, geology, presence of life, etc.) by studying material ejected from the surface during an impact event. For a number of impact scenarios, we estimate the escaping mass and assess its subsequent collisional evolution in a circumstellar orbit, assuming a Sun-like host star. We calculate the fractional luminosity of the dust as a function of time after the impact event and study its detectability with current and future instrumentation. We consider the possibility to constrain the dust composition, giving information on the geology or the presence of a biosphere. As examples, we investigate whether calcite, silica, or ejected microorganisms could be detected. For a 20 km diameter impactor, we find that the dust mass escaping the exoplanet is roughly comparable to the zodiacal dust, depending on the exoplanet's size. The collisional evolution is best modeled by considering two independent dust populations, a spalled population consisting of nonmelted ejecta evolving on timescales of millions of years, and dust recondensed from melt or vapor evolving on much shorter timescales. While the presence of dust can potentially be inferred with current telescopes, studying its composition requires advanced instrumentation not yet available. The direct detection of biological matter turns out to be extremely challenging. Despite considerable difficulties (small dust masses, noise such as exozodiacal dust, etc.), studying dusty material ejected from an exoplanetary surface might become an interesting complement to atmospheric studies in the future. Key Words: Biosignatures-Exoplanets-Impacts-Interplanetary dust-Remote sensing. Astrobiology 17, 721-746.
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Affiliation(s)
- Gianni Cataldi
- 1 AlbaNova University Centre, Stockholm University , Department of Astronomy, Stockholm, Sweden
- 2 Stockholm University Astrobiology Centre , Stockholm, Sweden
| | - Alexis Brandeker
- 1 AlbaNova University Centre, Stockholm University , Department of Astronomy, Stockholm, Sweden
- 2 Stockholm University Astrobiology Centre , Stockholm, Sweden
| | - Philippe Thébault
- 3 LESIA-Observatoire de Paris, UPMC Univ. Paris 06, Univ. Paris-Diderot , Paris, France
| | - Kelsi Singer
- 4 Southwest Research Institute , Boulder, Colorado, USA
| | - Engy Ahmed
- 2 Stockholm University Astrobiology Centre , Stockholm, Sweden
- 5 Royal Institute of Technology (KTH) , Science for Life Laboratory, Solna, Sweden
- 6 Stockholm University , Department of Geological Sciences, Stockholm, Sweden
| | - Bernard L de Vries
- 1 AlbaNova University Centre, Stockholm University , Department of Astronomy, Stockholm, Sweden
- 2 Stockholm University Astrobiology Centre , Stockholm, Sweden
- 7 Scientific Support Office, Directorate of Science, European Space Research and Technology Centre (ESA/ESTEC) , Noordwijk, The Netherlands
| | - Anna Neubeck
- 2 Stockholm University Astrobiology Centre , Stockholm, Sweden
- 6 Stockholm University , Department of Geological Sciences, Stockholm, Sweden
| | - Göran Olofsson
- 1 AlbaNova University Centre, Stockholm University , Department of Astronomy, Stockholm, Sweden
- 2 Stockholm University Astrobiology Centre , Stockholm, Sweden
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19
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Abstract
The number of confirmed exoplanets now exceeds 3000, with an additional nearly 5000 exoplanet candidates awaiting confirmation in the NASA Exoplanet Archive ( 2016 ). Nearly weekly we hear about the detection of a new exoplanet similar in mass to Earth and located in the so-called habitable zone around its host star. The excitement is understandable given our desire to find a second Earth. However, the excitement should not lead to an over-interpretation of the findings, because the claim can only be to have some crude similarity to Earth based on a few selected geophysical parameters. Only a very small fraction of these planets will actually be Earth-like. Earth-like would imply multiple environmental habitats and presence of a sizable biosphere and complex ecosystems, without which Earth, as we experience it, would not exist. Thus, it should be clearly communicated to the public that we are probably still many years away from having the technological capability to detect an Earth-like planet or Earth 2.0 with adequate certainty. Key Words: Habitable zone-Second Earth-Habitable planet-Habitability-Life. Astrobiology 16, 817-821.
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Affiliation(s)
- Dirk Schulze-Makuch
- 1 Center for Astronomy and Astrophysics, Technical University Berlin , Berlin, Germany
- 2 School of the Environment, Washington State University , Pullman, Washington, USA
| | - Edward Guinan
- 3 Department of Astrophysics and Planetary Science, Villanova University , Villanova, Pennsylvania, USA
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20
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21
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Abstract
Modeling studies of terrestrial extrasolar planetary climates are now including the effects of ocean circulation due to a recognition of the importance of oceans for climate; indeed, the peak equator-pole ocean heat transport on Earth peaks at almost half that of the atmosphere. However, such studies have made the assumption that fundamental oceanic properties, such as salinity, temperature, and depth, are similar to Earth. This assumption results in Earth-like circulations: a meridional overturning with warm water moving poleward at the surface, being cooled, sinking at high latitudes, and traveling equatorward at depth. Here it is shown that an exoplanetary ocean with a different salinity can circulate in the opposite direction: an equatorward flow of polar water at the surface, sinking in the tropics, and filling the deep ocean with warm water. This alternative flow regime results in a dramatic warming in the polar regions, demonstrated here using both a conceptual model and an ocean general circulation model. These results highlight the importance of ocean salinity for exoplanetary climate and consequent habitability and the need for its consideration in future studies.
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22
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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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23
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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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24
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Heller R, Williams D, Kipping D, Limbach MA, Turner E, Greenberg R, Sasaki T, Bolmont É, Grasset O, Lewis K, Barnes R, Zuluaga JI. Formation, habitability, and detection of extrasolar moons. ASTROBIOLOGY 2014; 14:798-835. [PMID: 25147963 PMCID: PMC4172466 DOI: 10.1089/ast.2014.1147] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/20/2014] [Accepted: 06/05/2014] [Indexed: 06/03/2023]
Abstract
The diversity and quantity of moons in the Solar System suggest a manifold population of natural satellites exist around extrasolar planets. Of peculiar interest from an astrobiological perspective, the number of sizable moons in the stellar habitable zones may outnumber planets in these circumstellar regions. With technological and theoretical methods now allowing for the detection of sub-Earth-sized extrasolar planets, the first detection of an extrasolar moon appears feasible. In this review, we summarize formation channels of massive exomoons that are potentially detectable with current or near-future instruments. We discuss the orbital effects that govern exomoon evolution, we present a framework to characterize an exomoon's stellar plus planetary illumination as well as its tidal heating, and we address the techniques that have been proposed to search for exomoons. Most notably, we show that natural satellites in the range of 0.1-0.5 Earth mass (i) are potentially habitable, (ii) can form within the circumplanetary debris and gas disk or via capture from a binary, and (iii) are detectable with current technology.
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Affiliation(s)
- René Heller
- Origins Institute, Department of Physics and Astronomy, McMaster University, Hamilton, Canada
| | - Darren Williams
- The Behrend College School of Science, Penn State Erie, Erie, Pennsylvania, USA
| | - David Kipping
- Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA
| | - Mary Anne Limbach
- Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey, USA
- Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, USA
| | - Edwin Turner
- Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey, USA
- The Kavli Institute for the Physics and Mathematics of the Universe, The University of Tokyo, Kashiwa, Japan
| | - Richard Greenberg
- Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
| | | | - Émeline Bolmont
- Université de Bordeaux, LAB, UMR 5804, Floirac, France
- CNRS, LAB, UMR 5804, Floirac, France
| | - Olivier Grasset
- Planetology and Geodynamics, University of Nantes, CNRS, Nantes, France
| | - Karen Lewis
- Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan
| | - Rory Barnes
- Astronomy Department, University of Washington, Seattle, Washington, USA
- NASA Astrobiology Institute—Virtual Planetary Laboratory Lead Team, USA
| | - Jorge I. Zuluaga
- FACom—Instituto de Física—FCEN, Universidad de Antioquia, Medellín, Colombia
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25
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Cullum J, Stevens D, Joshi M. The importance of planetary rotation period for ocean heat transport. ASTROBIOLOGY 2014; 14:645-50. [PMID: 25041658 PMCID: PMC4126269 DOI: 10.1089/ast.2014.1171] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/28/2014] [Accepted: 05/20/2014] [Indexed: 05/27/2023]
Abstract
The climate and, hence, potential habitability of a planet crucially depends on how its atmospheric and ocean circulation transports heat from warmer to cooler regions. However, previous studies of planetary climate have concentrated on modeling the dynamics of atmospheres, while dramatically simplifying the treatment of oceans, which neglects or misrepresents the effect of the ocean in the total heat transport. Even the majority of studies with a dynamic ocean have used a simple so-called aquaplanet that has no continental barriers, which is a configuration that dramatically changes the ocean dynamics. Here, the significance of the response of poleward ocean heat transport to planetary rotation period is shown with a simple meridional barrier--the simplest representation of any continental configuration. The poleward ocean heat transport increases significantly as the planetary rotation period is increased. The peak heat transport more than doubles when the rotation period is increased by a factor of ten. There are also significant changes to ocean temperature at depth, with implications for the carbon cycle. There is strong agreement between the model results and a scale analysis of the governing equations. This result highlights the importance of both planetary rotation period and the ocean circulation when considering planetary habitability.
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Affiliation(s)
- J Cullum
- 1 Centre for Ocean and Atmospheric Sciences, University of East Anglia , Norwich, UK
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26
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Armstrong J, Barnes R, Domagal-Goldman S, Breiner J, Quinn T, Meadows V. Effects of extreme obliquity variations on the habitability of exoplanets. ASTROBIOLOGY 2014; 14:277-91. [PMID: 24611714 PMCID: PMC3995117 DOI: 10.1089/ast.2013.1129] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/09/2013] [Accepted: 02/05/2014] [Indexed: 05/21/2023]
Abstract
We explore the impact of obliquity variations on planetary habitability in hypothetical systems with high mutual inclination. We show that large-amplitude, high-frequency obliquity oscillations on Earth-like exoplanets can suppress the ice-albedo feedback, increasing the outer edge of the habitable zone. We restricted our exploration to hypothetical systems consisting of a solar-mass star, an Earth-mass planet at 1 AU, and 1 or 2 larger planets. We verified that these systems are stable for 10(8) years with N-body simulations and calculated the obliquity variations induced by the orbital evolution of the Earth-mass planet and a torque from the host star. We ran a simplified energy balance model on the terrestrial planet to assess surface temperature and ice coverage on the planet's surface, and we calculated differences in the outer edge of the habitable zone for planets with rapid obliquity variations. For each hypothetical system, we calculated the outer edge of habitability for two conditions: (1) the full evolution of the planetary spin and orbit and (2) the eccentricity and obliquity fixed at their average values. We recovered previous results that higher values of fixed obliquity and eccentricity expand the habitable zone, but we also found that obliquity oscillations further expand habitable orbits in all cases. Terrestrial planets near the outer edge of the habitable zone may be more likely to support life in systems that induce rapid obliquity oscillations as opposed to fixed-spin planets. Such planets may be the easiest to directly characterize with space-borne telescopes.
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Affiliation(s)
- J.C. Armstrong
- Department of Physics, Weber State University, Ogden, Utah
| | - R. Barnes
- Department of Astronomy, University of Washington, Seattle, Washington
| | | | - J. Breiner
- Department of Astronomy, University of Washington, Seattle, Washington
| | - T.R. Quinn
- Department of Astronomy, University of Washington, Seattle, Washington
| | - V.S. Meadows
- Department of Astronomy, University of Washington, Seattle, Washington
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