This work reviews factors which are important for the evolution of habitable Earth-like planets such as the effects of the host star dependent radiation and particle fluxes on the evolution of atmospheres and initial water inventories. We discuss the geodynamical and geophysical environments which are necessary for planets where plate tectonics remain active over geological time scales and for planets which evolve to one-plate planets. The discoveries of methane–ethane surface lakes on Saturn’s large moon Titan, subsurface water oceans or reservoirs inside the moons of Solar System gas giants such as Europa, Ganymede, Titan and Enceladus and more than 335 exoplanets, indicate that the classical definition of the habitable zone concept neglects more exotic habitats and may fail to be adequate for stars which are different from our Sun. A classification of four habitat types is proposed. Class I habitats represent bodies on which stellar and geophysical conditions allow Earth-analog planets to evolve so that complex multi-cellular life forms may originate. Class II habitats includes bodies on which life may evolve but due to stellar and geophysical conditions that are different from the class I habitats, the planets rather evolve toward Venus- or Mars-type worlds where complex life-forms may not develop. Class III habitats are planetary bodies where subsurface water oceans exist which interact directly with a silicate-rich core, while class IV habitats have liquid water layers between two ice layers, or liquids above ice. Furthermore, we discuss from the present viewpoint how life may have originated on early Earth, the possibilities that life may evolve on such Earth-like bodies and how future space missions may discover manifestations of extraterrestrial life.

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What makes a planet habitable

We show that the escape of hydrogen from early Earth's atmosphere likely occurred at rates slower by two orders of magnitude than previously thought. The balance between slow hydrogen escape and volcanic outgassing could have maintained a hydrogen mixing ratio of more than 30%. The production of prebiotic organic compounds in such an atmosphere would have been more efficient than either exogenous delivery or synthesis in hydrothermal systems. The organic soup in the oceans and ponds on early Earth would have been a more favorable place for the origin of life than previously thought.

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A hydrogen-rich early Earth atmosphere

Acknowledgements Foreword Introduction I BACKGROUND Long-term astrophysical processes Evolution theory and the future of humanity Millenial tendencies in responses to apocalyptic threats Cognitive biases potentially affecting judgement of global risks Observation selection effects and global catastrophic risks Systems-based risk analysis Catastrophes and insurance Public policy towards catastrophe II RISKS FROM NATURE Super-volcanism and other geophysical processes of catastrophic import Hazards from comets and asteroids Influence of Supernovae, gamma-ray bursts, solar flares, and cosmic rays on the terrestrial environment III RISKS FROM UNINTENDED CONSEQUENCES Climate change and global risk Plagues and pandemics: past, present, and future Artificial Intelligence as a positive and negative factor in global risk Big troubles, imagined and real IV RISKS FROM HOSTILE ACTS Catastrophe, social collapse, and and human extinction The continuing threat of nuclear war Catastrophic nuclear terrorism: a preventable peril Biotechnology and biosecurity Nanotechnology as global catastrophic risk The totalitarian threat Author's biographies Index

Global Catastrophic Risks

Ke yW ords bioastronomy, cosmobiology, exobiology, SETI ■ Abstract Astrobiology is the study of the living universe. Astronomy provides the context for the origin and evolution of life on Earth. Conversely, discoveries about the terrestrial biosphere—from extremophilic microbes to the evolution of intelligence— inform our thinking about prospects for life elsewhere. Astrobiology includes the search for extraterrestrial life via in situ exploration, spectroscopy of solar and extrasolar plan- etary atmospheres, and the search for extraterrestrial intelligence. This review situates astrobiology within philosophical issues of the definition of life and the biological compatibility of the universe. It reviews the habitability of the Galaxy in general and of planets and moons in particular, and summarizes current controversies in origins- of-life research and in evidence for the earliest life on Earth. It critiques certain "rare Earth" and "anthropic" arguments, and considers four approaches to deciding whether intelligent life exists elsewhere in the Galaxy. It concludes that astrobiology must also speak to the future of human civilization.

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ASTROBIOLOGY: The Study of the Living Universe

Life is generally believed to emerge on Earth, to be at least functionally similar to life as we know it today, and to be much simpler than modern life. Although minimal life is notoriously difficult to define, a molecular system can be considered alive if it turns resources into building blocks, replicates, and evolves. Primitive life may have consisted of a compartmentalized genetic system coupled with an energy-harvesting mechanism. How prebiotic building blocks self-assemble and transform themselves into a minimal living system can be broken into two questions: (1) How can prebiotic building blocks form containers, metabolic networks, and informational polymers? (2) How can these three components cooperatively organize to form a protocell that satisfies the minimal requirements for a living system? The functional integration of these components is a difficult puzzle that requires cooperation among all the aspects of protocell assembly: starting material, reaction mechanisms, thermodynamics, and the in...

Experimentally Tracing the Key Steps in the Origin of Life: The Aromatic World

Stellar nucleosynthesis of heavy elements such as carbon allowed the formation of organic molecules in space, which appear to be widespread in our Galaxy. The physical and chemical conditions—including density, temperature, ultraviolet (UV) radiation and energetic particles—determine reaction pathways and the complexity of organic molecules in different space environments. Dense interstellar clouds are the birth sites of stars of all masses and their planetary systems. During the protostellar collapse, interstellar organic molecules in gaseous and solid phases are integrated into protostellar disks from which planets and smaller solar

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L. Becker

Papers

Astrophysical and astrochemical insights into the origin of life

Astrophysical and astrochemical insights into the origin of life