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A Sourcebook for the Worldwide Discovery of a Creative Organic Universe
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Genesis Vision
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Organic Universe
Earth Life Emerge
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III. Ecosmos: A Revolutionary Fertile, Habitable, Solar-Bioplanet Incubator Lifescape

H. Prolific ExoWorlds, Galactic Dynamics, Solar Orrerys, Habitable Zones, Biosignatures

Simpson, Fergus. An Anthropic Prediction for the Prevalence of Waterworlds. arXiv:1607.03095. As myriad orbital objects of every possible kind are being detected, the University of Barcelona, Institute for Cosmic Sciences, researcher notes that in contrast to a default state of wholly wet or dry surfaces, Earth’s mottled mantle of ocean and land is a rare anomaly. By way of a “planetary fecundity,” life has been able to evolve from primitive rudiments to human observers, thus an anthropic explanation. We include longish quotes to catch the gist, which appends another reason why this home Earth is so uniquely precious.

Should we expect most habitable planets to share the Earth's marbled appearance? Terrestrial planets within the habitable zone are thought to display a broad range of water compositions, due to the stochastic nature of water delivery. Such diversity, taken at face value, implies that the surfaces of most habitable planets will be heavily dominated by either water or land. Convergence towards the Earth's equitably partitioned surface may occur if a strong feedback mechanism acts to regulate the exposure of land. It is therefore feasible that the Earth's relatively balanced division of land and sea is highly atypical amongst habitable planets. We construct a simple model for the anthropic selection bias that may arise from an ensemble of surface conditions. Across a broad class of models we consistently find that (a) the Earth's ocean coverage of 71% can be readily accounted for by observational selection effects, and (b) due to our proximity to the waterworld limit, the maximum likelihood model is one where the majority of habitable planets are waterworlds. This 'Dry Earth' scenario is consistent with results from numerical simulations, and could help explain the apparently low-mass transition in the mass-radius relation. (Abstract)

On a purely statistical basis, one naıvely expects to find a highly asymmetric division of land and ocean surface areas. A natural explanation for the Earth’s equitably partitioned surface is an anthropic selection process. We have highlighted two mechanisms which could be responsible for driving this selection effect. First of all, planets with highly asymmetric surfaces (desert worlds or waterworlds) are likely to produce intelligent species at a much lower frequency. Secondly, planets with larger habitable areas are capable of sustaining larger populations. Both of these factors imply that our host planet has a greater habitable area than most life-bearing worlds. (7) It has been argued that the apparently unique and special properties of the Earth is indicative of the sparsity of life in the Universe - the so-called ‘Rare Earth hypothesis’. However this interpretation fails to account for one of the factors which controls the fecundity: the number of observers produced by each planet. This amplifies the already considerable observational selection effects associated with the emergence of life. The parameters of an observer’s host planet are heavily skewed in favour of those conditions which maximize the abundance of life, not just the probability of its emergence. The apparent fine-tuning of the Earth’s parameters need not reflect the sparsity of life in the cosmos, but on the contrary, it may be driven precisely because we are a small piece within a vast ensemble. (8)

Simpson, Fergus. The Longevity of Habitable Planets and the Development of Intelligent Life. International Journal of Astrobiology. 16/3, 2017. The University of Barcelona cosmologist adds one more qualification for organisms to evolve, develop, proceed so as to reach a stage of global, knowledgeable sentience. It is not so much a “difficult” process as a necessarily “slow” pathway that seems to requires some billion years from microbes to cities to play out.

Why did the emergence of our species require a timescale similar to the entire habitable period of our planet? Our late appearance has previously been interpreted by Carter (2008) as evidence that observers typically require a very long development time, implying that intelligent life is a rare occurrence. Here we present an alternative explanation, which simply asserts that many planets possess brief periods of habitability. We also propose that the rate-limiting step for the formation of observers is the enlargement of species from an initially microbial state. In this scenario, the development of intelligent life is a slow but almost inevitable process, greatly enhancing the prospects of future search for extra-terrestrial intelligence (SETI) experiments such as the Breakthrough Listen project. (Abstract)

Sramek, Ondrej, et al. Thermal Evolution and Differentiation of Planetesimals and Planetary Embryos. Icarus. 217/339, 2012. Czech, American, and French astrogeologists contribute to Great Earth’s discovery of myriad worlds such as our own across galactic and cosmic celestial ages, could one muse as if a uterine universe. And it is always intriguing in such studies how often gestational imageries are employed.

Stevenson, David and Sean Large. Evolutionary Exobiology: Towards the Qualitative Assessment of Biological Potential on Exoplanets. International Journal of Astrobiology. Online October, 2017. A Carlton le Williams Academy, Nottinghamshire, UK (search DS and the school) biologist and a University of Exeter physics consider this READ nascent exploratory phase of Earthkind’s cosmic census of potential near and far neighbors. A novel parameter of the relative “information density” of planetary life is added, along with tidal-locking from a good moon. See also Evolutionary Exobiology II by D. Stevenson in this journal (July 2018), and his 2017 Springer book The Nature of Life and Its Potential to Survive, which develops this informational essence.

A planet may be defined as habitable if it has an atmosphere and is warm enough to support the existence of liquid water. These are a basic set of conditions that allow it to develop life similar to ours, which is carbon-based and has water as its universal solvent. While this definition can allow a broad range of possibilities, it does not address whether any life forms will become complex or intelligent. In this paper, we seek a qualitative definition of which subset of these ‘habitable worlds’ might develop complex and interesting life forms. We identify two key principles in determining the capacity of life to breach certain transitions on route to developing intelligence. The first is the number of potential niches a planet provides. Secondly, the complexity of life will reflect the information density of its environment, which in turn is influenced by available niches. We use these criteria to place the evolution of terrestrial life in a mathematical framework based on environmental information content. Our model links the development of complex life to the physical properties of the planet, something currently lacking in all evolutionary theory. (Abstract edits)

Stojkovic, Neda, et al. Galactic Habitability Re-Examined: Indications of Bimodality. arXiv:1909.01742. Astronomical Observatory, Belgrade astrophysicists including Milan Cirkovic post an extensive contribution which seeks ways to better to quantify preferential places for living systems to form and evolve. As the quotes allude, this requires factoring in a range of stellar and galactic types, sizes and active shapes. See also Habitability of Galaxies and Application of Merger Trees in Astrobiology at arXiv:1908.05935 and What can Milky Way Analogues Tell us About the Star Formation Rate of Our Own Galaxy? at 1909.01654 for concurrent papers. Again how fantastic is it that homo to anthropo sapiens, phoenix-like out of war zones, can come together and move on to learn about our celestial neighborhood. The paper merits some extended quotes.

The problem of the extent of habitable zones in different kinds of galaxies is one of the outstanding challenges for contemporary astrobiology. In the present study, we investigate habitability in a large sample of simulated galaxies from the Illustris Project in order to at least roughly quantify the hospitality to life of different galactic types. In particular, we find a tentative evidence for a second mode of galactic habitability comprising metal-rich dwarfs similar to IC 225, LMC or M32. The role of the galactic environment and the observation selection effects is briefly discussed and prospects for further research on the topic outlined. (Abstract)

Hence, we have essentially two major views on the habitability of galaxies so far: (i) the conventional view based to a large degree on the “rare Earth” thinking of Gonzalez, Ward, and Brownlee limiting life to large spiral discs analogous to the Milky Way Population I, and (ii) the radical view, emerging since about 2015, that it is mostly quiescent early-type galaxies and dwarfs more similar to the local Population II that are the best abodes for life. Details may vary from study to study, but this dilemma is quickly becoming one of the central issues of astrobiology.(3)

Studies of galactic habitability are obviously in their infancy. There is a large number of galactic properties which may influence the habitability score in ways currently ill-understood, and even those whose influence is somewhat understood (like the mean metallicity or the global star-formation rate in the present study) are still only roughly represented in the models. Thus, we hereby propose an emerging picture of galactic habitability which is bimodal: high-metallicity dwarfs on one hand, and quiescent spiral discs on the other hand, represent the peaks of galactic habitability in the local universe at present. (15)

If the emerging picture is correct, one mode of habitability is based upon the following factors: large galaxies with active star formation, similar to the Milky Way, with habitable planets forming around the Pop I analog stars with high metallicities. This mode is characterized by many different kinds of planetary systems, a high level of chemical evolution and, presumably, easier routes for advancement of prebiotic chemistry in both interstellar/circumstellar medium and on planetary surfaces. Once life appears, however, it is subject to strong abiotic selection pressure of its astrophysical environment in form of frequent irradiations by supernovae and GRBs, perturbations due to the spiral-arm and galactic-plane crossings, higher cosmic-ray fluences, and other astrobiological regulation mechanisms. Part of these perturbing influences may overflow into the “Gaian windows”, which are likely to be shorter for biospheres in the giant spiral systems. (17)

Tamayo, Daniel, et al. A Criterion for the Onset of Chaos in Compact, Eccentric Multiplanet Systems. arXiv:2106.14863. We cite this June entry by Princeton University and University of Toronto astrophysicists including Scott Tremaine and Joshua Winn as another frontier instance whereof whole host star and orbital arrays are being treated as a unified assembly.

We derive a semi-analytic criterion for the presence of chaos in compact, eccentric multiplanet systems. We show that the onset of chaos is due to the overlap of two-body mean motion resonances (MMRs), like it is in two-planet systems and the secular evolution causes the MMR widths to expand and contract. For closely spaced two-planet systems, a near-symmetry suppresses this secular modulation. We make routines for evaluating the chaotic boundary available to the community through the open-source SPOCK package. (Abstract excerpt)

Tinetti, Giovanna. Galactic Planetary Science. Proceedings of the Royal Society A. 372/20130077, 2014. The University College London astrophysicist introduces this grand 2010s vista unto explorations of a novel animate universe which populates itself with myriad orbital worlds in starry arrangements. Since our own solar system is realized to be especially orderly and stable, one might suggest such studies might be seen as a “Galactic Positioning System” as we Earthlings come to realize our special situation and import.

The University College London astrophysicist introduces this grand 2010s vista unto explorations of a novel animate universe which populates itself with myriad orbital worlds in starry arrangements. Since our own solar system is realized to be especially orderly and stable, one might suggest such studies might be seen as a “Galactic Positioning System” as we Earthlings come to realize our special situation and import.

Tinetti, Giovanna, et al. The EChO Science Case. arXiv:1502.05747. With some 350 authors and 50 pages, this is a manifesto for an Exoplanet Classification Observatory project over the next decade as a scientific survey of a grand new genesis universe that fills itself with stars and planets so that an intelligent species can achieve its own self-comprehension. A sample of diverse subjects might be: exoplanet core to atmospheric composition, how many large gaseous planets, what is the range of planetary spin, orbit shape, dynamic movement, and can planets around low mass stars keep their atmospheres. And akin to the views of Elke Pilat-Lohinger above, a significant finding is just being realized. Our own solar system which contains a bioplanet able to do this is not typical at all, and especially conducive due to its long term stability. Whatever to make of this?

EChO has been conceived to address the following fundamental questions:• Why are exoplanets as they are? • What are the causes for the observed diversity? • Can their formation and evolution in history be traced back from their current composition? EChO would provide spectroscopic information on the atmospheres of a large, select sample of exoplanets allowing the composition, temperature, size and variability to be determined at a level never previously attempted. This information can be used to address a wide range of key scientific questions relative to exoplanets: • What are they made of? • Do they have an atmosphere? • What is the energy budget? • How were they formed? • How do they evolve? • How do weather conditions vary with time? And of course: • Do any of the planets observed have habitable conditions?

Conclusion: Our knowledge of planets other than the eight “classical” Solar System bodies is in its infancy. We have discovered over a thousand planets orbiting stars other than our own, and yet we know little or nothing about their chemistry, formation and evolution. Planetary science therefore stands at the threshold of a revolution in our knowledge and understanding of our place in the Universe: just how special are the Earth and our Solar System? It is only by undertaking a comprehensive chemical survey of the exoplanet zoo that we will answer this critical question.

Tosi, Nicola, et al. The Habitability of a Stagnant-Lid Earth. Astronomy & Astrophysics. Online July, 2017. A ten person exoplanetologist team from Technische Universitat Berlin, and the German Aerospace Center, note that mobile crustal land masses are a minority case, with stationary mantles more prevalent. While Earth’s continental movements may be more conducive for evolutionary life, a “stagnant-lid” world could yet have living systems.

Plate tectonics is a fundamental component for the habitability of the Earth. Yet whether it is a recurrent feature of terrestrial bodies orbiting other stars or unique to the Earth is unknown. The stagnant lid may rather be the most common tectonic expression on such bodies. To understand whether a stagnant-lid planet can be habitable, i.e. host liquid water at its surface, we model the thermal evolution of the mantle, volcanic outgassing of H2O and CO2, and resulting climate of an Earth-like planet lacking plate tectonics. We used a 1D model of parameterized convection to simulate the evolution of melt generation and the build-up of an atmosphere of H2O and CO2 over 4.5 Gyr. Our results suggest that stagnant-lid planets can be habitable over geological timescales and that joint modelling of interior evolution, volcanic outgassing, and accompanying climate is necessary to robustly characterize planetary habitability. (Abstract excerpts)

Tremaine, Scott. The Statistical Mechanics of Planet Orbits. arXiv:1504.01160. It is noteworthy in this post-Kepler era of myriad solar systems that scientists such as this Institute for Advanced Study, Princeton University, astrophysicist and leading expositor of galactic dynamics, can consider a common physical theory by which planetary bodies arrange themselves. On this e-print site and across the astro- journals, articles like Spacing of Kepler Planets: Sculpting by Dynamical Instability (1502.05449) and Consolidating and Crushing Exoplanet (1502.06558) now treat solar system formations as a valid subject amenable to theoretical explanations.

The final "giant-impact" phase of terrestrial planet formation is believed to begin with a large number of planetary "embryos" on nearly circular, coplanar orbits. Mutual gravitational interactions gradually excite their eccentricities until their orbits cross and they collide and merge; through this process the number of surviving bodies declines until the system contains a small number of planets on well-separated, stable orbits. In this paper we explore a simple statistical model for the orbit distribution of planets formed by this process, based on the sheared-sheet approximation and the ansatz that the planets explore uniformly all of the stable region of phase space. The model provides analytic predictions for the distribution of eccentricities and semimajor axis differences, correlations between orbital elements of nearby planets, and the complete N-planet distribution function, in terms of a single parameter that is determined by the planetary masses. The predicted properties are generally consistent with both N-body simulations and the Kepler catalog of extrasolar planets. A similar model may apply to the orbits of giant planets if these orbits are determined mainly by dynamical evolution after the planets have formed and the gas disk has disappeared. (Abstract)

Turbet, Martin, et al. CO2 Condensation is a Serious Limit to the Deglaciation of Earth-like Planets. arXiv:1703.04624. Sorbonne, Bordeaux, and Nantes University astroresearchers, including Francois Forget, identify another critically poised exoplanetary feature vital for life and evolution. Intricate, geochemical dynamics produce and interact with carbon dioxide levels in exoatmospheres, which then affects the degree of colder or warmer conditions. Another window of viability thus seems to be balanced between a frigid frozen or hot gaseous world. Search here for Haqq-Misra, Jacob, et al for another angle on this.

It is widely believed that the carbonate-silicate cycle is the main agent to trigger deglaciations by CO2 greenhouse warming on Earth and on Earth-like planets when they get in frozen state. Here we use a 3D Global Climate Model to simulate the ability of frozen planets to escape from glaciation by accumulating enough gaseous CO2. We find that Earth-like planets orbiting a Sun-like star may never be able to escape from glaciation if their orbital distance is greater than ∼ 1.27 AU (Flux < 847 W m−2), because CO2 would condense at the poles forming permanent CO2 ice caps. This limits the amount of CO2 in the atmosphere and thus its greenhouse effect. Our results may have implications for the search for life-suitable extrasolar planets orbiting in the Habitable Zone of Sun-like stars.
(Abstract excerpts)

Valencia, Diana. Composition and Internal Dynamics of Super-Earths. Karato, Shun-ichiro, ed.. Physics and Chemistry of the Deep Earth. Chichester, UK: Wiley-Blackwell, 2013. Into this 21st century, it is worth notice that the composite collaboration of a sentient, linguistic, technological species now of global cast and import, can proceed to so explore, name, quantify, and describe these depths of these myriad orbiting worlds. We cite this chapter by a Sagan NASA postdoctoral fellow (mellow) at MIT, to illustrate how our novel collective abilities can span the planet-filled galactic reaches.

Though the deep interior of the Earth (and other terrestrial planets) is inaccessible to humans, we are able to combine observational, experimental and computational (theoretical) studies to begin to understand the role of the deep Earth in the dynamics and evolution of the planet. This book brings together a series of reviews of key areas in this important and vibrant field of studies. A range of material properties, including phase transformations and rheological properties, influences the way in which material is circulated within the planet. This circulation re-distributes key materials such as volatiles that affect the pattern of materials circulation. The understanding of deep Earth structure and dynamics is a key to the understanding of evolution and dynamics of terrestrial planets, including planets orbiting other stars. This book contains chapters on deep Earth materials, compositional models, and geophysical studies of material circulation which together provide an invaluable synthesis of deep Earth research. (Publisher)

My research comprises the characterisation of the low-mass planets: super-Earths and mini-Neptunes. The former are planets that are mostly solid, either rocky or icy in composition, while the latter posses also a volatile envelope. My goal is to determine if planets with masses between 1-15 Earth-masses are scaled up versions of Earth, or scaled-down versions of Neptune in terms of their composition, evolution and physical properties. To date, out of the ~600 discovered exoplanets, and 1000+ planet candidates reported by space mission Kepler, there are a handful of low-mass planets with measured masses and radii. This number will continue to increase as new data from Kepler arrives, and new discoveries are reported from other missions such as ground-based MEarth, or space mission CoRoT, as well as follow-up work to radial velocity planets. (DV website)

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