VII. Pedia Sapiens: A Genesis Future on Earth and in the Heavens
C. An Earthropic Principle: Novel Evidence About a Special Planet
Schwieterman, Edward, et al. A Limited Habitable Zone for Complex Life. arXiv:1902.04720. UC Riverside and NASA Astrobiology Institute scientists quantify another significant variable with regard to biospheric and atmospheric concentrations of carbon dioxide and carbon monoxide. While aerobic life from microbes to mammals requires a viable, stable CO2 range over time, CO levels are highly toxic for all organisms. Since numerous K and M-type dwarf stars are prone to CO, they are less habitable. Our G-type sun is a better place to be, if CO2 can be sustainably kept in a safe, conducive range.
The habitable zone (HZ) is defined as the range of distances from a host star within which liquid water may exist at a planet's surface. Substantially more CO2 than present in Earth's modern atmosphere is required to maintain clement temperatures. However, most complex aerobic life on Earth is precluded by CO2 levels of just a fraction of a bar. At the same time, most of the HZ volume resides in proximity to K and M dwarfs, which are more numerous than Sun-like G dwarfs but have greater abundances of atmospheric CO, a toxic gas for organisms. Here we show that the HZ for higher fauna is significantly limited relative to that for microbial life. These results cast new light on the likely distribution of complex life in the universe and the search for biosignatures and technosignatures. (Abstract excerpts)
Seppeur, Sonja. Impact of Gas Giant Instabilities on Habitable Planets. arXiv:1802.05736. A Goethe University, Frankfurt, astrophysicist posts an extensive study to date of the better or worse effect that gaseous worlds can have by their common presence and temporal movements upon solar system habitability zones. In regard, as Alessandro Morbidelli (search) and colleagues have found, our own hot Jupiter has been quite conducive by moving inward and then back so as to clear out the usual crunch of close-in rocky planets. The outward served a well-spaced orbit for Earth. One more feature amongst the cosmic contingencies is added to an especial significance for this home bioworld.
The detection of many extrasolar gas giants with high eccentricities indicates that dynamical instabilities in planetary systems are common. These instabilities can alter the orbits of gas giants as well as the orbits of terrestrial planets and therefore eject or move a habitable planet out of the habitable zone. In this work 423 simulations with 153 different hypothetical planetary systems with gas giants and terrestrial planets have been modelled to explore the orbital stability of habitable planets. Planetary systems consisting of two giant planets are fairly benign to terrestrial planets, whereas six giant planets very often lead to a complete clearing of the habitable zone. Observed gas giants with eccentricities higher than 0.4 and inclinations higher than 20 degrees have experienced strong planet-planet scatterings and are unlikely to have a habitable planet in its system. (Abstract excerpts)
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)
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 applies mathematical finesse to figure how much duration is actually necessary for life to evolve and emerge from microbes of a collaborative sapience able to do this. As a result, another vital condition is added of an extended length of time it seems to require, some billion years in our Earthly case.
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 SETI experiments such as the Breakthrough Listen project. (Abstract)
Smith, Howard. Alone in the Universe. Zygon: Journal of Religion and Science. 51/2, 2016. In an Exoplanets and Astrotheology section, the Harvard-Smithsonian Center for Astrophysics scientist and philosopher updates his conclusion, as broached in American Scientist for July-August 2011, that based on a 2010s multitude of cosmological findings we human beings are most likely the only sapient personage. As this section along with Astrobiology, ExoEarths and elsewhere reports, such an epochal realization is in fact dawning upon us. A May 2016 Scientific American article Born of Chaos (Batygin), for example, describes our own solar systems as a uniquely ordered anomaly well suited for a long term habitable Earth. And from a Jewish perspective, the author notes that in his 2006 book Let There Be Light he evoked John A. Wheeler to say that we peoples might be the universe’s way of self-observation so as to bring into full creation.
We are probably alone in the universe—a conclusion based on observations of over 4,000 exoplanets and fundamental physical constraints. This article updates earlier arguments with the latest astrophysical results. Since the discovery of exoplanets, theologians have asked with renewed urgency what the presence of extraterrestrial intelligence (ETI) says about salvation and human purpose, but this is the wrong question. The more urgent question is what their absence says. The “Misanthropic Principle” is the observation that, in a universe fine-tuned for life (“Anthropic Principle”), the circumstances necessary for intelligence are rare. Rabbis for 2,000 years discussed the existence of ETI using scriptural passages. We examine the traditional Jewish approaches to ETI, including insights on how ETI affects our perception of God, self, free-will, and responsibility. We explore the implications of our probable solitude, and offer a Jewish response to the ethical lessons to be drawn from the absence of ETI. (Abstract)
Spalding, Christopher, et al. The Resilience of Kepler Systems to Stellar Obliquity. arXiv:1803.01182. Cal Tech planetary scientists CS, Noah Marx and Konstantin Batygin add still another highly variable feature of solar systems whence an axial tilt of its sunny star has a controlling impact on the number, orbital paths, and stability of any entrained worlds.
The Kepler mission and its successor K2 have brought forth a cascade of transiting planets. Many of these planetary systems exhibit multiple members, but a large fraction possess only a single transiting example. This overabundance of singles has lead to the suggestion that up to half of Kepler systems might possess significant mutual inclinations between orbits, reducing the transiting number. Here, we investigate the ubiquity of the stellar obliquity-driven instability within systems with a range of multiplicities. We find that most planetary systems analysed, including those possessing only 2 planets, underwent instability for stellar spin periods below ~3 days and stellar tilts of order 30 degrees. (Abstract excerpts)
Spohn, Tilman. Special Issue: Planetary Evolution and Life. Planetary and Space Science. 98/1, 2014. An introduction to this edition with six co-editors including Helmut Lammer and Frances Westall as we Earthlings began our near and far cosmic census of habitable ecobiospheres. With this home world as a measure, the 21st century project going forward involves quantifying an array of features such as core composition, geochemistry, outgasings, atmospheres, mantle hydration, and much more. See for example herein Plate Tectonics on Rocky Exoplanets, Earth-Like Habitats in Planetary Systems, and Biotic vs. Abiotic Earth. But an inkling seems to carry through the papers that this precious animate orb looks increasingly special and maybe unique.
Stern, Robert J.. Is Plate Tectonics Needed to Evolve Technological Species on Exoplanets? Geoscience Frontiers. 7/4, 2016. The UT Dallas geoscientist states a thorough case why a long term, global surface condition of relatively balanced, mobile land and ocean ratios is a vital necessity for life to make it all the way from single cells to a sapient species able to learn this and begin to look outward. See also Stagnant Lid Tectonics: Perspectives from Silicate Planets, Dwarf Planets, Large Moons, and Large Asteroids by the author in this journal (9/2, 2018).
Stern, Robert J.. The Evolution of Plate Tectonics. Philosophical Transactions of the Royal Society A. Vol.376/Iss.2132, 2018. Plate tectonics is a very unusual convective style for a silicate planet. All other active silicate bodies are encased in a single lithospheric lid. (17). In a Earth Dynamics and the Development of Plate Tectonics issue, the UT Dallas geoscientist reconstructs ancient crustal comings and goings in graphic stages of asthenosphere and lithosphere convections and subductions. Comparisons are made to single lid Venus and Mars, along with Europa and Io moons. Only Earth has a mobile, “fragmented” mantle, which then has major influences on animal evolution. Amongst the 15 papers are Magma Oceans as a Critical Stage in Tectonics, Biogeodynamics, and Geological Archive of Plate Tectonics. A century after its discovery by Alfred Wegener (1880-1930), the edition achieves a comprehensive noosphere verification. See also Biogeodynamics: Bridging the Gap between Surface and Deep Earth Processes by Aubrey Zerkle in this collection.
To understand how plate tectonics became Earth's dominant mode of convection, we need to address three related problems. (i) What was Earth's tectonic regime before the present episode of plate tectonics began? (ii) Given the preceding tectonic regime, how did plate tectonics become established? (iii) When did the present episode of plate tectonics begin? The tripartite nature of the problem complicates solving it, but, when we have all three answers, the requisite consilience will provide greater confidence than if we only focus on the long-standing question of when did plate tectonics begin? Earth probably experienced episodes of magma ocean, heat-pipe, and increasingly sluggish single lid magmatotectonism. A Neoproterozoic transition (~1,000 to 540 mya) from single lid to plate tectonics also explains kimberlite ages, the Neoproterozoic climate crisis and the Neoproterozoic acceleration of evolution. (Abstract excerpt)
Stevenson, David. Planetary Diversity. Physics Today. April, 2004. An introduction to a special issue on why the scientific study of how planets form has come into recent prominence. Foremost is the ability to detect by observation and inference the presence of many extrasolar planets, along with a growing understanding of extraterrestrial geology and atmospheres. From these advances a wider definition of what constitutes a planetary object has occurred. Stevenson goes on to imagine that a Darwinian selection might apply whereby planetary accretion by angular momentum seeks out all possible solar orbital niches. In this view, only a precious few such our Earth might become fertile with sentient life.
Stevenson, David S.. Planetary Mass, Vegetation Height and Climate. International Journal of Astrobiology. Online January, 2019. The British biologist (search) continues his unique studies so as to add another factor that would affect the relative habitability of an Earth to super-Earth size planet. As the Abstract says, a preferred, optimum arboreal height is a necessity for floral and faunal life to devolve and develop.
The maximum height trees can grow on Earth is around 122–130 meters. The height is constrained by two factors: the availability of water, and where water is not limiting, the pressure available to drive the column of water along the xylem vessels against the pull of gravity. In turn the height of trees impacts the biodiversity of the environment in a number of ways. On Earth the largest trees are found in the maritime temperate Pacific Northwest coasts of northern California and southern Oregon. These forests provide many secondary habitats for species and serve as moisture pumps that return significant volumes of water to the lower atmosphere. In this work, we apply mathematical rules to show how super-terran planets will have significantly smaller trees, with concomitant effects on the habitability of the planet. (Abstract)
Stevenson, David S.. Planetary Mass, Vegetation Height and Climate. International Journal of Astrobiology. Online January, 2019. The British biologist (search) continues his insightful studies which can add still another factor that would effect the relative habitability of an Earth to super-Earth size planet. As the Abstract says, an middle-range arboreal height is vital for life to be able to evolve and develop.
The maximum height trees can grow on Earth is around 122–130 meters. The height is constrained by two factors: the availability of water, and where water is not limiting, the pressure available to drive the column of water along the xylem vessels against the pull of gravity. In turn the height of trees impacts the biodiversity of the environment in a number of ways. On Earth the largest trees are found in the maritime temperate Pacific Northwest coasts of northern California and southern Oregon. These forests provide many secondary habitats for species and serve as moisture pumps that return significant volumes of water to the lower atmosphere. In this work, we apply simple mathematical rules to illustrate how super-terran planets will have significantly smaller trees, with concomitant effects on the habitability of the planet. We also consider the impact of varying tree height on climate models. (Abstract)