VII. Pedia Sapiens: A Genesis Future on Earth and in the Heavens
C. An Earthropic Principle: Novel Evidence About a Special Planet
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)
Tegmark, Max. We're Not Insignificant After All. www.edge.org. In this response to the 2007 Edge question: "What Are You Optimistic About," the MIT cosmologist offers a most positive vista whereby conscious, informed living entities might be able to expand into and transform the future universe. Human beings may be a rarity but yet possess such a potential to influence the fate of cosmic destiny. Indeed, as the quote avers, the next century could decide whether this path is taken. Here is a diametric alternative to the moribund physical multiverse whereof people are of no account. Our hope for this website is to document welling support for this grand option, to wit therefore choose life and earth.
Moreover, this brief century of ours is arguably the most significant one in the history of our universe: the one when its meaningful future gets decided. We'll have the technology to either self-destruct or to seed our cosmos with life. The situation is so unstable that I doubt that we can dwell at this fork in the road for more than another century. If we end up going the life route rather than the death route, then in a distant future, our cosmos will be teeming with life that all traces back to what we do here and now. I have no idea how we'll be thought of, but I'm sure that we won't be remembered as insignificant.
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?
Tobin, John, et al. Astro2020 Science White Paper: The Formation and Evolution of Multiple Star Systems. arXiv:1904.08442. A project proposal by 16 astronomers from across the USA to study these stellar duplexes because “nearly half of Solar-type stars are have been found to lie in binary or higher-order multiple systems.” An inference, one might add, is that such common pairing would be another obstacle for a long-term habitability.
Turbet, Martin, et al. The Runaway Greenhouse Radius Inflation Effect. arXiv:1906.3527. In a paper to appear in Astronomy & Astrophysics, University of Geneva and NASA Goddard scientists report another finely-tuned window that a habitable bioworld need remain within and pass through for life to evolve and emerge over a long time span. As the Abstract says, a narrow balance of solar radiation and surface conditions must be maintained for this purpose.
Planets similar to Earth - but slightly more irradiated - are expected to enter into a runaway greenhouse state, where all surface water rapidly evaporates, forming an optically thick H2O-dominated atmosphere. For Earth, this extreme climate transition is thought to occur for a ~6% increase only of the solar luminosity. In general, the runaway greenhouse is believed to be a fundamental process in the evolution of Earth-size, temperate planets. Using 1-D radiative-convective climate calculations accounting for thick, hot water vapour-dominated atmospheres, we evaluate the transit atmospheric thickness of a post-runaway greenhouse atmosphere, and find that it could possibly reach over a thousand kilometers. This abrupt radius inflation - resulting from the runaway-greenhouse-induced transition - could be detected statistically by ongoing and upcoming space missions such as TESS, CHEOPS and PLATO. This could provide an empirical measurement of the irradiation at which Earth analogs transition from a temperate to a runaway greenhouse climate state. This astronomical measurement would make it possible to statistically estimate how close Earth is from the runaway greenhouse. (Abstract excerpt)
Unterborn, Cayman, et al. Stellar Chemical Clues as to the Rarity of Exoplanetary Tectonics. arXiv:1706.10282. We quote an extended Abstract by a US and UK astrogeophysicst team because it highlights how important are mobile continental plates, along with certain metallic and atmospheric compositions, for life to appear and evolve. See also The Star-Planet Connection I by Natalie Hinkel and C. Unterborn at 1709.08630.
Earth's tectonic processes regulate the formation of continental crust, control its unique deep water and carbon cycles, and are vital to its surface habitability. A major driver of steady-state plate tectonics on Earth is the sinking of the cold subducting plate into the underlying mantle. This sinking is the result of the combined effects of the thermal contraction of the lithosphere and of metamorphic transitions within the basaltic oceanic crust and lithospheric mantle. The latter of these effects is dependent on the bulk composition of the planet, e.g., the major, terrestrial planet-building elements Mg, Si, Fe, Ca, Al, and Na, which vary in abundance across the Galaxy.
Veras, Dimitri. The Fates of Solar System Analogues with One Additional Distant Planet. arXiv:1608.07580. As the list of uniquely favorable features of our home planet and solar system for regnant life and humanity continues to grow, a University of Warwick astrophysicist theorizes that this eight world orrery – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, sans dwarf Pluto – would become unstable if a further ninth planet was present. See also by DV the papers Relating Binary-Star Planetary Systems to Central Configurations (1607.08606), and Post-Main Sequence Planetary System Evolution (1601.05419).
The potential existence of a distant planet ("Planet Nine") in the Solar system has prompted a re-think about the evolution of planetary systems. As the Sun transitions from a main sequence star into a white dwarf, Jupiter, Saturn, Uranus and Neptune are currently assumed to survive in expanded but otherwise unchanged orbits. However, a sufficiently-distant and sufficiently-massive extra planet would alter this quiescent end scenario through the combined effects of Solar giant branch mass loss and Galactic tides. Here, I estimate bounds for the mass and orbit of a distant extra planet that would incite future instability in systems with a Sun-like star and giant planets with masses and orbits equivalent to those of Jupiter, Saturn, Uranus and Neptune. I find that this boundary is diffuse and strongly dependent on each of the distant planet's orbital parameters. Nevertheless, I claim that instability occurs more often than not when the planet is as massive as Jupiter and harbours a semimajor axis exceeding about 300 au, or has a mass of a super-Earth and a semimajor axis exceeding about 3000 au. This instability scenario might represent a common occurrence, as potentially evidenced by the ubiquity of metal pollution in white dwarf atmospheres throughout the Galaxy. (Abstract)