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A Sourcebook for the Worldwide Discovery of a Creative Organic Universe
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VII. Pedia Sapiens: A Genesis Future on Earth and in the Heavens

C. An Earthropic Principle: Novel Evidence About a Special Planet

Stojkovic, neda, et al. Habitability of Galaxies and Application of Merger Trees in Astrobiology. arXiv:1908.05935. University of Belgrade astrophysicists NS, Branislav Vukotic and Milan Cirkovic (search) advance a latest, extensive consideration of how relatively conducive might myriad galactic neighborhoods be for living, evolutionary systems. Their studies draw upon mathematic and geometrics method as described Exploring the Cosmic Evolution of Habitability with Galaxy Merger Trees by Elizabeth Stanway, et al in the Monthly Notices of the Royal Astronomical Society (475/2, 2018).

Galaxies represent the main form of organization of matter in our universe. Here we present a systematic attempt to list and categorize major causal factors playing a role in an emergent galactic habitability. We argue that the methodology of cosmological merger trees can be useful in delineating what are systematic and lawful astrobiological properties of galaxies at the present epoch vs. those which are due to historical contingency. In a sense, this approach is directly complementary to using large-scale cosmological simulations to study habitable zones of individual galaxies with high mass/spatial resolution. Altogether, these endeavors serve to advance a new era of synergy and synthesis between cosmology and astrobiology. (Abstract edit)

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?

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.

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.

We present thermodynamic phase-equilibria calculations of planetary differentiation to calculate both melt composition and mantle mineralogy, and show that a planet's refractory and moderately-volatile elemental abundances control a terrestrial planet's likelihood to produce mantle-derived, melt-extracted crusts that sink. We find only 1/3 of the range of stellar compositions observed in the Galaxy is likely to host planets able to sustain density-driven tectonics compared to the Sun/Earth. Systems outside of this compositional range are less likely to produce planets able to tectonically regulate their climate and may be inhospitable to life as we know it. (Abstract)

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)

The consequences for other planetary systems are profound. Multiple planets beyond about 5 au (such as analogues of Jupiter, Saturn, Uranus and Neptune) may be common, but are so far unfortunately effectively hidden from detection by Doppler radial velocity and transit photometry techniques, the two most successful planet-finding techniques. If more distant, trans-Neptunian-like planets are also common, then the ingredients may exist to regularly generate instability and a frequently-changing dynamical environment during white dwarf phases of evolution. (14)

Waltham, Dave and Lewis Dartnell. Is the Earth Special? Astronomy & Geophysics. Vol. 53, August, 2012. A report on a Royal Astronomical Society, and Astrobiological Society of Britain, December 2011, conference by this title. In light of the recent exoplanet findings and other research studies, a 21st century universe seems graced by a propensity to fill (seed) itself with solar systems and potential bioplanets. With Jim Kasting and Helmut Lammer as keynoters, a stellar array such as Monica Grady, Nick Lane, and Richard Nelson scanned the range of cosmic and evolutionary causes and contingencies in search of an answer. If an early consensus might be entered, unicellular microbes are likely to be widespread. But many steps over a billion year planetary life span, such as plate tectonics and a good moon, are required to reach sentient, intelligent beings (as if a cosmos trying to gain its own vision and voice) which serves to distinguish our precious orb. So (great) Earth may indeed be an extraordinary occasion in the galactic heavens.

Recent developments in cosmology suggest an unimaginably vast `multiverse' generated by the process of cosmological inflation. According to ideas from fundamental physics, such as string theory, in such a vast space physical laws themselves may vary from region to region. In at least some cases, theories may allow a probabilistic determination of cosmological parameters (in the same way one might attempt to determine the probability distribution for the number of planets orbiting a typical star). However, such probabilities, and hence the properties of our region of the Universe, may be strongly preconditioned by our own existence as observers. As an example, these arguments --- sometimes referred to as the anthropic string landscape --- currently provide the most compelling explanation for the Universe's observed acceleration. The same ideas may have implications in assessing the likelihood of intelligent life on other planets. (Andrew Liddle “Special Earth, Special Universe?”)

Complex and intelligent life has arisen late in the history of the Earth, towards the end of the period that it will be habitable by complex organisms. I have argued that this is consistent with such life being contingent on a sequence of intrinsically unlikely events, and that we can even be quantitative about some aspects of these events. We’d conclude that the universe is very sparsely populated with such life, but this is not primarily due to the rarity of postulated astronomical Goldilocks factors. Rather it is due to the fact that an unlikely sequence of coupled biological and whole-system evolutionary steps must occur on a planet for it host the transition from basic chemistry through to conscious beings, as has happened here on Earth. (Andrew Watson “Goldilocks: Biology or Astronomy?”)

The Fermi Paradox refers to the non-detection of extraterrestrial intelligence (ETI), and in particular the apparent non-intereference with the Earth's biosphere by ETI over Earth's history. The 'Rare Earth Hypothesis' is a popular solution to the Fermi Paradox because, if the Earth is uniquely special as an abode of life, ETI will necessarily be rare (or even non-existent). However, while accepting that the Fermi Paradox very likely does indicate that ETI are rare in the Galaxy, I shall argue that this is unlikely to be due to unique properties of the Earth as a planet. Rather, even if Earth-like planets are common (which seems increasingly likely), I shall argue that intelligent life is probably rare owing to the low probability of either the origin of life itself or, more likely, of key evolutionary events (such as endosymbiosis in the origin of eukaryotic cells) which have occurred here but which may not happen even on essentially identical planets. In other words, I shall argue that the abundance (and probable rarity) of intelligent life in the universe depends more on the quirks of evolutionary biology than uniquely favourable astronomical or planetary conditions. (Ian Crawford “Does the Fermi Paradox Imply that the Earth is Special?”)

Waltham, David. Half a Billion Years of Good Weather: Gaia or Good Luck? Astronomy & Geophysics. 48/3, 2007. Earth’s climate over the past 550 million years of the Phanerozoic era of “visible animal life” has been an order of magnitude more stable than previous eons. A University of London geologist finds such consistency in accord with a biosphere that maintains atmospheric conditions favorable to its flora and fauna. A large Moon and a gap between Jupiter and Saturn further contribute to a preferential “anthropic” milieu. In an expanded cosmic perspective then, the human imperative not to upset this conducive global environment becomes even more critical.

Perhaps most importantly of all, this discovery would imply that climate stability is far from inevitable and that human-induced (or other) climate change could be far more dramatic and damaging than we have previously believed. (26)

Waltham, David. Lucky Planet: Why Earth is Exceptional – and What That Means for Life in the Universe. New York: Basic Books, 2014. The University of London astrobiologist and geologist contends that our relatively stable biosphere climate over a billion years is why we are here to witness. But such a stretch of benign weather is so improbable as to make it statistically unique. By one man’s analysis and opinion, no other neighbors are out there, this Earth is it. But could we just as well proffer that Earth is a “Plucky Planet” whereupon innate Gaian feedback processes have served to regulate, and life’s cooperative intelligence has keep evolution on an ascendant course?

As we discover countless exoplanets orbiting other stars, we become ever more hopeful that we may come across extraterrestrial life. Yet even as we become aware of the vast numbers of planets outside our solar system, it has also become clear that Earth is exceptional. In Lucky Planet, astrobiologist David Waltham argues that Earth’s climate stability is one of the primary factors that makes it able to support life, and that nothing short of luck made such conditions possible. Describing the three factors that typically control a planet’s average temperature—the heat received from its star, how much heat the planet absorbs, and the concentration of greenhouse gases in the atmosphere — Waltham paints a complex picture of how special Earth’s climate really is. Citing factors such as the size of our Moon and the effect of an ever-warming Sun, Waltham challenges the prevailing scientific consensus that other Earth-like planets have natural stabilizing mechanisms that allow life to flourish. (Publisher excerpts)

Waltham, David. Star Masses and Star-Planet Distances for Earth-like Habitability. Astrobiology. 17/1, 2017. The Royal Holloway University of London exoplanet researcher and author of Lucky Planet: Why Earth is Exceptional (2014) discusses current studies about how conducive stellar types and solar systems may or may not be conducive for life to originate, inhabit and evolve.

Wang, Haiyang, et al. The Volatility Trend of Protosolar and Terrestrial Elemental Abundances. arXiv:1810.12741. Australian National University, Canberra astrophysicists including Charles Lineweaver provide a detailed quantification of stellar and planetary chemical affinities via dynamic out-gassings over the ages of solar system evolution. See also Enhanced Constraints on the Interior Composition and Structure of Terrestrial Exoplanets by this group at arXiv:1810.04615 In regard, still another variable is involved with the relative habitability of a candidate exoEarth.

We present new estimates of protosolar elemental abundances based on an improved combination of solar photospheric abundances and CI chondritic abundances. These new estimates indicate CI chondrites and solar abundances are consistent for 60 elements. We compare our new protosolar abundances with our recent estimates of bulk Earth composition, thereby quantifying the devolatilization in going from the solar nebula to the formation of the Earth. (Abstract)

To first order, Earth is a devolatilized piece of the solar nebula. Similarly, rocky exoplanets are usually devolatilized pieces of the stellar nebulae out of which they and their host stars formed. If this is correct, we can estimate the chemical composition of rocky exoplanets by measuring the elemental abundances of their host stars, and then applying a devolatilization algorithm. The main goal of this paper is to go beyond the usual comparison of the silicate Earth with CI chondrites. We do this by comparing the bulk elemental abundances of Earth and Sun, and thus calibrate this potentially universal process associated with the formation of terrestrial planets. (1)

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