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Thread: Astrobiological papers from Arvix and everywhere

  1. #91
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    A potentially fascinating history of the question of whether we are alone in the universe, but behind a paywall.

    https://ui.adsabs.harvard.edu/abs/20......3C/abstract
    The Extraterrestrial Life Debate from Antiquity to 1900
    Crowe, Michael J. ; Dowd, Matthew F.

    This chapter provides an overview of the Western historical debate regarding extraterrestrial life from antiquity to the beginning of the twentieth century. Though schools of thought in antiquity differed on whether extraterrestrial life existed, by the Middle Ages, the Aristotelian worldview of a unified, finite cosmos without extraterrestrials was most influential, though there were such dissenters as Nicholas of Cusa. That would change as the Copernican revolution progressed. Scholars such as Bruno, Kepler, Galileo, and Descartes would argue for a Copernican system of a moving Earth. Cartesian and Newtonian physics would eventually lead to a view of the universe in which the Earth was one of many planets in one of many solar systems extended in space. As this cosmological model was developing, so too were notions of extraterrestrial life. Popular and scientific writings, such as those by Fontenelle and Huygens, led to a reversal of fortunes for extraterrestrials, who by the end of the century were gaining recognition. From 1700 to 1800, many leading thinkers discussed extraterrestrial intelligent beings. In doing so, they relied heavily on arguments from analogy and such broad principles and ideas as the Copernican Principle, the Principle of Plenitude, and the Great Chain of Being. Physical evidence for the existence of extraterrestrials was minimal, and was always indirect, such as the sighting of polar caps on Mars, suggesting similarities between Earth and other places in the universe. Nonetheless, the eighteenth century saw writers from a wide variety of genres—science, philosophy, theology, literature—speculate widely on extraterrestrials. In the latter half of the century, increasing research in stellar astronomy would be carried out, heavily overlapping with an interest in extraterrestrial life. By the end of the eighteenth century, belief in intelligent beings on solar system planets was nearly universal and certainly more common than it would be by 1900, or even today. Moreover, natural theology led to most religious thinkers being comfortable with extraterrestrials, at least until 1793 when Thomas Paine vigorously argued that although belief in extraterrestrial intelligence was compatible with belief in God, it was irreconcilable with belief in God becoming incarnate and redeeming Earth's sinful inhabitants. In fact, some scientific analyses, such as Newton's determination of the comparative masses and densities of planets, as well as the application of the emerging recognition of the inverse square law for light and heat radiation, might well have led scientists to question whether all planets are fully habitable. Criticism would become more prevalent throughout the nineteenth century, and especially after 1860, following such events as the "Moon Hoax" and Whewell's critique of belief in extraterrestrials. Skepticism about reliance on arguments from analogy and on such broad metaphysical principles as the Principle of Plenitude also led scientists to be cautious about claims for higher forms of life elsewhere in the universe. At the start of the twentieth century, the controversy over the canals of Mars further dampened enthusiasm for extraterrestrials. By 1915 astronomers had largely rejected belief in higher forms of life anywhere in our solar system and were skeptical about the island universe theory.
    Do good work. —Virgil Ivan "Gus" Grissom

  2. #92
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    So, panspermia was not such a dumb idea after all?

    https://arxiv.org/abs/2001.02235

    Transfer of Life by Earth-Grazing Objects to Exoplanetary Systems
    Amir Siraj, Abraham Loeb
    (Submitted on 7 Jan 2020)

    Recently, a 30 cm object was discovered to graze the Earth's atmosphere and shift into a Jupiter-crossing orbit. We use the related survey parameters to calibrate the total number of such objects. The number of objects that could have exported terrestrial microbes out of the Solar System is in the range 2×10^9 − 3×10^11. We find that 10^7−10^9 such objects could have been captured by binary star systems over the lifetime of the Solar System. The total number of objects carrying living microbes on them upon capture is 10−10^3.
    Do good work. —Virgil Ivan "Gus" Grissom

  3. #93
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    Radiation might be a good thing to make some planets habitable for a long period of time.

    https://arxiv.org/abs/1912.02862

    On the Habitable Lifetime of Terrestrial Worlds with High Radionuclide Abundances
    Manasvi Lingam, Abraham Loeb
    (Submitted on 5 Dec 2019 (v1), last revised 24 Jan 2020 (this version, v2))

    The presence of a liquid solvent is widely regarded as an essential prerequisite for habitability. We investigate the conditions under which worlds outside the habitable zones of stars are capable of supporting liquid solvents on their surface over geologically significant timescales via combined radiogenic and primordial heat. Our analysis suggests that super-Earths with radionuclide abundances that are ≳ 10^3 times higher than Earth can host long-lived water oceans. In contrast, the requirements for long-lived ethane oceans, which have been explored in the context of alternative biochemistries, are less restrictive: relative radionuclide abundances of ≳ 10^2 could be sufficient. We find that this class of worlds might be detectable (10σ detection over ∼10 days integration time at 12.8μ m) in principle by the James Webb Space Telescope at distances of ∼10 pc if their ages are ≲ 1 Gyr.
    Do good work. —Virgil Ivan "Gus" Grissom

  4. #94
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    Black hole sun? Could be, says this author, but aliens on planets around black holes would be in deeeep gravity wells. https://arxiv.org/abs/2001.10991
    Do good work. —Virgil Ivan "Gus" Grissom

  5. #95
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    What might life be like on Titan? A new paper peers into the possibilities by looking at how cell membranes might function there.

    https://www.airspacemag.com/daily-pl...urs-180974104/

    On Saturn’s Moon Titan, Living Cells May Be Very Different From Ours
    Yet another good reason to visit this exotic world.
    By Dirk Schulze-Makuch
    airspacemag.com January 31, 2020 1:30PM

    In a new paper published in Science Advances, Hilda Sandström and Martin Rahm from Chalmers University of Technology in Gothenburg, Sweden, consider the possibility of life on Titan—and, more specifically, whether living creatures would need cell membranes to survive.

    ==

    https://advances.sciencemag.org/content/6/4/eaax0272

    Can polarity-inverted membranes self-assemble on Titan?
    H. Sandström and M. Rahm
    Science Advances 24 Jan 2020: Vol. 6, no. 4, eaax0272

    Abstract: The environmental and chemical limits of life are two of the most central questions in astrobiology. Our understanding of life’s boundaries has implications on the efficacy of biosignature identification in exoplanet atmospheres and in the solar system. The lipid bilayer membrane is one of the central prerequisites for life as we know it. Previous studies based on molecular dynamics simulations have suggested that polarity-inverted membranes, azotosomes, made up of small nitrogen-containing molecules, are kinetically persistent and may function on cryogenic liquid hydrocarbon worlds, such as Saturn’s moon Titan. We here take the next step and evaluate the thermodynamic viability of azotosome formation. Quantum mechanical calculations predict that azotosomes are not viable candidates for self-assembly akin to lipid bilayers in liquid water. We argue that cell membranes may be unnecessary for hypothetical astrobiology under stringent anhydrous and low-temperature conditions akin to those of Titan.
    Do good work. —Virgil Ivan "Gus" Grissom

  6. #96
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    Quote Originally Posted by Roger E. Moore View Post
    Black hole sun? Could be, says this author, but aliens on planets around black holes would be in deeeep gravity wells. https://arxiv.org/abs/2001.10991
    More on the possibility that black holes can have habitable planets. Still freaks me out to think of it. Infalling debris would clobber those worlds, wouldn't it?

    https://www.sciencemag.org/news/2020...bit-black-hole
    Do good work. —Virgil Ivan "Gus" Grissom

  7. #97
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    Quote Originally Posted by Roger E. Moore View Post
    What might life be like on Titan? A new paper peers into the possibilities by looking at how cell membranes might function there.

    https://www.airspacemag.com/daily-pl...urs-180974104/

    On Saturn’s Moon Titan, Living Cells May Be Very Different From Ours
    Yet another good reason to visit this exotic world.
    By Dirk Schulze-Makuch
    airspacemag.com January 31, 2020 1:30PM

    In a new paper published in Science Advances, Hilda Sandström and Martin Rahm from Chalmers University of Technology in Gothenburg, Sweden, consider the possibility of life on Titan—and, more specifically, whether living creatures would need cell membranes to survive.

    ==

    https://advances.sciencemag.org/content/6/4/eaax0272

    Can polarity-inverted membranes self-assemble on Titan?
    H. Sandström and M. Rahm
    Science Advances 24 Jan 2020: Vol. 6, no. 4, eaax0272

    Abstract: The environmental and chemical limits of life are two of the most central questions in astrobiology. Our understanding of life’s boundaries has implications on the efficacy of biosignature identification in exoplanet atmospheres and in the solar system. The lipid bilayer membrane is one of the central prerequisites for life as we know it. Previous studies based on molecular dynamics simulations have suggested that polarity-inverted membranes, azotosomes, made up of small nitrogen-containing molecules, are kinetically persistent and may function on cryogenic liquid hydrocarbon worlds, such as Saturn’s moon Titan. We here take the next step and evaluate the thermodynamic viability of azotosome formation. Quantum mechanical calculations predict that azotosomes are not viable candidates for self-assembly akin to lipid bilayers in liquid water. We argue that cell membranes may be unnecessary for hypothetical astrobiology under stringent anhydrous and low-temperature conditions akin to those of Titan.
    I confess to not being able to understand the details in these papers at all, except that cell membranes might not be needed for life on Titan. That's pretty radical. Here's a slightly dumbed-down version that goes over my head toward the end of the first paragraph.

    https://phys.org/news/2020-02-polari...turn-moon.html
    Do good work. —Virgil Ivan "Gus" Grissom

  8. #98
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    Even if this habitable-zone planet is not like Earth, it could be quite friendly to life... very weird life.

    https://arxiv.org/abs/2002.11115

    The interior and atmosphere of the habitable-zone exoplanet K2-18b
    Nikku Madhusudhan, Matthew C. Nixon, Luis Welbanks, Anjali A. A. Piette, Richard A. Booth
    (Submitted on 25 Feb 2020)

    Exoplanets orbiting M dwarfs present a valuable opportunity for their detection and atmospheric characterisation. This is evident from recent inferences of H2O in such atmospheres, including that of the habitable-zone exoplanet K2-18b. With a bulk density between Earth and Neptune, K2-18b may be expected to possess a H/He envelope. However, the extent of such an envelope and the thermodynamic conditions of the interior remain unexplored. In the present work, we investigate the atmospheric and interior properties of K2-18b based on its bulk properties and its atmospheric transmission spectrum. We constrain the atmosphere to be H2-rich with a H2O volume mixing ratio of 0.02−14.8%, consistent with previous studies, and find a depletion of CH4 and NH3, indicating chemical disequilibrium. We do not conclusively detect clouds/hazes in the observable atmosphere. We use the bulk parameters and retrieved atmospheric properties to constrain the internal structure and thermodynamic conditions in the planet. The constraints on the interior allow multiple scenarios between rocky worlds with massive H/He envelopes and water worlds with thin envelopes. We constrain the mass fraction of the H/He envelope to be ≲6%; spanning ≲10^−5 for a predominantly water world to ∼6% for a pure iron interior. The thermodynamic conditions at the surface of the H2O layer range from the super-critical to liquid phases, with a range of solutions allowing for habitable conditions on K2-18b. Our results demonstrate that the potential for habitable conditions is not necessarily restricted to Earth-like rocky exoplanets.


    https://www.sciencealert.com/astrono...y-be-habitable
    Last edited by Roger E. Moore; 2020-Feb-27 at 11:25 PM.
    Do good work. —Virgil Ivan "Gus" Grissom

  9. #99
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    New place to look for aliens?

    https://arxiv.org/abs/2003.09231

    The subsurface habitability of small, icy exomoons
    J. Tjoa, etal.
    (Submitted on 20 Mar 2020)

    Assuming our Solar System as typical, exomoons may outnumber exoplanets. If their habitability fraction is similar, they would thus constitute the largest portion of habitable real estate in the Universe. Icy moons in our Solar System, such as Europa and Enceladus, have already been shown to possess liquid water, a prerequisite for life on Earth. We intend to investigate under what circumstances small, icy moons may sustain subsurface oceans and thus be "subsurface habitable". We pay specific attention to tidal heating. We made use of a phenomenological approach to tidal heating. We computed the orbit averaged flux from both stellar and planetary (both thermal and reflected stellar) illumination. We then calculated subsurface temperatures depending on illumination and thermal conduction to the surface through the ice shell and an insulating layer of regolith. We adopted a conduction only model, ignoring volcanism and ice shell convection as an outlet for internal heat. In doing so, we determined at which depth, if any, ice melts and a subsurface ocean forms. We find an analytical expression between the moon's physical and orbital characteristics and the melting depth. Since this expression directly relates icy moon observables to the melting depth, it allows us to swiftly put an upper limit on the melting depth for any given moon. We reproduce the existence of Enceladus' subsurface ocean; we also find that the two largest moons of Uranus (Titania & Oberon) could well sustain them. Our model predicts that Rhea does not have liquid water. Habitable exomoon environments may be found across an exoplanetary system, largely irrespective of the distance to the host star. Small, icy subsurface habitable moons may exist anywhere beyond the snow line. This may, in future observations, expand the search area for extraterrestrial habitable environments beyond the circumstellar habitable zone.
    Do good work. —Virgil Ivan "Gus" Grissom

  10. #100
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    Life on Mercury? Maybe once, perhaps.

    https://www.nytimes.com/2020/03/24/s...ife-water.html
    Do good work. —Virgil Ivan "Gus" Grissom

  11. #101
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    And yet more on the habitability of black-hole planets.

    https://arxiv.org/abs/1912.01518

    Effects of the general relativistic spin precessions on the habitability of rogue planets orbiting supermassive black holes
    Lorenzo Iorio
    (Submitted on 3 Dec 2019 (v1), last revised 25 Mar 2020 (this version, v3))

    Recently, the possibility that many starless telluric planets may form around supermassive black holes (SMBHs) receiving an energy input from the hole's accretion disk which, under certain not implausible circumstances, may make them habitable in a terrestrial sense has gained increasing attention. In particular, an observer on a planet orbiting at distance r=100 Schwarzschild radii from a SMBH in a plane slightly outside the equator of the latter would see the gravitationally lensed accretion disk with the same size of the Sun as seen from the Earth. Moreover, the accretion rate might be set in such a way that the apparent disk's temperature would be identical to that of the solar surface. We demonstrate that the post-Newtonian (pN) de Sitter and Lense-Thirring precessions of the spin axis of such a world would change, among other things, its tilt ε to its orbital plane by tens to hundreds of degrees over a time span of, say, just Δt=400yr , depending on the obliquity η∙ of the SMBH's spin to the orbital plane. Thus, such relativistic effects would have a relevant impact on the long term habitability of the considered planet. Other scenarios are examined as well.
    Do good work. —Virgil Ivan "Gus" Grissom

  12. #102
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    Tatooine is not likely. Sorry.

    https://arxiv.org/abs/2003.11682

    Earth-size planet formation in the habitable zone of circumbinary stars
    G. O. Barbosa, O. C. Winter, A. Amarante, A. Izidoro, R. C. Domingos, E. E. N. Macau
    (Submitted on 26 Mar 2020)

    In this work is investigated the possibility of close-binary star systems having Earth-size planets within their habitable zones. First, we selected all known close-binary systems with confirmed planets (totaling 22 systems) to calculate the boundaries of their respective habitable zones (HZ). However, only eight systems had all the data necessary for the computation of the HZ. Then, we numerically explored the stability within the habitable zones for each one of the eight systems using test particles. From the results, we selected five systems that have stable regions inside the habitable zones (HZ), namely Kepler-34, 35, 38, 413 and 453. For these five cases of systems with stable regions in the HZ, we perform a series of numerical simulations for planet formation considering disks composed of planetary embryos and planetesimals, with two distinct density profiles, in addition to the stars and host planets of each system. We found that in the case of Kepler-34 and 453 systems no Earth-size planet is formed within the habitable zones. Although planets with Earth-like masses were formed in the Kepler-453, but they were outside the HZ. In contrast, for Kepler-35 and 38 systems, the results showed that potentially habitable planets are formed in all simulations. In the case of the Kepler-413 system, in just one simulation a terrestrial planet was formed within the habitable zone.
    Do good work. —Virgil Ivan "Gus" Grissom

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