Re: searching for alien life

From: Amara Graps (amara@amara.com)
Date: Wed Aug 06 2003 - 04:24:54 MDT

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    >> It seems to me that the people conducting these searches are limiting
    >> themselves by putting a "habital" zone around a star.

    Anders:
    >Most searches are system-wise, so the exact extent of the lifezone is
    >not important. But plenty of arguments about the probability of life
    >depends on its average size, and a larger lifezone makes life more
    >likely.

    >However, people are indeed expanding the range of places where life
    >would not be unexpected. Some have suggested moons of gas giants a la
    >Europa, and brown dwarves might have lifezones of their own.

    Yes, nowadays, there is a large variety for where people are thinking to
    find stable and life-supportable planets, as well as how life could
    seed other places. A sample:

    Amara

    P.S. I discovered today that to the Jet Propulsion Laboratory, I'm
    a "twice-removed alien", that is, a "third-party foreign national"
    (foreigner to a foreign company). International ITAR agreements are
    particularly sticky for us 'aliens' (who in fact holds a citizenship
    of the same country (USA) that is issuing the ITAR agreement... )
    This is the strange world of aliens we live in.

    ------------------------------------------------------------------------

    Astrophysics, abstract
    astro-ph/0303186

    From: Marc Jason Kuchner <mkuchner@cfa.harvard.edu>
    Date: Sun, 9 Mar 2003 10:20:55 GMT (20kb)

    Volatile-Rich Earth-Mass Planets in the Habitable Zone

    Authors: Marc J. Kuchner
    Comments: 12 pages, including 1 figure. Submitted to ApJ letters March 9,
    2003

          Planets that form beyond the snow line with too little mass to
          seed rapid gas accretion (<~ 10 Earth masses) should be rich in
          volatile ices like water and ammonia. Some of these planets should
          migrate inward by interacting with a circumstellar disk or with
          other planets. Such objects can retain their volatiles for
          billions of years or longer at ~1 AU as their atmospheres undergo
          slow hydrodynamic escape. These objects could appear in future
          surveys for extrasolar Earth analogs.

    ------------------------------------------------------------------------
    Astrophysics, abstract
    astro-ph/0307009

    From: Rudolf Dvorak <dvorak@astro.univie.ac.at>
    Date (v1): Tue, 1 Jul 2003 13:01:44 GMT (190kb)
    Date (revised v2): Tue, 8 Jul 2003 15:59:22 GMT (143kb)

    A study of the stability regions in the planetary system HD 74156 - Can it
    host earthlike planets in habitable zones?

    Authors: R. Dvorak (1), E. Pilat-Lohinger (1), B. Funk (1), F. Freistetter
    (1) ((1) Institute of Astronomy, University of Vienna, Austria)
    Comments: submitted to A&A, 4 pages, 5 figures

          Using numerical methods we thoroughly investigate the dynamical
          stability in the region between the two planets found in HD 74156.
          The two planets with minimum masses 1.56 M_JUP (HD 74156b) and 7.5
          M_JUP (HD 74156c), semimajor axes 0.276 AU and 3.47 AU move on
          quite eccentric orbits (e=0.649 and 0.395). There is a region
          between 0.7 and 1.4 AU which may host additional planets which we
          checked via numerical integrations using different dynamical
          models. Besides the orbital evolution of several thousands of
          massless regarded planets in a three-dimensional restricted 4-body
          problem (host star, two planets + massless bodies) we also have
          undertaken test computation for the orbital evolution for fictive
          planets with masses of 0.1, 0.3 and 1 M_JUP in the region between
          HD74156b and HD74156c. For direct numerical integrations up to
          10^7 years we used the Lie-integrator, a method with adaptive
          stepsize; additionally we used the Fast Lyapunov Indicators as
          tool for detecting chaotic motion in this region. We emphasize the
          important role of the inner resonances (with the outer planet) and
          the outer resonances (with the inner planet) with test bodies
          located inside the resonances. In these two "resonance" regions
          almost no orbits survive. The region between the 1:5 outer
          resonance (0.8 AU) and the 5:1 inner resonance (1.3 AU), just in
          the right position for habitability, is also very unstable
          probably due to three-body-resonances acting there. Our results do
          not strictly "forbid" planets to move there, but the existence of
          a planet on a stable orbit between 0.8 and 1.3 AU is unlikely.

    ------------------------------------------------------------------------

    Astrophysics, abstract
    astro-ph/0211289

    From: Rudolf Dvorak <dvorak@astro.univie.ac.at>
    Date (v1): Wed, 13 Nov 2002 15:50:25 GMT (101kb)
    Date (revised v2): Mon, 18 Nov 2002 12:24:52 GMT (101kb)
    Date (revised v3): Fri, 29 Nov 2002 14:38:22 GMT (85kb)

    Planets in habitable zones: A study of the binary Gamma Cephei

    Authors: R. Dvorak (1), E. Pilat-Lohinger (1), B. Funk (1), F. Freistetter
    (1) ((1) Institute of Astronomy, Vienna, Austria)
    Comments: 4 pages, 5 figures, changed 2 references made minor changes due to
    referees advice

          The recently discovered planetary system in the binary GamCep was
          studied concerning its dynamical evolution. We confirm that the
          orbital parameters found by the observers are in a stable
          configuration. The primary aim of this study was to find stable
          planetary orbits in a habitable region in this system, which
          consists of a double star (a=21.36 AU) and a relatively close
          (a=2.15 AU) massive (1.7 Mjup sin i) planet. We did
          straightforward numerical integrations of the equations of motion
          in different dynamical models and determined the stability regions
          for a fictitious massless planet in the interval of the semimajor
          axis 0.5 AU < a < 1.85 AU around the more massive primary. To
          confirm the results we used the Fast Lyapunov Indicators (FLI) in
          separate computations, which are a common tool for determining the
          chaoticity of an orbit. Both results are in good agreement and
          unveiled a small island of stable motions close to 1 AU up to an
          inclination of about 15 deg (which corresponds to the 3:1 mean
          motion resonance between the two planets). Additionally we
          computed the orbits of earthlike planets (up to 90 earthmasses) in
          the small stable island and found out, that there exists a small
          window of stable orbits on the inner edge of the habitable zone in
          GamCep even for massive planets.

    ------------------------------------------------------------------------

    Astrophysics, abstract
    astro-ph/0211022

    From: Barrie W Jones <b.w.jones@open.ac.uk>
    Date: Fri, 1 Nov 2002 17:13:52 GMT (21kb)

    The Orbits of Terrestrial Planets in the Habitable Zones of Known
    Exoplanetary Systems

    Authors: Barrie W Jones, P Nick Sleep
    Comments: 5 pages, 1 Figure. To appear in the ASP Conference Series,
    'Scientific Frontiers in Research on Extrasolar Planets', edited by Deming
    and Seager

          We show that terrestrial planets could survive in variously
          restricted regions of the habitable zones of 47 Ursae Majoris,
          Epsilon Eridani, and Rho Coronae Borealis, but nowhere in the
          habitable zones of Gliese 876 and Upsilon Andromedae. The first
          three systems between them are representative of a large
          proportion of the 90 or so extrasolar planetary systems discovered
          by mid-2002, and thus there are many known systems worth searching
          for terrestrial planets in habitable zones. We reach our
          conclusions by launching putative Earth-mass planets in various
          orbits and following their fate with a mixed-variable symplectic
          integrator.

    ------------------------------------------------------------------------

    Astrophysics, abstract
    astro-ph/0209385

    From: Charles H. Lineweaver <charley@bat.phys.unsw.edu.au>
    Date: Thu, 19 Sep 2002 01:58:16 GMT (37kb)

    What can rapid terrestrial biogenesis tell us about life in the universe?

    Authors: Charles H. Lineweaver, Tamara M. Davis (School of Physics,
    University of New South Wales and the Australian Centre for Astrobiolgy,
    Sydney, Australia)
    Comments: 4 pages, 2 figures, IAU Symposium 213: ``Bioastronomy 2002: Life
    Among the Stars'' edt. R. Norris, C. Oliver and F. Stootman, Astron. Soc.
    Pac. Conf. Ser. Hamilton Island, Great Barrier Reef, Australia July 8-12,
    2002

          It is sometimes asserted that the rapidity of biogenesis on Earth
          suggests that life is common in the Universe. We critically
          examine the assumptions inherent in this argument. Using a lottery
          model for biogenesis in the Universe, we convert the observational
          constraints on the rapidity of biogenesis on Earth into
          constraints on the probability of biogenesis on other terrestrial
          planets. For example, if terrestrial biogenesis took less than 200
          Myr (and we assume that it could have taken 1 billion years) then
          we find the probability of biogenesis on terrestrial planets older
          than ~ 1 Gyr, is > 36% at the 95% confidence level. However, there
          are assumptions and selection effects that complicate this result:
          although we correct the analysis for the fact that biogenesis is a
          prerequisite for our existence, our result depends on the
          plausible assumption that rapid biogenesis is not such a
          prerequisite.

    ------------------------------------------------------------------------

    L.E. Wells et al. / Icarus 162 (2003) 38-46

    Reseeding of early Earth by impacts of returning ejecta during the late
    heavy bombardment

    Lloyd E. Wells,a, * John C. Armstrong,b and Guillermo Gonzalez c
    a Center for Astrobiology and Early Evolution and the School of
    Oceanography, University of Washington, Box 357940, Seattle, WA
    98195, USA
    b Center for Astrobiology and Early Evolution and Department of
    Astronomy, University of WA, Seattle, WA 98195, USA
    c Department of Physics and Astronomy, Iowa State University, Ames,
    IA 50011, USA
    Received 5 April 2002; revised 24 October 2002

    Abstract
    Mounting attention has focused on interplanetary transfer of
    microorganisms (panspermia), particularly in reference to exchange
    between Mars and Earth. In most cases, however, such exchange
    requires millions of years, over which time the transported
    microorganisms must remain viable. During a large impact on Earth,
    however, previous work (J.C. Armstrong et al., 2002, Icarus 160,
    183196) has shown that substantial amounts of material return to the
    planet of origin over a much shorter period of time (< 5000 years),
    considerably mitigating the challenges to the survival of a living
    organism. Conservatively evaluating experiments performed [by
    others] on Bacillus subtilis and Deinococcus radiodurans to
    constrain biological survival under impact conditions, we estimate
    that if the Earth were hit by a sterilizing impactor 300 km in
    diameter, with a relative velocity of 30 km s-1 (such as may have
    occurred during the Late Heavy Bombardment), an initial cell
    population in the ejecta of order 10^3-10^5 cells kg-1 would in most
    cases be sufficient for a single modern organism to survive and
    return to an again-clement planet 3000-5000 years later. Although
    little can be said about the characteristics or distribution of
    ancient life, our calculations suggest that impact reseeding is a
    possible means by which life, if present, could have survived the
    Late Heavy Bombardment.

    © 2003 Elsevier Science (USA). All rights reserved.

    -- 
    ***********************************************************************
    Amara Graps, PhD
    Istituto di Fisica dello Spazio Interplanetario, INAF - ARTOV,
    Via del Fosso del Cavaliere, 100, I-00133 Roma, ITALIA
    tel: +39-06-4993-4384       |fax:  +39-06-4993-4383
    Amara.Graps@ifsi.rm.cnr.it  | http://www.mpi-hd.mpg.de/dustgroup/~graps
    ************************************************************************
    I'M SIGNIFICANT!...screamed the dust speck. -- Calvin
    


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