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