Page 14 of 19 FirstFirst ... 41213141516 ... LastLast
Results 391 to 420 of 545

Thread: What do you think is the most likely explanation for the Fermi paradox?

  1. #391
    Join Date
    Apr 2005
    Posts
    2,474
    Quote Originally Posted by Noclevername View Post
    Explain please?
    If the dust density was so high it caused drag on orbiting bodies, we would hardly see the spiral arms in visible light. The extinction would be too high.

  2. #392
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by kzb View Post
    If the dust density was so high it caused drag on orbiting bodies, we would hardly see the spiral arms in visible light. The extinction would be too high.
    I don't think that's correct. Numbers?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  3. #393
    Join Date
    Jun 2005
    Posts
    13,687
    Quote Originally Posted by Noclevername View Post
    What is the dust density in the spiral arms?
    Just my two cents, but you claimed that the density would get high enough to create drag, and kzb claimed that it wouldn't get that high. I think that if the two of you are interested in this very important issue, then the two of you should work together to figure that out. I don't think it is really fair to expect kzb to do all the homework. Of course, if you are busy, you might say, "I'm sorry, I think the numbers are high enough but I haven't had time to find them yet, so could you please do the math for us?" then sure, I can understand.
    As above, so below

  4. #394
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Regarding transparency of dust:
    https://pages.astronomy.ua.edu/keel/galaxies/dust.html
    A flurry of work from various directions in the 1990s addressed this whole issue of the optical opacity of disk galaxies (as described in the proceedings of the Cardiff workshop The Opacity of Galaxy Disks. There were improved statistical analyses of various surface-photometry samples, models of radiative transfer in clumpy media, (ahem) studies of overlapping galaxies, and new measurements of far-infrared and submillimeter dust emission. The upshot was the most participants in the meeting could agree with the "Welsh Model" ( I Model Cymraig), which has the disks centrally quite optically thick, falling to transparent at the edges, with dusty spiral arms and considerable varation among galaxies.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  5. #395
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Regarding dust density, I have not yet been able to find the cites I was looking for. But as I've said I stink at Google research.

    *(And other search engines.)
    Last edited by Noclevername; 2019-Jun-06 at 01:34 AM.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  6. #396
    Join Date
    Apr 2005
    Posts
    2,474
    Quote Originally Posted by Noclevername View Post
    Regarding dust density, I have not yet been able to find the cites I was looking for. But as I've said I stink at Google research.

    *(And other search engines.)
    I've got to say I've not seen any articles which posit "dust drag" as a reason for increased rate of impacts.

    I've seen papers which posit climate change as a result of passing through clouds, but that was not considered in the paper we are discussing.

  7. #397
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by kzb View Post
    I've got to say I've not seen any articles which posit "dust drag" as a reason for increased rate of impacts.

    I've seen papers which posit climate change as a result of passing through clouds, but that was not considered in the paper we are discussing.
    I was able to get articles on gas causing drag, but not dust.

    I withdraw the suggestion, then. I recall seeing it in articles but I can't find them so I can't present any data on it.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  8. #398
    Join Date
    Sep 2004
    Posts
    3,199
    I went to the ADS homepage ( http://cdsads.u-strasbg.fr/ ) and searched there for articles mentioning "mass extinction" and spiral arms. Here is part of what I got. I cannot say I am convinced; most of the papers come from only a few people. Also, many of the mass extinctions studied have been found to have reasonable causes not connected with impacts (the Cretaceous one being an exception, in part).

    ===

    http://cdsads.u-strasbg.fr/abs/1998NewA....3...51L

    Mass extinctions and the sun's encounters with spiral arms
    Authors: Leitch, Erik M.; Vasisht, Gautam
    Affiliation: AA(California Institute of Technology, 105-24, Pasadena, CA 91125, USA), AB(California Institute of Technology, 105-24, Pasadena, CA 91125, USA)
    Publication: New Astronomy, Volume 3, Issue 1, p. 51-56. (NewA Homepage)
    Publication Date: 02/1998

    The terrestrial fossil record shows that the exponential rise in biodiversity since the Precambrian period has been punctuated by large extinctions, at intervals of 40 to 140 Myr. These mass extinctions represent extremes over a background of smaller events and the natural process of species extinction. We point out that the non-terrestrial phenomena proposed to explain these events, such as boloidal impacts (a candidate for the end-Cretaceous extinction) and nearby supernovae, are collectively far more effective during the solar system's traversal of spiral arms. Using the best available data on the location and kinematics of the Galactic spiral structure (including distance scale and kinematic uncertainties), we present evidence that arm crossings provide a viable explanation for the timing of the large extinctions.

    ===

    http://cdsads.u-strasbg.fr/abs/2013SerAJ.187...43F

    Mass Extinction and the Structure of the Milky Way
    Authors: Filipovic, M. D.; Horner, J.; Crawford, E. J.; Tothill, N. F. H.; White, G. L.
    Affiliation: AA(University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 2751, Australia) AB(School of Physics, University of New South Wales, Sydney 2052, Australia; Australian Centre for Astrobiology, University of New South Wales, Sydney 2052, Australia) AC(University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 2751, Australia) AD(University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 2751, Australia) AE(University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 2751, Australia)
    Publication: Serbian Astronomical Journal, vol. 187, pp. 43-52 (SerAJ Homepage)
    Publication Date: 12/2013

    We use the most up-to-date Milky Way model and solar orbit data in order to test the hypothesis that the Sun's galactic spiral arm crossings cause mass extinction events on Earth. To do this, we created a new model of the Milky Way's spiral arms by combining a large quantity of data from several surveys. We then combined this model with a recently derived solution for the solar orbit to determine the timing of the Sun's historical passages through the Galaxy's spiral arms. Our new model was designed with a symmetrical appearance, with the major alteration being the addition of a spur at the far side of the Galaxy. A correlation was found between the times at which the Sun crosses the spiral arms and six known mass extinction events. Furthermore, we identify five additional historical mass extinction events that might be explained by the motion of the Sun around our Galaxy. These five additional significant drops in marine genera that we find include significant reductions in diversity at 415, 322, 300, 145 and 33~Myr ago. Our simulations indicate that the Sun has spent ˜60 per cent of its time passing through our Galaxy's various spiral arms. Also, we briefly discuss and combine previous work on the Galactic Habitable Zone with the new Milky Way model.

    ===

    http://cdsads.u-strasbg.fr/abs/2014ebi..confP2.44R

    Galactic Habitable Orbits
    Authors: Rahimi, A.; Mao, S.; Kawata, D.
    Affiliation: AA(NAOC, UCL, ara@nao.cas.cn), AB(NAOC), AC(UCL)
    Publication: Search for Life Beyond the Solar System. Exoplanets, Biosignatures & Instruments. Online at http://www.ebi2014.org, id.P2.44
    Publication Date: 03/2014

    The fossil record shows that the Earth has experienced several mass extinctions over the past 500 million years1, and it has been suggested that there is a periodicity in extinction events on timescales of tens1 and/or hundreds of millions of years. Various hypotheses have been proposed to explain the cause of the mass extinctions, including the suggestion that the Earth's ozone layer may have been destroyed by intense radiation from a nearby supernovae2- 3, exposing the Earth's surface to damaging UV radiation. Recent observations of cores taken from the ocean floor revealed atoms of a very rare isotope of iron (60Fe) believed to have arrived on Earth around 2 million years ago as fallout from a nearby supernovae4. Astronomical evidence for that past supernovae was recently found in the debris of a young cluster of massive stars5, by tracing its past orbit, putting it at the right place at the right time to explain the mild extinction event. Here we report new high-resolution (both in space and time) N-body chemodynamical simulations (carried out with our novel code GCD+6) of the evolution of a model Milky Way Galaxy, tracing the orbit of èsun-like' stars over a 500 million year period, checking the proximity to supernovae throughout the history of the orbit and comparing the times when this occurs with past mass extinctions on Earth. We additionally explain the important effects of the spiral arm pattern, radial migration of stars and Galactic chemistry on habitability.

    ===

    http://cdsads.u-strasbg.fr/abs/2014RMxAC..44R.163P

    Mass extinctions, galactic orbits in the solar neighborhood and the Sun: a connection?
    Authors: Porto de Mello, G. F.; Dias, W. S.; Lépine, J. R. D.; Lorenzo-Oliveira, D.; Siqueira, R. K.
    Affiliation: AA( Observatório do Valongo, Universidade Federal do Rio de Janeiro.)
    Publication: XIV Latin American Regional IAU Meeting (Eds. A. Mateus, J. Gregorio-Hetem & R. Cid Fernandes) Revista Mexicana de Astronomía y Astrofísica (Serie de Conferencias) Vol. 44, pp. 163-164 (2014) (RMxAC Homepage)
    Publication Date: 10/2014

    The orbits of the stars in the disk of the Galaxy, and their passages through the Galactic spiral arms, are a rarely mentioned factor of biosphere stability which might be important for long-term planetary climate evolution, with a possible bearing on mass extinctions. The Sun lies very near the co-rotation radius, where stars revolve around the Galaxy in the same period as the density wave perturbations of the spiral arms. Conventional wisdom generally considers that this status makes for few passages through the spiral arms. Controversy still surrounds whether time spent inside or around spiral arms is dangerous to biospheres and conducive to mass extinctions. Possible threats include giant molecular clouds disturbing the Oort comet cloud and provoking heavy bombardment; a higher exposure to cosmic rays near star forming regions triggering increased cloudiness in Earth's atmosphere and ice ages; and the destruction of Earth's ozone layer posed by supernova explosions. We present detailed calculations of the history of spiral arm passages for all 212 solar-type stars nearer than 20 parsecs, including the total time spent inside the spiral arms in the last 500 Myr, when the spiral arm position can be traced with good accuracy. We found that there is a large diversity of stellar orbits in the solar neighborhood, and the time fraction spent inside spiral arms can vary from a few percent to nearly half the time. The Sun, despite its proximity to the galactic co-rotation radius, has exceptionally low eccentricity and a low vertical velocity component, and therefore spends 30% of its lifetime crossing the spiral arms, more than most nearby stars. We discuss the possible implications of this fact to the long-term habitability of the Earth, and possible correlations of the Sun's passage through the spiral arms with the five great mass extinctions of the Earth's biosphere from the Late Ordovician to the Cretaceous-Tertiary.

    ===

    http://cdsads.u-strasbg.fr/abs/2009IJAsB...8..213B

    The evidence for and against astronomical impacts on climate change and mass extinctions: a review
    Authors: Bailer-Jones, C. A. L.
    Affiliation: AA(Max-Planck-Institut für Astronomie, Königstuhl 17, Heidelberg, Germany calj@mpia.de)
    Publication: Special issue Papers from ESLAB 2008 Symposium Cosmic Cataclysms and Life, Volume 8, Issue 3, p. 213-219
    Publication Date: 07/2009

    Numerous studies over the past 30 years have suggested there is a causal connection between the motion of the Sun through the Galaxy and terrestrial mass extinctions or climate change. Proposed mechanisms include comet impacts (via perturbation of the Oort cloud), cosmic rays and supernovae, the effects of which are modulated by the passage of the Sun through the Galactic midplane or spiral arms. Supposed periodicities in the fossil record, impact cratering dates or climate proxies over the Phanerozoic (past 545 Myr) are frequently cited as evidence in support of these hypotheses. This remains a controversial subject, with many refutations and replies having been published. Here I review both the mechanisms and the evidence for and against the relevance of astronomical phenomena to climate change and evolution. This necessarily includes a critical assessment of time series analysis techniques and hypothesis testing. Some of the studies have suffered from flaws in methodology, in particular drawing incorrect conclusions based on ruling out a null hypothesis. I conclude that there is little evidence for intrinsic periodicities in biodiversity, impact cratering or climate on timescales of tens to hundreds of Myr. Although this does not rule out the mechanisms, the numerous assumptions and uncertainties involved in the interpretation of the geological data and in particular in the astronomical mechanisms suggest that Galactic midplane and spiral arm crossings have little impact on biological or climate variation above background level. Non-periodic impacts and terrestrial mechanisms (volcanism, plate tectonics, sea level changes), possibly occurring simultaneously, remain likely causes of many environmental catastrophes. Internal dynamics of the biosphere may also play a role. In contrast, there is little evidence supporting the idea that cosmic rays have a significant influence on climate through cloud formation. It seems likely that more than one mechanism has contributed to biodiversity variations over the past half Gyr.
    Last edited by Roger E. Moore; 2019-Jun-06 at 01:44 PM.

  9. #399
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    OK, so leaving aside the matter of spiral arms, what about supernovae? It's been suggested upthread that we have only recently emerged from a more active SN period which might have put a damper on the party. Presently, according to this hypothesis, we get hit with less boom now, leaving more room and time for life to grow and evolve to complexity.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  10. #400
    Join Date
    Sep 2004
    Posts
    3,199
    Part of the answer to the Fermi Paradox might have been discovered. Toxic atmospheres are more common than once believed.

    https://phys.org/news/2019-06-narrow...-universe.html

    New study dramatically narrows the search for advanced life in the universe
    by University of California - Riverside

    Scientists may need to rethink their estimates for how many planets outside our solar system could host a rich diversity of life. In a new study, a UC Riverside–led team discovered that a buildup of toxic gases in the atmospheres of most planets makes them unfit for complex life as we know it.

  11. #401
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Roger E. Moore View Post
    Part of the answer to the Fermi Paradox might have been discovered. Toxic atmospheres are more common than once believed.

    https://phys.org/news/2019-06-narrow...-universe.html

    New study dramatically narrows the search for advanced life in the universe
    by University of California - Riverside

    Scientists may need to rethink their estimates for how many planets outside our solar system could host a rich diversity of life. In a new study, a UC Riverside–led team discovered that a buildup of toxic gases in the atmospheres of most planets makes them unfit for complex life as we know it.
    Wasn't Earth a planet with a toxic atmosphere, at first? Certainly it could not have supported complex multi-celled life even if it existed back then.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  12. #402
    Join Date
    Sep 2004
    Posts
    3,199
    Quote Originally Posted by Noclevername View Post
    Wasn't Earth a planet with a toxic atmosphere, at first? Certainly it could not have supported complex multi-celled life even if it existed back then.
    But life changed the atmosphere to the current mix. On other planets, no such change occurs, as the air is too toxic for more advanced life to develop. Check the article details and linked paper.

  13. #403
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Roger E. Moore View Post
    On other planets, no such change occurs,
    If the ET life is close to our chemistry and can't adapt to toxic gasses. Untested premises.

    Check the article details and linked paper.
    I did, hence the caveat above.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  14. #404
    Join Date
    Mar 2008
    Posts
    5,583
    I was watching a documentary on abiogenesis, interestingly there is not enough evidence to make any definitive conclusion on the odds of it occurring and developing into intelligent life. They went on to say that some studies suggest the odds could be as little as 1 in 1027 which means there could be significantly less chance of life ever developing into intelligent life than there are stars in the observable universe.

  15. #405
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by cosmocrazy View Post
    I was watching a documentary on abiogenesis, interestingly there is not enough evidence to make any definitive conclusion on the odds of it occurring and developing into intelligent life. They went on to say that some studies suggest the odds could be as little as 1 in 1027 which means there could be significantly less chance of life ever developing into intelligent life than there are stars in the observable universe.
    "Never tell me the odds!"

    How did they calculate that?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  16. #406
    Join Date
    Jun 2005
    Posts
    13,687
    Quote Originally Posted by Noclevername View Post
    If the ET life is close to our chemistry and can't adapt to toxic gasses. Untested premises.
    They do, I think, recognize that point. Quoting the actual article:

    The metabolic oxidation of organic matter with O2 produces significantly more free energy than any other plausible respiratory process, and O2 is the only high-potential oxidant sufficiently stable to accumulate within planetary atmospheres (Catling et al. 2005). As a result, it is likely that the centrality of molecular O2 in the emergence and expansion of a complex biosphere on Earth is a general phenomenon (Catling et al. 2005).
    So they seem to be saying that oxygen-based life is the most plausible, but that this is only what appears most likely, and they recognize that there could be other types of life, though they say it seems most likely that they would be oxygen based.
    As above, so below

  17. #407
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Jens View Post
    So they seem to be saying that oxygen-based life is the most plausible, but that this is only what appears most likely, and they recognize that there could be other types of life, though they say it seems most likely that they would be oxygen based.

    Well yes, but my point was we're only oxygen based (and therefore able to support complexity) because our atmosphere was extensively altered by simpler forms of life.

    For most of life's existence, it was anaerobic, thriving in a toxic stew. To my reading, the paper's authors seem to imply or assume that something similar wouldn't happen on an exoplanet. Simple life there might thrive in what we call toxic gasses (as ours did) and then something gloms onto photosynthesis (as ours did) or some similar process. As that life changes the atmosphere and release more free oxidizer (as ours did), life can now grow in new directions and has more available energy to potentially support large complex life.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  18. #408
    Join Date
    Apr 2005
    Posts
    2,474
    Quote Originally Posted by cosmocrazy View Post
    I was watching a documentary on abiogenesis, interestingly there is not enough evidence to make any definitive conclusion on the odds of it occurring and developing into intelligent life. They went on to say that some studies suggest the odds could be as little as 1 in 1027 which means there could be significantly less chance of life ever developing into intelligent life than there are stars in the observable universe.
    Yep I think the Anthropic principle may have mileage in it. The probability of abiogenesis may well be 1 in 10^27 or whatever small number.

    But however small, it is greater than zero. We live in a universe where this small chance actually occurred. Other universes are virtual.

    The probability is tiny but it does not actually preclude life elsewhere in our universe. But for arguments sake if we go with 1 in 10^27 as the probability, the probability of life starting in two places independently is 1 in 10^54. So it is extremely unlikely there is life elsewhere in our universe.

  19. #409
    Join Date
    Sep 2004
    Posts
    3,199
    Well, if there aren't any other aliens, then we're just going to have to roll up our sleeves and make some.

  20. #410
    Join Date
    Sep 2004
    Posts
    3,199
    https://iopscience.iop.org/article/1...38-4357/ab1d52

    A Limited Habitable Zone for Complex Life
    Edward W. Schwieterman, Christopher T. Reinhard, Stephanie L. Olson, Chester E. Harman, and Timothy W. Lyons
    Published 2019 June 10 • The Astrophysical Journal, Volume 878, Number 1

    Abstract: The habitable zone (HZ) is commonly defined as the range of distances from a host star within which liquid water, a key requirement for life, may exist on a planet's surface. Substantially more CO2 than present in Earth's modern atmosphere is required to maintain clement temperatures for most of the HZ, with several bars required at the outer edge. However, most complex aerobic life on Earth is limited by CO2 concentrations of just fractions of a bar. At the same time, most exoplanets in the traditional HZ reside in proximity to M dwarfs, which are more numerous than Sun-like G dwarfs but are predicted to promote greater abundances of gases that can be toxic in the atmospheres of orbiting planets, such as carbon monoxide (CO). Here we show that the HZ for complex aerobic life is likely limited relative to that for microbial life. We use a 1D radiative-convective climate and photochemical models to circumscribe a Habitable Zone for Complex Life (HZCL) based on known toxicity limits for a range of organisms as a proof of concept. We find that for CO2 tolerances of 0.01, 0.1, and 1 bar, the HZCL is only 21%, 32%, and 50% as wide as the conventional HZ for a Sun-like star, and that CO concentrations may limit some complex life throughout the entire HZ of the coolest M dwarfs. These results cast new light on the likely distribution of complex life in the universe and have important ramifications for the search for exoplanet biosignatures and technosignatures.

    QUOTES: The origin and diversification of complex life on Earth is fundamentally tied to the rise of oxygen (O2) in our atmosphere and oceans (Lyons et al. 2014; Planavsky et al. 2014; Reinhard et al. 2016). Physical and geochemical evidence for eukaryotes (complex cells with organelles, such as mitochondria and chloroplasts) is absent from the rock record until well after the earliest accumulation of free O2 in Earth's atmosphere ~2.3 billion years ago (Ga) (Knoll 2014; Luo et al. 2016), while the first metazoan (animal) life emerged only in the last ~700 million years (Erwin et al. 2011; Zumberge et al. 2018). Later, significant increases in biological complexity on Earth, such as the Cambrian Explosion (~542 Ma; Lee et al. 2013), occurred against the backdrop of a more strongly oxygenated planetary atmosphere.

    The metabolic oxidation of organic matter with O2 produces significantly more free energy than any other plausible respiratory process, and O2 is the only high-potential oxidant sufficiently stable to accumulate within planetary atmospheres (Catling et al. 2005). As a result, it is likely that the centrality of molecular O2 in the emergence and expansion of a complex biosphere on Earth is a general phenomenon (Catling et al. 2005). However, complex aerobic life can be strongly impacted by CO2 and CO—the latter of which is produced by CO2 photolysis and surface biological activity.

    Elevated CO2 levels can impose severe physiological stress on complex aerobic organisms (Pörtner et al. 2004; Wittmann & Pörtner 2013).

    Our results show that CO concentrations on planets in the traditional HZ of FGK stars are unlikely to reach known toxicity limits for humans, at least in oxygen-rich atmospheres. However, for stars with effective temperatures below about 3200 K—for example, Proxima Centauri and TRAPPIST-1—we predict that CO concentrations can reach and significantly exceed short- and long-term human exposure limits. Because CO lifetimes are driven by destruction timescales set by OH (in O2-rich atmospheres), dry planets orbiting interior to the traditional HZ may be problematic for complex life even around FGK stars (Abe et al. 2011; Zsom et al. 2013). In addition, cooler worlds with lower tropospheric H2O contents may have even higher CO concentrations than those shown here (Grenfell et al. 2007; Gao et al. 2015).

    Given variability in biochemical response across metazoan clades and frequent secondary adaptation to high CO2 conditions, it is challenging to prescribe a specific limiting CO2 value for complex life in the HZ. Nevertheless, a recent review focusing on anthropogenically elevated CO2 impacts on ocean ecosystems found universally negative impacts on extant marine life, including corals, echinoderms, mollusks, crustaceans, and fishes at pCO2 ≥ 1% (Wittmann & Pörtner 2013).

    Lastly, we have shown here and elsewhere that CO concentrations are likely to be high on Earth-like planets orbiting late-type stars with similar surface molecular fluxes of CO (Schwieterman et al. 2019). This result is consistent with the seminal findings of Segura et al. (2005), who showed that the low NUV radiation from stars with low effective temperatures would act to drastically reduce the generation of OH radicals in Earth-like atmospheres on planets orbiting these stars, with the consequence that trace gases like CH4 otherwise destroyed by OH could build up to high abundances.

    Our results have a number of important implications for the search for exoplanet biosignatures and complex life beyond our solar system. For example, our predictions of a more limited zone for complex life place constraints on the planetary environments suitable for the evolution of intelligence, if it requires free O2 and limited concentrations of CO2, CO, and other potentially toxic trace gases. One implication is that we may not expect to find remotely detectable signs of intelligent life ("technosignatures") on planets orbiting late M dwarfs or on potentially habitable planets near the outer edge of their HZs. These CO2 and CO limits should be considered in future targeted SETI searches (Tarter 2001, 2004; Turnbull & Tarter 2003).

  21. #411
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Roger E. Moore View Post
    Well, if there aren't any other aliens, then we're just going to have to roll up our sleeves and make some.
    AI and uplift. And post-human modifications.

    All will be monumental tasks and take possibly a very long time. So for the foreseeable future, pet your dogs and talk to humans, cause we're it.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  22. #412
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    A thread from a few years ago on the Fermi Paradox.
    Another one from a few years before that.
    A really long thread on it.

    One on Gamma Ray Bursts in Galactic history, with an arxiv link
    Another on Galactic habitability.
    Last edited by Noclevername; 2019-Jun-12 at 03:31 AM. Reason: spelling
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  23. #413
    Join Date
    Apr 2005
    Posts
    2,474
    A Limited Habitable Zone for Complex Life
    Edward W. Schwieterman, Christopher T. Reinhard, Stephanie L. Olson, Chester E. Harman, and Timothy W. Lyons
    Published 2019 June 10 • The Astrophysical Journal, Volume 878, Number 1

    The metabolic oxidation of organic matter with O2 produces significantly more free energy than any other plausible respiratory process, and O2 is the only high-potential oxidant sufficiently stable to accumulate within planetary atmospheres (Catling et al. 2005). As a result, it is likely that the centrality of molecular O2 in the emergence and expansion of a complex biosphere on Earth is a general phenomenon (Catling et al. 2005). However, complex aerobic life can be strongly impacted by CO2 and CO—the latter of which is produced by CO2 photolysis and surface biological activity.


    I think it is a bit Earth-centric.

    Even on Mars we have perchlorate in the soil, and that releases almost as much free energy as O2 when used as an oxidant. CO may not be toxic to life with very slightly different enzyme structures.

  24. #414
    Join Date
    Sep 2004
    Posts
    3,199
    Quote Originally Posted by Noclevername View Post
    A thread from a few years ago on the Fermi Paradox.
    Another one from a few years before that.
    A really long thread on it.

    One on Gamma Ray Bursts in Galactic history, with an arxiv link
    Another on Galactic habitability.
    It's fine to revisit the question over time. New knowledge is unveiled and new suggestions are offered. Still no good answers, but it does seem from recent research that we on Earth might have been a lot luckier than we thought.

  25. #415
    Join Date
    Mar 2008
    Posts
    5,583
    Quote Originally Posted by Noclevername View Post
    "Never tell me the odds!"

    How did they calculate that?
    They based it on all the known factors required for life as we know it to start, survive and evolve on our planet to get to us (humans). This included all the chemistry, environmental conditions, external influences, proximity to other bodies, time scales and so on... obviously and like I pointed out before, all this is based on the assumption that life would be just like here on earth and require all the same conditions and so on... But since we are not sure of exactly how life came to be even on Earth a good chunk of this calculation came from assumptions about how life began and evolved here in the first place.

  26. #416
    Join Date
    Mar 2008
    Posts
    5,583
    Quote Originally Posted by kzb View Post
    Yep I think the Anthropic principle may have mileage in it. The probability of abiogenesis may well be 1 in 10^27 or whatever small number.

    But however small, it is greater than zero. We live in a universe where this small chance actually occurred. Other universes are virtual.

    The probability is tiny but it does not actually preclude life elsewhere in our universe. But for arguments sake if we go with 1 in 10^27 as the probability, the probability of life starting in two places independently is 1 in 10^54. So it is extremely unlikely there is life elsewhere in our universe.
    Yes this pretty much summed up what they were saying. Obviously the value is greater than zero but it could be so small that the chances of life evolving similar to here on Earth could be extremely improbable. The point being that if life does evolve on average once every per 1027
    stars then its highly likely that it has only occurred once within our observable universe.

  27. #417
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by cosmocrazy View Post
    They based it on all the known factors required for life as we know it to start, survive and evolve on our planet to get to us (humans). This included all the chemistry, environmental conditions, external influences, proximity to other bodies, time scales and so on... obviously and like I pointed out before, all this is based on the assumption that life would be just like here on earth and require all the same conditions and so on... But since we are not sure of exactly how life came to be even on Earth a good chunk of this calculation came from assumptions about how life began and evolved here in the first place.
    Link please? I'd like to examine their data and base factors.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  28. #418
    Join Date
    Jun 2005
    Posts
    13,687
    Quote Originally Posted by Noclevername View Post
    Link please? I'd like to examine their data and base factors.
    The link was already given in post 410.
    As above, so below

  29. #419
    Join Date
    May 2007
    Location
    Earth
    Posts
    10,253
    Since I'm not a statistician, could somebody quantify the probability of abiogenesis if we find, say, solid evidence of an independent start of life on Mars?
    Information about American English usage here and here. Floating point issues? Please read this before posting.

    How do things fly? This explains it all.

    Actually they can't: "Heavier-than-air flying machines are impossible." - Lord Kelvin, president, Royal Society, 1895.



  30. #420
    Join Date
    Jan 2002
    Location
    The Valley of the Sun
    Posts
    9,467
    Maybe natural selection removes the desire that people have to explore and colonize once they've populated their entire planet. Such a desire would no longer be beneficial with nowhere left to go except into space which is so expensive that it might as well not be there for most of the population. So even after they're able to move out into space, none of them cares to do so.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •