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

  1. #91
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    Appears applicable here. Don't yet have an opinion on the paper, see what you think. His use of "tranquil" and "placid" is odd. [LATER: never mind, I get it.--REM] Not sure what to think of the values given to various probabilities.

    https://arxiv.org/abs/1904.11796

    Multiverse Predictions for Habitability: Fraction of Life that Develops Intelligence

    McCullen Sandora (Submitted on 23 Apr 2019)

    Do mass extinctions affect the development of intelligence? If so, we may expect to be in a universe that is exceptionally placid. We consider the effects of impacts, supervolcanoes, global glaciations, and nearby gamma ray bursts, and how their rates depend on fundamental constants. It is interesting that despite the very disparate nature of these processes, each occurs on timescales of 100 Myr-Gyr. We argue that this is due to a selection effect that favors both tranquil locales within our universe, as well as tranquil universes. Taking gamma ray bursts to be the sole driver of mass extinctions is disfavored in multiverse scenarios, as the rate is much lower for different values of the fundamental constants. In contrast, geological causes of extinction are very compatible with the multiverse. Various frameworks for the effects of extinctions are investigated, and the intermediate disturbance hypothesis is found to be most compatible with the multiverse.
    Last edited by Roger E. Moore; 2019-Apr-29 at 03:42 PM.
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  2. #92
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    Quote Originally Posted by Roger E. Moore View Post
    Appears applicable here. Don't yet have an opinion on the paper, see what you think. His use of "tranquil" and "placid" is odd. [LATER: never mind, I get it.--REM]

    https://arxiv.org/abs/1904.11796

    Multiverse Predictions for Habitability: Fraction of Life that Develops Intelligence

    McCullen Sandora (Submitted on 23 Apr 2019)

    Do mass extinctions affect the development of intelligence? If so, we may expect to be in a universe that is exceptionally placid. We consider the effects of impacts, supervolcanoes, global glaciations, and nearby gamma ray bursts, and how their rates depend on fundamental constants. It is interesting that despite the very disparate nature of these processes, each occurs on timescales of 100 Myr-Gyr. We argue that this is due to a selection effect that favors both tranquil locales within our universe, as well as tranquil universes. Taking gamma ray bursts to be the sole driver of mass extinctions is disfavored in multiverse scenarios, as the rate is much lower for different values of the fundamental constants. In contrast, geological causes of extinction are very compatible with the multiverse. Various frameworks for the effects of extinctions are investigated, and the intermediate disturbance hypothesis is found to be most compatible with the multiverse.
    QUOTE OF NOTE: "The viewpoint here is that 'it takes a village to raise a question': that is, that the consciousness you enjoy is not wholly your own, but is in part inherited from the whole history of society."
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    — Mark Twain, Life on the Mississippi (1883)

  3. #93
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    Quote Originally Posted by Roger E. Moore View Post
    QUOTE OF NOTE: "The viewpoint here is that 'it takes a village to raise a question': that is, that the consciousness you enjoy is not wholly your own, but is in part inherited from the whole history of society."
    What does this mean? Is consciousness simply knowledge or do we modernize it a tad with a touch of "woke"?

    They chose to avoid the number of observers factor but it's hard to imagine an intelligent society not having the chain of events of language--> writing/reading --> reading glasses for the older (and wiser like us ) --> voila! telescopes.
    We know time flies, we just can't see its wings.

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    ANOTHER QUOTE OF NOTE: "Our technique has been to use detailed criteria for what life needs to count the number of environments suitable for life in the universe, and to check how this depends on the fundamental constants of physics. This counting depends on the assumptions we make about what constitutes a habitable environment, and several of the choices we made imply the existence of much more fertile universes, which would host the majority of observers. When the use of a habitability criterion leads to a very small probability of our existence in this universe, we conclude that this criterion is incompatible with the multiverse hypothesis. Thus, the existence of the multiverse can be used to predict which notions of habitability are right or wrong."

    I completely do not see what the multiverse hypothesis has to do with any of this. The multiverse issue makes this paper rather opaque for me.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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  5. #95
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    Quote Originally Posted by George View Post
    They chose to avoid the number of observers factor but it's hard to imagine an intelligent society not having the chain of events of language--> writing/reading --> reading glasses for the older (and wiser like us ) --> voila! telescopes.
    For a species with eyes, we followed that path of development... in one culture out of thousands.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  6. #96
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    Quote Originally Posted by Noclevername View Post
    For a species with eyes, we followed that path of development... in one culture out of thousands.
    Yes, and I almost typed "evolutionary process" but that gets misapplied and would be in this case. As species become more intelligent the "evolutionary process" is no longer passive but far more active.
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by George View Post
    Yes, and I almost typed "evolutionary process" but that gets misapplied and would be in this case. As species become more intelligent the "evolutionary process" is no longer passive but far more active.
    Choices are made, and consequences paid. One way or another.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by Noclevername View Post
    Choices are made, and consequences paid. One way or another.
    Yep, "Life is choices.", pastor Bill Shockley, and many others no doubt.
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by Noclevername View Post
    Evolution Does Not Work That Way. It is all ad hoc, based on whatever random mutations pop up first that manage to continue to reproduce themselves. It's Rube Goldberg.
    The randomness you’re talking about is constrained by the fact that most mutations don’t manage to survive and reproduce.

    Case in point, Earth life has evolved billions of very successful solutions to survival that do not include any form of brain.
    Yes, there are many living things, ranging from archaea to trees, that flourish without a brain.

    Nonetheless, the process known as cephalisation — development of a head with a brain in it — has occurred in multiple lineages, including cephalopods, arthropods, vertebrates.

    It’s not the only evolutionary strategy, but it’s one of them.
    Last edited by Colin Robinson; 2019-Apr-29 at 11:06 PM.

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    Quote Originally Posted by Noclevername View Post
    For a species with eyes, we followed that path of development... in one culture out of thousands.
    The path from spoken language to writing to lens technology involved several cultures, surely?

  11. #101
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    Quote Originally Posted by Noclevername View Post
    No. You cannot generalize based on an example of one. Period!
    Sure you can, if you want to. Others don't have to accept it though.
    As above, so below

  12. #102
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    Quote Originally Posted by Colin Robinson View Post
    The path from spoken language to writing to lens technology involved several cultures, surely?
    Yes, I'm not sure why Noclevername felt confident enough to state that so categorically. First of all, as you said, there were technologies that were handed out through multiple cultures; for example many ancient cultures did work with glass. Secondly, the origin of the use of eyeglasses is somewhat controversial, as it is unclear whether they were developed in China and brought to the West, or vice versa, or whether they were developed independently in both the East and West. And then on top of that, there are lens-like historical artifacts from ancient civilizations in museums, and although we don't know what they were for, we cannot categorically reject the possibility that some might have been used as visual aids. So although I don't know what the truth is, stating categorically that there was only one path seems a bit careless to me.
    As above, so below

  13. #103
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    Quote Originally Posted by Colin Robinson View Post
    The randomness you’re talking about is constrained by the fact that most mutations don’t manage to survive and reproduce.



    Yes, there are many living things, ranging from archaea to trees, that flourish without a brain.

    Nonetheless, the process known as cephalisation — development of a head with a brain in it — has occurred in multiple lineages, including cephalopods, arthropods, vertebrates.

    It’s not the only evolutionary strategy, but it’s one of them.
    Of course it is, or we wouldn't be here to discuss it. But my point was, nothing in evolution is inevitable. Had animals not come about, cephalisation would not be a thing. Had DNA zigged instead of zagged, there would be nothing capable of having a head or brain.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Noclevername said on 2019-Apr-25, 12:26 PM
    We've found that our Solar System is atypical of planetary systems. Why would our biology not be as well?

    How is the Solar System atypical?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

  15. #105
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    Quote Originally Posted by Tom Mazanec View Post
    Noclevername said on 2019-Apr-25, 12:26 PM
    We've found that our Solar System is atypical of planetary systems. Why would our biology not be as well?

    How is the Solar System atypical?
    I may be wrong, but IIRC exoplanets have tended to be large super planets orbiting very close to their stars. What I’m not sure about is whether this is because ours is unusual or whether it is partly due to an observation bias, meaning that those are easier to detect.


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    As above, so below

  16. #106
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    Per the Kepler spacecraft, most worlds tend to be super-Earths or mini-Neptunes, all about the same size range but with varying amounts of atmosphere. Our Solar System is unusual in its lack of same. Reference:

    http://cdsads.u-strasbg.fr/abs/2019arXiv190305258H

    The Super-Earth Opportunity - Search for Habitable Exoplanets in the 2020s
    Hu, Renyu; Beichman, Charles A.; Brain, David; Chen, Pin; Damiano, Mario; Dawson, Rebekah; Friedson, A. James; Hasagawa, Yasuhiro; Howard, Andrew; Johnson, Robert; Kataria, Tiffany; Kidd, Richard; Kite, Edwin; Knutson, Heather; Lyra, Wladimir; Mischna, Michael; Planavsky, Noah; Reinhard, Chris; Schlichting, Hilke; Seager, Sara; Sotin, Christophe; Swain, Mark; Turner, Neal; West, Robert; Yung, Yuk; Zellem, Robert (03/2019)

    The recent discovery of a staggering diversity of planets beyond the Solar System has brought with it a greatly expanded search space for habitable worlds. The Kepler exoplanet survey has revealed that most planets in our interstellar neighborhood are larger than Earth and smaller than Neptune. Collectively termed super-Earths and mini-Neptunes, some of these planets may have the conditions to support liquid water oceans, and thus Earth-like biology, despite differing in many ways from our own planet. In addition to their quantitative abundance, super-Earths are relatively large and are thus more easily detected than true Earth twins. As a result, super-Earths represent a uniquely powerful opportunity to discover and explore a panoply of fascinating and potentially habitable planets in 2020 - 2030 and beyond.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    The problem in generalizing about the most common sort of exoplanet is that every method of detecting exoplanets is prone to detecting a certain type of them, leaving out all other types. We are not really sure what sort of exoplanet is the most common.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    — Mark Twain, Life on the Mississippi (1883)

  18. #108
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    Quote Originally Posted by Roger E. Moore View Post
    The problem in generalizing about the most common sort of exoplanet is that every method of detecting exoplanets is prone to detecting a certain type of them, leaving out all other types. We are not really sure what sort of exoplanet is the most common.
    I've not read many papers on this topic recently. But is it not still the case that probably no planet would've been detected in our solar system by any of these methods?

    Also there was a paper a couple of years back that concluded the average G-type star has 1.1 habitable planets. In other words, a small number of G-star systems will have 2 or more habitable planets.

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    There are at least two ways that planets in the Solar System could be detected by hypothetical aliens.

    1. TRANSIT: If a Solar System planet moves across the solar disk relative to an exoplanet far away with aliens viewing it, they could of course discover it through the transit method. Does not work for finding planets significantly out of plane (e.g., 55 Cancri system), also does not find exoplanets far from their star unless the transit observation program is VERY long. News article about this:

    https://www.nbcnews.com/mach/science...hat-ncna806546
    NBC: If Space Aliens Are Looking Our Way, Here's What They Might See

    2. RADIAL VELOCITY: a long-term radial-velocity program could discover Jupiter, maybe Saturn; these programs work a lot better when big planets are close in to their stars, like 51 Pegasi or ultra-hot Jupiters. Will not find Earth-sized planets or even super-Earths unless they are close to their stars AND stars are very low mass. Two recent papers finding possible Jupiter twins:

    https://arxiv.org/abs/1507.03998
    The Solar Twin Planet Search II. A Jupiter twin around a solar twin
    M. Bedell, et al. (Submitted on 14 Jul 2015 (v1), last revised 16 Jul 2015 (this version, v2))
    ABSTRACT: Through our HARPS radial velocity survey for planets around solar twin stars, we have identified a promising Jupiter twin candidate around the star HIP11915. We characterize this Keplerian signal and investigate its potential origins in stellar activity. Our analysis indicates that HIP11915 hosts a Jupiter-mass planet with a 3800-day orbital period and low eccentricity. Although we cannot definitively rule out an activity cycle interpretation, we find that a planet interpretation is more likely based on a joint analysis of RV and activity index data. The challenges of long-period radial velocity signals addressed in this paper are critical for the ongoing discovery of Jupiter-like exoplanets. If planetary in nature, the signal investigated here represents a very close analog to the solar system in terms of both Sun-like host star and Jupiter-like planet.

    https://arxiv.org/abs/0802.1731
    The Jupiter Twin HD 154345b
    J. T. Wright, G. W. Marcy, R. P. Butler, S. S. Vogt, G. W. Henry, H. Isaacson, A. W. Howard (Submitted on 12 Feb 2008 (v1), last revised 12 Jan 2009 (this version, v2))
    We announce the discovery of a twin of Jupiter orbiting the slightly metal-poor ([Fe/H] = -0.1) nearby (d = 18 pc) G8 dwarf HD 154345. This planet has a minimum mass of 0.95 Jupiter masses and a 9.2 year, circular orbit with radius 4.2 AU. There is currently little or no evidence for other planets in the system, but smaller or exterior planets cannot yet be ruled out. We also detect a ~ 9-year activity cycle in this star photometrically and in chromospheric emission. We rule out activity cycles as the source of the radial velocity variations by comparison with other cycling late-G dwarfs.

    .

    https://en.wikipedia.org/wiki/Lists_...s_of_detection
    According to this page in Wikipedia (check references & sources), about 96% of all exoplanets are detected by radial-velocity and transit methods.
    Last edited by Roger E. Moore; 2019-Apr-30 at 06:02 PM.
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  20. #110
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    Quote Originally Posted by kzb View Post
    Also there was a paper a couple of years back that concluded the average G-type star has 1.1 habitable planets. In other words, a small number of G-star systems will have 2 or more habitable planets.
    Don't recall this paper, will poke around for it. I have reservations about that figure.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    — Mark Twain, Life on the Mississippi (1883)

  21. #111
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    Quote Originally Posted by Jens View Post
    I may be wrong, but IIRC exoplanets have tended to be large super planets orbiting very close to their stars. What I’m not sure about is whether this is because ours is unusual or whether it is partly due to an observation bias, meaning that those are easier to detect.
    Another paper discussing the commonality of super-Earths, a result of the Kepler spacecraft observations. I would have called these worlds mini-Neptunes, but whatever.

    https://arxiv.org/abs/1806.09282

    Understanding Super-Earths with MINERVA-Australis at USQ's Mount Kent Observatory

    Robert A Wittenmyer, Jonathan Horner, Brad D Carter, Stephen R Kane, Peter Plavchan, David Ciardi, the MINERVA-Australis consortium (Submitted on 25 Jun 2018)

    ABSTRACT: Super Earths, planets between 5-10 Earth masses, are the most common type of exoplanet known, yet are completely absent from our Solar system. As a result, their detailed properties, compositions, and formation mechanisms are poorly understood. NASA's Transiting Exoplanet Survey Satellite (TESS) will identify hundreds of Super-Earths orbiting bright stars, for the first time allowing in-depth characterisation of these planets. At the University of Southern Queensland, we are host to the MINERVA-Australis project, dedicated wholly to the follow-up characterisation and mass measurement of TESS planets. We give an update on the status of MINERVA-Australis and our expected performance.
    Last edited by Roger E. Moore; 2019-Apr-30 at 07:17 PM. Reason: fixes
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  22. #112
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    Trying hard to get a better grip on my issues with the Fermi paradox. It is my guess that there is not one or two but literally hundreds of reasons why we have not already detected other intelligent species external to our Earth. I've named a few, so why not a few more. By all means, pile on if my reasoning is faulty.

    We have a number of astronomy papers stating that super-Earths (defined in several ways, but usually 2-10 or 5-10 Earth masses and 1-4 Earth radii) are the most common exoplanets we know of. Are any of them in the habitable zones of stars? Yes! However, these super-Earths with habitable temperatures appear heavily weighted toward being in red M-dwarf systems. References (only a few, there are many):

    ===

    https://arxiv.org/abs/1704.05556

    A temperate rocky super-Earth transiting a nearby cool star

    Jason A. Dittmann, et al. (Submitted on 18 Apr 2017)

    M dwarf stars, which have masses less than 60 per cent that of the Sun, make up 75 per cent of the population of the stars in the Galaxy. The atmospheres of orbiting Earth-sized planets are observationally accessible via transmission spectroscopy when the planets pass in front of these stars. Statistical results suggest that the nearest transiting Earth-sized planet in the liquid-water, habitable zone of an M dwarf star is probably around 10.5 parsecs away. A temperate planet has been discovered orbiting Proxima Centauri, the closest M dwarf, but it probably does not transit and its true mass is unknown. Seven Earth-sized planets transit the very low-mass star TRAPPIST-1, which is 12 parsecs away, but their masses and, particularly, their densities are poorly constrained. Here we report observations of LHS 1140b, a planet with a radius of 1.4 Earth radii transiting a small, cool star (LHS 1140) 12 parsecs away. We measure the mass of the planet to be 6.6 times that of Earth, consistent with a rocky bulk composition. LHS 1140b receives an insolation of 0.46 times that of Earth, placing it within the liquid-water, habitable zone. With 90 per cent confidence, we place an upper limit on the orbital eccentricity of 0.29. The circular orbit is unlikely to be the result of tides and therefore was probably present at formation. Given its large surface gravity and cool insolation, the planet may have retained its atmosphere despite the greater luminosity (compared to the present-day) of its host star in its youth. Because LHS 1140 is nearby, telescopes currently under construction might be able to search for specific atmospheric gases in the future.

    ===

    https://arxiv.org/abs/1707.04292

    Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit

    R. Cloutier, et al. (Submitted on 13 Jul 2017)

    The bright M dwarf K2-18 at 34 pc is known to host a transiting super-Earth-sized planet orbiting within the star's habitable zone; K2-18b. Given the superlative nature of this system for studying an exoplanetary atmosphere receiving similar levels of insolation as the Earth, we aim to characterize the planet's mass which is required to interpret atmospheric properties and infer the planet's bulk composition. We obtain precision radial velocity measurements with the HARPS spectrograph and couple those measurements with the K2 photometry to jointly model the observed radial velocity variation with planetary signals and a radial velocity jitter model based on Gaussian process regression. We measure the mass of K2-18b to be 8.0 \pm 1.9 M_{\oplus} with a bulk density of 3.7 \pm 0.9 g/cm^3 which may correspond to a predominantly rocky planet with a significant gaseous envelope or an ocean planet with a water mass fraction \gtrsim 50%. We also find strong evidence for a second, warm super-Earth K2-18c at \sim 9 days with a semi-major axis 2.4 times smaller than the transiting K2-18b. After re-analyzing the available light curves of K2-18 we conclude that K2-18c is not detected in transit and therefore likely has an orbit that is non-coplanar with K2-18b. A suite of dynamical integrations with varying simulated orbital eccentricities of the two planets are used to further constrain each planet's eccentricity posterior from which we measure e_b < 0.43 and e_c < 0.47 at 99% confidence. The discovery of the inner planet K2-18c further emphasizes the prevalence of multi-planet systems around M dwarfs. The characterization of the density of K2-18b reveals that the planet likely has a thick gaseous envelope which along with its proximity to the Solar system makes the K2-18 planetary system an interesting target for the atmospheric study of an exoplanet receiving Earth-like insolation.

    ===
    https://arxiv.org/abs/1306.6074

    A dynamically-packed planetary system around GJ667C with three super-Earths in its habitable zone

    Guillem Anglada-Escudι, et al. (Submitted on 25 Jun 2013)

    Since low-mass stars have low luminosities, orbits at which liquid water can exist on Earth-sized planets are relatively close-in, which produces Doppler signals that are detectable using state-of-the-art Doppler spectroscopy. GJ 667C is already known to be orbited by two super-Earth candidates. We investigate whether the data supports the presence of additional companions. We obtain new Doppler measurements from HARPS extracted spectra and combined them with those obtained from the PFS and HIRES spectrographs. We used Bayesian and periodogram-based methods to re-assess the number of candidates and evaluated the confidence of each detection. Among other tests, we validated the planet candidates by analyzing correlations of each Doppler signal activity indices and investigate quasi-periodicity. Doppler measurements of GJ 667C are described better by six Keplerian-like signals: the two known candidates (b and c); three additional few-Earth mass candidates with periods of 92, 62 and 39 days (d, e and f); a cold super-Earth in a 260-day orbit (g) and tantalizing evidence of a ∼ 1 M ⊕ object in a close-in orbit of 17 days (h). We explore whether long-term stable orbits are compatible with the data by integrating 8Χ10 4 solutions derived from the Bayesian samplings. The system consisting of six planets is compatible with dynamically stable configurations. As for the solar system, the most stable solutions do not contain mean-motion resonances and are described well by analytic Laplace-Lagrange solutions. The presence of a seventh planet (h) is supported by the fact that it appears squarely centered on the only island of stability left in the six-planet solution. Habitability assessments accounting for the stellar flux, as well as tidal dissipation effects, indicate that three (maybe four) planets are potentially habitable.

    ===

    https://arxiv.org/abs/0705.3758

    The habitability of super-Earths in Gliese 581

    W. von Bloh, C. Bounama, M. Cuntz, S. Franck (Submitted on 25 May 2007 (v1), last revised 12 Oct 2007 (this version, v3))

    Aims: The planetary system around the M star Gliese 581 consists of a hot Neptune (Gl 581b) and two super-Earths (Gl 581c and Gl 581d). The habitability of this system with respect to the super-Earths is investigated following a concept that studies the long-term possibility of photosynthetic biomass production on a dynamically active planet. Methods: A thermal evolution model for a super-Earth is used to calculate the sources and sinks of atmospheric carbon dioxide. The habitable zone is determined by the limits of biological productivity on the planetary surface. Models with different ratios of land / ocean coverage are investigated. Results: The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star. In contrast, Gl 581d is a tidally locked habitable super-Earth near the outer edge of the habitable zone. Despite the adverse conditions on this planet, at least some primitive forms of life may be able to exist on its surface. Therefore, Gl 581d is an interesting target for the planned TPF/Darwin missions to search for biomarkers in planetary atmospheres.

    ===============

    The troubles with being a habitable planet around an M-dwarf are many: superflares, atmospheric destruction, etc. There are astronomers who think this might not be so big a problem, however, pointing out that Earth was exposed to much radiation in its youth.

    One recent issue that was raised about new troubles for a civilization around a super-Earth in a red-dwarf system is, you can't leave. You will have trouble getting into orbit from the super-Earth and especially in escaping from the nearby star because of gravity.

    ===

    https://arxiv.org/abs/1808.08141

    Limitations of Chemical Propulsion for Interstellar Escape from Habitable Zones around Low-Mass Stars

    Manasvi Lingam, Abraham Loeb (Submitted on 24 Aug 2018 (v1), last revised 29 Aug 2018 (this version, v2))

    The habitable zones of low-mass stars are characterized by escape speeds that can be a few times higher than the Earth's orbit around the Sun. Owing to the exponential dependence of the required fuel mass on the terminal speed for chemical rockets, interstellar travel may not be easy for technological species inhabiting planets around M-dwarfs.

    ===

    https://arxiv.org/abs/1804.03698

    Interstellar Escape from Proxima b is Barely Possible with Chemical Rockets

    Abraham Loeb (Submitted on 10 Apr 2018)

    A civilization in the habitable zone of a dwarf star might find it challenging to escape into interstellar space using chemical propulsion.
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    If it turns out, as it might, that super-Earths are just as good or better cradles for life than worlds like ours, part of the Fermi paradox is right there. The aliens can't get off-world. At the very least, it will be tough.

    Oh, here is the reference I forgot from earlier. My fault.

    https://arxiv.org/abs/1804.04727

    Spaceflight from Super-Earths is difficult

    Michael Hippke (Submitted on 12 Apr 2018 (v1), last revised 18 May 2018 (this version, v2))

    Many rocky exoplanets are heavier and larger than the Earth, and have higher surface gravity. This makes space-flight on these worlds very challenging, because the required fuel mass for a given payload is an exponential function of planetary surface gravity. We find that chemical rockets still allow for escape velocities on Super-Earths up to 10x Earth mass. More massive rocky worlds, if they exist, would require other means to leave the planet, such as nuclear propulsion.
    Last edited by Roger E. Moore; 2019-Apr-30 at 07:25 PM. Reason: adds
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  24. #114
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    Quote Originally Posted by Roger E. Moore View Post
    If it turns out, as it might, that super-Earths are just as good or better cradles for life than worlds like ours, part of the Fermi paradox is right there. The aliens can't get off-world. At the very least, it will be tough.

    Oh, here is the reference I forgot from earlier. My fault.

    https://arxiv.org/abs/1804.04727

    Spaceflight from Super-Earths is difficult

    Michael Hippke (Submitted on 12 Apr 2018 (v1), last revised 18 May 2018 (this version, v2))

    Many rocky exoplanets are heavier and larger than the Earth, and have higher surface gravity. This makes space-flight on these worlds very challenging, because the required fuel mass for a given payload is an exponential function of planetary surface gravity. We find that chemical rockets still allow for escape velocities on Super-Earths up to 10x Earth mass. More massive rocky worlds, if they exist, would require other means to leave the planet, such as nuclear propulsion.
    Could the biological effects of additional acceleration of launching a BFR from a super-Earth be a factor? Picture a high gravity macroscopic lifeform adapted to 3G getting crushed by an additional 6X their weight during takeoff.

    Some structural capacity just doesn't scale up well.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by Noclevername View Post
    Could the biological effects of additional acceleration of launching a BFR from a super-Earth be a factor? Picture a high gravity macroscopic lifeform adapted to 3G getting crushed by an additional 6X their weight during takeoff.

    Some structural capacity just doesn't scale up well.
    another extension of the Goldilocks Fermi thought experiment: it hardly counts as a paradox, with no evidence when many of us would expect no evidence of "intelligent" life around other stars.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
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    Running the heavy-planet problem in the other direction, we have closed-off waterworlds, assuming Europa and similar worlds are capable of developing life in their warm (relatively speaking) underground oceans. The life down there might not survive breaking through the ice cap over the ocean, so we would be unaware of their existence for the longest time.

    We might not be the only intelligent life in the universe, but we might be one of the few species able to physically reach another planet.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Quote Originally Posted by profloater View Post
    another extension of the Goldilocks Fermi thought experiment: it hardly counts as a paradox, with no evidence when many of us would expect no evidence of "intelligent" life around other stars.
    Just a wild thought in the wee hours. Not enough coffee!

    I agree with you, though. Taking into account the massive Universal scale in space and time, there's no reason why we should expect recognizably intelligent life to pop up conveniently close enough to see from our neighborhood, in the brief instant that we've been looking.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by kzb View Post
    Also there was a paper a couple of years back that concluded the average G-type star has 1.1 habitable planets. In other words, a small number of G-star systems will have 2 or more habitable planets.
    I vaguely recall something like this result in an article back then.

    Using the Exoplanet Catalog, I found only a little under 14% (71 of 517) of systems (F7 to G7 stars) have a planet in the HZ (liberal at 0.75 to 3 AU) that is < 3x that of the mass of Earth. As the smaller planets (higher hanging fruit) become more and more detectable, this percentage will certainly improve.

    [Added: It is perhaps only around 12% for (F4 to M9), though my work was a bit rushed. My total is restricted to those with stated stellar temperatures in order to allow their luminosity determination.]
    Last edited by George; 2019-May-02 at 03:37 PM.
    We know time flies, we just can't see its wings.

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    Quote Originally Posted by Colin Robinson View Post
    The randomness you’re talking about is constrained by the fact that most mutations don’t manage to survive and reproduce.



    Yes, there are many living things, ranging from archaea to trees, that flourish without a brain.

    Nonetheless, the process known as cephalisation — development of a head with a brain in it — has occurred in multiple lineages, including cephalopods, arthropods, vertebrates.

    It’s not the only evolutionary strategy, but it’s one of them.

    Evolution is not deterministic. It has unpredictable changes as part of its very premise. Yes, it has patterns, but patterns based on selecting from chaotic factors. And those "multiple lineages" you cite sprang from a common ancestry. They're all branches off the same biological tree trunk. The recurrence of cephalization might very well be causally connected to this genetic brotherhood. But there would be no such relationship with hypothetical, independently evolved alien life. They might follow a different path altogether.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by Noclevername View Post
    Evolution is not deterministic.
    Hmm, I don't think he was saying it is... By condensing arguments into absolutes, doesn't it make it difficult to discuss the actual subtleties of it all? I doubt that he thinks it is deterministic (I certainly don't), but I certainly would consider the possibility that there are elements that could frequently arise (not inevitably, but frequently). For example, locomotion would seem to be a good strategy within many environments, given the need to get energy, and so the development of some kind of locomotion, like limbs, might be seen commonly (no, again, I am not saying "inevitably").
    As above, so below

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