Page 5 of 19 FirstFirst ... 3456715 ... LastLast
Results 121 to 150 of 545

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

  1. #121
    Join Date
    Aug 2010
    Posts
    2,235
    Quote Originally Posted by Noclevername View Post
    Of course it is, or we wouldn't be here to discuss it. But my point was, nothing in evolution is inevitable.
    When we're discussing the likelihood of smart living things on other worlds, the question isn't whether evolutionary steps are inevitable, it's how probable (or improbable) they are.

    Had animals not come about, cephalisation would not be a thing.
    True. Cephalisation is hardly imaginable in a single-celled species, because a head is a multicellular system. In sessile species like trees and mushrooms, cephalisation would offer little or no evolutionary advantage, because trees and mushrooms don't have to make decisions about which direction to swim or crawl, or about which objects to grab and swallow.

    So it's relevant to think about how probable or improbable is the emergence of a class of multicellular motile life. Was that a hugely improbable event, likely unique to Earth?

    I think the answer to that question is no, firstly because neither multicellularity nor motility is unique to animals. There are multicellular algae and fungi, and there are single-celled critters which are motile in the sense that they have flagella to swim about with. One such group of single-celled motiles, the choanoflagellates, are considered the closest living relatives to the animal kingdom. The first animals were presumably not much different from a bunch of flagellates swimming about as a colony.

    Why did the first animals appear when they did, several billion years after the first living cells?

    Like just about everything in evolution, multicellularity and motility have downsides as well as benefits. Multicellularity lowers the ratio of surface area to volume, therefore requires greater energy density. Motility also has an energy cost. The combination of multicellularity and motility happened at a time when available chemical energy was increasing due to rising oxygen levels, presumably accompanied by rising availability of sugars. After all, oxygen and sugars are both produced by photosynthesis...

    Had DNA zigged instead of zagged, there would be nothing capable of having a head or brain.
    A new ecological niche was waiting to be filled. If one species of flagellate hadn't gone multicellular then, another species probably would have.
    Last edited by Colin Robinson; 2019-May-03 at 03:48 AM.

  2. #122
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Colin Robinson View Post
    When we're discussing the likelihood of smart living things on other worlds, the question isn't whether evolutionary steps are inevitable, it's how probable (or improbable) they are.
    Which are unknown factors. We are still generalizing from a single tree of life with a single shared origin and much DNA in common.

    A new ecological niche was waiting to be filled. If one species of flagellate hadn't gone multicellular then, another species probably would have.
    Agree to disagree.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  3. #123
    Join Date
    Apr 2005
    Posts
    2,466
    Quote Originally Posted by George View Post
    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.]
    I think if 14% have DETECTED planets in the HZ, that is very encouraging. The probability of detecting an Earth orbiting a F-G class star is not large.

    Going by this 14% figure I think it is reasonable to expect a true value of 1.1 per star. Although, as I said, I have got a bit behind on this topic.

  4. #124
    Join Date
    Sep 2004
    Posts
    3,199
    Found something relevant while looking for something else.

    https://arxiv.org/abs/1902.06004

    The SOPHIE search for northern extrasolar planets. XIV. A temperate (Teq ∼300 K) super-earth around the nearby star Gliese 411
    R. F. Díaz, et al. (3/26/2019)

    QUOTE: Besides theoretical advantages, recent results show that rocky planets orbiting low-mass stars are abundant: based on results from the HARPS velocity survey, Bonfils et al. (2013) reported an occurrence rate above 50% for super-Earth planets on orbits with periods between 10 and 100 days; using Kepler photometry Dressing & Charbonneau (2015) reported an average of 2.5 planets with radii between one and four Earth radii per M-star host, on orbits with periods shorter than 200 days; Gaidos et al. (2016) concluded that M stars host an average of 2.2 planets per star with periods between 1.5 and 180 d. Furthermore, planets orbiting in the habitable zone of low-mass stars are also relatively common. Dressing & Charbonneau (2015) reported an average between 0.16 - 0.24 Earth-sized planets in the habitable zone of M dwarfs, based on Kepler photometry. Bonfils et al. (2013) reported a fraction 0.41 +0.54/-0.13, although this number is probably slightly overestimated, as it included a planet candidate later shown to be likely produced by activity (Robertson et al. 2014).

  5. #125
    Join Date
    Sep 2004
    Posts
    3,199
    Found something else. Many of the arguments pro and con presented here were offered earlier in some form in this paper. It might be of interest and worth debating.


    http://cdsads.u-strasbg.fr/abs/1980QJRAS..21..267T
    (direct to PDF, a short one) http://articles.adsabs.harvard.edu//...&filetype=.pdf


    Extraterrestrial intelligent beings do not exist [Not an inflammatory title at all--REM]

    Tipler, F. J.
    Royal Astronomical Society, Quarterly Journal, vol. 21, Sept. 1980, p. 267-281 (09/1980)

    ABSTRACT: The singularity vs. the plurality of inhabited worlds in the universe is debated. Attention is given to astrophysical constraints on the evolution of intelligent species and to motivations for interstellar communication and exploration. It is argued that it is plausible that there is only one inhabited planet in the universe.

  6. #126
    Join Date
    Sep 2003
    Posts
    12,764
    Quote Originally Posted by kzb View Post
    I think if 14% have DETECTED planets in the HZ, that is very encouraging. The probability of detecting an Earth orbiting a F-G class star is not large.

    Going by this 14% figure I think it is reasonable to expect a true value of 1.1 per star. Although, as I said, I have got a bit behind on this topic.
    Agreed, the high-hanging fruit is quite high but more and more is being snagged. I hope Roger can dig-up that older info on how the >1 per system was determined.

    I just thought it would be fun to see if I could derive something from that downloadable exoplanet database, and it was easier than I thought. I did have to add a few columns to determine luminosity to get an HZ fit. There wasn't enough data on the exoplanets to try to use surf. grav. in the determination. I required star temperatures to determine luminosity, but this limited the count to that given above. It wouldn't be that hard to assume some star temperatures since most are classified, but I was short on time.
    Last edited by George; 2019-May-03 at 06:07 PM.
    We know time flies, we just can't see its wings.

  7. #127
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    To say that NO other thinking minds would exist ANYWHERE in our vast Universe, is IMO overly pessimistic. I think they're probably rare, and tenuous, but not impossible. It's the other end of the scale problem... the Universe is so big, and rolls the dice so many times, that at any given time there have to be other successes somewhere. I just don't think they're near us enough to detect them.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  8. #128
    Join Date
    Sep 2004
    Posts
    3,199
    Quote Originally Posted by George View Post
    I hope Roger can dig-up that older info on how the >1 per system was determined.
    Still looking.

  9. #129
    Join Date
    Sep 2004
    Posts
    3,199
    Found the following which makes use of the "1.1" search item, but not sure if it is what you were looking for. It is a pessimistic paper, though.

    ======

    https://web.archive.org/web/20160322...able-zone.html
    How Many Exoplanets Have Been Found in a Habitable Zone?
    QUOTE: In a paper by Catanzarite and Shao (2011), the authors found that the occurrence rate of habitable planets around sun-like stars in the Kepler sample is 1.1 (+0.6,-0.3)% using a particular set of assumptions about the habitable zone. (In other words, 1.1 (+0.6,-0.3)% of sun-like stars are estimated to host a habitable planet). The same study found that using a habitable zone based on different (more optimistic) assumptions gave an answer of 2.8(+1.9,-0.9)%. (Note that the authors assumed a restricted set of parameters for the host stars in order to consider them to be favorable for hosting habitable planets.)

    Comes from:

    https://arxiv.org/abs/1103.1443

    The Occurrence Rate of Earth Analog Planets Orbiting Sunlike Stars
    Joseph Catanzarite, Michael Shao (Jet Propulsion Laboratory, California Institute of Technology)
    (Submitted on 8 Mar 2011 (v1), last revised 25 Jun 2011 (this version, v3))

    Kepler is a space telescope that searches Sun-like stars for planets. Its major goal is to determine {\eta}_Earth, the fraction of Sunlike stars that have planets like Earth. When a planet 'transits' or moves in front of a star, Kepler can measure the concomitant dimming of the starlight. From analysis of the first four months of those measurements for over 150,000 stars, Kepler's science team has determined sizes, surface temperatures, orbit sizes and periods for over a thousand new planet candidates. In this paper, we characterize the period probability distribution function of the super-Earth and Neptune planet candidates with periods up to 132 days, and find three distinct period regimes. For candidates with periods below 3 days the density increases sharply with increasing period; for periods between 3 and 30 days the density rises more gradually with increasing period, and for periods longer than 30 days, the density drops gradually with increasing period. We estimate that 1% to 3% of stars like the Sun are expected to have Earth analog planets, based on the Kepler data release of Feb 2011. This estimate of is based on extrapolation from a fiducial subsample of the Kepler planet candidates that we chose to be nominally 'complete' (i.e., no missed detections) to the realm of the Earth-like planets, by means of simple power law models. The accuracy of the extrapolation will improve as more data from the Kepler mission is folded in. Accurate knowledge of {\eta}_Earth is essential for the planning of future missions that will image and take spectra of Earthlike planets. Our result that Earths are relatively scarce means that a substantial effort will be needed to identify suitable target stars prior to these future missions.

    QUOTE: We estimate that 1% to 3% of stars like the Sun are expected to have Earth analog planets, based on the Kepler data release of Feb 2011.

    QUOTE: Taking both estimates together, we find that the most likely range of is between 1.1% to 2.8%, with 1- minimum of 0.8% and 1- maximum of 4.7%.
    Last edited by Roger E. Moore; 2019-May-03 at 06:57 PM.

  10. #130
    Join Date
    Sep 2004
    Posts
    3,199
    A more optimistic paper. Note that this paper and the previous one are relatively "old" in terms of data presented.

    https://www.pnas.org/content/110/48/19273

    Prevalence of Earth-size planets orbiting Sun-like stars
    Erik A. Petigura, Andrew W. Howard, and Geoffrey W. Marcy
    PNAS November 26, 2013 110 (48) 19273-19278; https://doi.org/10.1073/pnas.1319909110

    We find that 22% of Sun-like stars harbor Earth-size planets orbiting in their habitable zones.

  11. #131
    Join Date
    Nov 2009
    Posts
    296
    Good discussion: 2 unrelated points

    Economics strongly argue against initial massive investments required for early stages of colonization, without which later stages never occur. Unless you posit that a "future Earth" magically finds itself with an abundance of "extra resources" that can be harnessed for a payback that is thousands of years downstream, it won't occur. Mankind has showed limited willingness to invest in projects that won't payoff, if ever, for multiple, multiple generations in the future. The cumulative investment necessary to do this provides ZERO payback to the current (and their immediate offspring) generation of decision makers who would have to fund the investment. Sure, little probes here and there at modest distances, but no one will fund the investment necessary that may start to payback in a few thousand years.

    Larger planets with more gravity will never create flying birds, etc. The initial building blocks of air travel will be too challenging thus there is not way to jump ahead to the massive rocket technology that would be needed to produce realistic escape velocity.

  12. #132
    Join Date
    Sep 2004
    Posts
    3,199
    Quote Originally Posted by Noclevername View Post
    ... the Universe is so big, and rolls the dice so many times, that at any given time there have to be other successes somewhere.
    Agree, but given what we would call a success and what the cosmos would call a success, there might not be much overlap.

  13. #133
    Join Date
    Sep 2003
    Posts
    12,764
    Quote Originally Posted by Roger E. Moore View Post
    Found the following which makes use of the "1.1" search item, but not sure if it is what you were looking for. It is a pessimistic paper, though.

    ...
    QUOTE: We estimate that 1% to 3% of stars like the Sun are expected to have Earth analog planets, based on the Kepler data release of Feb 2011.

    QUOTE: Taking both estimates together, we find that the most likely range of is between 1.1% to 2.8%, with 1- minimum of 0.8% and 1- maximum of 4.7%.
    A few years can make a lot of difference in this new and exciting field. They were likely using the safe "low-hanging fruit" numbers. Speculating on what is in that older noise would not be prudent.
    We know time flies, we just can't see its wings.

  14. #134
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Roger E. Moore View Post
    Agree, but given what we would call a success and what the cosmos would call a success, there might not be much overlap.
    I meant, success in evolving thinking minds.
    Last edited by Noclevername; 2019-May-03 at 09:59 PM.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  15. #135
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    That brings up the next question. How detectable would an advanced civilization be, anyway? Could it be they are somewhere near, but just don't make much of an impression?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  16. #136
    Join Date
    May 2013
    Location
    Central Virginia
    Posts
    1,926
    How detectable would an advanced civilization be, anyway?
    Good question NCN. What were Fermi's expectations when he came up with his "paradox"? I believe he only lived for 2 more years so there were no follow ups.

  17. #137
    Join Date
    Aug 2010
    Posts
    2,235
    Quote Originally Posted by Noclevername View Post
    That brings up the next question. How detectable would an advanced civilization be, anyway? Could it be they are somewhere near, but just don't make much of an impression?
    That's something we can't be sure about, because we don't know how advanced civilisations will behave in the long term.

    Will they colonise whole galaxies in a short time compared to the age of the universe? That's Michael Hart's opinion. Or will some advanced civilisations not do interstellar colonisation at all, while others colonise one or two star systems and then stop, as Geoffrey Landis suggested?

    If Hart's view is right, then if they were anywhere near, our own solar system would probably have colonised by now. If Landis' view is right, then yes, they could be quite near in the sense of just a few light years away. We've detected plenty of exoplanets, but we still know very very little about them.

  18. #138
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Colin Robinson View Post
    That's something we can't be sure about, because we don't know how advanced civilisations will behave in the long term.
    Right. We can't even agree on how we will act, let alone total aliens.

    Will they colonise whole galaxies in a short time compared to the age of the universe? That's Michael Hart's opinion.
    Why would they? What expansionist motives would hold true over such a long period of cultural and biological evolution?

    Or will some advanced civilisations not do interstellar colonisation at all, while others colonise one or two star systems and then stop, as Geoffrey Landis suggested?
    Devil's Advocate now ... If it can be done once or twice, then someone found a way to make it practical. As post-colonial technology advances, it should become easier, not harder, to travel between stars.

    If Hart's view is right, then if they were anywhere near, our own solar system would probably have colonised by now.
    Assuming our type of star system is valued by them. Also, that we would see them. Anyone check Titan for methane breathers?

    If Landis' view is right, then yes, they could be quite near in the sense of just a few light years away.
    If so, quite a lucky coincidence for us to be in the same neighborhood.

    We've detected plenty of exoplanets, but we still know very very little about them.
    If our neighbors even still use planets.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  19. #139
    Join Date
    Sep 2004
    Posts
    3,199
    Quote Originally Posted by George View Post
    A few years can make a lot of difference in this new and exciting field.
    This is true. Every day I expect something mind blowing in the arXiv files, and once a week my mind is blown.

  20. #140
    Join Date
    Aug 2010
    Posts
    2,235
    Quote Originally Posted by Noclevername View Post
    Right. We can't even agree on how we will act, let alone total aliens.
    Very true.

    Why would they? What expansionist motives would hold true over such a long period of cultural and biological evolution?
    An argument I've seen is that living things in general will expand into available habitats when they can.

    Devil's Advocate now ... If it can be done once or twice, then someone found a way to make it practical. As post-colonial technology advances, it should become easier, not harder, to travel between stars.
    Landis' percolation model of expansion assumes that interstellar colonisation is possible, but (regardless of how advanced you are) it is sufficiently difficult and costly that some advanced civilisation don't do it. Also, that it's only feasible to directly colonise a limited number of nearby star systems, and only if those star systems are not already occupied by an advanced civilisation. This means that non-colonising advanced civilisations — including descendants of colonists, who choose not to colonise further — act a blockers to the civilisations which currently want to colonise.

    He doesn't assume these things in the sense of asserting they must be true, but in the sense of saying they are plausible enough to consider what the results are if they are true.

    If you'd like to know more about this model, here is a link to Landis' paper about it.

    Assuming our type of star system is valued by them. Also, that we would see them. Anyone check Titan for methane breathers?
    If there was a civilisation like ours on Titan, I think the Cassini probe would have detected the city lights...

    If so, quite a lucky coincidence for us to be in the same neighborhood.
    I don't necessarily expect the nearest intelligent aliens to be just a few light years away. I think they may well be much further away. I was simply saying that they could be that close without having been detected yet by us.

    If our neighbors even still use planets.
    True. They may prefer artificial habitats in space.
    Last edited by Colin Robinson; 2019-May-05 at 01:51 AM.

  21. #141
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Colin Robinson View Post
    An argument I've seen is that living things in general will expand into available habitats when they can.
    Yes, when and if, but an alien species may not have read that argument.

    Though definitions may vary on what is an "available habitat". Same with "when they can". They may count the cost as too high with too little return. Especially if enough efforts fail... and some will.

    Landis' percolation model of expansion assumes that interstellar colonisation is possible, but (regardless of how advanced you are) it is sufficiently difficult and costly that some advanced civilisation don't do it. Also, that it's only feasible to directly colonise a limited number of nearby star systems, and only if those star systems are not already occupied by an advanced civilisation. This means that non-colonising advanced civilisations — including descendants of colonists, who choose not to colonise further — act a blockers to the civilisations which currently want to colonise.

    He doesn't assume these things in the sense of asserting they must be true, but in the sense of saying they are plausible enough consider what the results are if they are true.

    If you'd like to know more about this model, here is a link to Landis' paper about it.
    "Blockers" implies barriers that open space simply does not have. Bypass any non-colonizing civilizations, go further out, and settle where they aren't. It's orders of magnitude more difficult than reaching nearby stars, but possible if a Sufficiently Advanced polity dedicates enough effort and energy.

    To me, "some might not do it" is not a very convincing proposition. As kzb said, it only takes one to buck the trend.

    If there was a civilisation like ours on Titan, I think the Cassini probe would have detected the city lights...
    If they have eyes.

    I don't necessarily expect the nearest intelligent aliens to be just a few light years away. I think they may well be much further away. I was simply saying that they could be that close without having been detected yet by us.
    True.

    They may prefer artificial habitats in space.
    Oort Cloud habs could be too far away for us to even detect their heat even if they use liquid-water biology.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  22. #142
    Join Date
    Aug 2010
    Posts
    2,235
    Quote Originally Posted by Noclevername View Post
    Yes, when and if, but an alien species may not have read that argument.

    Though definitions may vary on what is an "available habitat". Same with "when they can". They may count the cost as too high with too little return. Especially if enough efforts fail... and some will.
    True.

    "Blockers" implies barriers that open space simply does not have. Bypass any non-colonizing civilizations, go further out, and settle where they aren't. It's orders of magnitude more difficult than reaching nearby stars, but possible if a Sufficiently Advanced polity dedicates enough effort and energy.
    Reaching even nearby stars is itself orders of magnitude more difficult than anything we humans have done up till now. Do we know even that would be feasible?

    As for bypassing nearby stars to colonise more distant ones... even if it's possible for a sufficiently advanced civilisation, how long does it take to become sufficiently advanced, and how many advanced civilisations never reach that stage.

    Isn't it possible that there are several advanced civilisations in our galaxy but none sufficiently advanced to colonise systems which are not nearby?

    To me, "some might not do it" is not a very convincing proposition.
    To me, "some might not do it" is not a very convincing summary of Landis' argument.

  23. #143
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Colin Robinson View Post
    Reaching even nearby stars is itself orders of magnitude more difficult than anything we humans have done up till now. Do we know even that would be feasible?
    According to some of the most brilliant scientists and engineers our species has yet produced, yes.

    As for bypassing nearby stars to colonise more distant ones... even if it's possible for a sufficiently advanced civilisation, how long does it take to become sufficiently advanced, and how many advanced civilisations never reach that stage.

    Isn't it possible that there are several advanced civilisations in our galaxy but none sufficiently advanced to colonise systems which are not nearby?
    Very true.

    To me, "some might not do it" is not a very convincing summary of Landis' argument.
    I may be misinterpreting this:
    Landis' percolation model of expansion assumes that interstellar colonisation is possible, but (regardless of how advanced you are) it is sufficiently difficult and costly that some advanced civilisation don't do it.
    To my understanding, that sounds very similar to my summary.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  24. #144
    Join Date
    Aug 2010
    Posts
    2,235
    Quote Originally Posted by Noclevername View Post
    According to some of the most brilliant scientists and engineers our species has yet produced, yes.
    Even the most brilliant aren't necessarily right about everything... In relation to interstellar colonisation, even people like Fermi and Stephen Hawking are wannabes. Its feasibility won't be conclusively shown until we either actually do it, or detect another species that has.


    As for bypassing nearby stars to colonise more distant ones... even if it's possible for a sufficiently advanced civilisation, how long does it take to become sufficiently advanced, and how many advanced civilisations never reach that stage.

    Isn't it possible that there are several advanced civilisations in our galaxy but none sufficiently advanced to colonise systems which are not nearby?
    Very true.

    To me, "some might not do it" is not a very convincing summary of Landis' argument.
    I may be misinterpreting this:

    Landis' percolation model of expansion assumes that interstellar colonisation is possible, but (regardless of how advanced you are) sufficiently difficult and costly that some advanced civilisation don't do it
    To my understanding, that sounds very similar to my summary.
    OK, but you're treating part of an argument as if it were the whole deal. I went on to say:

    Also, that it's only feasible to directly colonise a limited number of nearby star systems, and only if those star systems are not already occupied by an advanced civilisation. This means that non-colonising advanced civilisations — including descendants of colonists, who choose not to colonise further — act a blockers to the civilisations which currently want to colonise.
    In other words...

    The hypothesis that some may not colonise, does not by itself explain why our own neck of the woods seems to be un-colonised.

    But we need to consider Landis' other hypotheses

    a. that advanced civilisations throughout the galaxy might find it unfeasible to directly colonise star systems that are not nearby — which you've agreed is possible.
    b. that they might also find it unfeasible to colonise a system which there is already an advanced civilisation.
    c. that a colony which survives and grows will effectively be a new advanced civilisation, which may not share the culture and the goals of its parent civilisation.

    In that case the outcome depends on what proportion of advanced civilisations (including colonies) decide not to colonise, or not to colonise further... A high proportion of non-colonising advanced civilisations may either choke off the colonising process completely, or else may result in a patchy colonisation process, in which some regions of the galaxy do get colonised but others don't.
    Last edited by Colin Robinson; 2019-May-05 at 05:55 AM.

  25. #145
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    Quote Originally Posted by Colin Robinson View Post
    Even the most brilliant aren't necessarily right about everything... In relation to interstellar colonisation, even people like Fermi and Stephen Hawking are wannabes. Its feasibility won't be conclusively shown until we either actually do it, or detect another species that has.




    OK, but you're treating part of an argument as if it were the whole deal. I went on to say:



    In other words...

    The hypothesis that some may not colonise, does not by itself explain why our own neck of the woods seems to be un-colonised.

    But we need to consider Landis' other hypotheses

    a. that advanced civilisations throughout the galaxy might find it unfeasible to directly colonise star systems that are not nearby — which you've agreed is possible.
    b. that they might also find it unfeasible to colonise a system which there is already an advanced civilisation.
    c. that a colony which survives and grows will effectively be a new advanced civilisation, which may not share the culture and the goals of its parent civilisation.

    In that case the outcome depends on what proportion of advanced civilisations (including colonies) decide not to colonise, or not to colonise further... A high proportion of non-colonising advanced civilisations may either choke off the colonising process completely, or else may result in a patchy colonisation process, in which some regions of the galaxy do get colonised but others don't.
    That part, I agree with.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  26. #146
    Join Date
    Sep 2004
    Posts
    3,199
    Well! In the midst of our discussion on how many habitable worlds a G-type star can have, this paper appears POOF!

    https://arxiv.org/abs/1905.01032

    Accounting for Multiplicity in Calculating Eta Earth

    Jon K. Zink, Bradley M. S. Hansen (Submitted on 3 May 2019)

    Using the updated exoplanet population parameters of our previous study, which includes the planetary radius updates from Gaia DR2 and an inferred multiplicity distribution, we provide a revised η ⊕ calculation. This is achieved by sampling planets from our derived population model and determining which planets meet our criterion for habitability. To ensure robust results, we provide probabilities calculated over a range of upper radius limits. Our most optimistic criterion for habitability provides an η ⊕ value of 0.34 ± 0.02 planets/star. We also consider the effects of multiplicity and the number of habitable planets each system may contain. Our calculation indicates that 6.4 ± 0.5% of GK dwarfs have more than one planet within their habitable zone. This optimistic habitability criterion also suggests that 0.036 ± 0.009% of solar-like stars will harbor 5 or more habitable planets. These tightly packed highly habitable system should be extremely rare, but still possible. Even with our most pessimistic criterion we still expect that 1.8 ± 0.2% of solar-like stars harbor more than one habitable planet.

  27. #147
    Join Date
    Sep 2004
    Posts
    3,199
    Footnote to previous post: η ⊕ (called Eta Earth) is the expected occurrence of Earth-sized planets within a star's habitable zone.

  28. #148
    Join Date
    Apr 2005
    Posts
    2,466
    Quote Originally Posted by Roger E. Moore View Post
    Well! In the midst of our discussion on how many habitable worlds a G-type star can have, this paper appears POOF!

    https://arxiv.org/abs/1905.01032

    Accounting for Multiplicity in Calculating Eta Earth

    Jon K. Zink, Bradley M. S. Hansen (Submitted on 3 May 2019)

    Using the updated exoplanet population parameters of our previous study, which includes the planetary radius updates from Gaia DR2 and an inferred multiplicity distribution, we provide a revised η ⊕ calculation. This is achieved by sampling planets from our derived population model and determining which planets meet our criterion for habitability. To ensure robust results, we provide probabilities calculated over a range of upper radius limits. Our most optimistic criterion for habitability provides an η ⊕ value of 0.34 ± 0.02 planets/star. We also consider the effects of multiplicity and the number of habitable planets each system may contain. Our calculation indicates that 6.4 ± 0.5% of GK dwarfs have more than one planet within their habitable zone. This optimistic habitability criterion also suggests that 0.036 ± 0.009% of solar-like stars will harbor 5 or more habitable planets. These tightly packed highly habitable system should be extremely rare, but still possible. Even with our most pessimistic criterion we still expect that 1.8 ± 0.2% of solar-like stars harbor more than one habitable planet.
    The fact is many publications give quite high occurrences of Earth like planets in habitable zones, and this is another one.

    The reference I was on about was this one:

    Kepler exoplanets: a new method of population analysis

    Wesley A. Traub 2016

    https://arxiv.org/abs/1605.02255

    the number of planets per G star, per natural log unit of period (days) and
    radii (Earths) at the period and radius of the Earth around the Sun, is about ��(G) = 1.1 ± 0.1.
    A related parameter, η, which in addition depends on the range of period and radius considered,
    is found to be η(G) ≃ 1.0 ± 0.1.

    This is where I recalled the average number of Earths for G-stars from, maybe I misinterpreted it?
    Last edited by kzb; 2019-May-07 at 05:17 PM.

  29. #149
    Join Date
    Sep 2004
    Posts
    3,199
    Quote Originally Posted by kzb View Post
    This is where I recalled the average number of Earths for G-stars from, maybe I misinterpreted it?
    No, what you said looks correct. Beats me, then, why some have small values and some very large values for the number of Earths per star.

  30. #150
    Join Date
    Apr 2007
    Location
    Nowhere (middle)
    Posts
    36,941
    There's always the classic FP explanation "Ice moons with life are common, surface-planet life is rare."

    Maybe a species with a roof would never develop space travel.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

Posting Permissions

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