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Speculations on Habitable Zone Waterworlds

What to make of Fergus Simpson’s new paper on waterworlds, suggesting that most habitable zone planets are of this type? If such worlds are common, we may find that most planets in the habitable zones of their stars are capable of evolving life, but unlikely to host technological civilizations. An explanation for the so-called ‘Fermi Paradox’? Possibly, but there are all kinds of things that could account for our inability to see other civilizations, most of them covered by Stephen Webb in his If the Universe Is Teeming with Aliens … Where Is Everybody? (2nd ed., Springer 2015), which offers 75 solutions to the problem.

Simpson (University of Barcelona) makes his case in the pages of Monthly Notices of the Royal Astronomical Society, arguing that the balance maintained by a planetary surface with large amounts of both land and water is delicate. The author’s Bayesian statistical analysis suggests that most planets are dominated either by water or land, most likely water. Earth may, then, be something of an outlier, with most planets over 90 percent covered in water.

waterworld_earth

Image: Continents on other habitable worlds may struggle to break above sea level, like much of Europe in this illustration, representing Earth with an estimated 80% ocean coverage. © Antartis / Depositphotos.com.

The unlikelihood of a planet maintaining a balance like the Earth’s gives the paper its direction. Simpson offers a pedagogical example of the Bayesian approach:

…imagine that you look at a kitchen worktop and notice some spilled coffee granules. One of those granules, selected at random, is found to lie within 0.1 mm from the edge of the 600 mm worktop. This proximity could of course be entirely coincidental. But it is much more likely that the bulk of the granules fell on the floor, and what you are seeing is merely the tail end of the distribution.

Is planet Earth at the tail end of the distribution? We don’t have enough exoplanet data to know. But the statistical model here does take note of what we have learned in our explorations of the outer Solar System. Think of Titan, where Stanley Dermott and Carl Sagan predicted in 1995 that instead of a surface covered in extensive ocean, the lack of circularization of the moon’s orbit from oceanic tidal effects pointed to a largely dry surface. We now know that the duo was correct: Titan’s liquid hydrocarbons account for about 1 percent of the total surface area.

Simpson finds this a remarkable call, but he adds that the prediction could have been made even without orbital data on Titan. Let me quote him on this, as it reiterates his method:

On a purely statistical basis, and in the absence of correlations, one expects the division of liquid and solid surface areas to be highly asymmetric. This is because the volume of liquid need not match the capacity of perturbations in the solid. The two quantities often differ by several orders of magnitude. If it is the liquid that dominates, the solid surface becomes completely immersed. Enceladus and Europa offer exemplary cases of this phenomenon. Beneath each of their icy crusts, a single ocean completely envelops a solid core (Kivelson et al. 2000; Waite et al. 2009). If, on the other hand, the liquid’s volume is subdominant, it settles into small disconnected regions, as was found to be the case on the surface of Titan.

To put this in context, we need to consider the delivery of water from icy objects originating beyond the snow line. Here we have to assume a huge variety of outcomes, planets in the habitable zone with a wide variation in the amount of water delivered to the surface. What happens over time? Some water will be found in the mantle, some lost through the upper atmosphere. Planetary topography shapes the distribution of resulting oceans.

The author goes on to consider feedback mechanisms including erosion and soil deposition as well as the deep water cycle by way of placing what happened on Earth in context. Across the galaxy, trillions of dice have been rolled, comments Simpson, and what we would like to know is whether the dice somehow favor a balance between land and water. He thinks not.

Here the anthropic principle weighs in. Does our Earth have the particular balance of water and land we see because planets without this balance — desert worlds or waterworlds — would produce intelligent land-based species at a much lower rate? In other words, if we did not live in a place with this fine-tuned balance, we wouldn’t be observers, something we should keep in mind as we consider the prevalence of worlds like our own. We can also assume that planets with larger habitable areas are more capable of sustaining larger populations. These factors imply to Simpson that the Earth has a greater habitable area than most life-bearing worlds.

We see a planet that could not be otherwise for us to be here, and assume other habitable zone worlds are similar. Our planet, argues the author, is itself close to the waterworld limit, and the water mass fraction among habitable planets could be a good deal higher. From the paper:

…numerical simulations based on delivering water from planetary embryos found a median water mass fractions of approximately 1 per cent (Raymond et al. 2007), 10 times higher than the terrestrial value. Extremely elevated water compositions have been associated with the inflation of planetary radii (Thomas & Madhusudhan 2016). This scenario, in which the Earth is among the driest habitable planets, could help explain the appearance of a low-mass transition in the mass–radius relation of exoplanets (Rogers 2015; Chen & Kipping 2017).

And if it turns out that the Earth is unusually dry for a habitable zone planet, various mechanisms can explain the result. It is possible that low eccentricities and inclinations of planetary orbits in our Solar System make for less effective water delivery. Perhaps water delivery was affected by migration of the gas giants (Simpson considers the ‘Grand Tack’ model in which Jupiter reversed its migration, but points out that this might also produce a higher rate of water delivery). Or perhaps we have a drier Earth simply because of the stochastic nature of the delivery of water. The trillions of dice throws gave us the numbers we live with and take to be commonplace. We can hardly, then, take the Earth as the norm.

71 percent of Earth is covered by water. In addition to the amount of water delivered (and the mechanisms of that delivery), we have to factor in where it will be stored, how ocean and mantle interact, and how topographical features affect water distribution. Life has been able to emerge and technological civilization take hold because all these factors prevent the planet from becoming a waterworld, a fine balance, says Simpson, that most habitable zone worlds lack.

Simpson’s paper is an intriguing read and one that should provide fodder for science fiction scenarios (I consider that all to the good), but it’s highly speculative given that our models of water delivery in the early Solar System are still works in progress. What we come back to again and again is the need for observation. In the Titan reference Simpson makes early in the paper, it was data that proved the point made by Dermott and Sagan, and data that are required to put such factors as the depth of Earth’s water basins in a cosmic context. So we can keep waterworld theories in mind as we begin studying the actual atmospheric content of exoplanets within coming decades, when new space- and ground-based resources come online.

The paper is Simpson, “Bayesian evidence for the prevalence of waterworlds,” Monthly Notices of the Royal Astronomical Society 468 (3) (2017), 2803-2815 (full text). Thanks to Phil Tynan for an early pointer to this paper.

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{ 49 comments… add one }
  • Mike Scott April 26, 2017, 11:51

    I’m not sure how we can conclude that the Earth is unusually dry for a habitable zone planet, given that we have detailed information available for exactly two such planets, and the other one (Mars, of course) is a lot drier than the Earth.

  • DougF Dwyer April 26, 2017, 12:34

    Who do you know who has won a large lottery , in the same way life on Earth may be the only winner

  • Jim Early April 26, 2017, 13:42

    If you look at the distribution of surface radii for Mercury, Venus and Mars you have a maximum near the average radius and somewhat smooth distributions falling to lower values for high and low radii. By contrast the Earth has a bi-modal distribution with most of the surface near sea level for the land and near the deep ocean bottom radius for the oceans. I’m sure somebody has published a reason for this distribution.
    If the Earth had half the ocean volume, would the oceans be shallower or would there be more land area? Understanding the answer to that question may tell us what to expect on other planets.

    • Joe April 27, 2017, 15:33

      It seems to me that if you include continental shelves as submerged parts of continents, then if Earth had half the ocean volume, the continents would be much bigger, thus more overall land area.

  • Robert G April 26, 2017, 15:03

    Paul, thank you for a very interesting and well written explanation of a paper and its implications.

    I wonder how far the JWST, TESS and (eventually) PLATO telescopes will be able to take us towards answering this. Will we be able to eventually image planets well enough to measure their albedos as a function of time?

    Even if the planet appears as just a dot, we might still measure how bright the dot is and how that brightness varies. I would trust our current atmospheric models to do a good job telling us what the weather systems on a 100% water ocean covered planet would look like with various mass/orbital parameters. Then we would know one if we saw one, similar for the converse.

  • john walker April 26, 2017, 15:29

    The first image in Paul’s post is illustrative of the likely truth of Simpson’s simple analysis. This is not a new revelation. And anyone who has considered the relatively shallow depth of our oceans even relative to the calculated depths of much smaller bodies like Europa and Enceladus might have intuitively drawn the same conclusion. To call the Earth nearly dessicate seems superficially absurd but given that water represents less than one tenth of one percent of its mass it is vastly more accurate than it’s common label as a water world. Doubling that miniscule volume would however optically and biologically make it one even if the oceanic depths still weren’t comparable to that of Europa. And at 1G 0°C pure H2O will transition to Ice VI at around 63km depth capping the rock/liquid interface. The necessary water mass would increase the mass ratio by about a factor of 25 or from todays 0.023% to 0.575%. Lack of hard exoplanetary data prevents me from placing this value in perspective, but it seems casually like a fairly low number to achieve in order to create not only a terran sized ocean world but one on which life would have difficulty arising. Soft conclusion: life threatening ocean worlds and dry worlds will vastly outnumber truely habitable ones.

    • DJ Kaplan April 27, 2017, 12:24

      The “Anartis” image seems to suggest that the rise of ocean levels above a certain creative point would provocatively cover most of Europe, thus “no civilization”; I suppose the rest of us would muddle through somehow.

      It does add something to the Rare Earth argument…

  • hiro April 26, 2017, 16:40

    Does the planet gravity affect the brain size of intelligent species? Super-waterworlds that have 3-5 Me might allow some kind of quasi-octopi exist, these creatures would have different brain size, life span etc…..

    • Joe April 27, 2017, 15:41

      Yes, but could intelligent quasi-octopi build a technological civilization on a water world? Might be difficult to discover fire for example.

      • Robert April 29, 2017, 12:28

        Does intelligence necessarily mean technology? Humans were probably fully modern as far as intelligence eons before they invented technology, including fire. Some consider Dolphins intelligent. Could such a species develop sophisticated communication and culture without technology as we know it? Just some questions to ponder.

      • hiro April 29, 2017, 19:44

        It’s much easier for them to develop DNA computing, biotech, fluid dynamics …. It’s very hard to speculate where these fields would head to but manipulating genetics to increase the intelligence (& life span) in some certain way is positive for the species in general. Probably these types of creature could be interstellar big pharmas, XD.

        Some people speculated the possibility of the existence of liquid quantum computer, it seems this type of idea still belongs in the domain of SF.

        • hiro April 29, 2017, 19:57

          On the other hand, if one of them discovered the formula of opiroid, then oh well….

      • Cat Zega May 1, 2017, 19:20

        We’re ‘Squanderers’! I have to share this little passage on the subject:

        ‘The Sceuri took great pride in having become a technological, space-faring species, given the obstacles they’d had to overcome. A classic water-world environment had almost no easily available metals. Any metal-bearing ores that a waterworld possessed tended to be locked away under all that ice, deep in the planet’s inaccessible rocky core. Waterworlders had to do what they could with what fell from the sky in the shape of meteorites. To get into space in the face of such a paucity of readily available raw materials was not easy, and the Sceuri regarded themselves as deserving considerable recognition and respect for such a triumph of intellect over scarcity. Accomplishing the same feat when you came from a rock-surface planet was a relatively trivial, expectable, even dismissible trick. The Sceuri called people from such planets ‘Squanderers’ as a result, though not usually to their face or other appropriate feature.”

        – Iain Banks, The Algebraist

    • Harry R Ray May 2, 2017, 9:17

      HMMMMMM! An interesting concept indeed! Abbotts and Costellos on LHS 1140b!!! A great idea for a sequel, too! “Arrival II”: They came here first. NOW, we are going THERE.

      • ljk May 2, 2017, 10:23

        They did say they were going to need our help in three thousand years, which presumably means we will have interstellar travel long before then.

  • Brett April 26, 2017, 16:42

    The water-mantle connection seems like a big factor here. How does higher levels of surface water affect plate tectonics, which seem to require water to function properly? Did Earth have more surface water, which has been steadily consumed by the mantle?

    Then there’s the continents. From what I can tell, the continents were not there at the beginning – Earth was totally covered in water aside from volcanic islands. Do worlds with higher levels of surface water simply have continental masses build up higher and higher to the point where the continental rock simply can’t maintain its shape under gravity?

    We do have examples of other dry rocky planets in the solar system. Venus doesn’t seem to have had as much water as Earth even before the greenhouse runaway, and the same goes for Mars before it lost most of its atmosphere.

  • Anton M. April 26, 2017, 16:49

    1. There is strong evidence of a large ocean at ancient Mars. If ocean/terrestrial planets are unlikely, finding two such planets in single system is even less likely.

    2. There is a feedback geological mechanism regulating water distribution between oceans and magma, namely olivine/serpentine cycle. For the planets with plane tectonics it might reduce the likely range of ocean coverage greatly.

    • EricSECT April 27, 2017, 6:55

      Anton: I was thinking the very same thing about ancient Mars and the evidence that it also had large amounts (but NOT a water-world’s worth) of water, possibly about half land and half ocean (at least as I’ve seen in renderings, anyways). Perhaps that points to a unique water delivery mechanism here in our early Solar system vs. others (as Paul mentioned in the article). AFAIK, there is only weak evidence for possible (aborted) tectonic plate activity on Mars (Valles Marineris).

      Either way, this is a unique and interesting resolution to the Fermi Paradox. The galaxy may be rotten with life, and even with intelligent life (dolphins, squid), but technologically advanced intelligent life is rare, as it needs needs dry land to make fire to smelt ores, etc. to get the technology ball rolling.

      • Anton M. April 27, 2017, 18:56

        Eric,

        It looks like a universe where divers are in short supply!
        Anyway, you and I know that Fermi’s paradox should work with tenacity of physical law to explain “where is everyone?” question. Life on Earth was content with happily swimming in the ocean but it took it just a few hundred million years from first vertebrates on land to them building a rocket, so all waterworlds should very diligently flood every potential island or mini-continent to keep all the aliens at bay.

        My two personal takes on Fermi’s are (1) the first replicator might be too rare to occur twice in the visible Universe and (2) we might have a serious scale issue searching for EIs, like the entire Universe is artificial and we are too stupid to understand it.

  • Geoffrey Hillend April 26, 2017, 16:56

    I don’t think our choices are just a roll of the dice of trillions especially if we consider the type of world necessary for intelligent extra terrestrial life. With science we should be able to eliminate many planetary systems just based on the size of their star, and other contingencies like a magnetic field which might not be possible without a large Moon like ours which caused Earth to have a fast rotation, the necessary angular momentum provided by from a collision with Theia.
    It is good if Earth is not the norm because we can use the Earth-Moon system as an absolute. If we use our Earth-Moon as an example, then how many stars in our galaxy might have an Earth sized planet in the life belt with a Moon like ours. There still might be a lot but not compared to Earth sized planets it the life belt of all stars like red dwarf stars. Due to the hill radius planets in the life belt around red dwarfs can’t have Moons which illimintes them. How many planetary systems have a G or K star, are Earth sized but don’t have a Moon. Could such a world still have plate tectonics with an ocean without a large Iron core given by a collision like Theia? It might not so it not sustain plate tectonics and moderate it’s atmosphere with a carbon cycle and it would become a either a dry world over long time or a frozen world which is only a speculation or hypothesis.
    If an Earth like world without a Moon still can have plate tectonics, then there is the problem of a lack of a magnetic field and the wide shifts of it’s axis of rotation like Mars which results in a great change of the weather and climate over only 50,0000 years. These might make it harder to life and intelligent life to evolve including more cosmic radiation from a lack of a magnetic field.

  • Geoffrey Hillend April 26, 2017, 17:08

    Another unknown is that can a Earth sized planet in the life belt have a magnetic field? If it is dense, it certainly might have a large iron core but what about it’s rotation speed which has to be fast in order to generate a magnetic field? It might need a Moon from a Theia like collision to give it angular momentum. There might be a lot more contingencies when considering whether or not a planet can intelligent ET life.

  • Alexander Tolley April 26, 2017, 17:11

    Another thing to consider is the abundance of life on water worlds. Unless the origin of life started at the deep ocean vents, landless water worlds may have no life. If they do have life, it may not be very abundant as the Earth’s oceanic regions are virtual deserts compared to the shallow waters on the continental shelves.

    The author makes an interesting case that we might have a bimodal distribution of planets with some land vs those with none.

    As always, we really need empirical data, rather than mathematical models that are based on relatively little knowledge of planetary formation and evolution.

  • Abelard Lindsey April 26, 2017, 18:12

    Parhaps our Earth is relatively dry because the giant impact that created our moon sent a lot of that water out into space where it never returned to the Earth. The primordial Earth could have been a waterworld.

  • Enzo April 26, 2017, 18:36

    Was there a discussion similar to this in David Brin’s Earth ?
    I vaguely remember some discussion about how likely alien earths were to have more water than our own.

    Also, higher gravity earths => harder to support higher terrains => more likely to be underwater.

  • jonW April 26, 2017, 21:02

    The article does seem very (depressingly) plausible, though it is strangely over-emphatic on the anthropic principle, as if this were a new form of argument or one that people might not understand. Actually its conclusions seem rather obvious in hindsight, though I had never thought about it before. Just another kill-joy factor to (implicitly) incorporate into the Drake equation… let’s hope the number of rocky planets in the habitable zone is big enough that the few percent of them that have a useable land/water ratio nonetheless add up to a large number. (Or, let’s hope that there are more ways for water dwellers to attain technological civilisation than we’ve currently been able to imagine.)

  • Ron S April 26, 2017, 23:37

    The paper is interesting. How accurate it is I can’t say but I like the kind of questions it asks and delves into.

    As Brett’s comment above hints at there is a problem in the paper. The saturation factor (S) is not fixed. The evidence from geology is that S varies quite a lot due to tectonic activity, where periods of low mountain making activity and the normal course of erosion tend to flatten the Earth so that the 2.51 height ratio in the paper is much lower, and therefore the habitable area has at times been much smaller.

    Another important driver of S is climate. Right now land area is relatively low due to the end of the most recent ice age. With much water locked up in ice caps if the climate keeps warming the amount of land area will further decline to values that their model would frown upon. Not that glaciation is all that great since much of the larger land area is barren.

    Habitability and fecundity in their model is therefore not so straight-forward. A snapshot of S in the current era misleads since S varies greatly even as the amount of water is nearly static.

  • Tony April 26, 2017, 23:48

    Our continental crust has grown while our oceanic crust has thinned over geological time. This lead to a constant deepening of the ocean basins and a constant increase in the area covered by continental crust. At some point we had a sudden transition from almost entirely water to large blocks of land. As such, it is not just a one-shot dice-roll that must be considered, but evolutionary trajectories.

  • David Ivory April 27, 2017, 1:46

    I recall a paper discussed sometime ago here on Centauri Dreams where it was posited that ocean worlds in the habitable zone would be subject to a runaway greenhouse effect due to the high water vapour content of the atmosphere. The water in the super hot atmosphere would be disassociated into hydrogen and oxygen at high altitude and so water is lost to space. Eventually the ocean world would dry out revealing continental rock which would start the weathering process and so reduce the greenhouse effect. I can’t recall the mechanism this required though.

    If this process is possible then the creation of a habitable planet like Earth is more likely – different initial conditions would evolve into a stable equilibrium that appears the same. This so called edge-condition is thus more like a self organising system than mere chance. It was a variation of the Gaia hypothesis.

    The anthropic principle is compelling but we should always question any call to our specialness and instead assume that we’re nothing unusual. We should search for mechanisms that might explain why we’re not unique instead of marvelling at winning the jackpot. Surely that is better science.

    • Harry R Ray May 1, 2017, 17:49

      Not necessarily. Case in point: The recent “climate model” of the TRAPPIST-1 planets, stating that planets f, g, and h would be “iced over” instead of being “runaway greenhouse” worlds.

    • hiro May 2, 2017, 16:40

      If Venus were in Mars’ orbit, it would no longer be “hot Venus” wouldn’t it?

  • ole burde April 27, 2017, 13:07

    If waterworlds are common in the habitable zone , it would not prevent either the existence of native life or the establishment of a thriving human settlement.
    Almost every raw material necesarry for the functioning of a (floating!) industrial civilisation can be found dissolved in the water , which on earth makes i possible for many aquatic lifeforms to extract most necessary materials directly from the seawater .
    40 years ago I remeber someone tryng to extract gold from the seawater , it worked but was too expensive …but on a waterworld , seawater -extraction would only be the the first baby-step on the way to more serious methods …

  • Adam April 27, 2017, 16:06

    The “Waterworld Self-Arrest” concept – the idea that ocean is lost until some land is exposed – is here:

    Indication of insensitivity of planetary weathering behavior and habitable zone to surface land fraction

    Dorian S. Abbot, Nicolas B. Cowan, Fred J. Ciesla
    (Submitted on 8 Aug 2012)

    Water content helps determine surface land fraction, which influences planetary weathering behavior. [..] This process is named a “waterworld self-arrest,” and it implies that waterworlds can go through a moist greenhouse stage and end up as planets like Earth with partial ocean coverage.

    • ole burde April 28, 2017, 1:23

      What this paper doesnt relate to , is the time frame of the oxygenation processes which are probably needed to start up the tectonic plate movements , without which there wil probably not be any high-rising continents… The time needed could be very long for a planet without life , it might take much, much longer than on earth, where life produced most of the O2 necessary…so , it may be a very old waterplanet we are looking for

  • ljk May 2, 2017, 8:50

    If we can understand dolphins using AI, a similar method for ETI is probably the smartest way to go – especially if we are surrounded by watery exoworlds:

    https://futurism.com/the-language-of-dolphins-could-be-translated-by-2021/

    To quote:

    “We hope to be able to understand dolphins with the help of artificial intelligence technology,” KTH adjunct professor and Gavagai co-founder Jussi Karlgren said in an interview with Bloomberg. “We know that dolphins have a complex communication system, but we don’t know what they are talking about yet.” Whether Gavagai can convince other researchers in marine biology remains to be seen.

    My comment: Why does he have to convince marine biologists about this? What harm is it going to cause if he tries to understand dolphins via AI? Disciplines need to stop staying inside their little departments and start collaborating if they ever want to make real progress in science. Astrobiology and its other related fields cannot work without input from experts in multiple fields. This is the 21st Century, science community. Start acting like it.

    The other quote:

    The idea of preparing to communicate with alien forms also comes to mind. One of the problems with finding extraterrestrial life, as exciting as the prospect is, is that we will need to find ways to communicate. Since Gavagai’s systems have already mastered human languages, moving on to other life forms seems like a smart next step if we someday hope to use AI and machine learning to communicate with who- and whatever we meet as we explore further through our Universe.

    The traditional SETI view is that aliens will use mathematics as a literally universal language to communicate across the stars with other species. Well, it does make sense for us, but truly alien minds may not feel obligated to follow our brand of logic. They would be alien, after all, not the Star Trek brand of alien which is basically us with funny noses and ears.

    • hiro May 2, 2017, 16:38

      About the last paragraph, if ETs’ math & physics & mechanics are based in the quaternion structures/systems, then the machines (hardware) & software are also built & created around the codes in quaternions too, they might not be compatible with machines over here.

      • ljk May 8, 2017, 9:38

        Hiro, would you please explain what a quaternion is? Thank you.

  • ljk May 2, 2017, 9:51

    https://arxiv.org/abs/1702.01137

    False negatives for remote life detection on ocean-bearing planets: Lessons from the early Earth

    C.T. Reinhard, S.L. Olson, E.W. Schwieterman, T.W. Lyons

    (Submitted on 3 Feb 2017)

    Ocean-atmosphere chemistry on Earth has undergone dramatic evolutionary changes through its long history, with potentially significant ramifications for the emergence and long-term stability of atmospheric biosignatures. Though a great deal of work has centered on refining our understanding of false positives for remote life detection, much less attention has been paid to the possibility of false negatives, that is, cryptical biospheres that are widespread and active on a planet’s surface but are ultimately undetectable or difficult to detect in the composition of a planet’s atmosphere.

    Here, we summarize recent developments from geochemical proxy records and Earth system models that provide insight into the long-term evolution of the most readily detectable potential biosignature gases on Earth – oxygen (O2), ozone (O3), and methane (CH4).

    We suggest that the canonical O2-CH4 disequilibrium biosignature would perhaps have been challenging to detect remotely during Earth’s ~4.5 billion year history and that in general atmospheric O2/O3 levels have been a poor proxy for the presence of Earth’s biosphere for all but the last ~500 million years.

    We further suggest that detecting atmospheric CH4 would have been problematic for most of the last ~2.5 billion years of Earth’s history. More broadly, we stress that internal oceanic recycling of biosignature gases will often render surface biospheres on ocean-bearing silicate worlds cryptic, with the implication that the planets most conducive to the development and maintenance of a pervasive biosphere will often be challenging to characterize via conventional atmospheric biosignatures.

    Comments: Accepted to Astrobiology, 17 pages, 4 figures

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:1702.01137 [astro-ph.EP]

    (or arXiv:1702.01137v1 [astro-ph.EP] for this version)

    Submission history

    From: Christopher Reinhard [view email]

    [v1] Fri, 3 Feb 2017 19:35:24 GMT (1059kb)

  • john lodge May 5, 2017, 6:44

    Species on planet Earth adapt to their environment. In desert regions where there is a little water, you can find a variety of life-forms from plants to lizards, birds and small mammals. These species thrive in arid regions and are able to live quite well. In the oceans again there is a myriad of species that cope in various environments ie oceanic hot vents, very cold and pressurised levels up to and over three miles underwater, bacteria can be found in high radiation areas like Nuclear reactors etc. Lots of water or little means nothing life will develop a strategy to allow it to thrive. Rocky planets in a habitable zone are probably quite common and the galaxy teeming with life of one sort or another. Intelligence itself is an adaption, higher life forms have more of a non-specific body shape that allows it to adapt to several different environments. Humans have two arms and two legs and a head that allows easy access for making tools and movement but the body is not really a specific adaption and can cope with differing levels of need. There is no reason why alien life-forms have to be a specific shape but equally, there is no reason they could not be Bipedal Humanoid either it just depends on evolution and environment. Look at Earth pre-history and how the planet has changed. Look at the various life-forms that have existed over 4 billion years and tell me the galaxy is not capable of reproducing the same kind of thing as on Earth. Life is probably as common as shit. Enough said.

  • Douglas F Dwyer May 5, 2017, 9:58

    Think how events occur.
    Small high probability bang spreading out to dispersing contamination. If underwater if the probability path it will be found. We will evolve it may take 100 millions of years to evolveBiros if necessary though I like chromophores as an underwater means of communications

  • David Ivory May 5, 2017, 22:19

    I like chromophores as an underwater means of communications.

    Chromophores could be a much higher bandwidth communication method than vocal communication. Octopods may think our method of talking is primitive. Elon Musk talks of increasing output bandwidth with a jack-plug to the brain – octopods have a video screen for skin.

    There might be many routes to technology that do not required fire. We are just now starting in on using biology to build novel things and trying to work towards practical nanotech. It might be that a aqueous medium is the best for moving in this direction. The problem will be that we may not recognise technology when it is grown from seeds.

    For instance in the movie Avatar the so-called primitives might in fact be the high-tech culture that has created their nirvana. Their environment is pervasively symbiotic and surprisingly useful (Cameron should run with this idea in the sequels). An aquatic culture thus may be the better place for the emergence of advanced technology.

    But could they get off planet?

    • ljk May 8, 2017, 9:54

      This is why the Na’vi rejected the humans’ efforts to give them education, roads, and medicine. They didn’t need them, to say nothing of the fact that they undoubtedly suspected their real motives.

      Of course in Avatar we never really got to know the other native tribes on Pandora, although one gets the feeling from the glimpses we did have of them as our hero rallied the troops that they all seemed to be painted with the same broad brush. Which is not the case with human “tribes” on Earth, but science fiction has this old hoary habit of painting alien species as being generally all the same in order to make a point.

      Would the Na’vi ever leave Pandora? Do they need to? I suppose if Eywa (their version of Gaia only really ramped up as a giant planetary “brain”) felt the need to spread itself to nearby worlds they might.

      Note the other moons circling that gas giant world named Prometheus shown briefly throughout Avatar. We are supposed to see them in detail in the sequels. That would be the most logical place for the Na’vi and/others of their species to go first if they ever achieve space travel. If nothing else the humans certainly made them aware of the concept of space travel and alien life. That awareness includes Eywa.

      http://james-camerons-avatar.wikia.com/wiki/Eywa

    • ljk May 8, 2017, 10:16

      That is the big question: If you live in a world that you are in balance with and provides all your needs, would you ever want or need to leave it? This assumes in part that your environment remains stable for many generations, which is certainly not the case on Earth.

      Did the last Ice Age “motivate” humanity to develop tools and other living strategies to survive? Are the natives of Pandora, who were happy and content – or so we are led to believe even though they have a dominant warrior culture (meaning they had martial conflicts) and a very brief and unexplained mention of a time of great sorrow – until the invading humans came along now going to advance culturally and technologically because they were kicked out of paradise? Or have they been done a disservice because look at what technology did to those humans, or so the story goes? Do species who become “smart” all go through this period of growing pains? Do any of them survive it? That is one question SETI is supposed to answer.

      Although it is not without its flaws, check out the original Star Trek episode from 1967 titled “The Apple”. Kirk and company come upon a world where the humanoid inhabitants lead long, peaceful, and protected lives, thanks to their “god” Vaal, a big alien computer. The main Enterprise officers have an interesting debate over whether the natives being so sheltered are truly happy or leading lives of pointless stagnation at the expense of safety, comfort, and blissful ignorance.

      http://memory-alpha.wikia.com/wiki/The_Apple_%28episode%29

  • Douglas Dwyer May 6, 2017, 10:50

    Chromophores evolved water powered flight evolved all with a short life cycle, advantageous? SPACE FARING OCTOPUS?

  • Douglas Dwyer May 6, 2017, 15:19

    Remember evolution does not know where its going . It all depends on chance luck probability and hunger. Our spacefaring octopus must be parasitic on Man and have something we need like the ability to see in the future .

  • Sean Robert Meaney May 7, 2017, 20:50

    Unless they get to the point where they live on an iceberg like penguins and hunt and eat. But water worlds that dont create an accessible nutrient system are unlikely to have lifeforms.

  • Athena Andreadis May 8, 2017, 21:59

    “Anthropic selection”? Just no, for reasons I covered in my review of Rare Earth.

    Cephalopods are in fact extremely intelligent — they play and use tools. The bottleneck in their case is that they die after reproduction, making cultural transmission of acquired knowledge impossible. Also, due to the specifics of their neuronal organization, they don’t have an integrative brain map. A bit more on their uniqueness and limitations, Note to Alien Watchers: Octopuses are Marvelous, but Still Terrestrial

  • Alexander Tolley May 9, 2017, 13:13

    The bottleneck in their case is that they die after reproduction, making cultural transmission of acquired knowledge impossible.

    That seems like a strange assertion to make.. Cultural knowledge is transmitted to living peers so that transmission can happen by overlapping generations. This would only apply is cephalopods were strictly socially isolated, and reproduction always happened at the same time, something that is evidently not true of all cephalopods. If there was a Lamarckian type transmission, e.g. by modifying the genome or epigenome, death of the parent would also be circumvented.

    Social cephalopods would most likely evolve to dominate if cultural knowledge had survival value and death of the parent at birth was not possible to avoid.

    If early human parents both died at birth, cultural transmission would be managed by other adults who had not yet given birth. It might even result in agreements that members of a social group give birth at different times or years, or not at all.

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