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The Prevalence of ‘Water Worlds’

The first time I ran into the term ‘water world,’ it had a seductive quality. After all, we think of habitable zones in terms of water on the surface, and a world with an overabundance of water suggested a kind of celestial Polynesia, archipelagos surrounded by a planet-circling, azure sea.

But we immediately run into problems when we think about planets with substantially more water than Earth. For one thing, we may have no land at all. Let’s leave aside the icy moons of our Solar System that may well contain oceans beneath their surface and concentrate on exoplanets in the interesting size range of two to four times the size of Earth. We have to ask what would happen if a planet were completely covered with water, with no run-off of nutrients from exposed rock. Such an ocean could be starved of key elements like phosphorus.

Or how about a planet with a high-pressure zone of ice effectively cutting off the global ocean from the rocky mantle? A world with enough water — 50 times that of Earth has been considered — could create enough pressure on the seafloor to prevent geological activity, blocking the kind of carbon-silicate cycle that adjusts the atmospheric composition we find here on Earth. Cayman Unterborn (Arizona State) thinks liquid water on the 5th planet in the TRAPPIST-1 system could be as much as 200 kilometers deep, 20 times deeper than the Marianas Trench.

Image: A water world as envisioned in a photo-illustration by Christine Daniloff/MIT/ESO.

We don’t want to be too doctrinaire about this. For instance, Ramses Ramirez (Tokyo Institute of Technology) and Amit Levi (Harvard-Smithsonian Center for Astrophysics), have argued that there are ways to exchange greenhouse gases between deep sea ice and the atmosphere (citation below). In other words, we can have a carbon cycle without rock weathering, subject to a number of constraints like stellar type (hotter stars work best here) and rotation rate (thanks to Alex Tolley for his overview of this paper in private correspondence).

This is a work I hope to write about soon, but for now, let me quote from an essay by Shannon Hall on the topic that cites both Ramirez and Edwin Kite (University of Chicago), with a useful reminder from both on not being too quick to limit our thinking to Earth-centric models:

“What I’ve taken away from this project is the inadequacy of working from Earth’s analogy,” says Kite, admitting this conclusion is ironic given that he is a geologist by training. “I love rocks and Earth-history, but you really need to build up from basic physics and chemistry, rather than relying on Earth’s analogy in order to tackle exoplanet problems.” This consideration will be important when astronomers have to determine which individual worlds to further assess with large telescopes like the James Webb Space Telescope or when they have to choose between future missions that would survey hundreds of worlds and those that would study a handful of Earth clones in detail. But there is no consensus yet. “I think it could be dangerous just thinking about everything in an Earth-mindset,” Ramirez says. “You might be missing out on other possibilities.”

Exactly so, and when telescope time is precious, as it will continue to be for all our space-based resources in particular, target selection is critical. Meanwhile, I’ve run across the presentation that Li Zeng (Harvard University) and colleagues made at the recent Goldschmidt conference in Boston. The researchers point to data from both Kepler and the Gaia mission indicating that many known exoplanets may contain as much as 50 percent water. Here we can definitely toss the Earth-centric model out the window. Consider that Earth’s water content is 0.02% by weight. If these data are correct, huge numbers of exoplanets are entirely awash.

Zeng and team have been developing a model of the internal structure of two kinds of exoplanets: Those with a radius averaging about 1.5 times that of Earth, and those averaging 2.5 times Earth’s radius. According to their developing model, those with a radius 1.5 times the Earth’s are generally rocky planets, perhaps five times as massive as the Earth. Those with a radius 2.5 times that of Earth, massing about 10 Earth masses, are likely water worlds.

The model the researchers have developed tracks the changes in mass and radius when planets grow from a rocky core and later accrete either ices or hydrogen/helium gas, with the observed radius and mass-radius distribution reproduced in the model’s simulations. Many of the interesting planets in this range turn out to be water worlds.

“Our data indicate that about 35% of all known exoplanets which are bigger than Earth should be water-rich,” says Zeng. “These water worlds likely formed in similar ways to the giant planet cores (Jupiter, Saturn, Uranus, Neptune) which we find in our own solar system. The newly-launched TESS mission will find many more of them, with the help of ground-based spectroscopic follow-up. The next generation space telescope, the James Webb Space Telescope, will hopefully characterize the atmosphere of some of them. This is an exciting time for those interested in these remote worlds.”

On the larger planets, we can throw out my fanciful ‘Polynesia’ model. The researchers believe the surface temperature here would be in the range of 200 to 500 degrees Celsius. We would see an atmosphere dominated by water vapor, with a liquid water layer beneath, and high-pressure ices below. Our next generation of telescopes can put these ideas to the test.

“It’s amazing to think that the enigmatic intermediate-size exoplanets could be water worlds with vast amounts of water,” says MIT planet-hunter Sara Seager. “Hopefully atmosphere observations in the future–of thick steam atmospheres—can support or refute the new findings.”

The abstract for the Zeng et al. presentation at the Goldschmidt Conference, “Growth Model Interpretation of Planet Size Distribution,” is here. The Unterborn paper is “Inward migration of the TRAPPIST-1 planets as inferred from their water-rich compositions,” Nature Astronomy 19 March 2018 (abstract). The Ramirez and Levi paper is “The Ice Cap Zone: A Unique Habitable Zone for Ocean Worlds,” Monthly Notices of the Royal Astronomical Society 477, 4 (2018), 4627-4640 (abstract / preprint).


Comments on this entry are closed.

  • Gary Wilson August 20, 2018, 14:18

    Isn’t it far too early to come to these types of conclusions? I realize they are developing a model to explain mass/radius relationships but do we know anything at all about the relative frequency of heavy metal cores such as our own? And how significantly does a metal core affect the mass/radius relationship? The water planet idea is very interesting but I don’t think we have enough data available to make even a general guess about what actually happens in planet formation. We have 8 planetary examples in our own solar system and none of them are truly water worlds. I realize we don’t have anything at about 10 earth masses to compare to here but I would like to see a lot more data to support this idea. Especially considering the idea that about 1/3 of the planets of approximately 10 earth masses (or 2.5 earth radii) are water worlds. Does this suggest that 2/3 are gas mini-giants or very large rocky planets or a mixture of both? I also come back to the question of where so much water would come from in the early formation of a solar system?

    • stefan August 21, 2018, 18:00

      Hydrogen is the most abundant element in the universe and in our galaxy. Oxygen is the third most abundant element. The presence of water is very likely to occur.

    • andy August 22, 2018, 14:56

      We have 8 planetary examples in our own solar system and none of them are truly water worlds.

      True, none of the 8 major planets in our solar system are water worlds but there are an awful lot of ice moons and icy dwarf planets out there.

  • John walker August 20, 2018, 14:35

    As regular readers may be aware I’ve gone on ad nauseum about the multiple astrobiological benefits of exoplanetary “near dessication” and the need to abandon the naive concept of the habitable “Super Earth” water world. Water to mass percentages over 1% will not be beneficial at Earth mass or greater. 25% or even 35% are beyond bad in that sense. Unfortunately, reactions in the popular press to Dr. Li’s neutral presentation in Boston were depressingly predictable. “Science discovers habitable worlds more common than previously thought” Truth be told his conclusions point in the opposite direction.

    Nevertheless, a good deal of his presentation concentrated on intermediate sized worlds between 1.5 and 2.5 Earth radii. Previous calculations had already drawn the conclusion that 1.6 radii was very possibly the cut off point for habitable rocky planets.

    This seems to support that position.

  • Alex Tolley August 20, 2018, 15:16

    Waterworlds with oceans deep enough to have ice isolating them from the crust or mantle are going to have a great deal of difficulty recycling needed trace elements. No hot smokers in their depths. Perhaps underwater volcanoes will need to fulfill that role. Can they generate enough heat to maintain an ice-free route to the ocean above?

    Even with a surface temperature of 200-500C, if there is ice at depth, it implies that there must be an equitable temperature somewhere in the water column. Can this be supplied with enough energy to maintain life of some sort? I doubt aerobic, multicellular life would be there, but anaerobes, even multicellular ones, might be possible if enough chemical energy was present for metabolism.

    If we detected CH4 in the atmosphere (without CO), would that be a good hint that such life exists somewhere in that vast ocean?

    • Nicky August 21, 2018, 5:18

      Oceanworld life is almost undetectable with future in-situ probe, let alone current technology.

      Volcanism can be inhibited at the depth of water-mantle boundary due to the high pressure of the ocean. The melting point of silicate rocks is strongly dependent on pressure. The melting temperature increases with increasing pressure. With a 200km thick ocean on a 5 Earth-mass planet (corresponding to radius of 1.55-1.60), melting can be fully suppressed even in the presence of plate tectonics, and it can also happen on a 3 Earth-mass planet (corresponding to radius of 1.35-1.40) with a 250km thick ocean. Even worse, the water mass fractions needed to produce such depths are less than 1 wt%.
      Can abiogenesis with our current understanding ever occur on these planets is questionable.
      See: dx.doi.org/10.1016/j.icarus.2016.05.009

      Life needs abundant nutrients, like phosphorous, potassium, calcium, magnesium, etc. These elements come from dissolutions of magmatic minerals. On Earth, continental rain and river runoff transports them to the coastal ocean where biological uptake takes place. In the absence of continental weathering, an oceanworld, the dissolution of ocean floor minerals is significantly slower. The ocean would quickly
      become nutrients depleted. This decrease would also reduce the biological productivity, which would cause the biosignature too low to be resolved even with future telescope and in situ probe. The biogisnature on Earth is mainly contributed from land life. In fact, if you look at the ocean productivity map, you would find the coastal areas are alway the highest.
      See hou.usra.edu/meetings/habitableworlds2017/pdf/4109.pdf

      • Alex Tolley August 21, 2018, 12:02

        If volcanoes cannot release gases at depth as the crust will not melt, then the usual argument for waterworlds being uninhabitable starts to fail, as the carbon cycle cannot even add CO2 to the atmosphere, let alone the issues of finding a carbon sink to maintain a reasonable temperature range.

        • Nicky August 22, 2018, 5:52

          No, suppressed volcanism only makes habitability even worse.

          Life needs a continuous source of nutrients. Geological activity is the only choice. Though hydrothermal vent is also possible, the direct emplacement and dissolution of fresh silicate rocks remain as the best way to provide sufficient nutrients. Lack of seafloor magmatism would greatly reduce the already weak biological productivity.

          In order to keep the water-mantle boundary ice-free at a large depth, seafloor volcanism is necessary because it increases ocean depth that can possibly maintain ice-free seafloor.

          If there is no mechanism that can balance atmospheric CO2 sink and surface CO2 degassing, the planet would soon directly enter either snowball or runaway state depending on the stellar flux (sweet spot exists but the probability is too small). Furthermore, especially in the case of G-dwarfs, rapid increasing stellar luminosity should be counterbalanced by the drawdown of CO2. For example, over 3000 ppm CO2 was needed to deglaciate back in 400-500 million years ago on Earth. If the CO2 regulation isn’t close to perfect, the surface temperature will be subject to stellar luminosity evolution, which renders the planet uninhabitable.

          • Alex Tolley August 22, 2018, 15:26

            No, suppressed volcanism only makes habitability even worse.

            I expressed myself badly. That is exactly what I meant.

    • Brett Bellmore August 21, 2018, 6:38

      ” if there is ice at depth, it implies that there must be an equitable temperature somewhere in the water column. ”

      Ah, not really: The ice is there because with enough pressure, water forms “ice” even at elevated temperatures. It’s really, really hot ice.

      Now, this does raise the question of whether or not life can originate at extremely high pressures and temperatures. Maybe, but it wouldn’t be our sort of life.

      • Alex Tolley August 21, 2018, 11:55

        Using this phase diagram for water, 2ookm of water column would equate to about 2 GPa, putting the ocean bottom in the Ice VI domain, with a melting point below 100C. Shallower depths would be liquid at 0C.

        As extremophile life on Earth is found at up to 122C, this does not seem to a priori rule out life.

        So my question on whether volcanoes can generate enough heat is not changed by Nicky’s point about melting points. The issue of recycling is indeed critical, but this phase diagram for silica suggests that the sea bottom of this hypothetical waterworld is nowhere near the point at which silica (and other crustal rocks?) will be below its melting point.

        If my understanding is correct, it seems to me that the higher temperatures of the ice/water at depth would mean that the HZ around a volcanic vent would be much farther away than that around a hydrothermal vent on Earth. This is a quantitative, but not a qualitative change.

  • Alex Tolley August 20, 2018, 16:03

    A water world with even a surface temperature of 200-500C but with ice at depth should have a zone in the water column that is equitable for life, should it exist.

    While ice on the seabed might prevent those hot smokers, I have to wonder if volcanoes would fulfill the need for recycling trace nutrients, as well as adding macronutrients to the ocean. Would they be hot enough, at least in places, to ensure an ice-free water column?

    Life at depth might never be photosynthetic, so I would guess any life would be anaerobic, whether single- or multicellular.

    If we detected high enough ratios of CH4 in the atmosphere (without CO), would that be a good hint that life does exist in the oceans?

    • Michael Fidler August 21, 2018, 2:33

      Ok, We need to look at tidally-locked planets and how the water, ice and geologic processes would play out over the eons. How would major impacts like the one 65 million years ago effect a water world. How would a large moon effect the evolution of such planets. As for radios, my first rocket was a two stage water propelled missile. We need to shrink the box and look at the real possibilities, the universe is not that simple or sterile. (Except if you are on one of those rocky little worlds!)

  • Geoffrey Hillend August 20, 2018, 16:47

    We learned the density of Mercury by its effect of gravity on NASA’s Mariner 10. Also by it gravitational interaction with Venus. The mass divided by the volume is the density. Mercury has a high density so it must have a large iron core. How it got the core is assumed to be a collision with another large body. We can apply this to all other planets.

    A water world with a temperature between 200C and 500C does not have ice caps. The evaporation temperature or water goes up at a higher pressure, but I find it had to support the idea that there will be much water vapor since Venus has very little water vapor; it lost most of it due to the high temperature and solar wind stripping.

    I have no doubt that there is the possibility of water vapor detection. The CH4 is different story which I don’t expect to see in spectral lines of Proxima B or other red dwarf exoplanets. Maybe there we be some O2 but I don’t give that a high probability unless there is a lot of surface water and water vapor.

    The rain does take some of the Co2 out of the air but without a Urey reaction and land, the Co2 might build up with a lot of volcanism. Without it, there might be a frozen world or ice caps. Live might evolve there, but one again, the loss of atmosphere, and the solar wind stripping becomes a problem for worlds without a Moon and fast rotation.

  • Localfluff August 20, 2018, 16:49

    I’m not much into science fiction, but let me speculate wildly as if I were.

    On a water world, a “civilization” couldn’t make use of fire or radio or electricity much. But there might be substitutes and ways to do it anyway! If there is volcanic activity on the sea floor, then lava flows could be used to melt metals. Maybe pure metals could be separated with a lava centrifuge. Molding could be done by simply digging out the shape wanted and let the lava fill it. Extinct volcanic formations, like lava tubes, might be naturally evacuated from water by volcanic gasses. Biological processes might seal them off and use them as a diverse habitation niche. Hollows could be dug deliberately to build non-water filled environments. We ourselves use wood for ships, so structures floating on the surface could appear naturally and aerial life on such floating islands work much like land life on Earth. Ships are made out of metal and even concrete, that might be molded on the sea floor, using volcanic gasses for boyance. Oil available for plastic constructions.

    • Robin Datta August 20, 2018, 22:43

      The presumptions are a human brain equivalent structure, appendages comparable in mobility and dexterity to human hands, a visual system with stereoscopic near vision, and a communication system the equivalent of human speech. In the case of humans, the evolutionary history of each of these involves the interaction of quite a number of varied preconditions, each essential to the outcome. Most of the human preconditions would not be operative in an aqueous medium.

      Of course, the limitations of our imagination may not be operative in the realm of xenobiology or non-organic or even non-chemical “biology”.

  • Andrew Palfreyman August 20, 2018, 17:09

    Building radios underwater is umm challenging

    • J. Jason Wentworth August 28, 2018, 1:48

      That’s true, although VLF (Very Low Frequency–3 kHz to 30 kHz) radio signals do propagate through the oceans, and through the Earth’s crust (the “fishbone” VLF antennas that are used to communicate with nuclear submarines are buried underground). Also:

      Oceanic biological creatures that, like whales, could communicate acoustically could conceivably produce and detect VLF radio frequencies, particularly if they had biological “battery packs” (like electric eels and electric rays). If they were–or became–intelligent and moved onto land (perhaps as amphibious beings), they might be able to produce “sensory imitative technology” VLF radio devices that they could use to communicate with their fellow beings in the seas.

  • Abelard Lindsey August 20, 2018, 18:27

    I was thinking about this while hiking this weekend. Waterworlds are likely to hostile in the extreme. As the article suggests, the atmospheric pressure will be quite high, a 100 bar, maybe a 1000 bar, with it being a lot of CO2 and superheated steam at 200-500C. This is nasty. Also, with the ocean being deep enough, there is no erosion and wash off from land masses, thus no dissolved nutrients in the water. Planetary oceans could be sterile, like the water you use in a clothes iron.

    I think even planets with 1.5 radii of the Earth, although rocky, will have deep oceans (100 kilometers or more) and dense atmospheres (tens to hundreds of bar of pressure) and would still be hot.

    I don’t see these planets being habitable at all.

  • Brett August 21, 2018, 3:24

    I hesitate to even call a planet with a water layer hundreds of kilometers thick a “water world”. It’s a gas dwarf, like a mini-Neptune with a far thinner “mantle” of water-ice and probably a hydrogen-rich atmosphere above that (since planets that big will do a much better job of holding on to hydrogen). I wouldn’t think you’d find life there any more than you’d find it on one of our ice giant planets.

    Those with a radius 2.5 times that of Earth, massing about 10 Earth masses, are likely water worlds.

    That seems about right. As John Walker pointed out correctly above, the research seems to suggest that above 1.5 times Earth radius, you’re most likely dealing with gas dwarfs/mini-Neptunes. I’ve seen research that was even more pessimistic than that, saying that the 1.2 to 1.5 times Earth radius range was a transitional one where you could get gas dwarfs or rocky planets because they could theoretically hold on to hydrogen (planets below 1.2 times Earth radius and twice its mass simply can’t hold on to much hydrogen).

    I wonder how much water they’d typically get. We just don’t know – Venus and Mars don’t seem to have had so much water in the beginning that they’d be water worlds even if either had somehow remained habitable long enough for plate tectonics.

  • Nicky August 21, 2018, 3:49

    Let me give an important note, Zeng et al. presentation is contrary to previous studies on planetary composition deduced from planet radius distribution.
    The previous studies I’m talking about here refer to:

    These studies have agreed upon the fact that majority of observed planets with radius between 1.6 and 3 is rocky and depleted in volatile in nature but abundant in gases.
    One reason for this is that volatile is resistant to photoevaporation, if the larger radius of planets is contributed mainly through accreting volatiles, the radius location of photoevaporation gap should move upward to 2.0 to 3.0 instead of staying at 1.5 to 2.5.
    This distribution leads to the robust conclusion that mini-Neptunes should be depleted in volatiles and rich in gases, contrary to Zeng et al. presentation.
    The presentation did cite one of the previous studies (doi.org/10.3847/1538-4357/aa890a), but did not explain the discrepancy. I would like to see the how Zeng et al reconcile it.

    • andy August 21, 2018, 15:57

      I believe the paper in question is “Survival function analysis of planet size distribution with Gaia Data Release 2 updates” by Zeng, Jacobson, Sasselov & Vanderburg, available on the arXiv here. This gives a bit more detail than what is in the conference abstract.

      • Nicky August 22, 2018, 17:35

        Now I actually doubt the validity of this volatile-rich super-Earths hypothesis after reading. Zeng et al ignored the fact that the radius gap location decreases with increasing orbital period and the absence of mini-Neptunes with orbital period less than 3 days, which present as major caveats to this hypothesis.

  • AlexT August 21, 2018, 5:00

    Very probably that numbers given in this article mostly reflecting the resolution and limitation of used for observations instruments, than objective reality…

  • Brett Bellmore August 21, 2018, 6:41

    Some water worlds might be candidates for colonization, at least, if their surface temperatures are suitable. We could introduce trace elements by asteroid impacts.

    That actually does raise the question: Trace elements can come from above, as well as from below: Wouldn’t these elements accumulate in the water column, as long as they didn’t reach saturation level? A water world with capped rocky surface doesn’t have to be pure H2O, if there’s a fair amount of incoming meteors.

    • Michael Fidler August 21, 2018, 9:26

      Good point, and there is still the possibility of panspermia. Water world’s would seem to be the perfect egg.

    • Michael August 21, 2018, 11:44

      You beat me to it, bigger worlds pull in more rock, perhaps enough to add trace elements in sufficient amounts for life.

  • Harry R Ray August 21, 2018, 9:48

    Before we wax hypothetical on life on these worlds, let’s just FIND some! Until JWST or Giant Magellan come on line(2021-2024),this may prove very hard to do. However, there may be a loophole! For almost 20 years, Christiano Cosmovici has attempted to find planets orbiting MV stars whose atmospheres emit WATER MASERS! He has tentatively found evidence for water maser emmission at the planet hosting systems, Epsilon Eridani, Lalland 21185, and Gliese 581. These are TENTATIVE detections, because he was unable to determine the orbital characteristics of any planets because his equipment was not sensitive enough to measure Dopplar shifts at water maser frequencies. However, ALMA may be sensitive enough to do this. The best target for ALMA to do this is the LHS 1140 system which is nearby AND has TWO potential water worlds.

    • Harry R Ray August 21, 2018, 10:52

      The first exoplanets discovered by TESS will be announced on August 30 at MIT. Will any of them turn out to be water worlds?

      • Localfluff August 23, 2018, 7:46

        At least follow-up observations will soon find that out. That’s the great strength of TESS, finding nearby easy to observed exoplanets.

        • Harry R Ray August 27, 2018, 12:29

          Oops: The announcements will be actually made September 5, and will be in the form of alerts, rather than formal discoveries.

  • Mark August 21, 2018, 9:48

    I don’t understand the physics / geology of why water world would require it necessarily have to be so hot (200-500C), but I’ll take your word for it. As a practical matter, perhaps it’s best that waterworlds be uninhabitable. Future spacecraft could use them as refueling stations, siphoning off precious deuterium and tritium to supply their antimatter reactors.

    BTW on a completely different matter, I have SOLVED the Fermi Paradox. You know, the reason why we don’t see other life in the universe despite the vast size and age of the place. I’ve considered and rejected all current thinking on the matter and have come to the conclusion that we are the unwitting guests of honor at a special kind of intergalactic party. Any minute now, watch the skies for the SURPRISE!

  • Michael Fidler August 21, 2018, 10:06

    The only examples we have are Uranus and Neptune, both of which have active mixing atmospheres. What are the nearest water world exoplanets? Those that are in the habitable zone should be a top priority for determining the atmospheric characteristics.

  • Gary Wilson August 21, 2018, 10:41

    Drawing too many (or any) conclusions from too little (or in fact no) data about specific water worlds is a dangerous game to play. I suppose we do it because we are impatient with the current state of knowledge. Once we have several carefully studied examples of true water worlds we might be able to draw some general conclusions that might be useful as rules of thumb. Or it may turn out that each water world is very different indeed from the others. Rocky planets have display a huge range of characteristics where life is concerned and I expect water worlds to be the same. We haven’t studied a single rocky planet other than earth thoroughly yet (Mars is available, hint, hint) let alone a vastly different type of planet found in another planetary system.

  • Spaceman August 21, 2018, 11:46

    With surface temperatures between 200 °C and 500°C, how would these water worlds be able to retain liquid water on their surfaces for any significant length of time??

    • Harry R Ray August 21, 2018, 14:07

      The EXTREME PRESSURE put on the surface of these oceans by the super-dense atmospheres would raise the boiling points of these oceans DRAMATICALLY!

  • Michael August 21, 2018, 12:39

    On these worlds floating ice sheet may be the only places where life could hang on, any dust from space would tend to hang around longer on the surface.

  • Geoffrey Hillend August 21, 2018, 16:56

    Quote by Geoffrey Hillend: “A water world with a temperature between 200C and 500C does not have ice caps. The evaporation temperature or water goes up at a higher pressure, but I find it had to support the idea that there will be much water vapor since Venus has very little water vapor; it lost most of it due to the high temperature and solar wind stripping. ”

    This was supposed to say I find it HARD to support the idea that there will be much water vapor between 200C and 500C since Venus has very little water vapor due to its high temperature and solar wind stripping over a long period of time.

    Space LASERS and MASERS are simply the emission of two photons in the emission spectral bands instead of one. The atom or molecule is emitting two photons in stead of one which happens when there is a population inversion. There are no powerful MASER beams coming out like Star Wars since to make a LASER or MASER with any power, there has to be a cavity with two mirrors on either side or and undulator as in a free electron laser in order for the photons to build up to the point where there can be a cascade of trillions of photons in one pulse depending on the power in watts put into the laser or maser.

    • J. Jason Wentworth August 28, 2018, 1:53

      I had wondered about that (water masers) in relation to the CO2 lasershine on Mars, which–if memory serves–wasn’t detected from Earth, but by Mars orbiters and/or landers because it isn’t extremely powerful.

  • Harold Shaw August 21, 2018, 19:11

    Could we expect cryovulcanism? I can picture plates of each ice type layered across each other grinding, catalyzing material into rich organic slurry, metabolising under miles think shields of water. Could cryovulcanism do something similar enough to rock weathering and provide a pathway complexity for life?

    Would a water world be obvious in some way? If we can speculate telling a forested world by how it reflects light; can’t we speculate a world wide ocean by how it reflects light?

    As a feat of geoengineering, water could be shared between worlds. Turn one water world and two dry worlds into three desiccated.

  • Spaceman August 22, 2018, 9:53

    Okay. So it was mentioned earlier in the discussion that high atmospheric pressure might help the oceans liquid on these large water worlds.

    I have a few more questions: would these oceans likely be sterile given the barriers to life’s emergence on the ice-covered sea floor, as there is something odd about imaging a planet with more liquid water than Earth but no life whatsoever!? Also, what would astronauts experience upon exiting a floating space capsule in one of these oceans…would it be like entering a sauna on steroids?

  • Geoffrey Hillend August 22, 2018, 16:43

    Super Earths which are water worlds are still only a hypothesis. I’ll believe them when I see the data; the larger than Earth mass and diameter and strong spectral bands of water in the atmosphere. It may be that the amount of water is limited to whether or not a planet has a magnetic field and the size of the planet. A strong gravity tends to give a planet a thicker atmosphere and a stronger greenhouse effect even without a lot of Co2 like a gas giant.

    There is also the solar wind stripping which removes an atmosphere over time. I like the idea of a huge ocean and a water world though. I think they are very rare based on conservative planetary science. Yes I am using our solar system as an example, but we can calculate the amount of water Venus might have had when the Sun’s brightness was much less billions of years ago. Most of the water is assumed to come from the mantel through volcanoes on terrestrial rocky planets so huge oceans might be rare.

  • ljk August 23, 2018, 9:37

    As usual we only have one data point to work on so far, but if intelligent life does develop on these water worlds, we probably will not have solid scientific evidence of their existence until we either ramp up our telescopic technology or send interstellar probes to these places.

    Earth possesses some very smart beings in our waters, many who have been around as species much longer than humanity. Cetaceans, for example, have existed for at least 30 million years. However, unless they are hiding their communications equipment or use telepathy (and hey, how hard would that be to keep secrets from a bipedal land primate who has only explored about 10 percent of Earth’s oceans?), whales, dolphins, and octopi do not appear to be conducting their own METI programs, at least at this time.

    Perhaps living submerged in the water all the time precludes one from wanting to explore other realms in any serious way, especially since it is not easy to function or move much outside that buoyant liquid for them. Then again, a similar thing might be said for creatures on land, too, yet one is at least trying to overcome their evolutionary limitations.

    • Alex Tolley August 23, 2018, 11:19

      Projecting our intelligence embodied in machines might be a good example of [human] life overcoming its evolutionary limitations to explore beyond Earth.

      The late Douglas Adams would get a kick out of the idea whales are telepathically communicating with aliens. unbeknownst to us. Maybe First Contact will have aliens bringing large quantities of fish.

  • Smoke August 24, 2018, 3:17

    What if the water world is a moon of a much larger primary, and is close enough to undergo massive tidal flexing? Like Io, but much more so. Would the tides be strong enough to crack even a thick layer of bottom ice, allowing volcanos to spew some minerals into the global ocean? If so, I can see life arising.

  • Harry R Ray August 24, 2018, 10:33

    Ninteen hours ago, Alex Teachey, the lead author of the Kipping et al paper on exomoon CANDIDATE Kepler 1625b I posted this rather cryptic tweet: “Paper resubmitted!”. I may be potting WAY TOO MUCH INTO THIS, but a couple of things intrigue me about this. ONE: There has been NO papers up on ArXiv with Alex Teachey as the lead author for a long time. A couple of months ago, David Kipping tweeted that he would “share” the October 29, 2017 observations of Kepler 1625 “soon”. Since then, however, all has been STRANGELY QUIET on the Kepler 1625b I front. My take on this: A paper was ORIGINALLY submitted to a MAJOR science journal which prohibits ArXiv PREPRINTS. A non-detection would probably have been submitted to a lesser journal, and thus an ArXiv preprint would have ALSO appeared! Finally, on October 29 of this year, due to the freedom of information act, the october 29, 2017 HST observations of Kepler 1625b MUST BE MADE PUBLIC! Seems to me that the exomoon DOES exist, and COULD be one of the above mentioned in this post, “waterworlds”. Hudathunkit!

  • Geoffrey Hillend August 24, 2018, 19:20

    Water world exoplanets can be postulated to exist using our solar system as a model. Earth was a water world before the collision of Theia and it lost a lot of water and much of its original atmosphere. Also Earth still had 26 percent more water than today on it’s surface after the collision when it cooled off since there were no continents and mountains and mostly water. P. 3 Weather, an illustrated guide Revkin and Mechaley.

    There might be some water left on those worlds if there was initially a lot of water even without a moon, and with a magnetic field and solar wind stripping over time.

    • Alex Tolley August 25, 2018, 14:05

      Wouldn’t our Moon still have a lot of subsurface water if Earth was a water world and the Moon aggregated a fraction of the collision mass lost from Earth and Theia?

  • Geoffrey Hillend August 25, 2018, 16:24

    I am not an expert in planetary astrophysics, but a lot of the water was vaporized into a gas from the heat. There was some water molecule splitting or thermolysis into H and O2 which would escape the gravitational field. The entire mantel was blasted into space to become the Moon so we can assume some material, gas and water reached escape velocity, but some had to be re collected by the debris that formed the Moon in orbit around the Earth. There is some subsurface water on the Moon, and more than scientists earlier assumed, so the answer is Yes.

  • Geoffrey Hillend August 25, 2018, 16:26

    Excuse me, I mean mantle not mantel

  • Patient Observer August 25, 2018, 19:25

    The moon’s interior is more wet than previously thought:


    And I had the same thought that the early Earth may have had much more water than today courtesy of Theia. Given the potential for Theia-like collisions during the early formation of any solar system, there may be a mechanism to shed excess water for a significant fraction of otherwise habitable planets. And the mechanics of such impacts (warning – total speculation) could result in a preference for the residual water to match what is on Earth today.

  • Harry R Ray August 31, 2018, 9:55

    Edwin Kite and Eric Ford have just published a paper in The Astrophysical Journal(unfortunately NOT up on ArXiv)claiming 10% of planets completely covered by water are potentially habitable despite the lack of cycling if minerals and gasses(primarily carbon)! This,COMBINED WITH Ramirez et al’s CO2 cold trap hypothesis could SIGNIFIGANTLY INCREASE the likelyhood of life on water worlds orbiting M dwarf stars!