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Are Planets Like Proxima b Water Worlds?

Those of us fascinated by dim red stars find these to be exhilarating days indeed. The buzz over Proxima b continues, as well it should, given the fact that this provocative planet orbits the nearest star. We also have detections like the three small planets around TRAPPIST-1, another red dwarf that is just under 40 light years out in the constellation Aquarius. These are small stars indeed, just 8 percent the mass of the Sun in the case of the latter, while Proxima Centauri is about 10 times less massive (and 500 times less luminous) than the Sun.

But just what might we find on planets like these? A new paper from Yann Alibert and Willy Benz (University of Bern) drills down into their composition. The researchers’ goal is to study planet formation, with a focus on planets orbiting within 0.1 AU, a range that includes the habitable zone for such stars. While a forthcoming paper will look at the formation process of these planets in greater detail, the present work studies planetary mass, radius, period and water content.

To do this, Alibert and Benz have developed computer simulations that model red dwarf planetary systems, assuming a central star with a tenth the mass of the Sun and a protoplanetary disk around each modeled star. Putting the model into motion, the scientists studied a series ranging from a few hundred to thousands of such stars, with 10 planetary embryos in each disk — each embryo was modeled as having an initial mass equal to that of the Moon, and the initial location of each planetary embryo was drawn at random.

The results, according to Alibert:

“Our models succeed in reproducing planets that are similar in terms of mass and period to the ones observed recently. Interestingly, we find that planets in close-in orbits around these type of stars are of small sizes. Typically, they range between 0.5 and 1.5 Earth radii with a peak at about 1.0 Earth radius. Future discoveries will tell if we are correct!”


Image: Artist’s impression of Earth-sized planets orbiting a red dwarf star. Credit: @ NASA, ESA, and G.Bacon (STScI).

The most striking aspect of this work is likely to be Alibert and Benz’ findings on the water content of small planets in the habitable zone. The amount of water is found to depend upon the location at which the planet has accreted planetesimals, their composition being dictated by the thermal structure of the disk and the location of the snowline, which varies depending on disk mass. Note this: A significant fraction of the planets modeled show more than 10 percent water. Contrast this with the Earth, whose fraction of water is roughly 0.02%.

The study shows a correlation between the mass of a planet and the water fraction, with planets that do not contain a high degree of water being lower in mass (generally below one Earth mass), while planets totally devoid of water are all less massive than one Earth mass. Alibert and Benz see this as the result of migration, with more massive planets migrating from further out in the system, thus collecting water-rich material from beyond the snowline.

Water may be a key to habitability, and the ‘habitable zone’ is defined as the zone in which liquid water can exist on the surface. But water to the extent of deep global oceans is problematic. A large enough water layer can produce high pressure ice at the bottom, preventing the carbonate-silicate cycle that regulates surface temperature over long timescales from operating. Without this mechanism, atmospheric CO2 cannot cycle through the weathering of rocks on the Earth’s surface and the eventual subduction of calcium carbonate. High temperatures and pressures eventually return CO2 to the atmosphere by processes like volcanism, regulating global temperatures.

We don’t know how much of a factor the loss of this process might be on planets around red dwarf stars, as the paper takes pains to note:

In the case of low mass stars, which evolve on much longer timescales, this may not be a major problem, as the stellar flux varies on timescales much longer than in the case of the Sun. In this situation, a process that stabilizes the surface temperature may not be necessary. The second reason [why large amounts of water may be detrimental for habitability] is connected to the fact that for planets with too much water an unstable CO2 cycle destabilizes the climate making habitability more challenging… Again, this was demonstrated for solar-type stars and a similar process may or may not exist for low mass stars.

So we have a computer model that produces planets similar in mass and radius to the interesting worlds we’re finding around some nearby red dwarfs, with a peak in the distribution of radius at about one Earth radius. The authors argue that the properties of the disk potentially correlate with the mass of the star, thus determining the water content of emerging planets. Deep oceans on planets in the habitable zone of red dwarfs may be the norm, in which case we need to know a great deal more about climate on such exotic worlds.

…our models show that the properties of the disk and their potential correlation with the mass of the star are the most important parameters determining the characteristics, in particular the water content, of the emerging planet population. In this context, observational constraints on mass and lifetime of discs in orbit of low-mass stars become of paramount importance.

The paper is Alibert and Benz, “Formation and composition of planets around very low mass stars,” accepted at Astronomy & Astrophysics (preprint).


Comments on this entry are closed.

  • Geoffrey Hillend October 26, 2016, 17:31

    Quote by Paul Gilster: “A large enough water layer can produce high pressure ice at the bottom, preventing the carbonate-silicate cycle that regulates surface temperature over long timescales from operating.” I wonder if that pressure could prevent plate tectonics and even reduce volcanism?
    An ocean ten times deeper than Earth’s would have a high pressure at the bottom. Some Co2 gas might still get through and without any land, plate tectonics does not remove any Co2 through the carbon cycle which needs land to work.

    In the carbon cycle, the rain takes the Co2 out of the air by becoming carbonic acid in soil pore water and combining with calcium silicate to become silica(chert) and calcium carbonate(limestone) and are transported by rivers to the ocean. The limestone builds up on the bottom of the ocean where the ocean floor plates eventually subduct beneath the crust where the limstone is melted to become molten and the Co2 is re-released through volcanism.

    If volcanism can still release Co2 into the ocean of a deep ocean water world it might be released into the atmosphere? Without a carbon cycle it might build up into the atmosphere creating a large greenhouse effect?

    There still can be loss of atmosphere by the solar wind stripping it away or the splitting of Co2 into oxygen and carbon and H2O into hydrogen and oxygen. The hydrogen will escape being the lightest gas.

  • Geoffrey Hillend October 26, 2016, 17:38
    • Harry R Ray October 27, 2016, 9:43

      I think the REAL questionS should be: Was Proxima b EVER a water world? If so, IS IT NOW? The argument FOR(as presented in the POSTING) is just a RE-ITERATION of the arguments made by Giullen et al in their TRAPPIST-1 paper(i.e., the only thing NEW is the PREDOMINANCE of Earth-sized worlds OVER smaller and larger worlds in these stars’ habitable zones). Ten percent water is a whole heck of a lot to boil off in just a few billion years, but the case for that happening or not is not closed yet. If the answer to question one above IS “yes”, then what PERCENTAGE of water would you have to boil off to relieve ENOUGH PRESSURE to convert the ice-7 to water, and thus release nutrients into the ocean above? Depending upon the answer to THIS QUESTION, life could have got STARTED very late on these planets. NOW: With respect to Proxima b ALONE; the stellar insolation being only 70% solar, if there are ABSOLUTELY NO silicate or carbon land masses poking above the surface of a putative “global ocean”. there may STILL be “land” in the form of a NEARLY GLOBAL ICE SHEET FLOATING on the water, i.e. an “Eyeball Earth”. In terms of EVOLUTION, a MAJOR questioned to be asked would be: Would multi-celled marine life-forms be able to MIGRATE from ocean to ice sheet, like they did here in Earth from ocean to land, and, if so; would they be able to CONTINUE TO EVOLVE to the point of intellegence? I have been waiting for a science-fiction story on this subject, and as of now, I have found NONE. If one exists, and any reader of the postings on this website know of one, please reply to this comment ASAP!

  • Neil S October 26, 2016, 21:35

    So settling on one of these over-hydrated worlds would mean living in boats or possibly submersibles, especially if there were high winds and waves. In extreme cases they would not have access to minerals from the world, just from space, and meteorites, natural or artificial, would just make a big splash and sink to the bottom. But then if the bottom were covered with hard ice the meteorites from geological ages past might be sitting on top of that, available to very deep divers.

    Life is conjectured to have formed around our mid-ocean rifts based on volcanic heat and chemicals. Perhaps the same processes would have allowed life to form on such a world if the volcanic heat could overcome the cold of the oceanic deeps and “melt” that ice. But the step from existing at the bottom around such volcanoes and existing at the top using a process like photosynthesis for energy would be a giant one so it would be reasonable that the only life there would be very deep and would only be encountered by meteorite divers. There, I’ve set out the background for someone’s SF story.

  • xcalibur October 27, 2016, 2:49

    One kind of exoplanet I’ve always imagined is a watery world with land in the form of archipelagos. I wonder how common those are?

    I can see how a very deep ocean with exotic ice in its depths would raise many issues for habitability. However, a planet could still be dominated by its ocean without reaching those extremes. How are their ocean depths distributed, and how does the extent/depth of their oceans influence their development? Alot of blanks need to be filled in.

    I think archipelago/water worlds could be habitable. They would tend to have a temperate but windy climate.

  • torque_xtr October 27, 2016, 4:51

    It seems the paper doesn’t take the luminosity evolution of M-dwarfs into account, and for UCD’s it’s crucial – they start with luminosities hundreds of times higher than at main sequence and support it for much longer times than it takes for planetary formation to complete. So, when the planets form, the snow line is located up to ten times farther than the MS-stage luminosity implies, and in the abscence of migration all the system would be completely dessicated except for the outer reaches. In abscence of more data I cling to the guess that most of the late M-dwarfs systems are scaled-up versions of Jovian system, but with the dessication line farther out (Jupiter had significant luminosity for much shorter period, comparaple to satellite formation timescale, so only Io ended up totally dessicated), and that earths ans sub-earths in the MS-HZ of late M- and L-dwarfs mostly are bone dry. Of course, somewhere there is the transition, between scaled-up Jovian-type systems and GJ-1214-type systems, with wet super-earths and sub-neptunes starting to appear at M4 class and earlier.

    Maybe gravitational lensing findings support this point, and several-Me cold superearths at the outer reaches of late M-dwarf systems are Ganymede and Callisto analogs…

  • DJ Kaplan October 27, 2016, 12:38

    I appreciate this approach. The more we learn about our own system, the more we may consider whether the complex road on Earth that led to intelligent life, may possibly be unique, or as unique as anything can get in an infinite universe.

  • Andrew Palfreyman October 27, 2016, 12:48

    We are on the verge of being able to construct artificial habitats in space. Similarly, we may be on the verge of discovering alien (albeit bacterial size) life elsewhere. It strikes me that much of the rhetoric in which our pro-space community engages contains a type of conflation that ultimately does not serve us very well; and that is to conflate life-bearing planets or moons with extraterrestrial places we’d like to live. They by no means need to be the same.

    • DJ Kaplan October 28, 2016, 12:38

      Or, for that matter, connecting an orbit that might permit water to exist in a liquid state (under certain circumstances) with humanoid space-faring civilizations.

    • Robert October 28, 2016, 13:11

      We’ve been ‘on the verge’ since the 1970’s but lack the will and more importantly, the necessity of doing such. O’Neill showed it could be done with 70’s technology but large solar power satellites, which were always the purported justification, simply do not make sense.

  • Bounty October 27, 2016, 13:17

    I wonder if some kind of cryovolcanism might exist on these planets. Might be in an underwater form like hydrothermal vents. I guess it all depends on the pressures and temperatures involved. If so it might make a useful way for chemicals to move about.

  • Geoffrey Hillend October 27, 2016, 15:45

    If Proxima b has a higher mass than Earth, then is most likely has a large ocean and atmosphere as written by Paul Gilster above. It still might have a large ocean and atmosphere. Life most likely started on Earth in the ocean by Sunlight and then adapted to the bottom of the ocean not the reverse. If Proxima b is tidally locked, the dark side of it might have adapted in the same way with more nocturnal life than diurnal life if there is too much solar radiation? The ocean currents would take it out of the day side.

    • Alex Tolley October 27, 2016, 22:13

      Life most likely started on Earth in the ocean by Sunlight and then adapted to the bottom of the ocean not the reverse.

      All models that I am familiar with require a mechanism to concentrate the organic compounds for life, whether RNA or metabolites. This cannot happen in the open ocean. The vent hypotehsis assumes metabolites concentrate in the rock pores, whilst the surface model assumes drying surfaces. While we do not yet know the mecghanism[s] of biogenesis, the various promising chemistries won’t work in large bodies of open water.

    • Eniac October 29, 2016, 23:12

      Life did not start with sunlight. Photosynthesis is a relatively late development as early life goes. The first organisms fed on something chemical. My bet is carbon monoxide, which, when reacted with water, can provide both energy and carbon. It also can be present in significant amounts in a high carbon atmosphere, such as that of early Earth. Or, perhaps, in volcanic rock. Hydregenotrophic methanogens react H2 and CO2 to CO and H2O, first, and might well represent the most ancient metabolism there is.

  • Phil October 27, 2016, 17:33

    Water worlds without plate tectonics might be able to generate ‘land’ via persistent hot spot vulcanism. Imagine an Olympus Mons growing for gigayears. No idea what the astrobiological significance of that might be.

    Also – wasnt the early Earth a ‘waterworld’ (see caveat above) until plate tectonics got going?


    • Harry R Ray October 28, 2016, 9:36

      The only problem with that is the EXTREME PRESSURE of a VERY DEEP OCEAN would FLATTEN any shield trying to form on the ocean bhottom, and a thick layer of Ice-7 would prevent ANY kind of eruption.

    • Brett October 29, 2016, 2:01

      I believe so, until cratons started forming at plate tectonics boundaries and then piling up over time. There’s not much evidence for continental crust before 3.5 billion years ago, and it seems to have grown over time (i.e. about 2.6 billion years ago, we had about 60% of the continental crust we have now).

      What I wonder is how a deeper starting world ocean would affect plate tectonics. Water gets subducted down into the mantle, so would worlds with deeper water layers and plate tectonics devour their surface water much more quickly?

      • Neil S October 29, 2016, 11:20

        Well as I understand these things if they have plate tectonics with subduction they should have more volcanoes due to more water being pushed into the sub-ducted plate and making it more volcanic. Subsurface volcanoes of course.

  • andy October 27, 2016, 18:22

    It may be possible to have a negative feedback for carbon dioxide on ocean planets even if the silicate-carbonate cycle doesn’t provide one: see Levi et al. (arXiv:1609.08185 [astro-ph.EP]) “The Abundance of Atmospheric CO₂ in Ocean Exoplanets: A Novel CO₂ Deposition Mechanism

  • Robert Feyerharm October 28, 2016, 11:17

    If a Proxima b water world is tidally locked, wouldn’t much of the water be piled up at the subsolar and antipodal points in a permanent “high tide”? Depending on the strength of the tidal forces and the depth of the ocean, there could be exposed dry land along the terminator of the planet.

    • DJ Kaplan October 28, 2016, 16:38

      That’s an interesting model. The water under the “subsolar” areas would experience more evaporation, be conducted to cooler regions by winds, and condense as rain elsewhere. There would still be considerable sea currents as well.

      We may not be able to check this out for decades, however.

    • Eniac October 29, 2016, 22:52

      No, this is not true. The whole planet will be elongated just a little bit under the same influence of gravity that causes the tide. Exactly until the water is the same depth, everywhere. Tides only exist because the crust can not change shape fast enough for a daily cycle.

    • Michael C. Fidler October 30, 2016, 10:09

      That’s a very good point and I’ve been thinking something along the same lines. If this planet formed at the distance it is now from Proxima could it have formed in an elongated state? Like binary stars that are close together could the molten material of Proxima B’s birth cause it to form a shape like an egg or a football. The material would be tidally locked to Proxima when it was formed, the only problem is why Mercury did not form in this shape. The mass of Proxima is 1/8 of the sun and Proxima B is at 1/7 the distance that Mercury is from our Sun. The question is; would there be stronger tidal forces at work in this miniature solar system? Now what would the oceans due with this condition? Boiling at the subsolar point and frozen solid at the antipodal? Could the planet still rotate along its elongated axis and could it also precess along that axis if its rotation was fast enough? Maybe the universe has more in store for us then we can imagine!

  • Robert October 28, 2016, 13:17

    After seeing Interstellar, I’m a little leery of water worlds in close orbit of massive objects.

  • Geoffrey Hillend October 28, 2016, 16:44

    Alex Tolley. “All models that I am familiar with require a mechanism to concentrate the organic compounds for life, whether RNA or metabolites. This cannot happen in the open ocean. The vent hypotehsis assumes metabolites concentrate in the rock pores, whilst the surface model assumes drying surfaces. While we do not yet know the mecghanism[s] of biogenesis, the various promising chemistries won’t work in large bodies of open water.”
    I stand corrected. Life probably evolved in a shallow pond. I forgot about the carbon and the right concentrations of molecules needed which cannot occur in the hydrosphere which will dilute those. It was then carried into the hydrosphere by water where it multiplied and mutated to higher forms.
    I never believed the thermal vent hypotheses since the temperature is too high and there is no sunlight at the bottom of the ocean

    • Eniac October 29, 2016, 22:56

      I doubt life evolved in a shallow pond. Ponds are too transient, the tend to disappear, quickly. My bet is on rock pores. At thermal vents or, perhaps, even in bulk rock.

      • Alex Tolley October 30, 2016, 21:08

        The surface model is life evolving around hot springs where the water can evaporate concentrating the compounds needed for life. However, it has been hard to come up with the right chemistry and conditions for this and the hot vent model with metabolism starting in the rock pores seems more viable. But you never know, it is literally an evolving field of study.

        • Eniac November 2, 2016, 21:03

          Evolving, indeed :)

          There are two fundamental reasons that push me away from surface models: 1) The transience of any environment that is exposed to weather, and 2) the much greater volume, and, yes, surface area, of bulk rock.

          Since we now know that there is life miles underground, the bulk model has become very plausible, indeed. The sheer size of the prebiotic habitat is important, as the chance of abiogenesis is, of course, proportional to the size of the environment that has the right conditions.

          Just as it is proportional to the number of available planets, and we all know how that is turning out.

    • Michael C. Fidler November 3, 2016, 5:06

      This is there mother of us all!

      “The genes chosen by scientists suggest that Luca lived without oxygen. Instead, it survived on carbon dioxide and hydrogen. They also believe that it thrived in extremely high temperatures and required the presence of metals.
      Scientists have always wondered if life began in hydrothermal vents and Luca’s characteristics seem to fit the living conditions in such an environment.”

  • Harry R Ray October 31, 2016, 16:34

    Has anyone seen or heard anything DEFINITE on the RECENT ATA observations of Proxima Centauri(nothing was EVER found from observations of Alpha Centauri AB, AND: Proxima Centauri IS in the TWO DEGREE RADIUS AROUND these two stars, but pointing the array DIRECTLY AT Proxima Centauri had NEVER BEFORE BEEN ATTEMPTED)a few weeks ago? The reason I ask is that IF Proxima b IS a “water world”, there should be PLENTY of lightning on it! Could ATA have picked up a VERY VERY FAINT signal at the same frequency as the false(PROBABLY)positive HAT-P 11b signal and are following up to detect ADDITIONAL signals BEFORE they publish anything?

    • Harry R Ray November 9, 2016, 10:26

      Breakthrough Listen will use the Parkes radio telescope to observe Proxima Centauri VERY SOON!

      • Paul Gilster November 9, 2016, 12:16

        The effort has already started. More about this in about an hour on the next post.