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Will We Find Habitable ‘Super-Earths?’

As the 223rd meeting of the American Astronomical Society continues in Washington, we’re continuing to see activity on the subject of mini-Neptunes and ‘super-Earths,’ the latter often thought to be waterworlds. Given how fast our picture of planets in this domain is changing, I was intrigued to see that Nicolas Cowan (Northwestern University) and Dorian Abbot (University of Chicago) have come up with a model that allows a super-Earth with active plate tectonics to have abundant water in its mantle and oceans as well as exposed continents.

If Cowan and Abbot are right, such worlds could feature a relatively stable climate even if the amount of water there is far higher than Earth. Focusing on the planetary mantle, the authors point to a deep water cycle that moves water between oceans and mantle, a movement made possible by plate tectonics. The Earth itself has a good deal of water in its mantle. The paper argues that the division of water between ocean and mantle is controlled by seafloor pressure, which is proportional to gravity.

As planetary size increases, in other words, a super-Earth’s gravity and seafloor pressure go up as well. Rather than a waterworld with surface completely covered by water, the planet could have many characteristics of a terrestrial-class world, leading Cowan to say: “We can put 80 times more water on a super-Earth and still have its surface look like Earth. These massive planets have enormous sea floor pressure, and this force pushes water into the mantle.”


Image: Artist’s impression of Kepler-62f, a potential super-Earth in its star’s habitable zone. Could a super-Earth like this maintain oceans and exposed continents, or would it most likely be a water world? Credit: NASA/Ames/JPL-Caltech.

The implications for habitability emerge when we regard super-Earth oceans as relatively shallow. Exposed continents allow a planet to undergo the deep carbon cycle, producing the kind of stabilizing feedback that cannot exist on a waterworld. A super-Earth with exposed continents is much more likely to have an Earth-like climate, with all of this being dependent on whether or not the super-Earth has plate tectonics, and on the amount of water it stores in its mantle. Cowan calls the argument ‘a shot from the hip,’ but it’s an interesting addition to our thinking about this category of planet as we probe our ever growing database of new worlds.

On the matter of the amount of water, not all super-Earths would fit the bill, and it shouldn’t be difficult to turn a planet into a waterworld — Cowan argues that if the Earth were 1 percent water by mass, not even the deep water cycle could save the day. Instead, the researchers are considering planets that are one one-thousandth or one ten-thousandth water. From the paper:

Exoplanets with sufficiently high water content will be water-covered regardless of the mechanism discussed here, but such ‘ocean planets’ may betray themselves by their lower density: a planet with 10% water mass fraction will exhibit a transit depth 10% greater than an equally-massive planet with Earth-like composition… Planets with 1% water mass fraction, however, are almost certainly waterworlds, but may have a bulk density indistinguishable from truly Earth-like planets. Given that simulations of water delivery to habitable zone terrestrial planets predict water mass fractions of 10-5 – 10-2… we conclude that most tectonically active planets — regardless of mass — will have both oceans and exposed continents, enabling a silicate weathering thermostat.

The paper is Cowan and Abbot, “Water Cycling Between Ocean and Mantle: Super-Earths Need Not be Waterworlds,” The Astrophysical Journal Vol. 781, No. 1 (2014). Abstract and preprint available.


Comments on this entry are closed.

  • James M. Essig January 8, 2014, 16:18

    Awesome post Paul;

    When considering the vast possible variations in geography including mountains, oceans, lakes, rivers, planes, valleys and the like on super-Earths, the possibility for vacation destinations for our deep future time descendants is awesome.

    Other super-Earth resources can include fissile fuels for nuclear fission powered space arks or even faster fission pellet runway powered spacecraft as well as perhaps the presence of natural stable yet to be discovered super-heavy elements and isotopes. I won’t hold my breath on the latter prospect but the possibility is intriguing especially where exotic neutron stars have collided or perhaps still unverified quark stars.

  • Wojciech J January 8, 2014, 20:20

    “Focusing on the planetary mantle, the authors point to a deep water cycle that moves water between oceans and mantle, a movement made possible by plate tectonics. The Earth itself has a good deal of water in its mantle.
    This reminds me of the SF novel “The Flood” by Stephen Baxter where the issue of water content of Earth in its mantle is touched upon.
    As to the article-very interesting topic, but like with the others, it’s just a theory…We need telescopes to start proving them right or wrong, even if we will start from the basics.

  • Ross Harrison January 9, 2014, 0:13

    Paul, I have enjoyed reading your articles for the last couple of years. The subject natter and in depth detail is truly educational an inspiring.

    I do have an observation on Super Earth’s should humans ever succeed on landing on one if these, what would the impact of gravity have on the human body?. I would think that if a Super Earth was 3 Earth masses the gravitational affect would be very noticeable for a human being used to an earth mass.

  • ljk January 9, 2014, 0:35

    Life on other planets could be far more widespread, study finds

    Jan 07, 2014

    Full article here:


  • Daniel Högberg January 9, 2014, 2:25

    I guess Im just stupid or something, but why dont all scientists look for habitable planets around stars that are closer to our own sun? Who cares if we find a habitable planet 600 lightyears from earth? Of course it would be interesting and all, but we can fantasy and dream about that 600 lightyear-away planet without spending billions of dollars and thousands of hours of research on finding a planet that we can never even send a message to and say hello (in case of inhabitants :)) because no one will care or be around 1200 years later when the message comes back., well if it ever does.

  • Lionel January 9, 2014, 7:25

    It’s worth remembering that is Earth is already denser than any other planet or moon within our solar system, and many solar systems will have a far smaller amount of heavier elements than our own, while others will have more. Also, within our own solar system there’s a wide variety – despite being much less massive than Earth, Titan is less than a third as dense

  • ljk January 9, 2014, 9:11


    Program Description

    It’s a golden age for planet hunters: NASA’s Kepler mission has identified more than 3,500 potential planets orbiting stars beyond our Sun. Some of them, like a planet called Kepler-22b, might even be able to harbor life. How did we come upon this distant planet?

    Combining startling animation with input from expert astrophysicists and astrobiologists, “Alien Planets Revealed” takes viewers on a journey along with the Kepler telescope. How does the telescope look for planets? How many of these planets are like our Earth? Will any of these planets be suitable for life as we know it?

    Bringing the creative power of veteran animators together with the latest discoveries in planet-hunting, “Alien Planets Revealed” shows the successes of the Kepler mission, taking us to planets beyond our solar system and providing a glimpse of creatures we might one day encounter.

  • Ronald January 9, 2014, 13:19

    With regard to the study in Planetary and Space Science, referenced in the article mentioned by Ljk: that ‘far more widespread’ life is then below the surface, so probably bacterial and the like.
    Interesting as well, but not advanced life, and probably hard to detect.

    Daniel Högberg: this present Kepler search for planets is not instead of searches for planets closer to us, but a systematic search of a large enough sample of stars, to get a statistically reliable impression of the prevalence of various types of planets.
    This method (the transit method), if it is to be applied to a large number of stars simultaneously, works best on a large group that is optically close together (i.e. many stars in a relatively small field of view), so: not too close to us.
    There are and will be several searches specifically focused on nearby stars (such as the HARPS program).

  • Ronald January 10, 2014, 6:44

    As I just commented under a previous post, it seems that the super-earths/mini-Neptunes are really the same as the Neptunes, but just with more eroded gas envelopes, there is a continuum in size (both R and mass) and density, as also shown by http://oklo.org/2013/11/16/all-over-the-map/.

    The real question then becomes whether the ones that have become ‘secondary’ terrestrial through envelope erosion (the real super-earths) will be suitable as habitable planets. In other words, will they have primordial atmospheres, surface water, plate tectonics, magnetic fields, etc., like true terrestrial planets?

  • Mike Lockmoore January 10, 2014, 13:45

    Wojciech J: Yes, it’s an interesting idea, but I was very disappointed in Baxter’s _Flood_ considering how big a fan I am of his earlier works. The “water in the mantle” idea and flooding trigger is only given some hand-wavey justification which I thought Baxter could have handled much better. I also found the characters in this novel flat, unbelievable, and/or unappealing.

    This book came up in an earlier post in Centauri Dreams post. I added an expanded form of this critique there (can’t seem to find that posting now). Other readers had a more favorable view of _Flood_ or its sequel.

  • ljk January 11, 2014, 0:57

    SF State astronomers discover new planet in Pisces Constellation

    SAN FRANCISCO, Jan. 9, 2014 — A team led by SF State astronomer Stephen Kane has discovered a new giant planet located in a star system within the Pisces constellation. The planet, perhaps twice the mass of Jupiter, could help researchers learn more about how extrasolar planets are formed.

    The star system harboring the new planet contains only one star, as do the other three systems with extrasolar planets analyzed by Kane, an assistant professor of physics and astronomy, and his colleagues. It is a surprising finding, given the high rate of multiple-star systems in our solar neighborhood.

    “There is a great interest in these stars that are known to host planets,” Kane explained, since astronomers suspect that planet formation in a multi-star system would be very different from planet formation in a single-star system like our own. Kane presented his findings today at the annual conference of the American Astronomical Society.

    A multiple-star system “might have not one but two planetary disks” where planets form, he said. “Or it could be that having an extra star would be disruptive, and its gravity could cause any protoplanets to pull apart.”
    Relatively few extrasolar planets have been found in multiple-star systems, “but we know that they are there,” Kane said.”

    In the four systems studied by the researchers, using optical imaging data collected at the Gemini North observatory in Hawaii, there were some intriguing signs that perhaps a second star — or something else — was present.

    Full article here:


  • Ashley Baldwin January 11, 2014, 15:52

    Dear Ross. I wouldn’t necessarily worry about Super Earths having sure gravitates. Gravity is dependent. On must mass, with density of a planet and distance from centre of gravity forming part of a complex equation forming many factors. Even a large mass planet will have bearable gravity if low in density and a large radius. The human body can certainly stand 1.5 G for long periods its only at 2g and above we run into problems. The book “Human Survival: Life and death in extreme environments ” covers this issue amongst others and is a popular read for those interested in interstellar travel where G force us like
    Y to be an issue, both low and high. A good read and cheaply available from Amazon ( whose staff work in extreme environments if you believe the news! )

  • Brett Bellmore January 13, 2014, 7:36

    “I guess Im just stupid or something, but why dont all scientists look for habitable planets around stars that are closer to our own sun? ”

    They’re looking for the keys under the street lamp, essentially. Maybe once we’ve got a giant orbital telescope with a Km aperture we’ll be able to spot any kind of planet we want, anywhere, but right now it takes the right conditions to detect a planet.

  • Michael January 13, 2014, 10:42

    For one thing there are many more smaller dimmer stars out there.


  • Tom Mazanec February 15, 2014, 19:18

    For an interesting design of a life bearing superEarth, see Lyr on the planetopia site. There are other interesting planets depicted there as well.

  • Tom Mazanec February 15, 2014, 19:22

    Correction: Planetocopia site