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Life Beyond the Snow Line

The nice thing about our conventional idea of a habitable zone is that liquid water can exist on the surface. The less helpful part of that definition is that water is more readily available much further out in a planetary system, where it usually shows up as ice. Think in terms of the ‘ice line,’ or the ‘snow line.’ Beyond it is the area around the still-forming star where temperatures are low enough to allow hydrogen compounds to condense into ice grains.

Of course, we’re living proof of the fact that planets in the inner system can be covered with oceans. It’s therefore plausible to think in terms of delivery mechanisms, with icy comets bombarding planets in the inner system to produce oceans like those on Earth. But we’re learning to extend our reach beyond conventional habitable zone notions to places much further out, an idea recently given credence by divers hands.

Consider the work of Scott Gaudi (Ohio State), Eric Gaidos (University of Hawaii) and Sara Seager (MIT), familiar names to long-time Centauri Dreams readers. Recognizing the wealth of water resources in outer solar systems, the trio look to cold super-Earths, planets whose water did not have to be delivered by external means. An internal heat source might keep a liquid water ocean viable under the ice, assuming a massive world in the right place, even if that planet were five times farther out than the Earth.

“It turns out that if super-Earths are young enough, massive enough, or have a thick atmosphere, they could have liquid water under the ice or even on the surface,” Gaudi said. “And we will almost certainly be able to detect these habitable planets if they exist.”

By ‘massive enough,’ Gaidos is talking about a super-Earth ten times as massive as the Earth. The scientist reported these results at the American Geophysical Union meeting in San Francisco on Monday. The issue of detection seems clear enough — we’re making such strides in finding exoplanets that tracking down new super-Earths is more or less a given, especially since some are saying that a third of all solar systems probably contain them.

Right now, gravitational microlensing seems to be the best method for detecting planets at 5 AU or more. The planetary signature is found in changes to the magnification caused when a star passes in front of a more distant one as seen from Earth. A planet around the nearer star creates a secondary boost in the lens-like magnification, allowing not just detections at some distance from the star, but also detections around stars much farther away than would be feasible using radial velocity or transit methods. Even so, recent direct imaging successes remind us that the next generation of telescopes may also deliver many a super-Earth.

Proving the astrobiological case for these super-Earths is a tricky matter indeed. It may well take a dedicated lander on the surface of Europa, for example, to tell us about possible life there by drilling into the ice. How do we resolve the question of life on a distant super-Earth? The issue will remain open for years to come, but in short order we’re going to be finding so many of these interesting worlds that we’ll have plenty to speculate about when it comes to life’s formation around other stars.

Comments on this entry are closed.

  • Adam December 18, 2008, 3:19

    Hi Paul

    Wonder what borderline should be drawn between Super-Earth, Ocean Planet and Ice Giant? Only takes a bit more water than Earth to flood the continents since the average elevation is just 850 metres. I suppose a Super-Earth still has land, and Ocean Planet is 10-50% high-pressure water-ice, while an Ice Giant is ~10% hydrogen/helium. Or something like that. A planet a bit cooler than Neptune would probably have a liquid water ocean beneath the hydrogen/helium, much like David Stevenson’s Interstellar Planets, though more massive. Several cosmogonic theories have several Neptune-class planets cast free of the Sun by mutual scattering. If Patryk Lykawa is right a 0.3-0.7 Earth mass Ice Planet is not too far out from the Sun past the Kuiper Belt. Wonder if it has an ocean?

  • James M. Essig December 18, 2008, 14:59

    Hi Paul and Adam;

    Given that the plausable range of possible morphological distributions in the elemental and molecular constituents of extra-solar planets seems to keep expanding, I am encourged that perhaps there will be devised many specific constuction techniques that permit we humans to produce habitats on these planets, either on the surface and/or in sub-surface enviroments.

    I can imagine that even planets or objects in the Oort Cloud with a mass similar to that of Earth or larger, might make for sub-surface habitats that are powered by residual internal heat generated by natural radioactivity, or perhaps by nuclear fission and/or nuclear fusion reactors.

    For planets that have a large thick frozen water ice and/or methane, hydrogen, and/or helium layer, potentially extraordinarilly long lived civilizations could flourish under the surface of such planets.

    The Earth recieves about the equivalent of 31,000 tons of matter converted into energy each year. Human civilization’s energy output per year is only the equivalent of about 3 metric tons of matter converted into energy per year or about 400 metric tons of hydrogen fused to helium per year. Thus, assumming that heat loss prevention mechanisms such as thermal and electromagnetic insulation systems could be developed in these sub-surface regions and that some such icy worlds have a ice sheet with a total mass of 4 x 10 EXP 20 metric tons, a civilization could power it self for 10 EXP 18 years. Even if the civilization required 4 million metric tons of fusion fuel per year, it could still power itself for 10 EXP 14 years.

    I am truely intrigued by the the growing agknowledged range in the possible compositions of extra solar planets. Exoplanetary; geology, hydrology, and atmospheric science is a rapidly evolving field that will no doubt continue to evolve with ever greater precision as we develope ever faster and more capable supercomputers.



  • Didac December 18, 2008, 16:20

    Adam, you do not really need to go as far as Kuiper Belt to find oceans in “ice planets”. Europa or Enceladus are probably good examples of ice planets with subsurface oceans. Anyway, extrasolar planet research shows that Sub-Neptunes and Super-Earths really exists. The gap between Giant Planets (Jupiter, Saturn, Neptune or Uranus) and medium-sized planets (Earth or Venus), so strikingly in our Solar System, may not be a general case in other star systems.

  • Adam December 18, 2008, 18:30

    Hi Didac

    Most large ice/rock bodies are likely to have sub-crustal oceans according to current modelling – Europa & Enceladus are just the most blatant. What I was really trying to discuss was how to distinguish them in compositional terms. A Super-Earth is likely to be wetter than Earth – if so, when is it an Ocean Planet? And what makes an Ocean Planet different to an “Ice” Giant? As we currently understand Neptune, and high-pressure phases of water, there is probably no ocean of liquid water, just a hydrogen/helium rich super-critical fluid “ionic ocean” overlying hot ice (~1000-2000 K) and the silicate core.

    Ocean Planets, as currently theorised, have thin liquid oceans overlying thick layers of high-pressure, but relatively low temperature (<600 K), mantles of ice, in turn covering the silicate/metal core.

    Patryk Lykawa’s hypothetical object is interesting because it might be a small Ocean Planet rather than an Ice-Moon. I propose the frozen version of an Ocean Planet has a layer of condensed atmosphere overlying the frozen ocean itself, while an Ice-Moon is too small to have ever retained an atmosphere to freeze-out.

    Triton, Eris and Pluto upset that nice dichotomy by having extensive nitrogen/methane ephemeral atmospheres that freeze out approaching aphelion. If any of them warmed up those atmospheres would escape rapidly by hydrodynamic outflow, not just the slow Jeans escape they currently experience.

  • andy December 18, 2008, 19:30

    Regarding the known cold super-Earth OGLE-2005-BLG-390Lb, here’s a study of whether this planet could plausibly retain an ocean. The conclusion reached is that while it is likely that the planet is probably now entirely frozen, it could have retained an ocean for about 5 Gyr after it formed.

    (Interestingly enough this planet is located in the galactic bulge which has an estimated age of ~10 Gyr. I’d guess that a subterranean ocean would be fairly well insulated against the kind of supernova nastiness that goes on in the galactic centre too…)

  • Christopher L. Bennett December 18, 2008, 23:31

    Okay, so here you’re talking about outer super-Earths possibly being habitable under their ice crusts, and a couple of days earlier you were talking about outer super-Earths possibly becoming habitable in an Earthlike sense once their stars swell into red giants. Could both be true of the same planet? Could there be superterrestrial planets out there where life initially evolves beneath the ice, and then comes onto land once the star swells and the planet warms? (I’m assuming that either the ocean wasn’t planetwide to begin with or enough of it vaporizes/outgasses to leave solid ground.)

    If so, that would mean that there’s more prospect than we thought for complex life around hotter F- and maybe A-type stars with short main-sequence lifespans. Maybe they wouldn’t last long enough on the MS for complex life to arise, but if life can thrive during the red giant phase, that would create a longer window. That depends, of course, on how long the red giant phase lasts compared to the MS phase. Which is something I don’t know offhand, because I never thought the red giant phase was exobiologically relevant.

  • philw1776 December 19, 2008, 16:34

    Red Giant phase timeframe is a small fraction of MS phase timeframe

  • Adam December 19, 2008, 20:10

    Hi Chris

    The Red Giant Phase is pretty rapid compared to the Main Sequence, but it comes in distinct phases – the Redwards Traverse is a pretty steady situation (in Boothroyd and Sackmann it lasted ~1 Gyr and luminosity went from 2.3 to 2.7 times present), then there’s the ascent to the Red Giant tip, during which luminosity jumps 1000-fold, but much of the rise (x100) is in the last ~50 Myr.

    If an Ice-Moon like Europa survives the heat burst, then it has about 100 Myr of steady light during the Helium Main Sequence. Unfortunately it probably won’t have any ice or ocean left. During the Rise to the RGB tip there’s about 10 Myr when Europa would defrost and be quite pleasant.

  • andy December 20, 2008, 8:48

    Another question is what the lower mass limit for ocean planets is: could any of the ice moons in our solar system actually retain enough atmosphere during the red giant stage to support surface oceans? Worlds like Europa have uncomfortably low escape velocities.

  • Ron S December 20, 2008, 14:54

    Adam said: “Only takes a bit more water than Earth to flood the continents since the average elevation is just 850 metres.”

    I did a quick calculation with a calculator to understand this better. Let’s assume a slightly more massive Earth-like planet with 1.1x Earth’s mass, with the same proportions of constituent material and about the same density. Such a planet would have a surface area ~1.065 Earths. Further assume that the water mass, which is also 10% greater than on Earth, like on Earth, tends to rise to the surface during planet formation, and so is now mostly found in the oceans, fresh water and mineralized in the crust. That is, the water is almost all at or very near the surface.

    The total water depth is therefore about 1.1/1.065 x ~2700 meters (Earth’s average water depth if distributed evenly, or about ~4100 meters. This is more than the 850 meters amount Adam mentions.

    This is interesting if it really says something about water depth on smaller and larger Earth-like planets: shallower on smaller worlds (e.g. Mars) and deeper on super-Earths. I’m no expert on planet formation so could someone tell me how I’m wrong if I am wrong?

  • Adam December 21, 2008, 18:09

    Hi Ron S

    If water is outgassed from a late veneer of impactor material then it’s related to the amount of geothermal energy the planet has from formation. But it really depends, stochastically, on the percentage of water in the late veneer which in turn depends on the arrival of impactors from further out in the forming star system. Systems without larger planets further out to excite the orbits of potential impactors generally form inner planets with lower amounts of water.

    Another variable is just how much water is already present in Earth’s mantle. No one really knows and it could be up to 10 times Earth’s surface ocean. That’s considered unlikely, but it hasn’t been ruled out AFAIK, so Earth could really be quite a wet planet – but even that vast amount is only 0.2% of Earth’s mass. Imagine the ocean if it was ~1% or so.

    But for planets of the same composition, as a general guesstimate, the internal energy of formation goes up with the mass squared, while the radius (and thus area) is a bit more complicated. Compression means the density rises as the mass rises, thus instead of a 1/3 power law, the radius scales according to ~0.26-0.27 for up to 10 Earth masses, and is about ~0.3 for between 0.01 and 1 Earth mass. Earth – and Venus’s – uncompressed density is about 4.1 versus Earth’s current 5.5148. Compared to the Moon’s 3.3 and Mars’s 3.7 uncompressed densities it means Earth has quite a bit more iron/nickel. Mercury’s is a whopping 5.3 (compressed 5.43) meaning it is nearly 2/3 iron core versus Earth’s 1/3.

    But – after factoring all that in – your final conclusion, that smaller worlds are drier, and larger ones are wetter, is pretty accurate. There’s just enough chance variation to make things interesting.

  • Ron S December 22, 2008, 16:59

    Adam, many thanks for that nicely detailed reply. It’s very helpful.

    On the topic of the mantle, I thought it was fairly well established that water is only present in negligible amounts, as determined from studies of xenoliths. Also, from models, light elements made an early exit from the depths during planet formation. I don’t know the credibility of the sources I quickly glanced at, but while oxygen is abundant in the mantle, hydrogen is absent except in trace amounts.

    As to post-formation bombardment, is there reason to believe that the added mass contained more water (by %) than that which accreted in the earlier formation phase? I know that there’s lots of water in KBO/comets now, but would that have been true 4 to 5 Gyr ago? That is, would the dynamics of planet formation tend to select against water accretion further back in history?

  • Adam December 22, 2008, 17:58

    Hi Ron S

    You said…
    As to post-formation bombardment, is there reason to believe that the added mass contained more water (by %) than that which accreted in the earlier formation phase? I know that there’s lots of water in KBO/comets now, but would that have been true 4 to 5 Gyr ago? That is, would the dynamics of planet formation tend to select against water accretion further back in history?

    …the late added mass came from further out in the nebula and thus had formed from wetter material according to several models. The vast bulk of material formed in the hotter inner planet region, but once the gas giants began moving around the outer asteroids (or inner comets, since many seem to be cometoids now) were stirred up into orbits that cross the inner system. Jupiter seems to have had a major role to play in this process and modelling of formation without a Jupiter results in dry inner planets.

    As for the KBOs and the Oort Cloud they also seem to derive from planetesimals in the feeding zones of the forming outer planets, which were thrown into their new orbits as the big planets migrated. Oddly enough the Oort Cloud objects came from a warmer part of the Nebula near Jupiter, while the KBOs were perturbed from near Neptune/Uranus. The best analogues of the Oort cometoids might be the “Main Belt Comets” that have been observed in recent years.

  • spaceman December 23, 2008, 2:31

    I’ve heard difference estimates for the abudance of Super-Earths. Does anyone know what the latest data have to say about how common are super-earths?

  • Ron S December 23, 2008, 11:30

    Adam, another great reply. Thanks.