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Exomoons: A Fine Line for Habitability?

Public interest in habitable moons around gas giant planets received a powerful boost from the film Avatar, where a huge world in an Earth-like orbit (Polyphemus) is accompanied by the extraordinary moon Pandora. We have no detections of such moons — exomoons — but as we’ve seen in earlier posts here, David Kipping (Harvard-Smithsonian Center for Astrophysics) continues the hunt through the HEK project (Hunt for Exomoons with Kepler). HEK looks for transit timing variations (TTV) and transit duration variations (TDV), the kind of perturbations that a substantial satellite would create in the orbital motion of the larger world around its star.

While we wait for the first exomoon discovery — a moon down to about 0.2 Earth masses should be detectable with these methods — we’ve just gotten a look at exomoon issues from a new study of magnetic fields around giant planets. The work of René Heller (McMaster University) and Jorge Zuluaga (University of Antioquia, Colombia) finds that merely being in the habitable zone is hardly sufficient protection for any lifeforms that might develop on such moons. Not only are there issues relating to tidal heating and the transport of energy in the moon’s atmosphere, but the magnetic environment in which these moons would move could be a show-stopper.

Heller and Zuluaga’s new paper calculates the size of the magnetospheres of giant planets located in the habitable zone of their host stars. Being inside a planet’s magnetosphere can shield a moon’s surface from high-energy cosmic rays and the effects of the stellar wind from its star, but particles trapped in the magnetosphere itself can create their own problems. The paper notes that the net effect on a moon’s habitability depends on its actual orbit, the extent of the planet’s magnetosphere, and other factors like the intensity of the stellar wind from the star.


Image: An artist’s concept of the Saturnian plasma sheet based on data from Cassini’s magnetospheric imaging instrument. Credit: NASA/JPL.

The researchers have pooled information about the formation and development of magnetic fields in both terrestrial and giant planets and use it to predict the intensity of those fields, based on models developed by Jonathan Fortney and his collaborators at the University of California. They chose to exclude planets around M dwarf stars because of flare activity and excluded G-class stars as being too bright and too massive to allow for exomoon detections in the near future. The compromise was to work with K-class dwarf stars of about 0.7 solar masses.

The work then considers Neptune-, Saturn- and Jupiter-class host planets. As to what may be detected in the ongoing exomoon hunt, the paper argues that moons roughly the mass and size of Mars are likely to exist and should prove detectable around K stars in the near future.

Having determined the scope of a magnetosphere, the other factor that comes into play is the distance of the exomoon from its host planet. Working with Rory Barnes (University of Washington), Heller has previously studied the minimum distance a moon could orbit while sustaining habitability despite the effects of tidal heating (see Assessing Exomoon Habitability for more on this recent work). Get too close to the planet — Barnes and Heller called this moving inside the ‘habitable edge’ — and runaway greenhouse effects can emerge. There is, in other words, a minimum distance an exomoon has to maintain from its planet to remain habitable.

But does the minimum distance conflict with the magnetosphere?

From the paper:

For modest eccentricities, we find that satellites around Neptune-sized planets in the center of the HZ around K dwarf stars will either be in an RG [runaway greenhouse] state and not be habitable, or they will be in wide orbits where they will not be affected by the planetary magnetosphere. Saturn-like planets have stronger fields, and Jupiter-like planets could coat close-in habitable moons soon after formation. Moons at distances between about 5 and 20 planetary radii from a giant planet can be habitable from an illumination and tidal heating point of view, but still the planetary magnetosphere would critically influence their habitability.

Perhaps the odds on finding a Pandora out there, around a Jupiter- or Saturn-class world, are not as good as we might hope. If far enough from its host planet to avoid runaway greenhouse issues and the disrupting effects of tidal heating, the exomoon could outrun the magnetic shielding of the parent world, exposing it to stellar and cosmic high-energy radiation. But Heller and Zuluaga acknowledge that the planet’s composition has much to say about conditions on any moon. The paper goes on to point to the direction of their future work on the subject:

Once a potentially habitable exomoon would be discovered, detailed interior models for the satellite’s behavior under tidal stresses would need to be explored. In a forthcoming study, we will examine the evolution of planetary dipole fields, and we will apply our methods to planets and candidates from the Kepler sample. Obviously, a range of giant planets resides in their stellar HZs, and these planets need to be prioritized for follow-up search on the potential of their moons to be habitable.

The paper is Heller and Zuluaga, “Magnetic shielding of exomoons beyond the circumplanetary habitable edge,” accepted at the Astrophysical Journal (preprint).


Comments on this entry are closed.

  • Christopher L. Bennett September 10, 2013, 11:32

    I think we’re too quick to assume that radiation is a dealbreaker for life. We know there are radiation-resistant bacteria like Deinococcus radiodurans. That suggests that the reason most Earthly life is vulnerable to radiation is simply because radiation exposure is uncommon enough that life here hasn’t needed to evolve greater resistance. On an exoplanet or exomoon with higher radiation levels, it seems likely to me that life would evolve mechanisms to withstand them.

  • Mike Lockmoore September 10, 2013, 12:24

    Don’t forget Yavin 4 as another habitable moon around a giant gas planet depicted in popular cinema: Star Wars Episode IV – A New Hope, in this case. Yavin 4 hosted the Rebel Alliance’s base.

  • David Cummings September 10, 2013, 12:56

    HEK sounds great but it’s too bad Webb isn’t taking over from where Hubble left off. I know the JWST is going to be doing other great things but I really think it’s important to maximize our search for planets and moons. Nothing will increase public awareness of and spending on interplanetary and interstellar technologies like finding signs of life on other worlds.

  • Anthony Mugan September 10, 2013, 13:26

    It will be interesting to see the size range of exomoons as they start to turn up. Possibly sufficiently large ones, with the right composition might have their own magnetic field?

  • Rob Henry September 10, 2013, 18:05

    Is it just me, or is this entire analysis misdirected. Habitable exomoons are NOT habitable exoplanets, and I would expect their parameters of their HZ to be completely different. My naive guess would be as follows.

    For the equivalent distance, moons will have a far higher inventory of volatiles and, and tend to have higher levels of geological activity than planets. Worlds with Io’s level of activity but Mars’ mass would retain sufficient portions of gas to replenish a descent atmosphere against loss, and for radiation in its atmosphere to produce high energy chemicals, that could rain down on the ocean below. We would want the ice crust above this ocean to have large holes from volcanic heat plumes that allows those high energy chemicals through, and provides additional potential for photosynthesis. I would not want them to freeze over too quickly, or be too small a size, but yes, these worlds will be susceptible to the runaway greenhouse effect.

    For the above reasons, I suspect that the equivalent HZ for exomoons may well prove to only start at Mars’ distance, but extend well past that that of Ceres. Anyhow, I have difficulty taking the thought seriously that it will prove identical to the HZ of planets.

  • Doug M. September 11, 2013, 3:08

    @Christopher, I suspect the issue is not radiation per se but rather its effect on the atmosphere. It’s believed that this is why Ganymede has no atmosphere despite being a bit more massive than Titan (which does): charged particles from Jupiter’s belts spalled its atmosphere away.

    Incidentally, I see at least one soft spot in this paper: they assume rapid photodissociation of atmospheric water vapor, but a K star would generate far less photodissociating UV. It’s hard UV — under 250 nm — that does the heavy lifting. And even an early K would put out an order of magnitude less hard UV than the Sun does. It’s a minor point, but it makes me say “hmm”.

    Doug M.

  • Doug M. September 11, 2013, 3:14

    Also: we don’t really know enough about magnetic fields to be able to say that a large moon wouldn’t have one. Mercury has half the mass of Mars and rotates only once every 56 days, but it has a perfectly respectable magnetic field. On the other hand, Venus has no magnetic field; on the other-other hand, there’s reason to believe Mars used to have a pretty good one, but lost it. It’s a complex topic that is still imperfectly understood.

    Doug M.

  • Tony Mach September 11, 2013, 4:14

    One stumbles upon this term sometimes, what does it actually mean for an planetary body to be “in an RG [runaway greenhouse] state”?

    Considering that the influx of energy is (more or less) stable, any process that lowers the outflux of energy will only raise temperature up to a point, when a new equilibrium is reached (as increased temperature leads to increased outflux). To get an true runaway climate, one would need to lower the outflux to zero, something that is only possible at absolute zero – a state that will be quickly left behind in the case of influx of energy.

    I can only assume that by saying “runaway” scientists actually mean “reaching a higher equilibrium temperature”. So to me the term “runaway greenhouse” is nonsensical, sensationalistic, misleading and anti-scientific, and the usage of such terms reflects badly on any scientists using such terms uncritically.

  • Doug M. September 11, 2013, 10:34

    it’s shorter than “post-runaway greenhouse”.

    we know that Venus used to be much cooler and wetter, because of the D-H ratio. at some point, it went through a very extreme transformation, which is now irreversible by natural processes.

    the term has gained universal acceptance in the scientific community. so, it’s kinda hard to see how it’s “anti-scientific”.

    Doug M.

  • Ronald September 11, 2013, 10:55

    Apart from the problem of tidal heating and resulting RG, if a moon is orbiting too close to its mother planet, there is the, at least as important, issue of the planetary radiation belt. The authors do mention this, however, but I did not see it mentioned in the post above. Or is that sort of included in the RG?
    If I understood matters correctly, this means that too small subgiant planets super-earths, mini-Neptunes, Neptunes), apart from seldom having large enough moons, will also have too narrow magnetospheres.

    Since these categories of planets seem to be by far the most common. this does not seem to bode well for habitable exomoons. Pity, there go Endor, Yavin 4, Pandora, …

    But there is still hope for very large giant planets in the HZ of solartype stars.
    An example of this is 16 Cygni Bb, which is estimated at about 2.4 Mj and within its star’s HZ (well, part of its orbit, which is very eccentric).

    Other examples are (with minimum masses given): 47 Ursae Majoris b (2.5 Mj), 55 Cancri f (0.14 Mj), Upsilon Andromedae d (3.8 Mj), HD 28185 b (5.7 Mj), and maybe (barely) HD 37124 c (0.6 Mj).

  • FrankH September 12, 2013, 15:59

    A moon with the mass of Mars (at the limit of a non-capture moon mass) and the density of Ganymede (1.9gm/cm, typical of a large icy moon) might retain a Co2 atmosphere at 270K for a few billion years under the most benign conditions… but with that low a density, it will loose its inventory of typical volatiles (water, nitrogen, ammonia, methane) in short order, losing mass in the process and accelerating the loss of the other volatiles.
    If the Exomoon is much denser (like Mars) it may retain a Co2 atmosphere, but the effects of the host star and the host planet will still cause it to lose much of its atmosphere.

    Big moons around exoplanets is a sexy subject, but you can only put so much lipstick on a pig.

  • Doug M. September 13, 2013, 6:43

    1) Why would you assume Mars is the limit of a non-capture moon mass?

    2) A moon at 270K won’t be icy, pretty much by definition. (Unless it formed further out and migrated in, and even then it’s dubious.)

    3) Putting aside the issue of spalling loss from interaction with a planetary particle field and/or losses from the lack of a planetary magnetic field, a Mars-sized world at 270K should have no problem hanging on to a 1 bar atmosphere over astronomical time. That’s why we’re spending a couple of hundred million dollars to send MAVEN to Mars next year… Mars could have a respectable atmosphere, and we’re still not quite sure why it doesn’t.

    There are still a lot of open questions about exomoons. But right now, Earth-sized moons with shirtsleeve environments are still firmly within the error bars.

    Doug M.

  • FrankH September 13, 2013, 20:13


    #1 :
    The Canup and Ward paper:
    “A common mass scaling for satellite systems of gaseous planets”

    Puts a limit to the maximum size of a gas giant’s moon at around a Mars mass. The Heller and Zuluaga paper (in this post) even references it.The Canup and Ward paper is widely cited; the limits to a moon mass seem reasonable and match the moon systems of our gas giants.

    #2 – That’s my point. A Mars mass icy moon that forms past the snow line won’t stay a Mars mass moon for long at 270K.

    #3 – Mars is close to the Jean’s limit for Co2 loss; that’s without other means like hydrodynamic loss, external heating or mechanical losses (solar wind ablation, asteroid impacts). It’s escape velocity is very low compared to the escape velocity of common atmospheric gases, so a little energy (impacts, solar wind ablation) and the gas will go. There’s no solid data showing that it ever had a thick, warm or wet atmosphere; in fact the data points to a cold, dry world that lost its thick atmosphere early on (if it even had a thick atmosphere to begin with).

    The only reasonable way you’re going to get an Earth-sized moon with a shirtsleeve environment is if the gas giant captures an Earth-like planet on its way into the inner system. That’s extremely unlikely.

  • Rob Henry September 15, 2013, 0:11

    FrankH writes “Mars is close to the Jean’s limit for Co2 loss”

    Let’s just put that comment in context. The Jeans parameter for CO2 on the 210K surface of Mars is four times that of CH4 on the 94K surface of Titan. Thus Doug M is the more correct with “Mars could have a respectable atmosphere, and we’re still not quite sure why it doesn’t”

  • Doug M. September 16, 2013, 5:15

    um… Canup and Ward are saying that a *Jupiter mass* Jovian wouldn’t have a moon larger than Mars mass. But we know there are lots of joivans out there with more mass than Jupiter.

    Current thinking is that the total mass of a Jovian’s moon system will scale with the mass of the primary (and Canup and Ward affirm this). So, a ten-Jupiter-mass Jovian should have enough enough mass in its moon system to make half a dozen Marses.

    Doug M.

  • Doug M. September 16, 2013, 5:22

    “There’s no solid data showing that it ever had a thick, warm or wet atmosphere”

    actually, yes there is. The atmospheric isotope ratios reported by MSL/Curiosity in the last year show significant enrichment of O18, C13 and Ar40. That makes it pretty much certain that Mars had a thick atmosphere at some point in its history. “How thick” and “when” and “for how long” are still very open questions, but “was there” is no longer an issue.

    Doug M.

  • FrankH September 16, 2013, 15:46

    Doug – I misspoke; I was referring to the “thick, warm, wet” and life hospitable Mars and conflated it with a thick(er) initial atmosphere. Mars did lose it’s thick (where thick is undefined but >> than current) atmosphere early on; the Webster et al MSL paper even says as much:

    “Modeling estimates of escape processes and atmospheric stability during Mars’ initial history point to catastrophic loss of atmospheric mass, and suggest that many atmospheric species carrying records of early isotopic evolution did not survive beyond approximately 3.7 to 4 Ga ”

    Mars has probably been a cold, mostly dry – except for geologically short periods – and mostly airless world almost from the beginning.

    Canup and Ward calculate that the largest moon would be about 10-4 * mass of the planet. To get an Earth mass moon, you would need a gas giant of about 30 Jupiter masses; this is already well into the brown dwarf mass range.
    A 13J mass gas giant/baby brown dwarf might have a moon about half the mass of Mars, if the ratio holds.

    The environment and rules for planetary formation around brown dwarf stars may be different (the Canup and Ward paper doesn’t cover BD masses) , but if the ratio holds, it doesn’t preclude Earth mass (or larger) worlds. One can only hope most of that mass is rock.

    @Rob Henry – Titan is at the triple point for methane; what isn’t liquid or ice gets disassociated or lost from the upper atmosphere – same for ammonia. Neither gas is stable at Titan and must be replenished.

  • Rob Henry September 16, 2013, 17:45

    @FrankH. The subject in hand is atmospheres on moons of Jovian planets. The way and place they form attracts a much higher proportion of volatiles than terrestrial planets have, so your noting how easily replaced these are in Titan’s atmosphere rather underscores my point. For warmer moons, those replacing volatiles will be ammonia and water, not methane. Released ammonia will quickly photolyse to dinitrogen, but how quickly this will happen for H2O is more complex, given that an O2 atmosphere tends to form an ozone layer atop a cold trap.

    So… we are back where we started and, by analogy, we cant question the likelihood of a massive exomoon atmosphere by counterexamples of our own system. In fact, a Mars spewing out volatiles from its interior at an exomoon rate begins to look as if it already has twice the escape velocity it needs for the gas CO2 (I’m just illustrating the point). That Jeans parameter difference of a factor of four is still a killer.

  • Doug M. September 17, 2013, 4:31

    “Canup and Ward calculate that the largest moon would be about 10-4 * mass of the planet. To get an Earth mass moon, you would need a gas giant of about 30 Jupiter masses”

    Mars is about 0.11 Earth masses. Jupiter’s Galilean moons range from about 0.23 Mars masses (Ganymede) to 0.075 Mars masses (Europa); their total mass is about 0.6 Mars masses, or about 0.065 Earth masses.

    Scale everything up by five , and you have a 5J mass Jovian that’s far short of a brown dwarf. Ganymede is now bigger than Mars by about a fifth. Alternately, if all the large moons are combined in one, as with Saturn/Titan, you have a moon that’s around three Mars masses or about a third of Earth.

    Scale everything up by ten, and you’re still not in brown dwarf territory. You now have three moons that are comfortably bigger than Mars, or — if you go the Titan route — one supermoon that’s about six Marses or 2/3 an Earth mass.

    Does this actually happen? We don’t know! The models suggest it could. The error bars are still really large.

    I guess I’m just reacting to the “lipstick on a pig” thing. That’s just an overstatement. There’s no compelling reason to think that >Mars mass exomoons with atmospheres don’t exist. Even the models are still being worked out.

    Doug M.

  • Doug M. September 17, 2013, 4:36

    FWIW, my strong suspicion is that we won’t find a lot of rocky Mars-sized worlds, whether planets or moons, with thick atmospheres around 270 degrees. While they’re well within the Jeans limit, the example of Mars suggests that they’re awfully fragile.

    OTOH, I also suspect that there’s an inflection point, somewhere between Mars mass and Earth mass, where suddenly dense atmospheres become a lot more common.

    Doug M.

  • FrankH September 17, 2013, 15:20

    Doug, I just used the 10^-4 ratio from the Canup & Ward paper to find the largest moons for various gas giant masses.

    While you’re reacting to my “lipstick on a pig” comment, I’m reacting to the non-critical thinking that sees an Earth-like Avatar around every gas giant in a habitable zone. Even a marginally viable large moon (which is probably going to be icy/have a low density) is probably going to be on the unlikely-to-be-habitable-or-stable-for long end of the scale.

    If you’re going to look for habitable moons, don’t waste too much time on the lower mass gas giants and focus on the higher mass (5Mj or greater) planets and brown dwarfs. Telescope time is a precious commodity.

    I think the inflection point where a dense(r) atmosphere can be retained at 270K against most onslaughts is around 0.4 – 0.5 Earth mass (or 3 – 5 Mars mass). IIRC this range was in a recent paper. A rocky planet in this mass range also has an escape velocity well above the Jeans escape for all the interesting gases.

  • Rob Henry September 17, 2013, 15:58

    “Ganymede is now bigger than Mars by about a fifth”

    No, but Mercury is four fifths the volume of Ganymede.

  • Doug M. September 18, 2013, 4:14

    @Rob, go back and read for context.

    @Frank, the 10^-4 parameter seems likely but is not yet confirmed. Even if it is, we really don’t know how tight the distribution curve is around that figure. Are moon systems with (say) 2 x 10^-4 masses unheard of? or possible, but very rare? or just a couple of standard deviations out, unusual but not terribly uncommon? We don’t know, and we won’t for a while.

    If I were allocating telescope time, I wouldn’t bother looking for habitable moons yet. Look for any exomoons at all first, and start building up a population. Then we’ll begin to have some idea what the parameters are.

    Doug M.

  • FrankH September 18, 2013, 15:08

    @ Doug – No disagreements with your comments. I think that the number of Earth-like Avatar worlds around gas giants is probably very, very close to 0.

    I think finding exomoons would be helpful in pinning down the moon to planet mass ratio. Also the moons of our gas giants seem to mirror the layout of the majority of known planetary systems (baring the hot Jupiter systems) while our solar system does not. It would be interesting to find out why.

  • Eniac September 18, 2013, 22:55

    I bet we will be as surprised about the exomoons we find as we were about the exoplanets. There will be ones much larger than the Canup & Ward model predicts.

    The point about tidal heating strikes me as silly. If tidal heating warms the place up too much, just move it out a little further from the star, and everything is back in balance.

    A direct effect of radiation on life is equally silly. An atmosphere, ocean, or rocky crust provide more than enough shielding to block the fiercest of radiation environments you can imagine, providing plenty of safe space for life to evolve.

    Finally, loss of atmosphere, whether through radiation or otherwise, can easily be counteracted by making the moon just a little bit bigger. Which in turn can be achieved, as has been observed by others above, by increasing the size of the planet, even if you believe the Canup & Ward model is strictly correct.

  • Rob Henry September 19, 2013, 18:34

    “The point about tidal heating strikes me as silly. If tidal heating warms the place up too much, just move it out a little further from the star, and everything is back in balance.”

    I would go much further than that. To me, it not only moves the HZ out, but also stabilises it. Unlike terrestrial equivalents, they won’t be so vulnerable to ‘snowball earth’ episodes.

  • Doug M. September 20, 2013, 17:44

    Let’s not get carried away. Tidal heating can change dramatically over geological time thanks to orbital evolution and other factors. (We’re pretty sure this has happened in Jupiter’s moons.) So, it may not be such a great thing to have a lot of your heating coming from that direction.

    Doug M.

  • ljk December 20, 2013, 9:40


    Exomoon Candidate Detected

    News of discovered exoplanets seems to be an almost daily occurrence lately. Our methods of detection are consistently increasing and it seems, there is a lot to see beyond the boundaries of our solar system.

    What we haven’t done is detect a moon beyond our solar system. Until now. It seems intuitive that exomoons would exist. There are so many moons in our solar system; of course they would exist elsewhere. But they’ve always been too small to detect.

    David Bennett of the University of Notre Dame, reported earlier this week that he and his colleagues may have discovered the first known exomoon via a process called micro-lensing. What the team saw were two objects passing in front of a star, a large object and then an hour later, a smaller one. In truth there are two possible explanations of what they saw and only one of them would mean they detected a planet moon system.

    The objects in question may consist of a planet roughly four times the mass of Jupiter and a moon roughly half the mass of earth. If this is the case the objects measure at 1800 light years from earth. It’s also possible that the objects are much farther away and consist of a brown dwarf and a planet approximately the size of Neptune, in which case the discovery of first exomoon is still up for grabs.

    If the planet/moon model is correct the excitement doesn’t stop there. The objects seem to be too far from any nearby stars to be orbiting them. This could mean they were ejected from their system, killing any Endor-like paradise that may have previously existed. So long Ewoks.

    Unfortunately this means that this system won’t be passing in front of the star again and our opportunity to observe it again is next to nil. The question of whether or not this was in fact the first exomoon is up in the air, we may never know. But the excitement of discovery still stands. We don’t even have to go to a galaxy far, far away.

    Source: First Exomoon glimpsed – 1800 light years from Earth:


    Full article here: