≡ Menu

Assessing Exomoon Habitability

Yesterday’s post on exomoons and their possibilities as abodes for life leads naturally to new work from René Heller (Leibniz Institute for Astrophysics, Potsdam) and Rory Barnes (University of Washington). We’re finding planets much larger and more massive than Earth in the habitable zone, as the recent findings of the Planet Hunters project attest. What can we say about the habitability of any large moons these planets may have? In their paper, Heller and Barnes look at the issues that separate exomoon habitability from habitability on an exoplanet itself.

heller

If Earth-sized satellites of giant planets exist, they may have certain advantages over terrestrial planets in the same orbit, depending on the host star. We know that M-class dwarfs are by far the most common kind of star in the galaxy, and that habitable zone planets around one of these will probably be tidally locked, with one hemisphere permanently facing the star and the other in permanent darkness. Extreme weather conditions would result, creating severe limitations on the size of any habitable regions. But an Earth-mass exomoon around a gas giant will be locked not to the star but to the planet, a configuration that could stabilize the climate and prevent the dark side atmosphere from freezing out and the bright side atmosphere from evaporating.

Image: Artist’s conception of two extrasolar moons orbiting a giant gaseous planet. (Credits: R. Heller, AIP)

The stability of the atmosphere remains an issue, though, dependent on the size of the moon, the type of atmosphere and the intensity of incoming radiation. The researchers believe a nitrogen-dominated atmosphere would be stripped away by ionizing extreme ultraviolet radiation (EUV) in some habitable zone situations. This is interesting so let me quote the paper:

If Titan would be moved from its roughly 10AU orbit around the Sun to a distance of 1AU (AU being an astronomical unit, i.e. the average distance between the Sun and the Earth), then it would receive about 100 times more EUV radiation, leading to a rapid loss of its atmosphere due to the moon’s smaller mass, compared to the Earth. For an Earth-mass moon at 1AU from the Sun, EUV radiation would need to be less than seven times the Sun’s present-day EUV emission to allow for a long-term stability of a nitrogen atmosphere. CO2 provides substantial cooling of an atmosphere by infrared radiation, thereby counteracting thermal expansion and protecting an atmosphere’s nitrogen inventory.

For that matter, how massive does a moon need to be to create the magnetic shield that can sustain a long-lived atmosphere, and to drive tectonic activity and support a carbon-silicate cycle? The authors peg the figure at somewhere above 0.25 Earth masses, a figure that can be adjusted depending on the moon’s composition and structure. As I mentioned yesterday, the ongoing Hunt for Exomoons with Kepler (HEK) project pegs 0.2 Earth masses as the minimum size for an exomoon detection using current technology.

How large exomoons form is a question we’ve been kicking around here in the comments. If a gas giant is assumed to have formed well beyond the snow line and then migrated to its position in the habitable zone, would it bring with it a massive enough moon to sustain habitability? One recent paper (Canup and Ward, 2006) has shown that moons formed in the circum-planetary disk of a gas giant will have masses in the range of 10-4 times that of the planet’s mass. The paper runs through formation and capture scenarios and notes the work of Takanori Sasaki (Tokyo Institute of Technology), who suggests the formation of Earth-mass moons is indeed possible. We’re left with evolving ideas in what is clearly an active area of research.

Tidal heating is also a key factor in exomoon habitability, capable of causing intense magmatism and resurfacing on the moon’s surface in close orbits and enough volcanic activity to render the moon uninhabitable. But Heller and Barnes see scenarios where a moon becomes habitable only because of tidal heating, such as when the host planet orbits outside the outer edge of the habitable zone. For that matter, tidal heating can drive plate tectonics. Here the authors point to Europa, where tidal effect provides the heat to sustain a subsurface ocean of liquid water.

A theoretical model emerges, estimating the minimum distance a moon can be from its host planet and still allow habitability. The authors call the inner part of the circum-planetary habitable zone the ‘habitable edge.’ Moons inside the edge can run into runaway greenhouse effects because of tidal heating, while those outside the habitable edge are habitable by definition:

Similar to the circumstellar habitable zone of extrasolar planets… we conclude that more massive exomoons may have somewhat wider habitable zones around their host planets – of which the inner boundary is defined by the habitable edge and the outer boundary by Hill stability – than do less massive satellites. In future investigations it will be necessary to include simulations of the moons’ putative atmospheres and their responses to irradiation and tidal heating. Thus, our irradiation plus tidal heating model should be coupled to an energy balance or global climate model to allow for more realistic descriptions of exomoon habitability.

The minimum distance model for a planet to orbit and still be habitable should come in handy if we do find some candidate moons through the HEK project. “This concept will allow future astronomers to evaluate the habitability of extrasolar moons. There is a habitable zone for exomoons, it’s just a little different than the habitable zone for exoplanets,” Barnes said. But getting spectroscopic biosignatures in the atmospheres of any detected exomoons awaits next-generation space telescopes, so for now we’re left to use orbital configurations and studies of exomoon composition to assess potential habitability. One upcoming positive is the 2022 launch of ESA’s Jupiter Icy Moons Explorer mission, which should offer insights into the massive Galilean moons from tidal effects to surface chemistry and useful data on their interiors.

The paper is Heller and Barnes, “Exomoon habitability constrained by illumination and tidal heating,” accepted by Astrobiology (preprint). A news release from the Leibniz Institute for Astrophysics is available.

tzf_img_post

Comments on this entry are closed.

  • Malcolm Ramsay January 11, 2013, 12:21

    Could the habitability problems of an exomoon in the HZ be mitigated by the formation of a gas torus like Titan’s? The atmospheric gases might escape the moon’s gravitation, but be unable to escapes the primary body’s and thus be available for recycling back into the bulk atmosphere. Please note that I have done no research into the feasibility of this idea, I’m just chucking it out there for one of the clever types to play with…

  • d.m.falk January 11, 2013, 12:58

    If past events give any indication, then we should surmise that we should not arbitrarily rule anything out. :)

    (Pic reminds my of the habitable moon of Yavin, from “Star Wars”… ;) …And remember, we’ve now found Tatooine analogues! :D )

    d.m.f.

  • Abelard Lindsey January 11, 2013, 13:27

    The subject of habitable exomoons has come up again.

    The issues are the radiation belts around the gas giant, that the moons are not likely to be larger than Mars and thus limited atmospheres, and the moons will be rotationally locked with the gas giant, meaning that the “day” will be quite long (from say 60 hours up to 2-3 Earth weeks). None of this bodes well for habitability.

  • FrankH January 11, 2013, 13:34

    The Sasaki article mentioned:
    “ORIGIN OF THE DIFFERENT ARCHITECTURES OF THE JOVIAN AND SATURNIAN SATELLITE SYSTEMS”
    Sasaki, T., Stewart, G. R., Ida, S. 2010, ApJ, 714, 1052

    http://sasakitakanori.com/wp-content/uploads/2010/04/apj_2010_satellite.pdf

    Looking at figure 5 mentioned in the Heller and Barnes paper, the upper limit for the most massive moon around a Jupiter-like planet looks like it’s around 4 x 10^-4 (.0004) times the mass of the planet, or about 13% the mass of the Earth and about 30% rock.

    The Sasaki et al. article does a great job of expanding on the Canup and Ward 2006 work by calculating the expected internal composition of the moons as well as their formation, in detail.
    They do a credible job of explaining the differences between the Jovian and Saturnian moon systems.

    “private communication with Takanori Sasaki, formation of Mars- or even Earth-mass moons around giant planets is possible.”

    I’d rather see the details in an article like the one quoted above.

  • andy January 11, 2013, 14:48

    But an Earth-mass exomoon around a gas giant will be locked not to the star but to the planet, a configuration that could stabilize the climate and prevent the dark side atmosphere from freezing out and the bright side atmosphere from evaporating.

    On the other hand, around an M-dwarf that will subject the moon to some quite large stellar tides. The Hill radius of the parent planet will be quite small as well, and the timescale for tidal evolution of the moon quite short.

    In any case M-dwarfs don’t tend to have close-in gas giants (Gliese 876 is one of the freakish exceptions). Moons therefore do not seem to be a particularly good way of improving the habitability of M-dwarfs.

  • GaryChurch January 11, 2013, 15:01

    “how massive does a moon need to be to create the magnetic shield that can sustain a long-lived atmosphere, and to drive tectonic activity and support a carbon-silicate cycle? The authors peg the figure at somewhere above 0.25 Earth masses,-”

    The possible scenario that arises from the conditions necessary for another Earth is there may be two predominant types of life bearing bodies; Earth size goldilocks rocks with a magnetic field or icy moons with subsurface oceans.

    I am at odds with most space advocates on these forums for a couple reasons- besides my loathing of private space I find Mars to be a place that has zero attraction for me. I strongly believe we should be going to places that might have ocean life. Which means our next space explorers will probably be more like Jacques Cousteau than Neil Armstrong.

    Ceres, the Galilean moons, the moons of Saturn, Uranus, and Neptune all have a higher possibility of complex life than Mars. While Mars may seem like it is “just close enough” for chemical propulsion I am skeptical. The interesting destinations are years away even with atomic bomb propulsion.

  • Peter Chapin January 11, 2013, 19:43

    Setting aside for the moment the question of how likely habitable exomoons might be, I can’t help but wonder what it would be like to live on one. Suppose that, like Jupiter, the host planet had several moons of approximately similar size. How incredible would it be to grow up in a place where another habitable, life-bearing body was only an arm’s length (tentacle’s length?) away?

  • Rob Henry January 11, 2013, 19:47

    Malcolm Ramsay says “Could the habitability problems of an exomoon in the HZ be mitigated by the formation of a gas torus like Titan’s?” and he seems to have it all wrong.

    Correct, it is possible for such a gas torus to form, but it is not easy to return a significant proportion of that gas to the moon. Consider this: that ring is trying to dissipate everywhere over a huge surface area held back in one dimension by tidal forces, and unconstrained in the other. So how can we make the *Ramsay scenario* useful? I suggest the following minimum requirements

    1) Tidal force of the primary must be at least comparable to the gas velocity over the diameter where the torus is a sizable fraction of its maximum density.
    2) The moon must still be outside its Roche limit.
    3) Only the gravitational field of the moon can prevent dissipation in the direction normal to the plane of the torus, so that moon should be as massive as possible.

    If I have time, I will look at this problem further, and will let you know if I find anything interesting.

  • Rob Henry January 11, 2013, 20:20

    Gary Church writes “The possible scenario that arises from the conditions necessary for another Earth is there may be two predominant types of life bearing bodies; Earth size goldilocks rocks with a magnetic field or icy moons with subsurface oceans.”

    And I have two things to say about that. On the positive side, higher life only appeared on Earth after an Europa-like snowball Earth episode, and pretty soon after too. On the negative side, a hydrothermal driven biosphere in Europa should be 100 million times less powerful than ours.

    Thus, to me, that second HZ very much needs to include zones where surface ice is either melted through in places by volcanoes or sufficient insolation for photosynthesis, or it is so soft and thin that biogeochemical cycles can rapidly transport uv generated high energy chemicals across it to those subsurface oceans.

  • amphiox January 11, 2013, 20:42

    A few thoughts:

    Giant planets are likely to have their own fairly powerful magnetic fields. What effect would that have on an exomoon’s atmosphere? Could the planet’s magnetic field protect a moon’s atmosphere even if the moon itself does not have one.

    Volatiles escaping an exomoon’s atmosphere would still be subject to the giant planet’s gravity well. So is it possible that volatiles escaping from an exomoon could end up trapped by the planet’s gravity in an orbital region that is regularly crossed by the exomoon, such that the exomoon gets its supply of lost volatiles regularly replenished? What about a situation with two or more exomoons, wherein the volatiles lost by one ultimately get transferred to another?

    Could an exomoon retain sufficient volatiles to keep an atmosphere and surface water via this constant replenishment mechanism even if it is itself too small to gravitationally hang on to its original atmosphere?

    And as for the question of a captured large exomoon. In our own solar system at least one such large moon (Triton) is captured. Even if it is a very rare phenomenon, giant planets typically have lots of moons, and it only takes one single capture event in such a solar system to produce a large habitable exomoon.

  • Steven Rappolee January 11, 2013, 23:29

    this paper released yesterday by kipping on exomoon detection by kepler

    http://arxiv.org/pdf/1301.1853.pdf

  • Steven Rappolee January 11, 2013, 23:42

    or should I say lack thereof :):)

  • James Jason Wentworth January 12, 2013, 10:11

    Rings might help the exomoon habitability situation. If a jovian planet possessing an Earth-size moon orbited close to an M dwarf (red dwarf) star, the exomoon would have to orbit quite close to its primary due to gravitational tides from the close-by star. The close-orbiting exomoon might be sufficiently shielded by the jovian planet’s own magnetosphere (thus making a magnetosphere generated by the exomoon itself unnecessary to protect it). Now:

    If the jovian planet also had a ring system (particularly rocky/metallic rings, that close to the red dwarf star), the rings should–as is the case with Saturn–sweep up electrons and nuclei from the star that would ordinarily form strong radiation belts trapped by the planetary magnetic field. If the jovian planet also had other sufficiently large moons in the right orbits, the ring system could be stable despite the stellar tidal forces. This hypothetical, habitable exomoon depends on an admittedly rather large number of factors being “just right,” but we enjoy a no more likely set of circumstances.

  • Abelard Lindsey January 12, 2013, 13:37

    Looking at figure 5 mentioned in the Heller and Barnes paper, the upper limit for the most massive moon around a Jupiter-like planet looks like it’s around 4 x 10^-4 (.0004) times the mass of the planet, or about 13% the mass of the Earth and about 30% rock.

    That’s Mars size, but with a lot less rock. A Mars sized moon with 70% volatiles in the habitable zone will result in those volatiles boiling off and lost to space over 1-2 billion years. This paper suggests such moon are unlikely to be habitable.

  • Steven Rappolee January 12, 2013, 13:46

    Kepler can not detect moons smaller then mars so the Kipping paper above is the beginnings of the process of confirming Canup and Ward

  • Alex Tolley January 12, 2013, 13:58

    There is a misconception that because Earth has life at hydrothermal vents, that planets or moons with these vents will/could support life.

    We do not know the conditions for the genesis of life, thus we do not know if even the simplest life (bacteria) evolved in situ or migrated there. The macrolife that we see at these vents have migrated there from other oceanic ecosystems.

    Therefore even if we find hydrothermal vents (or similar) under the ice of these icy moons, they are most likely to be either sterile, or inhabited by bacteria only. No exotic macrolife.

    Bottom line – water is a necessary, but insufficient condition for life.

  • Abraham Samma January 12, 2013, 14:42

    The issue of the gas giant’s magnetosphere and how it will affect habitability is still an untouched issue as 1 comment mentioned. Also what kind of gas giant would be safe to form at (think jupiter versus Saturn and the rest)?

  • coolstar January 12, 2013, 16:57

    The sentence “For an Earth-mass moon at 1AU from the Sun, EUV radiation would need to be less than seven times the Sun’s present-day EUV emission to allow for a long-term stability of a nitrogen atmosphere. ” just makes no sense to me as the EARTH obviously has a predominately nitrogen atmosphere…..Do you need an ozone layer also to protect the N2? seems unlikely.

  • coolstar January 12, 2013, 17:26

    Can answer my own question about needing an ozone layer to sustain an N2 atmosphere: apparently NO, since the earth had a predominately N2 atmosphere for billions of years before that atmosphere had a substantial amount of O2 or ozone.

  • Rob Henry January 12, 2013, 21:16

    Coolstar, it strikes me that Earth could have lost 20-30% of its primordial N2 inventory, yet this would still be too insignificant to mentioned in planetary modeling. After all, about this order of magnitude (20% of that in the atmosphere) of our current inventory of nitrogen is nitrates in bedrock.

  • GaryChurch January 12, 2013, 23:02

    “Therefore even if we find hydrothermal vents (or similar) under the ice of these icy moons, they are most likely to be either sterile, or inhabited by bacteria only. No exotic macrolife.”

    That would be the best situation for the aquafarming industry. I am hoping for complex life though.

  • Wojciech J January 12, 2013, 23:03

    I find this discussion enjoyable to read.
    Especially the comments that life can’t exist due to various theories created by people arguing this. With implication that we shouldn’t be looking.
    The bottom line is that no matter how elegant the theory and calculations-it will be the empirical proof that will give us the final answer as to the question about life and Earth-like planets.

  • Rob Flores January 13, 2013, 3:10

    Yes, I agree that we might find a somewhat suitable exo-moon as an abode
    for life, But I suspect they will be pretty rare. Ganymede is the largest moon in Our System. It is bit larger than Mercury, though not near
    it’s mass. You would have to merge all the 4 Galilleans sallellites plus the smaller minor satellites of jupiter. to get a moon that comes near the neighborhood of even half earth’s volume. (assuming that this much volume with a density similar to Earth’s would help keep it’s atmosphere from bleeding into space.)
    We have four Gas Giants in our backyard and two have sizeable satellites. We might note, that the Neptune “class” planets do not. (Triton doesn’t count it’s a captured K.B.O.)

  • andy January 13, 2013, 10:11

    @coolstar: well they do also go on to note the protective effects of carbon dioxide on the atmosphere’s nitrogen inventory. If you ignore that part of the sentence then yes it does seem ridiculous. But after all, we don’t have a pure nitrogen atmosphere…

  • Tarmen January 13, 2013, 10:23

    If the Jupiter system were down in the orbit of Mars, we would have 3 ocean moons to study right here. So it seems certain that many Super Jupiter exoplanets would have a clutch of super large moons . And a subset of these super planet-moon systems will be in a goldilocks orbit. Huge deep oceans, terrific volcanism, warm sunlight, stir and wait a billion years for the wigglies. We have to assume some planetary configurations are even more(!) hosptitable to life than our Earth Luna duo at 1AU. Why would ours be the best anywhere?

  • GaryChurch January 13, 2013, 13:39

    “You would have to merge all the 4 Galilleans sallellites plus the smaller minor satellites of jupiter. to get a moon that comes near the neighborhood of even half earth’s volume.”

    Yes, but low gravity makes the ocean volume important; it is not as hostile pressure-wise as Earth’s oceans. I have no numbers but I believe you could scuba dive straight down for miles and not suffer any limiting saturation.

  • jkittle January 13, 2013, 13:59

    I think that the arguments on moon size are interesting but recall that with SO MANY systems out there, there are bound to be a LOT of outliers. Look at earth! Few models can easily accommodate the formation of such a large moon in orbit of an earth-sized planet in the habitable zone of a yellow star like ours… yet … there it is! Large satellites may be relatively rare in Jovian class systems around other stars but, multiply rare times many billion and there are still a lot of earth sized planets in orbit of giant planets out there. Also being near a gas giant probably shifts the habitable zone out further away from the primary star a bit. Remember that the gas gaint magnetic field only traps solar particle radio, not high energy cosmic rays (nor UV for that matter) . Thus a thick atmosphere will handle any increased radiation at the cloud tops… it will never reach the surface. My only real concern is that the moons of a giant planet tend to be bombarded by asteroids and have collisions with other orbiting debris, all because of the focusing effect of thee gas giants enormous gravity well. I would expect single cell life forms. I doubt if it is a great place to evolve a civilization. still…if we get there I am sure we might cope.. My intuition is that gas giants of larger stars may be habitable, where the zone is further form the Primary and there is likely fewer asteroids formed to begin with. such systems may have a dearth of habitable zone earth sized planets. – So these moons may be the ticket in systems with higher mass stars.

  • coolstar January 13, 2013, 21:00

    Rob Henry: That could be but it’s still inconsistent with their statement IMO.
    andy: true, but our atmosphere has been predominately N2 for a LONG time. I’ve seen one reference that says only a 1% O2 atmosphere (which could come about by photo dissociation of water and not require life) could give rise to an ozone layer…..Not being a chemist (as most of the readers of this paper won’t be, either), I think statements like the one I quoted really need more explanation that that given.
    The paper has been accepted for publication in A&A (I’ll admit to only skimming the paper, as aside from that sentence, it seems to be solid work but likely to be unimportant as I strongly suspect the number of habitable moons to be several orders of magnitude (3 to 6 maybe?) lower than that of habitable planets.)

  • coolstar January 13, 2013, 21:05

    GaryChurch: pressure just goes as density*g*depth, so the lower g doesn’t really help all that much.

  • nullzero January 13, 2013, 21:10

    What about double planets? We don’t know how frequent they are. It is even possible that a lot of known exoplanets are doubles. In a way, even the Earth-Moon system is considered a double.
    True doubles are orbiting a common center of mass that is in space, being tidally locked to each other, not to their star.
    This option also makes it possible to have habitable planets orbiting red dwarves without being tidally locked. Except if double planets are not stable in that kind of system. What do you think about that?

  • Rob Henry January 13, 2013, 21:15

    Jkittle says “a thick atmosphere will handle any increased radiation at the cloud tops”. And that reminds me, that on a world, it is traditional to talk of atmospheric thickness in terms of surface pressure. This may be perfect for calculating breathing or flying potential, but we must divide this by surface gravity to find its shielding potential. Thus titan’s atmosphere is not 40% thicker, but a whole order of magnitude “thicker” than Earth’s. Perhaps ice moon atmospheres of comparable thickness are common??

    He goes on to note “My only real concern is that the moons of a giant planet tend to be bombarded by asteroids and have collisions with other orbiting debris, all because of the focusing effect of thee gas giants enormous gravity well.” And I would agree, but for one thing.

    The largest extinction on Earth was the end Permian one, and impact evidence seems decidedly lacking there. The K-T event, is the largest such known event, and the consensus is that it had a net positive impact on the rate of evolution. To me this must frustrate your “expect single cell life forms” postulate a tad.

  • jkittle January 14, 2013, 14:46

    Rob- good point about pressure vs sheilding. that is one reason why mars atmosphere shielding is adequate ( or almost adequate) for humans on the surface. Minimal additional shielding is needed ( a foot of ice in an Igloo-like dome for example.) while I agree not all mass extinctions have been linked to large bolide impacts, the converse IS true, any large impact should lead to extinction… and like mutations on a single organism, to many extinctions and evolution may be thwarted, too few and … well there are likely Other reasons for mass extinction. . again. We need more data points. Evolution science witll Really get specific when we have 100 diverse planets with at least microbial life to compare ( note the” When”) .

  • andy January 14, 2013, 15:18

    One question about ocean worlds (including oceanic exomoons) is whether a climate supporting liquid water at the surface is stable. They do not have the silicate-carbonate cycle operating (if the ocean floor is rocky as opposed to high-pressure ice, then seafloor weathering probably has a much weaker dependence on the ocean surface temperature), so they may be much more vulnerable to collapse into either a snowball state or a moist/runaway greenhouse. The paper by Abbot, Cowan and Ciesla (also discussed on Centauri Dreams here) suggests that a land fraction above ~1% is necessary to stabilise the climate. So ocean worlds may not be a good bet for long-term habitable conditions.

    On the other hand the same paper suggests that an ocean planet may be able to recover from the moist greenhouse state when sufficient land surface area is exposed due to water loss. So maybe an initially oceanic moon can end up with a more Earthlike balance between land and sea, though what the hot greenhouse state would do to the moon’s supply of organics I have no idea.

  • Daniel January 14, 2013, 15:43

    nullzero Said:
    “What about double planets? We don’t know how frequent they are. It is even possible that a lot of known exoplanets are doubles. In a way, even the Earth-Moon system is considered a double.
    True doubles are orbiting a common center of mass that is in space, being tidally locked to each other, not to their star.
    This option also makes it possible to have habitable planets orbiting red dwarves without being tidally locked. Except if double planets are not stable in that kind of system. What do you think about that?”

    I find your question very interesting,a very interesting possibility Double planets in the M Dwarf HZ ,then I search and this was what find:

    “Imagining the dance of double planets”

    http://exep.jpl.nasa.gov/ave/index.cfm?FuseAction=ShowBlog&NewsID=380

    It’s seems there not a deep scientific study on this case,would be very interesting if Kepler or other ongoing or future projects,detect a double planet transit a Red dwarf star habitable zone

  • GaryChurch January 14, 2013, 18:25

    ” -pressure just goes as density*g*depth, so the lower g doesn’t really help all that much.”

    Well…..how much does it help? Give me something simple like a water column on Callisto vs. a water column on Earth. If I am a thousand feet down on Callisto what is the pressure compared to Earth?

  • coolstar January 14, 2013, 23:41

    nullzero: true double planets have to be quite rare or they would have already shown up in the Kepler data as their transit signatures would be quite striking. As has been discussed here before, tidal heating could also pose a big problem.

  • Kytshar January 15, 2013, 15:25

    What about HD 28185 b? It’s a gas giant in the habitable zone of a G5V star with a mass of at least 5.7 M_Jupiter. If exomoons of gas giants are in the order of 10^-4 the mass of their parent planets (as previously suggested), then HD 28185 b should easily have moon more massive than Mars.

  • Ronald January 15, 2013, 17:26

    Further to andy’s comment with regard to the rarity of giant planets around M dwarfs, as I mentioned under the previous post about gas giants in the HZ, there was another recent paper by Heller (Submitted on 1 Sep 2012), titled ‘Exomoon habitability constrained by energy flux and orbital stability’, which mentions that:
    “Gravitational perturbations by the star, another planet, or another satellite induce eccentricities that likely make any moon uninhabitable”, and:
    “resources should not be spent to trace habitable satellites around them”, and:
    “Deleterious effects on exomoon habitability may occur up to about 0.5M_sun”.

    So it looks like M dwarfs are not only rather unsuitable with regard to habitable planets, but also for habitable moons.

  • Tarmen January 17, 2013, 9:51

    This idea of “eccentricities that likely make any moon uninhabitable” is too broad a brush, given the 100 billions of candidates in the M class systems and the surprising variety of what we’ve already found. Some of those myriad oceans would have a great moderating effect, allowing some moons’ climate to ride out sudden heat or cold shocks.

  • Ronald January 17, 2013, 9:55

    @Kytshar: yes, the star is estimated at 7.5 gy old and has a luminosity of 1.15 solar. Which means that its luminosity has increased significantly over fairly recent times and the planet is now approaching the inner edge of the HZ. We have to hurry :-)

    As I mentioned under the previous post about gas giants in the HZ, another example is 16 Cygni B, a solar type star, G2.5, about solar mass, 1.3 times solar luminosity. It has a very large planet (b) of about 2.4 Mj at almost 1.7 AU.
    This one is on the outskirts of its HZ. 16 Cygni B is very old, at least 8 – 10 gy, so its HZ has moved outward, which means that planet b has moved into the HZ in recent times, the opposite situation of HD 28185 b.

  • Ronald January 17, 2013, 12:35

    @Tarmen: strictly speaking you are right.

    However, ref. to recent posts, comments and referenced articles, the % of M dwarfs with gas giants has been convincingly found to be quite low, probably around 1%. Similarly, relatively few of those planets are in the HZ.
    Let’s say that there are some 200 billion M dwarfs in our galaxy, that would mean about 2 billion with a gas giant. let’s also assume that 10% of those are in the HZ, that leaves some 200 million.
    How many of those do not have too deleterious eccentricity? Pure guesswork, but let’s again assume 10%, leaving some 20 million.
    This still does not take the other potential showstoppers into account, foremost the need for a sufficiently large moon, but also stellar flares, gas giant radiation, …
    I would be really surprised if there were more than a few million potentially suitable candidates among M dwarf exomoons in our galaxy.

    But then again, the galaxy has surprised us before (and it definitely does not care about my ideas).

  • andy January 17, 2013, 15:57

    @Kytshar: yes, the star is estimated at 7.5 gy old and has a luminosity of 1.15 solar. Which means that its luminosity has increased significantly over fairly recent times and the planet is now approaching the inner edge of the HZ. We have to hurry :-)

    But how certain is that age estimate? According to the Geneva-Copenhagen Survey re-analysis the allowable range is quite large.

    What about HD 28185 b? It’s a gas giant in the habitable zone of a G5V star with a mass of at least 5.7 M_Jupiter. If exomoons of gas giants are in the order of 10^-4 the mass of their parent planets (as previously suggested), then HD 28185 b should easily have moon more massive than Mars.

    Actually the relationship is more to do with the total mass of the moon system, rather than the individual moons themselves. Planets that are massive enough to open gaps in their protoplanetary discs are predicted to be more likely to form multiple large satellites, so the mass could easily be partitioned between several objects of a few lunar masses, too small to maintain habitable conditions.

  • FrankH January 17, 2013, 16:38

    @Kytshar: Even assuming that the exo-moon has the same mass AND DENSITY as Mars, at 300K (roughly HZ temps) the moon would not be able to hold on to the water, nitrogen and just barely be able to hold on to its oxygen in its atmosphere.
    If you assume that it’ll have about the same density as Ganymede (not unreasonable, considering that it would have formed past the ice line) and the same mass, the radius goes up to about 4200 – 4300 km, but the escape velocity drops from around 5km/sec to 4.5km sec. The moon would only be able to hold on to CO2… at least until its mass drops as the water melts evaporates and escapes.
    At HZ temperatures, the moon would have to have about 40% of the mass of the Earth (or 4x that of Mars) to even have a chance of retaining a significant atmosphere.

  • Thomas Hackney January 18, 2013, 7:57

    Underground habitations seem the most likely scenario, since atmospheric and cosmic weather become non-issues. They would essentially be self-sustaining groundships. Mining operations could pay for these facilities. Once you have a viable foothold inside a suitable planet or moon many other things become possible, but this is how you start.

  • coolstar January 18, 2013, 12:30
  • Rob Henry January 18, 2013, 16:39

    FrankH, says “Even assuming that the exo-moon has the same mass AND DENSITY as Mars, at 300K (roughly HZ temps) the moon would not be able to hold on to the water, nitrogen and just barely be able to hold on to its oxygen in its atmosphere.” And it seems to me that this makes several errors of over simplification.

    Firstly, water is peculiar in that it is usually retained so efficiently through a cold trap mechanism that it does not obey the same laws. Here, it helps immensely if an ozone layer forms atop the troposphere, especially around a star with relatively high uv.

    Secondly, we find that dinitrogen is permanently retained against thermal loss at 300K at escape velocities below 3.1 km/s. This equates to a planet a third the mass given by Frank still retaining its N2.
    However, given sufficiently high uv levels, monatomic N might have to be retained, and we might indeed need at least 3.7km/s, and at up to 4.4km/s to protect against Jeans loss at 300K.

    Thirdly, as I alluded to above, it is my belief that the most powerful and interesting exomoon ecosystems might occur at around the equivalent distance of Ceres, perhaps with volcanism levels about 10% that of Io punching multiple holes in the thin ice.

    Finally, we should only really be talking of exobase temperatures, not surface temperature. Even if Mars was placed where Venus is, and attained a Cytherean exobase temperature of around 400K, this would still only raise the above-considered escape velocities by 15%.

    So the calculations are more complex than you think, as thus for (I think) the prospects of an exomoon retaining an atmosphere in the HZ.

  • FrankH January 18, 2013, 19:22

    @Rob Henry
    We’ve argued this before – I don’t understand the math or physics you’re using to come up with your numbers. Sure, atmospheric loss is a complex process but Jean’s escape is the basic, classical calculation and in some ways the most benign.

    If you’re going to talk about atmospheric loss, the temperature will always refer to the gas, since that’s what’s going to get lost.

    At 300K, atmospheric N2 would be lost in a quickly (less than a few 100 million years) from a world with an escape velocity under 5km/sec.

    The Moon is at around 300K and its escape velocity is 2.4km/sec, yet it has no atmosphere to speak of. At 400K, Mars would be close to loosing what little CO2 it has.

  • Rob Henry January 20, 2013, 5:16

    Yes Frank, Jeans escape is basic. And the figures I calculated should give minimal loss over timespans of billion of years. For retentions in the order of 100 million years that you alluded to, we can lower those escape velocities I quoted by about 15% again.

    And when we went over it before, you referred me to a web site that stated that it used 5x rms molecular velocity as that for minimal retention (this is correct since it comes straight out of theoretical calculation for retention timespans in the order of 100 million years) then states, without reference, or derivation, or even listing the assumptions they made, that it could require up to 10x rms velocity for retention against atmospheric loss.

    As I mentioned to you earlier, the only way that I could reconcile this with theory, was if the assumed a diatomic gas that completely dissociated in the exosphere. Alternatively they could have predicated their model on the evidence of the few systems with atmospheres around Sol, and so this might not generalise well to other systems.

    I hopes this helps explain to you the some of the multiple assumptions implicit in you position.

  • Rob Henry January 20, 2013, 6:08

    Actually Frank, I have just twigged to a possible reading of your above comment
    “If you’re going to talk about atmospheric loss, the temperature will always refer to the gas, since that’s what’s going to get lost.”

    While true (and what I was, of cause, working from), your wording can be seen as implying that the atmosphere is not loosing particles as if in an evaporation, but ablating as if it is flowing away as a gas. This second type is called hydrodynamic escape, and has been written about much recently even though (to my knowledge) it has never been observed. If you are confusing one with the other you have my sympathies. The area where Jeans phases into the other gets very complicated but should only happen at much lower rms/escape velocity ratios to those we have considered here.

    Also, you mention our moon. If we give this an Earth-like atmosphere, I recall the retention times are in the order of 1000 to 10,000 years, but at such low jeans parameter values things really might get complicated by hydrodynamic escape, so it seems pointless to calculate. Surely a study has already been done by some enthusiasts if you really want an answer there.