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Habitable Moons: Background and Prospects

While I’m in Houston attending the 100 Year Starship Symposium (about which more next week), Andrew LePage has the floor. A physicist and freelance writer specializing in astronomy and the history of spaceflight, LePage will be joining us on a regular basis to provide the benefits of his considerable insight. Over the last 25 years, he has had over 100 articles published in magazines including Scientific American, Sky & Telescope and Ad Astra as well as numerous online sites. He also has a web site, www.DrewExMachina.com, where he regularly publishes a blog on various space-related topics. When not writing, LePage works as a Senior Project Scientist at Visidyne, Inc. located outside Boston, Massachusetts, where he specializes in the processing and analysis of remote sensing data.

by Andrew LePage


Like many space exploration enthusiasts and professional scientists, I was inspired as a child by science fiction in films, television and print. Even as a young adult, science fiction occasionally forced me to think outside of the confines of my mainstream training in science to consider other possibilities. One example of this was the 1983 film Star Wars: Return of the Jedi which is largely set on the forest moon of Endor. While this was hardly the first time a science fiction story was set on a habitable moon, as a college physics major increasingly interested in the science behind planetary habitability, it did get me thinking about what it would take for a moon of an extrasolar planet (or exomoon) to be habitable. And not “habitable” like Jupiter’s moon Europa potentially is with a tidally-heated ocean that could provide an abode for life buried beneath kilometers of ice, but “habitable” like the Earth with conditions that allow for the presence of liquid water on the surface for billions of years with the possibility of life and maybe a technological civilization evolving.

A dozen years later, the first extrasolar planet orbiting a normal star was discovered and a few months afterwards on January 17, 1996, famed extrasolar planet hunters Geoff Marcy and Paul Butler announced the discovery of a pair of new extrasolar giant planets (EGPs) opening the floodgate of discoveries that continues to this day. One of these new EGPs, 47 UMa b, immediately caught my attention since it orbited right at the outer edge of its sun’s habitable zone based on the newest models by James Kasting (Penn State) and his colleagues published just three years earlier. While 47 UMa b was a gas giant with a minimum mass of about 2.5 times that of Jupiter and was therefore unlikely to be habitable, what about any moons it might have? If the size of exomoons scaled with the mass of their primary, one could expect 47 UMa b to sport a family of moons with minimum masses up to a quarter of Earth’s.

I was hardly the first to consider this possibility since it was frequently mentioned at this time by astronomers whenever new EGPs were found anywhere near the habitable zone. But this realization did get me seriously researching the scientific issues surrounding the potential habitability of exomoons and I started preparing an article on the subject for the short-lived SETI and bioastronomy magazine SETIQuest, whose editorial staff I had recently joined. While working on this article, I started corresponding with then-grad student Darren Williams (Penn State) who, it would turn out, was already preparing a paper on habitable moons with Dr. Kasting and Richard Wade (Penn State). Published in Nature on January 16, 1997, their paper titled “Habitable Moons Around Extrasolar Giant Planets” was the first peer-reviewed scientific paper on the topic. They showed that a moon with a mass greater than 0.12 times that of Earth would be large enough to hold onto an atmosphere and shield it from the erosive effect of an EGP’s radiation environment. In addition, tidal heating could potentially provide an important additional source of internal heat to drive the geologic activity needed for the carbonate-silicate cycle (which acts as a planetary thermostat) for much longer periods than would otherwise be possible for such a small body in isolation.

I published my fully-referenced article on habitable moons in the spring of 1997. In addition to incorporating the results from Williams et al. and related work by other researchers, I went so far as to make the first tentative estimate of the number of habitable moons orbiting EGPs and brown dwarfs in our galaxy based on the earliest results of extrasolar planet searches: 47 million compared to the best estimate of the time of about ten billion habitable planets in the galaxy (estimates that are in desperate need of revision after almost two decades of progress). Since my research showed it was likely that habitable moons would tend to come in groups of two or more, I further speculated about the possibilities of life originating on one of these moons being transplanted to a neighbor via lithospermia. And since I did not have to contend with scientific peer-review for this article, I even speculated about the effects multiple habitable moons would have on a spacefaring civilization in such a system with so many easy-to-reach targets for exploration and exploitation.


Image: An artist’s conception of a habitable exomoon (credit: David A. Aguilar, CfA).

After SETIQuest stopped publication and I published a popular-level article on habitable moons in the December 1998 issue of Sky & Telescope, my scientific and writing interests lead me in other directions for the next decade and a half. But in the meantime, scientific work on exploring the issues surrounding habitable bodies in general and habitable moons in particular has continued. The current state of knowledge has been thoroughly reviewed in the recent cover story of the September 2014 issue of the scientific journal Astrobiology, titled “Formation, Habitability, and Detection of Extrasolar Moons” by a dozen scientists active in the field including one of the authors of the first paper on habitable exomoons, Dr. Darren Williams.

Even after 17 years of new theoretical work and observations, the possibility of habitable exomoons still remains strong. The authors show that exomoons with masses between 0.1 and 0.5 times that of the Earth can be habitable. A review of the available literature shows that exomoons of this size could form around EGPs or could be captured much as Triton is believed to have been captured by Neptune in our own solar system. Calculations also show that such exomoons, habitable or otherwise, are detectable using techniques that are available today, especially direct detection by photometric means like that employed by Kepler and by more subtle techniques such as transit timing variations (TTV) and transit duration variations (TDV) of EGPs with exomoons. As the authors state in the closing sentence of their paper:

In view of the unanticipated discoveries of planets around pulsars, Jupiter-mass planets in orbits extremely close to their stars, planets orbiting binary stars, and small-scale planetary systems that resemble the satellite system of Jupiter, the discovery of the first exomoon beckons, and promises yet another revolution in our understanding of the universe.

The fully referenced review paper is René Heller et al., “Formation, Habitability, and Detection of Extrasolar Moons”, Astrobiology, Vol. 14, No. 9, September 2014 (preprint).


Comments on this entry are closed.

  • RobFlores September 19, 2014, 14:55

    Thinking out the possibilities for life in moons orbiting Jovian planets
    additional scenarios we might look is factors that will mitigate the tide locking effects said moons. (tide locking being IMO a hostile environmental condition)

    If some of these Jovians are massive and hot (even being short of brown
    dwarf territory) would they emit enough energy and light to make a sizeable
    energy input to a large moon.
    Additionally with a Jovian at around 1.7 – 2.7 AU, there would be some energy reflected back out from the Jovian’s upper atmosphere to space to additionally heat up any close by moon.
    In combination, or individually maybe with these effects the temperature can be raised to above the freezing point on the surface of a moon. As a bonus maybe that moon’s atmosphere will depleted very slowly compared Earth or even Mars, due to distance from primary, assuming G or near G sized star.
    It may turn out that the extrapolated Kepler data indicating Earth sized planets “should be there in the HZ” are discovered to be a phantoms and such planets are indeed rare. In such a case think extra solar Jovian moons may provide additional hope of placing Earth Like planets out of the endangered species list.

  • Andrew Palfreyman September 19, 2014, 15:11

    “one could expect 47 UMa b to sport a family of moons with minimum masses up to a quarter of Earth’s.”
    I had a bit of trouble parsing that one!

  • Brett September 19, 2014, 15:37

    What worries me about big exo-moons is the possibility of a really high impact rate due to the mother planet’s far greater gravity well. That gas giant is going to pull in a lot of asteroids.

  • Michael September 19, 2014, 16:12

    Although there are many moons that orbit gas giants and these moons could be large enough to have long lived atmospheres I would be a little worried about an Io type inner moon. Volcanic material thrown off its surface would be accelerated by the gas giants magnetic field and the problem here is that the more massive the planet the faster its spin and higher the velocities of those charged particles. These charged particles could possibly erode an atmosphere because even large moons around gas giants would be unlikely have a magnetic dynamo as they would be tidally locked. But having said that the view would be astounding!

  • ljk September 19, 2014, 16:48

    By a cosmic coincidence, here is today’s Astronomy Picture for the Day:


    Astronomy Picture of the Day

    Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

    2014 September 19

    Potentially Habitable Moons

    Image Credit: Research and compilation – René Heller (McMaster Univ.) et al.

    Panels – NASA/JPL/Space Science Institute – Copyright: Ted Stryk

    Explanation: For astrobiologists, these may be the four most tantalizing moons in our Solar System. Shown at the same scale, their exploration by interplanetary spacecraft has launched the idea that moons, not just planets, could have environments supporting life. The Galileo mission to Jupiter discovered Europa’s global subsurface ocean of liquid water and indications of Ganymede’s interior seas.

    At Saturn, the Cassini probe detected erupting fountains of water ice from Enceladus indicating warmer subsurface water on even that small moon, while finding surface lakes of frigid but still liquid hydrocarbons beneath the dense atmosphere of large moon Titan.

    Now looking beyond the Solar System, new research suggests that sizable exomoons, could actually outnumber exoplanets in stellar habitable zones. That would make moons the most common type of habitable world in the Universe.

  • Andrew LePage September 19, 2014, 17:39

    @RobFlores September 19, 2014 at 14:55
    “Thinking out the possibilities for life in moons orbiting Jovian planets
    additional scenarios we might look is factors that will mitigate the tide locking effects said moons. (tide locking being IMO a hostile environmental condition)”

    There has been a lot of work done over the past couple of decades that strongly suggests that tidal locking is not an issue with planetary habitability especially for planets with dense atmospheres (which would be a natural result of the carbonate-silicate cycle) and/or oceans. Ignoring that for the moment, however, a synchronously rotating planet is NOT the same as a synchronously rotating moon. While they both present the same face towards the primaries they orbit, that primary is not the sun in the case of a moon. A synchronously rotating moon will have a “day” comparable in length to its orbital period as is the case with Earth’s Moon. Assuming that the angular momentum of systems of moons scales approximately with the mass of the primary as it is observed to do in our solar system, that means that large exomoons would be expected to have orbital periods/days on the order of a couple of days to a couple of weeks long. Such a rotation period is enough to forestall the effects of atmospheric collapse assuming that current thinking on the habitability of synchronous rotating planets proves to be wrong. And the presence of a dense atmosphere, clouds and an ocean would help moderate the temperature extremes of such long days.

    @Michael September 19, 2014 at 16:12
    The erosion of a moon’s atmosphere as a result of its primary’s radiation environment can certainly be an issue with the potential habitability but it is not insurmountable. It has been shown that a magnetic field will provide adequate shielding with observations and theoretical work strongly suggesting that large exomoons are more likely to possess such fields than comparable size planets in isolation. In addition, even moons without strong magnetic fields can maintain dense atmospheres as is the case with Titan in our own solar system.

    RobFlores and Michael, all of these issues are addressed in the paper referenced above as well as in articles I have written on this subject available via the following link:


  • andy September 19, 2014, 17:41

    What kind of constraints on the occurrence rate of large moons orbiting gas giants are there from Kepler? From what I’ve seen, the HEK project has focussed on sub-Neptunes/super-Earths rather than jovians as exomoon hosts, presumably because these would lead to easier detections. And is anyone aware of any studies that have tried looking for ring systems in the Kepler data?

    Also seems like co-orbital planets haven’t shown up either, other than a false alarm regarding KOI-730 (now Kepler-223). Maybe the migration of planets into the inner system ends up destroying such configurations?

  • Joëlle B. September 20, 2014, 1:47

    Further reading of interest:

    pop description: Research trio suggests exomoon atmospheres could cause false-positive signs of life on exoplanets
    preprint: http://arxiv.org/abs/1404.6531

    Transit of Exomoon Plasma Tori: New Diagnosis
    preprint: http://arxiv.org/abs/1404.1084
    (this is also referenced in the Heller et. al paper)


  • Joëlle B. September 20, 2014, 2:59

    We should also keep in mind that spectral analyses could hint toward extreme volcanism of a transiting system no longer active in the present, therefore limiting accurate knowledge of an atmospheric representation of a moon. So a moon once in an insulation-tide HZ may now be uninhabitable. Also, if a moon’s tidal activity modified its orbit and no other planetary bodies are present to perturb it like in the Jovian system, eccentricity could reach zero, limiting radiogenic heating, hindering habitability, as well.

    I would highly doubt a runaway, free-floating moon as a candidate for (active) life; though, a free-floating planetary companion could yield more promising (and diversely interesting!) results, especially if we ever get the chance to observe a moon without a star directly influencing its biosphere. Spooky creatures may be in store. There’s also a chance for a moon to be recaptured, and depending on lithopanspermia (or dormant indigenousness) , may obtain or regain a detectable biosphere.

  • Alex Tolley September 20, 2014, 11:39

    Terrestrial type moons orbiting a gas giant are obviously, as mentioned in the post, far rarer than habitable terrestrial type planets, and we don’t have any habitable planets definitely confirmed yet. I really don’t understand the interest in looking for the rare planetary formation events that, with just the right mix of fluke conditions might generate a habitable world under these conditions. If icy moons harbor life, then they are obviously of interest to exo-biologists, but terrestrial type moons?

    AFAICS, the only advantage of such a situation is that it extends the HZ outwards a little. Energetically, for photosynthetic driven ecosystems this doesn’t help beyond pushing out the orbit before the world freezes. Is there any benefit for the creation of life: we cannot know without knowing how life started on Earth.

    Fun speculation perhaps, but we need data about life bearing worlds.

  • ashley baldwin September 20, 2014, 13:23

    Rene Heller has published extensively on , as yet, undiscovered exomoons. His work backs up the original Kasting work that shows that a circumplanetary disk could accrete up to Mars size moons around ten Jupiter mass gas giants. Better still, larger moons could be captured terrestrial planets as gas or ice giants migrate inwards during the interplanetary billiards of early stellar systems. Ice giants with their lower mass and gravity would be a better bet as closing velocities between planet and ” moon” would be slower and more likely to result in capture rather than destruction , especially with a binary incoming system, with one planet captured and the other expelled. This seems to have happened with Triton at Neptune. All very well and good. What troubles me , and it is very apparent in the recent ( available on arxiv and excellent) Heller, Kipping, Greenberg paper , is the issue of a protective magnetic field . To be protected by the parent planet’s field , the moon has to be close , making it vulnerable to trapped radiation in the planet’s magnetic field, as with Europa at Jupiter and also heating tidal gravity effects too, as with Io. Stay further out and you are subjected to stellar radiation ,as being tidally locked to your planet you cant rotate fast enough to generate your own field easily. Hopefully , with time and discovery, this will be answered . But it hasnt as yet . Early days though. Maybe a big moon tidally locked to a fast rotating planet will create sufficient field that a thick atmosphere will supplement sufficiently to allow surface life. (As opposed to in a sub surface ocean)

  • Michael September 20, 2014, 13:41

    @Andrew LePage

    Great article by the way

    ‘It has been shown that a magnetic field will provide adequate shielding with observations and theoretical work strongly suggesting that large exomoons are more likely to possess such fields than comparable size planets in isolation. In addition, even moons without strong magnetic fields can maintain dense atmospheres as is the case with Titan in our own solar system.’

    The problem as I see it is that if you look at Jupiter with an Io type moon is that the ion flux is not only over a hundred times more but they also have 3 to 5 times higher energies than the solar wind, that is a significant atmospheric erosion hazard. Now I am not to sure about the magnetic field generation of a tidally locked moon but it has got to be less likely than an isolated rotating mass 1AU from a stellar mass sun. Combine lower probability magnetic fields, low moon gravities and with the potential ion creating cascade events high in a moons atmosphere which would more than likely be pulled away from it by a rapidly rotating magnetic field sweeping the moon which can be 50 km/s or more from a massive gas giant I can only imagine an atmosphere on these moons as being transitory of a short order.

  • Dave Moore September 20, 2014, 15:51

    Having given the paper a quick read, I have a couple of thoughts:

    There is a summary of a series of simulations that show that at if a moon is within a certain distance from its primary, tidal heating causes a run-away greenhouse. This means, of course, that if the primary is located outside the habitable zone, tidal friction can heat the moon to make it habitable, so searches for habitable moons should not stop at the outer edge of the habitable zone.

    The other point I want note is that no one has looked at the Kozai effect. What if the primary has a significant axial tilt? This would result in the moons attempting to trade orbital inclination for eccentricity. I can speculate two effects from this: even more tidal heating from when the moon’s orbit is at its most elliptical; and a possible consolidation of multiple moons into one larger body. Any thoughts?

  • Joëlle B. September 21, 2014, 5:22

    @ Alex

    I, for one, don’t believe it’s particularly dire to know how life started here, but rather what steps need to be taken to (re)create abiogenic scenarios under direction of the human hand, due to time constraints and efficiency. And we may never be able to find out how, conclusively, without some form of optical time travel or amazing luck for being in the right place at the right time, over a long period of time; as of right now, quantum tunneling seems to give the most thorough implication for such a feat in enabling thermonuclear reactions, insolation habitable worlds and low tunneling probability for low mass stars (like Sol) to perpetuate multicellular organisms via its own light and geothermal light directly from a world for anoxygenic photosynthesis. Surface reactions on interstellar dust grains leading to prebiotic molecules and high-energy chemistry of prebiotic stuff in upper atmospheres, like say, of Titan, and the perdurable radiogenic heating and tidal friction of say, Enceladus, also increase the variability of neccessary or favorable conditions outside of what we see on Earth.

    With that in mind, what’s more immediately important is whether or not a moon or planet (or any space foreign to Earth) is habitable–able to foster, nurture, and support life long enough for that life to adapt, much in the same way DNA/RNA lifeforms here have successfully habituated over billions of years. The focus is realizing the long (My-Gy) geolocigal/ecological time scales lifeforms have to work with.

    From the present evidence, it would appear that we can predict many forms of life inhabiting Earth as being up for the job. This is motivating in its own right, but the only “benefit” proving practical would be that of the interest of humanity’s, of course.

  • Andrew LePage September 21, 2014, 10:07

    @Alex Tolley September 20, 2014 at 11:39

    “Terrestrial type moons orbiting a gas giant are obviously, as mentioned in the post, far rarer than habitable terrestrial type planets”

    I’d be VERY careful about drawing any conclusions about the relative occurrence rates of terrestrial-type habitable planets or moons based on the 17 year old estimates I quoted in this essay and which I clearly stated are in desperate need of revision. The estimated number of habitable planets, for example, was from a 20 year old study based on simulations specifically designed to produce planetary systems that resemble our own solar system – an assumption that has since been proven to be wrong. And my exercise to estimate the number of habitable moons was just a SWAG based on the very earliest data about EGPs and a lot of assumptions. The continuing analysis of Kepler data as well as other investigations should allow astronomers to generate more accurate estimates of these numbers based on a lot more empirical data and fewer assumptions.

    If terrestrial-type habitable moons turn out to be less common than habitable planets (which I suspect to be true), that is certainly not a valid reason to dismiss the scientific investigation of such bodies. Part of the motivation of the study of potentially habitable moons (which is just one issue in the larger study of exomoons in general – a subject worthy of study in its own right for a range of reasons) is that it probes different aspects of habitability testing our assumptions about habitability in general in the process. Such investigations over the last 20 years and the recognition that Europa as well as other moons in our own solar system might be abodes for life have already forced scientists to rethink their ideas about “habitability”. Science is filled with examples where the study of rare phenomena sheds light on the processes responsible for more common phenomena.

  • Alex Tolley September 21, 2014, 17:22

    @Andrew LePage. I would suggest that there is a gulf between scientific investigation and speculation. Lowell crossed that gap when writing about Mars based on his relatively crude observations. Von Braun did something rather similar half a century later in his novel about traveling to Mars, where his logic about Martian conditions proved widly wrong, e.g. those beautiful winged landers being suitable based on a much denser atmosphere. Europa, on our doorstep, is perhaps another example, where speculation about life is rife, based on only the calcula6tions (ander later observations of plumes) of liquid water and the logic of “follow the water” when looking for life. Modeling can give us some clues to reality, but they are probably missing crucial variables and provide merely the slimmest indications of boundaries.

    By all means do theoretical calculations about possible conditions on exoplanets and moons, even comparing them to terrestrial conditions. But once one starts talking about living worlds as depicted in SF and movies, based as yet on not a shred of data, then I call this speculation, not investigation.

    Now I am very interested in the search for extra-terrestrial life, both locally and in the galaxy. So far we have an absence of evidence only. I’m hoping that we may have evidence within 20 years, based on spectrographic data. Such living worlds will need to be in their respective HZs. We cannot detect life in icy moons by remote observation as shown by our own solar system. Inhabited exomoons will still likely need to be in or close to the primary star’s HZ, unless there is any evidence that a gas giant can supply a viable, terrestrial type HZ. I think that all this means is that at best if no terrestrial world in the HZ is detected, that maybe an exomoon around a detected gas giant in the HZ may work. I would suggest that actual detection and characterization of terrestrial extremophiles, plus the Venter’s work on gene sequencing from ocean sampling has given biologists far more information about life than any amount of speculation about “habitability”. On the contrary, I would argue that it is terrestrial life that has expanded our horizons on habilitability.

    IMO, there is far too much emphasis placed on human colonization of the galaxy. Apart from the moral dilemmas raised about inhabiting living worlds, it seems far more likley that our machines will be the “colonizers” and that humans [post-humans?], if they explore, may opt for building artifical worlds, which makes potentially habitable worlds (for humans) almost moot.

  • Geoffrey Hillend September 21, 2014, 17:55

    I like the idea of a habitable moon around a gas giant like in Star Wars. It still has to be in the life belt, the area where liquid water can exist. Also what about asteroids and meteorite threats due to the large gravity of the gas giant? Too many collisions can threaten life or at least long term evolution into higher forms.

  • Harry R Ray September 22, 2014, 9:41

    A recent report by Kepler scientists said there was no evidence for exomoons in the first three years of data. They employ two methods in their search; multiple simultaneous transits in the event of a planet-planet capture scenario, and TTV-TDV analysis (with EMPHASIS om the TDV’s) for smaller mo0ons that form in a similar way to those in OUR solar system. I recommend a THIRD search strategy, one which takes into account ANOTHER rarer kind of planet-planet capture, a polar (or NEAR polar) capture where the orbital period of the captured planet around the primary planet is EQUAL to the orbital period of the primary planet ITSELF! In this scenario, the primary rlanet DOES NOT TRANSIT the host star, but JUST MISSES! Thus only the captured planet transits while the primary planet dips under (or over) the star from our line of sight. To CONFIRM this scenario, you would need follow-up RV observations. These observations may reveal an IMPOSSIBLY MASSIVE small planet! One such incedence MAY have ALREADY occures Marginal (AT BEST) RV observations of Kepler 131c show an ASTOUNDING mass of 77g/cm3! The Kepler 131 system is CURRENTLY being observed by HARPS-North, primarily due to Kepler 131b being a potential mega-earth. I am waiting with baited breath for the results! If confirmed, I would favor the above scenario, however unlikely it may be, to a planet made up of material in a density range that is currently unknown to science, and appears to be IMPOSSIBLE!

  • Andrew LePage September 22, 2014, 14:36

    @Alex Tolley September 21, 2014 at 17:22

    “I would suggest that there is a gulf between scientific investigation and speculation. Lowell crossed that gap when writing about Mars based on his relatively crude observations. Von Braun did something rather similar half a century later in his novel about traveling to Mars, where his logic about Martian conditions proved widly wrong, e.g. those beautiful winged landers being suitable based on a much denser atmosphere.”

    Percival Lowell most certainly crossed over from the realm of good science and into pure fantasy with his speculations about Mars. However, your characterization of von Braun and the design of his Mars landers is ill founded at best. When von Braun and other engineers in the US, USSR and elsewhere were designing the first spacecraft to land on Mars during the 1950s and early 1960s, the general consensus of the astronomical community was that the Martian atmosphere had a surface pressure of ~85 mb – over an order of magnitude higher than it actually is. This was based on decades of photometric and polarimetric measurements interpreted with the best models available at the time. The Mars lander concepts of von Braun and others at this time were based on this assumption and could have worked under those conditions.

    It was not until late 1963 and into 1964 that the analysis of IR spectra of the martian atmosphere showed that these earlier measurements were off by a factor of four or more. Radio occultation measurements by Mariner 4 in 1965 confirmed this and subsequent missions further refined our view of the martin atmosphere to that we have today. It turns out that the earlier analyses of the data had not properly taken into account the light scattered by the large amounts of fine dust in the martian atmosphere. A full account of how the knowledge of the martian atmosphere changed at this time and how it affected at least Soviet Mars lander missions is discussed here:


    This is NOT an example of bad engineering on the part of von Braun or other engineers. Their designs were based on the best scientific knowledge about Mars at the time. And when that information changed, their designs had to change. This particular episode is NOT even a case of bad science. In fact, it is a perfect case of how science is suppose to work!

    Science seeks to find natural explanations for observed phenomena. Part of this process is formulating hypotheses that not only explain the available data, but make predictions of what future observations will find in order to prove or disprove those hypotheses. If a hypothesis fails a test, it is either modified or discarded in favor of a new hypothesis that better explains the observations and the process starts again

    In this case, the photometric and polarimetric observations of Mars made from the 1920s to 1950s were best explained by a simple Rayleigh scattering model of a martian atmosphere with a surface pressure of ~85 mb. When the first IR spectral data from 1963 failed to find what was predicted, a new explanation had to be found that better explained the observations: A thinner martian atmosphere filled with dust.

    In the case of habitable moons, there are certainly those who have gone beyond science and into the realm of pure speculation. However, based on what science currently knows about planets, moons, habitability, etc., it would seem possible that habitable moons exist. Work like that described here synthesizes everything we currently know and makes predictions of what should be found and where. Future observations will either prove or disprove these predictions and we will learn more either way it turns out. That isn’t bad science. That’s how the self-correcting process of science works.

  • andy September 22, 2014, 17:21

    About the impact problem, isn’t there an anticorrelation between the presence of massive giant planets and circumstellar debris discs? In which case, the systems that produce habitable moons may well have lower impact rates due to clearing out most of the system’s debris relatively early on. Rarer impacts that hit a lot harder, perhaps?

  • Alex Tolley September 23, 2014, 0:13

    @ Andrew LePage
    My take is somewhat different than yours on Von Braun. This is definitely based on his novel which IIRC, had an exposition that layed out the logic of atmospheric density. This logic was incorrect, as you point out, but it was used at that time to justify the mission, ship design, etc. , both in the novel and in the technical booklet, “Das Mars Projekt”. But I see this as a salutary lesson for today’s speculation on exomoon habitability. We know so little, and our models are likely to be so flawed, that the predictions they make are likely to be wide of the mark. Of course we will eventually correct this with observations.

    Wherever knowledge is poor, speculation can abound. I think we still know so little that we almost use the models we build and their results as the modern equivalent of the fantastic stories that were woven about conditions and peoples on distant worlds.

    Science will eventually correct our interpretations, although I suspect it will be a long time before the reality can bound the speculations on life and ETIs on these distant moons.

    None of the above in any way is meant to imply that I disliked your piece, per se. I read much of what you write that gets posted on the BIS’s Facebook page and enjoy it, especially your historical pieces.

  • ljk September 23, 2014, 9:10
  • Robert van de Walle September 24, 2014, 14:28

    Just for more fun speculation, let’s continue extrapolating about multiple habitable moons and the evolution of a space-faring civilization. Since a mass of less than .5E is sufficient to support an atmosphere, these low-mass, low-gravity worlds would be able to reach orbit earlier in their technology revolutions. Getting to orbit is less expensive for them than for us. They would have very different resources available to solve global issues of pollution and economics; the resources of their local space environment!

    Suppose Life is common, and many instances of ETI arise on these moons, around many stars. Should they develop interstellar travel and meet, they will all have things in common, especially when compared to a species such as us, from a high-mass, singleton world.

    For example, they would not need bones as strong as ours. They possibly would not need nervous systems as quick as ours. They would have come to sentience in an orbital system under which it is immediately obvious they are *not* the center of the Universe.

    Here’s how that works: as humans bootstrapped ourselves towards sentience, we looked up and saw that the heavens revolved around us. Motions in the sky happen so slowly that it appears there is a dome over our planet and there is a show projected upon it for us. Every human civilization that left records explains that the Earth is the foundation. We *know* we are special. I suggest we are a species in service to our egos because we evolved thinking we are at the center of everything.

    However, ETI of a moon tidally locked to its gas giant, sees that the gas giant hangs in one spot in the sky, spinning madly, and everything else in the Universe is “pulled” along with it. Their moon(s) travel in the same direction as the rotation, their sun is further out and describes an epicyclic path, and the stars flow evenly by, projected on a sphere with the gas giant at the center.

    As ETI comes towards sentience, what sorts of mythologies and creation stories will they form and tell? ETI will have a profoundly different opinion of themselves and how they fit into the Universe than we do. Their heavens have two gods; a distant sun god and a nearer… center god? Plus the goddess of the soil beneath their feet?

    I predict universal hero stories shared in cantinas across the galaxy. We share common cultural things like the Great Flood within our own species, but they will *all* have a story such as the hero who goes sets out from the part of the world where you cannot see the gas giant and has an epiphany when one day the hero sees it rolling in place at the horizon.

    We are profoundly different from them.

  • Mark Zambelli September 29, 2014, 11:45

    To be fair, Titan and Europa get a lot of mention as they are perhaps top-of-the-list for habitability but what about our Ganymede? Yes it’s big but it also has an intrinsic magnetosphere, an exosphere and possible ionosphere. With a similar induced magnetic field to Europa and the subsurface ocean this implies I can’t help but wonder if Ganymede would tick all the boxes if Jupiter had migrated into Sol’s HZ?

    A few years ago I read an article about the changes that would come about if Titan were moved to near Earth’s orbit and I seem to recall the takehome-message was that with the increased insolation its atmosphere would inflate and be quickly lost. If Titan had an intrinsic or sufficiently powerful induced magnetic field would the prospects be better? If so, and if both Ganymede and Titan were nearer the Sun then perhaps our Solar System has two such examples for habitability and as such the chances for exomoons bearing fruit seems more likely.

    I remember being fascinated when the 12yr-old ‘me’ saw Endor in ROTJ and I’m glad that my son and daughter’s generation got their own inhabited exomoon as portrayed in the film ‘Avatar’ (with its reduced gravity and non-human-standard air it’s an added realism that is even more thought-provoking than Endor although Lucas’ decision to film RedWoods helped show that things should grow bigger on a moon with reduced gravity… although that doesn’t explain why the Ewoks were so small ha ha)

  • Andrew LePage September 29, 2014, 22:16

    Mark Zambelli September 29, 2014 at 11:45

    If we are making a list of moons in our solar system with buried oceans, Ganymede most certainly belongs on the list as you point out. As for Titan, a number of years ago there was a paper published by Ralph D. Lorenz, Jonathan I. Lunine and Christopher P. McKay titled “Titan Under a Red Giant Sun: A New Kind of ‘Habitable’ Moon” (in Geophysical Research Letters, Vol. 24, No. 22, pp. 2905-2908, November 15, 1997). I wrote a review summarizing this paper 16 years ago that can be found on the second page of the following:


    In a nutshell, the paper dealt with the potential habitability of Titan when the Sun becomes a red giant allowing Titan, with only 6% of Earth’s present solar flux, to become a very cold but potentially habitable world with a liquid ammonia water ocean for maybe 500 million years. I extended that work and speculated about cold habitable Earth-size, Titan-like planets far beyond the standard HZ with similar conditions around cool M and K-type stars (G-type stars and hotter would produce too much UV that would quickly destroy ammonia). That 1998 article, “The Extremes of Habitability”, can be found in the following:


  • Mark Zambelli September 30, 2014, 11:24

    @Andrew LePage

    Thankyou for the additional links, very interesting indeed. The Titan under a Red Giant Sun idea I’m familiar with and have always found intriguing as well as a little sad… sad I suppose as Titan gets its chance to flourish but only right at the end… anything evolving there has doomed prospects just as it starts (assuming that any Titanian evolution follows roughly the timeline of Terrestrial evolution).

    That being said, it should provide a temporary port-of-call for any of our far distant descendants who have decided to stick around home to witness those days.

    Thanks again for the follow-up Andrew, I’m off to pore over those links in more detail and imagine the view of a swollen orange disc imperceptibly rising above the hilly Titanian horizon and shining through scudding water-clouds on a balmy spring morning a few years before the onset of summer.

  • Mark Zambelli September 30, 2014, 12:28

    Ok, so my Titanian hills will need revising under a global ocean of water and ammonia and it looks like Titan will never be balmy… there go my childhood imaginings, ha ha.

    It strikes me that any tidally-locked moons that are caught in orbital resonances (such as the Io, Europa, Ganymede 4:2:1) should receive additional tidal heating as their orbits remain non-circularized and should provide energy to drive plate tectonics, keep the water liquid and the atmosphere gaseous. If this is the case, I wonder how far outside the star’s planetary HZ would be the line that denotes the outer edge of any HZ for moons of EGPs. I can imagine that we might find different width HZs depending on whether we are interested in isolated planets or moons of gas giants. With all the variables (gas giant masses, moon masses, orbits, composition etc) these HZs are becoming much more difficult to assess. Or are they?

  • andy October 22, 2014, 11:44

    A new work on the potential for large satellites to form around superjovian planets: Heller & Pudritz “Water ice lines and the formation of giant moons around super-Jovian planets” – suggests that icy moons with masses comparable to Mars may form around the most massive gas giants.