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On Potentially Habitable Moons

Looking through a recent Astrophysical Journal paper on gas giants in the habitable zone of their stars, I found myself being diverted by the distinction between a conservative habitable zone (CHZ) and a somewhat more optimistic one (OHZ). Let’s pause briefly on this, because these are terms that appear frequently enough in the literature to need some attention.

The division works like this (and I’ll send you to the paper for references on the background work that has developed both concepts): The OHZ in our Solar System is considered to be roughly 0.71 to 1.8 AU, which sees Venus as the inner cutoff (a world evidently barren for at least a billion years) and Mars as the outer edge, given that it appears to have been habitable in the early days of the system, perhaps some 3.8 billion years ago. ‘Habitable’ in both HZ categories is defined as the region around a star where water can exist in a liquid state on a planet with sufficient atmospheric pressure (James Kasting has a classic 1993 paper on all this).

The CHZ’s inner edge is considered to be at the ‘runaway greenhouse limit,’ where the breakdown of water molecules by solar radiation allows free hydrogen atoms to escape, drying out the planet at approximately 0.99 AU in our own system. Its outer edge, says the paper:

…consists of the maximum greenhouse effect, at 1.7 AU in our solar system, where the temperature on the planet drops to a point where CO2 will condense permanently, which will in turn increase the planet’s albedo, thus cooling the planet’s surface to a point where all water is frozen (Kaltenegger & Sasselov 2011).

It goes without saying that boundaries like these are going to vary from one planetary system to another, and it’s likewise clear that most of our thinking about habitable zone planets has gone in the direction of small rocky worlds as we mount the search for Earth analogues. What Stephen Kane (University of Southern Queensland), working with an undergraduate student at the university named Michelle Hill as well as colleagues at the University of California, Riverside has done is to identify 121 giant planets in Kepler data that could host habitable moons.

To be sure, the gas giants themselves aren’t considered candidates for life as we know it (though obviously we can’t rule out exotic species adapted to extreme conditions, like Edwin Salpeter’s ‘gasbags,’ free-floating lifeforms that might populate dense atmospheres — see Edwin Salpeter and the Gasbags of Jupiter for more). But the real focus is on those rocky moons that occur in such abundance in our own system.

“There are currently 175 known moons orbiting the eight planets in our solar system. While most of these moons orbit Saturn and Jupiter, which are outside the Sun’s habitable zone, that may not be the case in other solar systems,” says Kane. “Including rocky exomoons in our search for life in space will greatly expand the places we can look.”

Image: This is an artist’s illustration of a potentially habitable exomoon orbiting a giant planet in a distant solar system. Credit: NASA GSFC: Jay Friedlander and Britt Griswold.

As we consider the different dimensions of habitable zones around other stars, we should also keep in mind the fact that the moons that may emerge in these systems can be as various as our own. Earth’s Moon, for example, seems to be the result of a giant impact early in the system’s formation. Most moons are thought to have formed by accretion within the dust disks around planets, but others can be captured by a planet’s gravitational pull — Triton seems to be an example of this. Thus we could find moons of considerably different composition than their host planet. Considering how many moons we see orbiting our gas giants, the assumption that moons exist around other such worlds in exoplanetary systems seems reasonable.

We still have no exomoon detections, but the search continues, and I always scan the latest papers from the Hunt for Exomoons with Kepler project that David Kipping runs with anticipation, along with those of exomoon theorist René Heller. Having a database of the giant planets we’ve identified thus far as being in the habitable zone of their star may help us target future observations to refine the expected properties of their moons, assuming these exist. Such moons would receive energy from the primary star, of course, but would also receive reflected radiation from the planet they orbit. René Heller has proposed that exomoons in a habitable zone could provide a better environment for life than Earth itself. Let me quote the Hill paper:

Exomoons have the potential to be what [Heller] calls ”super-habitable” because they offer a diversity of energy sources to a potential biosphere, not just a reliance on the energy delivered by a star, like earth. The biosphere of a super-habitable exomoon could receive energy from the reflected light and emitted heat of its nearby giant planet or even from the giant planet’s gravitational field through tidal forces. Thus exomoons should then expect to have a more stable, longer period in which the energy received could maintain a livable temperate surface condition for life to form and thrive in.

Discussing the difficulties of exomoon detection, such as the fact that multiple moons around a single planet may eliminate a useful transit timing signal (this is Jean Schneider’s work) and the problems of direct imaging, it’s interesting to see that microlensing remains a candidate. It’s also intriguing to ponder the fate of exomoons, as this paper does, in terms of migrating gas giants and the likelihood that their moons will be lost. We still have much to learn about the movement of giant planets and the effect of their migration upon their own moons as well as other planets.

Once we have a firm exomoon detection, we can begin to characterize the possibilities. As we await improvements in our technology, deepening our knowledge of potential exomoon host planets is the best we can do, and that would begin, as this paper suggests, with radial velocity follow-up observations on gas giant habitable zone candidates like the ones compiled by the authors.

The paper is Hill et al., “Exploring Kepler Giant Planets in the Habitable Zone,” The Astrophysical Journal, 2018; 860 (1): 67. Abstract / preprint. The Kasting paper mentioned above is “Habitable Zones around Main Sequence Stars,” Icarus Vol. 101, Issue 1 (1993), pp. 108-128 (abstract). For René Heller’s work on ‘superhabitable’ moons, see Heller & Armstrong, “Superhabitable Worlds,” Astrobiology January 2014 (preprint). Jean Schneider’s paper on exomoon detection problems is Schneider & Sartoretti, “On the detection of satellites of extrasolar planets with the method of transits,” Astronomy & Astrophysics. Suppl. Ser. Vol. 134, No. 3 (1 February), pp. 553-560 (abstract).


Comments on this entry are closed.

  • Thomas R Mazanec June 20, 2018, 12:32

    There are no confirmed detections, but at least there are four candidates:

  • Alex Tolley June 20, 2018, 15:00

    What is clear in our one solar system example is that all but one of the moons is not large enough to hold an atmosphere for long (Titan being an exception, but way outside the HZ). Is there a reason for this? Do super gas giants have commensurately larger moons, or is there an aspect of moon formation that precludes large rocky moons except in rare cases or planet capture? While we should certainly look to find such moons (and large ones are much easier to find than small ones) we may just find that there are very few, if any, that meet the size criterion.

    [Of the 4 in the possible exomoon list on Wikipedia, all have tiny masses or may not be moons at all.]

  • H. Floyd June 20, 2018, 15:45

    Exomoons might catch a break with regard to at least two suspected Great Filters: axial stability and cosmic bombardment.

    Terrestrial planets might have a hard time developing complex life without the stabilizing partnership of a large (and unlikely) moon, but you might sidestep that dilemma by -being- a moon: https://www.space.com/12464-earth-moon-unique-solar-system-universe.html

    A host gas giant should also decrease the frequency of catastrophic collisions: https://archive.nytimes.com/www.nytimes.com/2009/07/26/weekinreview/26overbye.html

    I’d also hoped to find that terrestrial worlds nested within the magnetospheres of gas giants would not need magnetospheres of their own — which seems to be one of Mars’ lethal deficits. Instead, a terrestrial exomoon might need one all the more: https://www.astrobio.net/news-exclusive/hiding-from-jupiters-radiation/amp/

    It’d be fascinating to re-evaluate Great Filters as they’d specially relate to terrestrial exomoons.

    • Paul Gilster June 20, 2018, 16:29

      “It’d be fascinating to re-evaluate Great Filters as they’d specially relate to terrestrial exomoons.”

      That had never occurred to me. What an interesting thought!

    • Michael June 21, 2018, 2:06

      There are still a number of issues with moons around gas giants in a HZ’s. For one the orbital velocities around the giant planets are high, this together with the high orbital velocity of the planet in its orbit gives rise to very high impact velocities. Two, the magnetic fields of gas giants are powerful and can accelerate ions to high velocities stripping atmospheres. These ions could also cause a cascade effect where ions are released by photo ionisation and then accelerated by the giant planets magnetic field to re-impact the moons atmosphere and release even more ions. Thirdly is if a techalien species was to develop on one of these moons it would find it difficult to get away from the system due to the high escape velocities these giant worlds have.

    • Mauldred June 21, 2018, 4:03

      Unfortunately, recent models suggest that an exomoon around a gas giant would actually need a magnetosphere of its own. Being protected by the magnetosphere of a gas giant like Jupiter would mean being too close from it to be habitable (excessive tidal heating and illumination from the host planet would conspire to make it more Io-like or Venus-like). https://phys.org/news/2013-03-habitable-edge-exomoons.html#nRlv

      Perhaps for Earth-sized or Mars-sized exomoons, it is likely for them to generate their own magnetosphere if their metallic core is at least partially fluid due to residual heat or some moderate tidal stirring, and/or if they aren’t tidally locked and have a sufficient rotation rate. It looks hard to get both tidal heating and not being tidally locked, though. In the case of Jupiter, Callisto is well outside the magnetosphere of Jupiter and does not experience significant tidal heating but is still tidally locked to it despite its large orbit.

  • Andrew Joron June 20, 2018, 16:21

    These optimistic assessments of exomoon habitability seem to overlook the fact that radiation belts around giant planets have the potential to sterilize any moons inside the belts. Jupiter’s big moons, for example, are all bathed in lethal radiation.

    • J. Jason Wentworth June 21, 2018, 1:21

      I don’t think Callisto (which orbits about 1.1 million miles from Jupiter) is bathed in lethal levels of radiation. If a Jovian-type exomoon in a similar situation was made of silicates with an iron/nickel core, its rotation rate (tidally locked, almost certainly) could be rapid enough to generate an indigenous magnetic field. Also:

      As Alex Tolley mused above, maybe the much more massive Jovian-type exoplanets have proportionately larger and more massive moons. If so, their likely rapid rotational rates (and sidereal days) could easily be about one Earth day, or much less. Even long days might not result in excessive stellar heating, with the frequent and long eclipses likely produced by the Jovian exoplanets (except in the cases of such planets that might have more Uranus-type axial tilts).

      • Mauldred June 22, 2018, 10:28

        Callisto is actually the only Galilean moon where the possibility of installing an outpost for human exploration has been seriously studied by NASA (in their study called Concepts for Human Outer Planet Exploration), because indeed its surface receives a more manageable amount of radiation (300 times less than on Europa, but still about 10 times more than the average on Earth).

  • Geoffrey Hillend June 20, 2018, 16:54

    I used to think that gas giant had to form within the ice line outside the life belt. There could be a gas giant around a star and an Earth sized exomoon within the life belt. The problem is that the exomoon might not be able to have it’s own Moon so it can’t have the angular momentum or fast rotation or would be tidally locked. It can’t be to close to the gas giant or there will be too much tidal heat. The habitable exomoon might have it’s own moon if it orbits the gas giant at a large distance. Earth sized habitable exomoons might be very rare in the galaxy, but not impossible.

    Titan is able to retain an atmosphere because it is far from the Sun and very cold. Warmer temperatures propel a gas molecule faster so that a planets ability to retain an atmosphere is mostly based on escape velocity which is dependent on the mass and size of the planet. The other factor is the temperature. If you moved Titan into the life belt it would loose all its methane and most of it’s atmosphere in a short time. Molecules have different weights and the heavier gases like Co2 can be retained by a planet with a lower escape velocity. The hotter the temperature, the faster the gas moves. I recall the escape velocity of the planet has to be higher than the escape velocity of the gas. An Earth sized planet would retain it’s atmosphere but might become a Jupiter in the outer solar system. Also the freezing point of Co2 is also based on the atmospheric pressure. The lower the atmospheric pressure, the lower the temperature Co2 freezes. If Earth had Mars’s orbit, then Earth would not have as much frozen Co2 as Mars because the Earth’s atmosphere is more than one hundred times denser than Mars. I wonder what such a frozen world might be like and could life evolve there? There would be some liquid water. With a moon, it might have a fast rotation and magnetic field to deflect the solar wind retain it’s atmosphere and water.

  • Geoffrey Hillend June 20, 2018, 17:02

    The escape velocity of Titan is low only 2.6 kilometers per second which only a little more than our Moon’s escape velocity: 2.3 km/s. Mars 5.03 km/s, and Earth’s 11.186 km/s. Escape velocity, Wikipedia.

  • Ronald June 21, 2018, 10:49

    Exomoons remain a fascinating topic. However, I find the idea of super-habitability in relation to exomoons somewhat premature.
    I think it would be a fantastic achievement already to find any exomoon that reasonably fits the criteria for habitability.

    As others here have also noted, there is the issue of the giant planet’s radiation belt and strong tidal forces, both necessitating the moon to be orbiting at sufficient distance.

    The question is whether a sufficiently large moon can exist: even Ganymede is only 2.5% of Me, Titan only 2.25%. If I remember well, there is an estimate that the largest moons would be about 1/5000th of the mass of their giant planet (Ganymede is about 1/12000th of Jupiter, Titan about 1/4000th of Saturn).
    If this holds true, then a moon of 0.5 Me would require a giant planet of about 8 Jupiter masses.

  • andy June 21, 2018, 14:48

    On the subject of exomoons, the candidate exomoon around Kepler-1625b got a reanalysis in this arXiv preprint by Rodenbeck et al.. Looks a bit iffy, but they don’t have the HST data being collected by the HEK team. David Kipping has posted a response on Twitter.

  • Geoffrey Hillend June 21, 2018, 15:54

    The principle that could control the size limitation of the moons of gas giants might be based on how they are made. Gas giants are formed in an accretion disk like stars. The star and gas giant having the largest mass and most gravity hog most of the gas and dust?

  • ljk June 22, 2018, 9:33

    Wouldn’t it be funny if one of those exomoons actually turned out to be a Jupiter brain:


    Of course it will take another Tabitha Boyajian to elevate this idea into the professional astronomy realm to even be discussed seriously.

  • Sean M. Brooks June 25, 2018, 21:58

    I’m not an expert in physics or astronomy, so all I can offer are some analogies from science fiction. This article about possibly humanly habitable exomoons of gas giants reminded me of Poul Anderson’s novel VIRGIN PLANET, featuring the planet Atlantis, a moon of the gas giant Minos. I’ve also thought of Wayland, an uninhabitable but metal rich exomoon of another gas giant in another novel of Anderson called A CIRCUS OF HELLS.

    Here we see real world scientific discoveries coming close to vindicating science fictional speculations!

  • Harry R Ray June 26, 2018, 11:48

    Add one more to the list, AND IT IS A LULU!!! EPIC248847494 is a VERY EVOLVED G star of mass: 0.9+/-0.09 Msun(now TECHNICALLY a K subgiant)with a radius of 2.70 +/- 0.12Rsun located 1800 light years from Earth. EPIC248847494b is a 1.1 Rj JUPITER ANALOG with a Teff of 180K for its MOST LIKELY ORBITAL PERIOD of 10 years. HOWEVER: the orbital period COULD be as short as 7 years. Rene Heller stated that there could be Mars sized exomoons orbiting it! What’s REALLY exciting about this is that Mars sized Super Europa analogs could have developed life BILLIONS of years ago but by NOW may have made it to the SURFACE if these Super- Europa analogs have had their ice crusts MELTED by now! Since the ice crusts started melting only a few hundred million years ago, their atmospheres have probably NOT completely melted away, providing a scenario for any organisms present to START to EVOLVE!

  • Sean M. Brooks June 26, 2018, 17:45

    Dear Mr. Ray: I like these comments of yours and I hope they are borne out! And I hope humanly habitable planets which are exomoons of gas giants are found nearer to us.

  • Harry R Ray June 27, 2018, 10:22

    Why this one is SO IMPORTANT(and why it should be ADDED to the recently released list of 121)is that we KNOW that it is NOT a brown dwarf! The reason for this is: DUE to an INCREDIBLE stroke of good luck, a FULL 54 day transit was observed within the framework of an eighty day K2 observation run during campaign 14!!! Follow-up radial velocity observations indicate that this object’s mass is less than 13 Mj, meaning that this object is of planetary origin.

  • ljk September 20, 2018, 12:16


    Survivability of Moon Systems Around Ejected Gas Giants

    Ian Rabago, Jason H. Steffen

    (Submitted on 15 Sep 2018)

    We examine the effects that planetary encounters have on the moon systems of ejected gas giant planets. We conduct a suite of numerical simulations of planetary systems containing three Jupiter-mass planets (with the innermost planet at 3 AU) up to the point where a planet is ejected from the system. The ejected planet has an initial system of 100 test-particle moons.

    We determine the survival probability of moons at different distances from their host planet, measure the final distribution of orbital elements, examine the stability of resonant configurations, and characterize the properties of moons that are stripped from the planets.

    We find that moons are likely to survive in orbits with semi-major axes out beyond 200 planetary radii (0.1 AU in our case). The orbital inclinations and eccentricities of the surviving moons are broadly distributed and include nearly hyperbolic orbits and retrograde orbits.

    We find that a large fraction of moons in two-body and three-body mean-motion resonances also survive planetary ejection with the resonance intact. The moon-planet interactions, especially in the presence of mean-motion resonance, can keep the interior of the moons molten for billions of years via tidal flexing, as is seen in the moons of the gas giant planets in the solar system.

    Given the possibility that life may exist in the subsurface ocean of the Galilean satellite Europa, these results have implications for life on the moons of rogue planets—planets that drift through the galaxy with no host star.

    Comments: 8 pages, 7 figures. Submitted to MNRAS

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    Cite as: arXiv:1809.05639 [astro-ph.EP]
    (or arXiv:1809.05639v1 [astro-ph.EP] for this version)

    Submission history

    From: Ian Rabago [view email]

    [v1] Sat, 15 Sep 2018 03:15:56 GMT (2679kb,D)


    • ljk September 20, 2018, 12:21

      There was an article from a few years ago speculating that the four big Galilean moons of Jupiter were just the “survivors” of the evolution of many moons around the gas giant. The rest either sunk into the planet, smashed into each other, or were flung out into the Sol system.

      How could we determine which ones that left Jupiter came from there, assuming they survived? Could any of them have been flung strongly enough to leave the Sol system entirely?