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Liquid Water on a Free Floating Planet’s Moon?

As we learn more about how planetary systems form, it’s becoming accepted that a large number of planets are being ejected from young systems because of their interactions with more massive worlds. I always referred to these as ‘rogue planets’ in previous articles on the subject, but a new paper from Patricio Javier Ávila (University of Concepción, Chile) and colleagues makes it clear that the term Free Floating Planet (FFP) is now widespread. A new acronym for us to master!

There have been searches to try to constrain the number of free floating planets, though the suggested ranges are wide. Microlensing seems the best technique, as it can spot masses we cannot otherwise see through their effect on background starlight. Of these, the estimates come in at around 2 Jupiter-mass planets and 2.5 terrestrial-class rocky worlds per star that have been flung into the darkness. This is a vast number of planets, but we have to be wary of mass uncertainties, as the cut-off between planet and brown dwarf (usually around 13 Jupiter masses) comes into play.

Image: An artist’s conception of a free floating planet. Credit: JPL/Caltech.

Any chance for life on a world like this? It’s hard to see how unless it’s something exotic indeed, but it’s Friday, so let’s play around with the idea. A major paper on rogue worlds is a 1999 discussion in Nature by David Stevenson (Caltech), which assumes a hydrogen-rich atmosphere. I’m just going to pull this out of the abstract before moving on to the Ávila paper:

Pressure-induced far-infrared opacity of H2 may prevent these bodies from eliminating internal radioactive heat except by developing an extensive adiabatic (with no loss or gain of heat) convective atmosphere. This means that, although the effective temperature of the body is around 30 K, its surface temperature can exceed the melting point of water. Such bodies may therefore have water oceans whose surface pressure and temperature are like those found at the base of Earth’s oceans. Such potential homes for life will be difficult to detect.

To say the least. Let’s also note a later paper by Steinn Sigurðsson and John Debes that has shown that among terrestrial class planets ejected from their stars, a good number may retain a lunar-sized moon. Citations for both these papers are below.

But let’s think bigger. Ávila and colleagues go after Jupiter-sized worlds with large, terrestrial-sized moons (far larger than any we see in our Solar System, where Ganymede, larger than Mercury but much smaller than Earth, reigns supreme). They model the chemical composition and evolution of CO2 and water in an attempt to discover the kind of atmosphere that would allow liquid water on the surface. CO2 is found to produce more effective atmospheric opacity (governing atmospheric absorption) than Stevenson’s choice of molecular hydrogen.

From the paper:

…to the best of our knowledge, there are no detailed models of the chemical evolution of the atmosphere of a moon orbiting an FFP. Within this context, we introduce here an atmospheric model to tackle this limitation. We assume that in the absence of radiation from a companion star, the tidal and the radiogenic heating mechanisms represent the main sources of energy to maintain and produce an optimal range of surface temperatures.

The authors simulate the atmosphere of an Earth-sized moon in an eccentric orbit around a gas giant, analyzing its thermal structure and determining the mechanisms that can keep it warm. The assumption is that carbon dioxide accounts for 90% of the moon’s atmosphere. The model relies on radiogenic heating along with tidal factors as the main energy sources while invoking an atmosphere under changing conditions of cosmic ray ionization, chemistry, pressure and temperatures.

In a setting like this, the cosmic-ray ionization rate (CRIR) drives chemistry in the atmosphere. A bit more on this:

Due to the absence of impinging radiation, the time-scale of water production is driven by the efficiency of cosmic rays in penetrating the atmosphere. Higher CRIRs reduce the water formation time-scale when compared to low-CRIR models, implying that they play a key role in the chemical evolution, by enhancing the chemical kinetics. However, due to the attenuation of cosmic rays, in the lower layers of the atmosphere, the water production is also affected by the density structure, that determines the integrated column density through the atmosphere. This causes an altitude-dependent abundance of water as well as of some of the other chemical species, as CO, H2 and O2.

The authors’ model assumes an initial 10% molecular hydrogen and measures changes depending on atmospheric pressure, semi-major axis and eccentricity, the latter generating tidal heating. In the best scenario, we wind up with an amount of water on the surface of the moon that is about 10,000 times smaller than the volume of Earth’s oceans, but 100 times larger than found in Earth’s atmosphere. Thus we have a conceivable way to keep water a liquid on the surface, offering the possibility of prebiotic chemistry:

“Under these conditions, if the orbital parameters are stable to guarantee a constant tidal heating, once water is formed, it remains liquid over the entire system evolution, and therefore providing favourable conditions for the emergence of life.

Keeping that orbit eccentric enough to produce the needed tidal forces is a challenge. The authors’ research indicates that while moons around ejected gas giants may exist up to 0.1 AU from the planet, closer orbits in the range of ≲ 0.01 AU are more probable (Jupiter’s largest moons are within 0.01 AU). Is a single moon in this configuration not going to circularize its orbit, or can earlier orbital resonances survive the ejection? A good science fiction writer should have a go at this scenario to see what’s possible.

The paper is Avila et al., “Presence of water on exomoons orbiting free-floating planets: a case study,” International Journal of Astrobiology published online 08 June 2021 (full text). The Sigurðsson and Debes paper is Debes & Sigurðsson, ”The Survival Rate of Ejected Terrestrial Planets with Moons,” Astrophysical Journal Vol. 668, No. 2 (2 October 2007) L 167 (full text). The Stevenson paper is “Life-sustaining planets in interstellar space?” Nature 400 (6739):32 (1999). Abstract.

{ 29 comments… add one }
  • Alex Tolley June 11, 2021, 14:51

    What I find interesting is that this represents a possible edge condition for life. The FFPs are not going to be easily observable as they are not starlike, so finding them may require some searching, although an ongoing all-sky gravitational microlensing survey might turn up a lot if the numbers are to be believed. But what is really interesting is that unlike our icy moons that astrobiologists will ignore because they will not have detectable biosignatures, this is very much not the case with the worlds around these planets if they are large enough to maintain liquid water on the surface. How to observe them will be interesting as they will only reflect and emit IR. Unlike planets around a star, the planets/moons around FFPs may well be warmer and stronger signal sources than the FFP itself, depending on its type.

    An observer on one of these moons around a gas giant would only see a dark disk blotting out the stars, although the disk would show lightning flashes, comet impacts, and possibly dim details in the IR. The moon itself would be similarly dar except in the IR and longer wavelengths unless there was a nearby star that was a dim source of light. My sense is that any life would have to be chemotrophic as there would not be sufficient energy in the IR to allow any sort of photosynthesis, but one never knows. The energy supply would be very low, making this world quite sparse in any sort of autotroph, although the chemotrophs might be common. There would likely not be any O2 in the atmosphere unless the FFP emitted enough ionizing radiation to split water in the atmosphere.

    Definitely ample scope for world-building.

  • Geoffrey Hillend June 11, 2021, 16:23

    Since rogue gas giants are thought to be ejected, most likely they won’t have moons. If they do have earth sized moons or larger, then we will run into the problem of loss of atmosphere through freezing. The areas where there is tidal heating will only be effective if the surface has some volcanism. Most of the surface of the moon would be very cold accept around the areas where there is lava and volcanism. We can use the surface temperature of Io as an example and Io has a lot of tidal heating since it is very close to Jupiter, so the moon would have to be close to get the tidal heating.

    The whole atmosphere would have to be heated by the meager infra red coming from lava and cyrovolcanoes. The best one could hope for is a frozen Titan with some atmosphere which already had a lot of hydrocarbons. A cold temperature does tend to retain an atmosphere Vapor pressure still has to be considered though, since we would need some kind of atmosphere so liquid water could exist. Without the hydrocarbons there would be a problem because there would not be a thick enough atmosphere. Ganymede has lost it’s atmosphere through jeans escape since it is closer to the Sun than Titan. An atmosphere of the moon of a rogue gas giant without a star might be in a deep freeze with most of its surface frozen after deep time.

  • Robin Datta June 11, 2021, 16:25

    They ain’t floatin’. Archimedes’ Principle does not apply.

    • Alex Tolley June 12, 2021, 11:56

      While technically correct, it is rather pedantic. The use of “float[ing]” in reference to objects including astronauts in freefall. “Float” can describe the movement of an object – e.g. the balloon floats up into the air, or a static state where the object is at equilibrium, e.g. the shop floats on the sea. In the former case there is movement, in the latter no movement. “Floating in space” uses the lack of movement as a metaphor, even though the term is not used in a technical sense. A google search shows how frequently the float and space are combined. So FFP seems like a reasonable use of the term, just as black holes are neither [technically] black nor holes.

      Patient Observer uses: ”Gravitationally liberated planet”, but that doesn’t fit either. The planet is still bound to the galaxy’s gravity, and we certainly don’t think that the planet itself has no gravity either. “Non-star centric” seems more accurate, but either “rogue” or “free floating” is more evocative and has a romance attached to it, making them more likely to capture the imagination and common parlance.

    • Kevin A June 12, 2021, 12:05

      Yep. Seems a more appropriate name would be “Interstellar Planet”.

      • Robert June 12, 2021, 15:40

        How about Intersteller Watering Hole since ‘hole’ seems to be in common usage in astronomy.

    • Robert June 12, 2021, 15:36

      Sure they are. The word has more meaning than buoyancy. Also, in a sense they are floating on the level surface (zero curvature due to another body such as a star) of spacetime.

  • Patient Observer June 11, 2021, 22:24

    “Rogue” planet sounds menacing. “Free floating planets” is wrong per Datta. “Non-star centric” or ” Gravitationally liberated planet” seems more in keeping with modern parlance.

    If Venus became gravitationally liberated and wandered into interstellar space like our moon did in “Space 1999”, would its internal heat be sufficient to maintain a surface temperature significantly higher than the cold of interstellar space or would its CO2 atmosphere freeze out ending its potential as a sci fi story.. Tried to find a Mollier diagram for CO2 but soon realized I was in over my head.

  • gbaikie June 12, 2021, 4:48

    “Any chance for life on a world like this? It’s hard to see how unless it’s something exotic indeed,
    but it’s Friday, so let’s play around with the idea. ”

    Can make living with a Free Floating Planet?
    Need energy- no solar, so nuclear.
    How about an asteriod belt as depicted in movies.
    1 million km cube of space: volume 1 million times 1 million times 1 million
    a 100 km cube is 100 x 100 x 100 = 1,000,000 cubic km
    So 1 million km cube has 1 trillion 100 km cubes.
    Say there was 1 trillion rocks was was 10 km in diameter [or less]. 1 billion between 10 to 30 km in diameter.
    1 million between 30 to 50 km in diameter, 10,000 between 50 to 100 km and 100 larger than 100 km in diameter.
    So all are in within radius of 1 million km. Lots or all are rubble piles. And there is a lot dust. Within the
    1 million km radius.
    Nothing could just fly thru it- at least without hitting some dust.
    What happen if a 1 km diameter rock go thru it at say 50 km/sec?
    Say hits a 40 km diameter rock.
    You get nuclear flash type explosive.
    A 1 km diameter sphere has 5.24×10^8 cubic meter and say density of 2″ : 1.084 x 10^9.
    50,000 m/s x 50,000 m/s x 5.24×10^8 kg is 2,500,000,000 or 2.5 x 10^9 x 5.24×10^8 = 1.31 x 10^18 joules of heat
    1.4×10^18 J Yearly electricity consumption of South Korea as of 2009
    1.2×10^19 J Explosive yield of global nuclear arsenal
    So about 1/10 energy of exploding global nuclear arsenal
    Couple question, is movie type asteriod feild possible. And how offen the high velocity rocks go thru about 1 million km
    cube volume of space.
    Compared cross section of Earth [which is tiny in comparsion] Earth get hit small rocks every few weeks, but 1 km diameter rock is
    tens thousands of years but difference is about 1500 time larger area. So hundreds of years. But could be fast moving dust or 1 meter
    size rocks. Hundreds tons of dust hits earth. And if like Earth, hundreds thousand of tons hit it per year.
    Now, not thinking life evolving in it, rather intelligent life find it, and uses it.

    Or have planet with large atmosphere. If Earth was as warm as earth, lightyear from our sun, how big would it’s atmosphere be?
    If include Earth exosphere: “In principle, the exosphere covers distances where particles are still gravitationally bound to Earth, i.e.
    particles still have ballistic orbits that will take them back towards Earth. The upper boundary of the exosphere can be defined as the
    distance at which the influence of solar radiation pressure on atomic hydrogen exceeds that of Earth’s gravitational pull.
    This happens at half the distance to the Moon”
    So don’t have the sun’s radiation pressure. And most abundant element in universe is Hydrogen. So, instead hydrogen escaping from Earth
    the warm earth should be able to collect hydrogen when far from a star.
    And another fairly abundant element is oxygen- our star releases oxygen, but doesn’t make it, but other stars make oxygen.

  • torque_xtr June 12, 2021, 11:16

    Very difficult to imagine how easily-condensable CO2 might survive as the main component of a free-floating planet atmosphere. One of the outer edges of HZ is defined as the insolation where CO2 condenses at the poles, preventing sufficient greenhouse. In the darkness of space, condensable atmosphere will act as a heat pipe, rapidly transporting heat from the surface to the tropopause, and releasing it from there straight to space, no matter how thick is the atmosphere below. So the minimum required geothermal heat flow would be equal to the thermal radiation of the upper cloud deck composed of condensing gases. This is not less than 100 K for CO2 (temperature of CO2-ice clouds on the nightside of the upper atmosphere of Venus), corresponding to 5 W/m2, and likely quite higher. CO2 condensation point is 130 K at 10 Pa, which gives 15 W/m2 and still almost no greenhouse in above-lying layer (On Earth, partial pressure of CO2 is 40 Pa). So the atmosphere would freeze unless heat flow is many times higher than on Io. Other less condensable greenhouse gases combined with massive hydrogen atmosphere might be another story, requiring possibly just several bars total compared to 1 kbar of pure hydrogen.

    In case of volcanic worlds, most of the surface is still cold, heated only by lithospheric heat conduction which cannot go much higher than Earth-like values (corresponds to 30 K effective temperature, very likely not above 50K) , so the gases are still prone to condensation even at very high averaged heat flows.

    IMO, the best habitable rogue world is a sub-Earth 0.1-0.3 Earth masses. This would cope with diminishing residual accretion heat, leaving mainly radiogenic sources which are considerably more stable. Combined with 10-50 bars of H2/He and several per cent methane, this would provide a decent environment for billions of years (not unlike the lower atmosphere of Uranus where internal heat flow is lower than on Earth but 273 K isotherm is crossed at several tens of bars)

    Radiation levels around a rogue giant would likely not be significantly less than around Jupiter, because it’s magnetosphere would trap interstellar medium particles much like solar wind. It would be also more expanded due to lower surrounding density – no Callistos with benign radiation levels there. Brown dwarfs with their huge fields and frantic rotation are even worse – at the distance needed for liquid water, radiation levels are likely the highest among all classes of temperate planets (exceps WD and pulsar worlds)

  • torque_xtr June 12, 2021, 11:47

    PS an afterthought after new comments appeared. Any civilization on FFP would be severely limited in energy usage because the insulation needed to bring surface temperatures to the range of liquid water goes with very high sensitivity to heat sources on the surface!

    Here on Earth we cannot go above several petawatts of any type of energy. After this we’ll get global warming in it’s earnest due to additional heat sources becoming comparable to the solar input. But on FFPs the constraints would be much, much tighter, limited to the small fraction of geothermal heat instead. If the internal heat flow of 100 mW/m2 gives surface temperature of 298 K, then additional 10 mW/m2 will raise surface temperatures by several degrees, and our current gross energy production is already higher than that. According to https://ourworldindata.org/energy-production-consumption, 10 mW/m2 threshold was passed around 1960: 40000 TWh per year is 4.5 TW. Divided by the surface area of Earth, 510 million sq.km, this gives 9 mW/m2.

    • Mike Serfas June 13, 2021, 16:27

      @torque_xtr: This is true, but it is the smaller part of a big problem. Insulating a planet and providing organisms with *comfortable temperatures* is not really the same as providing them with *energy*. A terrarium full of plants, animals and fungi will not sustain life for long in the dark. If the planet has that low of a rate of heat efflux, the question becomes whether it is possible to route that effectively enough into individual organisms, cells, and proteins such that the free energy for an ATP-powered reaction is still there in one time and place. The requirement for free, thermodynamically useful energy to lift a finger or fix nitrogen has not changed.

      I think the civilization would be able to manage its heat budget well, because cooling is also a source of free energy. Conceptually, if an insulated copper cable is attached to a kite that flies in the steady Coriolis winds of their dense atmosphere, a heat engine can tap into the differential of temperature the same way as it might with a geothermal well, and the effect on temperature between the two can cancel out. Practically, I suppose they would use compact windmills for power – I have no idea what it would look like for windmill usage to quell the atmosphere of a planet to the point where it can’t exchange heat. But the turbulence of air is a complex science, and there ought to be some solution that encourages a more effective convection system in the atmosphere if that is needed.

      Side Track: I’m voting for “Rogue”. Planets wild and independent, subject to no one star’s law of gravity, wandering by stealth until they emerge from Planet Nine grade gloom to startle the natives of Earth with their city lights, perhaps. We need to stop strip-mining the poetry of our language to replace it with TLAs no one can remember.

  • Geoffrey Hillend June 12, 2021, 15:43

    I don’t recall saying anything about floating. I was thinking about the physics of a gas giant and it’s moon(s) being ejected and assuming that process potentially can trip away a moon. Also the size of the moon matters although there is no solar wind to strip away the atmosphere, the planet will still get really cold without sunlight. Imagine the Earth free somewhere halfway between the nearest star. It’s oceans and atmosphere would freeze. Now imagine it in close orbit around a rogue gas giant in interstellar space, the only thing heating it would be the tidal forces if it was close enough to the gas giant. The atmosphere would be heated only by the infra red thermal radiation from hot spots on the surface from lava or volcanoes in a few areas but most of its surface would be really cold. On the other hand due to the tidal forces some areas in the crust might be warm enough to melt the oceans in some areas near the ocean floor.

    • gbaikie June 12, 2021, 19:58

      “Imagine the Earth free somewhere halfway between the nearest star. It’s oceans and atmosphere would freeze.”

      Imagine Earth within a million years of it’s formation {particularly if the Moon is much nearer Earth}. Imagine Earth as starship good for only a few million years.
      Imagine answer to fermi paradox is our sun is bad star, and wrong time period to deal with this bad sun- If Sol was at time when closer the other solar systems, maybe then our bad sun is not so much of problem.
      So in terms habitability, one evolution could be quicker. And two, whatever looking at, didn’t need life to evolve on it- spacefaring civilizations could have many habitable worlds. We are not even spacefaring and we imagine we might be able to live on Mars. Main issue with Mars is, is it economical to live on Mars. Same goes for Mercury, Venus, our Moon, and etc.
      Way I look at it, Earth is not economical to live on- but we evolved on it.

  • Alex Tolley June 12, 2021, 18:07

    Reading the Stevenson scientific correspondence piece in Nature, it seems to me that while the planet is described as being interstellar, there is no reason why such Earth-sized planets could not also be in the outer reaches of the solar system. They just need enough mass and a kilobar CO2 atmosphere to maintain liquid water on the surface. If so, then this would indicate that the HZ would be much further out than hitherto calculated. This seems unlikely.

    The Avila paper, by contrast, assumes that the moon is Earth-sized with a dense CO2/H2 atmosphere and that it orbits an ejected Jupiter. The main source of heat is not radioactive decay, but tidal heating, like that of Europa. This requires not just an ejected gas giant, but also its retinue of moons, one of which must be of comparable size to Earth. If Geoffrey Hillend is correct, such an arrangement is unlikely.

    So not only unlikely but as Stevenson states:

    Such potential homes for life will be difficult to detect.

  • Thomas R Mazanec June 12, 2021, 18:59

    Wikipedia still uses “Rogue planet”.

  • Michael June 13, 2021, 0:18

    There is also magnetic field heating, if the gas giant is a fast rotator a current will be induced into any conductive materials on the moons. An intelligent species can utilise that power with conductive coils say at the poles of the moons and they could be quite small.

  • Wojciech Jacyk June 13, 2021, 8:22

    Some two interesting findings related to potential interstellar travel, panspermia and space colonization.

    Research shows that mammalian sperm can survive up to 200 years in environment like ISS


    Bdelloid Rotifers are claimed to be successfuly revived after being frozen for 24,000 years.


  • Michael Fidler June 13, 2021, 11:11

    A large super Earth is all that is needed, will be hot internally and H/CO2 to keep it nice and comfy. Nothing like a deja vu, think this might be where all those aliens are living, right next door… )

    • Alex Tolley June 13, 2021, 22:36

      That and the revival of dormant microorganisms 100 million years old suggests that [directed] panspermia is possible in theory.

      Imagine a civilizational program to place containers of [engineered] microbe populations inside every icy body in the Oort cloud so that exchanges of these bodies with other stars as the galaxy rotates offer the possibility of seeding a sterile world. A huge program, but one suited to [self-reproding] robotic execution. [Von Neumann replicators but confined to the Oort.] It might be suitable for mass production of small craft to reach their icy body target using some form of propulsion suited to the technology of that civilization. This is a very altruistic program, although it just might be a spinoff of a more prosaic goal of ensuring life depots for habitats and slow boat starships in the far future.

  • DCM June 13, 2021, 12:36

    They’re probably lifeless if they weren’t always, so they’re possible targets for modification and use — in the very long run of course.

  • gbaikie June 13, 2021, 15:20

    Intelligent creatures can live anywhere. But we connected to our natural
    world, so we could need to build environments for our life [and if we like, alien life} but such environment don’t need much energy- or they don’t use much energy on our planet. Or most energy used for Earth is regions with little life- deserts [1/3 of land] and most ocean surface nearly sterile- or called sterile ocean “deserts”. Or life uses little energy and most energy available is “wasted”. And our star wastes basically all it’s energy in comparison to amount used by life. Or just in terms of all matter of solar system, the sunlight has only very small portion warming any and all of the matter in solar system. And of all matter, warm, some very small percentage has life even near it.
    In terms of energy, we have nuclear fusion of Sun, other nuclear energy and orbital energy [energy related to gravity] and beyond solar system there is orbital energy and some stars emit so much energy- one can even consider solar energy when nearer such stars.

  • Geoffrey Hillend June 13, 2021, 16:51

    I agree with Alex Tolley. Another problem is the ice line since usually Earth sized bodies turn into gas giants since whether an atmosphere can keep an atmosphere is also based on it’s distance from the star and temperature i.e., an Earth size moon around a gas giant is very rare or it might not even be possible since it looks as though the larger body, the gas giant forms it own accretion disk to make the moons which might be small because the larger gas giant grabbed all the gas and dust just another contingency to add to the rareness of such an Earth sized exoplanet.

    I agree with large mass and the kilobar CO2 atmosphere because the temperature of deep interstellar space is very low on a exoplanet’s surface. Pluto, the temperature is only 375 to 40o below zero Fahrenheit and the Sun looks only like a bright star from cold, distant Pluto. Most of the atmosphere will freeze out of the air onto the land. The vapor pressure even matters on a small moon like Io or some other moon sized body or smaller with some sunlight. If you open up can of liquid water an poor it onto the ground, some of it will quickly freeze or most will instantly evaporate into water vapor or ice crystals. With no atmosphere or vapor pressure, there can’t be any liquid water, but only frozen or vapor which is what happens on the surface of Mars that has a thin of 6 or 7 millibars which is less than one hundred of Earth’s average surface pressure of 1,000 millibars. Consequently liquid water would have to be under the surface on Io and Mars.

  • AlexTru June 14, 2021, 2:51

    I am sure there are no any realistic models are standing after this hypothesis, all idea based on very good imagination of authors and not supported by any observations, calculations and/or models.
    Still good narrative for Sci-Fi story, no more.

  • stephen June 14, 2021, 15:04

    Could a hot Jupiter be perturbed out of its orbit, and retain plenty of heat for a long time? What moons could it have, and what moons could it retain after being perturbed out of orbit?
    As it’s being expelled from the solar system, it might perturb the orbits of other planets in the same system, and they all get expelled together…?

  • Michael T. June 14, 2021, 23:42

    Actually, the term “planet” is almost sufficient in itself, as it means “wanderer” anyway. But “interstellar planets” or “free planets” or “roaming planets” seem good. “Free floating planets” is evocative, though I could object to “float” if I was predantic.

    Or you could say, “Not in a relationship” – just dating every few million years.

    “Rogue planet” seems a bit perjorative unless it’s passing through a solar system you’re fond of .

  • Edwin June 15, 2021, 6:47

    Survival of exomoons around exoplanets

  • Marshall Eubanks June 15, 2021, 16:13

    I have always like the term nomadic planets, as they are indeed wandering.

    The best way to sustain substantial tidal heating is through a multi-body resonances causing forced eccentricities. That (and their Laplace resonance) is why Io, Europa and Ganymede haven’t long ago circularized their orbits. If nomadic planet moons are going to be really warm due to tidal heating, I would look for multiple moons in a resonance.

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