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A Habitable Exomoon Target List

Are there limits on how big a moon can be to orbit a given planet? All we have to work with, in the absence of confirmed exomoons, are the satellites of our Solar System’s planets, and here we see what appears to be a correlation between a planet’s mass and the mass of its moons. At least up to a point – we’ll get to that point in a moment.

But consider: As Vera Dobos (University of Groningen, Netherlands) and colleagues point out in a recent paper for Monthly Notices of the Royal Astronomical Society, if we’re talking about moons forming in the circumplanetary disk around the young Sun, the total mass is on the order of 10-4Mp. Here Mp is the mass of the planet. A planet with 10 times Jupiter’s mass, given this figure, could have a moon as large as a third of Earth’s mass, and so far observational evidence supports the idea that moons can form regularly in such disks. There is no reason to believe we won’t find exomoons by the billions throughout the galaxy.

Image: The University of Groningen’s Dobos, whose current work targets planetary systems where habitable exomoons are possible. Credit: University of Groningen.

The mass calculation above, though, is what is operative when moons form in a circumplanetary disk. To understand our own Moon, we have to talk about an entirely different formation mechanism: Collisions. Here we’re in the fractious pin-ball environment of a system growing and settling, as large objects find their way into stable orbits. Collisions change the game: Moons are now possible at larger moon-to-planet ratios with this second mechanism – . our Moon has a mass of 10−2 Earth masses. Let’s also consider moons captured by gravitational interactions, of which the prime example in our system is probably Triton.

What we’d like to find, of course, is a large exomoon, conceivably of Earth size, orbiting a planet in the habitable zone, or perhaps even a binary situation where two planets of this size orbit a common barycenter (Pluto and Charon come closest in our system to this scenario). Bear in mind that exoplanet hunting, as it gets more refined, is now turning up planets with masses lower than Earth’s and in some cases lower than Mars. As we move forward, then, moons of this size range should be detectable.

But what a challenge exomoon hunters have set for themselves, particularly when it comes to finding habitable objects. The state of the art demands using radial velocity or transit methods to spot an exomoon, but both of these work most effectively when the host planet is closest to its star, a position which is likely not stable for a large Moon over time. Back off the planet’s distance from the star into the habitable zone and now you’re in a position that favors survival of the moon but also greatly complicates detection.

What Dobos and team have done is to examine exomoon habitability in terms of energy from the host star as well as tidal heating, leaving radiogenic heating (with all its implications for habitability under frozen ocean surfaces) out of the picture. Using planets whose existence is verified, as found in the Extrasolar Planets Encyclopedia, they run simulations on hypothetical exomoons that fit their criteria – these screen out planets larger than 13 Jupiter masses and likewise host stars below 0.08 solar masses.

Choosing only worlds with known orbital period or semimajor axis, they run 100,000 simulations for all 4140 planets to determine the likelihood of exomoon habitability. 234 planets make the cut, which for the purposes of the paper means exomoon habitability probabilities of ≥ 1 percent for these worlds. 17 planets of the 234 show a habitability probability of higher than 50 percent, so these are good habitable zone candidates if they can indeed produce a moon around them. It’s no surprise to learn that habitable exomoons are far more likely for planets already orbiting within their star’s habitable zone. But I was intrigued to see that this is not iron-clad. Consider:

Beyond the outer boundary of the HZ, where stellar radiation is weak and one would expect icy planets and moons, we still find a large number of planets with at least 10% habitability probability for moons. This is caused by the non-zero eccentricity of the orbit of the host planet (resulting in periodically experienced higher stellar fluxes) and also by the tidal heating arising in the moon. These two effects, if maintained on a long time-scale, can provide enough supplementary heat flux to prevent a global snowball phase of the moon (by pushing the flux above the maximum greenhouse limit).

More good settings for science fiction authors to mull over!

Image: This is Figure 2 from the paper. Caption: Habitability probability for exomoons around known exoplanets on the semi-major axis – stellar effective temperature plane. Planets with known masses (with or without radius data) are marked with circles, planets with known radii only are marked with triangles. Colours of the markers correspond to the fraction of habitable moons and the sizes of the markers represent the sizes of the planets as shown in the legend. Note that the legend only shows three
representative sizes (Earth, Neptune and Jupiter), while the size of the markers in the plot is scaled to the real size of the planets. Green curves represent the borders of the circumstellar habitable zone for a 1 Earth-mass planet: dark green for the consevative HZ (Con. HZ) and light green for the optimistic HZ (Opt. HZ). Credit: Dobos et al. 2022.

Given that the spectral type of over half of the stars in the Extrasolar Planets Encyclopedia is not listed, there is a good deal of play in these results, although the authors point to the mitigating effect of gas giant magnetospheres as shields against incoming stellar radiation for potentially habitable moons. Even so, stellar type is clearly an important factor, and it’s also noteworthy that while the paper mentions planet migration, its effects on exomoons are not under consideration. This is about as much as the authors have to say about migration:

It is likely that the giant planets in the circumstellar HZ were formed at larger distances from the star and then migrated inwards to their current orbit (see for example Morbidelli 2010). During the orbital migration they can lose some or all of their moons, especially if the moon orbit is close to the planet (Namouni 2010; Spalding et al. 2016). Depending on the physical and orbital parameters of the planet and the moon, as well as on the starting and final semi-major axes of the planet, some moons can survive this process, and new moons can also be captured during or after the migration of the planet.

Just how migration would affect the results of this study is thus an open question. What we do wind up with is what the authors consider a ‘target list’ for exomoon observations, although one replete with challenges. Most of these potential exomoons would orbit planets whose orbital period is in the hundreds of days, planets like Kepler-62f, with a 268 day period and a 53 percent habitability probability for an exomoon. This is an interesting case, as stable moon orbits are likely around this 1.38 Earth radius world. But what a tricky catch for both our exomoon detection techniques.

Because many of the planets in the target list are gas giants, we have to consider the probability that more than a single moon may orbit them, perhaps even several large moons where life might develop. That’s a scenario worth considering as well, independent emergence of life upon two moons orbiting the same exoplanet. But it’s one that will have to wait as we refine exomoon scenarios in future observations.

The paper is Dobos et al., “A target list for searching for habitable exomoons,” accepted at Monthly Notices of the Royal Astronomical Society 05 May 2022 (abstract / preprint). Thanks to my friend Antonio Tavani for the heads-up on this work.

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{ 27 comments… add one }
  • Edwin May 17, 2022, 6:35

    Wow

    Yes this is one of my favorite subjects. I have a great collection of papers on the subject too, Thanks for the great post Paul.

    Cheers Edwin

  • Drew LePage May 17, 2022, 9:35

    Habitable moons has always been one of my favorite subjects!
    https://www.drewexmachina.com/2018/12/01/habitable-moons-two-decades-later/
    Glad to see work continuing in this area after over two decades and that we are on the cusp of actually finding such worlds.

  • Michael Fidler May 17, 2022, 10:48

    Why can’t Jupiter mass planet have a collision with a Saturn mass planets and form a Hycean moon?

    Hycean planets might be habitable ocean worlds.
    https://earthsky.org/space/hycean-planets-exoplanets-habitability/

    • Adam Crowl May 17, 2022, 22:17

      Hi Michael
      Why do they have to collide?

      • Michael Fidler May 18, 2022, 6:16

        Well, hopefully they can form from the original formation of the gas giant but collision where common when the earth formed our moon so it may make a much larger moon around Jupiter size planets. The main question is would this need to take place early to get rocky material from the core of the gas giant. Either way a Hycean ocean moon could be habitable as far out as Neptune around a sun type star.

  • Alex Tolley May 17, 2022, 12:06

    If there are several habitable moons around a planet, if any acquires life, all of them will, in short order, due to cross0contamination. A bacterial/viral load will be in a donut for each moon’s orbit, with a tail in the direction away from the star. This could, in turn, cross-contaminate moons and planets even further outwards in the system. On top of that, there will be lithopanspermia due to impacts throwing the ejecta into random orbits around the system.

    This would be unlike our system’s icy moons where any life will quickly freeze in space, and life from Earth (or possibly the early Venus and Mars) will fall on inhospitable icy surfaces.

    I do wonder if there are habitable moons around a Jupiter in the HZ whether they might also be subject to very high radiation levels and whether that will impact the sort of life that could evolve. Would it have very effective repair mechanisms and protection for the macromolecules including its genetic storage, or would it evolve more robust biology? If the biology is very different, could we detect it directly, or only infer it via chemical proxies, like gas disequilibria?

    • Brett May 18, 2022, 16:38

      That was my concern as well. A Jovian in an HZ orbit around a Sun-like star is going to have some pretty intense radiation belts, although perhaps that won’t be too much of a problem if the Earth-like moon is orbiting out at Callisto-esque difference (although that would also mean that the moon’s solar insolation would vary considerably over the course of an orbit, and it would have week-long days and nights).

      You’d also have to worry about it pulling in higher numbers of asteroids and comets, causing a greater impact frequency.

      It’d be easier if it was more like a double planet, like an Earth-sized moon orbiting an ice giant planet like Uranus.

  • Henry Cordova May 17, 2022, 13:09

    How likely is an exomoon to be tidally locked to its primary planet? I presume this would depend on the relative masses of the two worlds, and their orbital separation, and perhaps even the distance to the primary star.

    In the event the planet is tidally locked to the star (that is, keeps the same face to it, (or is locked in some rotational/revolutionary resonance) if the moon is rotationally locked to the planet as well then the insolation pattern might be quite complex. What are the consequences for life on the moon?

    How stable, over evolutionary time scales, would these potential configurations be?

    Is anyone aware of any detailed speculations on these issues, either in speculative fiction or peer-reviewed monographs? Different combinations of orbital configurations for exomoons might yield an extraordinary variety of of diverse niches for life. Perhaps its a good thing…

  • andy May 17, 2022, 13:53

    I’m surprised they state that Kepler-62f has a measured mass: as far as I’m aware there’s only an upper limit of around ~35 Earth masses.

  • Robert May 17, 2022, 16:34

    A bit of a tangent but I found this older article by Gregory Benford about terraforming our moon which I thought might be interesting.

    https://slate.com/technology/2014/07/terraforming-the-moon-it-would-be-a-lot-like-florida.html

    • Brett May 18, 2022, 16:40

      The Moon would be a plausible candidate for “para-terraforming” – IE covering it in interconnected cylinders or heavy anchored canopies. Wouldn’t need as much air, either, and you could make it as comfortable as you want.

    • Mike Serfas May 19, 2022, 10:04

      This Slate article seems like a bait and switch: it describes a Russian proposal for a moon base several times, but the terraforming project the headline is about seems solely its author’s idea. Yes, this article may be the best source quoted by Wikipedia about how to terraform the moon … but I’m not convinced the numbers work out. The proposal is to add 100 Halley’s Comets (2.2E+14 kg x 100) of air+water, whereas I vaguely remember seeing a figure of tens of thousands in the past. By comparison the Earth’s atmosphere is 5.1E+18 kg. The Moon has 0.074 as much surface area (1/14), so layering the same amount of air on the Moon would demand only 3.8E+17 kg of air, but the comet material is still only 6% of the total – not even the oxygen, if you could find comets of solid oxygen. Worse though, the Moon has a much taller scale height – because of the excesses of heat and cold, it is hard to be sure how much, but at least 6x expanded to account for the low gravity. That puts the comets at less than 1% of what we need, probably much less.

      The article also talks about breaking up comets so they land softly … even as they are used to spin up the Moon to a 60-hour rotation. This seems like spitting on a bowling ball to redirect it.

      Our actual “terraforming” might be very similar to what we should do on Earth and other planets – dig into the Moon, below radiation, impactors, and nuclear weapons, not to mention the extremes of temperature. Mine out vast caverns, some for farming or industrial use, some with natural ecosystems that can exclude invasive species and infectious diseases. We need one tech that no law of physics bans – a way to interactively shape sound so that it fractures deeply into stone in controlled planes, so that tunneling is very cheap. The depth should allow air and water produced locally to be retained. In time, as volatiles leak out, it becomes profitable for colonists to retrieve them with specialized pumps, and the same mechanism can be used to distribute new volatiles if a comet is intentionally crashed into some sacrificial zone on the Moon.

      • Alex Tolley May 19, 2022, 11:47

        The below-ground living is richly described in Ian McDonald’s Luna novels.

        John Varley’s Steel Beach had lunar cities carved out below the lunar craters. IIRC, the most expensive residences were nearest the surface until an impactor killed a lot of the wealthy residents. Thereafter, it was the deepest residences that became most desirable.

        I am in full agreement that living below the surface is by far the best idea, starting with the lunar caves. As far back as the 1950s the exploration of the Moon suggested siting the habitats in lunar crevices to protect from the radiation and micro-meteorites as shown by this Fred Freeman illustration for Colliers (1952).

  • Geoffrey Hillend May 17, 2022, 18:18

    I still like the idea on Star Wars of a habitable Earth sized planet around a larger one like a gas giant. I agree with the scaling up of the gas giant by 10 times Jupiter’s mass to get an Earth sized moon around it through accretion. It might be tidally locked, so the indigenous life might be only be microbial due to the excess radiation from a large magnetic field of the massive gas giant. It might be far away enough to have reduced radiation. Such a world would be certainly interesting.

    • Adam Crowl May 17, 2022, 22:19

      Hi Geoffrey Hillend
      Ganymede’s intrinsic magnetic field protects most of its surface from the radiation flux from Jupiter, so there’s no reason why tidally locked objects can’t have protective magnetic fields.

      • Geoffrey Hillend May 18, 2022, 16:30

        Adam Crowl, Ganymede’s magnetic field is much weaker than Earth’s magnetic field and it does not deflect ions or radiation from Jupiter. I think that some of the radiation from Jupiter is accelerated to close to light speed, so it would even go through a magnetic field as strong as Earth’s, and a gas giant with a mass of ten times Jupiter would have an even stronger magnetic field. The Further away from the gas giant the Earth sided exomoon formed, the better. Since Ganymede is tidally locked, it’s magnetic field must come from the Coriolis deflected convection currents from tidal heating from Jupiter’s gravity. Since an Earth sized exomoon must be tidally locked it probably must also have a weaker magnetic field than our Earth which has a fast rotation generated magnetic field. These would not rule out life given enough water and atmosphere. Tidal heating would also become a problem for such a exoplanet and that might cause more vulcanism and carbon dioxide in the atmosphere heating the planet. Microbial like might adapt to that.

        • Geoffrey Hillend May 18, 2022, 20:57

          Excuse me the mistake. The magnetic field of Ganymede is probably made by convection currents caused by Jupiter’s tidal forces, but not Coriolis deflection which needs a fast rotation like in Earth or Jupiter.

  • Michael Fidler May 18, 2022, 7:03

    While we are looking for moons around exoplanets maybe we should also look for artificial structures. What would a orbital array of large colonies orbiting an exoplanet look like from transit data and could the stacking of the that data obliterate embedded transit changes for such arrays? Thinking of a system around the earth made from the moon or asteroids of the tube or cigar shaped structures as I commented on in the “Dyson Spheres: The White Dwarf Factor” article. This would seem to be an optimum structure similar to ships in water but best design in space. Something in the realm of 2,000 miles in diameter by 20,000 miles long. This may leave room for 20 that would fit well in the orbit of the moon without colliding if two of them where to turn sideways. These would have a surface area inside of 6,000 miles by 20,000 miles each and may hold a population of up to 50,000,000,000. The rotating
    long axis would be aligned to the central star/sun so its light could illuminate the interior.

    Now twenty such objects should give a very unusual curve on the transits but may change over time depending on the inclination or eccentricity of their orbit. This would be much easier to to identify in individual transits but would most likely be considered rings around the planet. I would be very pleased if someone could model this to see if any definitive features would show up in transits of such objects… ;-}

  • wdk May 18, 2022, 10:12

    Over here, not the brightest bulb on the subject of magnetic fields, but I suspect that manifestation or detection is the result of relative motion: induction. If a planetary magnetic field is offset from a rotational axis, then when a planet revolves on its axis, rotation causes inductive interaction with another field. E.g., the Earth and sun produce a dynamo as a result of the Earth’s 24 hour rotation – and perhaps the sun’s rotation period of about 30 days. The Earth has an obliquity to the ecliptic plane, so the magnetic field in 24 hours is waved like a wand. Jupiter has an offset both in inclination to its rotation axis and its center of mass. Its rotational axis is not oblique, however, but perpendicular to the ecliptic plane. Saturn is about as oblique as the Earth but its magnetic field is closely aligned with the rotational axis…
    Both Jupiter and Saturn rotate rapidly ( in about half a day), but one would have to wonder why Saturn’s field is so nearly akin to Jupiter’s.
    Is it perhaps that the interior properties are more conducive to a magnetic field than the field’s geometry? Does that have anything to do with the helium precipitation? Going back to the case of a Galilean moon or another secondary body being phase locked in rotational rate with orbital, there are still rotational effects to consider with regard to magnetic fields or E-M induction: the sun, another stellar primary or a jovian planet has a spin rotation rate to consider – and a rotational rate the same as the orbital rate still constitutes rotation. A possible worry though is that if EM forces exist, then they could be dissipative for a secondary body. Dissipative in the sense that they could cause a planet or moon to spiral into the primary eventually. So, if as discussed in another section, the Trappist-1 system has been around for 8 billion years, and if any of the planets within had a significant magnetic field, then that of itself could have been a significant disrupter from the current state of the system. Unless one assumes that none of the Trappist-1 planets have surviving magnetic fields…

    Observers in previous centuries had many reasons to be puzzled by what they observed. That trend we can extrapolate into this century with confidence, I am sure.

  • FrankH May 18, 2022, 15:13

    If you get an exomoon forming “naturally” (ie, not via capture of a terrestrial planet or via collision), the moon will likely have a large % of water and other ices… which will reduce it’s density and therefore reduce its escape velocity.
    Mars lacks the mass to retain a thick atmosphere (**even if it had a magnetic field **) and it’s in the outer reaches of the Sun’s HZ.

    A light, icy moon in a warmer part of a star’s HZ is not going to be a great place for life, if the temperature is above the freezing point of water and the moon lacks the mass to retain water vapor.

  • Jeff Wright May 19, 2022, 3:46

    What if you have not one but two protective gas giants-one a hot Jupiter that takes the brunt of the abuse? It occults the star as the exomoon of the farther gas giant is at perigee-then Molniyas out behind its host for the duration?

  • Mike Serfas May 19, 2022, 9:19

    How about an exo-Theia? Imagine a smaller Earth still had an intact Mars-sized Trojan, or swapped orbits with it like Janus and Epimetheus. While the situation should be rare, it might be an environment that selects for organisms capable of crossing space between two thermally and geologically similar worlds. Also, the exo-Theians might have unusual motivation to take to the airwaves, should their world finally move toward its long-denied rendezvous.

  • Robin Datta May 19, 2022, 9:57

    David Kipping on John Michael Godier’s video podcast:

    Habitable Exomoons? w/ David Kipping of Cool Worlds

  • Michael May 19, 2022, 14:57

    A problem with moons around jupiter massed planets or larger is the powerful magnetic field causing the moons atmosphere to be stripped off by a cascade type erosion.

  • stephen May 19, 2022, 21:34

    A Jupiter-mass planet with a Neptune-mass moon might have its rotation slowed down, so it could have a weaker magnetic field. So another moon has life evolving without dealing with such an intense magnetic field. How much of an advantage would that be?

    The moon with Neptune’s mass might not be tidally locked, and might have a larger rocky component, and it might have a solid surface with life forms with a fairly “normal” day-night cycle.

    Is a brown dwarf with a moon the mass of Jupiter unlikely? It might not be tidally locked, it might rotate slowly and have a weaker magnetic field, and start out with a larger rocky component. So it gets cooked more gently than the Hot Jupiters do, and produce some interesting biochemistries.

    How massive a planet could plausibly have a solid surface? (Ocean worlds excluded.)

  • Michael Fidler May 20, 2022, 9:42

    [Submitted on 19 May 2022]
    Pandora: A fast open-source exomoon transit detection algorithm.
    Michael Hippke, René Heller

    We present Pandora, a new software to model, detect, and characterize transits of extrasolar planets with moons in stellar photometric time series. Pandora uses an analytical description of the transit light curve for both the planet and the moon in front of a star with atmospheric limb darkening and it covers all cases of mutual planet-moon eclipses during transit. The orbital motion of the star-planet-moon system is computed with a high accuracy as a nested Keplerian problem. We have optimized Pandora for computational speed to make it suitable for large-scale exomoon searches in the new era of space-based high-accuracy surveys. We demonstrate the usability of Pandora for exomoon searches by first simulating a light curve with four transits of a hypothetical Jupiter with a giant Neptune-sized exomoon in a one-year orbit around a Sun-like star. The 10 min cadence of the data matches that of the upcoming PLATO mission and the noise of 100 parts per million is dominated by photon noise, assuming a photometrically quiet, mV=11 Sun-like star for practicality. We recovered the simulated system parameters with the UltraNest Bayesian inference package. The run-time of this search is about five hours on a standard computer. Pandora is the first photodynamical open-source exomoon transit detection algorithm, implemented fully in the Python programming language and available for the community to join the search for exomoons.

    https://arxiv.org/abs/2205.09410

    This is #Pandora, the 1st public computer code to correctly model #transits of #extrasolar planets with moons (#exomoons). Pandora generates transit light curves (~10,000/sec) & videos: https://youtu.be/TDbj5AnjDU8 Paper: https://arxiv.org/abs/2205.09410 Code:
    https://github.com/hippke/Pandora
    @hippke

    https://twitter.com/DrReneHeller/status/1527559746751954946

  • stephen May 27, 2022, 21:25

    Could microorganisms from a habitable moon of a Jovian enter the Jovian’s atmosphere and continue to evolve, giving the Jovian a complex ecosystem? Or microorganisms from another planet eventually reach the Jovian?

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