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The majority of exoplanets found so far are comparable in size to Jupiter or Saturn. Which, as we know, are problematic for human habitation. But their moons have a lot of potential.

Some of these large exoplanets are in the Goldilocks zone. Given what we know about the moons of Jupiter & Saturn, if we should find duplicates of them circling in the Goldilocks zone of another star, would the moons be habitable?

By habitable, I mean more likely to support independent/aboriginal life than Mars, or requiring significantly less supportive technology than Mars.

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Presumably if Mars was closer to the sun, it would be warmer but also have less atmosphere. If you want to use Mars comparatively in your answer, moving it in and adjusting atmosphere is acceptable. –  James Jenkins Apr 7 at 16:27

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For Jovian and Saturnine moons, the simplest answer is no, at least not much more habitable than our own Moon is, because none of these moons have their own magnetic field or sufficient mass and their atmosphere would eventually thin out via the ionospheric hydrogen loss due to the solar wind. A bit more difficult answer is how many of these moons would be well within the parent planet's own magnetic field and still sufficiently protected from solar winds and cosmic radiation to sustain a breathable atmosphere, liquid water, and life as we know it.

Being within the goldilocks zone also means that these celestials would orbit well within the star's snow line, so their surfaces would form completely differently than they indeed have in the outer Solar system, past the snow line. They wouldn't acrete as much of water ice as they did, and the liquid water would be more exposed to escaping hydrogen (hydrogen atoms aren't really strongly bound to oxygen atoms in water molecules, and UV radiation in upper Earth's atmosphere is sufficient to produce extremely light hydrogen ions that move to ionosphere).

So it's difficult to say how such moons would even look like. Would they be barren rocks? Or could they hold onto enough water and sustain a breathable atmosphere? Would they even form at all from the protoplanetary disk, or would we mostly see captured planetesimals orbiting goldilocks Jupiters? I'm not sure that anyone can even answer these questions, there's enough of discussion even over our own Solar system's formation, but I'm fairly confident that there's not much point in discussing if Titan, Enceladus, Europa and alike moons of Jupiter and Saturn could support own, water-based lifeforms or be suitable for human colonisation, unless we can establish with a fair degree of certainty that they could even exist. For all we know, it might not be possible for them to hold onto so much water as some of the Jovian and Saturnine moons do. So it's quite possible we'd see much smaller, rocky moons and moonlets, with even smaller mass and surface gravity than we currently do.

But I'm not going to say it's impossible. Jupiter has incredibly strong magnetic field, likely sustained by the metallic hydrogen within its core, and is large enough to stretch nearly to other neighbouring planets. But how all this hydrogen actually moves in protoplanetary disks as their stars evolve, increase their temperature and push the snow line and blow lighter particles away, while at the same time consume a great deal of hydrogen to sustain their nuclear fusion, is still a bit of a mystery, as is the source of all the Earth's water. Perhaps we gained most of it via Late Heavy Bombardment of icy asteroids as the Earth's surface started to solidify? So what I'm saying is, that if such Jupiters form closer to their parent star, and within the goldilocks zone, we don't really know if they'd even be able to sustain a strong enough magnetic field. Composition of their inner cores might be entirely different than of our Jupiter. Even other gas giants (which leaves Saturn really, because Uranus and Neptune are ice giants) in our own Solar system don't support such strong magnetic field as Jupiter.

Anyway, these are just some thoughts on the subject, and why I think it's too speculative and we lack sufficient understanding of even our own Solar system's formation, to transpose this knowledge onto exoplanets we know even less about. I guess the proof of the pudding will have to be in the eating. I.e. we'll know, when we detect some.

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" because none of these moons have their own magnetic field or sufficient mass" This seems speculative. Is there a mass ceiling on how large a gas giant's moons can be? Ganymede and Titan are nearly as large as Mars. It seems plausible there are gas giants in other star systems with earth sized moons. Venus is less massive than the earth, has a much weaker magnetic field. Since it's closer to the sun, solar winds are stronger. Yet Venus has a thicker atmosphere than earth. –  HopDavid Apr 7 at 20:31
    
@HopDavid I clarified that I meant for Jovian and Saturnine moons (in our own Solar system). I didn't say it's not possible in other worlds, what I meant was that none of the moons of the gas giants in the Solar system have their own magnetic fields capable of preventing loss of atmosphere through Solar wind. I didn't assume anything about some exoplanetary moons (exomoons) we know nothing about. I just explained that it's less likely for them to acrete as much of water as they would, if they formed beyond the snow line, and that they would have problems holding onto it. That's all. –  Noordung Apr 7 at 20:57
    
On rereading the question I see James Jenkins is asking about duplicates of Jupiter's and Saturn's moons. Yes, your answer is more accurate for Jovian or Saturnine moons. But I still don't fully agree. As I mentioned, if atmospheric thickness were a function of temperature, solar wind, gravity and magnetic field, then Venus should have a thinner atmosphere than earth. Each body has a different history that makes it what it is. –  HopDavid Apr 8 at 0:50
    
@HopDavid Venus isn't really within the goldilocks zone, it's slightly too close to the Sun, and the hydrogen escape is a big factor there. Even most of proposed ways of terraforming Venus include introducing loads of hydrogen into its atmosphere in some way. Hydrogen is not only relevant to water but also reduces atmospheric carbon oxides via Sabatier and/or Bosch reactions. But it would still be losing it due to no magnetic field (short of a slight one in its ionosphere due to proton gradient ionospheric "tail" somewhat slowing down hydrogen loss). –  Noordung Apr 8 at 13:06
    
I'll probably update my answer to include another point - tidal bulging. If the parent planet would have less hydrogen in its core to sustain a massive magnetic field, then the moons would have to orbit it a lot closer to still be within the magnetic field. With larger volumes of liquid water, that then becomes a problem and the tidal forces would be tremendous. If it can keep the Enceladus' interior liquid, imagine what it would do if its surface was also liquid. I'm not sure it wouldn't just end up stretching the whole moon only to have larger surface exposed to solar winds and insolation. –  Noordung Apr 8 at 13:10

On the first reading I thought you were asking about possible earth like moons orbiting gas giants in habitable zones of other star systems. This is fairly plausible in my opinion. I can't think of a reason why moons of about earth's mass couldn't be orbiting some of the gas giant exo-planets

But on rereading it seems like you're asking about duplicates of Jupiter and Saturn's moons. This is somewhat less plausible. Most of these moons don't have much air or magnetic field (as TildaWave points out).

Titan has a thick nitrogen atmosphere, with some liquid ethane and methane that evidently evaporates and rains back down on occasion. But Titan gets about 1/100 of the sunlight we do. Move it to a warmer place and I'm not sure Titan would hang on to those volatiles.

Which isn't to say Titan or Ganymede sized moons couldn't have an atmosphere in a habitable zone. Each moon has a different history that made it what it is today. I believe it's possible there are Titan sized moons in habitable zones with a history that's resulted in lots of volatiles. But Titan or Ganymede sized moons with a different histories wouldn't be "duplicates".

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I was trying to keep the question from being overly opinion based and stay to knowledge available at this time. We know the Jupiter/Saturn size planets do occupy the Goldilocks zone of other systems and we know what moons circle these planets in our system. If the question is to open to speculation then The Integral Trees become optional answers and the question is out of scope. While it is unlikely that we will find exact duplicates, it is most likely we will find close similarities. –  James Jenkins Apr 8 at 10:39
    
Your premise is self contradictory. You imagine duplicates that have formed from completely different conditions. It's like asking us to imagine Hawaiian Islands duplicates but without a subsurface mantle plume. This would pop my WSOD quicker than a typical Niven story. Given 25 or 100 times more insolation, inner Jovian or Saturnine would cool slower. But like the moons we know, you'd still have tidal flexing. In spite of that, let's assume the cores of rotating liquid metal have long since frozen and the moons no longer have a magnetic field. –  HopDavid Apr 8 at 16:45

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