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Thanks for taking a moment to help me understand the feasibility of this scenario. Essentially, what I am looking at is an Earth-like world that would be between 1.3x and 1.6x the mass of our own planet, but unlike our own planet this world has multiple natural satellites, each of which are the size of, or smaller than our moon and also capable of supporting life.

Stars

This star system is intended to be a binary system comprised of two red dwarf stars. The idea here being that this would allow the system to last for an incredibly long time as to my understanding, red dwarves are some of the longest lasting types of stars in the known universe. I also like the idea of seeing two stars in the sky of some of these worlds during the day.

Earth-Like Planet

Almost identical to our own world with polar ice caps, large oceans, but more continents of various sizes and shapes. The idea is that there will be many different population centers, isolated from one another, allowing for different levels and types of technologies to be developed almost as if each of these continents are their own little world.

Moon 1

The largest of the satellites, this moon is mostly covered in tundra and glaciers, a very cold climate dominated by a species specially adapted to the harsh conditions. This world is intended to have normal "day-night" cycles like we have on Earth, or at least relatively normal, but with days becoming incredibly short or non-existent on parts of the world during what would be described as the "winter months". This world would have mostly frozen lakes and little to no "oceans".

Moon 2

The second largest of the natural satellites, this world is meant to be humid, mostly covered in jungle and swamp, but with a few pockets of temperate forest sprinkled here and there, probably near the poles. Though this world would be on average warmer than our Earth, I don't think the heat and humidity would get to levels that would be uninhabitable for life, or at least that is the intent. This world may have one moderate sized ocean, but plenty of lakes and marshes.

Moon 3

A smallish sized moon, this world would be mostly comprised of a desert enveloping the majority of its surface, with large oases and several rivers with more fertile soil near the banks/flood plains available to allow for sustaining decent sized population centers. Some portions of the world would be more like barren "scrublands" or a steppe-like envrionment. Similar to the second moon, the idea is that this world doesn't become uninhabital in areas due to the heat (but perhaps due to limited sources of water in some locations), nor would it become too cold at night to kill off the entire population.

In Summary

Giving the information provided (as limited as it may be), would it be possible for a world such as this to exist in a way that populations could not only live, but thrive, on each of the mentioned moons as well as the world they orbit? Would the gravitational pull of these moons be obstructed or manipulated by the binary stars their host orbits, considering the planet would be far closer to them than our own Earth is to its sun because of the nature of red dwarf stars? Could a stable orbit exist for all of these entities and still allow for relatively normal day/night cycles, even when taking the multiple stars into account?

If any more information is required, and I can supply it, I would love to do so. Honestly, astrophysics is not one of my strong points :) Thanks for taking the time to read my vague but long-winded descriptions!

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So there are basically three problems to deal with here:

Atmosphere.

Your three moons need to maintain separate atmospheres, and this requires them to be large as well as probably magnetic. Planets are constantly losing atmosphere mass due to the sun heating up their surfaces which heats up the gases in the atmosphere: as random thermal kicks give atoms too much kinetic energy, they fly off to infinity free from the gravitational well. You can basically model this as a ratio of temperatures: there is a well-defined temperature where the random thermal kicks are always enough to kick off molecules of a certain molecular mass $m$, versus the temperature of the surface. The first number is given by $k_\text B T = G M m / R,$ for Earth and N2 gas it's something like 210 000 kelvin as opposed to the actual surface temperature of ~300 kelvin, giving a factor of 700 between them. Getting closer to the hydrogen that we don't have too much of in our atmosphere you see that this factor reduces to 50 or so.

Note that neither Venus nor Mars have magnetic fields, so it's helpful to analyze them by this criterion. In both cases their atmospheres are mostly CO2. We see that Venus keeps a super-thick carbon dioxide atmosphere with a temperature ratio of ~400, while Mars has almost no atmosphere with a temperature ratio of ~90. So if the minimum temperature ratio to keep a meaningful atmosphere were something like, say, 100-200, then you can't make these much smaller than 1/3 to 1/7th of Earth's mass. You can maybe make the central planet bigger but you can't make the moons much smaller than this.

Temperature and day length

If you're going to have had these planets in mutual orbit for a long time, then probably most of these moons will be tidally locked. Tidal locking describes something that our Moon has done with us, where it always points one face towards us, rotating exactly once about its axis per revolution about the planet. As the name states, it does this because the tidal force of the Earth deforms the Moon slightly, just like its tidal force does to our water, causing our tides. This always causes friction, and in the Moon that friction has met its ultimate conclusion: the Moon is now as "stationary" as its conserved angular momentum will allow it to be. That's what tidal locking is about.

Well, basically, the day on the Moon lasts the 28 days of a lunar cycle as seen from Earth. So the cycles you're going to see are actually going to consist of a sort of monthly summer-winter-summer-winter cycle or so, especially if you do not have something like a big ocean to absorb a lot of that variability. So you want to work on explanations for why the temperature doesn't fluctuate rapidly on each of these moons, because the expectation is that it would.

Chaos and where the moons came from.

The Moon is surprisingly large relative to us, more than 1% of our mass. That's incredibly uncommon among the moons we know and is usually explained by a huge catastrophic collision turning us into a big soupy ball of magma while it ejected a huge chunk of Earth's mass into orbit around it.

Fortunately the Sun and other planets are very far away and we can kind of ignore their effects upon the Earth-Moon system. It's very likely that other moons, we could not thus ignore. Remember that they have to have relatively large masses compared to the Earth, and this creates a nasty situation in the equations that is called the "three-body problem": we do not have analytical solutions for more than 2 bodies orbiting each other in celestial mechanics. In fact under many situations of comparable masses, those 3-body problems become nasty chaotic things: and life tends to need predictable circumstances otherwise evolution cannot easily adapt that life to those changes.

So you want to space the moons out as much as you can, and you are limited in this because the furthest moon needs to still be well within the Hill sphere to be comfortably considered a satellite.

Even then, you risk a lot because those inner planets can often see some serious tidal forces from the combination of the other moons and the central planet. For example Io is the most volcanic object in the solar system because of this sort of situation; it can't come to a nice tidally locked equilibrium because of these other planets.

Binary systems and larger

Part of the problem is that you are describing "moons" rather than "planets", and moons are by definition small. There is no reason that within the Earth-Moon system you can't make the Moon significantly larger, until it is about the size of the Earth and they both orbit a central point. Then they might be tidally locked but they might also be co-orbiting this central point once per day, in what's called a "double planet" system. In fact we have observed triple stars with planets and so we know that triple-planetary systems are possible, and that they could even potentially have a moon.

(Side note: please use caution when googling for examples of triple planets as you will get deceptive answers. We have only recently been able to detect planets orbiting distant stars -- so "triple planet" right now generally means "the wobble in the signal appears to be unfittable with 2 sine waves, so there must be at least three planets around that star". Actually seeing light illuminating one of these rocks and discovering that it's actually three rocks orbiting a common center, is not possible right now, to my knowledge: even if you saw the "bulge" in the star from its reflected light I don't think you would see the "gap" between the planets.)

Multiple orbits

You can also have multiple planets orbiting a central star within the Habitable zone on different orbits, if they're many Hill radii apart from each other so that they do not mess with each other too much. This blog post has 6 concentric orbits in the Sun's habitable zone but also comes up with a hypothetical alien-engineered system which could have as many as 416 in orbit around a Sun-sized star. It's artificial -- it wouldn't form in nature -- but it points out that this is a really tangible possibility.

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  • $\begingroup$ Thank you very much for this detailed response! I especially liked the information about the tidal locking scenario with my moons, something I didn't think about. It does lead me to an interesting scenario that would actually fit with my intended world, that being that the residents of these moons would probably have semi-regular migrations to the sunny/day side of their world. Though that is still something of an issue because these moons will have to be larger than I originally intended, which means travel would be an issue. It still gets me thinking, and I appreciate that :) $\endgroup$ – Varwulf Dec 19 '16 at 18:07
  • $\begingroup$ Minor problem: The L1, L2, and L3 Lagrange points are not stable orbits. Solution: Put the secondary moons at the L4 and L5 points, 60 degrees ahead of and behind the Earth-like moon. Alternatively, the moons could be locked in orbital resonances, which would allow the moons to move across the sky relative to each other, yet still be in stable orbits. $\endgroup$ – Someone Else 37 Apr 7 '17 at 20:42
  • $\begingroup$ @SomeoneElse actually I think the L4 and L5 points we what I originally intended and I just misnamed them... $\endgroup$ – CR Drost Apr 8 '17 at 3:02
  • $\begingroup$ Isn't it possible (though not naturally likely) to put the moons in the same orbit but just separated by 120 degrees? My limited understanding of orbital mechanics tells me that it's stable. $\endgroup$ – Muuski Feb 13 '18 at 16:22
  • $\begingroup$ @Muuski I need to update this, but yes: there are many "resonance orbits" similar to how the L4/L5 points work. The real issue is that one would properly call those (and my L4/L5 "moons") "planets" rather than "moons" because they do not make full revolutions about another rock in space. $\endgroup$ – CR Drost Mar 1 '18 at 16:02
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Short answer. It may be possible for a broadly defined Earth-like planet to have one or more moons large enough to have external biospheres their surfaces.

But it seems difficult for even huge gas giant planets like Jupiter to have moons large enough to have substantial atmospheres and thus external surface biospheres (like Earth has, instead of being under ice or rock).

It is theoretically possible for a giant planet to orbit in the habitable zone of a star (a "hot Jupiter") and have one or more giant moons large enough to have Earth-like biospheres. There are no known examples of any giant planets having moons that large.

And it is theoretically possible for an Earth-like world with an Earth-like biosphere to have one moon also large enough to have an Earth-like biosphere. That should be much rarer than giant planets having such large moons. It would be an example of an Earth-like planet have a moon system with a much greater total mass than the moon systems of giant planets like Jupiter or Saturn.

But the planet-moon relationship would not be so obvious in that case. The sizes of the two bodies would be similar enough that people would tend to think of it as a double planet, just as Earth and the Moon and Pluto and Charon have been described as double planets.

If you add a third body large enough to have an Earth-like biosphere it gets even weirder. once again you have a tiny Earth-like planet having a moon system with a total mass of several times the total mass of the moon system of a giant planet like Jupiter or Saturn. Of course one might suppose that the Earth-like planet is so small because it split off so much of its mass forming giant moons.

I wrote an answer to a similar question recently about an Earth-like plane with an Earth-like moon or (smaller sister planet).

How would an earth-like planet with a habitable moon work and how to get there?

In it I give suggestions for how to not seem silly while writing about an Earth-like planet with a single moon large enough to have an Earth-like biosphere.

Note that I quote a source that suggests that a habitable Earth-like world -whether a planet or a moon of a planet, should have a mass beteen 0.25 and 2.0 that of Earth. Thus the planet could have at most about four to eight times the mass of one of it's habitable moons - making them seem like a double planet, and if it has several habitable moons it may not have more mass than the total mass of the moons.

Thus you might want to think of your set up as a multiple planet system, roughly analogous to a multiple star system. Multiple star systems usually have hierarchical orbital arrangements. The two innermost stars orbit each other at a close distance, and another star or pair of stars orbits them at several times their distance, and a third star or pair of stars orbits them at several times the distance of the second star or pair.

For example, the star system of Castor has three double star pairs that are each very close. The star pair Castor A and the Star Pair castor B are separated by many times the separation of the stars within the pairs, and the pair Castor C orbits at perhaps ten times the distance between A and B.

So maybe your planet and moons should be thought of as a quadruple planet, consisting of two double planets separated by a distance at leat several times the orbital distance of each pair.

For example, each pair cold be separated by 50,000 miles and the to pairs could be separated by 500,000 miles.

Another version would be to have the "planet" and the "inner moon" orbit each other about 50,000 miles apart, have the "second moon" orbit the pair at a distance of about 100,000 miles, and the "third moon" orbit the pair at a distance of about 200,000 miles.

But I suspect that an expert on orbital dynamics could find flaws with those suggestions too, demonstrating why they were impossible just as much as your original suggestion.

It might be much more plausible to have a hot Jupiter type planet with four habitable moons, one the "planet" in your question and the other three the "moons in your question.

How would an earth-like planet with a habitable moon work and how to get there?

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The first thing to remember is that in your own universe you can do anything you wish. However, when it comes to realism, your scenario is highly, highly unlikely.

First of all, consider that in order to retain their atmospheres, each satellite will require a fairly strong magnetic field (at least compared to our own Moon). Those atmospheres will also keep the surface of each moon from being microwaved by the Sun. This implies a fairly strong atmosphere, and thus a molten core, like Earth's. However, that doesn't work too well for "small" moons orbiting a moderately sized planet. You need a larger planet than that to "fit" a molten core into.

Second, you're talking about each of these moons having quite different biomes. One frozen, one very hot and dry, another very hot and wet, etc. However, these moons will all be spinning around this world of yours, and be exposed to the same (or at least a very similar) amount of heat from the Sun.

And so, I don't think that a world the size you're describing would be able to support three moons large enough that each would have a molten core, and a gravity field strong enough to generate an atmosphere which would be able to retain enough gasses for Earth-like climate, and fauna to evolve.

Furthermore, even if these moons did form in such a way as to retain atmospheres, and life did bloom on each one, I don't see how they might each have such different temperature ranges (dry vs humid might be explained by the amount of water available, but not the temperature).

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  • $\begingroup$ Not going to post my answer now as it contains all the same points (Ninja'd), but you might want to add planetary eclipses prevent normal day/night cycles. $\endgroup$ – Joe Bloggs Dec 19 '16 at 16:15
  • $\begingroup$ Great comment and explanation, I wasn't entirely sure if the location of each moon relative to the locations of the stars might have an impact (if the cold moon spends more time behind the planet, effectively being hidden from most of the radiation by the planet it orbits) might have an impact or not on the temperature, or if it'd even be possible because that would indicate that each moon is orbiting at a different speed/path? As I said, astrophysics are not my strong point, but I do appreciate the detailed answer :) $\endgroup$ – Varwulf Dec 19 '16 at 16:30

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