I'm making five habitable planets the size of earth as moons for a gas giant the size of Jupiter.

Then I want to add many more smaller moons, Our Jupiter has 67 moons, I want as many small moons possible and 5 earth-sized moons for mine.

What conditions of this planetary system would threaten habitability on the five earths?

How many moons can there be and still have a 100% perfect stable system?

How many moons would be visible from each of the earth sized planets?

What is the safest distance from a earth like moon to a small moon to make it possible for plankton/aero-plankton ''traveling'' from a moon to another.

Oh almost forgot, at least 60% of the small moons have to be good potential colonies for the humans and aliens that live in those 5 earth like planets.

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    $\begingroup$ I didn't address it in my answer below, but space plankton in such a system is a fascinating concept. I think the key to supporting plankton would be the densely packed atmospheres. 5 habitable planets plus a gas giant all with atmospheres in a small radius seems like the kind of place a space plankton would develop, if one were to develop at all. That would be an interesting question to ponder in its own thread... $\endgroup$
    – kingledion
    Aug 3, 2016 at 12:28
  • $\begingroup$ I wrote two blog posts that cover a lot of these issues. You are welcome to take a look here: planetplanet.net/2014/05/22/… and here: planetplanet.net/2014/11/18/…. Also, some of the risks of such a setup here: planetplanet.net/2017/12/19/exo-moons-innocent-bystanders $\endgroup$ Jan 4, 2018 at 21:33

2 Answers 2


I'm going to assume that the planet may be both Jupiter-like, and Saturn-like to give us more real world examples to work with. I'm also going to assume that said gas giant is at 1 AU from the sun so there is sufficient heat for Earth-like life. Imagining life in a cold Europan ocean is too broad for this topic. As for gas giant formation that close to the sun, there is ample evidence for that in exoplants. I don't think that we understand solar system evolution well enough to say that Jupiter in Earth's orbit is unrealistic.

Possibility of formation

Jupiter's moons are assumed to be the four left of a larger set of moons. As Jupiter's ancient moons formed, the proto-planetary disk around the sun was still relatively dense. This cause the moons to lose velocity and spiral into Jupiter. The four remaining were far enough away that they maintained orbit until the sun sucked up or drove off most of the free dust. So if (big if) your Earth-sized moons developed far enough off, then they could have formed. I suspect a larger planet than Jupiter would be needed to form moons of that size though...I don't know.

Saturn's inner moons apparently underwent a cataclysmic collision around the time of the dinosaurs that destroyed them. The debris then reformed into Saturns larger inner moons (basically all of them except Titan and Iapetus). You don't want that to be happening on your moons...unless you want four to destroy and reform while life is evolving on the outermost moon...the denizens of which could colonize the inner moons. There is no explanation as to why these inner moons collided...so there's that.

So the answer to, "could these moons form" is...maybe? Lots of mystery here. But the larger the gas giant the more likely it is to happen, in my opinion.

Hill Spheres and Moon's Moons

The Hill sphere is the region of space where a planet or moon can keep a satellite. Earth's is 1.5 million km, while the moon is 384,000 km away. So close the Jupiter, the hill sphere is smaller. An Earth-like planet in orbit of Jupiter at the orbital distance and eccentricity of the four large moons (Io, Europa, Ganymede, Callisto) would have a Hill sphere of 42, 36, 87, and 152 ,000 km. This is significantly smaller than the distance to our own moon. However it is larger than the distance to Deimos (23,000 km) the farther of Mars' two moons. So you could have Moon's moons, but they'd be smaller than our moon, and closer in too.

The farthest out large moon of Jupiter or Saturn is Iapetus, at 3.6 million km. If Earth was at this distance from Jupiter, its Hill radius would be 352,000 km. If it were circling Saturn (a smaller gas giant), it would be 526,000 km. This is the only configuration that could conceivably support a moon at Earth distance.

This just covers Hill spheres, there are plenty of other orbital dynamics that could disturb secondary moons forming around primary moons. In general, let me express my doubt that any of the moons could have their own moon, and let me be especially doubtful that any of those moons could be larger than a tiny moonlet.

The co-orbital moon idea is also interesting. Tethys and Dione (two moons of Saturn) have 'Trojan' moons in leading and trailing orbits. These moonlets are from 2–35 km in size.


Jupiter's satellites get...a lot. Io gets 3600 rem/day, Europa 500 rem/day, Ganymede 8 rem/day, Callisto 0.01 rem/day. 1000 rem is deadly, 100 rem will give you radiation sickness, 0.1 rem is on the order of a normal dose on Earth over a YEAR. So on Callisto you'd be fine, on Ganymede you'd die in a month, Europa a couple of days, and Io immediately.

Saturn's satellites on the other hand don't get very much. Jupiter has a powerful magnetic field, but that causes the radiation. Your gas giant will need a weak field like Saturn. On the other hand, the planets' inhabitants need to be defended from cosmic radiation sources, so each planet will need its own magnetic field. Alternatively, one the planets of which life developed could have magnetic fields, while the rest are colonized and have structures built underground to protect them from cosmic radiation.

Looking at moons from other moons

I calculated all these myself with a spreadsheet and trigonometry, so there may be some errors. Here are some reference numbers: The full moon from Earth is 31 arcmin, which is just over half a degree in the sky. The sun is about the same, although it is obviously much brighter. For the large satellites of Jupiter and Saturn, the largest a planet appears is Jupiter from Io at 549 arcmin or almost 9 degrees of the night sky. Another way to put this is 1/20th of the way from horizon to horizon or 18 times the size of the moon. Thats almost too big to comprehend. The smallest a planet appears is Saturn from Iapetus, at 56 arcmin, or still twice the size of the moon. All your moons will have significant light from the gas giant.

If Earth-sized planets were in the orbit of Io and Europa, at closest approach they would appear to be 175 arcmin from each other, or 3 degrees, or 6 times the size of the moon. If Earth sized planets had an average orbit around where Iapetus is, they would appear to be 6 arcmin from each other, 1/5 the size of the moon.

Colonies on small moons

One word: don't. Humans live at 1 g of gravity. Gas giants don't have surfaces, Venus has .9 g at the surface, Mars and Mercury have 0.38g. Our anatomy and physiology is designed for 1 g, it helps our bones calcify properly, helps our blood circulate, and does countless other things we may not even have discovered yet. Humans maybe could live a lifetime on Mars and Mercury. It is very unlikely they could live a lifetime at an even lower gravity. Luna, Titan, and the 4 Jovian moons have gravity of 0.12–0.18 g. Pluto, Triton, and various Kuiper belt objects are below 0.10.

A medium sized moon (like Rhea of Saturn, say) might have 0.027 g. This is way not enough for a human to live, but annoyingly much if you want to, say, launch things into space. Humans are better off living in space colonies that generate artificial gravity by rotating. If you want to exploit a moon, you are much better off with one that has a much lower surface gravity. Phobos, for example, has a radius of about 11 km, and a surface gravity of .0006 g, but still has $10^{16}$  kg of usable materials to mine.

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    $\begingroup$ An excellent answer. I wish I had the math in me to be able to do such calculations too. $\endgroup$ Aug 3, 2016 at 1:20
  • $\begingroup$ I just wish I had the willpower to invest so much time in song the calculations. $\endgroup$ Aug 18, 2016 at 3:30

Umn, I'm gonna assume that you are gonna ignore the fact that not much( at least to our knowledge can live at that kind of distance from the sun).

First thing first,

Whizzing meteors and comets

If your inhabitants haven't died of freezing and frostbite and starvation. Then they will have plenty of extra-terrestrial debris that can wipe out earth to worry about. Which can be seen from here that Jupiter gets hit by extinction-causing debris more often than not. This means that in all likelihood, your moons are living in a very dangerous neighborhood. With the asteroid belt towards the sun and the Oort Cloud to your back, I would not want to live near a planet that actually attracts them.

Jupiters 67 moons

This is a superficial fear but I'm gonna say that there's that likelihood that a fellow moon can and could wreck your nice planet moons. So you better place them well.

Jupiters gravity

According to this article, Jupiter's gravity affects the entire solar system. As I'm no physicists nor astronomer, this is just a theory but I have a feeling that your moons might be a time bomb. I have no idea when Jupiter gravity could tear them apart and you better be careful.

Now for the topic of moons for your earth moons

Co-orbital moons

Your best bet, considering you can fit more than a thousand earths in Jupiter. The chances of your earth moons meeting other moons on a daily occurrence is small as A: we have no idea how fast their orbits and B: the orbit would take forever. Meeting other moons would be something of a miracle. So therefore having moons that orbit your earth moon and vice versa would be a better bet.

Instead of many small moons

Did you know? Jupiter has a ring, a ring of dust and rocks. the only problem is that it's not as visible as saturns ring. Now all you need to do is make this ring more visible instead of having a billion little rocks that are all the more likely to murder your earth moons.


You know how I said that Jupiter can fit a thousand earths? Your gonna need a really really fast orbit to reproduce earths 24 H day and night,this is because your earth moons need to get around Jupiter to get any form of light at all. The number of days it would take to orbit around the sun[one year] would be the same as Jupiter which is 12 years, which means that your have 12 years worth of summer, spring, winter and autumn[Provided you have them].

More extra

I really hope you have thought through how your inhabitants will survive the with the measly sunlight and warm * cough * temperatures.

space plankton??

Not possible. No. Just 99.9% impossible . How are plankton going to travel across your earth moons anyway if we took 3 days to get to the moon safely. They will need cold immunity, immortality and be very very patient to wait for that 0.00000000000000000001x10^-9999999999999 chance to actually get there. Unless your plankton can build spaceships

  • $\begingroup$ Jupiter's gravity isn't enough to tear apart its existing moons, why would it tear apart Earth-like planets? Jupiter gets hit with lots of rocks because it is big. Its moons are not, and they have not gotten torn apart by space debris. upiter's existing 67 moons aren't careening into each other. I think your post is lacking in some science. $\endgroup$
    – kingledion
    Aug 2, 2016 at 18:39
  • $\begingroup$ Note that there is markup for headers shown on the edit toolbar. Don't fake it with all-bold paragraphs. Note that possesive takes an apostrophe: Jupiter’s gravity etc. $\endgroup$
    – JDługosz
    Aug 2, 2016 at 19:44
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    $\begingroup$ Note that the question does not state the distance of the Jupiter-sized planet from the sun. So there's no reason not to assume it is not in the habitable zone of its star. $\endgroup$
    – celtschk
    Aug 3, 2016 at 5:27

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