I know Jupiter has 63 but only the big four are really major moons. The other 59 being little more than former asteroids. How many big moons could a gas giant have that remained in stable orbits for prolonged periods? I've toyed with a story where a large gas giant had more than 150 moons. Including fully two dozen that were Mercury to Mars sized. Including 10 with substantial atmospheres.
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@Dayton3 That's the setting of Firefly. All the planets the people on Serenity visit are terraformed moons of a gas giant. I'd say this: I don't know how many moons a gas giant can have, or what distribution of sizes, compositions, atmospheres, etc. they could have. NEITHER DOES ANYONE ELSE. If you need 150 moons around a gas giant for your story, by golly, HAVE 150 moons around a gas giant. Just make it sound plausible. Sci-fi stories involving futuristic space travel are inevitably going to contain some not-totally-realistic concepts. But if one must be a specialist in orbital mechanics to identify them, most readers aren't going to notice.
I think the two concepts are related. Moons will only form where they can fit. I look at it this way (which is not wholly scientific): Jupiter has 60-someodd moons. So a gas giant with that number of moons is plainly possible. Do I have any reason to believe that Jupiter is anything other than a typical gas giant? Not really. So, is 150 moons too big a stretch? I don't think so, especially if you're not very explicit about the details of the system: how massive the gas giant is, how big ALL the planets are, what their orbital distances and inclinations are, etc. If I read this in the opening of a sci-fi story... ...that would be sufficient for me. Unless the details of the system are necessary for some reason, I'd leave it at that.
It seems to me that depending on various factors about the gas giant and its closeness to the star, that nights on one of these moons would be truly spectacular. And if they had liquid oceans, some bizarre tidal action.
There might also be some crazy temperature variances. If your story involves life on a significant number of the moons, especially indigenous life, you'll have a chance to get very creative about how these issues can affect and format living conditions.
Not exactly. The opening narration of the film makes it clear that the planets and moons are in one system with "dozens of planets and hundreds of moons." A much more detailed explanation of "the Verse" exists at http://en.wikipedia.org/wiki/List_of_Firefly_planets_and_moons
You can have as many moons as you want, but if you want to remain scientifically plausible two dozen moons that size would probably not exist. Jupiter has 4 large moons, but they're in orbital resonance. Which means that their orbit in a 1:2:4, or more specifically a Laplace Ratio. This ratio works to keep their orbits stable, however orbital resonance can also destabilize orbits. One or more moons can cause the orbits of other moons the speed up to where they will get flung out of orbit, or slow down enough where they can enter the Roche Limit. The Roche Limit is the point where a moon will get torn apart, but form a planetary ring instead of raining down as meteors. It's possible to have other orbital resonances as 1:3, 2:5, 3:5, 4:7 which obviously involve pairings. The planets of our solar system also have Laplace ratios, which work to keep their orbits stable. Any other planets that existed before the ratio stabilized were either thrown out of the solar, collided and formed larger planets, or were caught as moons by larger bodies. If you want to have a scientifically plausible gas giant, keep this in mind and set up your planetary orbits in such ratios. A ratio that would work is a 3:4:6:8. Also keep in mind that most gas giants are found outside the ice zone of a solar system. So the planets would need to be close enough that the gas giant can keep the moons warm by the thermal radiation it would give off. Jupiter radiates more energy than it takes in from the sun. Or that tidal action would keep the moons warm enough to maintain liquid water. There are hot Jupiters, which are gas giants that have orbits inside the ice zone of their solar system. This most likely happened because they were pushed out of their orbits by the influence of another star that passed close by the system. These planets will almost always be tidally locked to their star. But can still have their moons as they would have been affected as a whole. Hot Jupiters can pass through the orbital zones of other planets without affecting their orbits. This would also allow the moons to have an atmosphere that is in the habitable range for human life. The orbits of such moons would be fairly fast around their planet, the four Galilean moons of Jupiter orbit in roughly 2, 4, 8, and 16 days. So you could have a gas giant with planets that are in 3:4:6:8:12:16 resonance giving it's moons orbits of 2, 6, 12, 16, 24, and 32 days. But this is highly improbable, and would be nearly impossible to have 1o such moons orbiting a gas giant in such a resonance, since the odds of the moons being flung out or pulled in and destroyed is much higher. So in those terms, having 2 dozen Mars-Mercury planets sounds cool, in reality it's nearly impossible for this to happen.
I'm also curious if you could have moons in polar orbits. Orbiting basically in orbit perpendicular to most of the other moons?
According to this, all moons' orbits tend toward equatorial over time: http://curious.astro.cornell.edu/question.php?number=211
What about a dual planetary system? If you had two super Jupiters orbiting each other could they have more small moons or would it be too unstable?
With two large gravity wells like that, any moons would most likely be torn into asteroids by the gravitational forces when they get in between the two planets if they two giants are close enough to orbit each other. Any moons that do survive would probably orbit the gas giants without passing in between them. If any moons did orbit both, then what I explained above would still hold true.
Fair enough. This actually provides opportunity to make the story more interesting. So long as it doesn't get too far into details of the moons exact locations it can actually acknowledge that it's an impossible long term situation. A character could hang a lampshade on the problem, something like "We've run simulations of the moons orbits and it's not stable. They're just too densely packed, within the next forty thousand years half will be ejected from the system entirely. That's not the strangest thing though, we ran the simulation backwards and it didn't work either. According to the computer, this system can't exist."
@ed629 seems like he's on solid ground on the science of orbits, but I still say...if your story needs several large moons, just have several large moons. I like what @Bailey suggested: if you've really got to be a stickler for accurate physics, lampshade it. In any event, Jupiter has four pretty large moons...does your story REALLY need more than that, @Dayton3 ?
Yavin, Endor, Andor, and LV-223 are examples of sci-fi worlds that are moons of gas giants, but IIRC all three seemed to have Earth-like rotation periods, which wouldn't be the case IRL. I'm not sure, but Endor may not orbit a gas giant, but a huge terrestrial planet(?).
