World Size

[Baron Opal]

Is there any theoretical limit as to how large a moon of a gas giant can be? I was kicking around an idea for a system with three inhabitable worlds, the warm planet, the icy planet and the inhabital moon. The strange fields around the jovian planet purturbed by its moons, allow the inhabitable moon to be sufficently warm.

 

[Quantarum]

Natural satellites don't have a specific size limit, if the sizes of the two objects are large enough they are considered a double body (like Pluto and Charon). The largest moons in our solar system are around 5,000 km in diameter, so an Earth-sized satellite is not beyond the pale by any means.

-Q.

 

[Charwoman Gene]

What genre is this in?

 

[Mercurius]

What genre is this in?

I was going to ask that myself. In fantasy there is no limit--do whatever you want, just make it believable. In scifi, the same applies but you just have to work harder to make it believable.

 

[Umbran]

As a general posit - a moon should be notably smaller than it's primary. And by "smaller" I really mean "less massive". If we assume that the moon is made of stuff similar to the planet, this also generally means it is of smaller size, but one can construct edge cases.

People think in terms of a moon orbiting a planet. That is technically not quite correct. Both the moon and the planet orbit their common center of mass (aka "center of gravity"). If the planet is much heavier than the moon, the center of mass is probably within the body of the planet, so it looks much like the moon orbits the planet. For our Earth and Moon, the center is about 1700 km under the surface of the Earth (so, some 4700+ km from the center of the Earth itself).

If the two objects are of equal mass, the center of mass is right between them, and it looks more like the two objects orbit that point, or that they "orbit each other".

And, clearly, if the moon is actually more massive than the planet, you've mixed up which one is the primary, and which is the satellite.

 

[Bumbles]

Is there any theoretical limit as to how large a moon of a gas giant can be? I was kicking around an idea for a system with three inhabitable worlds, the warm planet, the icy planet and the inhabital moon. The strange fields around the jovian planet purturbed by its moons, allow the inhabitable moon to be sufficently warm.

Well, given that Jerry Pournelle had a whole book/anthology series about the world of Haven, and he's one of the more scientific types among science fiction authors, I'd say it's certainly going to pass the whiff test.

I'm sure there's somebody out there who has done some real number-crunching but you will have to look for that on your own.

 

[Theo R Cwithin]

No real upper size limit. A moon around a gas giant has lots of potential to be inhabitable, even without strange and mysterious fields. Gravity alone can justify a lot of geology, as well as possibly liquid oceans with tides, etc. And if the moon is big enough, it could support a substantial atmosphere, which provides weather and protects inhabitants from the the nastiest radiation of space. The primary itself would also shield the moon from most meteoric bombardment, and it's magnetic field would protect against ion storms coming off the systems sun. And the sky would be AWESOME to see! Yes, a veritable paradise.

One thing to keep in mind, if you're interested in "realism": If your moon is much smaller than the primary, it will always keep one face toward the primary, just as our moon always keeps one side toward Earth.

 

[Umbran]

No real upper size limit. A moon around a gas giant has lots of potential to be inhabitable, even without strange and mysterious fields. Gravity alone can justify a lot of geology, as well as possibly liquid oceans with tides, etc.

More simply - there's now evidence out there that gas giants don't need to forever live far away from their parent star. If it is close in, you don't need to invoke anything special aside from sunlight.

One thing to keep in mind, if you're interested in "realism": If your moon is much smaller than the primary, it will always keep one face toward the primary, just as our moon always keeps one side toward Earth.

This is not true. Tidal locking is certainly common, but not strictly required, and relative size is not the only issue at hand. For example, a satellite with an eccentric orbit (a highly elliptical orbit, rather than a nearly circular one) can fall into a different orbit resonance.

The Earth's Moon has a 1:1 rotation to orbit resonance - it takes just as much time for it to revolve around its own axis as it does to orbit the Earth. This is "tidal locking", keeping one face always towards the primary. However, Mercury is 3:2 (3 rotations for every 2 orbits). Other resonances are possible, and anything other than 1:1 means the satellite does not always keep the same face to the planet.

 

[Theo R Cwithin]

Tidal locking is certainly common, but not strictly required, and relative size is not the only issue at hand. For example, a satellite with an eccentric orbit (a highly elliptical orbit, rather than a nearly circular one) can fall into a different orbit resonance.

Aha! I'd forgotten about other resonances. I stand corrected - and can imagine a lot of other nifty effects for inhabitants of such a moon! (*sigh* I haven't done celestial mechanics in over 10 years )

More simply - there's now evidence out there that gas giants don't need to forever live far away from their parent star.

Yeah, if there's one thing astronomers have learned about solar systems in the last 15 years or so, it's that ours is hardly a representative example. It sometimes seems that "anything goes". All the more room for imagination, I suppose! Cheers!