Gas Giant satellite system limits

[Anthony]

As a rule, if we assume earth-density (which is slightly high for smaller satellites), maximum size for 0.01% is (16 * mass/MJ)^1/3, thus limiting Jupiter to size 2.The minimum size for a reasonable terrestrial world is 4-5, requiring a gas giant in the 4-8MJ range.

 

[Malenfant]

Actually looking at the paper itself it seems that the limit is really of the order of 0.01% of the mass of the jovian. The actual limits in practice are between 0.01% and about 0.07% of the primary's mass depending on the numbers you put in (Jupiter and Saturn's systems are 0.021% and 0.025% respectively, Uranus' is 0.011%. So that gives us a bit of wiggle room.

Here's what I get. Find the mass of the jovian or BD in the first column, and the number in the columns on the right shows the maximum possible diameter of its satellites (depending on their density) expressed as a Traveller size code. I'm assuming here that the limit is 0.03% of the primary's mass.

Icy = satellite density of 2000 kg/m3 (Titan/Ganymede)
Rocky = satellite density of 3500 kg/m3 (Europa/Mars)
Earthlike = satellite density of 5500 kg/m3 (Earth/Venus)

So a 1 MJ jovian could have a single size 5 icy satellite, or a single size 4 rocky or earthlike one. Or it could have more satellites that are less massive that add up to the same mass.

</font><blockquote>code:</font><hr /><pre style="font-size:x-small; font-family: monospace;"> -------Size Code-------
MJ icy rocky earthlike
0.01 1 1 1
0.05 2 2 1 - Uranus,Neptune
0.1 2 2 2
0.2 3 2 2
0.3 3 3 2 - Saturn
0.4 4 3 3
0.5 4 3 3
0.6 4 4 3
0.7 5 4 3
0.8 5 4 3
0.9 5 4 3
1 5 4 4 - Jupiter
2 6 5 5
3 7 6 5
4 8 7 6
5 9 7 6
6 9 8 7
7 A 8 7
8 A 8 7
9 B 9 8
10 B 9 8
11 B 9 8
12 C A 8
15 D A 9 - smallest BD
20 E B A
30 (16) D B
40 (17) E C
50 (19) (16) D
60 (20) (16) E
70 (21) (17) F - largest BD</pre>[/QUOTE]So Jupiter could actually have a large (size 5) icy satellite, or a few smaller ones (which it does). Saturn (at 0.3 MJ) 's works out too - a single large satellite and lots that are a lot less massive.

The interesting thing is that a Brown Dwarf of any mass can have pretty much any size satellite around it (the lowest mass BDs miss out on the largest possible size code in standard Traveller). However, the more massive BDs could have satellites that are so massive that they actually turn into gas giants themselves. So we could get gas giants around brown dwarfs...

 

[RandyT0001]

So a GG that is six times as massive as Jupiter could have moons that are similar in size and density to Earth around them? I am assuming that such moons would have to be located in the first few (say 1-4) orbits around the the 6x-12x larger gas giant. I wonder how much effect the higher gravity will have on such a moon. Would such a moon be in constant volcanic upheavel like Io(?) is? Would it ever be able to settle into a stable geology long enough for liquid water to form oceans and seas? Would it ever be stable enough for life to form (even simple single cell life)?

 

[Malenfant]

Well actually you'd need about 9-10 MJ to get a moon with earth mass and density - 6 MJ only lets you have an earth-sized icy or rocky moon, which are much lower in density.

It looks like most of the major gas giant satellites in our solar system are within 30 radii of their primaries. There's too many factors determining the spacing of the moons around the planet though - Jupiter's large moons in regularly spaced orbits because they're in orbital resonances, but Saturn's system is much more spread out in both size and distance.

All moons will be tidelocked to the primary though, so their 'day' is going to be the same as their orbital period around the planet. Volcanic upheaval depends on the tides, and that depends on whether or not there are any other large satellites in nearby orbits. Left to its own devices a satellite's orbit will want to become circular around a planet, but if there's something giving it a regular tug (ie a nearby moon) then that causes the orbit to become eccentric, and that means that you get tidal heating.

I guess that ideally you'd just want one large earthsize satellite (maybe another one a lot further out) and a few much smaller ones, that way the earthsize satellite can be nice and stable.

 

[Anthony]

Jupiter's Hill Sphere, if it were at 1 AU from the sun, would only be at around 10 million km. Long-term stable orbits are only within 3-4 million km (20-25 diameters).

You may get some tidal heating due to sun/GG interactions, even if there are no other large moons.

In theory, a jovian planet could capture a terrestrial planet, thus evading the restrictions on GG moon size. In practice, this is probably extremely unlikely.

 

[aramis]

Mal:

It is a wild approximation at best; recent articles in several science journals cite that none of the current generation of accretion software winds up covering the whole spectrum from star to moonlets.

Further, with only 4 datapoints available, and only 3 cited (Jupiter, Saturn, and Uranus), it is at best an interesting situation, which they have come up with an interesting model for.

Accepting that the simulation is proof is like saying that I can prove a man can jump 20 feet using a simulation. The simulation is not proof; it is at best a test for instant rejection upon the basis of the math not working.

The general validity level, according to several statistics texts, is n=30+. We have 4 observable; not enough to make valid statistical analyses of the GG's and their moons. (Educators and Astronomers have a strong tendency to use low-sample-size studies... which are indicative of a trend, but not good enough for validity. Both with good reasons, but in both cases, it's not really good enough for proof.)

 

[Malenfant]

Originally posted by Anthony:
You may get some tidal heating due to sun/GG interactions, even if there are no other large moons.

This is one thing I draw a frustrating blank on... I don't really have a handle on what happens to gas giant satellites if the planet is close enough to the star to be significantly affected by solar tides.

I know that if the planet was solid then its moon would slow to be tidelocked to it, and then the planet's rotation would slow so that it was as long as the moon's orbital period (if the moon - which is being pushed into a higher orbit all the time - isn't lost to interplanetary space by this stage), and then the stellar tides would basically suck energy out of the system so that the moon and planet approach eachother until they crash or one is broken up (that only happens after a VERY long time though).

But gas giants aren't solid and can dissipate the tidal heating much more efficiently. Basically if you put a gas giant and a terrestrial planet at the same distance close to a star, the terrestrial world will become tidelocked pretty rapidly while the gas giant won't be slowed down much at all (it will lock eventually, but it takes MUCH longer). So with a gas giant, you can and will get the satellites tidelocking to the planet, but the planet will take a lot longer to tidelock to the star.

What follows is guesswork: I think what would happen is that the stellar tides will still act on the satellite orbit somehow. They could act to make the orbit eccentric (and maybe the long axis of the orbit will always orient towards the star?), because the satellite will always have this varying tidal force on it from the star depending on where it is on its orbit around the planet.

I really don't know though. I do know that the planet won't be tidelocked to the star and the satellite won't go crashing into the planet though, because the stellar tides effectively can't slow the gas giant's rotation down.

 

[Malenfant]

Originally posted by Aramis:
[QB]It is a wild approximation at best; recent articles in several science journals cite that none of the current generation of accretion software winds up covering the whole spectrum from star to moonlets.

So? This article has nothing to do with covering that spectrum.

It is not in the slightest way a "wild approximation at best". I don't know why you insist that planetary science is "at best" completely inaccurate guesswork, but it is nothing of the sort. Frankly, I find that sort of statement incredibly offensive and I'm deeply insulted - as any scientist would be - that you make these claims.

These results have been peer-reviewed by people that are much more knowledgeable about the subject and more rigorous than you. If they had reason to believe during that process that these results were invalid then they would have rejected them. So just accept that and learn from them, or ignore the whole thing if that's what you prefer - but it doesn't make it go away. Call me weird, but I listen to experts because they know how things work - that's how I learn things. Maybe you should try that sometime.

Further, with only 4 datapoints available, and only 3 cited (Jupiter, Saturn, and Uranus), it is at best an interesting situation, which they have come up with an interesting model for.

Again, this is based on accretion theories that are pretty well understood. Sure, we could do with more data but it works for what we have so far and the physics is sound enough that it should generally work for anything else we find too. You seem to think that the universe beyond the solar system won't work the same way as it does within it, but I can assure you that it does. What applies here applies elsewhere too.

Accepting that the simulation is proof is like saying that I can prove a man can jump 20 feet using a simulation. The simulation is not proof; it is at best a test for instant rejection upon the basis of the math not working.

Nobody said it was proof. It's a simulation, a model. But it's one based on our current understanding of physics, which is based on observation and data.

At the end of the day, if you want to think that science is "wild guesswork" then that's up to you, but don't try to claim that scientists are taking wild shots in the dark all the time and don't know what they're talking about. Sure, there is some guesswork and assumption involved but it's educated guesswork and assumption based on what we know already. But that is not remotely the same as "wild ass guesswork".

The general validity level, according to several statistics texts, is n=30+. We have 4 observable; not enough to make valid statistical analyses of the GG's and their moons. (Educators and Astronomers have a strong tendency to use low-sample-size studies... which are indicative of a trend, but not good enough for validity. Both with good reasons, but in both cases, it's not really good enough for proof.)

So what would you have us do, just not bother studing the problem at all? We have four gas giants here, that's all we have. Until someone comes up with anything better, then we're going to work with what we've got and these models are the best we have so far.

Not that it really matters though, since the physics applies everywhere. I fully expect that if and when we do see other gas giants satellite systems, most of them will conform to these predictions. If they don't, well it's tough luck on us, but we'll figure out new models based on the new data. If you don't believe that what's presented here is useful then that's up to you, but again that is no reason to say that the science is "wild guess work".

If you think you can come up with better then by all means formulate your own model, and write a paper on it and publish it - otherwise clam up. If you want to question the validity of science then go elsewhere to do it. Now I'd appreciate it if you could kindly stop thread-crapping and leave this thread to the people who don't think that scientists and experts don't know what they're talking about.

 

[Malenfant]

Originally posted by Malenfant:
This is one thing I draw a frustrating blank on... I don't really have a handle on what happens to gas giant satellites if the planet is close enough to the star to be significantly affected by solar tides.

Just to answer my own question... I happened upon a paper after I wrote this that might answer the question.

http://arxiv.org/abs/astro-ph/0205035

I'm still trying to digest it, but it seems to basically depend on how quickly it takes for the jovian to tidelock to the star (remember that they will eventually tidelock, and if they're very close to the star (within 0.5 AU) they can do so within a few billion years).

But it looks like either the satellites will be lost if they evolve outwards beyond about 0.36 hill radii from the planet, or they'll spiral in and crash into the planet if the planet gets tidelocked (they might evem go out and then back in if the planet gets tidelocked as they're evolving outwards). The timescale of that depends on a lot of things, so it's something that one would have to crunch for each system... (unfortunately they don't print a table showing the timescales involved).

Still, it's something for us to go on for now. I'll see if I can crunch some numbers at some point (assuming of course that people don't immediately poo-poo it because it's planetary science

)

 

[notagoodusername]

Okay, this topic has been reported to me as it is close to getting personal again. Everyone heed Sigg's advice and stick to debating the topic and stay nice. Last warning.

Shane

 

[Laryssa]

Originally posted by Malenfant:
I'm not saying they're impossible at all. Again, read what is said.

You keep bringing things up that are totally irrelevant to the matter at hand. Star types are irrelevant. Terrestrial worlds are irrelevant. All we're talking about here are satellite systems of gas giants.

Seeing no evidence for something is no reason whatsoever to believe that it can exist. Computer models are based on the real world, and given that we've never seen earth-mass satellites around Jupiter-mass or less gas giants I'm inclined to go with the computer models that say that outcome should be expected. The models fit the observations - your assertions do not, and aren't even *based* on observations or evidence.

Your logic is designed to make sense to you only, Tom - it's quite baffling to anyone else. You claim that it's possible to have a gas giant orbiting a gas giant and then claim that if that's possible then it can have an earth-sized moon - but you don't even have evidence that it's possible for a gas giant to orbit another one in the first place. Perhaps it is possible for that to happen, but that's a very abnormal situation, and there's certainly no guarantee if that did happen that it's suddenly magically possible for them to have earth-mass satellites.

Instead of making the wrong extrapolations based on your ignorance, why don't you actually take the time to read up and learn about the subject? Gas giants are not terrestrial planets and they are not stars - they form differently and so do their satellites, and that is what this paper is about. Accept that and maybe we might get somewhere, because your opinion certainly isn't based on fact, but their models are.

I look at it this way. A gas giant is simply a star that began forming and then ran out of material before it became massive enough to become a star Typically that's called a brown dwarf. I don't know if their is a lower limit on what you call a Brown dwarf, but what if that potential brown dwarf ran out of material just when it accumulated enough material to make a Jupiter-sized body?. The gas clouds that make up a star, a brown dwarf of a gas giant are the same stuff, its just a question of how much their is available. The diffence between a brown dwarf and a gas giant is subjective, we decide what is what, nature could care less, to nature its just a pile of molecules held together by gravity, in some cases that pile is massive enough to create enough gravity and pressure to cause fusion at its core and in other cases the mass is insufficient. The processes that go to form binary star systems could also play out on a smaller scale to form binary gas giants. Maybe the computer model your talking about says that's impossible, but then again maybe that computer model takes into account only one sort of gas giant formation model. You assume the gas giant forms as part of a star system, but then it might form separately in the same stellar nursery where the stars formed. Many stars are created at the same time in that stellar nursery, those stars have varying masses, and no doubt some of those masses will be insufficinent to be stars. Now if you have a binary star system forming and the companion runs out of material before it becomes a star, then what you have is either a brown dwarf or a planet. Alot of binaries have elliptical orbits, so in this case you simply have a binary star system where one of the "Stars" doesn't have enough mass to become a star, and maybe this little gas ball has a binary companion of its own.

I prefer to keep an open mind about such matters, someone's computer model is not enough to prove to me that such happenings in the Universe are impossible, and their are so many stars out their, that I'm sure their are gas giants with Earth-sized companions. Astronomy is an observational science and so far our observational capabilities aren't sufficient to determine what sort of gas giants are out there and how many have Earth-sized moons. If we find that none of them do have them, then I'll agree that your right, but until we have actual observational data, and not just computer model projections, I'll have to keep an open mind about the possibility, that is all.

 

[Laryssa]

Originally posted by Shane Mclean:
Okay, this topic has been reported to me as it is close to getting personal again. Everyone heed Sigg's advice and stick to debating the topic and stay nice. Last warning.

Shane

The whole thing is a tempest in a teapot, stars and planets are distant things, nothing to get personal over. I could care less if Gas Giants had Earth-sized moons or not, I have no personal investment in it, its just that my gut instinct tells me not to let someone's theory prescribe what sort of worlds to include. To me the only thing that is important is observational data, and we don't have enough observational data about enough gas giants that are out their to really be sure that we've got a handle on how they form, how many ways they can form, and what size satellites they may have. Its all too easy for me to imagine some scientist painstakingly building a model on how star systems form, and then later on some astronomer discovers a star system that defies his prediction, its happened before with all the "hot Jupiters" that have been found recently, it happened once again when someone discovered that the Universe's expansion was accelerating, when computer models suggested that gravity should be slowing it down.

Now where would we be if we just stuck with what the computer models said about the Universes expansion rate and never took measurements to confirm it? I'm sure whoever wrote that computer model was a smart guy, but even smart guys can make mistakes, they leave things out, and they may not have enough information to make an accurate model in the first place even if they think they do. Scientists have been wrong so many times before, so all predictions must be verified eventually, we can't just take them as a matter of faith.

 

[Malenfant]

Let me make something clear - I post science stuff here so that people can learn about new ideas in astronomy and discuss them in the context of Traveller. I'm sure that's why other people do too. If you're not interested in learning or discussing that then don't read or comment on these posts, and don't use these threads as an excuse to foist your opinions on science on people because this isn't the place for it.

I am sick to the back teeth of people insulting my profession, denigrating science, claiming they know better than scientists, propagating their own ignorance and wacky worldviews, or just plain ignoring everything that is said in favour of their own personal beliefs that are not based on reality.

This sort of environment is very science-hostile and there's no damn reason for it to be like that at all. Right now I'm seeing little reason for me or anyone else to continue to waste their time by posting science links or discussing science here at all given that sort of hostility.

So make up your minds, CotI. Either you want an environment where people can freely discuss interesting scientific developments in the context of Traveller (and I know that there are people who are interested in this), or you want an environment where any advances in knowledge are going to be shouted down by a loud minority of people who want everyone to wallow in ignorance.

It's up to you.

 

[notagoodusername]

To quote the great Albert Einstein (Seeing as we are on a scientist topic):

Albert said:
Laws alone can not secure freedom of expression; in order that every man present his views without penalty there must be spirit of tolerance in the entire population.

However, that spirit of tolerance is now gone from this thread, so this thread is gone too.


Shane