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moon of a moon


TheNewTeddy
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I am writing a fiction and need some science help.

Lets presume that somewhere out there, there is a huge gas giant planet.

Orbiting it is a super-earth http://en.wikipedia.org/wiki/Super-Earth

First, could that super-earth be a water planet?

Regardless

It needs a moon itself. IE the moon of a moon. That moon can be small, but needs to support life.

The question is: what kind of day-night cycle would that moon have presuming everybody is tidally locked?

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Bodies orbiting gas giants, or brown dwarfs or small stars can't have moons on their own because the gravitational perturbations from the parent body are just too big. Rule of thumb, if something becomes tidally locked, it can not have moons. Though this is not an exact derivation, if the perturbing body has enough influence to tidally lock the moon, then it has enough force to kick anything out of orbit over long time too.

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Yeah, it could. But you must watch for SOI( http://en.wikipedia.org/wiki/Sphere_of_influence_(astrodynamics)).

My advice is hot(or to say, warm) Jupiter model, so you're free to decide the distance. The farther your Earth from Jupiter, the larger SOI of Earth can you have for the place of a moon. BTW the Earth is likely to be tidal locked by Jupiter, and if your Earth is big enough, they may be tidal locked by each other like Duna and Ike.

Edited by Cesrate
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In all likelihood, a satellite of large enough size to be worth mentioning wouldn't be dynamically stable over long time scales. You could maybe play around in Universe Sandbox or a similar program to get a feel for what configurations would or would not be stable. Not sure if it models tidal effects though.

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As I said, such thing would most likely not be gravitationally stable. To maximize stability, the ocean world would have to be very far from the brown dwarf and the habitable sub-moon very close to the ocean moon. I would still not be stable, but it will hold together a few million years till the blue star explodes and everything will be obliterated anyway.

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say orbital period 14 days for the ocean world and six hours for the sub-moon.

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( habitable means reductive atmosphere and some bacteria in hot springs, in the short period nothing more manages to evolve, except when it is terraformed )

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Temps on earth range from -50 to 50, so I'd be interested to create a planet where temps range from 50 to 150.

Also, I asked this on another forum, and someone told me to just do whatever I need to for plot purposes. My answer to that was: "I am very very careful not to do this. I want real science. If it's not real science, my literary needs change."

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50 to 150.

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earths average temperature is cca 15 degrees C. that world would require average global temperature 115 degrees C. while of course the boiling point of water can be increased vastly through pressure, there would be still a lot of water vapor in the atmosphere that would trap heat and create a wet runaway greenhouse. If your world is gonna to have water oceans, it most probably can't go much above say 30 degrees C average temperature without going all the way to hundreds of degrees.

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earths average temperature is cca 15 degrees C. that world would require average global temperature 115 degrees C. while of course the boiling point of water can be increased vastly through pressure, there would be still a lot of water vapor in the atmosphere that would trap heat and create a wet runaway greenhouse. If your world is gonna to have water oceans, it most probably can't go much above say 30 degrees C average temperature without going all the way to hundreds of degrees.

if earth is 15 above freezing, my world should be 15 below boiling

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if earth is 15 above freezing, my world should be 15 below boiling

He's saying that a water planets plausible average temperature comes in the form of a J-curve where the margin of maintaining 0-15C isn't the same, in terms of stability, as 85-100C. Because as the temperature passes, say, 30C average, the evaporated water vapor creates a compounding greenhouse effect that actually increases that number substantially with no measure of stability. At least, that's how I read MBobrik's response.

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He's saying that a water planets plausible average temperature comes in the form of a J-curve where the margin of maintaining 0-15C isn't the same, in terms of stability, as 85-100C. Because as the temperature passes, say, 30C average, the evaporated water vapor creates a compounding greenhouse effect that actually increases that number substantially with no measure of stability. At least, that's how I read MBobrik's response.

The pressure doesn't have to be the same as it is on Earth. It could be more like Titan, which has an atmosphere with about 1.5 times the pressure of Earth's. This would increase the boiling point of water, which would increase stability. The dense atmosphere could also help explain the hot temperatures.

About the actual moon of a moon idea, I think it could work. Let's say the large moon isn't tidally locked to its planet, and is a fair distance away. The small moon could be very close to the larger moon, which would also increase stability because gravity from the larger moon would be much stronger than gravity from the planet. It still wouldn't be perfectly stable, and you couldn't have the larger moon tidally locked, but otherwise I doubt it would be realistic.

Edited by ZingidyZongxxx
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Your problem finds a simple solution:

Yes, its possible. Given your statement of a huge gas giant, you have a very large SoI to work with, which makes your life easier. Lets assume this gas giant is a peaceful one, lacking a radiation belt (Jupiter will irradiate you into a piece of space poop within minutes). I'm going to use numbers now.

Radius of the gas giant = 70,000 Km at equator.

Radius of the super earth = 7,000 Km at equator.

Radius of the moon at equator = 700 Km at equator.

SoI of the gas giant is (using 700 as the multiplier) therefore 49,000,000 Km

SoI of the super earth would be 1,050,000 Km if it were alone in orbit around the Sun (using 150 as the multiplier)

I deem the SoI small enough to proceed anyway.

SoI of the moon is 26,600 Km, this will suffice.

Say the super earth is out at 25 million Km from its parent gas giant, where its SoI would grow to about (25/49 = 51% of it if it were alone, so 535,500 Km.

of 535k, 26.6k is a minor fraction (barely 5% of the super earth's SoI radius). If the moon orbited close to the super earth, creating a system of 2 bodies with significant mass orbiting a common barycenter (this effectively makes them stay together, despite the gas giants gravity).

With some rudimentary math and a bit of logic, you can explain anything. I would personally go with the binary system with the barycenter, as it's the easiest way to prevent capture of the moon by the gas giant.

Now, as you can tell, this doesn't compare to the Jool-Tylo system at all, where Tylo's SoI doesn't even reach beyond 10,000 Km. This comes from the proximity of Tylo to Jool. This system of gas giant-super earth abuses the distance and extremely high gravity for a moon (Earth's gravity is extremely strong given its size) to create a stable system. I would say that they remain tidally locked, even to the point of the super earth slowing the gas giants rotation gradually, and introducing tidal forces to the planet.

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As I understand it the region of stability for orbits is dependant on both the distance and the relative masses of the bodise involved. The larger you make the jovian, the further out you can put the super earth, but the further out you need to.

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In order for this to work your ocean planet need to be very far from the gas giant. Having a planet that big so far from it's planet would properly not be possible unless the gas giant was super massive and by that time it would properly be a brown dwarf. If you chance the gas giant to a brown dwarf it could work. A brown dwarf compared to a blue main sequence star would be so small it would behave like any other planet.

Also a tidally locked planet's days would be as long as its orbit. If you put it close enough to still be stabil you could have a day of 2-3 earth days depending on the main body's size.

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The problem here is not so much "would such a system be gravitationally stable" (the answer is yes, for the right configuration) - the problem is "how would such a system form."

Planets form by accretion of material from a protoplanetary disk. Basically, collisions happen at random in the available material until you get a large enough "seed" or "embryo" that it starts pulling in the rest of the material around. From there on, it gains mass relatively rapidly through gravitational accretion until it runs out of stuff to accrete.

Okay, now what about moons? Well, there are a couple of ways for a planet to gain a moon. It can acquire one through gravitational capture of a passing object (such as Neptune's capture of Triton) or through a "second generation" coalescence of material captured by a forming planet - a proto-munar disk so to speak. But here's a problem: in order for a planet to form its own accretion disk, it has to gravitationally capture material with sufficiently high angular momentum that that material begins to orbit the planet rather than accreting directly. But, since this material is coming from the primary stars' own accretion disk, anything in a similar orbit will have nearly zero angular momentum once it's captured, so you've got to capture material from relatively far away (or, you've got to find another method of generating the disk - this is what the Earth likely did, via collision). This means you've got to be big. This is why Jupiter and Saturn have multiple large moons, but the small worlds other than Earth do not (and there were special circumstances for the Earth).

Okay, for your scenario, you have that: a blue giant star which will likely have a huge protoplanetary disk with lots of stuff for forming planets, and a super-Jovian with lots of gravitational pull, capable of creating a large secondary accretion disk of its own. Now here's the tricky part: we need a third generation. It's no problem to buy that the super-Jove will pull in enough material for an Earth-sized or larger satellite. But, how is that satellite going to form a tertiary accretion disk so it can have a moon of it's own? The super-Jove's secondary disk will be large, but not that large, and the sphere of influence of the moon won't be big enough to attract material that doesn't simply accrete directly onto it. (Remember, not even the Earth is large enough to attract sufficient material for forming a large moon through direct accretion - and that's without a nearby gas giant to mess things up.) So, now we're looking at a collision type scenario similar to what happened for the Earth - basically, you'd need two near Earth mass moons to form in super-Jovian orbit and then collide. But, even if this happened, that nearby super-Jove is going to screw up the accretion process by tidally perturbing the disk, which means your super-Earth moon might have a pretty ring for a few million years but never a moon of it's own.

And then there's the whole problem of that blue parent star going "boom" in a few million years anyway (barely enough time to get this planet business started, frankly).

Basically, in order to get what you want, you have to scale up that super-Jove until it's not a super-Jove anymore.

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A second question that may need to be considered is how much radiation is coming form this gas giant.

You may need a small gas giant to keep from killing any life on this world unless it has evolved to adapt to it.

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He's saying that a water planets plausible average temperature comes in the form of a J-curve where the margin of maintaining 0-15C isn't the same, in terms of stability, as 85-100C. Because as the temperature passes, say, 30C average, the evaporated water vapor creates a compounding greenhouse effect that actually increases that number substantially with no measure of stability. At least, that's how I read MBobrik's response.

:confused:

What I'm saying is that the planet's temp will pivot around boiling as ours does around freezing.

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In order for this to work your ocean planet need to be very far from the gas giant. Having a planet that big so far from it's planet would properly not be possible unless the gas giant was super massive and by that time it would properly be a brown dwarf. If you chance the gas giant to a brown dwarf it could work. A brown dwarf compared to a blue main sequence star would be so small it would behave like any other planet.

Also a tidally locked planet's days would be as long as its orbit. If you put it close enough to still be stabil you could have a day of 2-3 earth days depending on the main body's size.

I'm not opposed to making a brown dwarf.

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:confused:

What I'm saying is that the planet's temp will pivot around boiling as ours does around freezing.

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As I said and westair repeated, the situation around freezing point and boiling point is not symmetric. The point where the entire atmosphere becomes unstable due to wet runaway greenhouse effect is low compared to the boiling point and more pressure does have only limited effect because the thing depends on absolute water vapor amount in atmosphere, not its relative proportions. It increases slowly with pressure because denser air traps heat on its own, so it will require more water vapor till it becomes unstable, but that is very limited because you are racing against an exponential function. At earths pressures and insolation it is around 25 degrees Celsius, so, say at increased pressure and decreased insolation can it be 28 or 30 deg C, but almost surely not 85 deg or more.

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It's no problem to buy that the super-Jove will pull in enough material for an Earth-sized or larger satellite. But, how is that satellite going to form a tertiary accretion disk so it can have a moon of it's own?

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the same as earth. Smaller proto-moon forms in L5 or L4 of the large proto-moon, and grows until the configuration becomes unstable and it crashes sideways to the big moon. And there you have your tertiary accretion disc.

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Bear in mind, as alluded to on the first page, a extremely large supergiant star may well burn out before anything much more than microbes could form. If you put native life on it I don't think you'd be looking at very evolved or intelligent life.

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The point where the entire atmosphere becomes unstable due to wet runaway greenhouse effect is low compared to the boiling point

We don't actually know this, there being no examples of a "wet runaway greenhouse effect" we've been able to observe. Water vapor is a greenhouse gas, but the total effect of increased water vapor upon the atmosphere isn't well known, due to cloud formation. Atmospheres are complicated and poorly understood things; don't be so quick to claim that something can't happen.

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We don't actually know this, there being no examples of a "wet runaway greenhouse effect" we've been able to observe. Water vapor is a greenhouse gas, but the total effect of increased water vapor upon the atmosphere isn't well known, due to cloud formation. Atmospheres are complicated and poorly understood things; don't be so quick to claim that something can't happen.

Because this follows from basic physics, it can be understood through theoretical analysis and simulation. Low clouds serve as negative feedback due to high albedo. But high clouds on the contrary trap more heat. And you will get clouds all the way through stratosphere when the atmosphere is saturated with water like it would be with a 85 deg average temperature world.

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