Complicated Question

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Space_Goose

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Hi,

I was wondering,

What if the Earth were not a planet but a moon orbiting a huge gas Giant? If the Earth was the same in every way, same distance from the sun, same size but instead of having a moon, it was a moon of a gas giant like Jupiter or Saturn. How would our lives be different? What effects would this difference have on seasons, tides, weather ect. Also what possible effects would this have on evolution?

Thanks.
 
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crazyeddie

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Oy! "Complicated" is right.....so many possible variables. I think the only thing you could say for sure is that if our world orbited a gas giant, our day would coincide with the period of our orbit around it, meaning it would be quite long by our present standards: several days to weeks in duration, depending on the distance. If the gas giant has a powerful radiation belt of the type possessed by Jupiter, it's hard to imagine how life could gain a foothold on our world (unless it, too, had a powerful magnetic field to protect it), otherwise, it's surface would be constantly bathed in lethal radiation.

Isaac Asimov envisioned such a situation in his novel "Nemesis", where an Earthlike planet revolved around a gas giant, that in turn orbited a red dwarf star.
 
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Space_Goose

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Hello,

Thank you for the information.

I ask this question because I am writing a story for a creative writing class. In my story, a human is taken by aliens back to their home world which is a moon of a Gas Giant. Even though my story is of course fiction I would still like to add some scientific truth to it and accurately describe the aliens’ home world. This story is written like a journal cataloging this humans experiences so he describes in great detail the alien home world. I just wanted my description to be as scientifically accurate as possible. So I guess I asked the question wrong. Since this moon is fictitious, you could plug in any variables that you want. I am just looking for help in coming up with what the alien life would be like for my story and what the living conditions would be like on this moon.

Thanks
 
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Saiph

Guest
well, we can start from some basic assumptions:

The 'planet' is likely to be tidally locked. So one side will always face the gas giant, the other will face away. This will dictate the 'day night' cycle as it's based on the moon's orbit, not rotation.

The tidal forces will likely severely flatten any geological features. So you'll have lots of rivers and lakes, but no real ocean. Unless there's enough water to make the whole flat thing one big shallow sea.

You'll have some pretty nasty weather, as portions of the planet would be under the punishing glare of the 'sun' for day's or weeks at a time, before it rotating out of view by going around it's orbit. Unless you've got a dim star (or very distant star), and say, a large hot brown dwarf...then you'd get lots of light all the time from the dim star, and from the brown dwarf radiating as it cools.

For the planet facing side, even during the night phase, the gas giant will reflect a lot of the sunlight, not really keeping it warm (the IR is trapped by the gas giant, but optical is bounced), but also strongly illuminating the surface. Imagine a Full moon....x100! They wouldn't really appreciate a true dark night except for when the gas giant eclipses the sun once an orbit (for hours/days depending on moons' orbit).

The outward side wouldn't have that problem though...so you could have plants vary more by which hemisphere they exist on, than how close to the equator (or poles) they live on, though that would likely matter too.

The radiation band (like earths van allen belts) don't have to enclose the planet, but could sterilize other moons. The gas giants magnetosphere could mean high radiation levels though, possibly suggesting more life is aquatic or subterranian (to shelter it from what radiation there is). Or with thick, radiation resistent hides/metabolisms.

You'd probably have some spectacular, nearly constant (and likely very bright) aurora -- planet wide.

Odd little local custom touch: with the tidally locked planet, and prominent gas giant, they could very well define the gas giant to be their 'north pole', instead of having it along the axis of rotation/revolution like earth. I.e. earth's north pole is tilted 23 degrees to our orbit, theirs would be ~90.
 
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neilsox

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As Saiph suggested, tide lock makes your planet quite different than Earth. If the aliens are very advanced perhaps they can prevent tide lock allowing a 24 hour day. The year can be 365 days if the planet/moon is 20? million miles from the gas giant. That far away, total eclipse of the Sun will not occur, but the partial eclipses may last several days, thus cooling the planet a bit. Tide lock is easy to avoid that far away. The gas giant will however appear smaller than our moon, and other massive bodies in the solar system need to stay far away to avoid perturbing the planet/moon out of it's orbit. The period of the gas giant does not affect temperatures much, if the orbit of the gas giant around the sun is close to circular, and the planet/moon axis is close to perpendicular to the plane of the gas giant's orbit. I don't think the year for the gas giant nor the tilt of the gas giant axis affects the climate of the planet/moon. Please correct me if I have some of this wrong. Neil
 
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Space_Goose

Guest
Wow guys, very good stuff. This really gives me some options for my story. My Story is not due for another two weeks so please feel free to keep information coming guys if you have it. I appreciate all of the responce.
 
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crazyeddie

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You may have to explain how your primary gas giant planet came to orbit within the star's habitable zone. The generally accepted theory of planetary formation would not permit a gas giant to form so close to it's star. The "hot jupiter's" we've discovered in close orbits of other stars must have migrated there from the outer system due to interactions with planetesimals or gas accretion disks. I don't know how this migration would bode for your alien's world. It's possible that such a gas giant could not hang on to such a large moon and still be able to migrate inwards to the star's habitable zone.
 
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Saiph

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well, one dodge for the habitable zone question is lodged in my post, make the gas giant a brown dwarf. While not a true star, they do radiate quite a bit of IR as they cool and contract. This could easily give you a good fudge factor for a habitable zone, and planetary placement.

Or you could ignore the question entirely. It may be odd, but not worth commenting on :) Or you could reasonably expect the question never to cross most readers' mind.
 
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Space_Goose

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Hmmm, I have thought about this question and have already considered making the gas giant a brown dwarf. The only problem I came up with is the fact that a brown dwarf is a failed star so if the moon orbited a brown dwarf, would it be different from orbiting a gas giant, would I have to change more on the moon to make this scernerio work? Like stated with Jupiter before, wouldn't the brown dwarf imite large amounts of radiation? And also considering that Even a small brown dwarf is bigger than a gas giant, wouldn't I probably need to move the moon even further out to prevent tidal lock?

I also read that some believe Jupiter to be a brown dwarf. What if instead of all that, the moon orbited a normal gas giant but the moon had a thick ozone like but not quit to the extreme as Venus and the moon had a hot and active iron core and generated heat from its center. Would this idea work? Could a moon still support life under these condisions even though it was far enough away from the star that its gas giant could have formed?
 
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Saiph

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As long as you don't get to specific, this scenario isn't to bad. And I'd actually keep the tidal lock, makes the world interesting, especially for a shorter story. A world of marshes and lakes, aliens that are awed by oceans and mountains.


As for the radiation, say it's far enough to avoid the worst, and the thick atmosphere protects them somewhat and helps trap the weak sunlight and the IR light given by the brown dwarf.

Throw in some biological measures to help cope, thick hides (maybe sorta insect like), and things like laying young as eggs in caves or such beneath the surface. This protects the young during a very critical and vulnerable period of growth.
 
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Space_Goose

Guest
Ok, good stuff.

Thanks for all the information guys. I will post a reply when I get my grade back from my short story and let you all know who I did.

Thanks again.
 
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neilsox

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I don't think we have found a newly formed brown dwarf, but they are thought to radiate energy mostly in the visable and ultra violet. In a million years or so they cool to red hot, thus radiating in the infra red range = radiant heat. At this stage they would keep a planet warm that was about ten million miles away, but would produce almost no light and not enough heat after they cooled a few thousand more years.
100 years ago we thought that Mercury was tide locked to our Sun, but now we know it is not tide locked, nor is any other planet known to be tide locked. We suppose there are tide locked planets is some solar systems as we have found several tide locked moons.
We think a planet with a bit thicker atmosphere than Earth, might boil ocean water where the sun was directly over head, and have a giant ice cap on the opposite side, which would always be very dark. except for stars and perhaps northern lights. The twilight band would have comfortable temperatures for humans, but not enough light for robust photosynthesis A slightly thinner atmosphere would allow all the water on the planet to be trapped in the great ice cap. Other Suns in a binary system, come close rarely, but make the planet too hot for the few weeks they are close, or they don't come close enough to melt the ice cap significantly. Civilization is precarious on a tide locked planet unless the atmosphere is very thick. Possibly the surface of Venus is one degree warmer because of internal heat. Lots of internal heat likely means excessive volcanos, earthquakes and geysers, but again we have zero examples to test our theories. Neil
 
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Space_Goose

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Hello everyone,

I just wanted to let everyone know that I have gotten my short story back from the professor and I got an A. The professor actually asked me if I had done research on the subject or if I had made EVERY THING up. I told her I did a lot of research but also got a lot of help from the people at space.com forums. Needless to say, thanks guys for everything, Part of this A beings to all of you. I got to admit that I have never really enjoyed writting or reading but I really had fun writting this story. I enjoyed writting it so much that I have decided to use what I have already written and turn it into a proper sci-fi novel. Who knows if I will ever do anything with it once I get done but this is mainly just going to be a hobby something for me to do in my spare time. And who knows, one day this might be a novel or a movie even. All I can say is if it does, everyone on these forums has a free copy coming their way if they are still around at the time. :)

For the final paper I decided to take Saiph's advice. I kept the moon orbiting a gas giant. The moon is close enough to the gas giant that it is tidally locked which I agree with Saiph made the moon more interesting to read about. The moon was there for very swamp like with now mountains and shallow seas. It has a thick atmosphere and a molten Iron Core so it is geologically active which paired with the thick atmosphere makes the planet warm although the Gas Giant is outside of the Goldielocks zone. There were a copy of assumptions I had to make that I wasn't sure if they were scientifically accurate but it was last minute and I didn't have time to ask on the forums.

My first assumption was that since the moon was tidally locked, that the side facing the planet at all times would have deeper water and there fore, is pretty much one big shallow sea. I figured this due to tidal bulg, that the gravity of the gas giant would pull the tides toward it like the moon does on earth meaning that on the side of the moon facing the planet, it would always be "High Tide".

My second assumption was on the aliens them selves and what they looked like. In my story, they were a huge physically intemidating race. They had armor plats on thier bodies like some of the early Dinosaurs to help protect them from radiation of the Gas Giant. But was thinking, if they came from a very swampy world full of marshes lakes and shallows seas, would it make since for the aliens to have evolved into being a huge physcally intempidating creature? I don't mean Dinosaur sized, I mean, I said in my story that the average height for an adult was around 12 feet. Average earth weight was about 350 lbs. I also started thinking that in my story, their moon world is smaller than earth and less massive which means gravity would be weaker so once again I didn't know if it would make since or not. I discribed their world as being slightly smaler than Venus but bigger than Mars.

so for my novel. do I need to change any of this to make it more scientifically accurate?

Thanks guys.!
 
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kg

Guest
Congrats on your A!
I do wish that more sci-fi writers would do at least a little bit of research before starting to write. Nothing throws my suspension of disbelief out the window faster than a writer throwing out scienctificaly sounding words while having obviously no idea what they mean.
Have you given any thought about what the atmosphere is made of. Oxygen, Nitrogen, CO2?
 
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Space_Goose

Guest
The only real thought I have given to the composition of the atmosphere is only that it has a higher amount of oxegen than the earth thus allowing for bigger life forms. But once gain, I am not sure if it would be good evolution theory to have a big massive bulky animal/people evolve on a planet with slightly less gravity than earth and full of swamps lakes and shallow seas. The only evolutionary trait I did add to my story since the moon is tidally locked and faces long periods of darkness when the moon is on the opposite side of the gas giant from its star, the aliens who two sets of eyes, one set which sees like we see, then another set of smaller eyes which see in infered. That way they could n essence, see when during the long peiords of darkness.

I know they would have the technology to slave this anyway but I was till trying to think how they might evolve on this tidally locked moon.
 
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neilsox

Guest
For humans and most oxygen breathing earth animals the best oxygen partial pressure is about 4 plus or minus 1 psi = pounds per square inch, so 8 psi needs 50% oxygen, 40 psi needs 10% oxygen. Fire hazard increases rapidly above 4 psi oxygen partial pressure, and some humans have severe altitude sickness below 3 psi, even after adapting slowly. ie Aspen, Colorado, or Pike's Peak. Neil
 
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Space_Goose

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I was just reading an article about early earth which said that oxegen levels on earth use to be a lot higher than they are now which allowed the Dinosaurs to go to the size that a lot of them did. Maybe this would cause fires to be a bit more feroucious on the moon. That might make it even more interesting. ;)
 
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SpaceTas

Guest
Many Jupiters have been found in the habitable zone, so the idea of an Earth sized moon is a real possibility. There are searches under way for such "moons".

As for conditions:
Tidally locking is expected only if the "moon" orbits close to the giant planet. But there is no reason to expect this except for the examples within our solar system (Galliean moons of Jupiter, Titan of Saturn, counter example Triton Neptune).

If there is tidal locking the temperature difference between day and night can be evened out by a thick atmosphere. Models suggest that an atmosphere 1.5 as dense as Earths makes temperatures overall OK.

A strong "moon" magnetic field would be enough to protect surface and atmosphere from giant planet magnetic field. However the "Jupiter" magnetic filed may be quite weak, if it is tidally locked to the star; ie it has a slow rotation. There is debate amongst the modelers whether slow rotation necessarily means a weak field.

If there are other moons in the system, especially in orbital resonances (orbital periods multiples of each other) there may be lots of tidal heating and so volcanic activity (Io an extreme example). This could be enough to keep a planet-moon warm enough for life on the cold side of the habitable zone.

Fun !
 
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Saiph

Guest
::preens::

You took my advice, thats just neat.

You got your 'high tide' assumption right on the mark.

As for the size of your aliens. Looks like you thought it out, and it could work. Evolution is kinda weird and can fill in all sorts of strange designs. Look at Giraffes :)

Large is fine, as long as you consider a bit how they deal with their native terrain. Actually, and size/shape is fine as long as you consider this.

Throw in the high oxygen content and lower gravity and it makes plenty of sense to have larger critters. Though they may not be stronger, pound/pound than a human due to different muscle densities...

I like the multiple eye idea, though I will point out that IR sensistive eyes would tend to interfere with themselves (they give off their own IR!). The trick there is to have a way of cooling the eye, say, by having them on eye-stalks? Or have dedicated skin flaps that act as radiatiors, cooling the blood supply for them, helping keep the eyes cool. The flaps could also bunch up to help shield the eyes from excessive light & heat.

Just a couple tidbits, sounds like you really hit the ball out of the park. Congrats.
 
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