# The Forest Moon of Endor and Polyphemus **URL:** \ **Category:** Factual Questions\ **Created:** [March 9, 2010, 1:20pm UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846 "2010-03-09T13:20:59Z")\ **Posts on this page:** 12\ **Page:** 3
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [March 10, 2010, 12:54pm UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846/41 "2010-03-10T12:54:19Z")
> [@CalMeacham](#): > > Is there some seminal work, like a novel, influential short story, or some NASA report suggesting this as a real possibility? I’ve heard of a paper ( mentioned in a similar discussion on another forum ) called “Habitable moons around extrasolar giant planets” in Nature from 1996; [this](http://www.nature.com/nature/journal/v385/n6613/pdf/385234a0.pdf) ( PDF ) is the abstract, you’d have to pay to see more apparently. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [March 10, 2010, 2:23pm UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846/42 "2010-03-10T14:23:35Z")
> [@dtilque](#): > > As far as eclipses go, since the moon will be tidally locked, the life on it will have to be evolved to live on a world which gets very long nights. Much longer than any eclipse. And half the moon will not get any eclipses at all, since it will be night on that side during the eclipses. I don’t see the eclipses as being a problem. > [@chorpler](#): > > Regarding tidally locking: why would that make the nights long? Because of the orbital period around the planet? Wouldn’t that depend on how far the moon orbits from the planet? As **chorpler** suggests, the nights aren’t necessarily going to be that long. Io, for example, has an orbital period of only 42 hours, so its night will be only 21 hours. (Since it’s tidally locked, the day will be the same length as the orbital period.) The other Galilean satellites have orbital periods of from about 3.5 to 17 Earth days. I also don’t see the eclipses as being a significant problem, since as **dtilque** says they will normally be much shorter than the night on any moon. ---
**Author:** ![davekhps](https://avatars.discourse-cdn.com/v4/letter/d/a5b964/32.png) [@davekhps](https://boards.straightdope.com/u/davekhps)\ **Post date:** [March 10, 2010, 3:41pm UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846/43 "2010-03-10T15:41:42Z")
> [@DSeid](#): > > The discussion about the scientific plausibility or implausiblity aside, movie directors don’t care about the science; they care about the story, the characters, and the effects. If it sounds good or looks good then that is enough reason, even if it isn’t fully exploited. That’s certainly true for most directors, but this is James Cameron we’re talking about here. He _most definitely_ cares about the science. For example, [Link](http://www.aintitcool.com/node/43923) [Link](http://www.foxnews.com/scitech/2009/12/17/science-james-camerons-avatar/) ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [March 10, 2010, 3:51pm UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846/44 "2010-03-10T15:51:16Z")
For those of you with time to spare…papers from [arXiv.org](http://arXiv.org) on [Habitable moons](http://arxiv.org/find/all/1/all:+AND+Habitable+moons/0/1/0/all/0/1) ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [March 10, 2010, 5:35pm UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846/45 "2010-03-10T17:35:20Z")
> [@dtilque](#): > > One fictional example of a Earthlike moon of a gas giant is a story by Poul Anderson. I’ve forgotten the name of the story, but it involved an alien Sir-Francis-Drake-like sea captain and his crew. (As far as I can remember, there are no humans in the story.) This may be the earliest science fictional example of an inhabited moon of a non-solar system gas giant, although I wouldn’t be surprised if there were earlier examples. > > The moon they’re on is tidally locked (as are virtually all natural satellites) and the country the captain is from is on the farside of the moon (facing away from the planet). So this captain and crew are circumnavigating their world for the first time and are awestruck when they get to the nearside and have this big planet perpetually hanging in the sky. Don’t remember too much else about the story. > > As far as eclipses go, since the moon will be tidally locked, the life on it will have to be evolved to live on a world which gets very long nights. Much longer than any eclipse. And half the moon will not get any eclipses at all, since it will be night on that side during the eclipses. I don’t see the eclipses as being a problem. Yes, I remember that story. That was my first thought too, a moon will be tidally locked to the planet and the planet will become a perpetually stationary skiy object - location dependent. SPOILER They ran across a stranded (earthman?) alien who had spent decades trying to assemble enough mercury to repair his spaceship shuttlecraft in this primitive culture. “Drake” destroyed the man’s current stash of mercury because he thought that exploration was the greatest adventure; to have some magically advanced civilization come and hand the world to you would take all the romance and adventure out of it. I’m sure I have that story in the books buried in my basement. Someday… ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [March 10, 2010, 5:39pm UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846/46 "2010-03-10T17:39:19Z")
The other issue is - the theory why we have gas giants and terrestrial planets - the solar radiation close in to the sun would “boil off” the lighter elements over the eons. That is why hydrogen and helium dominate on the gas giants, but unless chemically bound are minimal on terrestrial rocky planets. Even water on Mars, the theory is that it has boiled off as solar radiation dissocites water molecules and pushes away the lighter hydrogen. The oygen has reacted with carbon etc. SO a giant planet close in would likely not retain much light-gas atmoshpere, would still be a rocky planet. Who knows? ---
**Author:** ![msmith537](https://avatars.discourse-cdn.com/v4/letter/m/d9b06d/32.png) [@msmith537](https://boards.straightdope.com/u/msmith537)\ **Post date:** [March 10, 2010, 5:45pm UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846/47 "2010-03-10T17:45:06Z")
What about tides? Earth experiences significant tides from a little moon 1/6 our mass. What would they be like if we had a planey the size of Jupiter or Saturn in our sky? ---
**Author:** ![EdwardLost](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@EdwardLost](https://boards.straightdope.com/u/EdwardLost)\ **Post date:** [March 10, 2010, 6:04pm UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846/48 "2010-03-10T18:04:20Z")
> [@md2000](#): > > The other issue is - the theory why we have gas giants and terrestrial planets - the solar radiation close in to the sun would “boil off” the lighter elements over the eons. That is why hydrogen and helium dominate on the gas giants, but unless chemically bound are minimal on terrestrial rocky planets. Even water on Mars, the theory is that it has boiled off as solar radiation dissocites water molecules and pushes away the lighter hydrogen. The oygen has reacted with carbon etc. > > SO a giant planet close in would likely not retain much light-gas atmoshpere, would still be a rocky planet. Who knows? Solar radiation would prevent a gas giant from forming near the star. The close-orbit giants that have been discovered must have formed farther out and then migrated in through “friction” with other mass in the star system. By then they had enough mass to hold on to their hydrogen even as they got hotter. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [March 10, 2010, 6:19pm UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846/49 "2010-03-10T18:19:21Z")
> [@md2000](#): > > SO a giant planet close in would likely not retain much light-gas atmoshpere, would still be a rocky planet. Who knows? We do. While it is [theorized](http://en.wikipedia.org/wiki/Chthonian_planet)some close giants will be [reduced to a rocky core](http://en.wikipedia.org/wiki/COROT-7b), we are reasonably certain [others](http://en.wikipedia.org/wiki/51_Pegasi_b)[haven’t](http://en.wikipedia.org/wiki/55_Cancri_b) [yet](http://en.wikipedia.org/wiki/70_Virginis_b) ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [March 11, 2010, 6:40am UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846/50 "2010-03-11T06:40:42Z")
> [@chorpler](#): > > Regarding tidally locking: why would that make the nights long? Because of the orbital period around the planet? Wouldn’t that depend on how far the moon orbits from the planet? Yes. I was thinking of Ganymede with its ~86 hour night when I wrote that. ---
**Author:** ![fiddlesticks](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fiddlesticks/32/18129_2.png) [@fiddlesticks](https://boards.straightdope.com/u/fiddlesticks)\ **Post date:** [March 11, 2010, 2:06pm UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846/51 "2010-03-11T14:06:26Z")
> [@msmith537](#): > > What about tides? Earth experiences significant tides from a little moon 1/6 our mass. What would they be like if we had a planey the size of Jupiter or Saturn in our sky? Tidal forces is one reason why Io is so active volcanically and Europa may have liquid water underneath that smooth layer of ice. So…tides would certain play a big role in the development of a theoretical habitable moon. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [March 11, 2010, 2:58pm UTC](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846/52 "2010-03-11T14:58:08Z")
> [@fiddlesticks](#): > > Tidal forces is one reason why Io is so active volcanically and Europa may have liquid water underneath that smooth layer of ice. So…tides would certain play a big role in the development of a theoretical habitable moon. Just a qualification: If a moon is tidal locked to it’s primary then there are no tides, or rather the tide is fixed in one position. The reason Io and Europa experience tidal heating is because they’re part of an orbital resonance between Jupiter’s moons, so the tidal forces they experience are the result of each other’s gravity. If our hypothetical Earthlike moon had no near neighbors other than it’s primary or small moonlets of negligable mass, it would experience no tidal heating. If there were other big moons orbiting in resonance with it, then yes substantial heating. [Previous page](https://boards.straightdope.com/t/the-forest-moon-of-endor-and-polyphemus/531846.md?page=2)