# Gliese 581g - A Tidally Locked DnD World



## Alaxk Knight of Galt (Oct 1, 2010)

So Gliese 581g was found.  Scientist believe it to be Tidally Locked (much like the moon).  One side of the planet is constantly facing the star while the other side of the planet would be in darkness.  A swath of the world would always be in a state of continual sunrise or sunset.

So here's the thought exercise:  What would a DnD tidally locked world be like?


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## possum (Oct 1, 2010)

Look at how the planet Ryloth was depicted in the Star Wars expanded universe prior to the Clone Wars tv series screwing it up.  The majority of society lives in the narrow band of perpetual twilight buffered against the freezing cold half of the world that faces away from the sun and the blazingly hot half that does.  This society is probably underground, as sometimes flame storms from the hot side trespass into the habitable band, and I wouldn't be surprised if the opposite is true with the cold half.

Thank you, Kevin J. Anderson.

Now, as for some plots; there's always some evil wizard worshipping the giant sun who wishes for it to reign supreme and will attempt some ritual to wipe out all life on the planet thanks to the flame storms.

There's also the good-intentioned mages or druids who will attempt to get the planet moving on a normal basis, which will naturally destroy all society and maybe even the planet itself.


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## Festivus (Oct 1, 2010)

Dark Side: Cold and Dark.  The creatures that lived there before the world stopped turning either adapted to the extreme cold (lots of fur or blubber) and have night vision or some form of illumination, or died and rose again to become undead.  Look to deep sea life here on earth, or drow for inspiration... so almost like the underdark on the surface, only set in Antarctica.  Plant life evolved into mushrooms, glowing lichens and other assorted things that don't require light (even perhaps carnivorous plant life)

Light Side: Hot and Arid, Far to bright to see.  The creatures that lived there before the world stopped turning either adapted to the environment or died and rose again to become undead.  Imagine "Dark Sun", only hotter.  Survival in the desert is nearly impossible for anyone considered normal.

Between the two of these realms is a swath of life that has a day that wobbles between sunrise and sunset, neither taking much time to accomplish.  Life in this belt flourishes.  The creatures the lived there before the world stopped turning had no need to adapt, but must now continually defend themselves from the marauders from the light and dark sides of the planet.


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## jonesy (Oct 1, 2010)

possum said:


> Look at how the planet Ryloth was depicted in the Star Wars expanded universe prior to the Clone Wars tv series screwing it up.  The majority of society lives in the narrow band of perpetual twilight buffered against the freezing cold half of the world that faces away from the sun and the blazingly hot half that does.  This society is probably underground, as sometimes flame storms from the hot side trespass into the habitable band, and I wouldn't be surprised if the opposite is true with the cold half.
> 
> Thank you, Kevin J. Anderson.



That's reminiscent of a planet whose name I can't remember from the Foundation series. Which makes sense, since Isaac Asimov was a huge influence on Anderson.


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## Umbran (Oct 1, 2010)

Festivus said:


> Dark Side: Cold and Dark.
> 
> ...
> 
> Light Side: Hot and Arid, Far to bright to see.




I'm approaching this by way of applying some basic science to the thing:  since it is a fantasy world, science doesn't have to apply, of course.

Being tidally locked does not make the sun _brighter_ on the day side.  It is merely more persistent.  The brightness in the center of the hot side is still only what it would be at Noon if the planet were spinning.

Also, Gliese 581, our example, is not like our sun.  It is a red dwarf, far smaller, dimmer, and cooler than our star.  The back-of-the-envelope estimates I've seen suggest that, if the planet had an Earth-like atmosphere, you might see high temps of around 160 degrees Fahrenheit on the day side, and -25 on the night side.  

The habitable band around the terminator between night and day, then, is probably pretty wide, as even the hottest and coldest points aren't all that hot or cold.  There are no fire storms - 160 degrees isn't hot enough to ignite most materials.  Water tends to evaporate quickly, but it doesn't outright boil.  The cold.. well, -25 is something that folks in the Northern US see not too infrequently.

Another thing you'll see: persistent wind.  On the day side air is heated, and will rise.  Cooler air from the night side will be sucked in to replace it.  So, near the ground, you get winds flowing dark to light, and up high you get flow from light to dark.  This convective mixing may mitigate the temperature difference significantly.  Storm patters would not be much like what we see on Earth.


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## jonesy (Oct 1, 2010)

How about creatures (perhaps intelligent) born on the central band which have now adjusted to both extremes. They spend time basking in the sun of the hot side, and then go on hunting trips in the cold side (perhaps because they find the hot side creatures inedible for some reason).


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## Festivus (Oct 1, 2010)

Umbran said:


> I'm approaching this by way of applying some basic science to the thing:  since it is a fantasy world, science doesn't have to apply, of course.
> 
> Being tidally locked does not make the sun _brighter_ on the day side.  It is merely more persistent.  The brightness in the center of the hot side is still only what it would be at Noon if the planet were spinning.
> 
> ...




I mistyped that, meant to say "far too bright to see without sunglasses" or similar optical evolutions.

I like the persistent wind thing, but for every action, there is an equal and opposite reaction.  Perhaps the weather pattern pushes moisture to the cold side, feeding it's glaciers.  The planet has a molten core which melts the underside of the galciers, which drips down through a massive honeycomb of caves to the molten core, where it evaporates to a gas and extrudes from the light side.  I dunno, something like that 

I agree the belt would be pretty wide, not unlike the tropic of cancer / capricorn in terms of width, but with the temperatures getting extreme to the edges of it.  While still habitable, people in the north of the US don't endure a year of -25f temps, there are seasons.  But then, if the year were shorter, it could be different.

Oh this is fun.


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## Huw (Oct 1, 2010)

Brian Lumley's vampire planet in the _Necroscope_ series had one side permanently facing the sun and the other (where the vampires live) away. I think that one actually span on its side, like Uranus in our own solar system, rather than was tidally locked with its star.


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## possum (Oct 1, 2010)

jonesy said:


> That's reminiscent of a planet whose name I can't remember from the Foundation series. Which makes sense, since Isaac Asimov was a huge influence on Anderson.




I have sadly not read that.  I'll see if I can find a copy at my library.


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## jonesy (Oct 1, 2010)

I've just been told by my girlfriend that I'm totally wrong and don't know what I'm talking about. 

Not about the Asimov influence. That's true.

But the planet I was thinking of was warm on the equator (which rotated straight towards the sun), and everything else on the planet was iced.


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## Umbran (Oct 1, 2010)

jonesy said:


> How about creatures (perhaps intelligent) born on the central band which have now adjusted to both extremes. They spend time basking in the sun of the hot side, and then go on hunting trips in the cold side (perhaps because they find the hot side creatures inedible for some reason).




Well, it isn't like there's this terminator line, and on one side of it it is bright hot sun, and just a few feet away it is dark and cold.  In a planet with an atmosphere, there's a broad band - full-sun shades to twilight shades to dusk shades to full-night, over the course not of feet, but of tens or hundreds of miles.  The temperature will have a similar gradient.

So, if there's an animal that does what you suggest, it probably isn't doing it as a short hop, but as a major journey - go to feed on the night side, come back to breed on the day side, or suchlike.

Which brings me to another note - unless there's a major moon or other body that drives a periodic change, this planet does not have "seasons", as we understand them.  The climate would have weather, but otherwise be "steady state" over the course of the year.


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## jonesy (Oct 1, 2010)

Umbran said:


> Well, it isn't like there's this terminator line...
> 
> So, if there's an animal that does what you suggest, it probably isn't doing it as a short hop, but as a major journey - go to feed on the night side, come back to breed on the day side, or suchlike.



If it's a magical world there could be. 

But now I'm thinking that the idea of a race which spends its entire life on the move crisscrossing between the sides is actually pretty neat. Maybe three months (how would you measure time on a world like this?) resting, breeding, and generally taking it easy on hot side, then off to another long trek through the cold.


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## wolff96 (Oct 1, 2010)

I think the more interesting idea is not that the planet *stopped* spinning, as someone mentioned, but that it's always been that way.

Creatures native to the world are used to this situation.  Outsiders could be drawn to this world _because_ of it's unusual properties.  

I'm thinking (off the top of my head):
An Outpost of the City of Brass, almost right in the middle of the sun-side.  
At least one undead (vampiric, probably) city on the night-side, enjoying the perpetual darkness.
Nomad elves, wandering from the dark-side to the light-side and trading with each -- furs and firey items to the cold-side, blocks of ice (via sled or magic) to the communities close to the temperate band.
The location on the planet reflecting the power of the gods -- clerics of Pelor, for instance, face significant penalties on the dark-side.  (Brings a whole new meaning to 'being a shining light in the darkness').

I could see this as a world where non-magical flight was relatively common, given the winds mentioned above -- hang-gliders, hot-air balloons, and the like would be very efficient means of travel, depending on where one wanted to travel.


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## Thornir Alekeg (Oct 1, 2010)

jonesy said:


> If it's a magical world there could be.
> 
> But now I'm thinking that the idea of a race which spends its entire life on the move crisscrossing between the sides is actually pretty neat. Maybe three months (how would you measure time on a world like this?) resting, breeding, and generally taking it easy on hot side, then off to another long trek through the cold.



 i was thinking about a race that establishes most of the society in the habitable band.  They can absorb and store energy from the sun.  In the habitable band there is enough energy to sustain them, but not enough for propogation of the species.  Adults travel to the bright side where they store up energy and mate with each other.  

Unfortunately newborn offspring are highly sensitive to the radiation from the sun, so after mating, adults travel to the dark side of the planet to bear their young and raise them until they are old enough to handle the normal radiation present in the habitable band.  Because of the dark and cold conditions, the energy stored from the bright side is essential to the survival of the parents.  

The journey from the dark side to the habitable band with their young children is the most dangerous part of their life cycle as the parents are low on stored energy and the children have few skills to survive on their own without their parents.  In the habitable zone when new families start to arrive it is a time of celebration, followed by a period of mourning those who did nto survive the journey home.


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## Desdichado (Oct 1, 2010)

Here's one example, from Leigh Brackett.


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## Someone (Oct 1, 2010)

Umbran said:


> Which brings me to another note - unless there's a major moon or other body that drives a periodic change, this planet does not have "seasons", as we understand them.  The climate would have weather, but otherwise be "steady state" over the course of the year.




Excuse me for the non-accurate terminology, but if the planet's rotation axis isn't perpendicular to the plane of the orbit, wouldn't the terminator vary with the "season"?


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## Alisair Longreach (Oct 1, 2010)

What if Athas is small, tidally locked world orbiting a small red star. The Tablelands is just below the terminator towards the day side, having an eternal day but still habitable. To the south the land gets more scorched and hostile to life. To the north the land gets more green and cooler and the light gradually turns to dusk until you reach the frozen wastelands shrouded in eternal night.


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## Festivus (Oct 1, 2010)

Someone said:


> Excuse me for the non-accurate terminology, but if the planet's rotation axis isn't perpendicular to the plane of the orbit, wouldn't the terminator vary with the "season"?




And does the world have to be spherical?  What about a torus shaped world that was in geostationary orbit around a red dwarf sun, but that rolled, rather like a wheel.  You would have differing gravities as well as environments.  Perhaps the outside of the wheel being more like that of the moon, and the inside being that of two earths.

In the original example, I imagined it might have a slight warble to this rotation, giving you the dusk/dawn effect in the central belt.


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## Umbran (Oct 1, 2010)

Someone said:


> Excuse me for the non-accurate terminology, but if the planet's rotation axis isn't perpendicular to the plane of the orbit, wouldn't the terminator vary with the "season"?




The process of becoming tidally locked will generally force the axis of rotation to be perpendicular to the plane of the orbit.  You can have some small wobble, but nothing like the 23.5 degrees Earth has.  



Festivus said:


> And does the world have to be spherical?




Actually, it won't be, in general.  A tidally locked body will be elongated - with a long axis along the line between the centers of the bodies.  So, like an egg, one end pointed at the star.



> What about a torus shaped world that was in geostationary orbit around a red dwarf sun, but that rolled, rather like a wheel.




Well, now you're talking about a shape that couldn't develop by real-world natural processes.  Magic is involved, so all bets are off!


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## Stormonu (Oct 1, 2010)

"Dark side" would probably have a wide variety of bioluminescant plant/animal life.


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## Festivus (Oct 1, 2010)

Umbran said:


> The process of becoming tidally locked will generally force the axis of rotation to be perpendicular to the plane of the orbit.  You can have some small wobble, but nothing like the 23.5 degrees Earth has.
> 
> 
> 
> ...




What if the hole was blown into the center of the world by some evil plot long ago, rendering the world into it's current state (torus shaped and spinning like a wheel in it's orbit around the sun).  Or if the planet was pierced by something else that managed to not totally destroy it.


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## I'm A Banana (Oct 2, 2010)

Yeah, you're going to see the most difference with weather patterns, nocturnal/diurnal evolutions, and suchlike. Depending on the plate tectonics such a world has, as well, you could see pretty even temperature distribution. 

What strikes me would be the wildlife.

Say, there is a continent on the Dark Side, basically isolated from the Light Side (especially seeing as winds would make it a little more difficult to do some migratory flying like birds do here). Why would creatures there need eyes? Or color? Imagine fish in those ice-locked oceans. Imagine the sloooooow metabolism of creatures over there. I mean, there'd be no plants -- no photosynthesis as we know it. But with some volatile plate tectonics there could be interesting chemosynthesis. Worms and bacteria and insects and fungi in a frozen world that never sees the light of day. GIANT things that feed on them, slowly, over aeons. 

Of course, the other side would be made with photosynthesis (assuming that it's not a unique Earth evolution). Rampant plant life. Rampant life in general -- heat and light and an explosive orgy of bugs and bacteria and fish and maybe even more complex life. After all, it may not have been coincidence that the first "sapient" life on Earth evolved along the equatorial line.


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## ajanders (Oct 2, 2010)

*Random Musings*

There won't be much astronomy on the light side. The dark side might have societies with intimate knowledge of the stars.

Societies on the light side could expand into the dark, with sufficient technology. Get a giant mirror high and you can reflect the sunlight onto the dark.  You can hang it from a kite or balloon (if you have a strong tether) or put it atop a big tower. The sun won't be moving as much, so it should be relatively easy to track.

Can a big mountain create a pocket of shadow on the light side?

A big mountain (really big) might create a rainshadow on the dark side and a rainfed water cycle on the light side. This is probably a pretty good spot for people to live.


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## Lanefan (Oct 2, 2010)

Umbran said:


> Which brings me to another note - unless there's a major moon or other body that drives a periodic change, this planet does not have "seasons", as we understand them.  The climate would have weather, but otherwise be "steady state" over the course of the year.



There's some other variables to consider:

How much water does this planet have, and where is most of it (e.g. light side, dark side, both)? (this would drive weather patterns)

Where is the land and how is it distributed? (ocean currents, if they exist, also drive weather and are a big transporter of heat/cold)

How much atmosphere is there, and how much cloud cover? (more of either means less temperature variance between light and dark sides)

I've been giving this some serious thought of late, mostly along the lines of what would happen to a world's weather if its axis had no tilt; and I suspect it'd be much less variable than ours in any given place even beyond the obvious removal of seasons; because so much of our weather is driven by trying to redistribute the uneven seasonal heating we get. If the planet's truly locked *and* with no tilt to its axis I think the weather patterns would also almost lock in - there'd be places where it's always sunny but 50 miles away there'd be endless rain...

Lanefan


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## Stormonu (Oct 2, 2010)

Getting away from science for a moment, you could have some interesting methods of "lighting" the dark side if you did not want perpetual night for some reason, or "points of light" (pun intended) in the darkness.  

As I previously stated, you could have bioluminescant creatures.  

Heavy (ashless) volcanic activity could provide a source of light/heat.

Constant thunderstorms from the mix of cold/hot air may light areas of the dark side in random flashes.

A orbitally locked moon that reflects light down to the land below.

------------------------------------
Similarly, you could have periods of "darkness" on the light side that provide protection from the glaring sun or "areas of darkness" (pun intended) to adventure in:

Heavy cloud cover/fog from rising water vapor obscuring the light

Regular eclipses from a quickly orbiting moon (Imagine raiding creatures from the night side following the path of the eclipse to attack settlements in the light side...).

Chasms, craters or other features angled just right to provide protection from the sun


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## Merkuri (Oct 2, 2010)

I'm imagining a world with life stacked on one side, steaming jungles with plant life so thick in places that you need to tunnel through it or climb over it to travel.

Civilization lives in the narrow band of twilight, the sun being a constant spot on the horizon, an unerring point of reference, like our north star.

And the other side, which lives in an underdark-like state of blackness except for the glowing sky, alight with stars that could not be imagined on the jungle side.  It is silent and dead.  The little bit of life there is on this side of the world is made up of super stealthy predators.  It is eat or be eaten on the dark side.  They only way for life energy to make its way around from the sun is through creatures stupid enough to wander their way into the black and be consumed.

The sun side draws the civilized folk because it's teeming with wood, exotic foods, spices, and medicinal wonders.

The star side might draw civilization because of the stars themselves.  It is a religious experience for these people to see the spots of light on that side of the planet.  Maybe they believe that both sides of the planet had a sun at one time, but some godling shattered the one on the dark side, and that the stars are the dust that was left over.

Or maybe the star side is rich in minerals or other wonders just below the surface, something that is impossible to reach under the thick plant life of the jungle side.  Metals and gems just sit on the surface of the dark side with no plant life to anchor the soil, but silent death waits among the stars for those who try for it.

Damnit, now I wanna scrap my old idea for a campaign setting and use this instead.


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## Umbran (Oct 2, 2010)

Festivus said:


> What if the hole was blown into the center of the world by some evil plot long ago, rendering the world into it's current state (torus shaped and spinning like a wheel in it's orbit around the sun).  Or if the planet was pierced by something else that managed to not totally destroy it.




As I said, I cannot think of a natural process that would end in that arrangement.  So, any unnatural process you like is fine.

I would note, though, that if I have the image in my head right - a wheel, one "face" of which faces the star at all times, spinning as if it were rolling along the path of its orbit - you aren't gaining much from that spin, unless it is wicked-darn fast.  And at that point you've got issues keeping an atmosphere....

But then, we invoked magic to create the thing, so I'm probably being too nitpicky about the atmosphere.



Kamikaze Midget said:


> Say, there is a continent on the Dark Side, basically isolated from the Light Side (especially seeing as winds would make it a little more difficult to do some migratory flying like birds do here).




Well, if the flying creatures can go very high, migrating might be easy - low-altitude winds help carry you sunward, high-altitude winds carrying them darkward.


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## tylerthehobo (Oct 2, 2010)

There was a kind of fun mini-series on Fox (in the states) years ago about a tidally locked planet with a ('good') Victorian style society on the light side, and a ('nefarious') Medieval society on the dark side.  A few noteworthy actors were even in it, but I guess since it bombed it never turned up on DVD:

White Dwarf (1995) (TV)


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## AbdulAlhazred (Oct 2, 2010)

Umbran said:


> The process of becoming tidally locked will generally force the axis of rotation to be perpendicular to the plane of the orbit.  You can have some small wobble, but nothing like the 23.5 degrees Earth has.




True in general. What about a world with a fairly good sized satellite? Maybe one in a fairly tight orbit. That would tend to allow for a greater tilt. It could also create a bit of darkness on the light side. Imagine it was large enough and close enough to eclipse the sun on each orbit. The dark would be pretty brief but it could have some fun implications.

There would also certainly be some degree of libration. If the orbit is a bit eccentric the terminator could move a substantial distance over the course of a year. That could be fun too.

So an area near the terminator could go from having a low sun angle to having the sun a bit below the horizon over the course of the year. All of these factors could combine to create some degree of variation in weather as well. It would almost certainly be less unstable than our weather is but in any case even unchanging conditions don't necessarily imply perfectly consistent weather all the time, just some sort of fairly steady state.

I doubt much would live on the dark side outside of some areas like deep sea vents, maybe hotsprings, or some area that received a fairly steady supply of nutrients from the light side. Any land area would most likely resemble the interior of Antarctica. With little in the way of flowing water on the surface the dark side would also probably be a rugged landscape. Mountains would erode very slowly. Without soil or much temperature variation to promote weathering the terrain would be somewhat different from what we're used to.

I could definitely imagine some creatures migrating out a ways onto the dark side to reproduce in an area where there would be few predators. It is hard to imagine any (mundane) larger organisms really making a living there on a full time basis though. Again think of Antarctica, but remember it is only in darkness half the year, yet there are virtually no higher land organisms at all. Penguins may haul out there and nest on the edges but they have an ocean to feed them too. Beyond that there are a couple other bird and seal species and a small number of arthropods (the largest land animal in Antarctica being 6mm).

Of course that's just the mundane backdrop...


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## LightPhoenix (Oct 2, 2010)

Plant-wise: most plants sun-side are probably going to be photosynthetic in some way.  Not necessarily green, but definitely converting solar energy to biochemical energy.  On the other hand, dark-side "plants" will almost certainly be chemosynthetic - converting chemicals in the ground to biochemical energy.  My guess though would be that the dark side is fairly lifeless when it comes to flora.  You might get some phytoplankton-like bacteria, _maybe_ some moss.  Nothing large though - chemosynthesis can't provide enough energy at ground level to support complex life.  If there were open lakes, chemosynthesis might be enough.  At temperatures that constantly cold I think they'd be frozen too deep for anything on the surface to get at.

Animal-wise, on the sun-side pretty much anything goes.  You could certainly have animals migrate out to the dark-side from time to time, but without food none would be permanent.  You might have some scavengers that survive off of animals that get too far away from the sun-side.


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## Dannyalcatraz (Oct 2, 2010)

One thing you can do to muck about with the light/dark thing is make the local neighborhood a bit more complex.  Think of Isaac Asimov's _Nightfall_- a planet in a Multi-star system in which night only comes every 1000 years (for entirely astronomical reasons).

IOW, not only could other bodies cause eclipses that bring darkness to lightside, but also, they could bring occasional illumination to the darkside.

And I'm not just talking about stars- a largish moon with a high albedo could turn a deep, dark night into the equivalent of a stormy day.  A powerful aurora effect could make nights spectacular...and relatively bright.


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## S'mon (Oct 2, 2010)

For a planet like Earth that's 70% water the light side may be lush, wet, tropical, teeming with life.  It won't be a burned-up desert unless the whole planet is uninhabitably dry - which could very well happen as the evaporated water from the light side falls as snow on the dark side, eventually there'll be none left on the light side and you get a planet that is half desert, half snowball.  

In fact I'm not sure what could prevent that process, unless most of the dark side can be raised above freezing at least occasionally.  The twilight zone would simply end up as temperate desert.


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## jonesy (Oct 2, 2010)

"This magical world isn't realistic." 


How about a big huge system of rivers that travels from the hotter side and brings warmth to the other side. Or even an Amazon-like system on the hot which then turns into an underground system on the cold. Then you could have hot springs and oasis smack in the middle of winter. Now just got to figure out where it comes from and how it gets back.


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## Bluenose (Oct 2, 2010)

S'mon said:


> For a planet like Earth that's 70% water the light side may be lush, wet, tropical, teeming with life.  It won't be a burned-up desert unless the whole planet is uninhabitably dry - which could very well happen as the evaporated water from the light side falls as snow on the dark side, eventually there'll be none left on the light side and you get a planet that is half desert, half snowball.
> 
> In fact I'm not sure what could prevent that process, unless most of the dark side can be raised above freezing at least occasionally.  The twilight zone would simply end up as temperate desert.




Atmospheric circulation should do some of that. I'd think, if it was 70% water, that a world relatively close to its star would have very strong tidal effects. Theoretically, snow on the 'darkside' would mostly fall close to the habitable zone. It could then be 'recycled' by tidal effects and/or melting glaciers. Most rivers would be cold, and running from the dark side towards the light side. While I don't think it's possible to work it out mathematically without more information about the planet, it also doesn't seem impossible for a world with a higher percentage of water coverage to have liquid water even at the dark pole.


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## S'mon (Oct 2, 2010)

If it's tidally locked then there won't be tidal effects from the star, though.

The water would have to precipitate as liquid in the twilight zone, maybe into a twilight sea that was linked to the day-side ocean.  Rivers need gravity so I don't see them being a reliable source of replenishment.

Edit:  I suspect that IRL there are good reasons why M-type stars aren't a good source of planets with rich, complex ecosystems, or else the galaxy would be teeming with complex life and probably lots of intelligent life.  The deletirious effects of tidal locking may be one of those reasons.


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## Man in the Funny Hat (Oct 2, 2010)

S'mon said:


> If it's tidally locked then there won't be tidal effects from the star, though.



Add a large moon and/or magic.


> The water would have to precipitate as liquid in the twilight zone, maybe into a twilight sea that was linked to the day-side ocean. Rivers need gravity so I don't see them being a reliable source of replenishment.



Actually, I was thinking that the planet could still be similar to earth with 75% of its surface as ocean - but with the vast majority of that on the "light" side.  As you travel to the dark side the land rises.  With the heat and convection there's a lot of moisture lifted into the atmosphere to travel towards the dark side - but it would precipitate not at the center of the dark side as snow, but in the habitable band between the sides where the land rises so you could have a lot of temperate rainforest and massive rivers flowing lightside.

Heading further darkside you get glaciers, a lot of snow, mountains.  Once you get to, say, 60-70 degrees lattitude it becomes barren, cold wasteland.

Really, because it's a fantasy world and not a SF project you can postulate all kinds of effects and terrain that grab your fancy.


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## I'm A Banana (Oct 2, 2010)

> Well, if the flying creatures can go very high, migrating might be easy - low-altitude winds help carry you sunward, high-altitude winds carrying them darkward.




So maybe not big things like birds, but small things like insects (and pixies!). 

Of course, there'd need to be something in the cold half to be appealing enough to ever travel there. Without sunlight, there's not gonna be a whole lot of life, but maybe it could be like the North Pole, where tidal action brings life that flowers on the light side into the darkside gradually, feeding great behemoths there. Only instead of tidal action, you've got wind action: insects hatching in the warm light side go high enough to avoid predators, mate, and lay their eggs up high in the winds, relying on gravity to draw those eggs down to the lower wind layer, to cycle them back to the light side where they can hatch. The adults, though, manage to get devoured hulking white constantly-flying balloon-whales. Mostly mouths, they gulp great clouds of small life, sustaining themselves on it like baleen whales.

If the dark half is volcanically active, it could sustain some permanent life, even close to the "antipode." Even though Antartica is so far away from sunlight much of the time, the real killer there is rock + ice = basically lifeless (aside from moss, lichen, extremophiles, whatever). If you've got a few active volcanoes breaking that up, or a more open ocean like the North Pole, the odds for life go up nicely: the ocean can breed large quantities of fish and larger life (even migratory life that is born on the light side, but feeds on the dark side), and the volcanoes create lakes and melt icecaps and provide heat. Less Antartica, more Greenland. 

Maybe in a D&D-style place you have the "light side" as the above-ground critters, and the "dark side" as the Underdark. Dwarves and drow and beholders and whatnot over in the shadow, Terrasques and T-rexes and Assassin Vines and whatnot in the light.


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## Merkuri (Oct 2, 2010)

Hmm... anybody think about how the light side could be like the feywild and the dark side could be like the shadowfell?


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## Goblyn (Oct 2, 2010)

Y'knw, I betcha that if one were a lich who wanted to be left alone do to one's studies, a great place to build a fortress would be the dark pole of the planet.


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## jonesy (Oct 2, 2010)

Goblyn said:


> Y'knw, I betcha that if one were a lich who wanted to be left alone do to one's studies, a great place to build a fortress would be the dark pole of the planet.



I'm starting to think that the dark pole is pretty prime real estate and only the biggest baddest* creature, being, or faction would have the muscle to own it.

*or maybe, just to subvert the dark is evil cliche, the most good.


Another idea for the dark side: a paradise forest somehow thriving and glimmering from neon plantlife in the deep night in the middle of a frozen desert.


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## UngainlyTitan (Oct 2, 2010)

Goblyn said:


> Y'knw, I betcha that if one were a lich who wanted to be left alone do to one's studies, a great place to build a fortress would be the dark pole of the planet.



THe Fortress of Solitude


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## Lanefan (Oct 2, 2010)

Another thought leaps to mind:

What if most of the life wasn't on the surface of this planet at all?  I'm thinking "Hollow World" style, where most civilization is on an inside surface with only occasional quasi-legendary expeditions to the very-hard-to-reach outside.

Can you imagine the arguments over legend and religion?  "The outside is dark and colder than bone!"  "The outside is bathed in endless warmth and light!"  "The outside doesn't exist, it's all a lie!"  The first two would both be right, of course; depending on where that legend's original expedition happened to reach surface.

And the campaign, natch, could be simply a long and protracted surface expedition...

Lanefan


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## El Mahdi (Oct 2, 2010)

The planet doesn't necessarily have to be all "land".  If it has enough water to have oceans, the ocean currents would also contribute to moving heat around the planet (just like air currents).  The Atlantic Gulf Stream is a good example.  You'd still have a very cold and icy darkside, but perhaps not the binary desert/snowball scenario.  It could possibly add a warming effect to the dark side, and a cooling effect to the light side.  Sea creatures could spawn on the warmer light side, and live/hunt on the cooler dark side.

Playing around with the shape and dispersal of your continents and oceans (setting up varied ocean currents) could set up all sorts of interesting ecologies for the campaign world.

Along with adding volcanic activity, and the effect of different kinds of moons and orbits, you could still have a good amount of variety in climates and ecologies.


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## Desdichado (Oct 3, 2010)

Lanefan said:


> There's some other variables to consider:
> 
> How much water does this planet have, and where is most of it (e.g. light side, dark side, both)? (this would drive weather patterns)
> 
> ...



Actually, I'd think the center of the lightside hemisphere would be the genesis of some pretty severe planet wide weather systems.  All that constant heating of the atmosphere will cause it to rise and cooler air will rush in to replace it, creating strong, sustained winds.  In general, I'd expect this to be a world plagued by very strong prevailing winds, unusual storm activity and severe weather in general.


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## Lanefan (Oct 3, 2010)

Hobo said:


> Actually, I'd think the center of the lightside hemisphere would be the genesis of some pretty severe planet wide weather systems.  All that constant heating of the atmosphere will cause it to rise and cooler air will rush in to replace it, creating strong, sustained winds.  In general, I'd expect this to be a world plagued by very strong prevailing winds, unusual storm activity and severe weather in general.



Yes, but those planet-wide weather systems, no matter how severe, would eventually reach a near-steady state and essentially never move after that.

The planet is not rotating, and coriolis force from Earth's rotation is a large part of what causes weather systems to move.

Yes, energy would be transferred by weather.  Evaporation would always occur *here*, an endless wind would transfer the moisture over *there* to where it never stops raining, rain would fall, and the water would flow back to the sea.  The main difference with Earth would be that the highs, lows, and fronts are permanent.

Lanefan


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## Nifft (Oct 3, 2010)

Lanefan said:


> Yes, but those planet-wide weather systems, no matter how severe, would eventually reach a near-steady state and essentially never move after that.
> 
> The planet is not rotating, and coriolis force from Earth's rotation is a large part of what causes weather systems to move.
> 
> Yes, energy would be transferred by weather.  Evaporation would always occur *here*, an endless wind would transfer the moisture over *there* to where it never stops raining, rain would fall, and the water would flow back to the sea.  The main difference with Earth would be that the highs, lows, and fronts are permanent.



 Mmm. I'm not so sure. Cloud albedo might be sufficient to generate unpredictable weather, or at least weather patterns.

So far the proposed weather model has cold air being drawn from the dark-side to replace rising air heated by the sun on the light-side. This means we have a planet-wide layer of warm, moist air over a layer of cold, dry air -- moving in the opposite direction. That's a recipe for nasty weather, and the trigger for precipitation is going to be the turbulence between the atmospheric layers.

- - -

An interesting thought about this world's weather patterns, if they work as described above: we've got a fairly reliable pair of winds acting in opposite directions. Normally airships are limited to following whatever wind prevails, but with two well-known opposing forces, it would be possible to design some fairly reliable low-tech airships. Sea travel might not be convenient (i.e. it might suck that you would ALWAYS be tacking on your way Darkward), so perhaps airships handle the Sunward / Darkward axis, while sailing ships tend to travel between ports that are Equisolar.

Cheers, -- N


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## Olgar Shiverstone (Oct 3, 2010)

Nifft said:


> Mmm.
> So far the proposed weather model has cold air being drawn from the dark-side to replace rising air heated by the sun on the light-side. This means we have a planet-wide layer of warm, moist air over a layer of cold, dry air -- moving in the opposite direction. That's a recipe for nasty weather, and the trigger for precipitation is going to be the turbulence between the atmospheric layers.




IIRC, one of Larry Niven's Known Space worlds -- We Made It -- was a tidally locked world where the human colonists crash-landed in the habitable zone.  All was well until the winds started.  It resulted in some interesting ecology, with most of the life going subterranean, but with some explicitly adapting to above ground life under high winds.  Some of the life forms adapted to having different ways to resist the wind; others adapted to using the wind to hunt and/or enable reproduction.

It would definitely make a great D&D setting.


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## alms66 (Oct 3, 2010)

Someone already mentioned that the winds would be constant in the twilight zone, however, I didn't see mentioned that the effect of that constant convection current in the twilight zone would be to turn it into a Sahara-like gigantic desert.  This is the effect that occurs over Africa that causes the actual Sahara.  Winds rise at the equator, flow north and cool, then drop down in North Africa and flow back south, drying the region.
So, I don't imagine the twilight zone being too inhabitable (only marginally so, like the Sahara).  The same goes for the light side, 160° is going to create intense heating at the "pole" of the light side, causing giant convection currents there as well, and much drying over the region.  But at the same time, it's going to fuel intense evaporation, so assuming there is a large ocean on that side as well, maybe the effects would balance out, or the high humidity would cause the light side to be covered in cloud constantly, making it perfectly habitable.  Sort of like a "rain forest planet" on that side - warm, but not unbearably so.

That's my 2 cents to the story.

Edit:


Nifft said:


> An interesting thought about this world's weather patterns, if they work  as described above: we've got a fairly reliable pair of winds acting in  opposite directions. Normally airships are limited to following  whatever wind prevails, but with two well-known opposing forces, it  would be possible to design some fairly reliable low-tech airships. Sea  travel might not be convenient (i.e. it might suck that you would ALWAYS  be tacking on your way Darkward), so perhaps airships handle the  Sunward / Darkward axis, while sailing ships tend to travel between  ports that are Equisolar.



I'd imagine if air ships were developed, that they'd only go in one  direction as well, and sea ships in the opposite.  So then, you'd want  to have an air ship that could convert to a sea ship when it was  necessary to go in the opposite direction.


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## Nifft (Oct 3, 2010)

alms66 said:


> I'd imagine if air ships were developed, that they'd only go in one  direction as well, and sea ships in the opposite.  So then, you'd want  to have an air ship that could convert to a sea ship when it was  necessary to go in the opposite direction.



 Airships could go in either direction (Sunward or Nightward) by changing altitude, or they could do something they can't do in our world: *move perpendicular to the winds*, using one atmospheric layer as the "keel" layer and the other as the "sail" layer.

Airships would be more flexible transportation than sailing ships on this world, and that's pretty much the opposite of our world.

Hmm. Well, I guess one could make a combination airship / sailing ship which put all its sails on a dirigible (moored to the ship), and then sent the dirigible to the higher atmospheric layer for the trip Nightward. That might allow one to sail the Sunward/Nightward axis reliably.

Cheers, -- N


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## tomBitonti (Oct 3, 2010)

Some stream of thought ...

What would happen if there was a large (pacific) type ocean that straddled the night/day terminator?  There would seem a large convection cell with surface water travelling towards the dark / cold side and deep water flowing towards the light/warm side.    At the surface, the winds are contrary to the water, and there would be a large energy transfer and moisture transfer from the water to the air.  Would you end up with one big convection cell, or several?  If the ocean reached far into both the warm and cold areas, how big of a temperature differential would you get?    Would coasts deep in the cold zone be like the coast of Antarctica, with large, frequently calving ice sheets?  As far as life goes, where would that ocean best support life?  Where would there be large plankton blooms?  I imagine that the energy transfer from the ocean to the air would largely be in the form of water evaporation.  That energy would have to be released somewhere for there to be a complete energy cycle.  Where would the energy be release?  Would it be mostly as precipitation as the air mass moved towards and into the cold area?  There should be predictable (but perhaps not steady) rains and snows near and past the terminator.  Towards the sun-ward pole and anti-pole, would there not be the strongest winds, as the vast air masses crowded together to transfer between the upper and lower streams?

Thx!


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## Stormonu (Oct 3, 2010)

Goblyn said:


> Y'knw, I betcha that if one were a lich who wanted to be left alone do to one's studies, a great place to build a fortress would be the dark pole of the planet.




Hmmm...or perhaps a lich who's powerful enough to tidally lock a planet and create a barren wasteland in which to conduct his studies and whatnot...


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## tankschmidt (Oct 3, 2010)

*Already published!*

Judges Guild put out an adventure on such a world back in 1981.  It's more like a miniature campaign setting, which is what adventures tended to be like back in those days.  

The title is "Portals of Twilight".  The adventure is similar to what others have already mentioned; there is a narrow strip of habitable land, an uninhabitable hot side, and a icy cold side with frost monsters.  North Pole City is the major outpost of civilization, and there is indeed a constant wind.

You can probably pick it up at a good game store or online for less than $10.


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## jonesy (Oct 3, 2010)

tankschmidt said:


> Judges Guild put out an adventure on such a world back in 1981.  It's more like a miniature campaign setting, which is what adventures tended to be like back in those days.
> 
> The title is "Portals of Twilight".



I'm guessing this one?
Portals of Twilight (1981) - Judges Guild | DriveThruRPG.com

I have to say it does look interesting.

Edit: oh look, the link broke, and is pointing at the wrong one.. hmm..

For some reason it won't let me link directly to it. It's here, and only $3:
http://rpg.drivethrustuff.com/index.php?manufacturers_id=31&filters=0_0_0_0


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## Xeviat (Oct 4, 2010)

*Rotation*

A tidally locked planet does rotate, though; it rotates once every time it orbits the star. So it's planetary day is equal to its year. Relative to the point of the star, it doesn't rotate, but it is most definitely rotating. Gliese 581 g itself orbits its star every 37 days, so it completes a rotation every 37 days. This is 37x slower than Earth's rotation, so the coriolis effect would be much smaller, but it's going to be there.

How would this affect air currents?

Oh, and those interested in tidally locked worlds should look at these sources:

Aurelia and Blue Moon - Wikipedia, the free encyclopedia
and
[ame=http://www.youtube.com/watch?v=CNeTxPgGJ7I]YouTube - Extraterrestrial[/ame]


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## Silverblade The Ench (Oct 4, 2010)

Undead and maybe illithids would have a field day in the "dark" areas, and plant creatures likewise in the "bright" ones., hm? 

you could also have odd or catastrophic "seasonal" effects if the orbit isn't very circular.
Like our Milankovitch Cycles, but worse. Imagine mega hurricanes encircling the twilight belt....


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## AdmundfortGeographer (Oct 4, 2010)

D&D had a tidally locked world.

It showed up in Spelljammer's Practical Planetology. Called Radole.


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## Klaus (Oct 4, 2010)

Back in 2e, my home world of Jera was much like this in the past.

Pyrois, the day side, had magma, desert islands ruled by blue dragons and all manner of fiery things.

Cryois, the night side, had icebergs, white dragons and all manner of cold stuff.

The terminator band was dense with vegetation, from tropical jungles in the middle to scrublands near Pyrois and tundra near Cryois.

The work of the legendary heroes Ironheart and Steelaxe made the world spin again.


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## Nifft (Oct 4, 2010)

Klaus said:


> The work of the legendary heroes Ironheart and Steelaxe made the world spin again.



 Just proves you can fix anything with magnets.

"_Iron-y_", -- N


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## Desdichado (Oct 4, 2010)

Lanefan said:


> Yes, but those planet-wide weather systems, no matter how severe, would eventually reach a near-steady state and essentially never move after that.
> 
> The planet is not rotating, and coriolis force from Earth's rotation is a large part of what causes weather systems to move.



I don't see that as a "but" but rather as an "in addition to.


			
				Lanefan said:
			
		

> Yes, energy would be transferred by weather.  Evaporation would always occur *here*, an endless wind would transfer the moisture over *there* to where it never stops raining, rain would fall, and the water would flow back to the sea.  The main difference with Earth would be that the highs, lows, and fronts are permanent.



There are plenty of mitigating factors that can be introduced, if you want to, though.  Tidal forces from Gliese 581 could stretch the planet, sorta how Io is stretched by tidal forces from Jupiter (although probably less extreme if we want to assume that this world is habitable.)  That can actually change the landforms sufficiently to alter weather patterns, plus it causes substantial vulcanism which would also disrupt weather patterns.  Considering that Io, for example, has numerous active volcanos at any given time, you'd probably not have much in the way of steady state weather.

Of course, that may make things more difficult to establish any kind of steady state environment at all, but that's either a big problem or an interesting challenge, depending on how you look at it.

This also assumes a circular orbit, which it doesn't have to be.  A more elliptical orbit could create seasons just by distance from the sun, which could cause weather to do unusual things as well.

Tidal pull from a moon or moon(s) around the planet could also be used to vary the weather somewhat if you wanted.

And finally, if you assume that there's no Jupiter or other large gas giant in a mid-level orbit around the star, then there's no shield to gravitationally capture or deflect errant comets.  The star itself becomes the main comet and asteroid attractor in the solar system, and a planet with a near orbit around it finds itself unwittingly in the line of fire.

I presume, of course, that this planet is actually closer to its sun than the Earth is to our sun, since Gliese 581 is a much smaller, cooler and dimmer star than our sun.  In order to get similar climate, the orbit would have to be much closer, but I confess that I don't actually know what the situation for the real Gliese 581g is.


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## Klaus (Oct 4, 2010)

Nifft said:


> Just proves you can fix anything with magnets.
> 
> "_Iron-y_", -- N



Zing!

To provide more context: Ironheart and Steelaxe were blatantly inspired by Fafhrd and the Gray Mouser. Ironheart was a human barbarian with a chainmail like Warduke's. Due to a curse, he had his heart replaced with an iron one. Steelaxe was a short human with two handaxes of returning.

The religion was basically the Egyptian Pantheon.

And this "pre-history" was also heavily inspired by Golden Axe.


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## Remathilis (Oct 4, 2010)

Interesting topic!

I confess I lack the science to add anything to this, but I have usually subscribed to the "Rule of Cool" over scientific accuracy. With that in mind...

1.) Duality: The world is slightly egg-shaped (facing the sun). At the bright side is the Bright Desert, a perpetual desert full of sand-giants, manscorpions, and the like. At the first tropic, the temperature becomes more temperate and seasonal monsoons allow human life to flourish. The people live in constant fear of the encroaching desert. Beyond that creates the Terminator line known as the shadowlands, full of howling winds and broken tablelands (but a lot of mineral wealth). The dark side is a cold waste heated by volcanic activity and full of the worst creatures you could imagine: drow & duergar, undead, aberrations, and the like. 

2.) The Sky: The world is mostly water, where the heat of the sun boils the water, creating perpetual storms. Humanity and the like learned how to escape the rains by using magic to lift the few remaining chunks of land to above the cloud line where the climate is less extreme. Airships allow for movement between the sky-isles, careful to avoid the dark side and the iceburgs that linger below the storm-line. 

Both keep the feel of a world being locked without worrying about the physics involved.


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## HalfOrc HalfBiscuit (Oct 4, 2010)

I also have nothing to add to the science side of the debate. However, I'm a tad surprised no-one has yet mentioned Roger Zelazny's novel _Jack of Shadows_. In the words of the author himself (from _The Illustrated Roger Zelazny_):



			
				Roger Zelazny said:
			
		

> The world on which it is set is distinctive in that one side of it constantly faces the sun. This daylight side is ruled by the laws of science and giant energy screens keep the population from frying and the land from being baked dry. The dark side where the laws of magic hold sway is preserved from the cold by a sorcerous matrix of perpetually renewed spells. The two realms have little contact with one another, though they figure prominently in each other's myths, folklore and legends.
> 
> Many of the darksiders possess idiosyncratic supernatural abilities, which are stronger or weaker in various locales. When one such individual finds a spot where his powers are at their highest he realizes this to be his special place of power and makes every effort to gain political and military control, to establish his own kingdom there. This may of course require considerable time and effort, for the territory may overlap with that of another sorcerous power.
> 
> But the darksiders possess time for considerable effort, in that each of them is endowed with more than one life - just how many being a closely guarded secret with each individual. Some time after dying, such a one finds himself strangely resurrected, naked, in the Dung Pits of Glyve at the world's darkest pole. This necessitates a perilous journey back to more congenial climes for whatever endeavour was underway there.


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## Dannyalcatraz (Oct 4, 2010)

In a little tangent, may I also suggest that persons enjoying this thread may also enjoy Hal Clements' stories of beings from a high-grav world, collected as _Heavy Planet._

Macmillan: Heavy Planet: The Classic Mesklin Stories Hal Clement: Books


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## Umbran (Oct 4, 2010)

Sorry, Nifft!  My bad!  I'm working to get your post back properly!


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## Nifft (Oct 4, 2010)

Umbran said:


> Sorry, Nifft!  My bad!  I'm working to get yout post back properly!



 If you had just left the broken post there instead of deleting it, I could probably have recovered the original contents using the edit history, which seems to be visible to the original poster.

- - -

@ *Hobo*: If there were a significant moon, it's very possible that the planet wouldn't be phase-locked in the first place. Significant moons may be rare, though.

Ignoring the moon issue, if our phase-locked little planet were subject to frequent meteorite impacts, I wonder if those impacts would tend to favor the iced-over side. If they did, the introduction of a bunch of energy into the hemispheric glacier system might do something interesting to weather, or to oceanic currents, or at the least it might prompt explorers to go to the dark side to get rare metals.

Cheers, -- N


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## Xeviat (Oct 6, 2010)

Is anyone still interested in working on this? I've been reading to put together as much information as I can about Gliese 581g specifically. I'm sure more information will come together later, but here is what I've found so far:

* It orbits Gliese 581 every 37 earth-days. Night-siders would know this because of the rotation of the stars; it seems like it would take 18 and a half days for a star to transit the entire sky, so this period should be important to the people.

* It is tidally locked, or nearly tidally locked. I'm going to stick with a full tidal lock, because that's the main element to this world. This means the planet is orbitting its access every 37 days, which will create some centrifugal effect. Venus, for instance, rotates (retrograde) its own access every 243 days, while it rotates the sun every 224.7 Earth days; sunrise to sunrise is 117ish Earth days. The point of all this is that Venus has very fast winds at the high altitudes, but very slow winds at low altitudes (average like 1 meter/second). The winds rotate in the same direction as Venus, but they're faster than the rotation. There's a lot going on here, but I mean to show that 581g's winds don't have to be only from solar-pole to night-pole. The convection-based wind from equator to pole is much slower than the wind that flows around the planet. I'd like to discuss this more to decide on a wind pattern for the world; since it doesn't rotate like Venus or Earth, whatever rotational winds that might be caused will probably be overriden by the convection. There might be a trend for air to move along the equator more, since the surface is rotating faster there then at the poles.

* Temperatures are expected to be around 160 at the day side and -25 on the night-side; I don't know if this was supposed to be F or C, so I'm still looking.

* As long as the world's atmosphere is at least 10% as thick as Earth's, it shouldn't "freeze out" (where the atmosphere freezes at the night-pole and dries out completely at the day-pole).

* Higher gravity will compress the atmosphere. Depending on what you want out of the setting, you could have a higher pressure atmosphere by having an atmosphere as relatively voluminous as Earth's, or you could have an earth pressure or lower atmosphere by having a lighter one. A thick atmosphere would facilitate flight, which would make the whole airship idea someone had earlier more possible.

* 581 g is more massive than Earth, but if it is as dense as Earth it will have a larger diameter. Gravity on a sphere is measured as if the mass was concentrated at the center, so being further away lessens gravity. Gravity on 581g should only be 1.1 to 1.7g's. With higher gravity, I'm leaning towards liking a thicker atmosphere so flight is still feasible.

Just some thoughts here. I'm going to use 581g as a model for a fantasy setting as well, but I like to model the mundane in reality.


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## Lanefan (Oct 6, 2010)

Hobo said:


> There are plenty of mitigating factors that can be introduced, if you want to, though.  Tidal forces from Gliese 581 could stretch the planet, sorta how Io is stretched by tidal forces from Jupiter (although probably less extreme if we want to assume that this world is habitable.)  That can actually change the landforms sufficiently to alter weather patterns, plus it causes substantial vulcanism which would also disrupt weather patterns.  Considering that Io, for example, has numerous active volcanos at any given time, you'd probably not have much in the way of steady state weather.



Good call on vulcanism, I hadn't considered that.  A major ash-laden eruption on the light side could severely impact the amount of solar radiation getting to the surface for a while; you'd get some great weather while that all tried to sort itself out.



> This also assumes a circular orbit, which it doesn't have to be.  A more elliptical orbit could create seasons just by distance from the sun, which could cause weather to do unusual things as well.



To a small extent.  To get real seasons like we're used to you'd have to have a pretty elliptical orbit, which might result in completely baking one side and-or freezing the other at different times of year.



> And finally, if you assume that there's no Jupiter or other large gas giant in a mid-level orbit around the star, then there's no shield to gravitationally capture or deflect errant comets.  The star itself becomes the main comet and asteroid attractor in the solar system, and a planet with a near orbit around it finds itself unwittingly in the line of fire.



And might end up with lots of little moons as a result, not to mention some big-donkey craters and squashed lifeforms.

Lanefan


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## Umbran (Oct 6, 2010)

Nifft said:


> If there were a significant moon, it's very possible that the planet wouldn't be phase-locked in the first place. Significant moons may be rare, though.




A moon may slow down the process of becoming locked, but ultimately will not (and cannot) prevent it.  The moon's going to be smaller than the planet, after all - if the star can mess with the planet's rotation and orbit, it can mess with the moon's orbit, too.  

Eventually, what happens is the period of the moon's orbit around the planet gets tweaked so that it does not interfere with the locking - generally by becoming some fractional multiple of the planet's day/year.  You get a sort of "beat" structure - the moon will have an orbit equal to the day/year, or 3/2, 5/2, or more rarely some number of thirds of that period.

The period in which the system is coming into it's locked configuration is expected to be short (on cosmological scales, anyway), and it would be a... geologically interesting time for anyone on the planet or moon.


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## Nifft (Oct 6, 2010)

Umbran said:


> A moon may slow down the process of becoming locked, but ultimately will not (and cannot) prevent it.  The moon's going to be smaller than the planet, after all - if the star can mess with the planet's rotation and orbit, it can mess with the moon's orbit, too.
> 
> Eventually, what happens is the period of the moon's orbit around the planet gets tweaked so that it does not interfere with the locking - generally by becoming some fractional multiple of the planet's day/year.  You get a sort of "beat" structure - the moon will have an orbit equal to the day/year, or 3/2, 5/2, or more rarely some number of thirds of that period.
> 
> The period in which the system is coming into it's locked configuration is expected to be short (on cosmological scales, anyway), and it would be a... geologically interesting time for anyone on the planet or moon.



 I think you are assuming that the planet-moon orbital plane lies within the planet-sun orbital plane.

Either way, though, "short cosmological" time is plenty for an intelligent species to evolve. Our own moon will fly away in "short cosmological" time, but that doesn't mean a heck of a lot to us brief candles.

- - -

Regarding weather: it's quite possible that you'll see local bands of convection rather than just a planet-long low-altitude "sunward" wind under a planet-long high-altitude "nightward" wind. Just like on Earth, you'll have counter-cycling winds in rings, but centered on the sun-pole rather than around the equator. According to that National Geographic video, there might be a giant storm system centered around the sun-pole. If so, that's a fine place to draw the first circle of convection bands.

- - -

Anyway, how do humans work on this world?

- Savage day-side humans sleep every 6 hours (or so) for about 20 minutes. There is no dangerous night full of predators -- every day-lit minute is equally dangerous! They hunt in teams of four: hunt for three hours, then two nap while two guard.

- Caves are their Stonehenge. Specifically, caves that face away from their sun -- especially near the twilight lands -- are how their mystics track stars, and thereby track time. People in the daylight jungle don't bother to keep track of time, they have enough on their hands just staying alive.

- They are even more river-dependent than we were, since there may be no such thing as seasonal storms. Perhaps they are great canal-engineers.

- However, mountains may be where civilization starts, rather than river deltas. Mountains may disrupt the weather patterns enough to generate regular rains, which allow terraced agriculture on the sunny side -- but they also offer a dark side which grants respite from the sun, and access to the stars.

- - -

Funky scifi S&S angle: imagine a giant solar mirror orbiting around L2. The night side would still be cold, but perhaps not freezing cold any more. The night side and twilight zone would have another timekeeping cycle aside from the stars. North Pole City might be the last bastion of lost technology before the colonists fell to savage barbarism.

Cheers, -- N


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## Xeviat (Oct 6, 2010)

Umbran said:


> Eventually, what happens is the period of the moon's orbit around the planet gets tweaked so that it does not interfere with the locking - generally by becoming some fractional multiple of the planet's day/year. You get a sort of "beat" structure - the moon will have an orbit equal to the day/year, or 3/2, 5/2, or more rarely some number of thirds of that period.




How big can this fraction get? Could it be something as big as 37 moon rotations for every one of the planet's rotation, making the moon rise every 24 earth hours?

I'm under the assumption that the same tidal effects that straighten the planet's axis will also pull the moon to orbit at the planet's equator. This leads me to believe a moon would net constant eclipses. This could be very very cool. On the night-side, you'd see the moon move and take a few phases, with more drastic changes depending on how close you are to the night pole. It would then pass into the world's shadow, creating "midnight" as the moon would all but go black.

On the dayside, there would also be a "midnight" with a solar eclipse. If the moon orbits slowly, these eclipses would last a considerable amount of time.

I'd still like to know if anyone knows how big the star would appear in the sky.


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## Umbran (Oct 7, 2010)

Nifft said:


> I think you are assuming that the planet-moon orbital plane lies within the planet-sun orbital plane.




I am assuming the usual mechanics occur, which will tend to drive the moon onto that plane, even if it doesn't start there.



> Either way, though, "short cosmological" time is plenty for an intelligent species to evolve. Our own moon will fly away in "short cosmological" time, but that doesn't mean a heck of a lot to us brief candles.




"Short cosmological" is more on the hundred million year range or less than the billion year range.  An object like Pluto (small, very far from the star) might take 10 billion years or more to come to be tidally locked.   An object like Mercury (small, close to the star) has reached it's steady spin-orbit resonance (locking is just a very simple such resonance) in well under 4 billion years.  An object like our moon (large on the scale of the two bodies in question) got tidally locked early on as well.

Can you place a game in a time during the process of locking?  Sure.  That can be cool because it would be... geologically interesting (earthquakes, volcanoes, tidal waves, and so on).  But the math would suggest that the locking would occur early in the arrangement, rather than later.



> Regarding weather: it's quite possible that you'll see local bands of convection rather than just a planet-long low-altitude "sunward" wind under a planet-long high-altitude "nightward" wind.




Yes.  That "low/high" pattern is only the general flow that's required. You can get to that with convection bands.




Xeviat said:


> How big can this fraction get? Could it be something as big as 37 moon rotations for every one of the planet's rotation, making the moon rise every 24 earth hours?




In general, the system will be driven to the closest state of strong resonance.  But the relative sizes, speeds, compositions and distances between the object matter a great deal.  



> I'm under the assumption that the same tidal effects that straighten the planet's axis will also pull the moon to orbit at the planet's equator. This leads me to believe a moon would net constant eclipses.




The moon would transit the sun frequently.  Whether that counts as an "eclipse" depends on relative sized and distances.  Our setup is peculiar, as our moon's just at the size and distance to just barely cover the sun's disk in the sky.  But that is a coincidence, not a thing that generally happens.



> I'd still like to know if anyone knows how big the star would appear in the sky.




I'll see if I can find the data to tell you.  I suspect it'd be much larger than our sun is in our sky.

Edit:  Quick back-of-envelope calculation, given what data I can find on Gliese 581 and 581g : Our sun has an apparent diameter of about 0.5 degrees when seen from Earth.  I think Gliese 581 would have an apparent diameter of about one degree, as seen from G.  So, something like 4 times the total area - big, but not super-jigundo.


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## Xeviat (Oct 7, 2010)

Umbran said:


> The moon would transit the sun frequently. Whether that counts as an "eclipse" depends on relative sized and distances. Our setup is peculiar, as our moon's just at the size and distance to just barely cover the sun's disk in the sky. But that is a coincidence, not a thing that generally happens.




So you say the moon will fall into a lower fraction of X/2 the planet's rotation/orbit? Would trying to get something like a "day" orbit of the moon might be a bit much? I'm just trying to figure out what might drive a sleep cycle for the humanoids.

Having a moon will create tides. A larger moon, as I mention below, would make very large tides. But a larger moon and large tides would also tug on the planet itself, which in turn tugs on the star, no? Would that slow the moon?




Umbran said:


> Edit: Quick back-of-envelope calculation, given what data I can find on Gliese 581 and 581g : Our sun has an apparent diameter of about 0.5 degrees when seen from Earth. I think Gliese 581 would have an apparent diameter of about one degree, as seen from G. So, something like 4 times the total area - big, but not super-jigundo.




So the apparent diameter of the star will be about twice as big as our sun? That means the moon would need to be closer or larger in order to fully eclipse it. If the math is sound, I wouldn't mind if the moon appeared larger than the star in the sky; this would mean a longer eclipse, and a brighter "day" on the nightside.

And thanks for answering all of our astronomical questions. This is tempting me to focus my studies in this direction.


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## Umbran (Oct 7, 2010)

Xeviat said:


> So you say the moon will fall into a lower fraction of X/2 the planet's rotation/orbit?




Some X/2 or X/3 or X/5 are most likely, but it isn't like anything is outright forbidden.  



> Would trying to get something like a "day" orbit of the moon might be a bit much? I'm just trying to figure out what might drive a sleep cycle for the humanoids.




If the thing you're looking for is a sociological effect, then just make the moon do what you want, and the science can go hang 

If I were using this for a setting, I'd not be looking for a way to enforce a standard 24 hour period.  The whole point of this setting is that it is different - the day side is day, and the night side is night - so why force it back to have some secondary day/night?  The impacts of *not* having those enforced limitations are what makes this unique.  




> Having a moon will create tides. A larger moon, as I mention below, would make very large tides. But a larger moon and large tides would also tug on the planet itself, which in turn tugs on the star, no? Would that slow the moon?




Gliese 581 is small, but it is still a star.  While the planets are detected by the star being tugged on, for first approximation, the star's still as rock-solid as it gets.

But, yes, that's how this works.  There's a planetary orbit, a planetary spin, a moon orbit, and a moon spin - those are all momenta that can interchange with each other somewhat, and will do so until they reach a stable configuration.  

And momentum that doesn't get interchanged among those objects ends up as energy deposited in the moon or planet body as heat - thus the "geologically interesting" period as they settle down.



> So the apparent diameter of the star will be about twice as big as our sun? That means the moon would need to be closer or larger in order to fully eclipse it.




Correct.


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## Lanefan (Oct 7, 2010)

I was talking about this with a friend last night and we realized there would be other massive differences in how such a world functioned, beyond physical things like weather, eclipses, and so forth:

Life.

The most basic things we take for granted would not happen on a locked world.  Someone already alluded to sleep cycles - this is one.  A creature's sleep cycle would base itself on factors other than day-night; perhaps even something so simple as the sleep cycle of its predators!  Seasonal migrations would not happen, though some other migrations still might e.g. a creature that migrates inland to mate and lay eggs but otherwise lives at the seashore.  Trees and plants would not have the seasonal bud-leaf-seed-dieback cycle we're used to, or at least not all in lockstep like in any given region on Earth; trees that shed their leaves would do so whenever they felt like it, and those cycles would be of widely-varying lengths depending on the needs of the particular plant and-or species. (the advantage here would be that one could probably find edible plants year-round)  Hibernation would work much differently - bears, for example, might not all hibernate at once but instead evolve to hibernate at different times, thus allowing a given area to support more bears in total.  And so forth.

Have fun designing that lot! 

Lanefan


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## Umbran (Oct 7, 2010)

Lanefan said:


> Seasonal migrations would not happen, though some other migrations still might e.g. a creature that migrates inland to mate and lay eggs but otherwise lives at the seashore.




There are some things that might create seasonal variation on a tidally locked world.  Even a locked world can have some "wobble" - thsi won't mean much at the noon and midnight poles, but could be a big thing for the twilight zone.  Also, the orbit can be eccentric (oval, rather than circular), leading to seasonal variation with distance from the star.

The tidal forces involved (especially with a large moon with an orbital resonance) might create geologic "seasons" of volcanism, earthquakes, or tidal waves, tha tmight lead to any number of seasonal behaviors for plant and animal life....

In fact, therein might lie your solution for the tendency for such planets to turn into deserts.  You expect the water tends to evaporate at Noon, and condense out and freeze at Midnight - eventually, it all ends up in an ice cap on the dark side.  If, on a regular basis, something geologic melts a lot of the water at Midnight, you might keep a water cycle rolling so the world doesn't become a dustball.




> Trees and plants would not have the seasonal bud-leaf-seed-dieback cycle we're used to, or at least not all in lockstep like in any given region on Earth; trees that shed their leaves would do so whenever they felt like it, and those cycles would be of widely-varying lengths depending on the needs of the particular plant and-or species.




Note that you can already see this in some areas of the tropics on Earth.  If there's enough rainfall that there isn't much of a  "dry season", plants just go at it.


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## Nifft (Oct 8, 2010)

Lanefan said:


> I was talking about this with a friend last night and we realized there would be other massive differences in how such a world functioned, beyond physical things like weather, eclipses, and so forth:
> 
> Life.
> 
> The most basic things we take for granted would not happen on a locked world.  Someone already alluded to sleep cycles - this is one.



 Yep, that was me. I'm assuming humans are humans rather than sleepless aliens, and that probably means we aren't native to this particular world. There are lots of RPG-appropriate ways to travel between worlds, including magic.

It's interesting to consider how Earth species might adapt. For example: Dolphins are adapted to not fully sleeping, so they might fare quite well.



Lanefan said:


> Seasonal migrations would not happen, though some other migrations still might e.g. a creature that migrates inland to mate and lay eggs but otherwise lives at the seashore.  Trees and plants would not have the seasonal bud-leaf-seed-dieback cycle we're used to, or at least not all in lockstep like in any given region on Earth; trees that shed their leaves would do so whenever they felt like it, and those cycles would be of widely-varying lengths depending on the needs of the particular plant and-or species.



 The tropics of Earth are like that already. Just model things on jungle species ...*that never sleep*.

- - -

On the topic of satellites, what if -- in addition to a moon or two -- the planet had a *ring*? That would attenuate light above and below the ring's axis (the "equator"), and would somewhat light parts of the twilight band and the night-side (more brightly as one moved away from the "equator").

Cheers, -- N


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## El Mahdi (Oct 8, 2010)

Nifft said:


> ...It's interesting to consider how Earth species might adapt. For example: Dolphins are adapted to not fully sleeping, so they might fare quite well...




That's a good point.  Dolphins and Orcas are like this.  Their brain hemispheres are able to sleep independently.  One side of the brain sleeps, while the other is still awake and able to remain alert for predators, etc.  Perhaps all animal life on the light side (or both sides) have evolved this way.


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## Aloïsius (Oct 9, 2010)

The planet is like hell.
1) all the water evaporated from the light side freeze to ice when it falls on the dark side. Thus, after millions of years, there is not a single drop of water on the bright side, and the dark side is covered with glaciers. The only place where you can find liquid water is where the glaciers move and melt when reaching the bright side.

2) as it's tidely locked, I doubt there is still a magnetic field strong enough to repel the sun radiations. The atmosphere is probably still there because gravity is stronger than on earth however.

3) which lead to the consequence of strong gravity. Animals and plants are smaller and "crawlier" than their Earth counterparts. Snakes, not giraffes. Depending of the density of the atmosphere, flying creatures will be rare too... 

4)... and they have to deal with nightmare winds between the two side of the world. Ice cold wind coming from the dark side, transforming into hot jet stream that go back at high altitude.

5) Few mountains to block the wind : gravity is stronger, so erosion must be stronger as well. It's a "flat" world.


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## Umbran (Oct 9, 2010)

Aloïsius said:


> 1) all the water evaporated from the light side freeze to ice when it falls on the dark side.




If we don't establish a mechanic to make it otherwise, yes, this is likely/



> 2) as it's tidely locked, I doubt there is still a magnetic field strong enough to repel the sun radiations. The atmosphere is probably still there because gravity is stronger than on earth however.




Well, a few points:

Gliese 581 is a red dwarf star.  They are cooler, and more placid than our own sun - the put out less UV (and harder) radiation, and probably less in the way of solar wind as well.  An atmosphere might well be enough.

Magnetic fields are created largely through movement of liquid metals deep within the planet - some dynamo effect may still be present.



> 3) which lead to the consequence of strong gravity.




We know Gliese 581g's mass, but not it's size.  The strength of gravity on the surface depends on both.  It is 3 times the mass of Earth.  If it is Earth-sized, yes, then there is stronger than gravity.  But, if it is 1.7 or more times the radius of Earth, the surface gravity will be _less than ours_.



> 5) Few mountains to block the wind : gravity is stronger, so erosion must be stronger as well. It's a "flat" world.




But you just said that the water was locked up in glaciers.  Wind itself doesn't erode much - it needs to carry particulates (like sand) to do the trick.  And that glacier-coated dark side you have there isn't letting go of much sand.


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## El Mahdi (Oct 9, 2010)

Umbran said:


> ...We know Gliese 581g's mass, but not it's size. The strength of gravity on the surface depends on both. It is 3 times the mass of Earth. If it is Earth-sized, yes, then there is stronger than gravity. But, if it is 1.7 or more times the radius of Earth, the surface gravity will be _less than ours_. ...




From Wikipedia:



> It is believed to have a mass of 3.1 to 4.3 times that of the Earth and a *radius of 1.3 to 2.0 times that of Earth* (1.3 to 1.5 times Earth's if predominantly rocky, 1.7 to 2.0 times Earth's if predominantly water ice). Its mass indicates that it is probably a rocky planet with a solid surface. The planet's surface gravity is expected to be in the range of 1.1 to 1.7 times Earth's, enough to hold on to an atmosphere that is likely to be denser than Earth's.


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## Xeviat (Oct 10, 2010)

There were articles showing that a small atmosphere will protect against "freezing out"; the atmosphere only needs to be 10% as thick as Earth's to prevent that.

Also, this planet rotates every 37 days; it is much slower than every 24 hours, but it still rotates. As long as the core is molten, it will have some dynamo effect. And as Umbran says, the star is less dangerous.

Another thought for a geological cycle would be the tides caused by the moon. Since I want solar and lunar eclipses, the moon is going to have to be bigger or closer. Too big or too close, though, seems like it would create a dual planet situation, rather than a planet and a moon, so I might have to go for partial eclipses (which would mean there is absolutely no darkness on the day-side). Either way, with a slow lunar rotation, the tides will move slower. A larger or closer moon will create more drastic tides, which will likely drive the lifecycles of aquatic life. Tidal shores will be big. If life started in these tidal shores, they could be the basis for a large food chain.

I'm not sure what I want to do about sleep cycles. Sleepless humanoids as the norm feels odd, and I wouldn't want any race to have such a widely different sleep cycle than the others as that would affect play.

Aside from the geology and astronomy of the world, I started putting some thought into the cultures of humanoids living here (as every gaming setting has some sort of multiple races). I think the night-siders, or at least those far enough into the twilight to see the stars, will be the more advanced, both religiously and technologically. They have the stars to watch, which leads me to believe they will have a deeper knowledge of mathematics. They will likely worship the omnipotent stars, and will most definitely have noticed the paths of the outer planets in their system. They will be able to keep time.

The day-siders seem like they'd be more primitive. I haven't given them too much thought yet, but I wanted to find a way to avoid the immediate thought of having the daysiders be good and the nightsiders be evil.

I also think that mountains in mid latitudes (pretending the terminator line is the equator, since it will be more apparent than the actual equator) will house the largest day-side cities in their shadows. The mountains will collect rain, and as long as the mountains are small enough, some of that rain water could flow to the shadow-side; especially if the people create canals to channel the water around the mountain.

I wasn't looking for the moon to create a "day and night" in a duration, just to give the day-siders something to keep time. I'm thinking to have the day-siders keep time by months, counting each time the moon rises.


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## Wik (Oct 10, 2010)

After reading this thread, and realizing just how damned cool it is, I can't help but think of an elven culture that lives in the gigantic sun-side jungles and makes holy pilgrimages to the dark side to look at the stars to foretell their future.  And these elves, after living their long lives in the sun side, would go to the dark side to die.

And then there are the drow elves, who have decided to pay all their attention to the stars and foresake the sun in preference for religious fanaticism.  And since the other elves "don't listen to reason", these drow want to kill their light-skinned cousins.

The idea of giving the planet a ring, perhaps like Eberron's dragon shards or whatever they're called, is also pretty cool.  

I also like the idea of a rogue planetoid that comes around every, say, thousand years or so, which essentially FLIPS the planet, so that the dark side effectively becomes the light side, and the light side goes dark.  This, of course, would make for a great campaign - imagine if the planet was destined to flip next year?


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## Darynal (Oct 10, 2010)

tomBitonti said:


> Some stream of thought ...
> 
> What would happen if there was a large (pacific) type ocean that straddled the night/day terminator?  There would seem a large convection cell with surface water travelling towards the dark / cold side and deep water flowing towards the light/warm side.    At the surface, the winds are contrary to the water, and there would be a large energy transfer and moisture transfer from the water to the air.
> 
> ...




I like your thinking!  Almost didn't read this because it was one big paragraph, so I separated it for my own sake =)

This is a pretty cool thing to consider about a tidally-locked setting.  It makes me think of Dark Sun's backdrop in terms of the unique kind of global phenomena that can be encountered.


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## Merkuri (Oct 10, 2010)

Wik said:


> I also like the idea of a rogue planetoid that comes around every, say, thousand years or so, which essentially FLIPS the planet, so that the dark side effectively becomes the light side, and the light side goes dark.




Is there something that would really do that?  In the real world, is it possible for a tidally locked planet to flip without something massively destructive, like a meteor strike?


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## Wik (Oct 10, 2010)

Merkuri said:


> Is there something that would really do that?  In the real world, is it possible for a tidally locked planet to flip without something massively destructive, like a meteor strike?




Probably not.  I am totally not a science person.  

That being said, uranus and pluto switch orbits, in part due to their proximity to one another at a certain point in their orbit around the sun.


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## Camber (Oct 11, 2010)

Xeviat said:


> Also, this planet rotates every 37 days; it is much slower than every 24 hours, but it still rotates. As long as the core is molten, it will have some dynamo effect. And as Umbran says, the star is less dangerous.




The "rotation" of 37 days only means that the planet rotates relative to a stationary observer, but not relative to the red dwarf. Theh same side always faces the red dwarf.

The National Geographic piece http://www.youtube.com/v/CNeTxPgGJ7I&hl  noted that red dwarfs are almost always flare stars--they would be very dangerous neighbors if you didn't have some way of protecting yourself from being cooked by the solar flares (especially when you are as close to the star as Gliese 581g appears to be).

There's a nice page answering some questions on the planet here.
One of the sidebars showed a comparison of this planet's orbit with Earth's, and you'll note how close it is to some large planets. I wonder if there could be any tidal pulling from the nearby planets? They seem to be rather tightly packed together compared to the setup we have at Sol.

I've run into some information (here is an example) on magnetic fields, vulcanism, and rotation that suggest that rotation and magnetism of a planet are connected. For example, Venus used to be volcanic, but appears to have cooled down internally to the point that its molten core didn't flow in currents anymore, which caused it to lose its magnetism, which in turn caused it to virtually stop rotating. Mars similarly seems to have lost a lot of its internal temperature, which has caused its vulcanism to slow down, but it is still rotating pretty rapidly (almost as fast as Earth).

Here's an interesting quote on the topic (again, from here): 







> Mars: Its in-between geology suggests it is possibly too cool  for molten core, or possibly just hot enough. Because of its relatively  rapid rotation (almost as fast as the Earth's), a molten core should  produce magnetic field, but only a minuscule field is observed, which  implies that it may be too cool (probably less than 5000 Fahrenheit  degrees) to have a molten core. However, fossil magnetism  at the surface suggests that the rocks which contain that magnetic  signature were formed at a time, 4+ billion years ago, when Mars had a  substantial magnetic field; and parallel striping of that fossil  magnetism in certain areas suggests that in that same time frame,  something occurred similar to the seafloor spreading and magnetic  striping caused by magnetic field reversals in the Earth. So although  Mars' core must be relatively cool and solid now, it was undoubtedly hot  enough to create an active magnetic field and drive some mantle  activity, in the very early days of the planet's history.




It leads me to think of the magnetism of a planet as being kind of like its life-force. When the planet cools down and the magnetism is lost, the planet has essentially died, and eventually you lose rotation also. This would lead one to say that Gaia is still alive, Mars is limping along, and Venus is essentially dead.

So a tidally-locked planet that has vulcanism because of outside forces (such as tidal forces) instead of from internal magnetism and magma currents might be sort of undead--an animated but dead planet.



Klaus said:


> Back in 2e, my home world of Jera was much like this in the past.
> 
> The work of the legendary heroes Ironheart and Steelaxe made the world spin again.




The idea of having a story focused around efforts to restart the rotation of a planet is one that is full of interesting possibilities. What if the thing that was needed was to reheat the core and get it magnetized again, which would then slowly jump-start the rotation.

I am envisioning it taking a great deal of power and energy, probably at great cost. Just exactly what that cost would be, and how the decision is made to make the necessary sacrifices, is what would make the story interesting to me.


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## El Mahdi (Oct 11, 2010)

Camber said:


> ...The idea of having a story focused around efforts to restart the rotation of a planet is one that is full of interesting possibilities. What if the thing that was needed was to reheat the core and get it magnetized again, which would then slowly jump-start the rotation.
> 
> I am envisioning it taking a great deal of power and energy, probably at great cost. Just exactly what that cost would be, and how the decision is made to make the necessary sacrifices, is what would make the story interesting to me.




A high-yield Cobalt_Thorium-G_Doomsday_Device buried very deep, should do the trick...


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## Xeviat (Oct 11, 2010)

Camber said:


> The "rotation" of 37 days only means that the planet rotates relative to a stationary observer, but not relative to the red dwarf. Theh same side always faces the red dwarf.




Just wanted to point out that movement is relative to stationary space. You may not feel that the earth is wizzing around the sun as fast as it is, but it is. You may not feel that it is spinning, but it is. 581g is rotating every 37 days; it's tidally locked, but there will still be a centrifugal effect.

For example (an example from a Physics book), if you spin a bucket full of water, you create a whirlpool in the bucket. If you suddenly stop the spinning, the water continues for some time; force has been imparted into it, and it takes it a bit longer to lose this. This spinning is in relation to space, not the objects around it. Otherwise, you would get the same spinning force if you spun the room around the bucket of water. I may be remembering the analogy incorrectly, but my point is that just because the planet isn't spinning in relation to the star doesn't mean it isn't spinning. Venus is nearly tide locked but it spins enough that its atmosphere super rotates.

I had an article that explored the coreolis effect on slowly rotating bodies; I cannot find it again since I do not know what google-fu lead me there, but it did show that even an object rotating every 365 days (a tidally locked Earth for example) would have centrifugal force suitable to create some sort of magnetic field.

As for the issue of flare stars, for a setting I'm going to assume that my world began to evolve after the star's flare stage was over, or that the star was more stable. Red Dwarfs live far longer than Yellows, so the world has plenty more time to evolve.


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## Camber (Oct 11, 2010)

Ah, I think I understand you now. So you aren't saying, for example, that the rotation would be such that it would cause the terminator line to move if a settlement were built on it (you could expect the terminator line to be fairly stable), but you are saying that the rotation that is present (once every 37 days) would be quite a bit and enough to possibly rev up some magnetism and volcanic activity?

I was doing some research on the relationship between magnetic fields and rotation, and it is amazing how little we know about it other than that there is a strong connection, and that it doesn't always hold true. Dynamo Theory is widely accepted as the best current explanation for the connection, but it doesn't adequately explain all of the energy produced in the core of a planet like the Earth (current mathematical models suggest that the magnetic effect would die down in 20,000 years--there is a great deal of sustaining energy that we can't account for). There's a very interesting non-mainstream theory called the Georeactor Dynamo Theory that suggests that uranium sinks to the planetary core, where it is concentrated enough that a low-level fission reaction is created. In gas giants such as Jupiter, which produces far more energy than it takes in from the sun, a low-level fusion reactor is proposed, in which hydrogren is fused under pressure but at cooler levels than those needed to turn it into a star. 

As has been said, red dwarfs are extremely long-lived, and it is likely that Gliese 581g planet could be on the latter end of the star's lifespan, perhaps a few billion years after the uranium or whatever else is helping to keep the dynamo going has been  burnt out. Or, in more supernatural terms, after the planet's spirit has died out.

Why is it that celestial bodies tend toward being tidally locked over time? Is there any reason other than gradual loss of energy, and is it inevitable that all planets will eventually be tidally locked with their star(s)?


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## Xeviat (Oct 12, 2010)

Camber said:


> Ah, I think I understand you now. So you aren't saying, for example, that the rotation would be such that it would cause the terminator line to move if a settlement were built on it (you could expect the terminator line to be fairly stable), but you are saying that the rotation that is present (once every 37 days) would be quite a bit and enough to possibly rev up some magnetism and volcanic activity?




Well that sucks; I had a whole post done last night and it didn't go through. I'll try to remember what I typed.

Yes, a 37 day rotation should be fast enough to get a liquid core spinning a bit. I don't know if being tidally locked will tide lock the core as well, though. I suspect not; Mercury is tidally locked for a large portion of its orbit, and it still has a magnetic field (1% the strength of Earth's). Mercury is smaller and it rotates slower than 581g (Mercury rotates 3 times for every 2 orbits around the sun; it's tide locked when it is close and rotates halfway when it's far); it rotates once every 59 days, or 60% the speed of 581.

Since 581g is larger, it could have more iron in its core. The larger size combined with the faster rotation means the core is spinning faster (37 days for a planet larger than earth is much faster than 59 days for mercury). I would suspect that 581g, if its core is still molten (and it is larger so it should hold onto heat longer), should have some magnetic field. Plus, this is where fantasy could jump in and assist.

I also suspect a thicker atmosphere, combined with the lower UV output of the star, and the higher gravity, should help 581g hang onto more of its atmosphere (If I'm correct, the biggest problem is photons from the sun energizing air molecules so much that they escape the planet's gravity).



Camber said:


> Why is it that celestial bodies tend toward being tidally locked over time? Is there any reason other than gradual loss of energy, and is it inevitable that all planets will eventually be tidally locked with their star(s)?




Wikipedia has a good article on this. The main issue is that a large body creates large tides on an orbiting body. That tidal bulge acts as an anchor, which the larger body is tugging on. This will slow the smaller body's rotation until that tidal bulge is directly between the two bodies. Like how the moon creates a tidal bulge in our seas, but since the moon is smaller it doesn't have enough gravity to use it as a break (I think it's actually causing the moon to move further away).

I was looking into some more indepth astronomy, and I hear there is a way to determine the optimal orbital distance for moons around a planet. Something about the gravity well it creates. Too close to the parent star, the planet's gravity well may be too shallow to hold a large moon. I'm just curious, as I'd like a moon or moons for appearance but I don't want them if I'm going to have to support them with magic.


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## Nifft (Oct 12, 2010)

Xeviat said:


> Yes, a 37 day rotation should be fast enough to get a liquid core spinning a bit. I don't know if being tidally locked will tide lock the core as well, though. I suspect not; Mercury is tidally locked for a large portion of its orbit, and it still has a magnetic field (1% the strength of Earth's). Mercury is smaller and it rotates slower than 581g (Mercury rotates 3 times for every 2 orbits around the sun; it's tide locked when it is close and rotates halfway when it's far); it rotates once every 59 days, or 60% the speed of 581.
> 
> Since 581g is larger, it could have more iron in its core. The larger size combined with the faster rotation means the core is spinning faster (37 days for a planet larger than earth is much faster than 59 days for mercury). I would suspect that 581g, if its core is still molten (and it is larger so it should hold onto heat longer), should have some magnetic field.



 Since we don't really understand the process by which the Earth maintains its magnetic field, it's well within the realm of possibility that a larger, more massive planet would be able to maintain one.



Xeviat said:


> I also suspect a thicker atmosphere, combined with the lower UV output of the star, and the higher gravity, should help 581g hang onto more of its atmosphere (If I'm correct, the biggest problem is photons from the sun energizing air molecules so much that they escape the planet's gravity).



 The problems are photons and ions, and the magnetic field generally takes care of the ions.



Xeviat said:


> I was looking into some more indepth astronomy, and I hear there is a way to determine the optimal orbital distance for moons around a planet. Something about the gravity well it creates. Too close to the parent star, the planet's gravity well may be too shallow to hold a large moon. I'm just curious, as I'd like a moon or moons for appearance but I don't want them if I'm going to have to support them with magic.



 If the moon is significant, it may be the result of an event which imparted spin to the planet. For a phase-locked world, tiny moons seem more plausible.

- - -

IMHO a pair of mutually phase-locked planets would be a VERY interesting setting. There would be a cool "twilight zone" on their inward-facing sides, with relatively normal days & seasons as you went to the away-facing sides. If they were close enough to allow Teleportation to move between the worlds, they'd be able to accommodate high-level travelers but not much in the way of normal trade.

Cheers, -- N


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## Xeviat (Oct 13, 2010)

Nifft said:


> IMHO a pair of mutually phase-locked planets would be a VERY interesting setting. There would be a cool "twilight zone" on their inward-facing sides, with relatively normal days & seasons as you went to the away-facing sides. If they were close enough to allow Teleportation to move between the worlds, they'd be able to accommodate high-level travelers but not much in the way of normal trade.




That could be a very interesting sci-fi-styled equivalent of alternate planes. I like that, a lot.


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## Aloïsius (Oct 13, 2010)

Nifft said:


> IMHO a pair of mutually phase-locked planets would be a VERY interesting setting. There would be a cool "twilight zone" on their inward-facing sides, with relatively normal days & seasons as you went to the away-facing sides.



I don't understand. Sure, there would be frequent eclipses, but nothing as a twilight zone... For each other to create a permanent shadowy area, they would need to be so close that they would have been destroyed by tidal forces, and merged into one in a end of the worlds scenario.




> If they were close enough to allow Teleportation to move between the worlds, they'd be able to accommodate high-level travelers but not much in the way of normal trade.
> 
> Cheers, -- N




If they are tidely locked, and if their orbit is a well circular one (and not elliptic), then you may have some kind of "space lift" between them. The heroic fantasy kind of space lift is probably not a carbon nanotube cable, but maybe a real "stairway to heaven" made of light or magical cristal. Or maybe some kind of plant, a two-way Ygdrasil.


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## Nifft (Oct 13, 2010)

Aloïsius said:


> I don't understand. Sure, there would be frequent eclipses, but nothing as a twilight zone... For each other to create a permanent shadowy area, they would need to be so close that they would have been destroyed by tidal forces, and merged into one in a end of the worlds scenario.



 Nah. The zone between the is called a "twilight zone" because they never get noon (from 10:30 AM to 1:30 PM they get an eclipse), and they hardly get any night (since the "moonlight" reflected off their sister planet is quite bright). Even during the daily eclipse, they get some light conducted through their sister planet's atmosphere.



Aloïsius said:


> If they are tidely locked, and if their orbit is a well circular one (and not elliptic), then you may have some kind of "space lift" between them. The heroic fantasy kind of space lift is probably not a carbon nanotube cable, but maybe a real "stairway to heaven" made of light or magical cristal. Or maybe some kind of plant, a two-way Ygdrasil.



 In my model, the planets are about 12,500 km apart at their closest point, which is about 8,000 miles. Close enough to teleport, but not close enough to share an atmosphere.

Of course, if you're applying magical heroic phlogiston, perhaps the lack of an atmosphere is no impediment to special magical plant life... or perhaps there was a great hero who threw a mighty harpoon from one world to the next, fixing them together in the heavens, and as proof we still have the harpoon & steel cable right here, and you can totally climb them if you have necklaces of adaptation or you are warforged or whatever. Trip takes about a month, air runs out around day two, once you get half way it's easier to just push yourself towards the sister planet and let go of the cable.

- - -

Er, I don't want to derail the Gliese 581g discussion.

Cheers, -- N


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## Xeviat (Oct 15, 2010)

Nifft said:


> Er, I don't want to derail the Gliese 581g discussion.




Don't worry about it. I think it derailed itself. Apparently the big planet finding group things F and G might be artifacts in the math, and are currently labled "speculative planets". But, we can still fiddle with the notion. I'm making my map soon, and still trying to figure out the global wind patterns (venus's winds largely move in one circular direction around the equator, with cyclones at the poles, and it rotates slower than 581g does) and the coriolis effect on the oceans.

As for the dual planets thing, could two planets really orbit that close to each other? Wait, you said 12.5 km and then said 8,000 miles ... I guess you meant 12.5 thousand km? Seems far more reasonable. You don't want the worlds cyphoning atmospheres off each other to leak into space.


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## Nifft (Oct 16, 2010)

Xeviat said:


> As for the dual planets thing, could two planets really orbit that close to each other? Wait, you said 12.5 km and then said 8,000 miles ... I guess you meant 12.5 thousand km? Seems far more reasonable. You don't want the worlds cyphoning atmospheres off each other to leak into space.



 D'oh! Yeah, I meant 12,500 km (which is ~8,000 miles). Good thing I'm verbose, or I'd look extra dumb.

There was a neat thread on here about binary planets a few years back.

Cheers, -- N


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## Aloïsius (Oct 16, 2010)

Isn't 12500 km too close... They would be destroyed by tidal forces ?

As for Gliese 581g, it's only a matter of time before we find the real stuff.


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## Nifft (Oct 16, 2010)

Aloïsius said:


> Isn't 12500 km too close... They would be destroyed by tidal forces ?



 What tidal forces? They're phase-locked to each other.

They get (normal-sized) tides from the sun, since they are rotating relative to the sun, but they aren't rotating relative to each other. If you can see the sister-planet from where you are, you can ALWAYS see it from that location.

Days & nights work normally (except for the common solar eclipses if a large chunk of your sky is occupied by the sister planet), as do seasons.

From the calculations of the previous thread -- which I've been searching for in vain -- was that the closest possible sister planet would occupy between 40°-65° of the sky. That's freekin' huge compared to the portion of the sky the sun occupies (0.5331°), so even with seasonal variation in the sun's position, the daily eclipse zone remains.

Man, I wish I could find that other thread.

Cheers, -- N


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## jonesy (Oct 16, 2010)

Nifft, would this be the one?
http://www.enworld.org/forum/genera...cs-major-here-willing-help-me-few-things.html


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## Nifft (Oct 16, 2010)

jonesy said:


> Nifft, would this be the one?
> http://www.enworld.org/forum/genera...cs-major-here-willing-help-me-few-things.html



 That's probably it, seeing as Umbran's answer encompasses the values that I recall... though he says the sister planet could be quite a bit closer than I put them, perhaps it was somewhere else that had the info I needed to keep both planets mostly spherical.

Roche limit, that's the ticket!

Thanks, -- N


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## The Shaman (Oct 16, 2010)

A few notes from the tide-locked planet Noscitur in my _Traveller_ campaign -


 Like Aurelia, Noscitur features a giant cyclone at the substellar point, called the The Eye by the Noscituri. A shallow sea ringed by mudflats surrounds the substellar point, fed by massive rivers that flow from the anti-stellar (dark) side to the stellar (light) side of the planet.
 The source of the massive rivers is glacial meltwater. As air rises at the subsolar point, creating the massive storm known as The Eye, the air moves outward. In time the air cools and descends, creating a zone of high pressure which is intensely arid; the area beneath this ring of high pressure is an arid desert. The descending air produces warm dry surface winds which radaite outward from the desert, toward the equitorial zone, a sort of perpetual Santa Ana or chinook condition.
 As winds cross the twilight equitorial zone, they meet the cold air mass of the anti-stellar and are forced upwards; the rising air, carrying moisture from the equitorial zone is forced upward by the cold air mass, resulting in frontal storms along the line of 'sunset.' These storms produce considerable precipitation, which in turn feeds the glaciers along the edge of the anti-stellar side and allowing them to flow toward the stellar side.
 The planet's rotation (and yes, even though it's tide-locked, it still rotates in the same period in which it revolves around the sun) produces a Coriolis effect which causes The Eye to rotate slowly and the storm front along the sunset line to oscillate back and forth, varying the weather in the twilight equitorial band. The rotation also creates a dynamo effect in the planet's core, creating a magnetic field which protects the planet from some of the impact of the cosmic wind.
 Native producers (plants) line the river channels and the mudflats along the edges of the shallow sea. Consumers (animals) are migratory, to prevent excessive pressure on producer resources; instead of the polar/antipolar migrations with which we're familiar, the migrations are circumsubstellar, a great circular movement around The Eye.
 Noscitur was originally colonized as a defensive planet along the frontier of the First Imperium. A series of settlements linked by a circumequitorial railroad system were established by the original colonists, dug in to the planet's crust. Over time surface settlements appeared and the 'ring-rail' lines expanded in number, and they continue to be the main source of surface transport across Noscitur; collectively the various settlements of the world are known as 'Sunset City' and the individual settlements are districts of the city. For a fantasy world, this might be more of a great circular caravan route.
 Heavy industry is largely located on the anti-stellar side of the planet, in the bottoms of the many asteroid impact craters which give this side a pock-marked appearance. Waste heat from industrial processing is readily dissipated in the frigid air of the anit-substellar high.
 Tidal-flexing gives the planet active vulcanism and tectonic movement, though not as strong as Earth's.
 The circle is the most commonly used decorative feature in Noscituri artwork and architecture.


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## Xeviat (Oct 17, 2010)

The Shaman said:


> The source of the massive rivers is glacial meltwater. As air rises at the subsolar point, creating the massive storm known as The Eye, the air moves outward. In time the air cools and descends, creating a zone of high pressure which is intensely arid; the area beneath this ring of high pressure is an arid desert. The descending air produces warm dry surface winds which radaite outward from the desert, toward the equitorial zone, a sort of perpetual Santa Ana or chinook condition.
> As winds cross the twilight equitorial zone, they meet the cold air mass of the anti-stellar and are forced upwards; the rising air, carrying moisture from the equitorial zone is forced upward by the cold air mass, resulting in frontal storms along the line of 'sunset.' These storms produce considerable precipitation, which in turn feeds the glaciers along the edge of the anti-stellar side and allowing them to flow toward the stellar side.
> The planet's rotation (and yes, even though it's tide-locked, it still rotates in the same period in which it revolves around the sun) produces a Coriolis effect which causes The Eye to rotate slowly and the storm front along the sunset line to oscillate back and forth, varying the weather in the twilight equitorial band. The rotation also creates a dynamo effect in the planet's core, creating a magnetic field which protects the planet from some of the impact of the cosmic wind.




I'm trying to get an image of the general global wind patterns. I will treat the sub-solar and anti-solar points as poles, called solar-pole and night-pole. Thus, I'll use latitude to describe the zones I'm trying to get my head around. I'm sure this measurement will account for the vast majority of climate bands, though the planet's rotations might affect wind around the actual rotational poles (Venus has cyclones around its poles, for instance).

So, there is low pressure right at the solar-pole, which lifts warm moist air and drives it out from the pole. You're saying it would fall as dry warm air to create a sub-tropical high/desert. I can see this, and it is similar to Earth's zones.

What would the latitudinal degrees be for these zones? You're going to have a tropical zone around the solar-pole, a desert circling it, and a temperate zone around the equator. Would you center these around 30 and 60 degrees, like on Earth?

And how do you think the coriolis effect will effect this? Will it just curl the winds slightly, or will it cause the winds to rotate the rotational-poles as well?


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## Umbran (Oct 18, 2010)

Camber said:


> There's a very interesting non-mainstream theory called the Georeactor Dynamo Theory that suggests that uranium sinks to the planetary core, where it is concentrated enough that a low-level fission reaction is created.




As I recall it, one need not invoke fission.  Simple radioactive decay releases heat.  Tidal friction with a large moon will also generate heat in the body of the planet.



> Why is it that celestial bodies tend toward being tidally locked over time? Is there any reason other than gradual loss of energy, and is it inevitable that all planets will eventually be tidally locked with their star(s)?




It isn't just "gradual loss of energy" - remember that objects in motion tend to stay in motion.  A ball spinning in vacuum (like a planet) doesn't lose energy unless something exerts a force upon it.  

Tides occur because the force of gravity decreases as you move away from an object.  The force a sun exerts on the near side of a planet is greater than that it exerts on the far side.  That tends to deform the planet sightly as it rotates.  That bulging asymmetry gives a sort of handle - the pull of gravity tends to make that bulge end up pointing towards the star.

In theory, given enough time every body of large enough size will eventually reach some form of spin-orbit resonance (of which tidal locking locking is one case).  However, that's only if something else doesn't happen to upset the process.  Major collisions with other objects, for example, can speed up the process of coming into resonance, or slow it down.

In some cases, it can take a very, very long time - "older than the life of the Universe" kind very long time.


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## Umbran (Oct 18, 2010)

Nifft said:


> What tidal forces? They're phase-locked to each other.




"Tidal force" is shorthand for "differential in gravitational forces due to distance from the source body".  That differential doesn't go away when you tidally lock the system.

If that differential exceeds the body's own gravitational strength, the body is ripped apart.


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## Nifft (Oct 18, 2010)

Umbran said:


> "Tidal force" is shorthand for "differential in gravitational forces due to distance from the source body".  That differential doesn't go away when you tidally lock the system.



 Ah, right. "Tidal" seems a bit over-used in describing planetary forces.

Anyway, according to this guy, that distance is NOT close enough to tear the planet apart, so my confusion is moot.

Cheers, -- N


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## Umbran (Oct 18, 2010)

Nifft said:


> =Anyway, according to this guy, that distance is NOT close enough to tear the planet apart, so my confusion is moot.




Yes, I admit I'm being picky about terminology.  Old habit.

There are any number of scenarios where the objects will outright collide before one is ripped apart by tidal forces.  

The usual case is where the smaller of the two objects is really very small, such that it is more held together by material strength than by gravitational forces, or just very small with respect to the primary - meteors are not torn apart by tidal forces when approaching Earth, for example.  Forwards "Rocheworld" is the other end of the spectrum, where both objects are of similar size and mass.

Some have raised questions about such an arrangement, with models that suggest that while Forward's final state is stable, getting into that state without destroying the planets may not be realistic.

For an RPG setting, though, it seems close enough to plausible to me.


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## Xeviat (Oct 23, 2010)

Anyone have an idea about what the maximum moon size for a planet like 581g would be? Earlier someone said a moon would have orbital resonance with the star (like mercury does) and be tidally locked to the planet, but I keep hearing terms that relate to minimum orbital distances and such.

A moon will make the planet wobble a little, but this shouldn't be enough to affect temperatures, right? And orbit so close will be circular? Or maybe not, since Mercury isn't circular at all.


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## Umbran (Oct 23, 2010)

Xeviat said:


> Anyone have an idea about what the maximum moon size for a planet like 581g would be?




Well, the moon has to be smaller than the planet.  Other than that, I can only speak to vague probabilities.

Given what I know of the detection methods, any moon of 581g would not just be smaller than the planet, but much, much smaller.  They found 581g using "radial velocity" measurements - basically, looking for tiny wiggles in the star's movements.  We know the mass of 581g from that wiggle - and that mass is actually the total mass of whatever is orbiting there, planet and moon combined.

581g is supposedly about 3x the mass of Earth.  Let's say that it was more like a planet of 2x, and a moon the size of earth.  Those are both still big things, and the two of them whirling around in that orbit would be different than a single large planet, in ways I think (educated guess there) would be detectable.

So, expect any moon to be more like Phobos or Deimos is to Mars, than our Moon is to Earth.  But that's just a guess.



> Earlier someone said a moon would have orbital resonance with the star (like mercury does) and be tidally locked to the planet, but I keep hearing terms that relate to minimum orbital distances and such.




If the planet is tidally locked, any moon is probably in a resonant orbit, yes.

The minimum orbital distance is a mostly unrelated issue - if the moon is large, there's a minimum distance it could be from the planet.  Closer than that and it'd probably get torn apart by tidal forces.



> A moon will make the planet wobble a little, but this shouldn't be enough to affect temperatures, right?




If the planet's wobbling more than a little bit, it isn't really tidally locked yet.



> And orbit so close will be circular? Or maybe not, since Mercury isn't circular at all.




Okay, you're talking about a star, a planet, and a moon - which orbit are you talking about?

Tidally locked orbits tend to be nearly circular.  Orbits with other resonances may be significantly more eccentric.


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## Nifft (Oct 23, 2010)

Umbran said:


> Yes, I admit I'm being picky about terminology.  Old habit.



 In a thread where tidal LOCK and tidal ENERGY are under discussion, having tidal FORCE be a perpendicular vector (and of a different nature) is strange. I wonder if we're seeing a new "cleave" in the formative stages.



Umbran said:


> Some have raised questions about such an arrangement, with models that suggest that while Forward's final state is stable, getting into that state without destroying the planets may not be realistic.



 Meh, then go ahead and destroy them. Just do it when they're hot & molten, and have them re-form into hot molten balls afterward. Nobody said they had to remain habitable during the events that brought them together.



Xeviat said:


> Anyone have an idea about what the maximum moon size for a planet like 581g would be? Earlier someone said a moon would have orbital resonance with the star (like mercury does) and be tidally locked to the planet, but I keep hearing terms that relate to minimum orbital distances and such.
> 
> A moon will make the planet wobble a little, but this shouldn't be enough to affect temperatures, right? And orbit so close will be circular? Or maybe not, since Mercury isn't circular at all.



 The big deal with a moon is that our proposed planet is much closer to its star than Earth is to ours, so the star's pull will tend to overwhelm the stability of the planet-moon system.

- - -

Perhaps a thick enough atmosphere would scatter enough violet & UV light to make the icy night side habitable. Depends on the emission profile of the host star, and I can't seem to find that, but it's only a small handwave to grant lots of violet & UV light to our fictional world.

Small plants slowly reaching towards a cold, violet sky, with the most visible light coming from a dancing green aurora. Yeah, that would be a funky place to try to live.

Would greenhouses be viable under such conditions? If a bunch of violet & UV light came through the glass, they'd radiate in part as heat, right? And the IR heat radiation would be trapped by the glass?

Cheers, -- N


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## Umbran (Oct 23, 2010)

Nifft said:


> The big deal with a moon is that our proposed planet is much closer to its star than Earth is to ours, so the star's pull will tend to overwhelm the stability of the planet-moon system.




I'm not sure exactly what you intend to say here, but if my first guess at your intent is correct, then your statement is incorrect.

There's not much reason to think a stable planet-moon pair can't be there.  There's some restrictions on it (basically, some orbital resonances), but otherwise, it's quite feasible.



> Perhaps a thick enough atmosphere would scatter enough violet & UV light to make the icy night side habitable. Depends on the emission profile of the host star, and I can't seem to find that, but it's only a small handwave to grant lots of violet & UV light to our fictional world.




Gliese 581 is a red dwarf, smaller and cooler than our Sun - the sun as a surface temperature of about 5780 Kelvins (9900 F), while Gliese 581 is about 3480 Kelvin (5804 F).  That means less violet and UV radiation than the Sun puts out - a lot less.

And, if there's that much UV in the atmosphere that enough gets scattered to make the night side habitable, the day side would be uninhabitable, as the exposure to intense direct UV would tend to break the bonds in complex molecules required for life.  

It would (imho) more plausible to simply hand wave and say that there's enough atmospheric mixing to spread heat around and leave the night side habitable.



> Would greenhouses be viable under such conditions? If a bunch of violet & UV light came through the glass, they'd radiate in part as heat, right? And the IR heat radiation would be trapped by the glass?




If the light gets absorbed by matter (either surface or atmosphere), it imparts energy to the that matter - it heats up, and most of the re-radiation will be IR.


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## Woas (Oct 23, 2010)

Wouldn't all the water be frozen on the dark side since it would be extremely cold, never receiving any solar energy? The water vapor would move across from the hot side to the cold, condense, and then eventually freeze generating a huge half-world sized glacier.


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## Umbran (Oct 23, 2010)

Woas said:


> Wouldn't all the water be frozen on the dark side since it would be extremely cold, never receiving any solar energy? The water vapor would move across from the hot side to the cold, condense, and then eventually freeze generating a huge half-world sized glacier.




That would be the tendency, yes.  Nifft and I are invoking a few potential mechanics for getting some heat to the dark side - make dark and cool, not dark and frigid, to avoid that problem.


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## Nifft (Oct 23, 2010)

Umbran said:


> I'm not sure exactly what you intend to say here, but if my first guess at your intent is correct, then your statement is incorrect.



 "I don't know what you mean, but I know you are wrong." 



Umbran said:


> There's not much reason to think a stable planet-moon pair can't be there.  There's some restrictions on it (basically, some orbital resonances), but otherwise, it's quite feasible.



 The effect of the moon can't violate phase lock, since that's our initial assumption, so the moon can't have much effect on the planet.



Umbran said:


> Gliese 581 is a red dwarf, smaller and cooler than our Sun - the sun as a surface temperature of about 5780 Kelvins (9900 F), while Gliese 581 is about 3480 Kelvin (5804 F).  That means less violet and UV radiation than the Sun puts out - a lot less.
> 
> And, if there's that much UV in the atmosphere that enough gets scattered to make the night side habitable, the day side would be uninhabitable, as the exposure to intense direct UV would tend to break the bonds in complex molecules required for life.



 Depends on the cloud cover of the day-side. We're not dealing with an entity of uniform composition. The cloud cover will vary drastically.

But if the star isn't going to emit the light in the first place, it's moot.



Umbran said:


> It would (imho) more plausible to simply hand wave and say that there's enough atmospheric mixing to spread heat around and leave the night side habitable.



 Warm + lightless isn't habitable. Food won't grow. This is my major beef with the Underdark.

On the other hand, habitable does include places that are frosty but where there is enough ambient energy that food can grow. We can build shelters to catch ambient non-heat energy and turn it into heat, but turning heat into useful energy is somewhat more challenging.



Woas said:


> Wouldn't all the water be frozen on the dark side since it would be extremely cold, never receiving any solar energy? The water vapor would move across from the hot side to the cold, condense, and then eventually freeze generating a huge half-world sized glacier.



 Apparently some new models show that to be incorrect.

There are a bunch of links to specific debunkings of that -- and the theory that the atmosphere would boil off on the light size and freeze solid on the night side -- in the thread.

Cheers, -- N


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## Umbran (Oct 23, 2010)

Nifft said:


> The effect of the moon can't violate phase lock, since that's our initial assumption, so the moon can't have much effect on the planet.




Define term, "much".  Also note that what doesn't seem like much to the planet might be major for the living things upon it.  A little bit of wobble may not make huge differences in temperatures at the Noon and Midnight poles.  However, it might give you something akin to seasons in the twilight regions that might set lots of time-dependent behaviors in the life there.



> Depends on the cloud cover of the day-side. We're not dealing with an entity of uniform composition. The cloud cover will vary drastically.




I don't think you're going to get enough UV scatter _above_ the cloud layer to heat the dark side. Above the clouds the air is thin, and thus doesn't scatter much.  Overall, the atmosphere isn't a fiber-optic cable.



> Warm + lightless isn't habitable. Food won't grow. This is my major beef with the Underdark.




Photosynthesis is not the only way to go.  Check out the deep sea near volcanic vents - no sunlight, but lots of life.  



> There are a bunch of links to specific debunkings of that -- and the theory that the atmosphere would boil off on the light size and freeze solid on the night side -- in the thread.




I only see one that references it, and it is more interested in whether the planet holds an atmosphere at all: 

Aurelia and Blue Moon - Wikipedia, the free encyclopedia

_
"Nonetheless, the scientists employed by the programme decided to test the traditional assumptions for such a planet and start a model out for it from a proplyd through to its eventual death. Their estimations suggested such a planet could indeed hold on to its atmosphere, although with freakishly unusual results by Earth standards. Half of Aurelia would be in perpetual darkness and would be in a permanent ice age. The other half would contain a giant, unending hurricane with permanent torrential rain at the point directly opposite the local star. In between these two zones would be a place suitable for life."_

Other links are about magnetic fields and fictional worlds.  Which ones am I missing?


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## Nifft (Oct 24, 2010)

Umbran said:


> Define term, "much".  Also note that what doesn't seem like much to the planet might be major for the living things upon it.  A little bit of wobble may not make huge differences in temperatures at the Noon and Midnight poles.  However, it might give you something akin to seasons in the twilight regions that might set lots of time-dependent behaviors in the life there.



 "Much" = enough to give significant tides, promote vulcanism, and keep the planetary dynamo working longer than it otherwise would have worked.



Umbran said:


> I don't think you're going to get enough UV scatter _above_ the cloud layer to heat the dark side. Above the clouds the air is thin, and thus doesn't scatter much.  Overall, the atmosphere isn't a fiber-optic cable.



 It isn't a *good* fiber optics cable, but a nice thick atmosphere can carry light around the curve of a sphere, and ours does.

Their atmosphere could easily be thicker, what with their planet having a higher surface gravity.

Forget UV and just think about violet & blue light. There could be crops which will grow in a twilight zone which never sees their sun.



Umbran said:


> Photosynthesis is not the only way to go.  Check out the deep sea near volcanic vents - no sunlight, but lots of life.



 Yep, but that energy *is not heat*. It's chemical. The paragraph after the one you quoted hints at this same point: 







			
				Nifft said:
			
		

> On the other hand, habitable does include places that are frosty but where there is enough ambient energy that food can grow. We can build shelters to catch ambient non-heat energy and turn it into heat, but turning heat into useful energy is somewhat more challenging.



 ... but sure, I could have spelled that out more explicitly for you; and yes, deep sea vents on both sides of the planet would have plenty of non-photosynthetic life.

But: so what? How are those vents relevant to life *on the surface* of the night side?



Umbran said:


> I only see one that references it, and it is more interested in whether the planet holds an atmosphere at all (...) Other links are about magnetic fields and fictional worlds.  Which ones am I missing?



 Here's one from a link on page 3: 







			
				Space.com said:
			
		

> Researchers also know that the planet is tidally locked to its star. That means one side experiences eternal daylight, and the other side experiences unending darkness. Such a locked configuration helps to stabilize the planet's surface climate, Vogt said.
> 
> *3-D global circulation models have shown that the temperature differences on the day and night sides of the planet would not be enough for water to either freeze or boil off. They also suggest that the atmospheric circulation and wind patterns would be relatively benign.*



 I've bolded the relevant passage.

It's in a post by *Camber*.


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## Umbran (Oct 24, 2010)

Nifft said:


> "Much" = enough to give significant tides, promote vulcanism, and keep the planetary dynamo working longer than it otherwise would have worked.




Well, we don't as yet know if natural satellites large enough to do that are common, in general, much less whether they're likely in this particular sort of case.  However, since you can get all that from a satellite that's in a resonant orbit, it is at least possible.



> It isn't a *good* fiber optics cable, but a nice thick atmosphere can carry light around the curve of a sphere, and ours does.




Well, it is specifically a pretty crappy fiber-optic cable.  Yes, some light does make it over the horizon, but not by much.  I thought you were talking about having light reach great distances toward the dark pole, sufficient to drive heating and growth there, and that's unlikely.  



> Their atmosphere could easily be thicker, what with their planet having a higher surface gravity.




We know the mass of the planet, but so far, we don't have a good handle on its size - so, we don't know yet if the surface gravity is higher.  It is probable to be higher, but not a surety, and not necessarily higher by much.



> Yep, but that energy *is not heat*. It's chemical.




Doesn't matter - the point is that life can use a variety of energy sources.

There's no particular reason why life can't use a heat engine, if that's all that's available.  So long as you have a temperature gradient, you can have  a heat engine.  You could imagine huge heat-engine beds of moss-like material blanketing the surface, using a gradient across hundreds or even thousands of miles... classic D&D brown mold!

Or, perhaps more simply, if there's significant atmospheric (and/or oceanic) mixing from one side to the other, the dark may get significant influx of complex organic material (or the equivalent, if they aren't carbon based).  So, the dark side could then use chemosynthesis like an undersea vents ecology, or be essentially saprophytic like a cave ecology.

Or, since we are talking about a fantasy world, you can have a system based on arcanosynthesis - magic as the underlying energy!  



> Here's one from a link on page 3:  I've bolded the relevant passage.
> 
> It's in a post by *Camber*.




Ah, got it.  Sorry, I thought you were referring to links more specifically about those models.


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## Xeviat (Oct 24, 2010)

I'm not so much concerned with specifically modeling 581g anymore, especially considering it is in the "unconfirmed" pile of planets again. I'm looking at a planet in a similar situation, more like Aurilia (though I don't want the night-side glacier to be so encompasing; I figure more ocean on the planet would lead to better heat transfer).

To get a moon that nearly eclipses a red dwarf in the sky of a tidally locked planet, the moon is going to need to appear larger than ours. This means it needs to be considerably closer, or considerably bigger. It could be bigger, as the planet can be bigger, but some have pointed out that large satelites appear to be rare from our own solar system's guidelines (our own moon is throught to have come from a massive colision). A similar situation could have occured.

Now, what I'm thinking could be interesting, but still simultaneously useful for time keeping and suitably different from Earth, would be if such a planet had a few moons. They would create more frequent eclipses of the sun, which would aid in the tracking of time. Additionally, at least closer ot the twilight, if the star has more planets further interior, these could be seen as stars moving in and out of the star, and they'd prove useful (but the twilight side will be able to see the actual stars, which will drift in the sky rather quickly).

I also think I will be using chemosynthesis as the primary source of food on the night side. Vast gyser fields and hot springs will support chemosynthetic bacteria and fungi that animals feed off of. Other life will be supported by organics brought in from the day side (the ocean should still get a good supply of plankton and such on the ever circulating ocean currents).

I'd prefer to keep the magical ecosystems to a minimum, but this is because of my preference towards worldbuilding with an eye to Sci Fi.

Thanks for everyone's help. This is a fun discussion.


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## Nifft (Oct 24, 2010)

Umbran said:


> Ah, got it.  Sorry, I thought you were referring to links more specifically about those models.



 I posted content from one link which specifically and directly answers the question posted. What more did you want?


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## Umbran (Oct 24, 2010)

Nifft said:


> I posted content from one link which specifically and directly answers the question posted. What more did you want?




You want a detailed answer?  The issue isn't what I wanted, but what I was looking for and/or expecting.  

You said, "There are a bunch of links to specific debunkings of that -- and the theory that the atmosphere would boil off on the light size and freeze solid on the night side -- in the thread."

When I looked, I didn't see that.  I saw a bunch of links to general articles on the planet (and some on geomagnetism).  Buried in one of the general articles was a short assertion of reaching a different result on the atmosphere.  

I was expecting were several links that were specifically and primarily about the atmospheric modeling, with more details and discussion of what the new model found, and how it differed from the old model.  For real debunking, I'd even expect to see evidence that the original result wasn't just inaccurate, but downright fakery with no science behind it at all.  It isn't like Einstein "debunked" Newton, after all.

So, when I went looking, I didn't see what I call "a bunch", "specific", or "debunking".  See my previous note about being my being (perhaps unfortunately) picky about terminology when talking science. _Mea culpa_.


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## Nifft (Oct 25, 2010)

Umbran said:


> For real debunking, I'd even expect to see evidence that the original result wasn't just inaccurate, but downright fakery with no science behind it at all.  It isn't like Einstein "debunked" Newton, after all.



 It *is* inaccurate, and there is no science behind the "theory" that the atmosphere would both boil off and freeze out, or that all the water would be locked on the night side. It's just a bunch of simplistic reductionist intuition wrapped in science-y terms.



Umbran said:


> So, when I went looking, I didn't see what I call "a bunch", "specific", or "debunking".  See my previous note about being my being (perhaps unfortunately) picky about terminology when talking science. _Mea culpa_.



 You saw one, and I've pointed you towards another, both of which *specifically* refute the notion of phase-locked planets being uninhabitable.

I'd like to see their atmospheric models in more detail too, but they're both quite *specific* in that NOT all water in any of their models ended up locked on the night-side.

Be as picky as you want with science terms -- rigor is good for us -- but please try to apply that rigor to yourself as well, and honestly tell me that this isn't a *specific* refutation of the the claim made by *Woas*: 







			
				Space.com said:
			
		

> 3-D global circulation models have shown that the temperature differences on the day and night sides of the planet *would not be enough for water to either freeze* or boil off.




This is the claim being debunked. Emphasis added:


Woas said:


> *Wouldn't all the water be frozen on the dark side* since it would be extremely cold, never receiving any solar energy?




- - -

Okay, now that we've firmly established whatever the hell that was about, let's get back to talking about cool phase-locked worlds for games.

Perhaps it's possible to have a layered atmosphere, since the planet is so much larger than the Earth. There could be a "high" atmosphere comparable to our own, where human-ish life can develop, but over the seas and in deep canyons there is a layer of something heavy and greenhouse gassy, which may or may not be toxic to people, but should be uncomfortable. This layer could carry heat deep into the Nightside, and could reach land-bound glaciers without being in danger of freezing out itself.

So, in addition to the atmospheric cycles going on in the normal earth-like layer, you'd have these hot chasms which might never see sunlight, but are home to howling winds and severe weather, including "lake-effect" rainfall where prevailing winds run over these canyons.


I like this idea for a few reasons:

1/ A great excuse for Mars-like canals.

2/ A great excuse for airships to cross over the deep, hot oceans of inhospitable mist.

3/ A great excuse for piping heat over to the dark side.

Cheers, -- N


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## Aloïsius (Oct 26, 2010)

I was wondering about a binary system... One far away star that would bring little warm and light, but still enough for some photosynthesis activity. And one close red dwarf, with which the planet would be tidely locked.

When the secondary star is in the "light" part of the planet sky, it would bring a very hot summer, while the dark side would plunge into freezing cold. When it is on the other side, their would be some kind of spring season on the darkside, while the light one would benefit from some cooler temperatures.


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## Umbran (Oct 26, 2010)

Aloïsius said:


> When the secondary star is in the "light" part of the planet sky, it would bring a very hot summer, while the dark side would plunge into freezing cold. When it is on the other side, their would be some kind of spring season on the darkside, while the light one would benefit from some cooler temperatures.




A planet in the "Goldilocks zone" of a red dwarf is very close in, which means the year is short.  As an example, Gliese 581g (if it exists) has an orbital period of 36.5 days or so.  Each season would be about 9 days.

I think having a secondary star, rather than a moon, might be a good way to inject some time dependencies that would be interesting, but it wouldn't be a season like we have on Earth.


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## Nyeshet (Oct 26, 2010)

Aloïsius said:


> I was wondering about a binary system... One far away star that would bring little warm and light, but still enough for some photosynthesis activity. And one close red dwarf, with which the planet would be tidely locked.
> 
> When the secondary star is in the "light" part of the planet sky, it would bring a very hot summer, while the dark side would plunge into freezing cold. When it is on the other side, their would be some kind of spring season on the darkside, while the light one would benefit from some cooler temperatures.




This would be difficult to set up realistically. If the stars are closer than about 3 AUs (if I recall correctly), then they act as a single barycenter around which the planet orbits. They would appear as two suns that are always near each other - rising and setting together, etc. 

If the suns were further apart, there is a risk that the gravity of one would upset the orbits of the planets that tried to form about of the other (either preventing formation or tossing them out of the system during formation) unless they are *much* further apart - so far apart that the one not orbitted by the planet appears star-like (or - more accurately - venus-like, in apparent size in the sky). It would still give off as much light as a full moon, more or less, but it would not obviously be a second 'sun' - let alone provide enough heat for a hotter summer, etc.



Edit: I found the link I was looking for: planet forming discs of dust do not seem to form in binary star systems that are between ~3 and ~50 AUs apart. The stars either have to be nearer than 3 AUs (and so act as a single star for orbital purposes) or farther apart than 50 AUs (in which case the second sun will appear star-like). 


http://solstation.com/habitable.htm
http://solstation.com/images/bi2sep.jpg

On the other hand, I have some little experience in working on the equations for luminosity of stars in such a system, as I once worked out what a red dwarf star (0.34 sol masses) at 108 AU away would look like from a planet at a mars like orbit (1.69 AUs) from the binary star (1.29 sol masses). The red dwarf would have a luminosity of about -11.7, compared to the full moon's luminosity of about -12.7 and the earth's sun's of -26.8. So it would be no larger than any given star in the night sky, but its brightness would be that of a gibbous moon. You will get some faint shadows from the reddish-star due to its brightness, but little else. 


If we presume a sun-like binary star companion at ~50 AUs, its brightness would likely be several times brighter than the full moon - but it still would be too faint to give any real extra heat to the world. 



Okay, I calculated by extrapolation the apparent luminosity of a sun-like G2 star at 50 AUs using the link below. It works out to -18.37. This is roughly equivalent to the brightness of the sun as seen from Pluto (-18.2) or Neptune (-19.3). Your red dwarf tidally-locked planet not get much heat from its second sun, but low light vision should see much farther than on an earth-like world, at least when the second 'sun' is in the night sky. Mythically, locals might view this second sun as instead the king of the stars, the local myth equivalent of Jupiter due to its brightness. 

http://www.johnbray.org.uk/planetdesigner/
http://en.wikipedia.org/wiki/Apparent_luminosity


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## Nifft (Oct 26, 2010)

Nyeshet said:


> This would be difficult to set up realistically. If the stars are closer than about 2 AUs (if I recall correctly), then they act as a single barycenter around which the planet orbits. They would appear as two suns that are always near each other - rising and setting together, etc.
> 
> If the suns were further apart, there is a risk that the gravity of one would upset the orbit of the other unless they are *much* further apart - so far apart that the one not orbitted by the planet appears star-like (or - more accurately - venus-like, in apparent size in the sky). It would still give off as much light as a full moon, more or less, but it would not obviously be a second 'sun' - let alone provide enough heat for a hotter summer, etc.



 Yeah. IMHO it's way easier to posit a phase-locked moon orbiting a Jovian-scale gas giant. The gas giant would also be tasked with maintaining a magnetosphere, since they seem good at that.


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## Umbran (Oct 26, 2010)

Nyeshet said:


> This would be difficult to set up realistically. If the stars are closer than about 2 AUs (if I recall correctly), then they act as a single barycenter around which the planet orbits. They would appear as two suns that are always near each other - rising and setting together, etc.




However, in this situation, it is unlikely the planet would become tidally locked, as the tides are not going to be consistent over time



> If the suns were further apart, there is a risk that the gravity of one would upset the orbit of the other unless they are *much* further apart - so far apart that the one not orbitted by the planet appears star-like (or - more accurately - venus-like, in apparent size in the sky).




I am not sure this is correct.  

Take a larger, bright star.  Put the Red dwarf in orbit around it.  Put the planet in close orbit around the Red dwarf.  A situation like our Sun, a Jovian planet, and one of it's moons, just make everything bigger.  I think you can still get significant illumination from the bright star without significantly perturbing the planet's orbit.  I'd need to spend some time with the math to see if it works out well for sure.

Mind you, the bright star will have a much shorter lifespan than the red dwarf, and when it dies, it may well wreck things on the poor little world so nearby.


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## Nyeshet (Oct 26, 2010)

Umbran said:


> A planet in the "Goldilocks zone" of a red dwarf is very close in, which means the year is short.  As an example, Gliese 581g (if it exists) has an orbital period of 36.5 days or so.  Each season would be about 9 days.
> 
> I think having a secondary star, rather than a moon, might be a good way to inject some time dependencies that would be interesting, but it wouldn't be a season like we have on Earth.



A tidally locked world should have little or no tilt - and so no seasons to speak of, much like its one face has no day and its other face no night to speak of. 

I'll admit I did once play with the idea of a more 'normal' world around a red dwarf star, but to make it normal I had to place a tiny half-earth mass 'moon' around a Jovian world just barely in the habitable zone of a red dwarf star. The result was a 'world' with a day 0.7x the length of an earth day (as it orbitted the Jovian at a Callisto distance, so as to keep radiation to an acceptable level) and a year of about 32 days. There were still two main problems, however. 

First, the tides were absolutely incredible. Between the Jovian and the Red Dwarf, the worldlet experienced tides of about 100 meters (72.63 ave from Jovian, 25.59 ave from star). By comparison, earth's ave mid-ocean tide height from the sun and moon together is about 0.54 meters). If earth's maximum tide heights are any example from which to extrapolate, then this worldlet has a maximum tide height around 1.2 km. Its 'Bays of Fundy' would be incredible to see. 

Second, when the Jovian was at its maximum distance from the red dwarf, the worldlet, during its few hours of night, would actually be just outside the habitable zone for a few hours. So once a month, for a few days, the nights on the night side would be colder than most arctic nights, or so I would presume. 

As a minor point, the night side would have twilight nights when facing the Jovian, as the reflected light from its surface bathed the world in light, but solar eclipses would happen every noon on the day side during those times of the month when the Jovian was between the worldlet and the red dwarf. No corona would be visible, however, due to how vast the Jovian would appear from the surface even when compared to the red sun. On the other hand, during these eclipses, the night side would also be experiencing something unusual: Stars would only be visible when the night side faced away from the Jovian for a few days each month - also, as already mentioned, the coldest days of the year. Thus the world would have something akin to seasons, even as it had little to no tilt.


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## Nyeshet (Oct 26, 2010)

Umbran said:


> I am not sure this is correct.
> 
> Take a larger, bright star.  Put the Red dwarf in orbit around it.  Put the planet in close orbit around the Red dwarf.  A situation like our Sun, a Jovian planet, and one of it's moons, just make everything bigger.  I think you can still get significant illumination from the bright star without significantly perturbing the planet's orbit.  I'd need to spend some time with the math to see if it works out well for sure.
> 
> Mind you, the bright star will have a much shorter lifespan than the red dwarf, and when it dies, it may well wreck things on the poor little world so nearby.




That is a problem. Also a problem is explaining how a planet formed between two stars so near each other in the first place. A red dwarf separated from a sun-like star - let alone a much more gravitationally powerful F, A, or even O type star - by only 5 AUs (Jupiter distance) would be near enough to have any dusty disc surrounding it torn away by the gravity of the companion star - even if the potential planet was as near as the red dwarf's habitable zone. Actually, with O type stars the situation is worse. I recall an article that spoke of dusty planetary discs around young stars being literally blown away (gradually) by the solar wind of O type stars a mere 5 to 10 light years distant. Having an O type star 5 to 10 AUs distant would likely prevent any planet from forming even around a sun-like star, let alone a red dwarf. 


However, red dwarves live a *very* long time. The eldest ones still have more than 10x the current age of the universe before there is any chance of them going out. So why not a captured one that formed its planets late and then joined another star (perhaps gathering a disc when passing through a nebula - long after its own flaring youthful couple billion years, then a few billion years later being captured by another star into a binary orbit - losing its outter planets in the process but keeping those one or two nearest its surface). Now we have a nice stable red dwarf with a planet within its habitable zone fully developed, and a binary companion that might be as close as 5 to 10 AUs. Such would have to be an extremely rare event, as no such binary system so close together and yet with a planet around one of the two has yet been found. 

Note, however, that even an F type star would not give a notable amount of heat at Jovian distances. You would need at least an A - maybe an O, and yes, when the younger larger star goes supernova, the tidally locked world is toast. (Actually, for an O type star, the world would be toast during the red giant phase, when the red dwarf - at 5 AU - would likely end up skimming almost over the surface of the red giant's outter envelope of atmosphere).


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## Umbran (Oct 26, 2010)

Nyeshet said:


> A tidally locked world should have little or no tilt - and so no seasons to speak of, much like its one face has no day and its other face no night to speak of.




Yes.  We touched on that earlier in the thread.  Lacking day and night, or seasons, some folks wanted something else to mark time in the system that the biology and cultures could key on - we've discussed moons, and now perhaps another star nearby, to help add something along those lines.



Nyeshet said:


> However, red dwarves live a *very* long time. The eldest ones still have more than 10x the current age of the universe before there is any chance of them going out. So why not a captured one that formed its planets late and then joined another star




Capture would be the obvious way to end up in this arrangement, yes.  



> Note, however, that even an F type star would not give a notable amount of heat at Jovian distances.




I don't think we are so much worried about heat as illumination - something  animals could see by, or that might run a slow plant ecology - as much visible light as given by a full Moon would probably be enough for our purposes.  

And, honestly, while we are using a real physical setup for inspiration, we're considering it for a *fantasy* game setting.  I don't think we are worried about exact stellar spectral types, or exact distances.  I brought up the Jovian example merely as something folks could easily visualize.


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## Nyeshet (Oct 26, 2010)

Umbran said:


> I don't think we are so much worried about heat as illumination - something  animals could see by, or that might run a slow plant ecology - as much visible light as given by a full Moon would probably be enough for our purposes.
> 
> And, honestly, while we are using a real physical setup for inspiration, we're considering it for a *fantasy* game setting.  I don't think we are worried about exact stellar spectral types, or exact distances.  I brought up the Jovian example merely as something folks could easily visualize.




Okay, but to be technically just one more time ... 

All below presume a world 5 AU distant from a star of the stated solar mass.

Mass. Type. Lum . . Notes

2.0 . A . . -26.8 . 1.25 Ba out of 1.44 Ba lifespan, sun as seen from earth

1.5 . A/F . -25.5 . 2.52 Ba out of 3.21 Ba lifespan, sun as seen from Mars (-25.6, aphelion)

1.25. F5. . -25.0 . 4.51 Ba out of 5.01 Ba lifespan, sun as seen from (almost) Mars

1.0 . G . . -23.45 .4.64 Ba out of 10.0 Ba lifespan, sun as seen from ~Jupiter


Considering the pictures returned from Mars, it looks like you can get as much illumination as you need with just an F type type (F0 to F5), but a Sun-like star will only be enough if you think photosynthesis would work on a warmer Europa. The full moon may give just enough light for dark adjusted eyes to notice shadows, but plants don't grow much under its light (and the sun as seen from Pluto is quite a bit brighter - and just the brightest star in the night sky from Pluto's point of view).


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## Umbran (Oct 26, 2010)

Nyeshet said:


> Considering the pictures returned from Mars, it looks like you can get as much illumination as you need with just an F type type (F0 to F5), but a Sun-like star will only be enough if you think photosynthesis would work on a warmer Europa.




Fantasy game, remember?  Fire breathing dragons, and such?  The biology already doesn't fit Earth norms.  

We don't need scientific accuracy, we need it plausible enough to not get in the way of drama and developing story.




> The full moon may give just enough light for dark adjusted eyes to notice shadows, but plants don't grow much under its light (and the sun as seen from Pluto is quite a bit brighter - and just the brightest star in the night sky from Pluto's point of view).




I don't know about where you live, but in Boston, the full moon on a clear night gives more than enough light to notice shadows; it gives enough light to read by - I was just noting that here two nights ago, even.  

I think that sounds like plenty for a magically endowed world.  Almost an unseelie fey feel, even, under that dim pale light in an otherwise eternal night...


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## El Mahdi (Oct 26, 2010)

_...under dim, pale light in an otherwise eternal night..._

Man! That sounds Cool!


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## Nyeshet (Oct 26, 2010)

You have good eyes, then.  I can (barely) read by full moon light if I've been well away from bright artificial lights long enough for my eyes to fully adjust to the lunar level of light. Granted, I also need to wear glasses, so I expect my eyes are less adept than might be typical. 

Anyway, I just finished a bit more research. Photosynthesis ceases around -20.5 luminosity (500-600 lux, due to no longer being able to keep up with respiration), so even a normal sun will be fine at Jupiter distance. In fact, at 5 AU, you can get down to lower mass K stars before the luminosity is too low to allow for photosynthesis. So you could have a binary composed of a red dwarf and an orange star and it would work (albeit being as dark as a *very* overcast day [~ -21] and having incredibly slow / weak plant growth).


Well, that's enough science from me today.  Sorry if I bored you, but I myself became curious as to the answer as I read the posts and researched possible answers.


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## Scholar & Brutalman (Oct 27, 2010)

Nyeshet said:


> I'll admit I did once play with the idea of a more 'normal' world around a red dwarf star, but to make it normal I had to place a tiny half-earth mass 'moon' around a Jovian world just barely in the habitable zone of a red dwarf star. The result was a 'world' with a day 0.7x the length of an earth day (as it orbitted the Jovian at a Callisto distance, so as to keep radiation to an acceptable level) and a year of about 32 days. There were still two main problems, however.




Apparently moons of close Jovians will be unstable over time: the paper I found was [astro-ph/0205035] Stability of Satellites Around Close-in Extrasolar Giant Planets 



> The tidal bulge that a satellite induces onits parent planet perturbs the satellite’s orbit (e.g., Burns 1986), causing migrations in semimajor axis that can lead to the loss of the satellite. For an isolated planet, satellite removal occurs either through increase in the satellite’s orbital semimajor axis until it escapes, or by inward spiral  until it impacts the planet’s surface (Counselman1973). In the presence of the parent star, stellar-induced tidal friction slows the planet’s rotation, and the resulting planet-satellite tides cause the satellite to spiral inward towards the planet (Ward&Reid1973; Burns1973). This eﬀect is especially important for a planet in close proximity to its star, and has been suggested as the reason for the lack
> of satellites around Mercury(Ward&Reid1973; Burns 1973).




It's only really a problem for habitable satellites of jovians around red dwarfs, a Jovian in Earth's orbit could still have habitable planets.


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## Aloïsius (Oct 27, 2010)

Nyeshet said:


> Anyway, I just finished a bit more research. Photosynthesis ceases around -20.5 luminosity (500-600 lux, due to no longer being able to keep up with respiration), so even a normal sun will be fine at Jupiter distance. In fact, at 5 AU, you can get down to lower mass K stars before the luminosity is too low to allow for photosynthesis. So you could have a binary composed of a red dwarf and an orange star and it would work (albeit being as dark as a *very* overcast day [~ -21] and having incredibly slow / weak plant growth).




That's really cool news by the way. It means that with a (very) powerful greenhouse system*, you can grow food on the moon of Jupiter.


*I don't think we have a technology allowing us to keep enough of the solar heat inside a greenhouse to raise the temperature at the appropriate level... We would need glass that let enter 100% or so of the sun light in one way, and reflect 100% or so of it the other way...


On a related note... There has been news of a new strange extrasolar planetary system : two giant gaseous planets orbiting a binary star. One of the star being a white dwarf, the other a red dwarf (IIRC). It means that either those planets survived a nova that should have engulfed them, or that they were created from the nova's leftover. 
In the first hypothesis, they were probably far away than they are now from the two stars, and migrated inward after the nova. 
Now, imagine... civilization A flourish on world B. World B is nearly destroyed by a nova, and then is frozen to death. Then, after a billions years, world B reach a warmer area in its solar system. Life reappear, a new civilization emerge... And discover strange ruins and artefact unbelievably old. And then, the Very Old Thing awaken...


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## Umbran (Oct 27, 2010)

Aloïsius said:


> *I don't think we have a technology allowing us to keep enough of the solar heat inside a greenhouse to raise the temperature at the appropriate level... We would need glass that let enter 100% or so of the sun light in one way, and reflect 100% or so of it the other way...




On Earth, visible light enters through the glass of a greenhouse.  Some small amount of that light is reflected around (so we can see), but the lion's share of it is absorbed by the opaque matter under the glass.  

Much of that energy is then re-emitted, as infrared radiation, instead of it's original color of light.  The glass is largely opaque/reflective to infrared, so the heat stays in the box.  The box needs to be transparent to what the plants need to grow, and opaque only to infrared - not all light.


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## The Shaman (Oct 27, 2010)

Xeviat said:


> What would the latitudinal degrees be for these zones? You're going to have a tropical zone around the solar-pole, a desert circling it, and a temperate zone around the equator. Would you center these around 30 and 60 degrees, like on Earth?
> 
> And how do you think the coriolis effect will effect this? Will it just curl the winds slightly, or will it cause the winds to rotate the rotational-poles as well?



The Coriolis effect is what creates our latitude zones, so on a planet with a slower rotation, the zones will be fewer in number and further apart. For Noscitur I put the dry subtropical zone at 60 degrees and the moist temperate zone at about 100 degrees - because of the strong "night-pole" high, I didn't feel that extending it all the way to 120 degrees made sense.

Because of time considerations I haven't backed any of this with actual calculations, though someday that might be fun to do; I just wanted to get something that made intuitive sense.


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## Xeviat (Nov 2, 2010)

The Shaman said:


> The Coriolis effect is what creates our latitude zones, so on a planet with a slower rotation, the zones will be fewer in number and further apart. For Noscitur I put the dry subtropical zone at 60 degrees and the moist temperate zone at about 100 degrees - because of the strong "night-pole" high, I didn't feel that extending it all the way to 120 degrees made sense.
> 
> Because of time considerations I haven't backed any of this with actual calculations, though someday that might be fun to do; I just wanted to get something that made intuitive sense.




Thanks for getting back to me. Now, I'm assuming those degrees treat the solar and lunar poles as north and south? Also, we won't get a cold moist zone, like on Earth, since most of the water will be dumped in the temperate zone?

I'm looking to have a world with life on the darkside as well, so I might need a thicker atmosphere that is more capable of transfering heat to the dark-side.


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