Gliese 581g - A Tidally Locked DnD World

[Remathilis]

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.

 

[HalfOrc HalfBiscuit]

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):

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.

 

[Dannyalcatraz]

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

 

[Umbran]

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

 

[Nifft]

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

 

[Xeviat]

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.

 

[Lanefan]

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

 

[Umbran]

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.

 

[Nifft]

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

 

[Xeviat]

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.