Gliese 581g - A Tidally Locked DnD World

[Umbran]

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.

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.

 

[Xeviat]

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?

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.

 

[Umbran]

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.

 

[Lanefan]

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

 

[Umbran]

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.

 

[Nifft]

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.

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

 

[El Mahdi]

...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.

 

[Aloïsius]

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.

 

[Umbran]

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.

 

[El Mahdi]

...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.