Gliese 581g - A Tidally Locked DnD World

[The Shaman]

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.

 

[Xeviat]

• 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?

 

[Umbran]

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.

 

[Umbran]

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.

 

[Nifft]

"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

 

[Umbran]

=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.

 

[Xeviat]

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.

 

[Umbran]

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.

 

[Nifft]

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.

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.

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

 

[Umbran]

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.