Gliese 581g - A Tidally Locked DnD World

[Nifft]

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.

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.

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.

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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

 

[Xeviat]

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.

 

[Aloïsius]

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.

 

[Nifft]

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.

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.

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

Cheers, -- N

 

[Xeviat]

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.

 

[Nifft]

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

 

[Aloïsius]

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.

 

[Nifft]

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

 

[jonesy]

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

 

[Nifft]

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