Gliese 581g - A Tidally Locked DnD World

[El Mahdi]

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

Man! That sounds Cool!

 

[Nyeshet]

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.

 

[Scholar & Brutalman]

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.

 

[Aloïsius]

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...

 

[Umbran]

*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.

 

[The Shaman]

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.

 

[Xeviat]

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.