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Old 03-13-2020, 08:09 AM   #11
YankeeGamer
 
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Default Re: [Space] Climate & habitability of tide-locked planets

I don't know if this is scientifically valid--I'm not enough of an expert--but it looks plausible--certainly sufficient for gaming. Here's a well thought out tide-locked world which keeps in mind the fact that orbits are usually not circular.
http://www.worlddreambank.org/L/LIB.HTM

http://www.worlddreambank.org/P/PLANETS.HTM has a lot of neat concepts :)
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Old 03-13-2020, 06:23 PM   #12
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Default Re: [Space] Climate & habitability of tide-locked planets

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Originally Posted by Crystalline_Entity View Post
Indeed, it's a little cloudy here today, and I'm still alive :)
Figure 2 in Merlis & Schneider (op. cit.) shows a small area (at the middle of the day side of the slow-rotating planet, made up of two narrow arcs north-east and south-east of the subsolar point on the fast-rotating one) with 62.5 mm of precipitation per day. That is very rainy. That is twice as much rain as the rainiest place on Earth. But it falls over only a very small area. Most of the zone indicated as getting an excess of rainfall over potential evaporation gets between 12.5 mm per day and 62.5 mm per day. That seems like a lot of rain to me the town where I live gets an average of 1128 mm per year or 3 mm per day. Seathwaite Farm in Cumbria gets 3350 mm per year (9.2 mm per year). No doubt I would find it dismal. But even Megahalaya in India is far from being incapable of supporting plant life.
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Old 03-13-2020, 06:40 PM   #13
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Default Re: [Space] Climate & habitability of tide-locked planets

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Originally Posted by YankeeGamer View Post
I don't know if this is scientifically valid--I'm not enough of an expert--but it looks plausible--certainly sufficient for gaming. Here's a well thought out tide-locked world which keeps in mind the fact that orbits are usually not circular.
http://www.worlddreambank.org/L/LIB.HTM
Well, there are a lot of things about it that I would like to ask the author about. I would have thought that the tides that braked that world's rotation would also have eliminated its obliquity. The eccentricity (0.2) seems rather large (it's equal to Mercury's, with stronger tides), and large enough that I would have expected a 2:1 spin:orbit resonance rather than synchronous rotation. The pattern of winds on Librata has equatorial easterlies, whereas the modelling I have seen shows superrotation (i.e. equatorial westerlies).

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http://www.worlddreambank.org/P/PLANETS.HTM has a lot of neat concepts :)
It has.
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Old 03-13-2020, 06:56 PM   #14
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Default Re: [Space] Climate & habitability of tide-locked planets

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Originally Posted by Agemegos View Post

Since permanent human settlement requires (a) photosynthetically active illumination, (b) an excess of precipitation over potential evaporation, and (c) an average temperature between 273 K and 303 K it seems to me that the habitation candidates among synchronously rotating planets are not those that are about as warm as Earth (settled in the twilight zone), but those that are on average a little cooler than Earth, which will be settled in the subsolar region where it rains and where the light is brighter.
I've never had the impression in 4th edition that people who live on tide locked planets are living in the twilight zone anyway. More like in the northern and southern latitudes where things are cooler than they are at the east pole anyway. The only effect on habitability is the -25% to hydrosphere.

Which overlooks the biggest issue impacting habitability there of course, the periodic blasts of radiation from the sun.

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Old 03-13-2020, 09:05 PM   #15
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Default Re: [Space] Climate & habitability of tide-locked planets

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Originally Posted by David Johnston2 View Post
I've never had the impression in 4th edition that people who live on tide locked planets are living in the twilight zone anyway.
Have you calculated the Habitability scores and Dayside temperatures of a lot of randomly-generated tide-locked worlds? I've had quite a lot of them drop out of my generator, and I don't like the look of them. The ones with high Habitability scores have uninhabitably hot day sides.

The thing that brought this issue to the forefront of my mind just now is trying to generate a society (and adventure) for Gliese 370 II "Persatuan", the highest-Habitability tide-locked world in Central Sector of my randomly-generated universe. It has an average temperature of 27 C, and a calculated dayside temperature of 67 C.

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More like in the northern and southern latitudes where things are cooler than they are at the east pole anyway.
I don't understand this. What do you mean by "East Pole"? The subsolar point? The intersection of equator and terminator east of the subsolar point?

What is it in Space that makes you think that latitude makes a significant difference? Did I miss something big, or are you bringing in more recent discoveries from other sources?

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Which overlooks the biggest issue impacting habitability there of course, the periodic blasts of radiation from the sun.
Yes, those x-ray flares and solar ion storms are nasty. The stripping of atmosphere by the intense solar wind is bad, too. Then there is the prolonged intense heating of the inner system as the star develops along the Hayashi track. Given those three challenges, the fact that the sunlight is rich in IR and poor in photosynthetically active radiation (meaning slow photosynthesis and delayed development from "Ocean" type to "Garden" type) is almost trivial by comparison.

There is still a lot to find out about the planets of M and late K -type dwarf stars twenty years ago it was still not clear that they were common at all! but I'm not really holding my breath on the discovery of planets habitable by humans in the system of anything cooler than about K5.
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Old 03-13-2020, 10:29 PM   #16
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Default Re: [Space] Climate & habitability of tide-locked planets

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Originally Posted by Agemegos View Post
Have you calculated the Habitability scores and Dayside temperatures of a lot of randomly-generated tide-locked worlds? I've had quite a lot of them drop out of my generator, and I don't like the look of them. The ones with high Habitability scores have uninhabitably hot day sides.
Well of course your random generator produces a huge number of tidelocked worlds with much higher average temperatures than the by the book world generation technique. When I randomly generate systems for smaller M-Class dwarfs, on the rare occasion when I actually get a stanard-sized world in the habitable zone they are always on the cold side of things. I've never gotten a "normal", much less a "warm"

Quote:
The thing that brought this issue to the forefront of my mind just now is trying to generate a society (and adventure) for Gliese 370 II "Persatuan", the highest-Habitability tide-locked world in Central Sector of my randomly-generated universe. It has an average temperature of 27 C, and a calculated dayside temperature of 67 C.
That's a bit on the warm side. I'm guessing that you rolled really high on hydrosphere.

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I don't understand this. What do you mean by "East Pole"? The subsolar point? The intersection of equator and terminator east of the subsolar point?
Why would the intersection of the equator and the terminator be east of the subsolar point?

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What is it in Space that makes you think that latitude makes a significant difference? Did I miss something big, or are you bringing in more recent discoveries from other sources?
The average temperature is roughly the seasonal average at a latitude of 45 degrees. Obviously it will be colder on average as you approach the poles and warmer as you get closer to the equator. But in the case of a tidelocked worlc that will actually be a roughly circular band of tolerable temperature all around the sun-side. Frankly I didn't didn't consider the possibility of a warm or even tropical average temperature sun locked world because it's apparently impossible to randomly generate them using the unmodified gurps method.

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There is still a lot to find out about the planets of M and late K -type dwarf stars — twenty years ago it was still not clear that they were common at all! — but I'm not really holding my breath on the discovery of planets habitable by humans in the system of anything cooler than about K5.
Oh, honestly M0s aren't that bad.
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Old 03-13-2020, 10:59 PM   #17
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Default Re: [Space] Climate & habitability of tide-locked planets

One possibility for a tide-locked planet that is not often talked about is a planet with a 90-degree axis that 'rolls' as it revolves around its star (the north pole is always pointed at the star, the south pole is always pointed away, and the equator is the terminus). In recent reports though, there are some rather massive differences in temperature between the night side and dark side of directly observed tide-locked planets with very dense atmospheres. I am not sure if the mathematical models of previous research actually reflect reality.

For example, there is a planet where molten iron replaces water, and it possesses a day side with temperatures 150% higher than the night side. The molten iron vaporizes in the dayside, rains down at the terminus, and freezes on the night side. Of course, iron gas is much less reflective than water vapor, so that would drive up the day side temperatures, but it is an amazing planet.

Another thing to consider is the placement of continents. A 'polar' ocean would moderate day side temperatures through evaporation and reflective cloud cover, as well as being an effective method of transferring heat to the night side. Conversely, a 'polar' continent would be an inferno, and the rest of the planet would likely be uninhabitable outside of the terminus because there would be insufficient heat transfer. Of course, the tidal bulge caused by the tide locking would likely result in a polar continent, meaning that most tide-locked planets are likely uninhabitable outside of the terminus.
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Old 03-14-2020, 12:05 AM   #18
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Default Re: [Space] Climate & habitability of tide-locked planets

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Originally Posted by David Johnston2 View Post
Why would the intersection of the equator and the terminator be east of the subsolar point?
A tide-locked world would have 6 identifiable poles:

- The subsolar and anti-subsolar points (which I like to call the day-pole and night-pole). The equator passes through both these points.
- The north and south poles. These point up and down from the planet's orbital plane, and are located on the terminator. The planet's rotational axis, due to its orbiting its star, passes through these poles.
- The east and west poles (for lack of clearer names). These would be the leading and trailing points in the planet's orbit. They'd be on the intersections of the equator and terminus. They might be significant for climatic patterns to gather at due to Coriolis forces.

These points might vary due to orbital eccentricity, libration, etc.

I'm not sure if the academic literature uses these terms exactly though.
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Old 03-14-2020, 12:57 AM   #19
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Default Re: [Space] Climate & habitability of tide-locked planets

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Originally Posted by David Johnston2 View Post
Well of course your random generator produces a huge number of tidelocked worlds with much higher average temperatures than the by the book world generation technique.
Why? It's a pretty damn faithful implementation.
Quote:
The average temperature is roughly the seasonal average at a latitude of 45 degrees. Obviously it will be colder on average as you approach the poles and warmer as you get closer to the equator. But in the case of a tidelocked worlc that will actually be a roughly circular band of tolerable temperature all around the sun-side.
Ah. I thought you were talking about people living on tide-locked worlds, because that was the topic. Referring to the geography of a normally-rotating world was rather confusing in context. So you were saying that people live in the mid-latitudes rather than at the east pole. What's the east pole on a normally-rotating planet?
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Why would the intersection of the equator and the terminator be east of the subsolar point?
There's got to be one such intersection to the east and the other to the west.
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Old 03-14-2020, 12:58 AM   #20
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Default Re: [Space] Climate & habitability of tide-locked planets

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Originally Posted by AlexanderHowl View Post
One possibility for a tide-locked planet that is not often talked about is a planet with a 90-degree axis that 'rolls' as it revolves around its star (the north pole is always pointed at the star, the south pole is always pointed away, and the equator is the terminus).
No, that is not a possibility, because it violates conservation of angular momentum.

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Another thing to consider is the placement of continents.
Yes. Hu & Yang acknowledged that in their paper, and I mentioned it in my original post. You will have continents and oceanic ridges and shallows deflecting currents, and mountain ranges deflecting winds, besides a patchwork of land where the models assume water. It's hard, perhaps impossible, to estimate the effects at the level at which the Space planet generator works, but I suppose that one near-universal is that geographical features will result in a greater variety of surface conditions than the models indicate.
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A 'polar' ocean would moderate day side temperatures through evaporation and reflective cloud cover, as well as being an effective method of transferring heat to the night side. Conversely, a 'polar' continent would be an inferno, and the rest of the planet would likely be uninhabitable outside of the terminus because there would be insufficient heat transfer.
According to the modelling by Hu & Yang that I linked in the OP the westerly current at the equator is most important for advecting heat from the light side to the dark side. Flows in the polar regions are much less significant.
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Of course, the tidal bulge caused by the tide locking would likely result in a polar continent
The mantle material conforms to the same geoid as the oceans do. Water conforms more quickly than mantle material does, producing the tides where applicable, but the mantle goes to the long-term average equipotential surface. You don't get a band of continents around the equatorial bulge of a planet like Earth, and you won't get continental bulges at the subsolar and anti-solar points on a tide-locked planet. Remember that tides affect the oceans.
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meaning that most tide-locked planets are likely uninhabitable outside of the terminus.
What do you mean by "outside the terminus"?

I really don't believe that the twilit band near the terminator is likely to be highly habitable, for reasons that I have explained at length. That area is going to be poor in light to drive photosynthesis (a problem exacerbated by the low photosynthetic efficiency of the light from cool stars). And the modelling all shows that it is likely to be arid, too.

The results of global circulation modelling suggest to me that the best prospect for settling on tide-locked worlds is on the sunlit faces of comparatively cool ones.
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