Calling all orbital mechanics specialists in the playground!

[Aotrs Commander]

I have a vision: Andorlaine, the Evenstar, a world of perpetual evening in living memory, where every several centuries, night falls, plunging the world into lasting darkness.

Before I can go any further, though, I need to work out the orbital mechanics as to how that can actually work.

Background:

My current campaign world, Dreemaenhyll, is now over a decade old, and while still far from complete, is functional and fairly extensive. But I'm once again feeling that urge to start a new thing from scratch - this time, another stab at something I've often wanted to try but never really managed: an entirely alien fantasy world. No humans, no familiar anything, just a whole world populated by alien lifeforms at typical fantasy tech-levels. Dreemaenhyll went as far as chucking out the 3.5 bestiary entirely, with everything re-imagined from scratch. This time, I want to (try to!) go even further.

I am still at the high-end concept stages, and I'm as much going to be solidifying my own ideas as I am bouncing my thoughts off you. So, the basic premise is that the world is evening and every 500 years or so, it goes dark (for umpteen hundred years or something). An event that is believed by some to be legend and so on and so on. Now, because I am a pedant above all else, my first port of call is to work out exactly how such an effect is achieved, and what effects it will have on the ecology of a world. Because this is, like, the single most important factor of the world that will determine how the rest of it works. So what I'm looking is anyone who has a better grasp of orbital mechanics and such to help me hash out a first-order feasibilty for the planetary structure. (And yes, it DOES have to be planets and stellar bodies!)

There are a couple of basic options, as I see it.

Option 1: Very slow planetary rotation (day-length > year). This would acheive the effect, yes, but raises other problems, in that the change would be gradual and constant. It would be known about, and would likely give rise to a very nomadic sort of civilisation, always moving ahead of the terminator between day and night, which is not reall what I think is ideal for a fantasy-sort of environment. In addition, of course, it means half the planet is boiling and the other half freezing (nevermind what other effects it has.) (It also means that there's also a perpetual morning, afternoon and such in living memory...)

Option 2: Tide-locked planet. This would give you a land of eternal darkness on one side, a boiling watse on the other, and the only habtiable land in the terminator. However, you then need something else to block the sun - presumably some stellar body over extended periods (like a jupiter-sized body in relatively close orbit for example, or maybe a cloud or nebulae or something.) That's sort of a tempting idea - but tide-locking opens up a host of new factors - no seasons, for one and no "day" as a short length of time.

So with the both simple solutions not being quite right, I'm looking at getting more complex - as in some of my first ideas, making it a binary star system. I am toying with the idea of a rotating planet following a figure-8 orbital pattern between two suns (so you'd only have true "night" when it was at the top or bottom and one sun blocked the other). That, of course, has problems of it's own, namely that you'd have to have them quite distant to make them not over-heat the planet and to make it "evening...!" all the time. And I don't know how completely ludicrous that sort of orbit would be (whether we're into "Discworld is more feasible" territory or just merely "it's a stretch but maybe it could work...")

Other ideas I'm considering are whether you could have the planet orbit not on the usual orbital plane (which might give you more of an "evening" angle of deflection.

At this stage, I'm still not 100% sure of what I want myself: I'm hoping using the playground as a sounding board and a discussion of orbital mechanics will make me ask the right questions to concrete it in my own mind.

Any help or suggestions (or even somewhere that might be populated with people who might have more of an idea!) would be greatly welcomed.

[Mando Knight]

Tidally locking a planet to its star also limits where you can put it... and "where you can put it" means ridiculously close (as in, literally a sun-scorched wasteland close) due to the gravitational gradients. Slowed planetary rotation is basically the same thing as a tidally-locked body, just "incomplete."

A stable figure-8 orbit might be possible, but that's a 3-body problem, and isn't conducive to being analytically solvable.

Anything Jupiter-sized would have to be ridiculously close to eclipse the star completely, to the point that you'd probably end up with an Endor or Yavin situation... and a 3-body problem. You won't find an orbital harmonic that is "off" for centuries and then "on" for centuries.

There is no orbital mechanic I can think of that would allow centuries of "evening" passing into centuries of "night" while allowing the planet to be habitable anywhere.

[Aotrs Commander]

Tidally locking a planet to its star also limits where you can put it... and "where you can put it" means ridiculously close (as in, literally a sun-scorched wasteland close) due to the gravitational gradients. Slowed planetary rotation is basically the same thing as a tidally-locked body, just "incomplete."

A stable figure-8 orbit might be possible, but that's a 3-body problem, and isn't conducive to being analytically solvable.

Anything Jupiter-sized would have to be ridiculously close to eclipse the star completely, to the point that you'd probably end up with an Endor or Yavin situation... and a 3-body problem. You won't find an orbital harmonic that is "off" for centuries and then "on" for centuries.

There is no orbital mechanic I can think of that would allow centuries of "evening" passing into centuries of "night" while allowing the planet to be habitable anywhere.

Fudge baskets.

(The tidal-locking idea came specifically from the mini-series Alien Planet, wherein the scientists theorised that a planet tide-locked to a red dwarf coud support life.)

Still, I have learned... actually quite a lot, since googling 3-body problem lead me to a fairly extensive wiki trawl through the solar system, which included learning they'd found another TNO even further out than Sedna.

Some ideas still floating around though - one thing that occurred to me wa a white dwarf, perhaps with a particularly dense planetary nebulae obscuring the planet every so often... But I'm not quite sure planetary nebulae Work That Way.

[Aotrs Commander]

If you aren't completely adverse to a tidally locked world, you could have the world wobble during its orbit, so that parts of the terminator swing in and out of day/night. You could also have glacial masses near the terminator force the crust to shift, plunging one region into eternal day, another into eternal night, and others into the day/night zones.

Hmmm. That has some potential, doesn't it? I'm not sure how volcanism would play a part in a tidal-locked world, but that could be a part too. That sort of thing could be an irregular occurance...

Oooooh. To steal an idea from one of my earliest sci-fi authors, Douglas Hill... In Day of the Starwind, the main character is on a planet that every so often comes into proximity with another planet in it's system, not close enough for a collision, but close enough to make the planet wobble and create terrible wind storms.

You could maybe adapt that idea and have another stellar body that passes close-ish that excerts a change in the gravitational pull that causes the crust to slip (either slowly or quickly, maybe?) There is some suggestion that crust-slipping has happened on Earth (according to a documentary I watched once anyway). And given what I've looked up on n-body problems, solar systems are quite chaotic systems anyway, so it wouldn't have to be at regular intervals. Heck, maybe even the pull of a more distant companions star or something could be the effecting factor, if the dark side has a larger build-up of ice or something...

Definitely some potential.



Edit: Gliese 667 C and Gliese 667 Cc in particular, on the other hand, make an interesting case for a "evening world" by just dint of a dimmer star (but more radiation in the infra-red range, making for a warmer, but maybe not as bright, world.) Also, this artist's impression of Gliese 667 Cc settles it - whatever, nominal evenstar world is definitely being in a multi-star system, because binary secondary suns is just visually awesome!
http://upload.wikimedia.org/wikipedia/commons/5/5f/Gliese_667_Cc_sunset.jpg

[sktarq]

It won't get you all there way to your goal but a tidally locked moon in an orbit that is on a shifting plane around its planet may be a place to start. Over ages the orbital plane shift brings the moon further in and out of the planets shadow which causes at least the planetward side to have near darkness for much of the time. With a reflective planet the amount of reflected "Earthshine" could well be in the "evening" levels and if the sun is farther away to compensate dim light would be the norm.

[Rakaydos]

Another idea is putting the planet in a wide eliptical orbit. With a sufficently bright star, the habitable zone could have a year a hundred thousand days long, half of which in in the inner habitable zone, half of which is in the outer habitable zone. (or "darkness")

Also there's the posibility of a manufactured occlusion- some ancient civilization put a massive sunshade up to protect against the "summer", eclipsing the sun. (in this case, the outer habital zone is "evening" and the shaded inner zone is "darkness") The sunshade would be in a 1:2 harmonic orbit with the planet, always blocking the sun when it got too hot.

[lightningcat]

Its kinda a ridiculous thought, but maybe make it a four body system.

The sun is a dim, but hot star. The world is stuck at the L1 position of a reflective gas giant. And every 500 years a second gas giant passes between the world and the sun, blocking both the light to the world, and to the reflecting gas giant.
I can make the idea work with a flashlight, beach ball, disco ball and orange. But how close the planets would actually have to be to work on a celestial scale, I don't know.

[Mando Knight]

Its kinda a ridiculous thought, but maybe make it a four body system.

The sun is a dim, but hot star. The world is stuck at the L1 position of a reflective gas giant. And every 500 years a second gas giant passes between the world and the sun, blocking both the light to the world, and to the reflecting gas giant.
I can make the idea work with a flashlight, beach ball, disco ball and orange. But how close the planets would actually have to be to work on a celestial scale, I don't know.

Problem: Lagrange points are particular solutions found from constraining the 3-body problem. Adding in a 4th body means that there aren't any Lagrange points. Making the 4th body large and close (which it has to be in order to eclipse the star) means that you're dealing with a 3-body problem even if you assume that the habitable planet is sufficiently small as to make no discernable effect (which is, incidentally, one of the constraints used to find the Lagrange points or including orbital effects from the Moon).

Rakaydos's suggestion of a brilliant star and a much more eccentric orbit might be possible. It'd be a large star, so that the habitable zone would be far away and so that the orbits would be very long. Some kind of blue supergiant, probably: far brighter than our sun, and so much larger. They're relatively short-lived, though, so life in such a stellar system would imply divine creation over evolution.

[Aotrs Commander]

Still thinking hard - and researching this. Nowt like a real skull bender...!



Problem: Lagrange points are particular solutions found from constraining the 3-body problem. Adding in a 4th body means that there aren't any Lagrange points. Making the 4th body large and close (which it has to be in order to eclipse the star) means that you're dealing with a 3-body problem even if you assume that the habitable planet is sufficiently small as to make no discernable effect (which is, incidentally, one of the constraints used to find the Lagrange points or including orbital effects from the Moon).

So not likely then... Darn...


Rakaydos's suggestion of a brilliant star and a much more eccentric orbit might be possible. It'd be a large star, so that the habitable zone would be far away and so that the orbits would be very long. Some kind of blue supergiant, probably: far brighter than our sun, and so much larger. They're relatively short-lived, though, so life in such a stellar system would imply divine creation over evolution.

Too short for my tastes - I'm trying to avoid any acts of creation (divine or technological).



Maybe... What about, thinking from lightningcat's idea, a habitable moon tide-locked to a large reflective gas giant, which is itself tide-locked to dim, but hot star (possibly a dwarf star or some sort, perhaps red or white?) Perhaps a planetary nebulae (or several) perioidcally passes through a closer orbit every so often, and sometimes when it does, it co-incides with a dull period in the dwarf star, rendering near darkness. Maybe the nebulae would be of a type that reflects visible light more than heat...? Or... thinking about the binary system thing, maybe the whole dwarf star system passes through a dense debris cloud left by the extinction of a larger star, in far orbit of the binary pair...?

[Tirunedeth]

Still thinking hard - and researching this. Nowt like a real skull bender...!




So not likely then... Darn...



Too short for my tastes - I'm trying to avoid any acts of creation (divine or technological).



Maybe... What about, thinking from lightningcat's idea, a habitable moon tide-locked to a large reflective gas giant, which is itself tide-locked to dim, but hot star (possibly a dwarf star or some sort, perhaps red or white?) Perhaps a planetary nebulae (or several) perioidcally passes through a closer orbit every so often, and sometimes when it does, it co-incides with a dull period in the dwarf star, rendering near darkness. Maybe the nebulae would be of a type that reflects visible light more than heat...? Or... thinking about the binary system thing, maybe the whole dwarf star system passes through a dense debris cloud left by the extinction of a larger star, in far orbit of the binary pair...?

Hmm. A planetary nebulae are on the scale of an entire stellar system, so the latter idea is a bit more plausible. I'm not sure it would be dense enough, though, to actually block enough light to make a difference, and I think you'd still have a simple periodic variation.

Something that might work is to have a planet tidally locked to a red dwarf star, both orbiting an irregular variable giant star. The giant star delivers a significant amount of light to the planet most of the time, but occasionally reduces in luminosity dramatically. Transport of heat from the light side of the planet keeps the planet from freezing over during these dark periods, but flares from the red dwarf prevent

It seems there is even a real class of variable star which might work fairly well: the R Coronae Borealis variables. R Coronae Borealis is a supergiant star which occasionally builds up clouds of carbon in its photosphere, decreasing its visual luminosity by about a factor of 400 (although, interestingly, its infrared luminosity changes relatively little). The typical time between these episodes is rather shorter than what you want for RCB, on a scale of months to years, but the class of stars is sufficiently poorly understood that you should be able to get away with having one with longer times between dim periods.

If this idea sounds like it might work, I can try to come up with a few more specifics if you'd like.

[Aotrs Commander]

Hmm. A planetary nebulae are on the scale of an entire stellar system, so the latter idea is a bit more plausible. I'm not sure it would be dense enough, though, to actually block enough light to make a difference, and I think you'd still have a simple periodic variation.

Something that might work is to have a planet tidally locked to a red dwarf star, both orbiting an irregular variable giant star. The giant star delivers a significant amount of light to the planet most of the time, but occasionally reduces in luminosity dramatically. Transport of heat from the light side of the planet keeps the planet from freezing over during these dark periods, but flares from the red dwarf prevent

Prevent...? Prevent what? Forest fires? Infection? Bananas? The people must know!

Ahem.


It seems there is even a real class of variable star which might work fairly well: the R Coronae Borealis variables. R Coronae Borealis is a supergiant star which occasionally builds up clouds of carbon in its photosphere, decreasing its visual luminosity by about a factor of 400 (although, interestingly, its infrared luminosity changes relatively little). The typical time between these episodes is rather shorter than what you want for RCB, on a scale of months to years, but the class of stars is sufficiently poorly understood that you should be able to get away with having one with longer times between dim periods.

If this idea sounds like it might work, I can try to come up with a few more specifics if you'd like.

Yeah, please - that does sound promising!

[avr]

How about the Sauron solution? Periodically the volcanoes open up and block out the sun. This could be triggered by astronomical events e.g. If another planet or star in the system has a millennia long, elliptical orbit and passes close enough for its gravity to influence your planet.

[sktarq]

I had a bit of a weird idea. what if a hot star was still forming its planets in a gas disk when it captured the home world of your adventures. So the world would be swamped in gas in a similar orbit as the gas builds up or filters away the visible light changes. Now I wouldn't call this a stable world on a geologic or even evolutionary time scale but if you don't need it to be THAT long a time period for a good game to run-A few thousand years.

[Mando Knight]

I think the long-term variable star might be the best bet mentioned so far.

It's even possible that stars like that exist, since centuries on/centuries off would be too long of a scale to expect any direct scientific/quantized observation on: 500 years ago would be before Galileo, and the numeric values for magnitude weren't formalized until the mid 1800s.

[Aotrs Commander]

How about the Sauron solution? Periodically the volcanoes open up and block out the sun. This could be triggered by astronomical events e.g. If another planet or star in the system has a millennia long, elliptical orbit and passes close enough for its gravity to influence your planet.

Too mundane. Volcanism is easily understandable from the perspective of the populace, at least in terms of what's going on, if not why.

(And also would probably drop the temperature far too much. It is volcanism that is often partly responscible for mass extinctions: that sort of evernt every five hundred years would probalbly render it completely unihabitable.)


I think the long-term variable star might be the best bet mentioned so far.

It's even possible that stars like that exist, since centuries on/centuries off would be too long of a scale to expect any direct scientific/quantized observation on: 500 years ago would be before Galileo, and the numeric values for magnitude weren't formalized until the mid 1800s.

So, having a good look at R Coronae Borealis... It's a yellow supergiant - sorta - (estimated 100 times solar radii), but only 0.8 solar mass, with ten thousand times the luminosity and a temperature of something between about the sun (data below) to 1.2 times the sun (wiki). It's got a 900k dust cloud starting 100 stellar radii from the star (which, if I'm understanding right, would be 200 solar radii from the centre of the R CrB), starting from 102 stellar radii from the centre of R CrB and dragging 25 light years behind it.

Okay.

To start with, assume it's just orbiting the star. According to wiki, circumstellar habitable zone is centred on very expromiximatly the square root of luminoisty (muliplied by the sun's distance of 1.34 AU). (So, in this case 185 AU, or there abouts.) Long way... Plenty enough to have be orbiting a red dwarf star. From what I can gather, yellow supergiants also don't last very long, 'cos they're shedding mass like no tomorrow and/or heading towards red giant status... But also they have far greater masses than R CrB does. Guh.

Okay then. I'm probably trying to work out with approximations, questions that would actually require professional scientists (maybe even Steven Hawking...!) to scratch their heads...



So... A planet tide-locked to a red dwarf star1 (and moderately distant from it), which orbits an RCB class star. Most of the light (and presumably, heat) comes from the RCB star, except during periods when the luminosity dims. The red dwarf keeps the planet's heat ticking over, as if were, during the occlusion periods (like putting the oven on to keep something warm), provides some dim light. The RCB star is slightly unusual by even RCB standards, having a long burn, with extended periods between its dark periods that are about twos orders of magnitude longer than "usual". We will assume that the RCB results from, as theoried, the collision of two white dwarf stars, previously a stellar binary. The greater distance from the red dwarf (by habitability standards, which is really, really close!) might reduce some of the problems caused by early flaring, though by this point in the system's life-cycle, you might expect it to have past that stage anyway.

Thought: Unless the red dwarf is itself tide-locked to the RCB star, you'd preumsably get a sort of day/night cycle as the dwarf-planet system orbits the RCB star Do we need the reflective gas giant in to keep the true "night" from occurring?


For artistic lisence, the system also contains a pair of bright binary stars, which orbit with the RCB star. (Essentially, at one point it would have been a system with two pairs of binary stars, one pair of which had a companion red dwarf star.)



Does that sound sort of vague right/plausible? Would we need a reflective gas-giant in there anywhere?

Would there be/ should there be a moon? (From a purely aethetic standpoint, I'm half-thinking of something Phobos/Deimos like - a small, irregularly-shaped chunk of rock fairly close. Though whether it would be able to orbit a tide-locked planet, or just be a static feature in the sky I don't know.)



Then this brings the big question! How long is a "day" in the activity cycle sort of things (obviousy there is never a true "day"). The first obvious one would be a "waning", i.e. with orbiting the RCB (and or the binary stars in the distance), there will be some sort of variance to the ambient light (the red dwarf is going to at some point have an eclipse of the light of the RCB star - unless the world is above or below the orbital plane of the two stars, which seems a bit odd), which might be a good first time indicator. (As I guess the rate of the orbital speed of the planet around the red dwarf could be petty much anything.)

One presumes the apparent orbital path of the binary stars would be very slow, hundreds or thousands of years, as the RCB and the binary pair orbit each other, and their position relative to the RCB star wouldn't change much.

Yes? No? Is this making any sense at all? (Damn, this is HARD!)



R CrB is much too far for direct parallax measure, but similar stars in the Large Magellanic Cloud (the largest of our nearby satellite galaxies, whose distance is well known) have luminosities around 19,000 times that of the Sun. If similar, "R" would have to be between 4500 and 5000 light years away. An estimated temperature of 5000- 5500 Kelvin would give it a radius of some 100 times solar, which is half the distance from Earth to Sun. Not an ordinary high-mass supergiant, R Cor Bor carries a weight of but 0.8 solar, its great luminosity coming from a highly advanced, but still mysterious, evolutionary state. The spectrum reveals only a trace of hydrogen, leading us to conclude that its outer layers are made largely of helium, the star's original hydrogen envelope long-since expelled. Supporting the idea, R CrB is especially rich in carbon, which is the result of helium fusion. The high luminosity drives heavy mass loss estimated at 100,000 times the rate in the solar wind. About one percent of the outbound gas condenses into carbon dust, but in discrete "puffs." if one comes off in the line of sight, the "soot star" then hides from our view. In support, we see an inner surrounding dust shell with a temperature of just under 900 Kelvin that starts 100 stellar radii from the star and that is encased in a huge, cold "fossil cloud" some 25 light years across. Why the star huffs and puffs is unknown. Moreover, the dust should be made where the gas sufficiently chills, at least 20 stellar radii out. However, the spectrum shows it to be made 10 times closer, perhaps as a result of outbound shock waves. The origins of such stars are unknown. One theory is that while developing into an ordinary white dwarf (the core of a once more-massive giant), the innards suddenly suffered a violent onset of helium fusion (to carbon) that expanded the outer layers to supergiant proportions. Another is that R Cor Bor stars are the result of mergers of two white dwarfs in orbiting binary systems, with the same effect. Friendly R CrB is clearly trying to tell us something. We just don't yet know what. But have fun waiting for its disappearing act.



1As opposed to a Red Dwarf star, which wuld be like, Chris Barrie or something2.

2I'm sorry.

[Tirunedeth]

Prevent...? Prevent what? Forest fires? Infection? Bananas? The people must know!

Me from completing my sentences, evidently. :smallredface:

I was trying to say that flares prevent the inhabitants of the planet from making use of the light side of the planet, to maintain the whole perpetual twilight thing. However, I'm thinking now that temperatures alone might be enough to prevent habitation of the light side, so I'm leaving flares as more of an optional thing.

Anyway, here's an attempt at some physical properties for this system. Mass, luminosity, and radius are in multiples of the values for the Sun; distances are in AU. I've heavily based the primary star on R Coronae Borealis, aside from the mean time between dimming events.
Primary: R Coronae Borealis variable star
Spectral type: G supergiant
Mass: 0.8
Luminosity: 20,000
Radius: 140
Drops in visual luminosity by a factor of ~1600 at irregular intervals, typically several centuries apart (~40% of luminosity is in visual spectrum).

Secondary: Red dwarf star
Spectral type: M6 dwarf
Mass: 0.1
Luminosity: 0.0009
Radius: 0.15
Orbits Primary at ~270 AU (period ~5000 years)

Planet
Orbits Secondary at ~0.0345 AU (period ~178 hours)
Ice caps from ~90 to 140 degrees west of substellar point, at +-45 degrees of latitude.
Water flows form the ice caps, evaporates as it travels to the day side.

Secondary delivers about 1000 W/m2 to Planet, while Primary delivers about 370 W/m2 (for comparison, the Sun delivers ~ 1366 W/m2, so the combined total is roughly the same).

Primary appears to rise and set every 7 1/2 days due to the rotation of Planet.

Primary has an angular diameter of about 1/4 of a degree as seen from Planet (about half the angular diameter of the Sun or Moon as seen from Earth), while Secondary has an angular diameter of about 4 degrees.

Realistically, Secondary's orbit would be eccentric, so there would also be some predictable variability in the amount of light it delivers, with a period of around 5000 years.

Flares can make the light side of Planet uninhabitable, although that can probably be achieve by temperature alone (and it might be worthwhile to have the light side be marginally habitable).

If you want high vulcanism, add a gas giant or two in resonance with Planet to drive its orbital eccentricity up.

Note that, as I said earlier, R CrB stars aren't well understood (there were only roughly 40 known in 1996, and maybe 100 known now), so there is some debate as to how they form. The current hypotheses are either a final stage of evolution for certain red giants before they become white dwarfs, or the merger of two white dwarfs. Either process could be problematic for habitability of a nearby planet, depending on the timescales required for such evolution to happen and how energetic the process is. Also note that R CrB stars may have very short lifetimes, on the scale of tens of thousands of years.


From what I can gather, yellow supergiants also don't last very long, 'cos they're shedding mass like no tomorrow and/or heading towards red giant status... But also they have far greater masses than R CrB does. Guh.

Okay then. I'm probably trying to work out with approximations, questions that would actually require professional scientists (maybe even Steven Hawking...!) to scratch their heads...

R CrB stars are rather bizarre, rare, and poorly understood, as I've already mentioned. I'm honestly kind of surprised to find out anything like them exists, although given some of the things out there I probably shouldn't be.


Would there be/ should there be a moon? (From a purely aethetic standpoint, I'm half-thinking of something Phobos/Deimos like - a small, irregularly-shaped chunk of rock fairly close. Though whether it would be able to orbit a tide-locked planet, or just be a static feature in the sky I don't know.)

Maybe, maybe not. Those same tidal forces that have locked Planet to Secondary would tend to destabilize the orbit of anything around Planet. However, the Hill sphere of Planet is around 150,000 km, which seems like it ought to be enough for a small moon or two. On the other hand, the Hill sphere is merely an approximation, and things inside it can still be unstable (I've tried moving the Moon to Venus in a gravity simulator, and that system ultimately becomes unbound).


Thought: Unless the red dwarf is itself tide-locked to the RCB star, you'd preumsably get a sort of day/night cycle as the dwarf-planet system orbits the RCB star Do we need the reflective gas giant in to keep the true "night" from occurring?

Hmm. Yeah, this is a bit of a problem. I'll think about it a bit more, but I'm honestly not sure how to get the eternal twilight (with no day/night cycle) with this setup. Can your concept of the world be altered somewhat, to light peaking at twilight instead of eternal twilight?

[Aotrs Commander]

Me from completing my sentences, evidently. :smallredface:

I was trying to say that flares prevent the inhabitants of the planet from making use of the light side of the planet, to maintain the whole perpetual twilight thing. However, I'm thinking now that temperatures alone might be enough to prevent habitation of the light side, so I'm leaving flares as more of an optional thing.

Anyway, here's an attempt at some physical properties for this system. Mass, luminosity, and radius are in multiples of the values for the Sun; distances are in AU. I've heavily based the primary star on R Coronae Borealis, aside from the mean time between dimming events.
Primary: R Coronae Borealis variable star
Spectral type: G supergiant
Mass: 0.8
Luminosity: 20,000
Radius: 140
Drops in visual luminosity by a factor of ~1600 at irregular intervals, typically several centuries apart (~40% of luminosity is in visual spectrum).

Secondary: Red dwarf star
Spectral type: M6 dwarf
Mass: 0.1
Luminosity: 0.0009
Radius: 0.15
Orbits Primary at ~270 AU (period ~5000 years)

Planet
Orbits Secondary at ~0.0345 AU (period ~178 hours)
Ice caps from ~90 to 140 degrees west of substellar point, at +-45 degrees of latitude.
Water flows form the ice caps, evaporates as it travels to the day side.

Secondary delivers about 1000 W/m2 to Planet, while Primary delivers about 370 W/m2 (for comparison, the Sun delivers ~ 1366 W/m2, so the combined total is roughly the same).

Primary appears to rise and set every 7 1/2 days due to the rotation of Planet.

Primary has an angular diameter of about 1/4 of a degree as seen from Planet (about half the angular diameter of the Sun or Moon as seen from Earth), while Secondary has an angular diameter of about 4 degrees.

Realistically, Secondary's orbit would be eccentric, so there would also be some predictable variability in the amount of light it delivers, with a period of around 5000 years.

Awesome!


Flares can make the light side of Planet uninhabitable, although that can probably be achieve by temperature alone (and it might be worthwhile to have the light side be marginally habitable).

That and/or, if we crib some idea from Aurelia as shown on Alien Planets, a torrential rainstorm/hurricane on the light side should discourage much of serious habitation aside from fairly primitive life.


If you want high vulcanism, add a gas giant or two in resonance with Planet to drive its orbital eccentricity up.

Noted.


Note that, as I said earlier, R CrB stars aren't well understood (there were only roughly 40 known in 1996, and maybe 100 known now), so there is some debate as to how they form. The current hypotheses are either a final stage of evolution for certain red giants before they become white dwarfs, or the merger of two white dwarfs. Either process could be problematic for habitability of a nearby planet, depending on the timescales required for such evolution to happen and how energetic the process is. Also note that R CrB stars may have very short lifetimes, on the scale of tens of thousands of years.

We can maybe take a few liberties here as assume that either this particular RCB star is borderline and burns much slower or something or perhaps because it has [exotic compound] is part of it's structure, though the fact we don't understand it well gives us some room anyway. And if we go with it being recently formed by a former binary pair of white dwarf stars that finally collapsed into each other, tens of thousands of years is enough time for life to have made adjustments and to be "forever" as far as the civilisations on the planet would know.




Maybe, maybe not. Those same tidal forces that have locked Planet to Secondary would tend to destabilize the orbit of anything around Planet. However, the Hill sphere of Planet is around 150,000 km, which seems like it ought to be enough for a small moon or two. On the other hand, the Hill sphere is merely an approximation, and things inside it can still be unstable (I've tried moving the Moon to Venus in a gravity simulator, and that system ultimately becomes unbound).

Small, irregular moon is very tempting if possible, if just because, well, it would look cool! It's the sort of image in my head that puts me in mind of the sort thing you'd see in the sort of atomic sci-fi art you saw around the 1970s period. (When Stewart Cowley wrote the Terran Trade Authority stuff, which is probably singularly responscible for making me what I am today!) It would further emphasise this is alien. (Also, if I choose to use D&D as the main engine (still not 100%, I might even choose Rolemaster!) having a moon will mean less re-writing of all moon-related gubbins!)


Hmm. Yeah, this is a bit of a problem. I'll think about it a bit more, but I'm honestly not sure how to get the eternal twilight (with no day/night cycle) with this setup. Can your concept of the world be altered somewhat, to light peaking at twilight instead of eternal twilight?

As mentioned, I don't think it would hurt to have some variety in the brightness (as for an RPG - among other things - a day-anologue time period is is going to be required), rather than perpertual unchanging twilight. (Otherwise, working out time is going to be a real bugger! No day, no seasons... Where would your primitive place start from!) So, more "no true night except when the primary goes dark" as opposed to "literally always twilight." I sort of figure even when the red dwarf passes in front of the primary relative to the planet, it's be more like a brief eclipse (maybe a "weekly" event if it occurs once every period.) Heck, a weekly eclipse could be your starter for ten for determining the time period. Or a subdivison... One assumes that it would happen sort of at the midday point, assuming they are in the same plane?

Actually, no, hang on, that would stupidly assume a perfect system! If there's some orbital eccentricty, the two may not always pass exactly through each other (though if I'm getting it right, the secondary would be much bigger in the sky than Sol or Luna, so proportionally the primary would have more apparent area to intersect.)



I was half planning to see if I could map something out in Celestia (or pick the brains of their forum as well) - though your data above more or less obviates the major need for that! - but the main website appears to be gone, sadly.

[Aotrs Commander]

So, I spent the last month or so talking to astrophysicists and reading all sorts of stuff and doing Hard Sums (and then, after a skullpalm, doing Hard Sum Onna Spreadsheet), so before I depart for a week, I thought I would post the initial alpha configuration of the new world (working title Andorliane the Evenstar).

The following then, is a first-order attempt at the solar system, based on my research. The single most interesting point, I think, was that as the stellar flux rises, on a planet with a thick atmosphere and lots of water vapour, the clouds that form around the subsolar point (i.e. The Bit Wot Is Closest To the Sun) actually raise the albedo and the greenhouse effect and keep the temperature cool - and also that atmospheric movement makes the temperature gradient MUCH less steep than you'd expect. (Granted, this was done primatily considering red dwarf stars, but as the wer modelling on stellar flux, the same principls apply...)



Primary: RCB Variable Star (G0)

Mass 0.8 Sol
Radius 96.25 Sol
Absolute Magnitude -5.85
Apparent Magnitude -26.78 (undimmed)
Angular Diameter 29.04'
Luminosity: 18818 Sol

Andorlaine (tide-locked to primary)
Mass 5.97219×1024 kg (= Earth)
Radius 6371km (= Earth)
Orbital Radius 15650000000km (≈ 104 AU)
Orbital Period 1197.13 years
Stellar Flux 2351.5 W/m²
Albedo (sunward side due to substeller point cloud formations) 0.54 (Green house effect 29K)
Global Mean Temperature 291.79K (19.64ºC)
Estimated Hot Pole temperature 311.79K* (39.64ºC)
Estimated Cold Pole temperature 271.79K* (-0.36ºC)

Andorliane's Moon
Mass 9.35563×1020 kg
Radius 610km
Density 984 kg/m²
Albedo 0.8
Orbital Radius 139798.15 km
Orbital Period 6.02 days
Angular Diameter 30'
Apparent Magnitude -13.98



Andorlaine from space will be dominated by a sunward cyclone. The upper atmosphere super rotates (like Venus). The lower atmosphere blows warm air at ground level from the day-side across the equatorial terminators, while cold air flows back to the day-side over the poles; thus the surface is subjected to continuous winds at about 20mph.

The global temperature is about +4ºC higher than Earth's, though temperature variation is likely to be more modally extreme over the surface.

The night-side has a large ocean, whose surface is frozen, but which contains liquid warer beneath. The sun-side is a more mountainous region, with heavy and continuous rain, and while it has some extremophile flora and fauna, there are few larger plants or animals able to survive there. The terminator between is most populated on the sunward side, where the plants get the most light. Flora closest to the subsolar point is more green, but darkens to black as it approaches the terminator, which is lit in a state of continuous evening. (The plants there are akin to what is found in Red Dwarf stars where luminosity is lower.)

Andorlaine's moon is a relatively recent capture. Composed almost entirely of water-ice, is thus has a high albedo. It appears significantly brighter than Luna, and the dim moonlight allows some limited, highly specialised and very slow-growing extremophile flora on the night side. The night-side thus paradoxically enjoys something of a day-night cycle, due to the six day orbital period of the moon.

Andorlaine is not quite completely tide-locked. Due to a combination of volcanicity (partly caused by orbital resonance of Andorliane's nearest naighboruing gas giant), dwindling angular momentum and the effect of the moon, the crust rotates almost imperceptibly, resulting in a day length of a few thousand years.

At irregular intervals with a period of 400-1100 years, Andorliane's primary dims significantly, dropping eight magnitudes in the space of a few months, due to excreted carbon dust. This period can last for up to a few hundred years (the time between dimming correlates to length of the dimming period). The precise reason Andorlaine's sun has dim periods considerably longer than other RCB variable stars, and why is has retained this stage much longer, is as yet unknown. Theories suggest exotic materials in the star are causing it to burn "slower" than a typical star. Other theories postulate that the ejected dust clouds are partical recycled by the distant orbit of the binary star companions of the primary.

During these periods, the sun is still visible, but is muted and much more red-shifted. Further, the amount of visible light reaching Andorlaine is slightly less than expected, making it appear even darker. The infrared radiation is lower, but not significantly so, so while the global climate does cool, it is a relatively mild and slow drop. This does causes a spread of the night-side flora across the terminator (and the ecology that follows) and a corresponding shift of the terminator flora towards the subsolar point.

The irregular frequency of these events is such that the native civilisations (of which there have been several) are often caught unawares, previous events being myth or legend. While the ecology of Andorlaine goes into minor upheaval during these events, primitive civilisations have all thus far been unable to cope with the additional pressures and collapse. Some of the more advanced civilisations (which arose in periods where the intervals between dimming were greatest) became aware of the phenomina, but had not reached a sufficient level to be able to adapt fast enough.

(Imagine the results of what would happen on Earth even with 21st century technology - which some of the civilisations approached - if the sun suddenly dimmed drastically.)



The moon was calcuated (eventually), based on Tethys in being mostly water-ice with a high albedo. Making a very small, irregular moon easily visible by the naked eye proved to be impractial, as you couldn't get it close enough without hitting the atmosphere - likewise, making one double the apparent size of Luna put considerable tidal acceleration on the planet. (Andorlaine's moon imparts 0.26 of Luna's tidal forces. I am assuming therefore a) the length of time it is captured will be in the same "billions of years" order of magnitude and b) there will be some tides in the sea, which will be much less noticable and possible more frequent.)

The sun being the same angular diameter was honestly more accident than design - that's just what it turned out at at the right distance for the solar flux (picked on mean global temperature) I wanted. (An averagely warmer-than-Earth planet seemed like it would stand the temperature shifts a bit better.)

The climate stuff is a bit more tenuous. The sources I looked at were, in some cases, reported results of a model, so is a bit more guesswork and extrapolation and I'm well aware that they could be wrong. (But then again, all of this is very theorhetical anyway - I have cme to realise I'm looking really at the bleeding edge of astrophysical theory! Still, let it not be said that I didn't make a spirited effort for plausibility!)



The numbers are not quite final yet - I will probably play with them a touch more to make them a bit more varied off the baselines (to make them a bit more "real" - beauty of a spreadsheet!), but I think this is my overall starter for ten, at least enough that I can spend my thinking time on holiday starting to consider what actually lives there.



Thoughts, feedback - especially on the more tenuous areas like climate, weather patterns and temperature I've been hazarding, or the sort of extrasolar flora/fauna that might arise are welcomed!



*Both these values are very, very approximate. They are pretty much eyeballed from a source that showed the minimum/maximum temperatures at up to 1400W/m², which were consistent with the sort of values as derived from the paper Bandersnath linked in post #2 (from which the greenhouse and albeo numbers are estimated), taking into account other estimation people had made which fell into a similiar range values, though I do not believe those calculations involved greenhouse effects. I thus haven't quite pulled them out of the ether, but I think they should be in the right sort of ballpark. Maybe. I have come to the conclusion though, that it may well be impossible to get the true answer without actually having some sort of appropriate computer model (since that's where the people who got their values got them!) I thus treat them as very dubious! (According to some other data, the absolute maximum for the hot pole would be √2 times the global mean temperature (which would be ≈371K), but that did not account for cloud formation.