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Thread: fictional: tidally locked planet situated between two stars...

  1. #1
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    Question fictional: tidally locked planet situated between two stars...

    Hi folks,

    I'm preparing a speculative fiction novel on a sapient species (a crustacean) that is confronted by a visiting alien spaceship. For reasons of narrative I'd prefer the crustacean's planet to be situated between the two stars of a binary system. As I am too imaginative to keep this extreme setting within the (stretched) boundaries of what's realistically possible, I turn to you guys for help.

    A first question I have is in regard to distances. I need the planet situated close/far enough to the stars for liquid water to be present, while their opposing gravitational pull should not pull the planet apart. My research led me to decide on a G-type and M-type star with the tidally locked planet being closer to the latter. You think that is a good setting, or are more suitable star type combinations around? And what distance range would you suggest?

    A second question is on geography and climate. As liquid water is doubtable on the M-type facing hemisphere I picture it entirely as bare rock dessert. The G-type facing hemisphere however I picture as one big ocean. The terminator zone, then, where almost all life forms have their habitat, begins from the ocean side as a landscape of dramatic fjords that over the course of thousands of kilometres change into chasms, canyons and ravines to end well into the M-type hemisphere by the abrupt high elevation of the rock dessert.
    I derived at this particular landscape by assuming that low, vapour rich winds would prevail in a G to M-hemisphere direction, carrying rain clouds with them that would shed their waters as they travel 'south'. The high elevation at the end prevents these clouds to reach the rock dessert. Heated up there, air would rise with high dry winds blowing back 'north'. Does all this seem plausible? Am I overlooking some major issues?

    Although I'd obviously prefer thought-through reactions, all replies, suggestions, comments are greatly appreciated. Oh, and more questions are likely to come... Greetings, Gufghur.

  2. #2
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    You appear to be envisioning a planet that is synchronized with both stars. That would be at the Lagrangian L1 point, which unfortunately is unstable. You would need station-keeping thrusters to keep any sort of small body at or near that point.

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    A planet tidally locked between two stars, maintaining (I assume) a balancing point between their two gravities, would not last. It would almost immediately fall toward one of the two stars and either drop into it or go on a (hyperbolic?) swing around it.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Quote Originally Posted by Roger E. Moore View Post
    A planet tidally locked between two stars, maintaining (I assume) a balancing point between their two gravities, would not last. It would almost immediately fall toward one of the two stars and either drop into it or go on a (hyperbolic?) swing around it.
    Neither of those options are likely - it would need a fairly large delta v to either decelerate and drop into the star, or accelerate to hyperbolic velocity.
    Instead, you'd get an initial elliptical orbit around one or other star, but strongly perturbed by the other. Repeated interactions with the two stars might eventually serve to eject the planet from the system, but that wouldn't happen on the first pass.

    Grant Hutchison

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    If a suggestion is okay, you might want to try an Earth-sized moon of a gas giant or brown dwarf that is also circling a sunlike star, or a Trojan arrangement with a star, gas giant/brown dwarf, and planet. Not quite the same but works better mechanically.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Quote Originally Posted by Roger E. Moore View Post
    If a suggestion is okay, you might want to try an Earth-sized moon of a gas giant or brown dwarf that is also circling a sunlike star, or a Trojan arrangement with a star, gas giant/brown dwarf, and planet. Not quite the same but works better mechanically.
    I think it breaks the scenario, however. The moon will likely tidally lock to the gas giant. If it somehow tidally locked to the star pair, it would end up with the stars 60 degrees apart in its sky, not on opposite sides of the planet.

    Grant Hutchison

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    How about a planet tidally-locked to a small red dwarf, which is orbiting a G-type star just outside the habitable zone?

    If the red dwarf is small enough, this would mean that the planet's year around the red dwarf would be on the order of one Earth day, and the heat from the G-Type star would be enough to give the planet a fairly Earth-like diurnal cycle. One side of the planet would be a bit too warm, while the other side would be a bit too cold- but the atmosphere could transfer heat from one side to the other to normalise temperatures a bit.

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    A figure 8 orbit might be very remotely possible, if the system only had 2 stars and only the one planet, and the planet had no moon, and the orbit of the plant around both stars, intersected the L1 point exactly. However the tiniest permutation by a fourth body, even something like a 100m asteroid passing near the planet, would case that orbit to fail, and it would likely assume an elliptical orbit around the largest star. It's mathematically possible, but the reality is such a clean three body systems would probably never form. Also the planet would not be tidal locked to either star.

    If you want a tidally locked planet around a red dwarf, that would be possible. Red Dwarf are actualy more active then a class G star, in that they have much more frequent CME's. The Red dwarf then could be in a far orbit (> 25000 AU's) around a binary star system consisting of a white dwarf and a red giant.

    Perhaps the aliens are visiting because of the fact that a white dwarf/red giant binary will eventually produce a type 1a super nova at some point, and they detected radio signals...


    You could have a lot of fun with such an arrangement.

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    Thanks for the clear answer, even though it's bad news. Stubborn as I am, though, I'll try to tweak the solar system according to the other replies and see whether a more plausible setting pops up. If you'd care to provide them, I'd highly appreciate your opinions also on those.

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    Thanks for the clear answer. I'm afraid station-keeping thrusters are way beyond reach of the local sentient's technological advancement. I'll try tweaking the solar system according to some of the other replies to see whether any more plausible settings pop up. If you'd care to provide them, I'd highly appreciate your opinions also on those.

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    Quote Originally Posted by Roger E. Moore View Post
    If a suggestion is okay, you might want to try an Earth-sized moon of a gas giant or brown dwarf that is also circling a sunlike star, or a Trojan arrangement with a star, gas giant/brown dwarf, and planet. Not quite the same but works better mechanically.
    Thanks for your reply. Your suggestion motivates me to hang on to my scenario for just a little while longer, even though Grant Hutchison seems to doubt your suggestion. I now wonder whether adding any number of other planets -- in orbit around either of the stars or even both -- help stabilize my planet in its fixed position between the two stars?

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    Quote Originally Posted by grant hutchison View Post
    I think it breaks the scenario, however. The moon will likely tidally lock to the gas giant. If it somehow tidally locked to the star pair, it would end up with the stars 60 degrees apart in its sky, not on opposite sides of the planet.

    Grant Hutchison
    Thank you for replying, although you quite managed to douse the short flare of hope I felt after reading Roger's suggestion. However, do you see any other constellation, with added planets orbiting either one or even both suns, that would help stabilize my planet between the two stars?

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    Quote Originally Posted by eburacum45 View Post
    How about a planet tidally-locked to a small red dwarf, which is orbiting a G-type star just outside the habitable zone?

    If the red dwarf is small enough, this would mean that the planet's year around the red dwarf would be on the order of one Earth day, and the heat from the G-Type star would be enough to give the planet a fairly Earth-like diurnal cycle. One side of the planet would be a bit too warm, while the other side would be a bit too cold- but the atmosphere could transfer heat from one side to the other to normalise temperatures a bit.
    Yes. This is the scenario I had in mind. But according to other replies, it seems highly unlikely.

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    Quote Originally Posted by dgavin View Post
    If you want a tidally locked planet around a red dwarf, that would be possible. Red Dwarf are actualy more active then a class G star, in that they have much more frequent CME's. The Red dwarf then could be in a far orbit (> 25000 AU's) around a binary star system consisting of a white dwarf and a red giant.

    Perhaps the aliens are visiting because of the fact that a white dwarf/red giant binary will eventually produce a type 1a super nova at some point, and they detected radio signals...


    You could have a lot of fun with such an arrangement.
    Thanks for your reply and the useful suggestions. If a fixed position between two stars isn't at all possible I need to choose between a planet orbiting both stars of a binary or a red dwarf tidal lock. I like your idea of drawing alien attention by means of a far away binary. Do I assume correctly you mean the alien is likely to observe the binary because it might turn super nova and that way detect the local sentient's radio signals?

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    Quote Originally Posted by grant hutchison View Post
    I think it breaks the scenario, however. The moon will likely tidally lock to the gas giant. If it somehow tidally locked to the star pair, it would end up with the stars 60 degrees apart in its sky, not on opposite sides of the planet.
    The original scenario was to have each side of the planet locked to a different star. If orbiting a brown dwarf and tidally locked to it, one side of the planet is warmed by infrared heat radiation. The other side would get irregular illumination and heat from the other star, sustained by heat transfer from the locked side. True, it's not the same, but interesting anyway.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    Quote Originally Posted by dgavin View Post
    A figure 8 orbit might be very remotely possible...
    This seems unlikely, much too unstable even in the short term. No matter how much I like the utods' homeworld orbiting various stars in The Dark Light Years, it still won't work.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    Thanks all for your contributions. I had replied to each individually but must have done something wrong for these messages fail to show. So lets retry with a bundled version:

    Little doubt remains the setting I had envisioned is impossible. Apart from eburacum45's reply -- which reformulates exactly the setting I had in mind -- consensus seems to be that a planet & red dwarf tandem orbiting a G-type star is unstable. Would you all agree?

    The thruster idea is inventive but the level of technological advancement of the local sentient prevents me from using it. Before giving up entirely, however, I wonder whether adding one or more planets to the binary system could help stabilize 'my' planet at its Lagrange point between both stars? My reasons for holding on so stubbornly are interlinked. One: it gives the sentient as small as possible an incentive for deriving at clock and calendar time (ageing will give a sense of duration, but that's about it: no diurnal rhythm, no circadian rhythm, no night sky), and two: because of this, the sentient is unlikely to focus on the universe -- it does not even know there is one -- but on its own scale of existence and smaller: chemistry, biology, genetics, the works.

    So, if any other setting allows for a solar system with a planet of perpetual daylight, I'd love to hear it!

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    Quote Originally Posted by gufghur View Post
    I had replied to each individually but must have done something wrong for these messages fail to show.
    gufghur

    You did nothing wrong. As part of our anti-spam measures, posts from "newbies" go into a 'moderation queue' and have to be approved by a moderator before they are shown. This usually happens in a couple of hours, but sometimes on weekends it takes longer. They have all now been approved and you are past the needs-approval point.
    At night the stars put on a show for free (Carole King)

    All moderation in purple - The rules

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    This is the forum at its best. A sci-fi writer is seeking advice on keeping the physics as realistic as possible and is getting useful feedback.

    ETA: On second thought I would suggest going with the L1 point. My dad was comfortable with science fiction that took liberties with a single point of physics. Disregarding the L1 instability in your story would be mild compared with warp drive in Star Trek, a highly regarded series.
    Last edited by Hornblower; 2019-Jan-14 at 02:33 PM.

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    Unless the planet is itself an (enormous) spacecraft capable of maintaining a position between two stars...

    ...or dark matter is being manipulated to keep the planet in position between the stars, for a reason that needs to be figured out...
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    Just a reminder: The creatures in the proposed story do not have the technology to do any such station keeping, as I think I understand it.

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    Then we are back to Isaac Asimov's "Nightfall".
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    Quote Originally Posted by gufghur View Post
    Thank you for replying, although you quite managed to douse the short flare of hope I felt after reading Roger's suggestion. However, do you see any other constellation, with added planets orbiting either one or even both suns, that would help stabilize my planet between the two stars?
    Roger seemed to have two suggestions. One is to place your planet in tidally-locked orbit around one body which orbits a central star. That's a viable situation, but it means your planet would have a normal, if slow, day-night cycle. The other is to place your planet at a Trojan point of two stars, which would be stable, but would place the stars 60 degrees apart in the sky, rather than on opposite sides of your planet, which is what you need. (The planet would also tend to wander in a tadpole or horseshoe orbit relative to the the stable Trojan point, and so would see movement of the two stars in its sky even if it had some sort of average tidal lock.) There isn't a way to stablize the unstable libration point between the two stars, unfortunately - any additional masses in your system would simply perturb your planet out of position faster.
    I think a tidal lock on a single star, with the night hemisphere facing empty space, is probably the nearest you can get in a physically realistic situation. Numerous SF authors have used that scenario before. (The tidally locked planet orbiting a gas giant orbiting a star, as mentioned by Roger, is another pretty common SF scenario.)

    Grant Hutchison
    Last edited by grant hutchison; 2019-Jan-14 at 05:11 PM.

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    Trojan points need at least a 25 to 1 ratio of mass between the two heavier bodies.

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    This is the forum at its best. A sci-fi writer is seeking advice on keeping the physics as realistic as possible and is getting useful feedback.
    Now that's a comment I like . And a rewarding collaboration it is... I've already learned quite a bit from researching on you guys' suggestions.
    So it appears some options remain for a perpetual daylight planet: 1A; force-feed the L1 position without mentioning its impossibility, 1B; same as 1A but reveal dark matter manipulation in a sort of deus ex machina development, 2; a Nightfall setting.

    Despite it being the easiest way out, at the moment 1A appeals to me the least. But you never know... if the other options turn out too complex I might go for it anyway. So I'll need decide between 1B and 2. And between those, I like 1B most. As this book is part of a series (three to six books in chronological order plus two 'sidekicks' --> yes, I'm frightfully ambitious), I doubt not the layered narrative has room for dark matter manipulation. It 'only' needs considering the potentialities and consequences for the series' overall structure.
    As far as 2, the Nightfall option is concerned: I found through wikipedia a link to a pdf wherein the scientific plausibility of the story is researched: https://arxiv.org/pdf/1407.4895.pdf.

    So, having made up my mind (somewhat) on using a deus ex machina L1 scenario, have you guys ideas/suggestions on distances? Based on ..., I came up with a habitable zone of 0.400 to 0.430 AU between planet and M-type and of 1.000 to 1.200 between planet and G-type. Does that sound reasonable for:

    a) weak enough gravitational forces so the planet is not pulled apart
    b) liquid water to exist
    c) a harsh climate that nonetheless could allow sentient life to evolve
    d) any other aspect my ignorance blinds me for

    Your replies are greatly appreciated.

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    Oops! See I forgot to provide the link to the habitable zone calculations:

    https://depts.washington.edu/naivpl/...files/hz.shtml

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    Quote Originally Posted by eburacum45
    How about a planet tidally-locked to a small red dwarf, which is orbiting a G-type star just outside the habitable zone?
    If the red dwarf is small enough, this would mean that the planet's year around the red dwarf would be on the order of one Earth day, and the heat from the G-Type star would be enough to give the planet a fairly Earth-like diurnal cycle. One side of the planet would be a bit too warm, while the other side would be a bit too cold- but the atmosphere could transfer heat from one side to the other to normalise temperatures a bit.
    Quote Originally Posted by gufghur
    Yes. This is the scenario I had in mind. But according to other replies, it seems highly unlikely.
    No; I must have miscommunicated somehow, but this scenario is one of the reasonable ones. Your planet could be tidally locked to the red dwarf, but still receive a respectable amount of heat from the larger star.

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    Actually now I notice that dgavin mentioned a similar scenario, but pointed out that the red dwarf star could be prone to flares and mass ejections. This might affect the habitability of such a planet considerably. But not all red dwarfs are flare stars.

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    Chaos causing noob, I am.

    My apologies for causing such confusion. When I initially read up on the tidal lock concept, I understood it as being similar to stationary orbit. Later I noticed this was a mistake but the damage was done: the image had settled in my head. So when I read:
    How about a planet tidally-locked to a small red dwarf, which is orbiting a G-type star just outside the habitable zone?
    , I imagined a red dwarf & stationary planet tandem orbiting a G-type, which is the setting I was looking for and have learned here is impossible for lack of stability.

    I'll try being more careful in the future...

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    No need for apology, the question generated a lot of interest. Good luck!
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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