Questions About Planets

Discussion in 'World Building' started by D. Gray Warrior, Sep 12, 2017.

  1. D. Gray Warrior

    D. Gray Warrior Master

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    I am building a world that is like earth for the most part, but with a few differences, and I want to know how that would affect life on this planet.

    The planet orbits two stars in figure 8 pattern, which I'd expect would lead to a period of the year where there is daylight on both sides of the world. How long would it take for it to complete an orbit? Assuming both suns are yellow medium stars like our sun, could I just take our year and double it?

    How would life be different if we orbited a red dwarf or a star that is somewhat brighter than our sun, assuming the planet is just the right distance to be able to support life?
     
  2. FifthView

    FifthView Istari

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    Maybe someone with more knowledge of astrophysics could provide a more definitive answer....but my suspicion is that this figure eight orbit is impossible.

    Zeta Reticuli is a binary system with two suns like ours, but they are approximately 3,750 AU apart. The Earth is only 1 AU from our sun. So you would need two stars only approx. 2 AU apart, in which case I'd guess the planet would be way, way overheated—it would receive 2X the energy Earth gets—or you'd need two stars much farther apart but this would cause the planet to go into deep freeze for very long periods. Perhaps because it receives 2X the energy, you could make the distance between the stars more than 2 AU. But still, they'd need to be very close in astronomical terms.

    Two stars orbiting each other that close would probably need to orbit fast just to keep them from colliding. I.e., in order to have a somewhat stable arrangement. And I suspect that a planet orbiting anywhere in that neighborhood would be torn apart by gravitational forces from the two in their tug-of-war on the planet. If not torn apart, there'd probably be very strong gravitational forces anyway leading to severe geological events. (This is me just stretching my imagination to its limits; I'm kinda pulling this outta my...)

    Plus, my guess is that only one star would capture the planet into its orbit, because there's always a tug-of-war and the stronger and/or nearest star to the planet would prevail. I can't see how this could be disrupted allowing for the other star to "steal" the planet, leading to a figure eight. We've found cases of an exoplanet circling two binary stars, I think, but the two stars orbit each other and the planet orbits around both of them—no figure eights. All other cases of binary systems have the planet circling only one of the stars.

    Edit: I just found this article which announced the closest-orbiting binary system discovered with planets (for each star), and the distance between the two is 360 AU: Astronomers Discover Three Giant Exoplanets in Twin Binary System | Astronomy | Sci-News.com
     
    Last edited: Sep 12, 2017
  3. Vaporo

    Vaporo Lore Master

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    Sorry, but I don't think that this system could exist in a physically realistic way. Orbits simply do not go in figure eights.

    However, there is something called the L1 gate, which is a midpoint between two orbiting bodies where it is slightly easier to pass out of the orbit of one body into the other. The closest thing that I thing that you could realistically get would be an object that periodically passes through the L1 gate. Such an object would be extremely unlikely. In order for it to be truly periodic instead of just randomly transferring in and out, the Sun, moon, and stars would literally have to align to make it work. I can almost assure you that the orbit would not be stable in the long term.

    Actually, that could be an interesting story. The planet transfers between the two stars at random, so the people of the world believe that different gods are in charge depending on which star they're orbiting. You could do a lot with this.

    Of course, this is fantasy. If you really want this kind of orbit, you can have it. It just means that you won't be following any real physical laws. In that case, you can make the orbital period be anything you want, since you're making up the rules.
     
  4. D. Gray Warrior

    D. Gray Warrior Master

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    I was kicking around a similar idea where the two suns are actually gods who are also twins with a sibling rivalry and they compete against each other for the planet and worshipers. This leads to religious tensions and holy wars between the inhabitants.
     
  5. FifthView

    FifthView Istari

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    I think you could still create a world that has two suns from the inhabitants' view, without needing to posit a figure eight orbit.
     
  6. Vaporo

    Vaporo Lore Master

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    Certainly! The planet could be orbiting a binary star system where the stars are extremely close together and the planet is relatively distant. Think of the two suns on Tatooine in Star Wars. That could realistically exist.
     
  7. pmmg

    pmmg Scribal Lord

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    I'm gonna sign on with those that say figure eight would be almost impossibly unlikely, but....given an infinite universe...



    >How would life be different if we orbited a red dwarf or a star that is somewhat brighter than our sun, assuming the planet is just the right distance to be able to support life?

    To be in the habitable zone would probably mean this would have a negligible effect (though superman might be affected if the sun was red). If the sun was brighter, we would likely have to be further away to stay habitable, and conversely for the red dwarf.
     
  8. CupofJoe

    CupofJoe Valar Lord

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    The Hellicona series of books by [the recently late and still lamented] Brian Aldiss is all about a planet in a binary system. Well that's not all it is about... It's a bit strange in places...
    It's been a while but I remember there was a very long periodicity to the binary orbit. Long enough for the last time to be come more myth and legend than history...
     
  9. Corwynn

    Corwynn Journeyman

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    I agree with the others that a figure-8 orbit between two stars in a binary system seems highly implausible. As others have pointed out, a lot of factors would have to align for the planet to be passed back and forth each time. There is also the fact that any planetary orbits of each star that cross over each other would eventually result in them colliding. In a best-case scenario, the larger planet would capture the smaller one(s) as a moon.

    However, the aforementioned Helliconia series provides a good alternative. In that series, planets orbit the smaller sun, Battallix, which in turn orbits the bigger sun, Freyr. The orbit of Battallix around Freyr is slow and elliptical, resulting in global seasons for the planets that last centuries; Summer when Battallix is closest to Freyr, and Winter when they are farthest apart. In addition to this, you could add elliptical orbits of planets around a star (which would lead to the same thing on a smaller scale), and/or regular seasons caused by axial tilt to create really odd and complicated climate patterns. How this affects life on a planet will depend on how long the cycle takes and how intense it is. In a faster or less extreme shift, creatures could migrate or tough it out until better weather comes around again. But if the cycle is longer or more extreme, you may see mass extinctions when the seasons turn, or perhaps life adapts to survive a wide range of climates.

    With a brighter or dimmer star, the habitable zone will move outward or inward respectively. A habitable planet orbiting a very bright star would have a longer orbit, and therefore longer years and longer seasons (if there are any). A habitable planet orbiting a red dwarf might have to be so close in that it becomes tide-locked, with permanent day and night sides. However, it could avoid this if it is the moon of a "hot Jupiter".
     
  10. FifthView

    FifthView Istari

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    Occasionally questions like the OP's lead me to ask whether the natives on the planet have any concept of astronomy as a science. If the inhabitants don't have telescopes, somewhat advanced mathematics, schools and intellectuals studying astronomy...then needing an exact scientific basis for the way suns, moons, and stars appear in the sky is somewhat unnecessary–for fantasy stories, at least.

    It's a little like GRRM's world for ASOIAF. Fans and amateur and pro theorists have tried to guess what kind of solar system that world has, one that would lead to extremely long winters and summers but without an exact recurring duration for those seasons. But GRRM doesn't give that information, and the inhabitants of that world have no clue about these things and don't seem to even imagine a scientific reasoning for the seasons.

    I think that a fantasy story doesn't necessarily have to include scientific reasoning for the appearance of these heavenly bodies. Perhaps some stories require a little possibility of such reasoning, just to avoid creating an obviously impossible or ridiculous world, but even then the reasoning may not need to be made explicit in the story.
     
  11. psychotick

    psychotick Dark Lord

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    Hi,

    You don't need a figure eight pattern to have the planet have both sides in the sun at the same time periodically. Normal binary systems have two stars orbiting one another at a distance greater than the planetary orbits around the star / s. So if our system was a binary the second star would be out somewhere beyond the orbit of Pluto. Now because inner planets generally orbit faster than outer ones, the inner planets will at different times have the second sun behind them while at other times it may be on the other side of the sun they orbit.

    Next the year length is anything you want it to be. So for example, Mercury's year is 88 Earth days, and Pluto's year length is 250Earth years. But a note - the length of a year relates to the orbit of a planet around its sun. Having a second sun doesn't change that, and because the second sun's year length is likely to be vastly longer than the planet's you won't actually see the second sun move in the sky. Likelihood is that decades will pass and you'll see the second sun move half an inch across the sky.

    Third - a red star. This has issues. First, everything on Earth survives because of photosynthesis. Luckily red light is one of the spectra that plants can use. But the plants will be different colours because of the different chlorophyll. Similarly if the main sun, ie the one your planet orbits, is the red sun, all your colours will be off. Probably because creatures / people would have evolved on the world with the light it won't matter so much. Then as was pointed out elsewhere, if the sun is red, it likely provides less heat / light than our yellow sun. So in order for the planet to get enough it has to be in a closer orbit to the sun, which means shorter years and quite possibly a smaller planet. Also your people would likely be very different because orbitting closer to the sun, means they have a different radiation exposure. So they may have thicker more heavily pigmented skin etc.

    Cheers, Greg.
     
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