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Exomoon in Permenant Shade?

  1. Jan 25, 2015 #1
    Hello, thank you for reading my post.

    For a story I am working on I am wondering about the plausibility of some star and planet configurations. I have done some web research on my own and learned a lot, but for some things I think my vocabulary and knowledge is too limited to search for exactly what I want.

    What I would like to have is a large gas planet (or a brown dwarf) as the lone planet circling a star. Orbiting this planet would be an exomoon, perhaps the size of Titan (a larger, spherical moon).

    Is it possible for a stable orbit to exist between the planet and moon where the moon is either

    1. Orbiting slowly enough so that it is locked between the star and planet, causing a solar eclipse on part of the planet's surface permenantly, or
    2. Behind the planet in a line with the star, causing the moon to be permenantly hidden from the star (permenantly in a lunar eclipse)?

    Is this the same as an object being in the gas planet's L1-L2 points? I have read that these points are not very stable, especially for large objects, for earth anyway, so I wonder how large a moon could exist in the L1-L2 points of a very large planet/brown dwarf. Ideally, for the story, it would be on the range of 3+ billion years.
    Last edited: Jan 25, 2015
  2. jcsd
  3. Jan 25, 2015 #2


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    Yes, that's the only case where it could work. But these are not just 'not very stable' - they're unstable. You can't hold a moon there in the same way as you can't make a needle stand on its tip. The satellites that use those points must correct their trajectories with thrusters to stay where they are.

    I'm afraid it can't be made to work. The scenario you described has been discussed in this section of the forum many times. Try looking through old threads for more details.
  4. Jan 25, 2015 #3
    I was worried that was the case. I tried mostly searching words like eclipse or lagrange/lagrangian, but I'll look around some more. Thanks for the quick reply.

    Edit: already found some. Lots of interesting discussion.
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