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Thread: Contact binary gas giant planets

  1. #1
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    Contact binary gas giant planets

    I'm asking to see if there is any chance that a pair of gas giants might form under any circumstances so that they are in contact with each other, and share the outer layers of their envelopes. I've made a speculative model of such a pair, and if this type of planet is at all possible, I can include it in our worldbuilding project
    (if not, not).
    Contact Binary worlds

  2. #2
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    It seems not much less likely to me than two red dwarfs forming a contact binary (shared envelope). So far, to my knowledge, we haven't seen one of these, but it should be possible. I think the way it would need to happen would be for them to form a close binary with a third nearby companion that interacts in a way to reduce the orbit of the inner pair.
    Forming opinions as we speak

  3. #3
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    Could the third component be a relatively nearby red dwarf primary star?

    Another mechanism I've seen for the formation of contact binary stars is magnetic braking. Maybe this could help bring a pair of gas giants, already in orbit around each other, close enough to share an envelope, or a surface.

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    Simple tidal interaction with the parent star can remove angular momentum from binary gas giants until they collide. I imagine the contact phase would be short-lived, in astronomical terms, because of the fierce tidal effect at that point.
    Useful formulae here.

    Grant Hutchison

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    Millions, or billions of years? Millions would make them rare. I expect that the last stages, when the planets are actually in contact, might only last a million years or so. That means you'd need to examine many thousands of systems to find just one example.

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    Quote Originally Posted by eburacum45 View Post
    Millions, or billions of years? Millions would make them rare. I expect that the last stages, when the planets are actually in contact, might only last a million years or so. That means you'd need to examine many thousands of systems to find just one example.
    The tidal evolution timescale depends on how far they are from their parent star.
    But the contact period is going to be a short part of a much longer evolutionary process. So to have contact persists for a longer time, you need to prolong all parts of the tidal evolution process (by placing your binary a long way from its parent star) - so then you're most likely to encounter contact binaries a long way out from elderly stars, and unlikely to encounter them in closer orbits.

    Grant Hutchison

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    You could get a feel for the time scale by treating your contact binary as one big gas giant, and plugging data into the tidal spindown formula (which I think we discussed before). The rotation speed would obviously be the orbital period of the contact binary, and you can plug in an estimate of the moment of inertia from the separation of centres.
    Obviously the binary will persist for only a fraction of the time it would take the corresponding "planet" to spin down to synchrony, but you could get a feel within an order of magnitude or two.

    Grant Hutchison

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  10. #10
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    Quote Originally Posted by eburacum45 View Post
    That's the formula in this post, I believe.
    https://forum.cosmoquest.org/showthr...54#post2405754
    That's the one.

    Grant Hutchison

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