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Thread: Elliptical orbits and the tail wagging the dog

  1. #1
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    Elliptical orbits and the tail wagging the dog

    I picked up the latest Sky & Telescope at lunch today and devoured the cover story about stars with planets. As usual, I found an interesting point in the article worth sharing.

    It seems that inner planets with highly elliptical orbits are more common around massive stars than smaller stars, like our Sun. The theory presented to explain this discrepancy was fascinating, to say the least. (Note to Spoons: this would qualify as fascination #9).

    Stars like our Sun have 3 general layers: the stellar core, a stable radiative layer, and a turbulent convective layer. Apparently larger stars are so hot that they dispense with the convective layer altogether.

    Anyway, the theory goes that all stars can form inner planets with highly elliptical orbits. But smaller stars can have their outer convective layer "torqued" into a new alignment by a sufficiently large inner planet while the axis for the core and radiative layer remained unchanged. In essence, the tail is wagging the dog.

    Larger stars without a convective layer are immune to this effect.

    Fascinating!

  2. #2
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    Quote Originally Posted by baric View Post
    I picked up the latest Sky & Telescope at lunch today and devoured the cover story about stars with planets. As usual, I found an interesting point in the article worth sharing.

    It seems that inner planets with highly elliptical orbits are more common around massive stars than smaller stars, like our Sun. The theory presented to explain this discrepancy was fascinating, to say the least. (Note to Spoons: this would qualify as fascination #9).

    Stars like our Sun have 3 general layers: the stellar core, a stable radiative layer, and a turbulent convective layer. Apparently larger stars are so hot that they dispense with the convective layer altogether.

    Anyway, the theory goes that all stars can form inner planets with highly elliptical orbits. But smaller stars can have their outer convective layer "torqued" into a new alignment by a sufficiently large inner planet while the axis for the core and radiative layer remained unchanged. In essence, the tail is wagging the dog.

    Larger stars without a convective layer are immune to this effect.

    Fascinating!

    I would not call the arrangement "wagging the dog" as much as I would say---"Monday-morning quarter backing" via synergistic interplay

  3. #3
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    I would expect massive stars to tend to have planets with more elliptical orbits. Circularization of elliptical orbits takes time, and more massive stars just don't live as long.

    I'm not sure what process you're describing with torquing the convective layer, though...you appear to be describing how a large planet around a small star can affect the internal structure of that star, not why planets around large stars are more likely to have higher orbital eccentricity. My best guess is that this realignment somehow greatly increases tidal effects that circularize the orbit of the planet, but I don't see why this would be so. Also, my understanding is that only the lowest mass stars are fully convective, and larger mass stars tend to have more layered structures, with little mixing of the layers until the end of their life.

  4. #4
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    But wouldn't this torquing interaction with a convective layer in itself serve to circularize the orbit, so one of the reasons why there are fewer small stars with highly elliptical planets is that highly elliptical orbits circularize faster around those?
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  5. #5
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    Quote Originally Posted by cjameshuff View Post
    you appear to be describing how a large planet around a small star can affect the internal structure of that star, not why planets around large stars are more likely to have higher orbital eccentricity.
    I think this theory is suggesting that this discrepancy is an illusion. Since we are comparing the planets' eccentricities to the visible rotation of the star (the outer layer only), we are missing all of the smaller stars whose internal rotation (core & radiative layer) are out of synch with their innermost planet.

  6. #6
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    Quote Originally Posted by HenrikOlsen View Post
    But wouldn't this torquing interaction with a convective layer in itself serve to circularize the orbit, so one of the reasons why there are fewer small stars with highly elliptical planets is that highly elliptical orbits circularize faster around those?
    A process that confines the angular momentum gained to the shell being torqued wouldn't necessarily increase overall torque...it seems more likely to reduce it by allowing the parts of the star most affected by the planet's tidal forces to partially synchronize with its orbit. And an effect that increases the overall coupling between the two would have to outweigh the more obvious increases from larger star diameter and mass.

    Some process that greatly increases tidal coupling seems to be the only answer, but I'm not clear on how that would occur.


    Quote Originally Posted by baric View Post
    I think this theory is suggesting that this discrepancy is an illusion. Since we are comparing the planets' eccentricities to the visible rotation of the star (the outer layer only), we are missing all of the smaller stars whose internal rotation (core & radiative layer) are out of synch with their innermost planet.
    What does orbit eccentricity have to do with the rotation, surface or internal, of the star?

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