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Thread: Can an exoplanet, by definition, orbit another exoplanet?

  1. #31
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    Quote Originally Posted by StupendousMan View Post
    If you are willing to view the eclipse from the surface of a moon, rather than the surface of a planet, there are plenty of combinations which yield "perfect" solar eclipses in our Solar System.

    http://spiff.rit.edu/richmond/asras/...pse.html#place
    Brilliant!

  2. #32
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    So, a moon can be a planet, too!


    https://arxiv.org/abs/1810.11060

    Catching a planet: A tidal capture origin for the exomoon candidate Kepler 1625b I

    Adrian S. Hamers, Simon F. Portegies Zwart (Submitted on 25 Oct 2018)

    The (yet-to-be confirmed) discovery of a Neptune-sized moon around the 17.2 Jupiter-mass planet in Kepler 1625 puts interesting constraints on the formation of the system. In particular, the relatively wide orbit of the moon around the planet, at ~40 planetary radii, is hard to reconcile with planet formation theories. We demonstrate that the observed characteristics of the system can be explained from the tidal capture of a secondary planet in the young system. After a quick phase of tidal circularization, the lunar orbit, initially much tighter than 40 planetary radii, subsequently widened gradually due to tidal synchronization of the spin of the planet with the orbit, resulting in a synchronous planet-moon system. Interestingly, in our scenario the captured object was originally a Neptune-like planet, turned into a moon by its capture.

    QUOTES: In this Letter, we argue that, although the (hypothetical) moon puts interesting constraints on the early dynamical evolution of the planet-moon system, its existence is not surprising. According to our understanding, the current moon was born a planet in orbit around the star 2MASS J19414304+3953115. This planet turned into a moon upon its tidal capture with the more massive planet. Further tidal interaction circularized and widened the orbit due to angular-momentum transfer from the spin of the planet to the orbit until synchronization. For convenience, we will keep referring to “planet” for the giant planet Kepler 1625b, and “moon” for its companion Kepler 1625b I, although both should be called planet according to this scenario.

    We argued that the planet-moon system in Kepler 1625 is the result of the tidal capture of a secondary planet by the primary planet around the star. As a result of scattering induced by convergent migration in a disk, the two planets approached each other on a low-energy hyperbolic or parabolic orbit, and passed each other within . 2.5(Rp + Rm). The tidal dissipation induced in this encounter subsequently led to the capture of the minor planet by the primary planet, turning the former into a moon. The first tidal encounter led to a highly eccentric and wide orbit, and for capture to be successful, the apocenter should have remained within the planet’s Hill sphere. The orbit then circularized to a tight orbit, in ∼ 10 yr. Over a much longer time-scale of ∼ Gyr, the primary planet subsequently transferred its spin angular momentum to the orbit, widening the latter until synchronization. We find that the primary planet must have had a primordial spin of at least ∼ 40% of critical rotation in order to deposit sufficient angular momentum into the planet-moon orbit to be consistent with the current orbit. We expect that the current orbit evolves very slowly, and that both the planet and moon are in almost synchronous rotation with the orbit. These captures are probably not uncommon, being some twice as often occurring as planet collisions. However, the precise frequency for this process to operate remains unclear. We expect that moon formation from tidal capture is not uncommon (see also Ochiai et al. 2014), and probably comparable to the number of planet collisions or ejections. The capture must have occurred early in the planetary system’s evolution (more than a Gyr ago) to allow tidal dissipation to synchronize the system to its current orbit. Our scenario can be tested by measuring the spins of both planet and moon, which should be synchronous with the orbit, and along the same axis as the orbital angular momentum of the planet-moon system.
    Do good work. —Virgil Ivan "Gus" Grissom

  3. #33
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    Quote Originally Posted by Roger E. Moore View Post
    So, a moon can be a planet, too!
    Or rather a planet can become a moon...

    CJSF
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  4. #34
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    So where does that leave Pluto and Charon? Dwarf moon?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  5. #35
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    Quote Originally Posted by Noclevername View Post
    So where does that leave Pluto and Charon? Dwarf moon?
    Pluto is a dwarf planet with a regular moon.

  6. #36
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    Quote Originally Posted by DaveC426913 View Post
    Pluto is a dwarf planet with a regular moon.
    But Pluto and Charon are within closer size ratio to each other than many of these exo-not-moons are to their planets. So...?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  7. #37
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    Quote Originally Posted by Noclevername View Post
    But Pluto and Charon are within closer size ratio to each other than many of these exo-not-moons are to their planets. So...?
    Sorry, I don't follow. How would exo-bodies change the status of bodies in our own system, which have already been defined?

  8. #38
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    Quote Originally Posted by DaveC426913 View Post
    Sorry, I don't follow. How would exo-bodies change the status of bodies in our own system, which have already been defined?
    What difference does it make where? If the status of a body as either planet or moon depends on its size relative to what it orbits, why should that be limited to only outside the Solar System, or not applied to smaller bodies? I'm just trying to find some consistency here in how to apply these ever changing rules.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  9. #39
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    So, the choices would be between 1] dwarf planet with moon, and 2] binary system.

    Both are valid.

    I guess it's a binary dwarf planet system.

  10. #40
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    Revised version of paper already mentioned, not sure what was changed. Perhaps of interest here.

    https://arxiv.org/abs/1810.11060

    Catching a planet: A tidal capture origin for the exomoon candidate Kepler 1625b I

    Adrian S. Hamers, Simon F. Portegies Zwart (Submitted on 25 Oct 2018 (v1), last revised 24 Nov 2018 (this version, v2))

    The (yet-to-be confirmed) discovery of a Neptune-sized moon around the ~3.2 Jupiter-mass planet in Kepler 1625 puts interesting constraints on the formation of the system. In particular, the relatively wide orbit of the moon around the planet, at ~40 planetary radii, is hard to reconcile with planet formation theories. We demonstrate that the observed characteristics of the system can be explained from the tidal capture of a secondary planet in the young system. After a quick phase of tidal circularization, the lunar orbit, initially much tighter than 40 planetary radii, subsequently gradually widened due to tidal synchronization of the spin of the planet with the orbit, resulting in a synchronous planet-moon system. Interestingly, in our scenario the captured object was originally a Neptune-like planet, turned into a moon by its capture.
    Do good work. —Virgil Ivan "Gus" Grissom

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