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B Binary/ternary system

  1. Apr 3, 2016 #1
    Dear PF Forum,
    Sorry for asking two questions in a row. Because I think my next question belongs to a new thread. After all it's Sunday here.
    My friend told me about Kelt - 4A. Orbitting 3 stars.
    So, in this galaxy (and universe) if let alone to the law of gravity and chance.
    1: Can there be a system where there's no a single star in it? Every object in the system orbits the most massive object in the middle, where the most massive object mass in only, say 10 Jupiter mass?
    2: Can there be a system where all of its "planet" are stars, because all of their mass is higher than the lowest possible star mass?
    It's just out of curiousity.
    And if those 2 questions are impossible, are there any law that govern that for a system, its orbit center must have at least 1 solar mass.
    Such as the law that for the minimum natural black hole is 3 solar mass (Chandrasekar? Paully Exception Principle?)
    Or the law that the lowest possible mass for a neutron star (always natural, right :smile:) is 1.4 solar mass
  2. jcsd
  3. Apr 3, 2016 #2


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    This is not correct. The planet Kelt-4A b orbits Kelt-4A. Kelt-4a is one of three stars in a ternary system.

    "KELT-4BC is a binary star system 328±16 AU away from KELT-4A and the projected separation between KELT-4B and KELT-4C is 10.30±0.74 AU.

    An object of 10 Jupiter masses will not hold much in orbit. Compare the sun's mass with 10 times that of Jupiter.
  4. Apr 3, 2016 #3
    Okay, perhaps my statement about Kelt - 4A is not correct.
    It's just that my friend mentions it, and that makes me think about something else.
    The composition of a star system, as I asked above.

    So, you're saying that a system can't have a 10 Jupiter mass object at its centre? (with all its satellites and its satellites' satellites). It needs higher mass object to hold them together?
    Last edited: Apr 3, 2016
  5. Apr 3, 2016 #4
    If the object is less than ≈13 Jupiter masses, then it is a planet. Yes, there are rogue planets. An object greater than ≈13 Jupiter masses can start fusing deuterium, and that makes it a brown dwarf. If an object is greater than ≈65 Jupiter masses then the brown dwarf can also start burning lithium. By the time an object exceeds ≈80 Jupiter masses it should be able to achieve a temperature of 2.5 × 106°K at its core and start fusing hydrogen, thus making it a star.

    The closest brown dwarfs are Luhman 16AB at a distance of 6.59 light years. Luhman 16A has a mass between 41.9 and 52.4 Jupiter masses, and Luhman 16B has a mass between 31.4 and 52.4 Jupiter masses.

    The closest known rogue planet is CFBDSIR J214947.2-040308.9 with an estimated mass between 4 and 7 Jupiter masses at an estimated distance of 130 ± 13 light years.

    Planets are not stars. Planets stop being planets and become brown dwarfs when they become larger than ≈13 Jupiter masses and start fusing deuterium. Brown dwarfs stop being brown dwarfs and become stars when they become larger than ≈80 Jupiter masses and start fusing hydrogen.

    The Chandrasekhar Limit of 1.39 solar masses is not a "law," but rather the approximate maximum mass of a white dwarf. In most cases any degenerate star larger than 1.39 solar masses becomes a neutron star. Neutron stars have a mass between ≈1.4 and ≈3.0 solar masses. The Tolman-Oppenheimer-Volkoff Limit estimates the maximum mass of a neutron star at ≈3.0 solar masses. Degenerate objects greater than the Tolman-Oppenheimer-Volkoff Limit are called black holes.
  6. Apr 3, 2016 #5


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    No, such a system could exist and almost certainly does, but it is not a star system since there is no star. If Jupiter was ejected from the solar system it would likely take at least a couple of its moons with it and become exactly this.
  7. Apr 3, 2016 #6


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    Jupiter would take moons, but not other planets. A Jupiter size planet or larger may have moons. I'm not sure how one would confirm that any exoplanet has satellites.

    This might address some of the issues raised - Investigating Low-Mass Binary Stars And Brown Dwarfs with Near-Infrared Spectroscopy

    The sun has some brown dwarf neighbors.
  8. Apr 3, 2016 #7


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    I think that would depend on the circumstances of its ejection. While the probability is almost certainly vanishingly small, I believe it is possible for Jupiter and one or more planets to be ejected and form a system. However, it's far more likely that such a system would naturally form during the collapse of a nebula.
  9. Apr 3, 2016 #8
    Ganymede is bigger than Mercury (in radius). So why is it not a planet?
    Are satellites of "rogue planets" defined as moons or as planets?
    In case of an eclipsing system, easily.
  10. Apr 4, 2016 #9
    Why Ganymede is not a planet?
    I think the definition is this.
    Planet is a celestial body which orbits the sun directly.
    Ganymede doesn't directly orbit the sun.
    Why comet is not a planet?
    Because the orbit of a comet interects other "planet" orbit and the orbit of this comet is more extreme than the orbit of the object it intersects.

    I think the definition of moon is this:
    A celestial body which orbit another celestial body which directly orbit the sun. I don't know if you can understand my English.

    And since we are in this topic. I'd like to ask if it is possible for a case like this:
    1: The sun orbits the centre of the galaxy. I call it 1st degree
    2: Jupiter oribts the sun directly. 2nd degree
    3: Eropa orbits Jupiter. 3rd degree

    Is it possible in our solar system where there are natural celestial object 5th degree? I don't know how to express this, but I hope the mentor/advisor here knows about this degree.
    Is it possible in our galaxy or in the universe to have 5th degree, celestial object not man made/technology.
    Or in the creation of a star system, nature cannot naturally have this object which orbits another which orbit another which orbit another which orbit another.
    Just out of curiosity
  11. Apr 4, 2016 #10


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    In the case of a Jupiter type planet revolving around a star, yes, but I don't believe this works for moons around a planet, at many light years from the observer.
  12. Apr 4, 2016 #11


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    No need to make stuff up.

    From here: http://www.iau.org/news/pressreleases/detail/iau0603/


    The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

    (1) A "planet" [1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

    Possible but very unlikely. The larger body that this object would be orbiting about would be so small that it would be nearly impossible to have a stable orbit.
  13. Apr 4, 2016 #12
    Expressly applies to our Solar System alone. Need to make stuff up is as urgent as ever.
  14. Apr 4, 2016 #13
    Yes, it's entirely possible to have systems with no stars. There exist rogue planets and brown dwarfs that could potentially have many moons. There are also exotic systems like neutron stars, pulsars, black holes...

    Yes, it's also possible to have a system that's all stars. In fact, that's probably far more likely, as the gravitational interaction between the stars would likely toss any planets out of the system (turning them into the rogue systems that I mentioned before.)
  15. Apr 4, 2016 #14


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    Well, we were talking about the solar system. But if you want to talk about exoplanets, then we have already have a "working definition" according to this link: https://media4.obspm.fr/exoplanets/pages_definition/appellation-planete.html

    You can find the actual text from the IAU here: http://w.astro.berkeley.edu/~basri/defineplanet/IAU-WGExSP.htm
    A note about the working defintion from the IAU:

  16. Apr 4, 2016 #15
    So, a brown dwarf is defined as not a planet.
    Is an object that does not orbit a "star" by any definition therefore not a planet by definition?
  17. Apr 4, 2016 #16
    I would define a "moon" as any object, regardless of mass, that orbits a planet. I would define a "planet" as any celestial object that has achieved enough mass to have an hydrostatic equilibrium (is approximately round), but less than ≈13 Jupiter masses (not big enough to begin fusing deuterium). Thus, if the rogue planet CFBDSIR J214947.2-040308.9 (which is between 4 and 7 Jupiter masses) has any satellites, they would be "moons." Otherwise you would have planets orbiting planets and that doesn't seem right.

    If there are celestial objects that orbit the binary brown dwarfs Luhman 16AB, and they are large enough to achieve hydrostatic equilibrium, but not so massive as to fuse deuterium, then they would be planets. Planets do not necessarily have to orbit stars. Besides rogue planets, we know of planets orbiting neutron stars, and there are most likely planets orbiting brown dwarfs and black holes.
  18. Apr 4, 2016 #17
    The IAU definition doesn't work. By their definition the second star in a binary system is a "planet" since they didn't bother to include a maximum limit. Also, there could be no exoplanets, since they do not meet the first requirement and orbit around the Sun. Furthermore, there are no "planets" in our solar system using the IAU definition since none of them have "cleared the neighbourhood around its orbit."

    The IAU was so desperate to keep out trans-Neptunian objects like Eris, Makemake, and 90377 Sedna from the official definition of "planet" that they concocted this really bad, and purely political, definition.
  19. Apr 4, 2016 #18
    A brown dwarf is not a planet. Planets do not fuse deuterium or lithium, brown dwarfs do. When a celestial object reaches ≈13 Jupiter masses, it begins fusing deuterium. At ≈65 Jupiter masses, the object begins fusing lithium. At ≈80 Jupiter masses the object begins fusing hydrogen. When the core temperature reaches 2.5 × 106°K it will begin to fuse hydrogen, and that is when we call it a "star."

    Objects that do not orbit a "star" can be many things - a planet, a moon, an asteroid, a comet, etc., etc. These objects can orbit brown dwarfs, white dwarfs, neutron stars, and/or black holes, or not orbit anything (e.g., "rogue").
  20. Apr 4, 2016 #19


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    The IAU definition of a planet only applies to bodies within our solar system. See my other post for their working definition of an exoplanet.

    As for whether or not the IAU definition actually applies, I choose not to try to argue either way and just accept it as the best we have for the time being. Such a discussion would be off topic for this thread anyways.

    I believe that would be called either a Rogue Planet or a Sub-Brown Dwarf, but I don't know if there is a specific definition for all cases. You can read the wikipedia article for more details: https://en.wikipedia.org/wiki/Rogue_planet
  21. Apr 4, 2016 #20
    You are absolutely right, such a discussion would be off topic. I will leave it for another time.
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