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

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    Question Can an exoplanet, by definition, orbit another exoplanet?

    https://www.sciencedaily.com/release...1003142931.htm

    The above is a link to the exomoon discovery for Kepler-1625 b, suspected to exist for over a year and apparently proven by transit. However, the moon of the Jovian exoplanet is as large as Neptune.

    Should a future definition of "planet" allow for planets to also be (large, spherical) moons of other planets?

    Note that I am not rearguing the IAU definition of planet within our Solar System.
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    I think that the IAU defined "Planet" in a technical way, but no group of scientists has tried to define "Exoplanet" yet. It is a loose term, and so far, there is no particular value in specifying an enforceable boundary between Exoplanets, and bodies which just slightly don't qualify as an exoplanet. I am guessing that there won't be such a need until long after we are observing complete populations of objects down to the dwarf planet size range around many star systems. It's not a theory, it's a definition, and so we should have enough facts to make the definition be meaningful.
    Forming opinions as we speak

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    I think it matters more what the mass of the moon is compared to the planet, rather than the size of the moon. The article linked above says the moon is 1.5% the mass of the planet, so it's practically certain that the barycenter of the pair is well inside the exoplanet (which is several times more massive than Jupiter). But who knows what the IAU might do, and what that might mean for astronomers (probably not much). I agreed that Pluto needed some sort of "reclassification", given what we now know, but what the IAU came up with is silly, in my opinion. Astronomers and planetary scientists will study Kepler-1625 b with the same glee and frustration whatever the "official" name of its component parts.

    CJSF
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    I'm not going to worry about what the IAU thinks, just really interested in some opinions and thoughts on how the planet-star-moon thing will eventually shake out.

    Kepler 1625 b seemed to be a good time to think about it.

    Me, I can see several definitions that cross over each other. For example, one starting place is here.

    =============

    https://www.hou.usra.edu/meetings/lpsc2017/pdf/1448.pdf
    A GEOPHYSICAL PLANET DEFINITION
    K.D. Runyon, S.A. Stern, T.R. Lauer, W. Grundy, M.E. Summers, K.N. Singer
    pdfLunar and Planetary Science XLVIII (2017)

    We propose the following geophysical definition of a planet for use by educators, scientists, students, and the public: "A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters."

    =========

    The lowest boundary could be borrowed from this next paper.

    =========

    https://arxiv.org/abs/1603.08614
    Probabilistic Forecasting of the Masses and Radii of Other Worlds
    Jingjing Chen, David M. Kipping (Submitted on 29 Mar 2016 (v1), last revised 3 Nov 2016 (this version, v2))

    "To begin, we first define our basic criteria for a data point (a mass and radius measurement) to be included in what follows. Since our work focuses on the MR relation, all included objects must fundamentally have a well-defined mass and radius. Whilst the former is universally true, the latter requires that the object have a nearly spherical shape. Low mass objects, for example the comet 67P/Churyumov-Gerasimenko, may not have sufficient self-gravity to overcome rigid body forces and assume a hydrostatic equilibrium shape (i.e. nearly spherical). The corresponding threshold mass limit should lie somewhere between the most massive body which is known to not be in hydrostatic equilibrium (Iapetus; 1.810^21 kg; Sheppard 2016) and the least massive body confirmed to be in hydrostatic equilibrium (Rhea; 2.3 10^21 kg; Sheppard 2016). This leads us to adopt a boundary condition of M > 2 10^21 kg for all objects considered in this work."

    This is a good starting place from my perspective: you gotta be a sphere to be a planet, and this is the lowest possible end.
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    Quote Originally Posted by CJSF View Post
    I think it matters more what the mass of the moon is compared to the planet, rather than the size of the moon.
    I'd agree with this (at least for my own personal classification, which probably doesn't influence anyone else ). So something that's the size of Neptune that is orbiting something a hundred times larger is a huge moon orbiting a truly enormous planet. Two objects both roughly the size of Neptune orbiting each other as they go around their star? That's a double planet.
    Conserve energy. Commute with the Hamiltonian.

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    Using mass, I would set the upper end of planet-hood at 0.08 solar mass, or the hydrogen-burning limit.

    Everything between the lowest spherical size/mass up to the hydrogen-burning limit is a planet, no matter how it was created or whether it burns deuterium.

    We do have different KINDS of planets, and some types melt into others, like brown dwarfs and Jovians. Burning deuterium is not a starlike quality, however.

    Again, just my own opinion.

    Defining a planet or moon based on size ratio between planet and moon could be problematic. An Earth-sized world orbiting a brown dwarf orbiting a giant F-type star, that's a situation in which you could wind up with no planets at all depending on definitions (if a brown dwarf is not a planet and the Earthlike body is only a moon).
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Oh, anything smaller than the lower mass level for spherical planets is a rock, or some other term (planetoid, asteroid, rubble pile, etc.).
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    Quote Originally Posted by Roger E. Moore View Post
    Defining a planet or moon based on size ratio between planet and moon could be problematic.
    But necessary. Mercury is generally accepted as a planet. Titan is bigger than Mercury, and generally accepted as a moon.
    If Mercury and Titan orbited each other, and together the Sun, would they be neither of them a planet, one of them a planet and the other a moon (and then which?), or both planets?
    Quote Originally Posted by Roger E. Moore View Post
    An Earth-sized world orbiting a brown dwarf orbiting a giant F-type star, that's a situation in which you could wind up with no planets at all depending on definitions (if a brown dwarf is not a planet and the Earthlike body is only a moon).
    If a brown dwarf is classified as not a planet, then because it is classified with stars. And then a satellite of a brown dwarf qualifies as a planet rather than a moon.

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    Those are current definitions, but evidence can change that. Titan is spherical, has a significant mass, and is bigger than Mercury. It even has an atmosphere and oceans and climate. What would go wrong if one day it was decided Titan was a planet that was a moon of Saturn? You can have it both ways.

    For example, from what I've seen in the literature, brown dwarfs and Jovians are more alike than they are different. They are about the same size but differ in mass. There appears to be a transition through the mass scale, no matter how the object is created. At times is seems a lot of effort is expended attempting to keep Jovians and brown dwarfs apart, when the distinction between them from about 0.6 Jupiter mass to 80 Jupiter masses is less than between an M-dwarf and Betelgeuse, yet both the latter are stars. Deuterium burning is not that big of a transition point.

    Going back to "PROBABILISTIC FORECASTING OF THE MASSES AND RADII OF OTHER WORLDS", by Jingjing Chen and David Kipping, for example:

    "[We] propose that ... a class of Jovians is taxonomically rigorous in the mass-radius plane. A defining feature of the Jovian worlds is that the MR power-index is close to zero (−0.04 0.02), with radius being nearly degenerate with respect to mass. We find that brown dwarfs are absorbed into this class, displaying no obvious transition (also see Figure 1) at ∼ 13 MJ, the canonical threshold for brown dwarfs Spiegel et al. (2011), as was also argued by Hatzes & Rauer (2015). When viewed in terms of mass and radius then, brown dwarfs are merely high-mass members of a continuum of Jovians and more closely resemble “planets” than “stars”."

    I am happy to post others papers (without links) to show that there is a significant group of researchers who seem willing to soften the barrier between the two.

    We are not calling a rabbit and an orange the same thing. Jovians and brown dwarfs are more alike (to me) than they are different.
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    http://adsabs.harvard.edu/abs/2015ESS.....350201Q
    Planets around Giant Stars: Results from the Lick Survey
    Quirrenbach, Andreas; Reffert, Sabine; Trifonov, Trifon; Bergmann, Christoph; Schwab, Christian
    12/2015

    Nu Ophiuchi, a giant star, has two brown dwarfs in planetary-like orbit around it, one at 24 M-Jupiter and one at 27 M-Jupiter.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Surely the distinction between a planet and a moon is that planets orbit stars while moons orbit planets. i.e. it;s a primary orbit versus secondary orbit thing.

    I don't see why size has to factor in. Why would a body have to be smaller than a certain size to be classified as a moon? A Neptune-sized body orbiting a planet is still a moon.

    The only sticking point, then, is how large must a moon be relative to its parent before it is classed as a double-planet system?

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    Quote Originally Posted by Roger E. Moore View Post
    Me, I can see several definitions that cross over each other. For example, one starting place is here.

    =============

    https://www.hou.usra.edu/meetings/lpsc2017/pdf/1448.pdf
    A GEOPHYSICAL PLANET DEFINITION
    K.D. Runyon, S.A. Stern, T.R. Lauer, W. Grundy, M.E. Summers, K.N. Singer
    pdfLunar and Planetary Science XLVIII (2017)

    We propose the following geophysical definition of a planet for use by educators, scientists, students, and the public: "A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters."
    I find this to be a terrible way to define a planet. "Anything large enough to be a sphere" will mean our solar system will have hundreds if not thousands of planets, which in my opinion renders the definition meaningless. "Anything large enough to be a sphere" is simply too small of a limit for planets. I like the current IAU definition of planets, the only thing they could do to improve it would be to place some actual numbers to their limits instead of just "cleared out it's orbit" change it to perhaps "cleared out it's orbit so that it accounts for 99% of the mass of anything which crosses it's orbit" or something like that. But the general idea of limiting planets to only the most significant objects in the system is a good one and should not be tossed aside.

    Quote Originally Posted by DaveC426913 View Post
    The only sticking point, then, is how large must a moon be relative to its parent before it is classed as a double-planet system?
    This seems a pretty simple one to me: If the barycenter is inside the bigger object, then it's a moon orbiting a planet. If the barycenter is located in the space between the objects, it's a double planet (assuming they meet the other criteria for "planet", that is).

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    Quote Originally Posted by Dave241 View Post
    This seems a pretty simple one to me: If the barycenter is inside the bigger object, then it's a moon orbiting a planet. If the barycenter is located in the space between the objects, it's a double planet (assuming they meet the other criteria for "planet", that is).
    Yeah. I thought of that too.

    (Great name, BTW. You must have been in line for the internet 426,672 spots ahead of me! )
    Last edited by DaveC426913; 2018-Oct-08 at 04:27 AM.

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    Quote Originally Posted by DaveC426913 View Post
    Surely the distinction between a planet and a moon is that planets orbit stars while moons orbit planets. i.e. it;s a primary orbit versus secondary orbit thing.
    Not just i. e.
    Quote Originally Posted by DaveC426913 View Post
    I don't see why size has to factor in. Why would a body have to be smaller than a certain size to be classified as a moon? A Neptune-sized body orbiting a planet is still a moon.
    Of course, a Proxima-sized body orbiting a bigger body that orbits a yet bigger body is too big to be a moon, because it is big enough to be a star. Then ahain, in tis case the secondary also has to be big enough to be a star.
    Quote Originally Posted by DaveC426913 View Post
    The only sticking point, then, is how large must a moon be relative to its parent before it is classed as a double-planet system?
    Quote Originally Posted by Dave241
    This seems a pretty simple one to me: If the barycenter is inside the bigger object, then it's a moon orbiting a planet.
    Not simple to me. This arbitrary criterion would tend to make double planets just because the distance between the components is larger. When are Earth and Moon due to become a double planet, while still bound, due to tidal evolution?

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    Quote Originally Posted by chornedsnorkack View Post
    When are Earth and Moon due to become a double planet, while still bound, due to tidal evolution?
    IIRC, the Sun will expand to swallow the Earth before that happens.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    The composition and mass of an object is more significant than its orbit.

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    Quote Originally Posted by agingjb View Post
    The composition and mass of an object is more significant than its orbit.
    Except when determining if it's a moon.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    Quote Originally Posted by chornedsnorkack View Post
    Not simple to me. This arbitrary criterion would tend to make double planets just because the distance between the components is larger. When are Earth and Moon due to become a double planet, while still bound, due to tidal evolution?
    I don't have a problem with this, conceptually. On the other hand, there are always going to be objects and situations that aren't completely clear. We're the ones insisting everything fit into bins for categorization.

    CJSF
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    I'm looking at how much information is carried in a word. A "planet" is at once perceived to be spherical, terrestrial up to gas giant in size, orbiting something else unless the word "rogue" is used as an adjective. It can be unusually small if the adjective "dwarf" is added. It has a significant mass and radius.

    A "moon" orbits a planet... but you know nothing else about it. Is it like an asteroid, a 1-meter rock, or a world the size of Neptune? You don't know.

    I suggest "planet" when considering the object by itself, then adding qualifiers to better describe it, such as "satellite planet" or "moon planet" to denote you've got a planet in orbit around another planet.
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    Surely if something would fulfil the definition of "planet" in our solar system would be an exoplanet in another system? And something that would fulfil the definition of a satellite moon in our system would be an "exomoon"?

    In other words we simply carry over the definitions from our system to others. Surely that is the correct thing to do.

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    Quote Originally Posted by kzb View Post
    Surely if something would fulfil the definition of "planet" in our solar system would be an exoplanet in another system? And something that would fulfil the definition of a satellite moon in our system would be an "exomoon"?

    In other words we simply carry over the definitions from our system to others. Surely that is the correct thing to do.

    No simple matter.
    Definitions in solar system happen to be simple because solar system is simple. A lot of hard cases that do not need resolving in Solar System because they happen not to happen here may occur elsewhere and need resolving then.

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    It should be possible for the giant moon to have a satellite or two of its own, right? A moon to the second power?

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    Quote Originally Posted by DonM435 View Post
    It should be possible for the giant moon to have a satellite or two of its own, right? A moon to the second power?
    In a paper that came out today on the exomoon, the moon of a moon is called a "submoon".
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    So there's already a name for it!

    I'll bet there's be some really impressive eclipses or transits visible from there.

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    Quote Originally Posted by Roger E. Moore View Post
    In a paper that came out today on the exomoon, the moon of a moon is called a "submoon".
    I think there was a discussion on here a little while back about moons of moons. There are no known examples in the solar system, and that is for a reason. The reason is the range between the Roche limit and the stable orbit limit is small to non-existent.

    That's not to say it can't possibly happen, but elsewhere would it be an exosubmoon, or is the exo redundant because there are no submoons in the solar system? There's a question for you.

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    We have artificial submoons, but that's as far as it goes, spacecraft orbiting (briefly) our Moon.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    Charon has 12% of the mass of Pluto in our system. I would like to say that these two worlds should be called a double planet, but the IAA don't recognise this term at the moment. Maybe the boundary should be set a bit higher, at 20-25%. Double planets might be fairly common, but we haven't found any yet.

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    Quote Originally Posted by DonM435 View Post
    I'll bet there's be some really impressive eclipses or transits visible from there.
    Transits, sure. Eclipses, less so.

    Our "Ring of Fire Eclipses" require the Sun-Moon distance ratio to be very close that of the Sun-Moon radius ratio (400:1 in our case). An improbable coincidence.

    You could have a moon that is much larger than the sun's disk. I wonder if that would be better described as an occultation though.

    I read a sci-fi book once where aliens from all over the interstellar neighborhood came to Earth for our spectacular eclipses because they did not occur anywhere else.
    Last edited by DaveC426913; Yesterday at 11:43 PM.

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    Of course, the fact that the Sun and Moon are about the same apparent size from Earth may very well be unique.

    I was thinking of an observer on a sub-moon watching a nearer sub-moon cross its primary while the latter is in transit across its sun. Maybe once in a billion years.

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