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Thread: Smash two planets together and you get.... ?

  1. #1
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    Question Smash two planets together and you get.... ?

    Puzzled by this paper's conclusion, because wouldn't a giant impact on a massive planet NOT create a new, lighter inner planet, but instead a new, lighter moon for the impacted planet? Like the Earth and Moon? The "post-impact" planets are in nearly circular orbits, too, which strikes me as weird given their creation.


    https://arxiv.org/abs/1902.01316

    A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system

    Aldo S. Bonomo, et al. (Submitted on 4 Feb 2019)

    Measures of exoplanet bulk densities indicate that small exoplanets with radius less than 3 Earth radii (R ⊕ ) range from low-density sub-Neptunes containing volatile elements to higher density rocky planets with Earth-like or iron-rich (Mercury-like) compositions. Such astonishing diversity in observed small exoplanet compositions may be the product of different initial conditions of the planet-formation process and/or different evolutionary paths that altered the planetary properties after formation. Planet evolution may be especially affected by either photoevaporative mass loss induced by high stellar X-ray and extreme ultraviolet (XUV) flux or giant impacts. Although there is some evidence for the former, there are no unambiguous findings so far about the occurrence of giant impacts in an exoplanet system. Here, we characterize the two innermost planets of the compact and near-resonant system Kepler-107. We show that they have nearly identical radii (about 1.5−1.6 R ⊕ ), but the outer planet Kepler-107c is more than twice as dense (about 12.6 gcm −3) as the innermost Kepler-107b (about 5.3 gcm −3). In consequence, Kepler-107c must have a larger iron core fraction than Kepler-107b. This imbalance cannot be explained by the stellar XUV irradiation, which would conversely make the more-irradiated and less-massive planet Kepler-107b denser than Kepler-107c. Instead, the dissimilar densities are consistent with a giant impact event on Kepler-107c that would have stripped off part of its silicate mantle. This hypothesis is supported by theoretical predictions from collisional mantle stripping, which match the mass and radius of Kepler-107c.
    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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    News item that just appeared, on same topic. Doesn't explain how to get an inner planet instead of a moon from a planetary collision.

    https://phys.org/news/2019-02-giant-...ollisions.html

    Giant impacts caused by interplanetary collisions
    February 5, 2019, University of Bristol

    ADDED NOTE: This paper seems to have hit all the science-news services at once today. Nothing new added, so far as I can tell.
    Last edited by Roger E. Moore; 2019-Feb-05 at 06:46 PM. Reason: note
    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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    Example of what I'm talking about: 2019 paper on Uranian giant impact concept making no reference at all to the formation of another planet, only to satellites of final body.

    https://arxiv.org/abs/1811.05234

    The Exchange of Mass and Angular Momentum in the Impact Event of Ice Giant Planets: Implications for the origin of Uranus
    Kenji Kurosaki, Shu-ichiro Inutsuka (Submitted on 13 Nov 2018)

    QUOTE: Safronov (1966) pointed out that Uranus experienced a giant impact event to reproduce its tilted rotation axis. An impactor of several earth mass may have transported the angular momentum to proto-Uranus via collision and tilted the rotation axis of proto-Uranus. Parisi & Brunini (1997) estimated the giant impact based on the conservation of angular momentum and energy and concluded that the minimum impactor mass is ∼ 1 − 1.1 M⊕. Such large impact event may also have produce a circumplanetary disk around the proto-Uranus, which might be the origin of small prograde satellites around Uranus (e.g., Parisi et al. 2008). A giant impact scenario is widely accepted as an explanation for terrestrial moon formation (Hartmann & Davis 1975; Cameron & Ward 1976) and recently suggested to apply to Phobos and Deimos formation (e.g., Citron et al. 2015; Hyodo et al. 2017a).
    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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    My understanding has always been than when an impact results in some
    material being flung away from the planet and not immediately falling back
    down, the large majority of the material escapes to solar orbit, with only a
    small portion going into orbit around the planet, if any. So in the giant
    impact which created Earth's Moon, most of the material which did not
    immediately fall to Earth escaped to solar orbit. The material which was
    vaporized was probably blown out of the Solar System by solar wind.
    Larger particles would have had their orbits degraded by solar light
    pressure in the Poynting-Robertson effect, causing them to fall close to
    the Sun where the intense sunlight vaporized them, allowing them to
    also be blown away by solar wind. Some of the particles would have hit
    Venus or Mercury instead. Many of the larger chunks-- probably most--
    would have eventually run into the Earth again since all of the material
    started out in an orbit around the Sun almost identical to Earth's orbit.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    Jeff, yes, what you said. Thank you.

    Having agreed, I did discover that the Hill Spheres of the two planets would be very small because of their proximity to their sun (have not done the math, but I have the equations around somewhere, not here). So a lot of material would escape if the hit was very hard. If the hit was "soft", I have trouble seeing two same-size planets result, you'd likely get one big heavy planet with an iron core (Earth analog) and a smaller world that is less dense (Moon analog). If the worlds cannot stay together, cannot say what orbits they would take, but it seems logical they might collide later or interfere with each other's orbit, right? Not take up different next-door orbits.
    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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    This may or may not be relevant.

    I have read that many simulations of planet collisions result in
    not one but two similar-size moons forming, in essentially the
    same orbit, which collide after something like a hundred orbits.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    Quote Originally Posted by Roger E. Moore View Post
    Jeff, yes, what you said. Thank you.

    Having agreed, I did discover that the Hill Spheres of the two planets would be very small because of their proximity to their sun (have not done the math, but I have the equations around somewhere, not here). So a lot of material would escape if the hit was very hard. If the hit was "soft", I have trouble seeing two same-size planets result, you'd likely get one big heavy planet with an iron core (Earth analog) and a smaller world that is less dense (Moon analog). If the worlds cannot stay together, cannot say what orbits they would take, but it seems logical they might collide later or interfere with each other's orbit, right? Not take up different next-door orbits.
    My intution tells me (supported byabundant evidence) that your use of intution (unsupported by any substantial evidence) to predict the likely outcome of planetary impacts is probably faulty. Jeff's reply, noting the outcome of simulations, represents the only (currently) meaningful way of assessing what is and is not likely.

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    Quote Originally Posted by Eclogite View Post
    My intution tells me (supported byabundant evidence) that your use of intution (unsupported by any substantial evidence) to predict the likely outcome of planetary impacts is probably faulty. Jeff's reply, noting the outcome of simulations, represents the only (currently) meaningful way of assessing what is and is not likely.
    I must readily agree. Are you aware of any "giant impact" research that indicates a result of anything other than a planet reforming with a moon around it? I am unable to find any papers of this sort. Everything I've seen in the SAO-NASA ADS search engine is oriented toward duplicating the Earth-Moon system or the Uranus-satellite system. A collision appears to always result in a new, combined-material planet with a heavy core, orbited by a light-core moon(s). You might briefly get two significant moons, also agree, have seen papers on that, but they collide.

    Nowhere do I see any research that indicates two separate planets, in two separate orbits, result from a planetary-scale collision. All papers mention that any material that does not result in a combined-material planet & moon is lost in the system.

    Your help and advice are appreciated.

    LATE NOTE: If there is any interest, I could post the names of papers I've examined with relevant (very short) excerpts from them on the topic.
    .
    Last edited by Roger E. Moore; 2019-Feb-11 at 08:27 PM. Reason: corrections & additions
    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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    The problem is that the paper, what is available of it, makes no mention of any test simulations that suggest or confirm that a giant-impact scenario between Earth-mass planets would result, under the conditions present in that planetary system, in the formation of two planets in separate orbits, with the heavier-mass planet in the outer orbit. Instead, it comes across that they made an untested hypothesis about the origin of the two planets in question, without bothering to go into any detail on how this could occur.

    A paper just came out discussing the difficulties in running computer simulations of giant-impact events. I was unaware of the great difficulty involved in running a mildly complex scenario, or that early scenarios did not account for lost matter resulting from a collision.

    https://arxiv.org/abs/1902.04052
    Can a machine learn the outcome of planetary collisions?
    Diana Valencia, Emaad Paracha, Alan P. Jackson (Submitted on 11 Feb 2019)
    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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    My apologies if I missed this point being made in one of the above posts, but I believe that one of the most important considerations, when determining the end result of a mashup between two bodies that are massive relative to each other, is the relative velocity of the two bodies at the time they collide. Remember that force is a function of two variables, namely mass and acceleration. This is why scientists believe that the Mars sized object that struck the proto Earth and created the Moon must have done so at a relative velocity no greater than four kilometers per second. Why? Because had the relative velocity of the two bodies been much greater, we would not have been here to ask the question; Earth would have been shattered and much of the debris would probably collide with the Sun, be blown away by the solar wind or accrete into smaller objects like asteroids. If I use my hand to fling a full metal jacket bullet at you, it would probably annoy you, but it would not hurt you. If I fired it with a gun, it would be an entirely different story, and the difference here is the relative velocity between yourself and the bullet. Funny thing is that if you were travelling in the same direction as the bullet but somewhat slower than the bullet, the bullet would catch up with you, but you would not die as the relative velocity at the time of impact would be too low to destroy the bonds that hold your vital organ etc together, or even pierce your skin, for that matter. It is thought that the object that struck Earth to create the Moon had an orbit very similar to Earth's, and with Kepler's laws we therefore know their velocities must have been very similar, if not exactly the same, but this means that when they collided, the effects were not as dramatic as they would have been if the Mars sized object had been on an eliptical orbit and its velocity much higher when it hit Earth; as a matter of fact, it probably could not have been on such an orbit as planets can only accrete matter when they are in relatively circular orbits.

    One of the insights we gain from this sort of reasoning is that it is not only the size of the asteroid we ought to worry about, but its relative speed to Earth. A 1 km asteroid in a retrograde orbit striking Earth would be way worse than its prograde orbit equivalent.

    Bit of a ranting answer, and English is not my first language, so sorry if it came off as somewhat incoherent!

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    Quote Originally Posted by HappyKoala View Post
    Bit of a ranting answer, and English is not my first language, so sorry if it came off as somewhat incoherent!
    Came out fine to me. Not sure it applies to the initial example, but good point.
    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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