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Enter the ‘Synestia’

What happens when giant objects collide? We know the result will be catastrophic, as when we consider the possibility that the Moon was formed by a collision between the Earth and a Mars-sized object in the early days of the Solar System. But Sarah Stewart (UC-Davis) and Simon Lock (a graduate student at Harvard University) have produced a different possible outcome. Perhaps an impact between two infant planets would produce a single, disk-shaped object like a squashed doughnut, made up of vaporized rock and having no solid surface.

Call it a ‘synestia,’ a coinage invoking the Greek goddess Hestia (goddess of the hearth, family, and domestic life, although the authors evidently drew on Hestia’s mythological connections to architecture). Stewart and Lock got interested in the possibility of such structures by asking about the effects of angular momentum, which would be conserved in any collision. Thus two giant bodies smashing into each other should result in the angular momentum of each being added together. Given enough energy (and there should be plenty), the hypothesized structure should form, an indented disk much larger than either planet.



Image: The structure of a planet, a planet with a disk and a synestia, all of the same mass. Credit: Simon Lock and Sarah Stewart.

Moreover, this process should be widespread (if generally short-lived) in young, evolving planetary systems. As planet formation ends, planetary collisions should produce rapidly rotating, partially vaporized rocky objects. The researchers developed a computer code called HERCULES that allows them to calculate the physical structures of bodies in varying temperatures and rotational states. There are combinations of rotational rate and thermal energy that make it impossible for a planet to rotate like a solid body. Instead, we get an inner region with its own rotation connecting to a disk-like outer region moving at orbital velocities.

The paper on this work notes that “…the structure of post-impact bodies influences the physical processes that control accretion, core formation and internal evolution. Synestias also lead to new mechanisms for satellite formation.” Moreover, Stewart and Lock believe that rocky planets are vaporized multiple times during their formation. Thus synestias should be a common outcome in young systems. From the paper:

…there is a corotation limit for the structure of terrestrial bodies that depends on mass, compositional layering, thermal state, and AM [angular momentum]. We have named super-CoRoL [corotation limit] structures synestias. Synestias typically consist of an inner corotating region connected to an outer disk-like region. By analyzing the results of N-body simulations of planet formation, we found that high-entropy, highly vaporized post-impact states are common during terrestrial planet accretion. Given the estimated range of planetary AM during the giant impact stage, we find that many post-impact structures are likely to be synestias.

Remember that a sufficiently large impact will have produced molten or gaseous material in vast quantities, expanding in volume and responding to all that angular momentum. One outcome, given the size of the impacting objects and the energy involved, could be a disk of material surrounding the impacted planet. But the researchers believe a synestia is likely at some point, perhaps lasting as little as a hundred years. For the same amount of mass, a synestia would be much larger than a solid planet with a disk of material around it.

Could we ever hope to observe such an object given how short its lifetime is expected to be? Perhaps, and not just by a stroke of good luck. For Stewart and Lock argue that a synestia formed from larger objects like gas giants or even stars could potentially last much longer. That would make a synestia a possible observable in young extrasolar systems. With that in mind, it will be interesting to see whether the HERCULES code produced in this work will find its way into new studies of planet formation and evolution.

The paper is Lock & Stewart, “The structure of terrestrial bodies: Impact heating, corotation limits, and synestias,” Journal of Geophysical Research: Planets 122 (2017). Abstract / preprint.


Comments on this entry are closed.

  • Daniel de França Diniz Rocha May 24, 2017, 12:15

    Maybe this thing is the kind of thing that is blocking Tabby’s Star.

  • Thomas W. Hair May 24, 2017, 12:35

    Given that this work is highly speculative and based upon computer simulation alone, but very well thought out, and not unlike the highly speculative theories surrounding Boyajian’s Star I would humbly submit that something like this be added to said list of theories…I know planet-sized collision have been brought forth as a possible explanation, but this adds meat to the bones. Could a certain input of parameters to their model produce result in the theoretical ball park of what we are seeing at KIC 8462852?

    • Alexander Tolley May 24, 2017, 15:13

      Wouldn’t such an object cloud show up in the IR that is missing in Tabby’s star? I don’t see how this could be an explanation.

      • thomas W. Hair May 25, 2017, 16:32

        Dust and granular material will certainly radiate in the IR, but rocks and boulders will not to anywhere near said level. Simulation (I know, there I go again) and basic physics suggests that static electricity will quickly absorb the dust from a major planet forming event.

  • Thomas W. Hair May 24, 2017, 12:41

    PS I meant produce result”s” in the theoretical ball park

  • Geoffrey Hillend May 24, 2017, 17:18

    I don’t think the addition of the angular momentum of the Earth and Moon will result in a doughnut shape like Synestia since the orbital inclinations of the debris are two high which might prevent the Moon from ever being formed. Synestia sounds like the processes in solar system formation but not collision of two planets like the Earth and Theia. The inclinations place the debris in orbits which are too far apart. The Moon is supposed to have coalesced in only a month after the Earth’s collision with Theia, so those high inclinations in the Synetia model are dubious. Consequently, we would still see a lot of debris in those inclinations today, but we don’t.

  • Robert Walker May 24, 2017, 17:42

    Interesting article. I wonder if such an object as it cooled down could produce a donut shaped planet? I know that the whole point in it is that the outer ring is rotating at orbital velocities so not all at the same speed, but as the spinning material accretes? And if the amount of the material is less than normal? Maybe central material even forms an inner moon?

    Just speculating for fun. This is a theoretical stable state for a planet, along with contact ternary and quarternary planets and contact quinternary rings. It would be stable only for as long as the planet spins so fast it is almost but not quite tearing itself apart. It can even also in theory have a Moon that bobs up and down through the hole in its center. The main issue is, how can it get into this state in the first place?

    I’m not sure what the latest is on this but here is a paper from 1986 which has such things as a 5 fold contact binary ring

    See my Surprising shapes of rapidly spinning planets

    • Harry R Ray May 25, 2017, 9:17

      Not a donut shaped one, but perhaps a pancake-shaped one, like Hal Clement’s “Mesklyn”, if it was rotating rapidly enough.

      • Jeff Wright May 26, 2017, 15:17

        Speaking of world-building

        What about a smaller version–but with a metal-rick Psyche type asteroid asteroid doing a close fly-by of the sun?

        What I want to do is to get a molten, gooey version of this–not a cloud–but a membrane.

        Then you hit the very center of this Red Blood cell shaped molten synestia with a hypervelocity object, and turn it into a gooey molten smoke ring than then freezes into the shape of a one piece, hollow torus.

        Can it be done?

        One piece ring station–just cut a hole–and fit it out on the inside.

        Instant cycler station.

  • Andrew Palfreyman May 25, 2017, 1:18

    Can these synestiae solidify? It would be gravitationally very interesting to live on one!

  • Michael May 25, 2017, 6:22

    If these impact shapes are common then moons are more likely to form around the planets.

  • Michael C. Fidler May 25, 2017, 19:23

    Something dawned on me this morning, looking at the article on “Tabbys Star”, “KIC 8462852: Will the Trojans return in 2021?”. Planets that formed in the early solar nebula would also have objects form at the five Lagrange points in there orbit around the sun. This material should coalesce at these points and form lunar size objects, because of the instabilities from Venus, Jupiter and Mars they would eventually collide or orbit with the main planet (Earth). The fact is would these Synestia be a much more common state for early planets in solar system? What about the tightly packed systems like TRAPPIST-1??? Has someone already put this thru the models for planetary formation in the super computers???

    Doppelgänger at L3 – One of my favorite movies: “Journey to the Far Side of the Sun (1969)”

    • Michael C. Fidler May 25, 2017, 22:09

      Just thinking out-loud, would these Lagrange points form like vorticities in the nebula? Would it be like the Fractal pattern in Chaos theory with smaller and smaller vorticities forming at each level as the objects coalesced? An example of this would be the Earth and the Moon being in the ‘Synestia’ state: the moon would form smaller moons at the Lagrange points that eventually collide with it and form the great lunar maria. (Talked about this before with the moon turning the maria toward earth as it became tidally locked.) Just imagine at the Trojan points many planetoids forming and smaller planetoids forming around them at there five Lagrange points, ad infinitum! Then comes the dragon king event: “The behavior of extreme events – very large fluctuations in a system that often leads to catastrophic results. These occur in many complex, chaotic systems: enormous rogue waves in the ocean, extreme weather in the climate, or global stock market crashes”. So the final jump up from the strange attractors is to a dragon king event then to a stable planetary system.

    • Thomas W. Hair May 25, 2017, 23:06

      Michael, I agree with you that there are too many unknown unknowns and we are in the land of blinded by science speculation, but I’ll hang my hat on a fairly recent planetary size collision in KIC 8462852 before I give into my desire for an alien mega-structure. Kepler was looking at quite a number of stars and it is not unreasonable to think that given we know truly next to nothing about exoplanetary systems and their formation beyond our very recent observations it should suggest that we entertain a plethora of fringe ideas.

  • Michael C. Fidler May 25, 2017, 22:24

    Could Mars have been at our L3 point and been thrown into a higher elliptical orbit by Jupiter’s influence?

  • stephen June 8, 2017, 10:13

    G. David Nordley wrote To climb a Flat Mountain, which was anthologized electrolyzed in Analog and published as a trade paperback.

    How plausible is Larry Niven’s Smoke Ring? Maybe we could. detect that by spectroscopy.

    Maybe a body hitting a young planet in the direction opposite to its rotation could slow it down so its rotation period could be very slow, hundreds or thousands of years. Would that be what happened to Venus?