Astronomers propose a new type of planetary object

From the American Geophysical Union and the “Pluto is still denied planetary status” department comes this idea:

WASHINGTON, DC — Scientists suggest in a new study the existence of a planetary object called a “synestia,” a huge, spinning, donut-shaped mass of hot, vaporized rock, formed as planet-sized objects smash into each other.

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

At one point early in its history, Earth was likely a synestia, said Sarah Stewart, a planetary scientist at the University of California Davis and co-author of the new study in the Journal of Geophysical Research: Planets, a journal of the American Geophysical Union.

Stewart and Simon Lock, a graduate student at Harvard University in Cambridge, Massachusetts and lead author of the new study, explore how planets can form from a series of giant impacts. Current theories of planet formation hold that rocky planets such as Earth, Mars and Venus formed early in the solar system when smaller objects smashed into each other.

These collisions were so violent that the resulting bodies melted and partially vaporized, eventually cooling and solidifying to the nearly spherical planets we know today.

Lock and Stewart are particularly interested in collisions between spinning objects. A rotating object has angular momentum, which must be conserved in a collision. Think of a skater spinning on ice: if she extends her arms, she slows her rate of spin. To spin faster, she holds her arms close by her side, but her angular momentum stays constant.

Now consider two skaters turning on ice: if they catch hold of each other, the angular momentum of each skater adds together so that their total angular momentum stays the same.

In the new study, Lock and Stewart modeled what happens when the “ice skaters” are Earth-sized rocky planets colliding with other large objects with both high energy and high angular momentum.

“We looked at the statistics of giant impacts, and we found that they can form a completely new structure,” Stewart said.

Lock and Stewart found that over a range of high temperatures and high angular momenta, planet-sized bodies could form a new, much larger structure, an indented disk rather like a red blood cell or a donut with the center filled in. The object is mostly vaporized rock, with no solid or liquid surface.

They have dubbed the new object a “synestia,” from “syn-,” “together” and “Estia,” Greek goddess of architecture and structures.

The key to synestia formation is that some of the structure’s material goes into orbit. In a spinning, solid sphere, every point from the core to the surface is rotating at the same rate. But in a giant impact, the material of the planet can become molten or gaseous and expands in volume. If it gets big enough and is moving fast enough, parts of the object pass the velocity needed to keep a satellite in orbit, and that’s when it forms a huge, disc-shaped synestia, according to the new study.

Previous theories had suggested that giant impacts might cause planets to form a disk of solid or molten material surrounding the planet. But for the same mass of planet, a synestia would be much larger than a solid planet with a disk.

Most planets likely experience collisions that could form a synestia at some point during their formation, Stewart said. For an object like Earth, the synestia would not last very long – perhaps a hundred years – before it lost enough heat to condense back into a solid object. But synestia formed from larger or hotter objects such as gas giant planets or stars could potentially last much longer, she said.

The synestia structure also suggests new ways to think about lunar formation. The moon is remarkably like Earth in composition, and most current theories about how the moon formed involve a giant impact that threw material into orbit. But such an impact could have instead formed a synestia from which the Earth and Moon both condensed, Stewart said.

No one has yet observed a synestia directly, but they might be found in other solar systems once astronomers start looking for them alongside rocky planets and gas giants, she said.


This research article is open access for 30 days. A PDF copy of the article can be downloaded at the following link:

116 thoughts on “Astronomers propose a new type of planetary object

    • Indeed. More wild speculation dressed as “science” which is supposed to mean knowledge.

      … the resulting bodies melted and partially vaporized, eventually cooling and solidifying to the nearly spherical planets we know today.

      The Earth is NOT solid most of it is still molten. The crust is flexible and deformed by the Earth’s rotation. The crust is relatively thin and sometimes leaks: look at Hawaii, for examples.

      • While the core is molten, this is mainly from heat caused by the extreme pressure from the weight of the materials above. But the earth is considered a solid mass as opposed to a Giant gaseous planet like Jupiter or Saturn (which also have solid cores of molten metal without which they and we would have no magnetic field)
        This toroid effect could explain at least some of the Trojan Asteroids which could have coalesced out of the materials in those regions as the materials cooled and recombined.

      • Bryan A
        I think you’s have a hard time demonstrating that pressing on a solid or incompressible liquid caused it to increase in temperature. Heat in the core is the result of nuclear decomposition and the insulation of thousands of miles of overburden.

      • Thanks Crispin
        I guess My Science teachers got it wrong as well.
        A quick search indicated at several physics sites that numerous people were incorrectly taught this by their Science Teachers

    • Exactly the way the planet Neptune was discovered in 1846. And later Pluto in 1930. Mathematically predicted first and discovered later. A lot of physics and astrophysics works that way. Absence of evidence is not necessarily evidence of absence. You should read more.

      • The observational anomalies that led to Neptune and Pluto were actual observations, so your comment is not quite on point.

      • Neptune was found by observing the perturbations in the orbit of Uranus. Pluto was found based on the residuals after the effect of Neptune was accounted for. However, it was a serendipitous discovery since Pluto is far too small to have any measurable effect on Uranus’ orbit.

        • scarletmacaw

          Neptune was found by observing the perturbations in the orbit of Uranus. Pluto was found based on the residuals after the effect of Neptune was accounted for. However, it was a serendipitous discovery since Pluto is far too small to have any measurable effect on Uranus’ orbit.

          A story often repeated.
          So, my question becomes: If the known perturbation in Uranus’ orbit (and that sound childishly weird, doesn’t it?) were NOT fully explained by Neptune’s mass and position in its orbit, and Pluto is too small to explain the perturbations by itself, then what (unknown planet-sized mass) DOES explain the perturbations in Uranus’s orbit? Or have they been “disappeared” into the aether?

    • I would trust the math of AstroPhysics over almost every other discipline out there. They do have a very good track record as others have pointed out.

    • True, but the physics is well understood, and the math is straight-forward. Since such formations are predicted to be short-lived, we’re going to have to be lucky to observe one. Which is what it will take to move this from hypothetical to observed phenomena.

    • I admire your chutzpah, but expect there is more in this than what can be dismissed like that.

    • This is exactly what scientists are supposed to do- think in a new way about something(E=mC^2) and figure out the implications. Voila’ a new hypthesis which can be tested against observations. So it might explain why the earth apparently has a mostly iron core- iron is a fairly dense, abundant element. In a gas-fluid structure it might tend to aggregate towards the center and retain enough energy to stay molten for a long time as the core of a planet.

    • “Math games about things no one has ever seen, or has much evidence for.”
      Just say ” Model ” and be done with it.

    • Yes, and just say astrophysics, rocket science, string theory and your bandwagon is quickly filled. Modeling without verification or validation data is fooling around.

  1. Seems that observing this would require studying a newly formed star system. Do we look towards the center of the universe?

    • Yes, these formations wouldn’t exist for very long at a time.
      The theory sounds plausible to me. Instead of a thin disk of debris being created after a collison, they are saying it might take the shape of a flattened sphere of debris.

      • I think the E-ELT will be large enough, but AHT (astonishingly huge telescope is probably being planned :). I value telescopes much, much over climate mitigation efforts.

      • We need to put some telescopes in the Earth/Moon and/or Earth/Sun Lagrangian areas, and network them together to create a telescope with a diameter the equivalent of a good percentage of the Moon’s orbit, or the Earth’s orbit.
        We already have telescopes that are networked and work in this way on Earth. We can spread the telescopes much farther apart in space and make a much bigger telescope which can see a lot more of the universe.
        We really do need a good orbital transfer vehicle. One that could visit such telescopes, should be a basic requirement.

      • I sometimes wonder why they didn’t outfit the Space Shuttles properly so that they could be left permanently in space? All they would need would be a fuel tank and fuel, which we should be able to shoot large ice bullets into orbit from rail guns by now that could be processed into hydrogen/oxygen fuel with solar electricity in space. It seems that so much more could have been done by NASA with the Shuttles, including converting them to orbital transfer vehicles, or even being able to go to the Moon and back. I am not a rocket scientist, so there is probably a reason why.

    • @Pop: There is no center of the universe. Or looking at it differently, everywhere is the center. It’s like asking what the center of the surface (not the volume) of a sphere is; any point on the surface can be considered its center, which is perhaps the same as saying no point on the surface is its center.

  2. >>
    A rotating object has angular momentum, which must be conserved in a collision.
    I remember a physicist made the claim that cats can’t land on their feet because angular momentum is conserved. Then they showed video after video of cats held upside-down with zero angular momentum and dropped from sufficient height landing on their feet. Apparently cats know more physics than that physicist.
    Lock and Stewart found that over a range of high temperatures and high angular momenta, planet-sized bodies could form a new . . . .
    No one has yet observed a synestia directly . . . .
    They’ve never seen one, but they know its temperature.

  3. This is something that I might actually want to read Dr. DeGrass-Thaistick’s thoughts on, as he is actually an astronomer.

    • If you mean Tyson, he’s an astrophysicist, but that’s a nit-pick distinction. He might know enough to comment on the computer simulation since he knows so much about climate models.

      • Astrophysics is very different from astronomy. To an astronomer, deriving a number “to astrophysical accuracy” means getting the decimal point in the right place. I’ve taken courses in both. The distinction is not nit-picking.

      • >>
        Astrophysics is very different from astronomy.
        Astrophysics is a branch of astronomy, so that statement isn’t exactly true. I would say that all astrophysicists are astronomers, but not all astronomers are astrophysicists.
        Well, it is not nit-picking for an astrophysicist.
        No argument there.

  4. If two skaters of identical mass are spinning in opposite directions at the same speed, and they catch hold of each other, they will both stop spinning.
    Theoretically, at least. It might be painful.

  5. Went and read the very long and rather dense paper. The new transitory structure emerges from a new method for computing simulated planetary collisions assuming 2/3 silicate shell and 1/3 iron core with varying angular momentum. The code is described in the S1. Best as can tell, not validated, and certainly not verified. Nor would protoplanets in newly forming solar systems necessarily have chemical compositions similar to ours and the simplified model Earths. Largely a speculative theoretical exercise.

    • I was tempted to read it but you’ve put me off – did they factor in gravitational attraction between the condensing matter?
      I’m interested in how likely large meteorite and cometary impacts are to dislodge chunks of crust into orbit. You’ve only got to get around 100km to be in orbit and with a 5 or 10 km object at typical collision velocities, the energies seem plausible, especially with an oblique impact. Think milk drop impacts on a solid surface.
      Anything large put into a low earth orbit from a process like that will be coming down fairly soon after – to wreak havoc all over again somewhere else on earth.

      • Think of a large body coming in and giving a glancing blow. Most of the material will fall back quickly, but some of it does reach orbital, or even escape velocity.

  6. It would have to be quite massive indeed for the hot vaporized rock to be unable to escape the local gravitation. This strongly resembles the Oort Cloud – except for the heat. One can hardly call it an ‘object’ as there is no direct physical continuity among the separate parts. It is more of a ‘fog’.
    Such a ‘structure’ is strictly temporary – one may say ephemeral – as radiation will cool the individual particles of rock quite quickly as the system approaches thermal equilibrium with its environment, and thereafter it is simply a matter of collisions between particles eliminating variations in their individual angular momenta from the mean before it is reduced to a classical planet/ring system.
    The earth and other minor planets would be round regardless of the early temperature; the malleability (‘plasticity’) of ‘solid rock’ is usually underestimated. A large enough specimen of any rock will readily bend under its own weight.

  7. As for Pluto’s status, the following improved definition for “planet” has been proposed by Runyon et al. (2017), and currently describes ~110 known planets in the solar system; this supersedes the flawed definition adopted by the IAU in 2006.
    “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 descibed by a triaxial ellipsoid regardless of its orbital parameters.”
    (abstract link:

    • That definition won’t fly. You’d end up with tens of thousands of “planets” in the solar system and interstellar space between it and other star systems. It would be meaningless, with real planets, dwarf planets, minor planets, moons, asteroids and other objects all lumped together.
      The IAU definition is good as is.

    • Thanks for that link, David.
      Here’s an excerpt I found very interesting. I think I argued pretty much the same thing [that zone clearing was a flawed requirement] in an earlier WUWT post about Pluto:
      “The IAU Definition: The planet definition adopted by the IAU in 2006 [1] is technically flawed, for several reasons. First, it recognizes as planets only those objects orbiting the Sun, not those orbiting other stars or orbiting freely in the galaxy as “rogue planets”. Second, it requires zone clearing, which no planet in our solar system can satisfy since new small bodies are constantly injected into planet-crossing orbits,”
      It seems now that the only qualification the IAU requires to be called a planet is to be big enough to form a spherical body. So I guess I can resume calling Pluto the ninth planet. 🙂

      • TA,
        Bodies orbiting other stars count as exoplanets if they meet the other criteria. Rogue planets orbiting the barycenter of the galaxy also count as planets.
        Small bodies crossing the paths of planets don’t disqualify them, nor do asteroid-like objects parked at gravitational nodes.
        The difference between having “cleared your neighborhood” and not having done so is huge. All the planets have absorbed or ejected from their vicinity other bodies by factors of 1000 to a million times greater than dwarf planets. Besides which, the orbits of Kuiper Belt objects resemble comets more than planets. Pluto, for instance, was sometimes the 8th planet (when it was a “planet”), and at other times the 9th, since its highly eccentric orbit carried it within Neptune’s path.

      • I don’t have a problem calling Pluto a dwarf planet, just like I don’t have a problem calling Jupiter a giant planet, but both of them are planets, according to the IAU, and Pluto is in position number nine in the solar system at the present time, so I don’t see any reason not to continue calling it the ninth planet.

      • If you count every body big enough to form a sphere then Pluto is around the 20th planet. You’d have to also count Ceres and all of the spherical satellites as planets inside the orbit of Pluto.

      • Scarlet,
        Yup. The biggest moon and smallest planet overlap, depending upon whether you go by mass or volume.
        Mercury’s mass is 0.055 Earths. Jupiter’s moon Ganymede’s mass is only 0.025 Earths, but its volume is 0.0704 Earths, v. Mercury’s 0.056 Earths. You’ll note how similar the densities of Earth and Mercury are.

      • “If you count every body big enough to form a sphere then Pluto is around the 20th planet. You’d have to also count Ceres and all of the spherical satellites as planets inside the orbit of Pluto.”
        Well, actually, Ceres would be the only fly in the ointment, since all the other bodies are moons of other planets which disqualifies them from being called planets. FWIIW, both Ganymede and Titan are larger than the planet Mercury, although Mercury is more massive than both moons combined. Mercury’s core is much thicker than that on Earth, and some think Mercury might have formed as a much bigger planet in the past.
        I have a simple solution to this problem: We should arbitrarily declare that Pluto is a planet, and any body as big or bigger than Pluto is also a planet, and anything smaller than Pluto is a dwarf planet or a minor planet. This will preserve our heritage, and the “Mysterious Giant Planet 9” is going to have to change its name to “10” or higher depending on when and if it is found.
        Please don’t complain about Eris being left out. We have to make the cutoff somewhere. 🙂

  8. Planet. Asteroid. Planetoid. Meteor. Meteoroid. Comet. Black hole. Galaxy. Universe. Cosmos. Kosmos. …
    Just call ’em all “space thingies”.
    Problem solved.

  9. A rotating object has angular momentum which must be conserved…

    No. Total angular momentum of the entire system must be conserved, and even in this case conservation
    applies only so long as the system is isolated. Externally applied torque changes angular momentum.
    I wonder if they have examined the stability of such an object. It does not appear like a stable configuration to me.

    • Looks to me like a ring system modified by the ring being gaseous, and fluid pressure blowing the ring away from the orbital plane. Once it cools it condenses into a conventional ring system. Very much a short lived structure.

    • No. Total angular momentum of the entire system must be conserved, and even in this case conservation

      This nit-pick calls for founding of the Planetary Angular Momentum Conservation Society, which aims to protect the threatened angular momentum. It has been projected that humanity may use up the angular momentum which will make working days longer, a serious health hazard that hits hardest the poor people.
      The stability is a major point. See the paper here.

  10. Skeptical for skeptic sake? Please. The authors are not claiming fact; they’re proposing a theory. It is up to scientists to disprove or poke holes in the theory – the scientific method. Theories are how we advance knowledge. Unlike the (non)science of “Climate Change”, astrophysics if far from a settled science. If the theory does not pass scientific muster, it will disappear into scientific oblivion. Just don’t start making political and economic decisions on this one just yet.
    BTW, the scientific theory in not dependent on proving truth, but allowing a theory to stand up to all challenges and questions and either survive or change. That is the problem with Climate Change (non) science. They are working inside out; opposite of the scientific theory; desperately trying to prove its existence and predict its future rather than letting the theory answer the challenges, which, to this point, have not. Instead of welcoming challenges like a real scientific method, proponents instead want to silence anyone who issues a legitimate challenge. At that point, we’re confusing faith, religion, and dogma with science.

    • fxk,
      I’m with you on this one. It is way out stuff, but that is what planetary scientists do.
      Beats having a taxpayer funded $700,000 climate change musical.

  11. Well, it’s possible. It’s mathematically coherent. Perhaps we should leave it in the philosophical logical domain until it is observed and replicated.
    The scientific logical domain is already stuffed with speculation about the distance past and the prophesied future, the universe and extra-dimensional frames, [human] life and storks that deliver us at our convenience.

  12. Of course, a good computer model could confirm it or not.
    Just build in an assumption that the truth proposed is there, and then all quantities inserted will always lead to this truth.
    Brilliant !

      • Chimp, the Stanford blurb you link makes NO sense. Dark matter is surmised through its gravitational interactions (like stars orbiting a galaxy center at rates impossible unless dark matter exists). So the essence of the surmise is gravitational attraction. Two galaxy clusters passing through each other will ‘slow down’ or ‘speed up’ (both via gravitational attraction) both luminous and dark matter. Not luminous only as the Stanford ‘observation’ asserts.
        The other more recent supposed ‘observation’ via gravitational lensing is also fraught.
        I have a simpler theory. There are more black holes (recently detected gravity waves from two merging) and they are more massive than we figured. And by definition they are dark but have gravitational interactions. Plenty of time (13.7 billion years) for lots of supermassive stars to burn themselves out quickly, form black holes, then feed on their surroundings. Lots and lots.

      • Rud,
        That some of the dark matter is baryonic has been confirmed by the discovery of rogue planets orbiting the barycenter of the galaxy. They’re too dark to be seen, since they don’t orbit a star, but have nevertheless been detected.

      • Jarryd Beck May 22, 2017 at 2:21 pm
        Gravitational lensing is not a theory. It’s a confirmed observation.

    • It hurts to see something like this. Physics has two parts: observing and formation of theory. This particular theory lacks observation so far. No reason to think it is not important.

      • My point is not to completely write off the formation of theories. My point is really that astronomy in particular has a history of making up a bunch of stuff that has never been observed. Ok, they made a model and ran some simulations and it came up with something new. But it’s already a leap to say “Most planets likely experience collisions that could form a synestia at some point during their formation”.
        Half the problem is the media, they just take every claim and go wild. Until this thing is observed it is just a theory.

      • Hugs
        The great thing about working with physicists, as I often do, is that they have no problem living with incompatible partial explanations of observable reality, and ‘getting on’ with further investigation.
        Contrast that with the ‘one cause fits all’ approach of ‘climate scientists’ which blame all earthly natural phenomena save a few on CO2, the magical gas. It is the inordinate desire to attribute to CO2 concentration a myriad effects with no space for normal replication and ‘on the other hand’.
        There are many huge problems with the current Standard Model of the universe. That doesn’t stop people from doing what they can with the tools available. We use shorthand tools to overcome inordinately complex ‘true proofs’ because they are ‘good enough for government work’.
        The proof that 1+1=2 is surprisingly complex. I am sure there is baryonic and non-baryonic matter, and even something I call ‘Darker Matter’ which has not yet hit the journals.
        We have a long way to go and should be very tolerant of the multiplicity of explanations of ‘how things work’. No one yet has a good grip on reality.

      • >>
        No one yet has a good grip on reality.
        I remember another physicist who said that he thought that the Laws of Thermodynamics would probably not be disproved. When someone says, “The science is settled,” I know I’m not talking to a scientist.

  13. Larry Niven wrote about a vast torroidal zone orbiting a small star. It is called “The Integral Trees” after the giant totally shaped trees that grow in the huge space. It would be pretty cool to find out Niven might have to say about this.

  14. “No one has yet observed a synestia directly, but they might be found in other solar systems once astronomers start looking for them alongside rocky planets and gas giants, she said.”
    I wonder if looking at the Asteroid Belt is evidence of a primordial ‘Synestia’? The asteroid belt must have been formed by a collision by two or more planets or dwarf planetoids, since we find in them pure iron and pure silicate that indicates that the asteroids must have been much larger astronomical bodies originally that allowed iron to sink to the core of the planet/planetoid, and then after the smash up, there is all these billions of chunks of rocks of varying materials and densities. Obviously, the asteroid belt did not form directly from a gaseous disc in that orbital position, rather, it appears logical that all the bits and pieces are a result of collision between two or more much larger bodies. Including a fairly small dwarf planet like Ceres that failed to sweep the asteroid belt material back into a planet. Or perhaps it is still sweeping its orbit…
    Ceres is the 33rd-largest known body in the Solar System, it is the only dwarf planet within the orbit of Neptune and the largest mass object in the asteroid belt, estimated to about 1/3 of the entire mass of the asteroid belt. Ceres is the largest object in the asteroid belt that lies between the orbits of Mars and Jupiter. Its diameter is approximately 945 kilometers (587 miles), making it the largest of the minor planets within the orbit of Neptune.
    Perhaps Saturn with its rings is also an example of a miniature synestia around a planet, although instead of pure rocky planetoids colliding, it was partial water/ice moons that collided with densities too low for them to re-solidify into one larger moon. Hence the icy rings around Saturn.

    • Ron, by what she wrote, there wouldn’t be all those chunks of rocks, they would have coalesced back into a planetoid.

      • Tom, Ok…I was wondering about that, hence why I was calling the asteroid belt a primordial synestia, perhaps still evolving back into a planetoid with Ceres attempting over the long term to clear its orbit. But perhaps it is evidence of a failed synestia, and the asteroids are evidence of that. From looking at pictures of Ceres, as as most other celestial bodies, it is heavily cratered. With a lot of abundant material in its orbit, perhaps it is accreting matter slowly with every smaller asteroid acquisition, which gives it more mass to attract more material. And so on…maybe in 2-3 billion years it has coalesced back into a larger dwarf planet.
        So maybe it is mid syntestia, as far as the hypothesis goes.

    • Why “MUST” the asteroid belt have been formed by the collision of two planets?
      Current theory is that the material is a failed planet, It was prevented from forming by the gravity of Jupiter.

  15. “Lock and Stewart modeled what happens”
    I stopped right there. Modeling is mathematical onanism, not science.

  16. I am rather fascinated by this deduction –
    “Most planets likely experience collisions that could form a synestia at some point during their formation, Stewart said.”
    Hard to believe that in the grand vastness of space, planets run around like bumper cars, looking to run into each other.

    • That’s how all planets formed in the first place, by playing bumper cars with each other.

  17. Angular momentum could also cancel between objects depending upon their spin directions and angle of impact. There are a semi infinite number of potential combinations of these factors so how do these “researchers” come up with their theoretical results?

  18. Another theory has the Earth heated by a decay of radioactive elements. We know a precious little about the topmost 10 kilometers of the crust and almost nothing about the remaining 6,360 km to the center. A great playground for a general speculation.

  19. I’m having a little trouble with the conception of how a synestia would work.
    A disk/ring of debris is one thing, as the range of velocity from inner edge to outer edge varies consistently to remain in orbit. But it seems to me the upper and lower boundaries of a synestia would either rip it apart very quickly or there has to be something strange going on.
    The upper and lower boundary orbital velocities are surely radically different to what it required to orbit with the central plane of the debris? Unless there is some internal synestia force operating to force the upper and lower boundary debris (& of course everything above and below the ‘ecliptic’ of the debris) to travel in lockstep with the central plane velocity, that non-central plane debris would assume radically different velocity.
    They appear to need something akin to the invention of Dark Matter, which supposedly causes the disks of galaxies to travel in non-Kepler fashion – so we’d have something causing galactic disks to be unusual, then planetary orbits which are ‘normal’ and then another ‘unusual version at a single planet scale.
    Or do I have my head off the planet? 😀

    • And what happens to all the angular momentum in the synestia as it collapses into a solid planet again? Surely the planet would be spinning like a top?

    • MarkMcD, interesting thoughts.
      I was also curious about these two statements:
      “If it gets big enough and is moving fast enough, parts of the object pass the velocity needed to keep a satellite in orbit, and that’s when it forms a huge, disc-shaped synestia, according to the new study.”
      This isn’t quite clear, but they seem to be suggesting that parts of the object will exceed escape velocity? If that is true, what would cause those parts to stay around? Once you’ve exceeded escape velocity, presumably you will continue on the same trajectory (unless otherwise disturbed), heading ever farther away from the initial collision point . . .
      Or maybe they aren’t talking about escape velocity at all and are just saying the particles will orbit farther out than our current satellites? But in that case in order to remain in orbit those farther particles would have to move slower than the closer particles, not faster . . .
      In any case, it seems poorly worded.
      “Most planets likely experience collisions that could form a synestia at some point during their formation, Stewart said. For an object like Earth, the synestia would not last very long – perhaps a hundred years – before it lost enough heat to condense back into a solid object.”
      Why would losing heat cause orbiting debris to condense back into a solid object? Is there any evidence that, say, the rings of Saturn are condensing back into a single object? The observational evidence we have for Saturn’s rings seems to suggest that larger objects are breaking up into smaller ones, not the other way around . . .
      I’m probably missing something obvious here, but just a couple of questions that come immediately to mind.

    • Other apes besides hominids have convergently evolved molars like our ancestors’, to include Gigantopithecus, a ten foot-tall, ground dwelling orangutan. Until fossils of this ape’s feet and other post-cranial anatomical features are found, this conclusion is tentative at best.
      As one wag joked, “I’d rather be African than Bulgarian”. Regardless of where the first hominid evolved, we’re still ultimately African great apes.
      But there is no reason why our ancestors couldn’t have traveled between Europe and Africa in the late Miocene and early Pliocene Epochs. They did in the prior Oligocene. Orangutans evolved in Asia earlier in the Miocene, as shown by Ramapithecus (Sivapithecus) of India c. 12 Ma and Gigantopithecus in China and SE Asia c. 9 Ma until only 100 Ka.

  20. Paper grabbed for later reading in detail. But I hope it does not have the assertion made in this post!
    Angular momentums (like any momentums) are additive – but not always (actually, very infrequently) with the same sign.
    Ice dancers know this one quite well. Watch any pair do a “romantic” piece, and you’ll probably see them rotating in opposite directions until they grab hold of each other, and “discover” their love. Slowly rotating around their now common center to display their expressions to the audience around the rink. (They could stop dead, too – but that is usually not part of the routines.)

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