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It is well known that planetary collisions can create moons orbiting the result of the merger if they happen in the correct way, and this is how the Earth's moon is believed to have been formed. See the animations on this Durham University page to get an idea of how the mechanism works http://icc.dur.ac.uk/giant_impacts/.

It seems to me that it should be at least theoretically possible for the same process to happen when neutron stars collide, which would produce bizarre extremely-high-metallicity (or rather high-average-atomic-mass) planets. However, I also know that the physics is very different in some ways: the colliding objects are much denser; the collision is much higher-energy; radioactive decay creates a burst of extra energy from any matter thrown off the objects; the gravity and velocity are high enough that relativity matters a lot; they probably are in very circular orbits spiraling toward each other rather than hitting each other from the angles that protoplanets do; etc.

It's also possible that most of the mass of the neutron star might be thrown away and leave a low-mass remnant that might expand into an high-atomic-weight planet or white-dwarf, or that some bit of ejected matter might be thrown out at similar enough velocities (speed AND direction) to eventually coalesce into a rogue planet.

I'm just wondering whether anyone has looked into this before, or if anyone has any input as to whether this would be more or less likely than moons forming from planetary collisions, or if anyone knows how to test this with simulations.

EDIT: I've just realized the reason why it is probably impossible for a planet to form in the same way the Moon formed around the Earth: The outward force is way stronger than gravity except for close objects, which would be inside the Roche limit of the resultant black-hole or neutron-star and thus form an accretion disc or ring rather than a planet (due to the fact that any potential planet would be ripped apart by tidal forces). I haven't done any math on this, and this is just my impression, though. In addition, this doesn't mean a planet couldn't form from the ejecta in other ways; for instance, the disc of matter close enough to be held in orbit after the initial explosion might be pushed out to include a planet-forming region outside the Roche limit during a later phase of the event.

EDIT 2: I've had an idea for how this might happen, but I think this might really be a different question. The idea is that, if another star was in the same system as a kilonova (collision between stellar remnants that ejects matter and radiation), the kilonova might leave enough of the star to stay in the system, or perhaps leave enough matter for the other star to capture it somehow. One thing about this scenario, though, is that the idea of another star being in the same system as a compact binary merger rather implies that this third star has already been hit by at least one supernova, possibly multiple and maybe several novae, depending on whether a parasitic binary was formed. (This wouldn't apply if the third star was captured into the system after both of the other stars had already died, though.) Supernovae are stronger than kilonovae in terms of energy that gets thrown out, so the previous supernovae would already have had a stronger affect on the object. I believe that kilonovae are thought to produce heavier elements than any type of supernova, so stars hit by kilonovae would be different in composition than ones hit by supernovae, but it's still basically the same question: What kind of remnants can survive from stars hit by supernovae/kilonovae/novae at close range. I think it's pretty obvious that this could form some kind of remnant, possibly depending on the distance to the third star, so that already answers my question, though I don't know what compositions are possible or what masses are likely, but I think this is really a different question that should probably be asked separately if I or anyone else want it answered on Stack Exchange.

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  • $\begingroup$ As I understand it the bulk of e.g. Gold is produced in neutron star mergers, so I think what we have now is pretty much the norm. I'm not sure how "extreme" you expect the concentrations of such elements to become. $\endgroup$ – StephenG Oct 15 '20 at 5:30
  • $\begingroup$ I'm not sure how extreme it would be, but I would think that it would be much higher than in most of the universe, since normally debris from neutron-star-mergers is mixed in with lots of other gas of elements produced in the Big Bang and in normal supernovae before it coalesces into other star systems. In any case, as I understand it, fusion of lighter elements to iron should happen spontaneously on the surfaces of neutron stars ( arxiv.org/abs/1803.03818 ), so one might expect most of the debris to be heavier than Iron, which is extremely unusual. $\endgroup$ – H. H. Oct 15 '20 at 7:06
  • $\begingroup$ Actually, I never actually read that paper all the way through, and now that I'm looking at it, I'm not sure if it says what I thought it did. $\endgroup$ – H. H. Oct 15 '20 at 7:08
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    $\begingroup$ Ok, I get it. Neutron star crusts also contain a lot of neutron-rich nuclides that would decay into iron if they were anywhere else, and at high enough pressures begin to undergo a kind of disproportionation that creates both lighter and heavier isotopes than iron-56, before eventually the line between separate nuclei becomes blurred. Also, another good paper about neutron star crusts is this one: link.springer.com/article/10.12942/lrr-2008-10 $\endgroup$ – H. H. Oct 15 '20 at 7:11
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    $\begingroup$ You might enjoy this question about supernova energy: physics.stackexchange.com/q/455526/123208 It's mostly about core collapse supernovae, though, not kilonovae. $\endgroup$ – PM 2Ring Nov 12 '20 at 8:34
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There do appear to have been some studies on the properties of potential fallback discs formed after neutron star mergers, for example:

These studies focus on explaining X-ray flaring in the aftermath of gamma-ray bursts rather than the potential to form exotic planets in these environments. It does seem fairly likely that some kind of disc does form around the remnant of a neutron star merger, but it's going to be extremely hot and likely so close to the remnant that it will be unable to form planets.

As noted in Menou et al. (2001) "Stability and Evolution of Supernova Fallback Disks", planet formation in fallback discs depends on the timescales for the disc cooling and how long it takes to spread beyond the Roche limit: if the disc becomes neutral before it spreads beyond the Roche limit, spreading becomes reliant on interactions within the remaining disc of rocks. While they consider the case of merging white dwarfs (noting that this scenario leads to a more favourable environment for planets than post-supernova fallback discs around black holes or neutron stars), they do not study the case of merging neutron stars.

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  • $\begingroup$ I haven't read those papers yet, but I'm wondering whether it would make a difference if one or both of the colliding objects were a magnetar and whether the resulting object was a neutron star or a black hole. $\endgroup$ – H. H. Oct 15 '20 at 13:19
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which would produce bizarre extremely-high-metallicity planets

My, fifteen cents: "neutron star" called so, because it consist with barely neutrons, not metals, like ferrum/iron which is core of The Earth.

10km in diameter, heavier than The Sun million times. It is very heavy bunch of neutrons on square like Moscow City.

The borders between atoms wiped out, and the whole star - like one big atom, with "tridizillion" of neutrons, each star may take 10^google*x - place in periodic table of Mendeleev.

Probably, even if you can by collide two of such bodies - extract part of its material in to the common orbit, it would not be metals, definitely not an Iron - which is product of burning of primary stars... It would be pure neutrons.. "Neutron Star".

By the way, black holes - are the results of such collides...

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    $\begingroup$ Thank you, but I seem know more about this than you. Most of the mass does form a black hole, but a small fraction is thrown out. It does not remain as pure neutrons, because pure neutrons are unstable to beta-decay at low pressures. It's not obvious that heavy metals would result, but astrophysicists believe that they do, and create most atoms of heavy elements. GW170817 and GRB 170817A in 2017 showed what appeared to be a neutron star collision ejecting matter. Due to high temp., velocities, & complexity, I think Strontium was the only heavy element confirmed in the debris. See my comments. $\endgroup$ – H. H. Nov 6 '20 at 20:06
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    $\begingroup$ BTW, neutron stars are not pure neutrons. See physics.stackexchange.com/a/275716/123208 and physics.stackexchange.com/a/105475/123208 $\endgroup$ – PM 2Ring Nov 6 '20 at 22:28

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