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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.

Mr. Nichan
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  • 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. – StephenG - Help Ukraine Oct 15 '20 at 05:30
  • 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 ( https://arxiv.org/abs/1803.03818 ), so one might expect most of the debris to be heavier than Iron, which is extremely unusual. – Mr. Nichan Oct 15 '20 at 07:06
  • 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. – Mr. Nichan Oct 15 '20 at 07:08
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    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: https://link.springer.com/article/10.12942/lrr-2008-10 – Mr. Nichan Oct 15 '20 at 07:11
  • In any case, the paucity of elements much lighter than magnesium would definitely make the planet weird, even if it turned out that it wasn't mostly heavier elements than iron and nickel. There probably would be some lighter elements than magnesium created by some kind of spallation or some such during the collision, though, and most of the mass would probably be built up directly from neutrons anyway, so that actually probably would lead to quite a lot of hydrogen and lighter elements. I don't know, though. – Mr. Nichan Oct 15 '20 at 07:17
  • Based on this paper: https://iopscience.iop.org/article/10.1088/2041-8205/738/2/L32, I would say that neutron-star ejecta is all over the place in terms of the elements from hydrogen to silicon, though with no single isotope much more common than 0.2 mass%, with most of the mass in elements between Tin and Mercury, with one peak centered around Xenon to Cerium and another centered around Iridium and Platinum, both about 0.5 mass% per nuclide at the top. – Mr. Nichan Oct 15 '20 at 12:06
  • I'm having difficulty believing those mass% numbers add up to 100%, but that is based on what a diagram in the paper says, and I don't think I'm misinterpreting it. Also, Any planet that did form would be forming in a gas/plasma that was cooling from very high temperature, so more refractory elements like tungsten and those near it would be favored. Thus the platinum-peak would be the strongest, though perhaps centered more on Osmium than Platinum or something. (Platinum Planet still sounds better, though.) It's possible that Boron, Carbon,or Oxygen might also be heavily represented. – Mr. Nichan Oct 15 '20 at 12:49
  • That paper also gives an estimate that it would take a rate of about 2~3 neutron star mergers in the Milky Way per 100 thousand years to explain all the Europium in solar system. That means 20~30 thousand collisions every billion years, so that there have probably been over a million in the Milky Way since the Earth formed. So that's potentially the base rate to which the likelyhood of planet-formation would have to be multiplied to get the commonness of such planets, should they exist. The likelyhood is likely low and more affected by other factors like companions and magnetic fields, though. – Mr. Nichan Oct 15 '20 at 23:28
  • By "companions", I mean that I think something might be possible in massive trinary star systems, e.g., if a lower mass star orbited two higher mass stars, which then went supernova at least once to form remnants which later merged in a kilonova. Whatever remained of the low-mass third member of the system might be interestingly high-atomic-mass and might even have most of it's mass stripped off and become more like a planet than a star. – Mr. Nichan Oct 16 '20 at 05:05
  • I guess that would fall under the general category of interesting remnants of "What happens when a star is hit by a supernova at close range?". – Mr. Nichan Oct 16 '20 at 05:12
  • You really should merge most of those comments into your question (or perhaps a self-answer). Comments should only be used for temporary or supplementary information, and they can be deleted at any time. – PM 2Ring Nov 04 '20 at 23:39
  • Only the last 3 are really relevant to my question, and the last two are kind of a different question. – Mr. Nichan Nov 06 '20 at 19:34
  • I added something related to those last two into my question. I don't know if I'm supposed to delete those comments now or anything. I might self-answer if I find something interesting after examining those papers antispinward gave or similar stuff and the variable-space of the situations considered in them. – Mr. Nichan Nov 06 '20 at 20:48
  • https://en.wikipedia.org/wiki/GW170817 is a little ambiguous on the remnant of that collision. It says: "A total of 16,000 times the mass of the Earth in heavy elements is believed to have formed, including approximately 10 Earth masses just of the two elements gold and platinum. A hypermassive neutron star was believed to have formed initially, as evidenced by the large amount of ejecta (much of which would have been swallowed by an immediately forming black hole). – PM 2Ring Nov 06 '20 at 22:36
  • (cont) "The lack of evidence for emissions being powered by neutron star spin-down, which would occur for longer-surviving neutron stars, suggest it collapsed into a black hole within milliseconds. Later searches did find evidence of spin-down in the gravitational signal, suggesting a longer-lived neutron star". – PM 2Ring Nov 06 '20 at 22:36
  • By the way, I can tell you that the gold and platinum numbers are simply from simulations, because I think I remember finding the quote in the sources, and I also know that strontium was the first and probably still the only heavy element actually spectrally identified. I think part of the reason why they couldn't do better is because the extreme temperatures and velocities of the debris broaden the spectral lines with lots of random red-shift and blue-shift so that it's hard to see anything. This happens with any supernova, and the variety of elements produced here would make it even worse. – Mr. Nichan Nov 07 '20 at 00:14
  • If by "remnant" you mean the black hole and not the ejected debris, then that's not really that relevant to my question, which makes me wonder if the "EDIT 2" I added, mentioning a different kind of "remnant" was confusing and lead you to think I was talking about the remnant of the merger itself rather that of stars hit by the ejecta, not that I'm offended by comments being only marginally relevant. – Mr. Nichan Nov 07 '20 at 00:21
  • Yes, I was mostly talking about whether the merger resulted in a neutron star or a BH, although I did also mention the ejecta. In either case, some of the ejecta would fall back on the remnant, but there'd be differences that (I think) should be observable. You'd probably get an accretion disk either way, but it's easier for the falling material to hit a neutron star because it's a much bigger target. – PM 2Ring Nov 12 '20 at 08:24
  • BTW, when replying to comments you need to use the @UserName syntax so that the person gets notified of the reply. You get an automatic notification because you're the author of the post. (It can be confusing because if only 2 people are involved in the comment thread they both get auto-notifications). – PM 2Ring Nov 12 '20 at 08:28
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    You might enjoy this question about supernova energy: https://physics.stackexchange.com/q/455526/123208 It's mostly about core collapse supernovae, though, not kilonovae. – PM 2Ring Nov 12 '20 at 08:34

2 Answers2

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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.

  • 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. – Mr. Nichan 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...

JordanoBrunoAlive
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    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. – Mr. Nichan Nov 06 '20 at 20:06
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    BTW, neutron stars are not pure neutrons. See https://physics.stackexchange.com/a/275716/123208 and https://physics.stackexchange.com/a/105475/123208 – PM 2Ring Nov 06 '20 at 22:28