How likely are planets to form after neutron star collisions?

How likely are planets to form after neutron star collisions?

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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

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 This Site.

There do appear to have been some studies on the properties of potential fallback discs formed after neutron star mergers, for example:

  • Rosswog (2007) "Fallback accretion in the aftermath of a compact binary merger "
  • Cannizzo et al. (2011) "Fall-back Disks in Long and Short Gamma-Ray Bursts "
  • Lyutikov (2013) "The Electromagnetic Model of Short GRBs, the Nature of Prompt Tails, Supernova-less Long GRBs, and Highly Efficient Episodic Accretion "

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.

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…

Astronomers just proved the incredible origin of nearly all gold, platinum, and silver in the universe

Platinum and gold are among the most precious substances on Earth, each fetching roughly $1,000 an ounce.

However, their allure may grow stronger — and weirder — thanks to a groundbreaking new finding about their violent, radioactive, and cosmic origins.

On Monday, scientists who won a Nobel Prize for their discovery of gravitational waves, or ripples in the fabric of space, announced the first detection of the collision of two neutron stars.

The team alerted astronomers all over the world to the event right after it happened, helping them point telescopes directly at the scene of the crash and record unprecedented observations of the aftermath in visible light, radio waves, X-rays, and gamma rays.

The two neutron stars most likely merged to form a black hole, though the tiny bit of neutron star that escaped — and formed new elements — could get recycled into planets like Earth where aliens may eventually dig up the metals as we have.

"The calculations we did suggest most of the matter that came out of this event was in a swirling disk around a black hole. Half of that matter fell in, and half of it got ejected," Brian Metzger, an astrophysicist at Columbia University who's one of roughly 4,000 researchers involved in the discovery, told Business Insider. "The matter that ended up in your wedding band could have just as well fallen in."

Astronomers detected the merger from 130 million light-years away, in the galaxy NGC 4993, on the morning of August 17.

"This is going to have a bigger impact on science and human understanding, in many ways, than the first discovery of gravitational waves," Duncan Brown, an astronomer at Syracuse University who's a member of the research collaboration, told Business Insider. "We're going to be puzzling over the observations we've made with gravitational waves and with light for years to come."

Larry Neutron Star Collision Q&A.jpg

Why is the optical discovery of the merger exciting?

What is exciting is not so much the optical discovery per se, but rather the combined discovery across the electromagnetic spectrum and gravity waves. This is exciting for many reasons: Observing the same event with both gravity waves and light allows much more information to be obtained than with a single method and really means that the field of gravitational wave astronomy has come into its own.

Radio observations point to likely explanation for neutron-star merger phenomena

Three months of observations with the National Science Foundation's Karl G. Jansky Very Large Array (VLA) have allowed astronomers to zero in on the most likely explanation for what happened in the aftermath of the violent collision of a pair of neutron stars in a galaxy 130 million light-years from Earth. What they learned means that astronomers will be able to see and study many more such collisions.

On August 17, 2017, the LIGO and VIRGO gravitational-wave observatories combined to locate the faint ripples in spacetime caused by the merger of two superdense neutron stars. It was the first confirmed detection of such a merger and only the fifth direct detection ever of gravitational waves, predicted more than a century ago by Albert Einstein.

The gravitational waves were followed by outbursts of gamma rays, X-rays, and visible light from the event. The VLA detected the first radio waves coming from the event on September 2. This was the first time any astronomical object had been seen with both gravitational waves and electromagnetic waves.

The timing and strength of the electromagnetic radiation at different wavelengths provided scientists with clues about the nature of the phenomena created by the initial neutron-star collision. Prior to the August event, theorists had proposed several ideas -- theoretical models -- about these phenomena. As the first such collision to be positively identified, the August event provided the first opportunity to compare predictions of the models to actual observations.

Astronomers using the VLA, along with the Australia Telescope Compact Array and the Giant Metrewave Radio Telescope in India, regularly observed the object from September onward. The radio telescopes showed the radio emission steadily gaining strength. Based on this, the astronomers identified the most likely scenario for the merger's aftermath.

"The gradual brightening of the radio signal indicates we are seeing a wide-angle outflow of material, traveling at speeds comparable to the speed of light, from the neutron star merger," said Kunal Mooley, now a National Radio Astronomy Observatory (NRAO) Jansky Postdoctoral Fellow hosted by Caltech.

The observed measurements are helping the astronomers figure out the sequence of events triggered by the collision of the neutron stars.

The initial merger of the two superdense objects caused an explosion, called a kilonova, that propelled a spherical shell of debris outward. The neutron stars collapsed into a remnant, possibly a black hole, whose powerful gravity began pulling material toward it. That material formed a rapidly-spinning disk that generated a pair of narrow, superfast jets of material flowing outward from its poles.

If one of the jets were pointed directly toward Earth, we would have seen a short-duration gamma-ray burst, like many seen before, the scientists said.

"That clearly was not the case," Mooley said.

Some of the early measurements of the August event suggested instead that one of the jets may have been pointed slightly away from Earth. This model would explain the fact that the radio and X-ray emission were seen only some time after the collision.

"That simple model -- of a jet with no structure (a so-called top-hat jet) seen off-axis -- would have the radio and X-ray emission slowly getting weaker. As we watched the radio emission strengthening, we realized that the explanation required a different model," said Alessandra Corsi, of Texas Tech University.

The astronomers looked to a model published in October by Mansi Kasliwal of Caltech, and colleagues, and further developed by Ore Gottlieb, of Tel Aviv University, and his colleagues. In that model, the jet does not make its way out of the sphere of explosion debris. Instead, it gathers up surrounding material as it moves outward, producing a broad "cocoon" that absorbs the jet's energy.

The astronomers favored this scenario based on the information they gathered from using the radio telescopes. Soon after the initial observations of the merger site, the Earth's annual trip around the Sun placed the object too close to the Sun in the sky for X-ray and visible-light telescopes to observe. For weeks, the radio telescopes were the only way to continue gathering data about the event.

"If the radio waves and X-rays both are coming from an expanding cocoon, we realized that our radio measurements meant that, when NASA's Chandra X-ray Observatory could observe once again, it would find the X-rays, like the radio waves, had increased in strength," Corsi said.

Mooley and his colleagues posted a paper with their radio measurements, their favored scenario for the event, and this prediction online on November 30. Chandra was scheduled to observe the object on December 2 and 6.

"On December 7, the Chandra results came out, and the X-ray emission had brightened just as we predicted," said Gregg Hallinan, of Caltech.

"The agreement between the radio and X-ray data suggests that the X-rays are originating from the same outflow that's producing the radio waves," Mooley said.

"It was very exciting to see our prediction confirmed," Hallinan said. He added, "An important implication of the cocoon model is that we should be able to see many more of these collisions by detecting their electromagnetic, not just their gravitational, waves."

Mooley, Hallinan, Corsi, and their colleagues reported their findings in the scientific journal Nature.

Last Year's Incredible Neutron Star Collision Likely Produced A Hypermassive Neutron Star

Last year researchers detected the first neutron star collision. It was an epochal discovery. The event was detected with both traditional observatories (from radio waves to gamma rays) and gravitational wave observatories, the first observation of its kind. Among the confirmation of many hypotheses, the merger also started a mystery. What kind of object was formed in this collision?

Based on its mass, it could be either a black hole or a neutron star. This summer, a team claimed that the merger most likely created a black hole. But a new analysis of the gravitational wave detection seems to suggest that the resulting object is a hypermassive neutron star. This is reported in the Monthly Notices of the Royal Astronomical Society.

Researchers Maurice van Putten of Sejong University in South Korea and Massimo Della Valle of Italy's Osservatorio Astronomico de Capodimonte looked at the data collected by the gravitational wave observatories and noticed that after the distinct “chirp” of the merger, there was a descending signal. This signal is consistent with a neutron star but not with a black hole.

“We’re still very much in the pioneering era of gravitational wave astronomy. So it pays to look at data in detail," van Putten said in a statement. "For us this really paid off, and we’ve been able to confirm that two neutron stars merged to form a larger one.”

The neutron star formed in this collision is 2.7 times the mass of our Sun, close to the possible upper limit of how big neutron stars can get before they collapse into black holes. Another neutron star, known as PSR J1748-2021B, has a similar size and the two scientists think that it might have had the same origin.

Previous work has looked at the light emitted by the object, which was not as bright as it should be if the object was a neutron star. Considering the mass and light profile, a black hole seemed to be the best bet. This study turns this picture on its head but clearly leaves us with more questions.

Neutron stars and black holes are complex objects and it is likely that we are missing several important factors to do with how they behave and evolve. More observations are necessary to strengthen our understanding of neutron star collisions. The three gravitational wave observatories – the two LIGO facilities and the Virgo interferometer – will soon be back online after a technical shutdown and next year they will be accompanied by the Kamioka Gravitational Wave Detector (KAGRA) in Japan. This is only just the beginning of gravitational wave astronomy.

We have two ways to measure the expansion of the Universe, measuring the brightness and speed of pulsating and exploding stars and looking at fluctuations in radiation from the early Universe. But both of these methods give different answers.

In 2019, JHU (John Hopkins University) published a study about the expansion of the Universe. The study confirmed that the Universe is expanding about 9% faster than expected based on its trajectory seen shortly after the big bang.

The study was led by Adam Riess, Bloomberg Distinguished Professor of Physics and Astronomy at The Johns Hopkins University and Nobel Laureate.

The team analyzed light from 70 stars in our neighboring galaxy, the Large Magellanic Cloud, with a new method that allowed for capturing quick images of these stars. The stars, called Cepheid variables, brighten and dim at predictable rates that are used to measure nearby intergalactic distances.

But this is still not satisfied scientists around the world, and they are still searching for an expansion rate, again in 2019 research was done by Prof. Wendy Freedman, a decorated Hubble constant veteran who first published her research in 2000.

Freedman and her team announced a new measurement of the Hubble constant using a red giant star. Their observations made with NASA’s Hubble space telescope indicate that the expansion rate of the Universe is just under 70 kilometers per second per megaparsec — slightly smaller than their previous measurement, source.

The Hubble constant is the cosmological parameter that sets the absolute scale, size, and age of the universe it is one of the most direct ways we have of quantifying how the universe evolves,” said Freedman, the John and Marion Sullivan University Professor in Astronomy and Astrophysics and a world-renowned astronomer.

Even though there is still an ongoing debate about how fast the Universe is expanding, many find it surd.

Mystery Bursts

Neutron stars are compact neutron-packed cores left over when massive stars die in supernova explosions. A teaspoon of neutron star would weigh as much as one billion tons. Their internal structure is not completely understood. Neither is their occasional aggregation into close-knit binary pairs of stars that orbit each other. The astronomers Joe Taylor and Russell Hulse found the first such pair in 1974, a discovery that earned them the 1993 Nobel Prize in Physics. They concluded that those two neutron stars were destined to crash into each other in about 300 million years. The two stars newly discovered by LIGO took far longer to do so.

The analysis by Berger and his team suggests that the newly discovered pair was born 11 billion years ago, when two massive stars went supernova a few million years apart. Between these two explosions, something brought the stars closer together, and they went on circling each other for most of the history of the universe. The findings are “in excellent agreement with the models of binary-neutron-star formation,” Berger said.

The merger also solved another mystery that has vexed astrophysicists for the past five decades.

On July 2, 1967, two United States satellites, Vela 3 and 4, spotted a flash of gamma radiation. Researchers first suspected a secret nuclear test conducted by the Soviet Union. They soon realized this flash was something else: the first example of what is now known as a gamma ray burst (GRB), an event lasting anywhere from milliseconds to hours that “emits some of the most intense and violent radiation of any astrophysical object,” Dent said. The origin of GRBs has been an enigma, although some people have suggested that so-called “short” gamma-ray bursts (lasting less than two seconds) could be the result of neutron-star mergers. There was no way to directly check until now.

In yet another nod of good fortune, it so happened that on Aug. 17, the Fermi Gamma-Ray Space Telescope and the International Gamma-Ray Astrophysics Laboratory (Integral) were pointing in the direction of the constellation Hydra. Just as LIGO and Virgo detected gravitational waves, the gamma-ray space telescopes picked up a weak GRB, and, like LIGO and Virgo, issued an alert.

A neutron star merger should trigger a very strong gamma-ray burst, with most of the energy released in a fairly narrow beam called a jet. The researchers believe that the GRB signal hitting Earth was weak only because the jet was pointing at an angle away from us. Proof arrived about two weeks later, when observatories detected the X-ray and radio emissions that accompany a GRB. This provides smoking-gun proof that normal short gamma-ray bursts are produced by neutron-star mergers,” Berger said. “It’s really the first direct compelling connection between these two phenomena.”

Hughes said that the observations were the first in which “we have definitively associated any short gamma-ray burst with a progenitor.” The findings indicate that at least some GRBs come from colliding neutron stars, though it’s too soon to say whether they all do.

Science: Pulsar 'cannibalised' another star's planets

A recently discovered planet that circles a pulsar may have been ‘canni-balised’
when the planet and its parent star collided with the pulsar, according
to astronomers in Cambridge. The star itself would have been destroyed in
the encounter.

Astronomers at Jodrell Bank discovered the pulsar with a planet earlier
this year (New Scientist, Science, 27 July). The planet circles the pulsar,
known as PSR 1829-10, at about the same distance as the Earth orbits around
the Sun.

Theorists have been baffled because a pulsar, or neutron star, is the
relic of a supernova, according to standard theories. By rights, the supernova
explosion should have destroyed any planet this close.

Philip Podsiadlowski and Martin Rees, at Cambridge’s Institute of Astronomy,
along with Jim Pringle, from the Space Telescope Science Institute in Baltimore,
Maryland, now suggest that an originally solitary pulsar acquired this planet
from another star (Nature, vol 352, p 783). If a neutron star were to pass
within a million kilometres of a star like the Sun, its gravity would tear
the star apart. The resulting gas would surround the neutron star to form
a large tenuous atmosphere, which would gradually blow off into space over
a period of about 100 000 years, to leave a system consisting of a neutron
star with one or more planets.


‘There’s a one-in-a-billion chance that the Sun will suffer a collision
like this,’ says Podsiadlowski. If most of the stars in the Galaxy have
planets, then collisions should have led to about 100 neutron stars with

But the rate of collisions would be much higher in globular clusters,
which are dense agglomerations of stars. According to Podsiadlowski, PSR
1829-10 could lie in a globular cluster: this part of the sky is thick with
interstellar dust that would hide the cluster. Cambridge astronomers plan
to look for a globular cluster with an infrared telescope that can see through
the dust.

Another possibility is that the pulsar was not born in a supernova explosion.
The Cambridge team has a second suggestion: that the system originally consisted
of two white dwarf stars stars orbiting each other. The gravity of the more
massive white dwarf ripped its companion apart, to form a dense disc of
gas. Some of this gas fell onto the white dwarf, making it collapse to become
a neutron star. Material remaining in the disc condensed into bodies the
size of planets.

In the same issue of Nature (p 763), Peter Dawson of Trent University
in Ontario suggests that this pulsar may have formed from the collapse of
a single white dwarf. This white dwarf had started life as a star like the
Sun, complete with a planetary system. The planets survived the gentle conversion
of the star into a white dwarf. The white dwarf was heavier than the natural
weight limit for this kind of star, but was initially kept stable by its
high temperature. As it cooled, it spontaneously collapsed into a neutron
star with planets.

6 Answers 6

I want to build on top of already existing answers:

First and foremost, the state of matter in a neutron star is something way out of the ordinary as to assume that "common sense" applies. It is formed by subatomic particles that do not form actual atoms.

In fact, you could compare the neutron star with the initial stages of the Big Bang, before atoms were formed.

Now, if you get to scoop a large enough dust of neutrons, what would happen? Mathaddict claims that it would explode I am not so sure but the most interesting part is that isolated neutrons have a half-life of 14 minutes and 42 seconds in a process that will produce a proton, and electron and an antineutrino.

And what is a proton + an electron? An Hydrogen atom. Maybe some of the protons would combine with (yet unconverted) neutrons to form deuterium, or even Helium by combining with other protons, but that is basically all that you would get from it (again, the comparation with the Big Bang).

Now, the final question would be if 100,000 years would be enough to build a gas giant (the only kind of planet that you could get) from just the Hydrogen and Helium. I am severely lacking in this aspect, but I doubt that -even accounting that the existence of other elements in the solar system could cause gravitational movements that increase the chance of the gas concentrating- 100,000 years would be enough.

A disting possibility, though, would be if the gas cloud was crossed by some already existing planet that served as a "nucleus" to "vacuum" all the gas around it. And even in this case, I am not sure that after 100,000 revolutions you would get little more than a "rock with a lot of hydrogen around it" and not a true gas giant.

For this to happen, you need the neutron star to be hit by something that will not merge with it. Good candidates are a gamma ray burst up close, or another, passing neutron star.

The escape velocity for neutron stars is in the relativistic range. Most of the mass will just fall back. Whatever mass is lost will leave the system at close to light speed. Such mass may reform as rogue planets leaving the galaxy, specially if going out of the galaxy plane.

As for the star, it will actually expand from the lost mass, because the degenerate pressure upon it will be reduced. Once it has lost enough mass it will revert to a regular, dead or dying small star. At this point escape velocities will be much lower, and some debris may reform as gas planets around it.

For the layman understanding I have of neutron stars, they are created once the gravity is strong enough to overcome the degeneracy pressure which keeps the nucleons apart in conventional atomic nucleus. Therefore every atoms collapses into more and more neutrons the closer to the center of the star it goes.

From this it follows that whatever substance venture onto or into a neutron star would be subjected to the same pressure, collapsing into neutrons, too.

So this pretty much rules out any matter based means (spoons and the like, explosives, etc.)

To overcome the gravitational attraction of a neutron star one could use a black hole, which is the following step in the cosmic monstrosity level. However, I am afraid that it would be easier for a camel to go through a neutron star than for a dromedary to escape a black hole.

Assuming one can carefully control the position of the black hole with respect to the neutron star, so that it is kept after the Roche limit and can disgregate but not fall in the black hole.

However I am afraid that the sudden release of the pressure would result in an energetic explosion triggered by the weak force. This might make for a fantastic strong bomb, but not for a planet. (for visual reference, minerals collected in the depth of the Earth crust also tend to explode due to the sudden release of pressure, and they do not deal with strong force at all)

That is quite a few questions. I think it is best to take them one at a time.

  1. Is the condition of the atoms permanent? First, they're not atoms at all, in a neutron star, it doesn't really make sense to talk about atoms. Next, permanent (as far as the state of matter), in the context of taking some away from the star and the gravity holding it in that type of state, no, it is not permanent.
  2. If you took a cup of it away from the star (never mind the how), would it expand to something approximating its original density? First, would it expand, yes, it would expand in a very large explosion in which there would be so much energy released that it wouldn't form a planet at all, just a giant explosion of exotic matter undergoing constant decay and causing more explosions as it decomposes. Second, it is unclear what you mean by its original density, if you were to collect all the exploded bits of the explosion after it all cooled down, it would have a density close to that of regular matter (my guess would be that with that much energy, it would be mostly hydrogen, but I don't think it's possible to know).
  3. How to do get this mass out of the neutron star by hitting it with something? Any method that has sufficient energy to break up the neutron star to remove pieces of it, would also provide the star with enough energy to break apart entirely. You would have to make up some sort of imaginary method to do this and to avoid the problems associated with the exploding mass in order to have this form a planet in the way you described.

Is it a true or false premise that the condition of the atoms at that point is not permanent? As far as we know, yes, it's true.

If you scooped out a cup of neutron star matter and tossed it a long distance away from the star. would it expand to something approximating its original density? Not likely (again, as far as we know).

Assuming this is believable, what mass + force could be brought against a neutron star to cause it to shatter such that the resulting debris does not fall back together quickly (quickly <= 100,000 years) but allows the mass to expand — thereby forming planets? Just about anything with mass moving at relativistic speeds, and hitting at the correct angle.

I liken this to the formation theories of the planet Mercury. Mercury has an unusual composition of elements, compared to what is expected in most planetary creation methods known. One predominant theory, for a while, was that Mercury had originally formed 'normally', but then had a head on collision with another planet sized object, causing the apparently missing elements from the planet's mantel to be vaporized and blown away by solar wind. But this 'head on collision' theory didn't account for some of the materials that were still on the planet's surface, which should also have been vaporized and blown away, and it didn't account for the pieces of the two planets that should then start orbiting the Sun, but aren't. So the theory was adjusted to a 'glancing blow' instead of a head on collision. This allowed most of the surface (the side away from the colision) to remain cool enough not to vaporize the stuff that the head on version would have blown away, and also greatly reduces the amount of shrapnel that would be orbiting the sun, most of it falling back to Mercury, or falling into the Sun, or following the other planet as it exited the solar system or whatever happened to it.

Now, if such a collision had taken place farther from the sun, the debris would not have been so easily absorbed by the sun. And this is actually what is widely regarded as the method the Earth and Moon were formed from. Earth(instead of Mercury) was impacted by something, but this time (most of) the Debris didn't get sucked in to the Sun, instead some fell back to Earth, some formed the Moon, and some flew away to who-knows-where.

Now we have the basis for the Neutron star collision. Something hits it, and it's either very big and moving very fast, or it's not so big and moving VERY fast.

Neutron stars are thought to be between 1.4 and 3 solar masses. Bigger and they become black holes, and smaller and they wouldn't form in the first place. However, they can theoretically be as small as just over 1 solar mass, and still maintain enough gravity to avoid becoming a nuclear explosion rivaling a supernova.

So, if you want to re-form this stellar system from scratch, then it's a head on collision, the Neutron star blows itself to Protons(mostly), and you've got a new proto-star cloud, and stellar and planetary accretion start over.

If you want the Neutron star to remain, then it's a glancing blow, a large chunk comes off, but a small enough amount that the main star has enough gravity to stay a neutron star. The broken chunk blows itself to protons(mostly), since it doesn't have enough gravity itself to avoid it, and you have an accretion disk around a neutron star, which can be used to form planets. Neutron stars are also thought to have a heavy element 'crust', not pure 'neutronium" surfaces, so that might even form rocky planets.

If you want the original Neutron star to remain, but revert (more-or-less-'immediately') back to some other type of more 'normal' star . sorry, no way to do that without much more hand-waving than I've already done here.

How likely are planets to form after neutron star collisions? - Astronomy

A Tour of A New Signal for a Neutron Star Collision Discovered
(Credit: NASA/CXC/A. Hobart)
[Runtime: 02:39]

A bright burst of X-rays has been discovered by NASA's Chandra X-ray Observatory in a galaxy 6.6 billion light years from Earth. This event likely signaled the merger of two neutron stars and could give astronomers fresh insight into how neutron stars &mdash dense stellar objects packed mainly with neutrons &mdash are built.

When two neutron stars merge they produce jets of high energy particles and radiation fired in opposite directions. If the jet is pointed along the line of sight to the Earth, a flash, or burst, of gamma rays can be detected. If the jet is not pointed in our direction, a different signal is needed to identify the merger.

The detection of gravitational waves &mdash ripples in spacetime &mdash is one such signal. Now, with the observation of a bright flare of X-rays, astronomers have found another signal, and discovered that two neutron stars likely merged to form a new, heavier and fast-spinning neutron star with an extraordinarily strong magnetic field.

Chandra observed the source, dubbed XT2, as it suddenly appeared and then faded away after about seven hours. The source is located in the Chandra Deep Field-South, the deepest X-ray image ever taken that contains almost 12 weeks of Chandra observing time, taken at various intervals over several years. The source appeared on March 22nd, 2015 and was discovered later in analysis of archival data.

This result is important because it gives astronomers a chance to learn about the interior of neutron stars, objects that are so dense that their properties could never be replicated on Earth.

A Quick Look at A New Signal for a Neutron Star Collision Discovered
(Credit: NASA/CXC/A. Hobart)
[Runtime: 1:08]

A bright blast of X-rays from a source in a distant galaxy has led astronomers to a fascinating discovery.

Neutron stars are dense stellar objects that contain extreme physical conditions that are impossible to replicate on Earth.

Occasionally, they can merge with each other. Until now, every neutron star merger seen has been followed by a flash of gamma rays.

This new source, dubbed XT2, is different. Scientists saw it changing in X-rays in Chandra data from 2015.

By comparing the Chandra observations to theoretical models, researchers identified XT2 as a neutron star merger.

This new finding will help astronomers learn more about the interior of neutron stars and give them a new method for finding mergers between them.

(Credit: NASA/CXC/Uni. of Science and Technology of China/Y. Xue et al)
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