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Simplistically, a neutron star is a celestial body with enormous mass crushed into a small volume. That crushing force is gravity and the result, one might think, is atoms packed much more closely than they want to be.

Background rumination in question form

Is it a true or false premise that the condition of the atoms at that point is not permanent? 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? (Yeah... not unlike opening a can-o-snakes).

The actual question

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?

(If this works, it would be a cool source of rogue planets.)

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    $\begingroup$ "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?" -- Yes! Actually, wait....it might explode. Perhaps it depends how quickly it is brought up the gravity gradient. I'll be watching this question! $\endgroup$ – Qami Sep 27 '18 at 19:15
  • $\begingroup$ I asked a question about something related to this once upon a time on Astronomy. $\endgroup$ – HDE 226868 Sep 27 '18 at 19:24
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    $\begingroup$ "That crushing force is gravity and the result, one might think, is atoms packed much more closely than they want to be": No, one may not think that. There are no atoms in a neutron star: it is made of neutronium; hence the name. $\endgroup$ – AlexP Sep 27 '18 at 19:53
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    $\begingroup$ @Renan: Aren't we on a web site which can safely be considered "popular literature"? Anyway, the point is that "neutron stars are composed almost entirely of neutrons", or at least so says the found of all knowledge. $\endgroup$ – AlexP Sep 27 '18 at 20:07
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    $\begingroup$ @Mołot, so what happens? Is the material stable? If a portion of it is withdrawn from the gravity well, what happens? $\endgroup$ – JBH Sep 27 '18 at 23:07
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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.

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  • $\begingroup$ From the linked Wikipedia page, "A very small minority of neutron decays (about four per million) are so-called "two-body (neutron) decays", in which a proton, electron and antineutrino are produced as usual, but the electron fails to gain the 13.6 eV necessary energy to escape the proton (the ionization energy of hydrogen), and therefore simply remains bound to it, as a neutral hydrogen atom (one of the "two bodies")." The rest disperse at relativistic speeds; probably more than escape velocity for the neutron star $\endgroup$ – JollyJoker Sep 28 '18 at 6:42
  • $\begingroup$ The neutrons in this case are anything but isolated. The cup full of neutrons is essentially a single atom with an very high atomic mass and very little charge. The only thing that was keeping that matter stable was the gravity of the star. $\endgroup$ – Mathaddict Sep 28 '18 at 15:28
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    $\begingroup$ Much of the neutron star's matter near the surface is nowhere near as dense as an atomic nucleus, but as you go down, the particles are more and more smashed together. As such, you will encounter neutron bunches of any possible size as you go down. Neutrons don't naturally repel each other, so these neutron bunches may stay stable while you lift them up until some neutrons start decaying into protons turning them into possibly heavy atomic nuclei. So, the result will not necessarily be almost pure hydrogen, but can also contain stuff like gold. $\endgroup$ – cmaster - reinstate monica Sep 28 '18 at 21:52
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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.

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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)

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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.
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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.

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A full, direct hit of a neutron star with a correctly sized black hole should suffice to scatter virtually the entire neutron star's crust into empty space. You won't survive this kind of event on any planet in the galactic vicinity, but the cloud of debris will be rich in heavy elements.

The trick is for the black hole to be fast and heavy enough to take a sizable fraction of the neutron star's mass out of the system never to return. Since the black hole's diameter will be much less than that of the neutron star, even though the black hole is much more heavy, the crust of the neutron star will simply not have the time to really react to the approaching black hole until after the collision is over.

The cloud will be hot directly after the hit. It will radiate insanely bright and start to expand immediately. However, as it spreads out, it will also cool down, and as it started really, really dense, parts of it might actually recollapse into planet sized celestial bodies. This will be helped by the asymmetries caused by the collision itself.

The resulting planets will very likely be rocky planets, perhaps even with metallic cores. A neutron star contains neutron clusters of all sizes, which will decay into all kinds of atoms, including the very heavy variants.

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