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Everywhere I can hear, that if we press anything to the right size, we create black hole.

Can we create neutron star by pressing, for example, our planet? And what if we press something bigger, like Jupiter?

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It depends on what you mean by "can".

If you want to make the Earth into a black hole, you would have to compress it down to about the size of a marble. That is impossible. That is not just impossible with the technology of today, that is impossible with anything less than magic.

However, if we did make an Earth-mass black hole, it would be stable. (Except to Hawking radiation)

Now if you could somehow compress the Earth into a slightly smaller space, you would need to overcome electron-degeneracy and at some point before making your black hole you would form neutron matter. This is impossible. Nothing can do this, not now and not ever.

An Earth-mass piece of neutron matter, however, would not be stable. Its own gravity is not sufficient to keep it from expanding, so it would very very rapidly explode. The same with Jupiter. A body needs to be larger than about 15% of the mass of the sun to have enough gravity as a neutron star to be stable. Jupiter has about 0.1% of the sun's mass

We can't create a small neutron star by compressing the Earth, unless we use magic, and if we stop squeezing it would explode.

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    $\begingroup$ 5th paragraph is inaccurate. A neutron star could be made that was about 0.15 solar masses. physics.stackexchange.com/questions/143166/… $\endgroup$ – Rob Jeffries Jan 3 '18 at 20:41
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    $\begingroup$ I wish the answer did not use the word "impossible" without the proper qualifiers. The first paragraph makes a very strong claim without backing it up. It might well be true, but that requires either an argument or the honest admission "that requires technology or some physics we do not know about". $\endgroup$ – Anders Sandberg Jan 3 '18 at 21:33
  • $\begingroup$ No that is done on purpose. The impossibilty of compressing matter into neutronium is not a technical limit. It is due to the fact that our tools are made of matter and are limited by their nature. The only way to create the compression required is to use tools that are not made of matter. Ie Magic. $\endgroup$ – James K Jan 3 '18 at 22:59
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    $\begingroup$ Impossible? Do you know clarke's three laws? :) "Any sufficiently advanced technology is indistinguishable from magic." $\endgroup$ – Martin54 Jan 4 '18 at 6:37
  • $\begingroup$ Try dropping Earth onto a large neutron star. Sure, it'd be messy, and a head-on collision may simply blast a lot of the matter back into space, but if you got the angle right I reckon you'd convert a fair proportion of the Earth matter to neutronium. (I specified a large neutron star so that the non-neutronium crust is minimal). $\endgroup$ – PM 2Ring Jan 4 '18 at 22:31

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