# can 'planet' become blackhole?

by Chitose
Tags: blackhole, planet
 P: 71 first, what I know about blackhole is; It has so much mass that collapse into itself create gravity field so strong that even light can't escape. and normally, blackhole born only in giant star explosion. but what 'IF' normal rocky planet happen to collect mass (by asteroid, gas or anything) until it has so much mass that equal to red giant? or even equal to some small blackhole? in pure theory, can it collapse into itself and become blackhole? If not How big rocky planet can get? I know it silly, but I really wonder. ------------------------------- English is not my native language, sorry if I'm wrong in spelling or gamma.
 P: 2,427 You can imagine some space faring engineers deciding to build a planet in the habitable zone of a sun. Using bulldozer rockets to push chunks of matter together to make a large enough planet only to discover that once they got the size they needed for their civilization... Each new chunk causes the planet to heat up a bit as matter compresses together and then... POOF! it collapses into first a compact star then a neutron star and then a blackhole losing contact with those engineers directing the matter accumulation on the planet. Ahh, Houston we have a problem... It looks like if the mass was 10^3 solar masses then it would collapse into a black hole.
 Sci Advisor PF Gold P: 9,090 Which would be awkward since the star you intended the new 'planet' to orbit would decide to orbit it. Long before you could achieve sufficient mass to form a black hole the core of the 'planet' would get hot enough to initiate fusion. This would not end well. If there was insufficient fuel for fusion, the poor thing would gravitationally collapse then detonate rather spectacularly.
P: 2,427

## can 'planet' become blackhole?

The birth of a sci-fi series here on PF!
 Sci Advisor PF Gold P: 4,781 Here's the way a super aliens commit suicide making a BH where nothing happens until too late: They attach magic propulsion to 2.5 * 10^26 largish asteroids (10^15 kg each, 5-10 km size) from all over the galaxy (or galaxies, as needed). They collect them in in one region, keeping them about than 1000 km apart. As soon as they are all assembled at this mutual distance, suicide has been achieved - they are within the collection's Schwarzschild radius. Singularity (classically) guaranteed. Ensuing pyrotechnics don't matter - nothing will escape the region, and (classically) all will soon reach the singularity. (Minor technical issue - that mass of asteroids is about a couple percent the mass of a very large galaxy; so probably need to farm many galaxies for enough asteroids.).
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 Quote by Chronos Which would be awkward since the star you intended the new 'planet' to orbit would decide to orbit it. Long before you could achieve sufficient mass to form a black hole the core of the 'planet' would get hot enough to initiate fusion. This would not end well. If there was insufficient fuel for fusion, the poor thing would gravitationally collapse then detonate rather spectacularly.
I wonder if you know the answer to this fanciful scenario. Suppose a primarily iron/nickel planet was bombarded with nothing but iron/nickel asteroids. Would it collapse to a neutron star, or explode in some fashion without leaving a neutron star? Or would it form a neutron star but blow off layers as well? The obvious idea is that no nuclear processes would occur.
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P: 10,776
 Quote by PAllen I wonder if you know the answer to this fanciful scenario. Suppose a primarily iron/nickel planet was bombarded with nothing but iron/nickel asteroids. Would it collapse to a neutron star, or explode in some fashion without leaving a neutron star? Or would it form a neutron star but blow off layers as well? The obvious idea is that no nuclear processes would occur.
It would eventually go supernova and turn into a neutron star.
 P: 50 Anything can become a black hole if you make it's density increase to a point that the object's escape velocity equals the speed of light. If the Earth was a black hole, for example, it's diameter would be around 2 cm
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 Quote by Drakkith It would eventually go supernova and turn into a neutron star.
The normal theory of a supernova starts from a star, and is collapse/explosion following a nuclear fuel cycle. The structure, energy, and composition are completely different from my proposal.

I know what GR says if you just treat it as ideal matter (e.g perfect fluid) of the indicated density: it just smoothly collapses when the amount of matter exceeds a threshold. But that is not what would happen in reality. The issue is that the atomic electron structure become unsustainable at some point. What I don't know, and was asking, is if somewhere in this breakdown (with different composition and much less starting energy than a star), enough energy would still be released for some type of explosion. The answer is not obvious at all, and I was hoping Chronos might know something about it.
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 Quote by gabriel.dac Anything can become a black hole if you make it's density increase to a point that the object's escape velocity equals the speed of light. If the Earth was a black hole, for example, it's diameter would be around 2 cm
There is no known process to achieve that for the earth. What you need is a way to add mass without an explosion ocurring. That is because the greater the mass, the less the density required for a BH because the Schwarzschild radius is proportional to mass, while the mass is proportional to radius cubed. Thus, for any density at all, there is an amount of mass such that it is inside the SC radius at that density. For example, for the Milky Way galaxy as a whole to become a BH, it would just have to be compressed to the point where its average density is 3.72 * 10^-8 gm/cc, that is 100,000 times less dense than air.
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 Quote by PAllen I wonder if you know the answer to this fanciful scenario. Suppose a primarily iron/nickel planet was bombarded with nothing but iron/nickel asteroids. Would it collapse to a neutron star, or explode in some fashion without leaving a neutron star? Or would it form a neutron star but blow off layers as well? The obvious idea is that no nuclear processes would occur.
I share your doubts. I don't know, but, my guess is a core collapse supernova is not triggered by the outer layers comprised of light elements, but, the nickel iron core - hence, the term core collapse supernova. The detonation event may not be spectacular without an outer layer of light elements to bombard with thermal neutrons, but, I'm fairly convinced it will occur.
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 Quote by Chronos I share your doubts. I don't know, but, my guess is a core collapse supernova is not triggered by the outer layers comprised of light elements, but, the nickel iron core - hence, the term core collapse supernova. The detonation event may not be spectacular without an outer layer of light elements to bombard with thermal neutrons, but, I'm fairly convinced it will occur.
I did some reading, and would guess that if mass increase was slow, as proposed:

- You would get core collapse, with huge energy release almost all as neutrinos
- but most other features of Type II supernova would be absent (no shell for a neutrinos or a shock wave to interact with)
- you would be left with a neutron star

If you kept bombarding with asteroids, you would eventually get a black hole.
 Sci Advisor PF Gold P: 9,090 Perhaps, or you might get a thermonuclear detonation of matter that accumulates on the surface and possibly destroy the star. Creating a black hole is not a trivial process. For example, pop III stars are generally considered capable of achieving incredible masses: hundreds, and perhaps thousands of solar masses. Yet, they obviously polluted the ISM with huge amounts of metals - suggesting few collapsed to form black holes without expelling an enormous amount of energy and mass in the process [e.g., GRB's].
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 Quote by Chronos Perhaps, or you might get a thermonuclear detonation of matter that accumulates on the surface and possibly destroy the star. Creating a black hole is not a trivial process. For example, pop III stars are generally considered capable of achieving incredible masses: hundreds, and perhaps thousands of solar masses. Yet, they obviously polluted the ISM with huge amounts of metals - suggesting few collapsed to form black holes without expelling an enormous amount of energy and mass in the process [e.g., GRB's].
Thermonuclear isn't possible in my fictitions scenario because all matter added is iron/nickel.

In the normal core collapse process you have a star 10 or more times the mass of the sun, only 10% of whose mass is Fe/Ni, almost all in the core. The core collapses and rebounds as soon as the Chandreshekhar limit is reached -that is, the core is 1.4 solar masses. So you have collapse and rebound of the core, with most of the star's mass blown away (most, but not all from the outer layers, leaving a neutron star in most cases). So it seems to me with incremental accretion of Fe/Ni to an Fe/Ni planet, you would have the core part of this happening, with no matter for the rest. Then, with further accretion onto a neutron star, if all the infall remains Fe/Ni as I propose, I don't see what could happen other than matter crushed to neutron star state with energy carried off by neutrinos and EM radiation, for each infall chunk. At some fixed mass, you would get BH.
 P: 2,427 The OP hasn't responded perhaps we can close this thread.
P: 50
 Quote by PAllen There is no known process to achieve that for the earth. What you need is a way to add mass without an explosion ocurring. That is because the greater the mass, the less the density required for a BH because the Schwarzschild radius is proportional to mass, while the mass is proportional to radius cubed. Thus, for any density at all, there is an amount of mass such that it is inside the SC radius at that density. For example, for the Milky Way galaxy as a whole to become a BH, it would just have to be compressed to the point where its average density is 3.72 * 10^-8 gm/cc, that is 100,000 times less dense than air.
That is a possibility too, but I'm not wrong. It is also impossible to bring more mass to Earth :p