<p>It won't ever collapse into a black hole, it will collapse to form a neutron star.</p>
<p>As you add mass to your planet, initially it will get larger, perhaps till it reaches a few times the size of the Earth, when it is a fraction of a Jupiter mass. After that, it will get <em>smaller</em> because it's structure is governed by electron degeneracy.</p>
<p>It will shrink until it has a radius of about 2000 km, when it is just over a solar mass. Beyond that it is unstable and will collapse. There will be a large explosion, but the remnant is not massive enough to avoid forming a stable neutron star.</p>
<p>More details here <a href="https://physics.stackexchange.com/q/196520/123208">https://physics.stackexchange.com/q/196520/123208</a></p>
<p>The only way you might do it is to add the mass very quickly so that the iron star becomes very hot and not supported by degeneracy pressure. It would need to be hotter than about <span class="math-container">$5\times 10^8$</span> K, just like the centre of a massive star. Then, if it were more massive than about 2.5 solar masses and you allowed it to cool, it <em>might</em> collapse to form a black hole.</p>

CC BY-SA 4.0
2023-02-25T19:40:02.557Z
2023-02-25T19:40:02.557Z
ProfRob
<p>Suppose we are building a pure iron-56 planet atom by atom, how large can it get in terms of radius before it collapses into a black hole?</p>

CC BY-SA 4.0
2023-02-25T02:24:24.453Z
Related: https://physics.stackexchange.com/q/196520/123208
PM 2Ring
CC BY-SA 4.0
2023-02-25T13:04:17.087Z
You might find [worldbuilding.se] a more useful site.
StephenG - Help Ukraine
<black-hole><planet><neutron-star><white-dwarf>
What is the maximum radius of a pure iron-56 planet?
EFanZh
# What is the maximum radius of a pure iron-56 planet?

(Asked by: username_0 on 2023-02-25)

Suppose we are building a pure iron-56 planet atom by atom, how large can it get in terms of radius before it collapses into a black hole?

**Tags:** black-hole, planet, neutron-star, white-dwarf

> **Comments:**
> **username_1**: Related: https://physics.stackexchange.com/q/196520/123208  
> **username_2**: You might find [worldbuilding.se] a more useful site.  

---

## 1 answers

### Answer by username_3 on 2023-02-25. Score: 8 | Accepted answer

It won't ever collapse into a black hole, it will collapse to form a neutron star.

As you add mass to your planet, initially it will get larger, perhaps till it reaches a few times the size of the Earth, when it is a fraction of a Jupiter mass. After that, it will get *smaller* because it's structure is governed by electron degeneracy.

It will shrink until it has a radius of about 2000 km, when it is just over a solar mass. Beyond that it is unstable and will collapse. There will be a large explosion, but the remnant is not massive enough to avoid forming a stable neutron star.

More details here <https://physics.stackexchange.com/q/196520/123208>

The only way you might do it is to add the mass very quickly so that the iron star becomes very hot and not supported by degeneracy pressure. It would need to be hotter than about $5\times 10^8$ K, just like the centre of a massive star. Then, if it were more massive than about 2.5 solar masses and you allowed it to cool, it *might* collapse to form a black hole.


---

