<p>Among the asteroids,  Vesta has a differentiated iron core.  Pallas (which is only slightly smaller) is thought not.</p>
<p>To become differentiated, the object needs to at least partially melt. In Vesta, the decay of <sup>26</sup>Al heated the interior sufficiently within a million years (or so) of its formation for the iron and nickel to melt and flow under their own gravity towards the centre.  In Pallas, by contrast, the evidence suggests that the interior didn't even reach the 820K needed to dehydrate silicates, and the interior is likely to be quite homogenous.</p>
<p>Both Vesta and Pallas are a little over 500km in diameter (roughly, neither is quite &quot;round&quot;) and so you can take this as a guide: most rocky objects in the inner solar system that are larger than this will have an iron core. Those that are smaller probably won't.  There may be exceptions, for example, if an impact causes melting, then differentiation could take place on a smaller body.</p>
<p>The very early solar system was a chaotic place, The iron meteorites will have derived from the core of some planetesmials in the early solar system. Most rocky meteorites are chondrites (about 85%), and contain undifferentiated material, from relatively small asteroids.  Achondrites (about 10%) contain material from differentiated bodies (including the Moon and Mars). Iron meteorites are about 5%. Meteorites are quite diverse in their composition indicating a variety of parent bodies.</p>

CC BY-SA 4.0
2024-11-01T11:34:25.560Z
So the size threshold marks where the radiative losses exceed the heat generation from radioactive decay? That would imply different size thresholds for bodies of different origin and hence composition.´(As an aside I find it intriguing that bodies that are not even properly round may have had a molten core.)
Peter - Reinstate Monica
CC BY-SA 4.0
2024-10-31T09:26:20.640Z
2024-10-31T19:31:01.067Z
2024-10-31T19:31:01.067Z
James K
<p>Nickel-iron meteorites imply that a very large planet was totally destroyed, like in Star Wars.</p>
<p><a href="https://i.sstatic.net/CbyK6nOr.webp" rel="nofollow noreferrer"><img src="https://i.sstatic.net/CbyK6nOr.webp" alt="" /></a></p>
<p>What's the minimum size that such a planet could have been?</p>
<p>Everybody else believes the asteroid belt was created by colliding planetsimals. These plantesimals must have been pretty damn big.</p>
<p>I think the existence of nickel-iron meteorites implies that the asteroid belt contained two full size planets that just got a little too attracted to each other.</p>
<p>And the planets that collided were Earth size, I would think the ratio of metal to chondrite meteors would mimic the ratio of iron core to mantle material in the Earth.</p>
<p>If there were no iron meteors, then we would know that the planetesimals were alway smaller than the minimum size I am asking about.</p>
<p>If the iron ratio in meteors is smaller than the ratio in the earth, that would indicate the planetecimals were smaller than the Earth.</p>
<p>Anyway, it seems to me that the iron ratio in Earth minus the iron ratio in meteors would indicate planetesimal size.</p>
<p><a href="https://i.sstatic.net/51hRMUfH.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/51hRMUfH.gif" alt="" /></a></p>
<p>Does it?</p>

CC BY-SA 4.0
2024-10-31T10:26:33.987Z
A very large planet? Unlikely. The total current mass of the asteroid belt is roughly 4% that of our Moon, so just under 0.05% of the mass of the Earth. Of course, some primordial asteroids may have been flung out of the inner system, and some eaten by Jupiter, as I mention here https://astronomy.stackexchange.com/a/35069/16685 Also see https://astronomy.stackexchange.com/a/49425/16685 which has an asteroid size distribution graph.
PM 2Ring
CC BY-SA 4.0
2024-10-31T14:36:01.300Z
Keep in mind that the mass of Vesta, which is widely thought to have a differentiated core, is only 0.004% of the mass of the Earth.
David Hammen
CC BY-SA 4.0
2024-11-01T18:07:13.400Z
What strikes me as odd in the animated picture at the bottom of this post is the horizontal ... scraped/depression lines running along the equator. Totally unrelated to this question, but it was something that immediately jumped out at me.
Greg Burghardt
CC BY-SA 4.0
2024-11-02T08:06:00.780Z
"Nickel-iron meteorites imply that a very large planet was totally destroyed, like in Star Wars" "Everybody else believes the asteroid belt was created by colliding planetsimals". "I think the existence of nickel-iron meteorites implies that the asteroid belt contained two full size planets that just got a little too attracted to each other". Can you articulate your reasons for rejecting the consensus of planetary scientists?
Simon Crase
CC BY-SA 4.0
2024-11-02T12:21:41.750Z
No, it's not a thesis I want to defend. I'm just trying to discover if nickel iron meteors necessitated large planets. Apparently, nobody else thinks so. I was hoping someone would would explain  what kind of temperature and pressure are required to condense iron to the purity and density that I saw in the meteorites that I reached out over the rail and touched in the Smithsonian Institution when I was in 8th grade. It was absolutely pure, shiny metal crystals. I was amazed. === But the hell with it. Answer accepted; case closed.
Miss Understands
CC BY-SA 4.0
2024-11-03T15:46:16.677Z
World of anything like Earth scale don't go splat, they merge.  To go splat the objects must be approaching at far above their escape velocity so that the debris leaves faster than escape for the merged bodies.  Anything else will leave a world behind.  And if they did hit that hard the pieces would go everywhere, not just make an asteroid belt.
Loren Pechtel
CC BY-SA 4.0
2024-11-04T14:48:06.110Z
To add onto @LorenPechtel 's observation, a collision between Earth and another planet about the size of Mars ~4.5 billion years ago is the leading theory for the formation of the Moon. The Earth wasn't splattered to pieces by it, it and the planet just mixed together and coalesced into a new sphere, with the rest of the debris itself coalescing into the Moon: https://en.wikipedia.org/wiki/Giant-impact_hypothesis
Idran
CC BY-SA 4.0
2024-11-05T02:46:35.370Z
@Idran Yup, that's the result of a glancing impact insufficient to completely stop the incoming body.  The end result was mass transfer from Theia to Earth, not the creation of an asteroid belt.  Theia was simply the last major impact, it's very unlikely it was the only one.  Anything that wasn't glancing just became part of the Earth.
Loren Pechtel
<planet><asteroids><collision><asteroid-belt>
How big does a planet have to be before it can form an iron core?
Miss Understands
# How big does a planet have to be before it can form an iron core?

(Asked by: username_0 on 2024-10-31)

Nickel-iron meteorites imply that a very large planet was totally destroyed, like in Star Wars.

[![](https://i.sstatic.net/CbyK6nOr.webp)](https://i.sstatic.net/CbyK6nOr.webp)

What's the minimum size that such a planet could have been?

Everybody else believes the asteroid belt was created by colliding planetsimals. These plantesimals must have been pretty damn big.

I think the existence of nickel-iron meteorites implies that the asteroid belt contained two full size planets that just got a little too attracted to each other.

And the planets that collided were Earth size, I would think the ratio of metal to chondrite meteors would mimic the ratio of iron core to mantle material in the Earth.

If there were no iron meteors, then we would know that the planetesimals were alway smaller than the minimum size I am asking about.

If the iron ratio in meteors is smaller than the ratio in the earth, that would indicate the planetecimals were smaller than the Earth.

Anyway, it seems to me that the iron ratio in Earth minus the iron ratio in meteors would indicate planetesimal size.

[![](https://i.sstatic.net/51hRMUfH.gif)](https://i.sstatic.net/51hRMUfH.gif)

Does it?

**Tags:** planet, asteroids, collision, asteroid-belt
> **Comments:**
> **username_1**: A very large planet? Unlikely. The total current mass of the asteroid belt is roughly 4% that of our Moon, so just under 0.05% of the mass of the Earth. Of course, some primordial asteroids may have been flung out of the inner system, and some eaten by Jupiter, as I mention here https://astronomy.stackexchange.com/a/35069/16685 Also see https://astronomy.stackexchange.com/a/49425/16685 which has an asteroid size distribution graph.  
> **username_2**: Keep in mind that the mass of Vesta, which is widely thought to have a differentiated core, is only 0.004% of the mass of the Earth.  
> **username_3**: What strikes me as odd in the animated picture at the bottom of this post is the horizontal ... scraped/depression lines running along the equator. Totally unrelated to this question, but it was something that immediately jumped out at me.  
> **username_4**: "Nickel-iron meteorites imply that a very large planet was totally destroyed, like in Star Wars" "Everybody else believes the asteroid belt was created by colliding planetsimals". "I think the existence of nickel-iron meteorites implies that the asteroid belt contained two full size planets that just got a little too attracted to each other". Can you articulate your reasons for rejecting the consensus of planetary scientists?  
> **username_0**: No, it's not a thesis I want to defend. I'm just trying to discover if nickel iron meteors necessitated large planets. Apparently, nobody else thinks so. I was hoping someone would would explain  what kind of temperature and pressure are required to condense iron to the purity and density that I saw in the meteorites that I reached out over the rail and touched in the Smithsonian Institution when I was in 8th grade. It was absolutely pure, shiny metal crystals. I was amazed. === But the hell with it. Answer accepted; case closed.  
> **username_5**: World of anything like Earth scale don't go splat, they merge.  To go splat the objects must be approaching at far above their escape velocity so that the debris leaves faster than escape for the merged bodies.  Anything else will leave a world behind.  And if they did hit that hard the pieces would go everywhere, not just make an asteroid belt.  
> **username_6**: To add onto @LorenPechtel 's observation, a collision between Earth and another planet about the size of Mars ~4.5 billion years ago is the leading theory for the formation of the Moon. The Earth wasn't splattered to pieces by it, it and the planet just mixed together and coalesced into a new sphere, with the rest of the debris itself coalescing into the Moon: https://en.wikipedia.org/wiki/Giant-impact_hypothesis  
> **username_5**: @Idran Yup, that's the result of a glancing impact insufficient to completely stop the incoming body.  The end result was mass transfer from Theia to Earth, not the creation of an asteroid belt.  Theia was simply the last major impact, it's very unlikely it was the only one.  Anything that wasn't glancing just became part of the Earth.  

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## 1 answers

### Answer by username_7 on 2024-10-31. Score: 20 | Accepted answer

Among the asteroids, Vesta has a differentiated iron core. Pallas (which is only slightly smaller) is thought not.

To become differentiated, the object needs to at least partially melt. In Vesta, the decay of 26Al heated the interior sufficiently within a million years (or so) of its formation for the iron and nickel to melt and flow under their own gravity towards the centre. In Pallas, by contrast, the evidence suggests that the interior didn't even reach the 820K needed to dehydrate silicates, and the interior is likely to be quite homogenous.

Both Vesta and Pallas are a little over 500km in diameter (roughly, neither is quite "round") and so you can take this as a guide: most rocky objects in the inner solar system that are larger than this will have an iron core. Those that are smaller probably won't. There may be exceptions, for example, if an impact causes melting, then differentiation could take place on a smaller body.

The very early solar system was a chaotic place, The iron meteorites will have derived from the core of some planetesmials in the early solar system. Most rocky meteorites are chondrites (about 85%), and contain undifferentiated material, from relatively small asteroids. Achondrites (about 10%) contain material from differentiated bodies (including the Moon and Mars). Iron meteorites are about 5%. Meteorites are quite diverse in their composition indicating a variety of parent bodies.

> **Comments:**
> **username_8**: So the size threshold marks where the radiative losses exceed the heat generation from radioactive decay? That would imply different size thresholds for bodies of different origin and hence composition.´(As an aside I find it intriguing that bodies that are not even properly round may have had a molten core.)  

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