<p>Essentially, a planet can be considered as a self-gravitating fluid. As was already known to <strong>Archimedes</strong>, under gravity low-density fluids are <strong>buoyant</strong> and rise, while high-density fluids sink. So, with some local exceptions, planets are always well structured from the densest material in the core to lower-density material further up.</p>

CC BY-SA 3.0
2017-03-31T17:35:36.470Z
2017-03-31T17:35:36.470Z
Walter
<p>Apparently you don't fully appreciate the results of  gravity.  Try to model a fluid sphere (for our purposes here, a collection of rock, liquid, and gas is semifluidic) such that somehow there's less gravitational pressure near the center than near the outermost shell.  Can't happen. 
Next, think about trying to make chunks of lead, or granite, float on top of a pond (of liquid water).  Won't happen. </p>

<p>Your last question is kind of pointless - there are "rocks" with densities both greater and less than the density of pure iron. </p>

CC BY-SA 3.0
2017-03-31T21:05:22.093Z
You explained by you didn't answer the question.
StellarExile
CC BY-SA 3.0
2017-03-30T12:41:37.307Z
2017-03-30T12:41:37.307Z
Carl Witthoft
<p>When planets are forming, does the densest material sink the core and the less dense material "float" on top? What I'm asking is that are the more dense materials more likely to appear in the core rather than the surface. I'm also wondering, can a planet have an iron surface and still have rock beneath the surface in its crust and core?</p>

CC BY-SA 3.0
2017-03-30T13:17:44.740Z
See [this question](http://physics.stackexchange.com/questions/1888/balloons-and-lifting-gases) which explains the source of less dense objects rising (and conversely more dense objects sinking).
zephyr
CC BY-SA 3.0
2017-03-30T19:22:33.727Z
In a world-building sense, a planet could be covered with oceans, and those oceans, rich in Iron, like the young Earth, then, much of the water evaporates and you're left with an Iron dust over the planet.  Mars is like that a bit.    Solid Iron, probably not possible, but an Iron rich coating is possible.
userLTK
<planet><planetary-formation><core>
Are planets more likely to have dense materials at their cores?
StellarExile
# Are planets more likely to have dense materials at their cores?

(Asked by: username_0 on 2017-03-29)

When planets are forming, does the densest material sink the core and the less dense material "float" on top? What I'm asking is that are the more dense materials more likely to appear in the core rather than the surface. I'm also wondering, can a planet have an iron surface and still have rock beneath the surface in its crust and core?

**Tags:** planet, planetary-formation, core
> **Comments:**
> **username_1**: See [this question](http://physics.stackexchange.com/questions/1888/balloons-and-lifting-gases) which explains the source of less dense objects rising (and conversely more dense objects sinking).  
> **username_2**: In a world-building sense, a planet could be covered with oceans, and those oceans, rich in Iron, like the young Earth, then, much of the water evaporates and you're left with an Iron dust over the planet.  Mars is like that a bit.    Solid Iron, probably not possible, but an Iron rich coating is possible.  

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## 2 answers

### Answer by username_3 on 2017-03-31. Score: 1 | Accepted answer

Essentially, a planet can be considered as a self-gravitating fluid. As was already known to **Archimedes**, under gravity low-density fluids are **buoyant** and rise, while high-density fluids sink. So, with some local exceptions, planets are always well structured from the densest material in the core to lower-density material further up.


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### Answer by username_4 on 2017-03-30. Score: 0

Apparently you don't fully appreciate the results of gravity. Try to model a fluid sphere (for our purposes here, a collection of rock, liquid, and gas is semifluidic) such that somehow there's less gravitational pressure near the center than near the outermost shell. Can't happen.
Next, think about trying to make chunks of lead, or granite, float on top of a pond (of liquid water). Won't happen.

Your last question is kind of pointless - there are "rocks" with densities both greater and less than the density of pure iron.

> **Comments:**
> **username_0**: You explained by you didn't answer the question.  

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