<p>Asteroids and smaller celestial objects tend to be odd-shaped, whereas planets are always spherical (or slightly oblate due to their spin). The reason obviously is that for sufficiently large masses/sizes the gravitational force overwhelms any resistance of even rocky materials to maintain a shape other than a sphere (which minimises the gravitational energy).</p>

<p>The characteristic scale at which this transition occurs depends on the material and is different for ice and rock as mentioned in <a href="https://astronomy.stackexchange.com/a/11924/633">this answer</a> (to a <a href="https://astronomy.stackexchange.com/questions/2092/what-is-the-minimum-mass-required-so-that-objects-become-spherical-due-to-its-ow">related question</a>) which also links <a href="https://en.wikipedia.org/wiki/Hydrostatic_equilibrium#Planetary_geology" rel="nofollow noreferrer">this useful Wikipedia article</a>. I wonder whether this limit can be estimated from simple physics (involving material constants for ice/rock).</p>

CC BY-SA 4.0
2019-05-06T13:20:48.837Z
@RoryAlsop Good catch (I didn't find that post). However, neither the question nor the answers ask/suggest any estimate for the limit.
Walter
CC BY-SA 4.0
2019-05-06T13:22:31.630Z
I think the read across from the answers there is that there is no specific limit. It depends on mass, density, rotation and makeup.
Rory Alsop
CC BY-SA 4.0
2019-05-06T14:55:02.147Z
The OP's question *I wonder whether this limit can be estimated from simple physics (involving material constants for ice/rock)* has an excellent answer here: [How did early estimates of a “potato radius” set 1 eV ~ GMμ/R and get 200 to 300 km?](https://astronomy.stackexchange.com/q/27875/7982) I've voted to re-open in order to close as duplicate of this instead.
uhoh
<planet><gravity><asteroids>
A quantitive estimate of the size above which celestial bodies must be near-spherical
Walter
# A quantitive estimate of the size above which celestial bodies must be near-spherical

(Asked by: username_0 on 2019-05-06)

Asteroids and smaller celestial objects tend to be odd-shaped, whereas planets are always spherical (or slightly oblate due to their spin). The reason obviously is that for sufficiently large masses/sizes the gravitational force overwhelms any resistance of even rocky materials to maintain a shape other than a sphere (which minimises the gravitational energy).

The characteristic scale at which this transition occurs depends on the material and is different for ice and rock as mentioned in [this answer](https://astronomy.stackexchange.com/a/11924/633) (to a [related question](https://astronomy.stackexchange.com/questions/2092/what-is-the-minimum-mass-required-so-that-objects-become-spherical-due-to-its-ow)) which also links [this useful Wikipedia article](https://en.wikipedia.org/wiki/Hydrostatic_equilibrium#Planetary_geology). I wonder whether this limit can be estimated from simple physics (involving material constants for ice/rock).

**Tags:** planet, gravity, asteroids

> **Comments:**
> **username_0**: @RoryAlsop Good catch (I didn't find that post). However, neither the question nor the answers ask/suggest any estimate for the limit.  
> **username_1**: I think the read across from the answers there is that there is no specific limit. It depends on mass, density, rotation and makeup.  
> **username_2**: The OP's question *I wonder whether this limit can be estimated from simple physics (involving material constants for ice/rock)* has an excellent answer here: [How did early estimates of a “potato radius” set 1 eV ~ GMμ/R and get 200 to 300 km?](https://astronomy.stackexchange.com/q/27875/7982) I've voted to re-open in order to close as duplicate of this instead.  

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