<p>When a planetary system is born, most material to build bodies is in fine dust(gas state/plasma) because it has termo-dynamic high , thus is easy that gravity force create regular spherical forms width very weight materials as iron (earth center) after that system planetary is yougest (lower termo-dynamic),it has planets and smal bodies with regular forms but with trajectory collision, after has collisions occurred , it has regular and irregular bodies, when it reach state like as solar system, it has regular forms(spherical) semi-regular(with flat surface but irregular form as phobos) and irregular (asteroids),</p>

<p>Other razons:   </p>

<p>because the main axis force from Centripetal force of solar system, has a direction to west-east axis (earth)</p>

<p>Some bodies(planets and satellites) has liquid state matter and this dragg solid matter in her surface</p>

<p>Tectonic plate changes create deformation in surface bodies( Monts etc.)</p>

CC BY-SA 3.0
2014-01-10T15:14:32.940Z
2014-01-10T15:14:32.940Z
Adrian R
<p>If the planet was made of gas or liquid, and was not spinning it would be a perfect sphere, because with each atom settling as close to the center as possible, that's the shape you get. We can see this on Earth whenever we take a ride in a boat and look out at the ocean. We see the surface of a sphere. (If the planet is spinning, it bulges out around the equator a bit.)</p>

<p>A rocky planet like Earth is a bit more complicated than a gas or water world (but only a bit).  Continents are lighter than the mantle, so they float in it, but they're rigid, so they can "stick up" a bit like a boat in water, rather than form a smooth layer over the mantle like oil on water.</p>

CC BY-SA 3.0
2014-01-10T06:03:52.270Z
2014-01-10T06:03:52.270Z
Marc
<p>That's mainly because of the gravity, and of size. Small objects, e.g. a stone, create only a very weak gravitation. The stone is much harder than necessary to withstand this force caused by its own gravity.</p>

<p>Earth is much larger. The gravity at the surface causes the weight we feel. A huge mountain on Earth would be flattened under its own weight, because the matter it's made of isn't hard enough to withstand this weight.
By hills filling up valleys the planet as a whole becomes spherical.</p>

<p>A fast-rotating planet gets a slightly flattened ellipsoid shape.</p>

CC BY-SA 4.0
2018-05-12T14:52:38.457Z
`A fast-rotating planet gets a slightly flattened ellipsoid shape` interesting.  is there any logical explanation I can read.
roottraveller
CC BY-SA 3.0
2014-01-09T18:22:52.720Z
2014-01-09T18:22:52.720Z
Gerald
<p>I am a complete novice when it comes to astronomy, but someone asked me the above question casually. It seems as though planets are all roughly spherical, whereas other celestial bodies aren't necessarily. Is there are particular reason for this?</p>

CC BY-SA 3.0
2014-01-09T18:03:56.113Z
Related: [What's the largest non-spherical astronomical object in the universe?](http://astronomy.stackexchange.com/questions/925/whats-the-largest-non-spherical-astronomical-object-in-the-universe)
called2voyage
CC BY-SA 3.0
2014-01-10T00:57:16.600Z
Also related: http://astronomy.stackexchange.com/questions/1102/what-are-the-current-accepted-theories-of-the-formation-of-the-uranian-moon-mira
oakad
CC BY-SA 3.0
2014-01-10T08:06:54.847Z
Also related (and partially answered there): http://astronomy.stackexchange.com/questions/917/does-a-planets-mass-or-gravity-affect-the-height-of-its-mountains/995#995
Envite
<planet>
Why are planets spherical?
LanceLafontaine
# Why are planets spherical?

(Asked by: username_0 on 2014-01-09)

I am a complete novice when it comes to astronomy, but someone asked me the above question casually. It seems as though planets are all roughly spherical, whereas other celestial bodies aren't necessarily. Is there are particular reason for this?

**Tags:** planet

> **Comments:**
> **username_1**: Related: [What's the largest non-spherical astronomical object in the universe?](http://astronomy.stackexchange.com/questions/925/whats-the-largest-non-spherical-astronomical-object-in-the-universe)  
> **username_2**: Also related: http://astronomy.stackexchange.com/questions/1102/what-are-the-current-accepted-theories-of-the-formation-of-the-uranian-moon-mira  
> **username_3**: Also related (and partially answered there): http://astronomy.stackexchange.com/questions/917/does-a-planets-mass-or-gravity-affect-the-height-of-its-mountains/995#995  

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## 3 answers

### Answer by username_6 on 2014-01-09. Score: 5 | Accepted answer

That's mainly because of the gravity, and of size. Small objects, e.g. a stone, create only a very weak gravitation. The stone is much harder than necessary to withstand this force caused by its own gravity.

Earth is much larger. The gravity at the surface causes the weight we feel. A huge mountain on Earth would be flattened under its own weight, because the matter it's made of isn't hard enough to withstand this weight.
By hills filling up valleys the planet as a whole becomes spherical.

A fast-rotating planet gets a slightly flattened ellipsoid shape.

> **Comments:**
> **username_7**: `A fast-rotating planet gets a slightly flattened ellipsoid shape` interesting.  is there any logical explanation I can read.  

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### Answer by username_4 on 2014-01-10. Score: 3

When a planetary system is born, most material to build bodies is in fine dust(gas state/plasma) because it has termo-dynamic high , thus is easy that gravity force create regular spherical forms width very weight materials as iron (earth center) after that system planetary is yougest (lower termo-dynamic),it has planets and smal bodies with regular forms but with trajectory collision, after has collisions occurred , it has regular and irregular bodies, when it reach state like as solar system, it has regular forms(spherical) semi-regular(with flat surface but irregular form as phobos) and irregular (asteroids),

Other razons:

because the main axis force from Centripetal force of solar system, has a direction to west-east axis (earth)

Some bodies(planets and satellites) has liquid state matter and this dragg solid matter in her surface

Tectonic plate changes create deformation in surface bodies( Monts etc.)


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### Answer by username_5 on 2014-01-10. Score: 1

If the planet was made of gas or liquid, and was not spinning it would be a perfect sphere, because with each atom settling as close to the center as possible, that's the shape you get. We can see this on Earth whenever we take a ride in a boat and look out at the ocean. We see the surface of a sphere. (If the planet is spinning, it bulges out around the equator a bit.)

A rocky planet like Earth is a bit more complicated than a gas or water world (but only a bit). Continents are lighter than the mantle, so they float in it, but they're rigid, so they can "stick up" a bit like a boat in water, rather than form a smooth layer over the mantle like oil on water.


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