<p>Gravity makes objects compress to their centers of mass, since gravity extends in all directions.</p>

<p>For planets, gravity continues compressing the object until the rocks cannot be compressed anymore, since the pressure will fight against gravity. At this point (called hydrostatic equilibrium), the object has become a spheroid. One of the requirements for an object to be called a "planet" is that it has reached hydrostatic equilibrium.</p>

<p>For black holes, the shape of their event horizons really depend on whether the black hole is spinning or not. For non-spinning black holes, gravity extends in all directions, so the event horizon will become spherical in shape. For spinning black holes, it will likely be an oblate spheroid.</p>

<p>Most galaxies are only spherical when they are forming. Their angular momentum usually causes them to flatten out over time. However, one part of galaxies, called the "halo", is spherical.</p>

<p>Wormholes are purely hypothetical, so I won't really entertain that.</p>

CC BY-SA 3.0
2016-03-23T19:55:00.427Z
As an extra note on galaxies in particular, I found [this question](http://physics.stackexchange.com/questions/25950/why-are-some-galaxies-flat) on the Physics site which explains why you can get some galaxies that aren't spherical.
Dean
CC BY-SA 3.0
2016-03-23T21:09:08.123Z
@Dean Most galaxies aren't spherical.
ProfRob
CC BY-SA 3.0
2016-03-24T11:03:51.263Z
@RobJeffries I know, I was just making the point that they aren't necessarily "round" as the OP says. I wasn't claiming to make a reference to the number of spherical vs non-spherical galaxies with my comment.
Dean
CC BY-SA 3.0
2016-03-23T19:16:37.497Z
2016-03-23T21:15:56.477Z
2016-03-23T21:15:56.477Z
Sir Cumference
<p>I was studying our solar system and all celestial objects are depicted as round objects, including black holes.. worm holes as hollow cylinders as a pathway across two universes.. even the galaxy is show circular.. Why does this happen? </p>

CC BY-SA 3.0
2016-03-23T15:37:31.540Z
Just a comment about wormholes: Do not believe the cylindrical shape you've seen in pictures - this is just a common method to attempt to illustrate them on two-dimensional paper. They **may not** even exist by the way, so do not believe everything you read about them, especially on the internet.
Andy
CC BY-SA 3.0
2016-03-23T16:12:34.137Z
Simple answer is gravity.
Dean
CC BY-SA 3.0
2016-03-23T18:08:04.617Z
@SirCumference, perhaps you should post that as an answer so that this subject doesn't show up in an unanswered search.
BillDOe
CC BY-SA 3.0
2016-03-24T15:39:22.003Z
Can least surface area be also a point?
Anubhav Goel
CC BY-SA 3.0
2018-01-25T14:38:13.373Z
The possible duplicate cited was asked almost two years later than this question and in fact that question has been linked as a possible duplicate of this question. Since this question came first, it should not be considered the duplicate.
zephyr
CC BY-SA 3.0
2018-01-26T15:16:48.520Z
@zephyr - There is no stackexchange rule that says that the newer of two questions that ask the same thing has to be closed as a duplicate of the older question. An older question can be closed as a duplicate of a newer question. Which is marked as a duplicate and which is left open is a judgement decision that depends on the quality of the two questions and on the quality of the answers to those two questions.
David Hammen
CC BY-SA 4.0
2018-05-24T08:57:36.667Z
Reopen voters: we should find a better dupe target to this post, or not close it as a dupe ez al. [Relevant meta discussion](https://astronomy.meta.stackexchange.com/q/443/1891).
peterh
<solar-system><planet>
Why are planets round?
AStrangeLettuce
# Why are planets round?

(Asked by: username_0 on 2016-03-23)

I was studying our solar system and all celestial objects are depicted as round objects, including black holes.. worm holes as hollow cylinders as a pathway across two universes.. even the galaxy is show circular.. Why does this happen?

**Tags:** solar-system, planet
> **Comments:**
> **username_1**: Just a comment about wormholes: Do not believe the cylindrical shape you've seen in pictures - this is just a common method to attempt to illustrate them on two-dimensional paper. They **may not** even exist by the way, so do not believe everything you read about them, especially on the internet.  
> **username_2**: Simple answer is gravity.  
> **username_3**: @SirCumference, perhaps you should post that as an answer so that this subject doesn't show up in an unanswered search.  
> **username_4**: Can least surface area be also a point?  
> **username_5**: The possible duplicate cited was asked almost two years later than this question and in fact that question has been linked as a possible duplicate of this question. Since this question came first, it should not be considered the duplicate.  
> **username_6**: @zephyr - There is no stackexchange rule that says that the newer of two questions that ask the same thing has to be closed as a duplicate of the older question. An older question can be closed as a duplicate of a newer question. Which is marked as a duplicate and which is left open is a judgement decision that depends on the quality of the two questions and on the quality of the answers to those two questions.  
> **username_7**: Reopen voters: we should find a better dupe target to this post, or not close it as a dupe ez al. [Relevant meta discussion](https://astronomy.meta.stackexchange.com/q/443/1891).  

---

## 1 answers

### Answer by username_8 on 2016-03-23. Score: 6 | Accepted answer

Gravity makes objects compress to their centers of mass, since gravity extends in all directions.

For planets, gravity continues compressing the object until the rocks cannot be compressed anymore, since the pressure will fight against gravity. At this point (called hydrostatic equilibrium), the object has become a spheroid. One of the requirements for an object to be called a "planet" is that it has reached hydrostatic equilibrium.

For black holes, the shape of their event horizons really depend on whether the black hole is spinning or not. For non-spinning black holes, gravity extends in all directions, so the event horizon will become spherical in shape. For spinning black holes, it will likely be an oblate spheroid.

Most galaxies are only spherical when they are forming. Their angular momentum usually causes them to flatten out over time. However, one part of galaxies, called the "halo", is spherical.

Wormholes are purely hypothetical, so I won't really entertain that.

> **Comments:**
> **username_2**: As an extra note on galaxies in particular, I found [this question](http://physics.stackexchange.com/questions/25950/why-are-some-galaxies-flat) on the Physics site which explains why you can get some galaxies that aren't spherical.  
> **username_9**: @Dean Most galaxies aren't spherical.  
> **username_2**: @RobJeffries I know, I was just making the point that they aren't necessarily "round" as the OP says. I wasn't claiming to make a reference to the number of spherical vs non-spherical galaxies with my comment.  

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