<p>The main force acting on start and on large bodies, such as planets and stars is gravity. Since it's symmetric, as long as the mass is large enough, gravity shapes those into an approximately spherical shape.</p>

<p>However, when rotation is considered, the sphere is slightly deformed into an <a href="https://en.wikipedia.org/wiki/Spheroid#Oblate_spheroids" rel="noreferrer">oblate spheroid</a>, approximately. That happens on Jupiter and Earth, just to give two examples.</p>

<p>Nevertheless, when looked closely, we can also perceive that astronomical bodies may present surface features, such as mountains, valleys or craters. And those are not symmetric.</p>

<p>Current theory accept that there is a symmetry of gravity and other forces and that the physical laws are the same across time and space (this is called <a href="https://en.wikipedia.org/wiki/Cosmological_principle" rel="noreferrer">isotropy</a>), and it is not expected that the physical laws change.</p>

<p>See also Scientific American answer on <a href="https://www.scientificamerican.com/article/why-are-planets-round/" rel="noreferrer">why are planets round?</a> and NASA's answer <a href="https://spaceplace.nasa.gov/planets-round/en/" rel="noreferrer">why are planets round?</a>.</p>

CC BY-SA 4.0
2019-11-22T09:47:45.160Z
2019-11-22T09:53:05.813Z
2019-11-22T09:53:05.813Z
Ertxiem - reinstate Monica
<p>Planets and stars are roundish due to gravity. The gravitational attraction of an object to other parts of itself grows as objects get ever larger. This self gravitational attraction is negligible for tiny objects. Tiny objects are not symmetric. Their shape is closer to fractal. Somewhat larger objects are shaped more or less like lumpy potatoes. How lumpy depends on size, composition, temperature, and luck. Even larger objects exhibit a mix of potato shape and axial symmetry. Even larger objects exithib more or less axial symmetry.</p>

<p>An object's self-gravitation becomes overwhelming at some point. This point is a radius of 200 to 400 km, depending on composition, temperature, and perturbing influences. Objects at or near the <a href="https://arxiv.org/pdf/1004.1091.pdf" rel="noreferrer">potato radius</a> such as the asteroid Vesta are roundish if one squints hard enough but still potato shaped if one looks carefully. As all of the planets are significantly larger than the potato radius, they are quite roundish.</p>

CC BY-SA 4.0
2019-11-22T14:43:59.210Z
Can you answer my edit part.
आर्यभट्ट
CC BY-SA 4.0
2019-11-23T08:33:44.897Z
@yuvrajsingh That edit completely changes the answer. You're not supposed to do that. If you want to ask about the distribution of matter in the universe, ask a new question - or better, search for an existing one, because this is one of the questions everyone asks here or on physics.SE.
Luaan
CC BY-SA 4.0
2019-11-23T08:54:04.070Z
@Luaan I think you are right I will update it.
आर्यभट्ट
CC BY-SA 4.0
2019-11-22T11:17:03.043Z
2019-11-22T11:17:03.043Z
David Hammen
<p>Gravity.</p>

<p>Things in space large enough to consider(i.e. Generally Speaking), can have only two possible shapes:
1- Perfect or Distorted, some form of a sphere. Ex: Planets, Moons, Big Asteroids...
2- flat as discs. Ex: Galaxies</p>

<p>And no one knows about the shape of the universe because we don't even know how big the universe is.</p>

CC BY-SA 4.0
2019-11-22T17:44:50.003Z
2019-11-22T17:44:50.003Z
Mrityunjay Kandwal
<p><em>Can quantum randomness account for observable discrepancy in the universe?</em> </p>

<p>Your sample size is too small. I think if we could zoom out far enough looking at the CMB, we'd begin to see patterns emerge. And if we could model the entire existence of the universe, we would find the catch-22 of everything being exactly where it should be because of where everything was.</p>

<p>Given that all spherical objects are symmetrical and that 'the circle' is the most common shape in the universe, the question of <em>why geometry</em> has to do with the certainty of the different, but specific, lattices that will form from given atoms, because the shape of something at the molecular level is often the same at the visual level. <em>It's turtles all the way down.</em></p>

<p>Now, if you ask <em>why</em> different atoms form specific geometries, that might not be exactly where the limit between quantum behavior and classical behavior is, but it's certainly the limit of my understanding.</p>

<hr>

<p>In classical mechanics, "The ‘randomness’ stems from ignorance of physical information in the initial toss or throw." - which is why I mention modeling the universe in its entirety.</p>

<p>"In diametrical contrast, in the case of quantum physics, the theorems [...] indicate that quantum randomness does not stem from any such <em>physical information</em>." - which is the stop where I get off....</p>

<p>– <a href="https://en.wikipedia.org/wiki/Quantum_indeterminacy#Logical_independence_and_quantum_randomness" rel="nofollow noreferrer">Quantum indeterminacy</a></p>

CC BY-SA 4.0
2019-11-22T22:40:47.663Z
2019-11-22T22:40:47.663Z
Mazura
<p>I have seen, in my text books, and on nasa sites, the all the planets are symmetrical. And universe also love symmetry. What cause this symmetry, why the symmetry is important. </p>

<p>As we know that universe is expanding day by day. Will the symmetry will change. I am a high school boy, and happy to have an answer on it. </p>

<p>Edit:) we do not know where is the limit between quantum behavior and classical behavior. Can we put the local asymmetry of the universe on the quantum randomness? To be specific, the fact that looking in one direction we see the constellation Centaurus, and in another direction the constellation Lyra, can be put on quantum randomness?</p>

CC BY-SA 4.0
2019-11-22T10:59:01.250Z
Related: [Why does gravity make everything round?](https://astronomy.stackexchange.com/questions/24698). I'm not going to mark this question as a duplicate of the question I marked as related because that other question was not asked well.
David Hammen
CC BY-SA 4.0
2019-11-22T15:46:20.037Z
Try to ask only one question per question posted; you have three questions here, one about planets, one about the change in the structure of the universe over time, and one about the source of asymmetry. Those are all potentially good questions, but if you ask them all at once, you'll probably only get answers to one of them.
Eric Lippert
CC BY-SA 4.0
2019-11-22T15:58:43.667Z
Symmetry at what level?  Our continents are seriously asymmetric.
Carl Witthoft
CC BY-SA 4.0
2019-11-22T16:06:03.570Z
At universe level.
आर्यभट्ट
CC BY-SA 4.0
2019-11-23T03:58:46.710Z
"At universe level" doesn't make sense.
user2357112
CC BY-SA 4.0
2019-11-23T04:04:20.267Z
I think I very well explained the question in edit part, so think before, closing or down voting.
आर्यभट्ट
CC BY-SA 4.0
2019-11-23T08:16:50.243Z
Your edit makes the question worse rather than better.
David Hammen
CC BY-SA 4.0
2019-11-23T08:21:36.717Z
That, s my doubt, so I wrote it in the question.
आर्यभट्ट
CC BY-SA 4.0
2019-11-24T13:37:37.937Z
I already request to close the question. Thanks.
आर्यभट्ट
<planet><universe>
What makes the planets to be symmetric
आर्यभट्ट
# What makes the planets to be symmetric

(Asked by: username_0 on 2019-11-22)

I have seen, in my text books, and on nasa sites, the all the planets are symmetrical. And universe also love symmetry. What cause this symmetry, why the symmetry is important.

As we know that universe is expanding day by day. Will the symmetry will change. I am a high school boy, and happy to have an answer on it.

Edit:) we do not know where is the limit between quantum behavior and classical behavior. Can we put the local asymmetry of the universe on the quantum randomness? To be specific, the fact that looking in one direction we see the constellation Centaurus, and in another direction the constellation Lyra, can be put on quantum randomness?

**Tags:** planet, universe

> **Comments:**
> **username_1**: Related: [Why does gravity make everything round?](https://astronomy.stackexchange.com/questions/24698). I'm not going to mark this question as a duplicate of the question I marked as related because that other question was not asked well.  
> **username_2**: Try to ask only one question per question posted; you have three questions here, one about planets, one about the change in the structure of the universe over time, and one about the source of asymmetry. Those are all potentially good questions, but if you ask them all at once, you'll probably only get answers to one of them.  
> **username_3**: Symmetry at what level?  Our continents are seriously asymmetric.  
> **username_0**: At universe level.  
> **username_4**: "At universe level" doesn't make sense.  
> **username_0**: I think I very well explained the question in edit part, so think before, closing or down voting.  
> **username_1**: Your edit makes the question worse rather than better.  
> **username_0**: That, s my doubt, so I wrote it in the question.  
> **username_0**: I already request to close the question. Thanks.  

---

## 4 answers

### Answer by username_5 on 2019-11-22. Score: 8

The main force acting on start and on large bodies, such as planets and stars is gravity. Since it's symmetric, as long as the mass is large enough, gravity shapes those into an approximately spherical shape.

However, when rotation is considered, the sphere is slightly deformed into an [oblate spheroid](https://en.wikipedia.org/wiki/Spheroid#Oblate_spheroids), approximately. That happens on Jupiter and Earth, just to give two examples.

Nevertheless, when looked closely, we can also perceive that astronomical bodies may present surface features, such as mountains, valleys or craters. And those are not symmetric.

Current theory accept that there is a symmetry of gravity and other forces and that the physical laws are the same across time and space (this is called [isotropy](https://en.wikipedia.org/wiki/Cosmological_principle)), and it is not expected that the physical laws change.

See also Scientific American answer on [why are planets round?](https://www.scientificamerican.com/article/why-are-planets-round/) and NASA's answer [why are planets round?](https://spaceplace.nasa.gov/planets-round/en/).


---

### Answer by username_1 on 2019-11-22. Score: 7

Planets and stars are roundish due to gravity. The gravitational attraction of an object to other parts of itself grows as objects get ever larger. This self gravitational attraction is negligible for tiny objects. Tiny objects are not symmetric. Their shape is closer to fractal. Somewhat larger objects are shaped more or less like lumpy potatoes. How lumpy depends on size, composition, temperature, and luck. Even larger objects exhibit a mix of potato shape and axial symmetry. Even larger objects exithib more or less axial symmetry.

An object's self-gravitation becomes overwhelming at some point. This point is a radius of 200 to 400 km, depending on composition, temperature, and perturbing influences. Objects at or near the [potato radius](https://arxiv.org/pdf/1004.1091.pdf) such as the asteroid Vesta are roundish if one squints hard enough but still potato shaped if one looks carefully. As all of the planets are significantly larger than the potato radius, they are quite roundish.

> **Comments:**
> **username_0**: Can you answer my edit part.  
> **username_6**: @yuvrajsingh That edit completely changes the answer. You're not supposed to do that. If you want to ask about the distribution of matter in the universe, ask a new question - or better, search for an existing one, because this is one of the questions everyone asks here or on physics.SE.  
> **username_0**: @username_6 I think you are right I will update it.  

---

### Answer by username_7 on 2019-11-22. Score: 0

Gravity.

Things in space large enough to consider(i.e. Generally Speaking), can have only two possible shapes:
1- Perfect or Distorted, some form of a sphere. Ex: Planets, Moons, Big Asteroids...
2- flat as discs. Ex: Galaxies

And no one knows about the shape of the universe because we don't even know how big the universe is.


---

### Answer by username_8 on 2019-11-22. Score: 0

*Can quantum randomness account for observable discrepancy in the universe?*

Your sample size is too small. I think if we could zoom out far enough looking at the CMB, we'd begin to see patterns emerge. And if we could model the entire existence of the universe, we would find the catch-22 of everything being exactly where it should be because of where everything was.

Given that all spherical objects are symmetrical and that 'the circle' is the most common shape in the universe, the question of *why geometry* has to do with the certainty of the different, but specific, lattices that will form from given atoms, because the shape of something at the molecular level is often the same at the visual level. *It's turtles all the way down.*

Now, if you ask *why* different atoms form specific geometries, that might not be exactly where the limit between quantum behavior and classical behavior is, but it's certainly the limit of my understanding.

---

In classical mechanics, "The ‘randomness’ stems from ignorance of physical information in the initial toss or throw." - which is why I mention modeling the universe in its entirety.

"In diametrical contrast, in the case of quantum physics, the theorems [...] indicate that quantum randomness does not stem from any such *physical information*." - which is the stop where I get off....

– [Quantum indeterminacy](https://en.wikipedia.org/wiki/Quantum_indeterminacy#Logical_independence_and_quantum_randomness)


---

