<p>Imagine the initial gas and dust cloud before it collapsed. It was enriched in heavy elements by supernovaes from other stars. As we could simplify a supernova as an "explosion", the shock waves of the novae alone will provide some momentum.</p>

<p>Additionally the sun did not form alone but is surrounded by other stars, which will also exert some force on the initial dust cloud.</p>

<p>The other fact to bear in mind is that even if the cloud collapsed towards it's center, the individual particles and gas molecules can and will have their own path through this cloud. The will collide with each other, be pushed around by radiation and gravity of other bodies.</p>

<p>If we set aside these forces and trace the path of one of this particles towards the center there are two things that could happen. It "hits" the center or it doesn't. If it doesn't hit the center it will continue it's path on a elliptical path around the center. As the gas cloud already has an initial momentum by the forced described above, all particles that do not follow this momentum are more likely to suffer a head-on collision, loose some or all of their momentum and therefore fall towards the center. The other particles will be more likely to follow their path. This will lead to a situation where all particles, that do not follow the overall momentum of the cloud, will fall towards the center and unify with our Proto-Sun. Therefore only the particles that followed the initial momentum of the cloud had the cance to form planets and asteroids.</p>

CC BY-SA 4.0
2018-07-23T14:39:46.073Z
2018-07-23T14:39:46.073Z
CKA
<p>Ontological/statistical answer:</p>

<p>Assume a great many clouds of matter, each of which collapses. They can start out in any configuration (speed, rotation, momentum; being more homogenous or more disturbed etc.) whatsoever. </p>

<p>There are comparatively few special overall configurations: the ones where there is no noticable net rotational momentum, and no appreciable local clumps with their own momentum. <em>These</em> few configurations will lead to not forming a solar system with planets; if all turns out well (highly unlikely), everything will just smack together in the middle and create a very lonely star.</p>

<p>All other cases will lead to some form of momentum, which will start to spin faster as matter gets drawn into a smaller volume (compare dancers who tuck their arms in).</p>

<p>So: lots and lots of statistical probability for a random start configuration to end up with some momentum; very very small probability for that being not the case.</p>

CC BY-SA 4.0
2018-07-23T16:09:15.447Z
"There is exactly one special overall configuration: the one where there is no net rotational momentum" Why?
Surely there are a very large number of states - the peak of the probability distribution - where the net angular momentum is zero. I understand the point that any non-zero net AM of the initial cloud will tend to be magnified as the cloud collapses into a star + planets, but something has to impart that initial AM.
A cloud of particles with randomly distributed velocity vectors, collapsing under gravity, will not end up spinning.
Tom Graham
CC BY-SA 4.0
2018-07-23T16:09:15.447Z
Clumps of matter with their own momentum will end up as planets - but all orbiting in different directions and planes.
Which statement is wrong?
Tom Graham
CC BY-SA 4.0
2018-07-23T17:42:53.380Z
Just as a note, I've converted Tom's answer to a comment, as it was meant to be. I had to do some editing and rolling back and such to get it to work correctly and obey the character limit - apologies if anything is confusing.
HDE 226868
CC BY-SA 4.0
2018-07-23T17:56:04.680Z
@TomGraham the largest number of states perhaps but that doesn't mean much here:  Consider flipping a million indistinguishable fair coins.  While the most common single outcome is exactly 500,000 heads and 500,000 tails, this is still only accounts for 0.08% of outcomes.  There's way more than a million things being collapsed into a star system, and there's more than two states for each.
Dan Uznanski
CC BY-SA 4.0
2018-07-24T11:15:09.463Z
@TomGraham, all of this depends on how we define the dimensions of the "state" (it does not need to have the individual atoms listed up etc.); but that is rather a technicality in regards to my answer. I've changed it a bit, maybe it's better now.
AnoE
CC BY-SA 4.0
2018-07-23T15:41:27.620Z
2018-07-24T11:12:08.450Z
2018-07-24T11:12:08.450Z
AnoE
<p>A particle's angular momentum around a point is the particle's mass times its distance from the point times its velocity around the point.  So a small velocity at a large distance can still mean a lot of angular momentum.</p>

<p>The bit of a <a href="https://en.wikipedia.org/wiki/Star_formation" rel="noreferrer">Giant Molecular Cloud which collapses to form one solar system</a> is typically on the order of a light-year across or somewhat less. It normally possesses some angular momentum -- just from random motions if nothing else.</p>

<p><a href="https://en.wikipedia.org/wiki/Star_formation" rel="noreferrer">When it collapses</a> to form a solar system, its size drops from maybe a light-year down to maybe 10-100 AUs. A light year is about 60,000 AUs (astronomical units), so it is collapsing to about <sup>1</sup>/<sub>1000</sub> the size. To conserve angular momentum, the circular velocity of the matter must <em>increase</em> by a factor of a thousand.  And the fraction of the matter which will form the star collapses by <em>another</em> factor of a thousand or so.</p>

<p>Not only does this give you plenty of speed, but for the star to form it actually needs to <em>shed</em> angular momentum.</p>

<p>See the <a href="https://en.wikipedia.org/wiki/Angular_momentum" rel="noreferrer">Wikipedia article on angular momentum</a> for a lot of good details.</p>

CC BY-SA 4.0
2018-07-23T15:33:39.250Z
The tidal angular momentum from the galaxy's gravity well ensures there's always some to start with.
Joshua
CC BY-SA 4.0
2018-07-23T15:36:51.507Z
... which would mean that we could just reframe the question "why does the galaxy's gravity well have an angular momentum"... ;) @Joshua
AnoE
CC BY-SA 4.0
2018-07-23T15:38:01.843Z
@AnoE: Because the cloud is orbiting it rather than having fallen to the center. The required orbital velocity for the inside and the outside of the cloud aren't the same.
Joshua
CC BY-SA 4.0
2018-07-23T14:43:29.297Z
2018-07-23T14:43:29.297Z
Mark Olson
<p>I know that Earth is always falling towards the Sun, but due to its sideways momentum it just misses always.  But how did it all start?  To go into detail, when the gas cloud with zero angular momentum collided, our solar system was made with planets and sun having their own angular momentum which makes them rotate.  But how did the revolution start in the beginning?  Were  they placed in such a position and distance that they started attracting each other but how ? .They were having only rotation .  But a revolution started which forced them to follow a elleptical path  so how this happened in the beginning?  Did some external force act on them and revolution started?  How was  this revolution started?</p>

<p>i have assumed dust cloud was having zero angular momentum but it can have any value that does not affect the question i think ? </p>

CC BY-SA 4.0
2018-07-23T13:44:48.030Z
Why do you assume that the cloud that collapsed had no angular momentum?
HDE 226868
CC BY-SA 4.0
2018-07-23T14:00:34.633Z
@JanDoggen Question is clearly different as i am not asking about rotation but about revolution as why sideways momentum started ? how these tangential force started to apply on planets its a complete different question i think
Amey
CC BY-SA 4.0
2018-07-23T14:06:28.533Z
The gas cloud will have initial angular momentum from the rotation of the Galaxy. This is conserved as the cloud collapses to form the star and planets
astrosnapper
CC BY-SA 4.0
2018-07-23T14:42:48.773Z
It's not a duplicate of *that* question, @JanDoggen. The referenced question asks about a planet's rotation, with the questioner understanding why planets orbit in the same direction. This question is asking about orbits.
David Hammen
CC BY-SA 4.0
2018-07-23T16:44:03.320Z
The sun is so much biggger than the earth because so much matter did fall into it. In fact all matter with too little momentum. - _i have assumed dust cloud was having zero angular momentum_ in general __nothing__ has zero angular momentum.
TaW
CC BY-SA 4.0
2018-07-23T18:11:47.677Z
Also see https://physics.stackexchange.com/questions/146857/what-is-the-origin-of-spin-of-celestial-objects?noredirect=1&lq=1
ProfRob
<the-sun><earth>
How did the solar system get its angular momentum? Why did Earth not fall into the Sun at the start?
Amey
# How did the solar system get its angular momentum? Why did Earth not fall into the Sun at the start?

(Asked by: username_0 on 2018-07-23)

I know that Earth is always falling towards the Sun, but due to its sideways momentum it just misses always. But how did it all start? To go into detail, when the gas cloud with zero angular momentum collided, our solar system was made with planets and sun having their own angular momentum which makes them rotate. But how did the revolution start in the beginning? Were they placed in such a position and distance that they started attracting each other but how ? .They were having only rotation . But a revolution started which forced them to follow a elleptical path so how this happened in the beginning? Did some external force act on them and revolution started? How was this revolution started?

i have assumed dust cloud was having zero angular momentum but it can have any value that does not affect the question i think ?

**Tags:** the-sun, earth

> **Comments:**
> **username_1**: Why do you assume that the cloud that collapsed had no angular momentum?  
> **username_0**: @JanDoggen Question is clearly different as i am not asking about rotation but about revolution as why sideways momentum started ? how these tangential force started to apply on planets its a complete different question i think  
> **username_2**: The gas cloud will have initial angular momentum from the rotation of the Galaxy. This is conserved as the cloud collapses to form the star and planets  
> **username_3**: It's not a duplicate of *that* question, @JanDoggen. The referenced question asks about a planet's rotation, with the questioner understanding why planets orbit in the same direction. This question is asking about orbits.  
> **username_4**: The sun is so much biggger than the earth because so much matter did fall into it. In fact all matter with too little momentum. - _i have assumed dust cloud was having zero angular momentum_ in general __nothing__ has zero angular momentum.  
> **username_5**: Also see https://physics.stackexchange.com/questions/146857/what-is-the-origin-of-spin-of-celestial-objects?noredirect=1&lq=1  

---

## 3 answers

### Answer by username_10 on 2018-07-23. Score: 12 | Accepted answer

A particle's angular momentum around a point is the particle's mass times its distance from the point times its velocity around the point. So a small velocity at a large distance can still mean a lot of angular momentum.

The bit of a [Giant Molecular Cloud which collapses to form one solar system](https://en.wikipedia.org/wiki/Star_formation) is typically on the order of a light-year across or somewhat less. It normally possesses some angular momentum -- just from random motions if nothing else.

[When it collapses](https://en.wikipedia.org/wiki/Star_formation) to form a solar system, its size drops from maybe a light-year down to maybe 10-100 AUs. A light year is about 60,000 AUs (astronomical units), so it is collapsing to about 1/1000 the size. To conserve angular momentum, the circular velocity of the matter must *increase* by a factor of a thousand. And the fraction of the matter which will form the star collapses by *another* factor of a thousand or so.

Not only does this give you plenty of speed, but for the star to form it actually needs to *shed* angular momentum.

See the [Wikipedia article on angular momentum](https://en.wikipedia.org/wiki/Angular_momentum) for a lot of good details.

> **Comments:**
> **username_11**: The tidal angular momentum from the galaxy's gravity well ensures there's always some to start with.  
> **username_7**: ... which would mean that we could just reframe the question "why does the galaxy's gravity well have an angular momentum"... ;) @username_11  
> **username_11**: @username_7: Because the cloud is orbiting it rather than having fallen to the center. The required orbital velocity for the inside and the outside of the cloud aren't the same.  

---

### Answer by username_6 on 2018-07-23. Score: 2

Imagine the initial gas and dust cloud before it collapsed. It was enriched in heavy elements by supernovaes from other stars. As we could simplify a supernova as an "explosion", the shock waves of the novae alone will provide some momentum.

Additionally the sun did not form alone but is surrounded by other stars, which will also exert some force on the initial dust cloud.

The other fact to bear in mind is that even if the cloud collapsed towards it's center, the individual particles and gas molecules can and will have their own path through this cloud. The will collide with each other, be pushed around by radiation and gravity of other bodies.

If we set aside these forces and trace the path of one of this particles towards the center there are two things that could happen. It "hits" the center or it doesn't. If it doesn't hit the center it will continue it's path on a elliptical path around the center. As the gas cloud already has an initial momentum by the forced described above, all particles that do not follow this momentum are more likely to suffer a head-on collision, loose some or all of their momentum and therefore fall towards the center. The other particles will be more likely to follow their path. This will lead to a situation where all particles, that do not follow the overall momentum of the cloud, will fall towards the center and unify with our Proto-Sun. Therefore only the particles that followed the initial momentum of the cloud had the cance to form planets and asteroids.


---

### Answer by username_7 on 2018-07-23. Score: 0

Ontological/statistical answer:

Assume a great many clouds of matter, each of which collapses. They can start out in any configuration (speed, rotation, momentum; being more homogenous or more disturbed etc.) whatsoever.

There are comparatively few special overall configurations: the ones where there is no noticable net rotational momentum, and no appreciable local clumps with their own momentum. *These* few configurations will lead to not forming a solar system with planets; if all turns out well (highly unlikely), everything will just smack together in the middle and create a very lonely star.

All other cases will lead to some form of momentum, which will start to spin faster as matter gets drawn into a smaller volume (compare dancers who tuck their arms in).

So: lots and lots of statistical probability for a random start configuration to end up with some momentum; very very small probability for that being not the case.

> **Comments:**
> **username_8**: "There is exactly one special overall configuration: the one where there is no net rotational momentum" Why? Surely there are a very large number of states - the peak of the probability distribution - where the net angular momentum is zero. I understand the point that any non-zero net AM of the initial cloud will tend to be magnified as the cloud collapses into a star + planets, but something has to impart that initial AM. A cloud of particles with randomly distributed velocity vectors, collapsing under gravity, will not end up spinning.  
> **username_8**: Clumps of matter with their own momentum will end up as planets - but all orbiting in different directions and planes. Which statement is wrong?  
> **username_1**: Just as a note, I've converted Tom's answer to a comment, as it was meant to be. I had to do some editing and rolling back and such to get it to work correctly and obey the character limit - apologies if anything is confusing.  
> **username_9**: @TomGraham the largest number of states perhaps but that doesn't mean much here:  Consider flipping a million indistinguishable fair coins.  While the most common single outcome is exactly 500,000 heads and 500,000 tails, this is still only accounts for 0.08% of outcomes.  There's way more than a million things being collapsed into a star system, and there's more than two states for each.  
> **username_7**: @TomGraham, all of this depends on how we define the dimensions of the "state" (it does not need to have the individual atoms listed up etc.); but that is rather a technicality in regards to my answer. I've changed it a bit, maybe it's better now.  

---

