Solar system formation? I've looked and I'm not satisfied the generally accepted explanation is satisfactory.

[GeoffonTour]

Ok I was trying to work out how solar systems are formed and couldn’t find a satisfactory answer.

Ok so we start with a cloud of dust and gas. Most people say it starts rotating (how??) and collapses into a disc (how? there was no explanation of what caused it to flatten that made sense to me in 3d space).

Ok the most common explanations start with a cloud of dust and gas. If it’s static then it will all collapse into a centre of mass and everything will fall into the centre and you only get a star. So I presume it has to be moving. I made the assumption that the movement would be random as I can’t think of any law of nature that would cause it to be ordered.

So all these particles are falling slowing towards the centre of mass and colliding. as they collide their vector properties would combine, destroying disorder and leading to a trend towards the same vector. Since this happense with every collision (and gravitational interactions would also reduce disorder I believe every time) and I can’t think of anything that would increase the disorder, that means that eventually everything would share one vector. This would explain why all planets end up on the same orbital plane with very little varation.

The trend towards one vector would be inevitable as soon as it begins, and progression towards the vector that everything ultimately shares would presumably become more and more powerful with every collision. This trend would be shared throughout the system by disordered particles, and I presume that must happen quite early on as there is nothing I can think of that would cause it to spread up and down orbital heights once the system becomes stable and everything has settled into circular orbits (circular orbits are I think essentially one dimensional as far as a momentary explanation of the movement of a point mass goes, you have prograde and retrograde, the circular result is not due to anything changing so it’s not necessary to include it I believe).

That would explain why orbits are cicular and on the same plane. There’s an object in our solar system that’s on a completely different orbit to everything else which could be a trend towards a different vector that didn’t get annihilated because it was further out so it had less interaction with all the matter that collided and led towards the dominant vector overall.

I kept thinking, and I couldn’t for the life of me explain why anything from a mostly static dust cloud could every gain enough transversal velocity (not sure if velocity is the right term there) to reach orbital speed. Even if it started with some, as it fell it would get reduced more and more as it falls towards the sun or the centre of mass overall that will eventullly be the sun.

I believe it’s more probable given my understanding of orbital mechanics that the matter that the planets are made from is extrasolar, which means the sun must have been established or far enough in it’s development to capture the material. The most likely source of that matter as far as I understand would be an exploding star, which means it would all arrive in our solar system with very similar vectors (assuming it all came from one star), which would again make it more probable that they would end up in similar orbits with one plane.

Then I thought about moons, and they I suppose could be manifestations of smaller trends towards order in which some differences to the dominant result survived the overall annihilation of differences in vectors long enough to form groups of matter around what became the planets.

I haven’t seen any explanation anywhere that suggests the extrasolar nature of the matter that made the planets so if I’m wrong please let me know.

Thanks,

Geoff Beint

[magnemoe]

I think the initial gas cloud is huge think an light year or more in diameter. So any sideways movements would be magnified many order of magnitude. And you would have movements because of tidal forces and light on the cloud. 
Something made it collapse after all. 

Now the planetary formation we don't understand well I think. The old theory was that the inner planets are of rock as the light elements did not stay as it was to hot but they stayed outside the asteroid belt so the got the giant planets. This makes some sense but then we found the hot Jupiter's 

[farmerben]

It is a bit curious why all the planets end up on the same disc.  

[GluttonyReaper]

As already mentioned, you get rotation just from angular momentum - the host clouds that stars form from can be on the order of ~few parsecs, and individual "clumps" that go on to form single star systems can easily be a few thousand AU wide. The closer in you draw in gas, the faster it rotates, because you're conserving angular momentum, and some of that gas has a long way to fall and thus a lot of momentum to contribute (the classic example of this is an ice skater drawing their arms in to pull themselves into a fast spin)

11 hours ago, GeoffonTour said:

Ok the most common explanations start with a cloud of dust and gas. If it’s static then it will all collapse into a centre of mass and everything will fall into the centre and you only get a star. So I presume it has to be moving. I made the assumption that the movement would be random as I can’t think of any law of nature that would cause it to be ordered.

So all these particles are falling slowing towards the centre of mass and colliding. as they collide their vector properties would combine, destroying disorder and leading to a trend towards the same vector. Since this happense with every collision

Funnily enough, I think you've described the generally accepted idea here - gas and dust particles collide constantly as they're drawn in closer, transferring angular momentum between each other until they've formed a natural distribution. The actual plane of rotation will depend purely on which angular momentum "wins" out in the end. Also to note: you don't need all of your gas and dust to hit orbital speed... in fact, most of it won't. Anything that's moving too slowly will just fall and become part of the star, and anything moving too fast will be ejected - whatever small fraction is left is plenty to make up a reasonable-sized disc.

11 hours ago, GeoffonTour said:

Most people say it starts rotating (how??) and collapses into a disc (how? there was no explanation of what caused it to flatten that made sense to me in 3d space).

The formation of discs is... somewhat more complicated. You get some of the disc shape from your rotation and friction, in the same way you get oblateness in planets - i.e. spinning your gas/dust clump forces some of the material to the plane of rotation, and then that overdensity creates a zone of higher friction that 'traps' particles as they cross though that plane, creating a greater overdensity... and so on. The factor you might have missed though, is magnetic fields.

There's still plenty we don't understand in terms of detailing exactly how magnetic fields contribute to disc formation (and star & planet formation in general) so I'd recommend having a read around, but a simple model is enough to understand how it can help:

A spherical gas clump forms with a close-to-linear magnetic field running through it, parallel to the axis of rotation (which comes first is beside the point for now...). The gas is partially charged, so is affected by the magnetic field. Crucially, charged particles really don't like moving across magnetic fields, rather preferring to move along them. In this case, it means the magnetic field helps to resist collapses along two dimensions, but has little effect in the remaining third dimension. Thus, you collapse your clump into a 2D shape... a disc.

There's plenty more complexity here that I haven't touched on (see: the magnetic breaking catastrophe) and some fun side-effects that we can more directly observed (e.g. outflows), but in general star formation just seems to be a very complicated problem. Regardless, there's enough mechanics to toy with that I don't think you need to toy with extra material injections or anything to form planets.

 

[Starshot]

It's a good question and good to ask these things.

There seems to me quite strong empirical evidence that clouds of dust and gas will collapse into a disc rather than capture of material already polarised - this is from looking at galaxy and exoplanets. Galaxies like our own are very approximately flat discs and are very common, so there must be something causing that at the galactic scale. Furthermore in exoplanetary systems we have observed exoplanets in approximately the same plane - like our own solar system - which would be unlikely if it was by coincidence of extrastellar capture that our solar system was flat.

[farmerben]

The angular momentum of a gas cloud has a single vector representing its net angular momentum.  Any momentum orthogonal to this vector is likely to be balanced by an equal amount going the opposite direction.  

[Terwin]

99.86% of the mass of our solar system is in the sun.

The remaining 0.14% is orbiting the sun using the net orbital momentum of the starting material.

When you only need to orbit 0.14% and you are pulling in particles from thousands of au away, a tiny amount of initial momentum is all you really need. Light pressure from distant stars may well be all it takes.

 

Anything not in the orbital disk will collide with something in the disk and either move closer to the disk, or closer to the sun/escape velocity.  

Only things with very few chances to interact with the orbital disk will be notably eccentric (like long period commets), as many interactions (collisions or gravitational interactions) will get them lined up with the disk.

Edited Wednesday at 08:56 PM by Terwin

[JoeSchmuckatelli]

https://www.hup.harvard.edu/books/9780674974968

This is a good, approachable book on gravity - easy place to start 

[Jacke]

On 1/29/2025 at 12:03 AM, GeoffonTour said:

Ok I was trying to work out how solar systems are formed and couldn’t find a satisfactory answer.

A long time ago I took a lot of Astrophysics.  The part covering the Formation of the Solar System has developed a lot, but the basics are still much the same.

A good starting place is this Wikipedia article:
https://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System

 

On 1/29/2025 at 12:03 AM, GeoffonTour said:

Ok so we start with a cloud of dust and gas. Most people say it starts rotating (how??) and collapses into a disc (how? there was no explanation of what caused it to flatten that made sense to me in 3d space).

The study of collapse of gas and dust clouds is complex, with some simple details.  It also links into the evolution of galaxies and their initial state.

Many of the clouds pass the requirement to collapse.  I learned this as the Jean's Mass, though it's more commonly referred to as the Jean's Instability now:
https://en.wikipedia.org/wiki/Jeans_instability

One trend as a cloud collapses, especially if the cloud has high Astrophysical Metallicity (Elements of greater Atomic Number than Helium), is that the Jean's Mass gets smaller.  Thus the cloud can break up into many smaller clouds, leading to several stars forming from the same initial cloud.  This also leads to higher average initial masses for Population II and III stars.  There's also the impact that supernovae can have to prompt cloud collapses.

The cloud spins because for the many possible ways the cloud can collapse, the vast majority of dynamic distributions has the cloud on average spinning.  This is also true for galaxy formation, which can lead to the clouds within it to spin as well.

After that, when studying a single stellar system, things get complex.  This has been studied by more and more complex simulations, most of them trying to determine how our Solar System evolved.  I suggest digging through the articles in Wikipedia.

Edited Thursday at 04:56 AM by Jacke

[Vanamonde]

As a cloud of particles is condensed by its own gravity, it would be strange if it didn't pick up some rotation. For there to be no rotation, all the particles would have to have, purely by chance, cancelled out each other's momentum. 

[Spacescifi]

According to google AI:

AI Overview
+2
Does the sun rotate clockwise? Science of solar rotation | Space
No, the Sun does not rotate clockwise; it rotates counterclockwise when viewed from the North Pole, meaning it spins from west to east, like most other planets in our solar system.
Explanation: This counterclockwise rotation is a result of the initial spinning motion of the gas and dust cloud that formed our solar system.
Key points about the Sun's rotation:

    Direction: Counterclockwise

Reason for this direction: Conservation of angular momentum from the collapsing gas cloud that formed the solar system.

 

Me: Basically it's the whole action/reaction thing happening. Cause and effect.