Planet Formation: Unsolved Questions in Solar System Origin

[stanz123]

I donʻt believe that a low-density nebula can rotate stably. Looking at a particle at high "latitude", in order to have stable rotation of the nebula as a whole, that particle would have to rotate in a small circle about the rotation axis. But there is no axial force that would maintain a small-circle rotation. Such a high-latitude particle with a non-zero velocity would, in fact, orbit around the center of mass of the nebula, in an inclined orbit (that is, inclined to the mean angular momentum of the nebula). I know this is an unpopular conclusion, but it appears to be unavoidable if one believes in the laws of physics. I would be happy to hear counter-explanations, but hopefully with more rationality than some of the popular ones!

 

[DaveC426913]

I donʻt believe that a low-density nebula can rotate stably. Looking at a particle at high "latitude", in order to have stable rotation of the nebula as a whole, that particle would have to rotate in a small circle about the rotation axis. But there is no axial force that would maintain a small-circle rotation. Such a high-latitude particle with a non-zero velocity would, in fact, orbit around the center of mass of the nebula, in an inclined orbit (that is, inclined to the mean angular momentum of the nebula). I know this is an unpopular conclusion, but it appears to be unavoidable if one believes in the laws of physics. I would be happy to hear counter-explanations, but hopefully with more rationality than some of the popular ones!

1] This thread is 4 years old.

2] Who suggested that nebulae behave the way you are saying?? You are describing the solar system as a spinning top - with all masses - including those above and below the protoplanetary disc - orbiting the common protoplanetary axis! No sane scientifically-educated person thinks that. Certainly no one in this thread suggested such a thing.

 

[Chronos]

It does appear logical the inner planets could not retain much of the original gasses that seeded the solor system. The atmosphere of earth, for example, is probably almost entirely self generated. The preponderance of rocky and metallic meteroids and asteroids in the solar system is indicative of the types of materials available to the inner planets during the accretion phase.

 

[D H]

I donʻt believe that a low-density nebula can rotate stably. Looking at a particle at high "latitude", in order to have stable rotation of the nebula as a whole, that particle would have to rotate in a small circle about the rotation axis. But there is no axial force that would maintain a small-circle rotation. Such a high-latitude particle with a non-zero velocity would, in fact, orbit around the center of mass of the nebula, in an inclined orbit (that is, inclined to the mean angular momentum of the nebula). I know this is an unpopular conclusion, but it appears to be unavoidable if one believes in the laws of physics. I would be happy to hear counter-explanations, but hopefully with more rationality than some of the popular ones!

You are creating a red herring. A nebula does not rotating as a sphere. The formation of a protostar causes the nebula to form a protoplanetary disk.

 

[stanz123]

Rather than name-calling, please read my post more carefully. It was written in response to an earlier post that did, in fact, talk about rotation of the proto-nebula. There are an unfortunate number of relatively sane people who discuss or imply nebula rotation, usually when discussing the evolution of the quasi-spherical nebula into the proto-planetary disk. Stephen Hawking, for example, mentions in passing the creation of a disk by the spinning of the nebula ("like sitting on a piano stool"). Others talk about gravitational collapse into a disk, which violates other physical laws (gravitational attraction). I could describe my own conclusions, which arenʻt much different from those of V. S. Safronov in his book and papers from the 70ʻs. It describes a very large number of inelastic collisions that eventually mutually cancel all out-of-plane angular momentum components, leaving a disk-shaped residue of high-angular-momentum aggregates and particles, and a large mass of low-angular-momentum particles added to the central proto-solar mass.

 

[stanz123]

Sorry - when I re-read *your* post I saw nothing in it like "name-calling"- donʻt know what I was imagining. I withdraw and apologize for that comment! But not for the physics!

 

[D H]

Rather than name-calling, please read my post more carefully. It was written in response to an earlier post that did, in fact, talk about rotation of the proto-nebula.

Which earlier post?

A couple of points about using this forum, stanz123.

1. Please think twice about dredging up old threads -- and this is a very old thread. Look at the dates by the posts.

2. Please use the quote button when you are replying to some post, particularly if the post to which you are replying is not the most recent post in the thread. That quote gives the essential context that the readers of your post need in order to know what you are talking about. I still don't know the earlier post about you are writing.

 

[stanz123]

Which earlier post?

A couple of points about using this forum, stanz123.

1. Please think twice about dredging up old threads -- and this is a very old thread. Look at the dates by the posts.

2. Please use the quote button when you are replying to some post, particularly if the post to which you are replying is not the most recent post in the thread. That quote gives the essential context that the readers of your post need in order to know what you are talking about. I still don't know the earlier post about you are writing.

Thanks for the instruction.

Are you saying that there are newer ideas of planetary disk formation than those I mentioned? Or, can Mentors not comment on content of posts as well as just monitor the flow?

 

[stanz123]

1] This thread is 4 years old.

2] Who suggested that nebulae behave the way you are saying?? You are describing the solar system as a spinning top - with all masses - including those above and below the protoplanetary disc - orbiting the common protoplanetary axis! No sane scientifically-educated person thinks that. Certainly no one in this thread suggested such a thing.

Please refer to my earlier "reply", my apologies that it was not actually replied to you. My question was about the evolution of a nebula into a disk, and several of the suggested processes implied such a spinning-top rotation. If there is a more recent explanation that does not refute physical law, please inform me, if you will. Or send a reference that I can read for myself. Iʻve searched and read a lot of papers, but none seem to see the need for simple physics - like conservation of momentum, or gravitational attraction. I am glad that you and I seem to agree on that requirement, anyway, if I understand you correctly.

 

[Kurdt]

There are other forces at work in the disk as well as gravity. There is a lot of self-interaction that is not fully understood. I think you're neglecting this.

 

[stanz123]

There are other forces at work in the disk as well as gravity. There is a lot of self-interaction that is not fully understood. I think you're neglecting this.

What sorts of forces are they? or, what sorts of observations are there that seem to point to other forces? Iʻve read some fairly recent work on planetary migration, for example, that seems a little dubious. The equations, as nearly as I can parse them, seem to imply an exchange of angular momentum with a cloud of surrounding particles, without explaining how the cloud remains in space without orbiting at the same velocity as the immersed planet (and hence having no difference in angular momentum to exchange.) And I can see a lot of self-interaction, by the inelastic collisions that I mentioned (from Safonov, and maybe others), as well as part-elastic collisions that may shatter the colliding particles. Understanding a multi-body multi-collision orbiting problem is a huge lot more than *I* can analyze, but I always hope that someone(s) else can do it/has done it. But these are all pretty elementary physics, in theory. Itʻs just the astronomical quantities of interactions that make it hard to understand in detail. Of course, we know what the mathematical solution is - weʻre right here, living on it!

 

[Kurdt]

A dust cloud is a fluid and subject to all the intermolecular forces you can expect to find in a fluid. It has viscosity, inter-particle forces, pressure and in the case of an ionised gas there are electromagnetic forces. Remember how weak gravity is compared with other fundamental forces.

 

[DaveC426913]

My question was about the evolution of a nebula into a disk, and several of the suggested processes implied such a spinning-top rotation.

Just so we're clear: there is nothing wrong with spherical nebulae evolving into a disc nebula.

But this:

Looking at a particle at high "latitude", in order to have stable rotation of the nebula as a whole, that particle would have to rotate in a small circle about the rotation axis.

...is not it. So I don't know why you put it forth.



In a spherical nebula, all particles will follow their own path in orbit around the centre of mass.

 

[stanz123]

Just so we're clear: there is nothing wrong with spherical nebulae evolving into a disc nebula.

But this:
...is not it. So I don't know why you put it forth.
In a spherical nebula, all particles will follow their own path in orbit around the centre of mass.

I guess I really wasnʻt making myself clear. The point of my comment was that I did *not* think that a low-density nebula could rotate as a cloud. I made that statement as a horrible example, to illustrate how wrong such an idea must be.

Thereʻs no doubt that clouds evolve into disks, there are lots of examples to observe. Itʻs the mechanism of the evolution that I am interested in, and none of the stuff Iʻve read describes any mechanism that harmonizes with physical laws. The density of the protonebula is too low (mean free paths too long) for the usual thermodynamics to work, except in the sense that thermodynamics is, in a way, a treatment of the multi-body problem of 10^xxx particles interacting on a molecular scale. But the scales here are too large for normal thermodynamics. Water molecules, to pick one molecule as an example, probably form by collisions of individual atoms or radicals, of high enough energy to overcome molecular repulsion but not high enough to disrupt the bond. Molecular astronomy finds large numbers of fairly complex molecules in huge molecular clouds - "more ethanol than has been produced on Earth in all the history of mankind!" - that presumably form in this way where the energy density and matter density are Just Right.

What Iʻm really trying to do is to find out what is/are the generally accepted mechanism(s) for planetary nebula-to-disk evolution, and see if any of them are physically reasonable.

>In a spherical nebula, all particles will follow their own path in orbit around the centre of mass.

Precisely! And it seems likely that there would be, in such a chaos of intersecting orbits, on astronomical time scales, an astronomical number of collisions, that would play a major part in the evolution into a disk, for example, by mutual cancellation of out-of-plane components of angular momentum.

This is an unsatisfactory explanation from an Academic sense, of course, no mathematical treatment is offered.

 

[DaveC426913]

I guess I really wasnʻt making myself clear. The point of my comment was that I did *not* think that a low-density nebula could rotate as a cloud. I made that statement as a horrible example, to illustrate how wrong such an idea must be.

Yes, that was obvious. I/we did not misinterpret that.

But since you don't purport it, and none of us purport it, it was a herring of the reddest kind.

 

[stanz123]

Yes, that was obvious. I/we did not misinterpret that.

But since you don't purport it, and none of us purport it, it was a herring of the reddest kind.

Thatʻs all?!

 

[DaveC426913]

Thatʻs all?!

Everything else you said I have no issue with (mostly because it's beyond my ken :blush:).

 

[stanz123]

Everything else you said I have no issue with (mostly because it's beyond my ken :blush:).

Thanks anyway - I would surely love to hash that stuff over with somebody whoʻs worked in the field but still has an open mind - i.e. willing to think about possibly stupid, non-standard-academic-published ideas without dismissing them out of hand as crank science! Guess Iʻll keep looking - and for heavenʻs sake, thereʻs no reason to blush!

 

[Chronos]

The problem with most such ideas is they tend to violate a well established principle of physics right out of the box. Lacking a reasonable, and well supported, explanation for such a 'violation' is the usual reason for dismissing them out of hand.

 

[stanz123]

The problem with most such ideas is they tend to violate a well established principle of physics right out of the box. Lacking a reasonable, and well supported, explanation for such a 'violation' is the usual reason for dismissing them out of hand.

Another problem, unfortunately, is that a few generally-accepted hypotheses also violate well-established principles of physics but are still accepted because everybody knows they must be right. Like the relativistic time-dilation paradox.

 

[russ_watters]

Another problem, unfortunately, is that a few generally-accepted hypotheses also violate well-established principles of physics but are still accepted because everybody knows they must be right. Like the relativistic time-dilation paradox.

Time dilation does not violate any well established principles of physics - only once-upon-a-time believed but now understood to be wrong principles.

 

[stanz123]

Time dilation does not violate any well established principles of physics - only once-upon-a-time believed but now understood to be wrong principles.

If you take a look at the thread "Spacetime / Time Dilation Question" you may note that "once upon a time" actually seems to be "right now". There's a lot of discussion in that thread within the past few weeks of how fast or slow a relativistic space ship's clock will be when it gets back to Earth/home base/etc. - based purely on special relativity.

On the other hand, the link to "time dilation" in your post is very good (I humbly suggest) in that it says something like "apparent" or "observed" time dilation, but "no real change in clock time" (which can be shown to be impossible by symmetry in any of several experiments with multiple space ships, etc.) But the Science fiction keeps creeping back into the Physics, impossible or not...

 

[Vanadium 50]

I think a) this thread is drifting, and b) it would be good for people to read the stickies in the Relativity sub-forum.

 

[stanz123]

I think a) this thread is drifting, and b) it would be good for people to read the stickies in the Relativity sub-forum.

a) you're quite right b)I just did, thanks for the pointer, interesting commentary