why are spins and orbits the same direction?

[dougettinger]

I can visualize and reason that the planets orbit the Sun in the same direction as the Sun's rotation due to the protostar disk falling inward. Similarly, I can reason why the satellites orbit their parent planets in the same direction as the rotation of their parent planet. I have much difficulty reasoning why the planets and satellites rotate in the same direction as their orbits.

The nebula hypothesis uses accretion to create large bodies in the solar system. The accretion process is random and cannot create a consistent type of rotation for all bodies. If one considers fluid dynamics with velocity gradients increasing toward the center and viscosity of fluid increasing toward the hotter center, then eddy currents should be set-up that create cells of material rotating in an opposing direction. The planetary rotations would be like the planetary gears of a mechanical system that rotate opposite to the sun and ring gears.

How does the nebular hypothesis justify common directions for planetary orbits and planetary spins (rotation) ?

Doug Ettinger
Pittsburgh, PA
01/26/2011

[BMAONE23]

Triton's orbit is retrograde. It is the only large moon to orbit "backwards", the only other moons with retrograde orbits are Jupiter's moons Ananke, Carme, Pasiphae and Sinope and Saturn's Phoebe all of which are less than 1/10 the diameter of Triton.

[bystander]

The spin of Venus is contrary to its orbit (its upside down) and Uranus is really on tilt.

[dougettinger]

I can visualize and reason that the planets orbit the Sun in the same direction as the Sun's rotation due to the protostar disk falling inward. Similarly, I can reason why the satellites orbit their parent planets in the same direction as the rotation of their parent planet. I have much difficulty reasoning why the planets and satellites rotate in the same direction as their orbits.

I fully realize that there exist anomalies in our solar system regarding this general characteristic.

The nebula hypothesis uses accretion to create large bodies in the solar system. The accretion process is random and cannot create a consistent type of rotation for all bodies. If one considers fluid dynamics with velocity gradients increasing toward the center and viscosity of fluid increasing toward the hotter center, then eddy currents should be set-up that create cells of material rotating in an opposing direction. The planetary rotations would be like the planetary gears of a mechanical system that rotate opposite to the sun and ring gears.

How does the nebular hypothesis justify common directions for planetary orbits and planetary spins (rotation) ?

Doug Ettinger
Pittsburgh, PA
01/26/2011

I am truly interested in any commentary or suggested specific reference that can help me with this question.

Doug Ettinger 02/04/11

[thoth]

Bear in mind the following is not the only way this can happen, it is just a physically intuitive way to explain.

OK, the easy answer is conservation of angular momentum or spin/orbit direction. A huge cloud of gas that rotates about some central axis collapses into a disk as it is gravitationally collapsing. The disk will spin about the same axis that the cloud was. If you imagine sticking a pole through the center of the disk this pole represents its angular momentum vector. Think of sitting on a horse in a merry go round going in one direction around the middle and you get the idea.

When the disk forms it pretty much sets the orbital direction of the planets. When the planets form they also tend to form around a smaller region but with the same angular momentum vector and hence the same spin direction as orbit direction, to reverse this means a large local reversal of angular momentum which can come about by collisions or other methods.

Now a bit more detailed. We have a disk around a protostar where the whole disk is rotating counterclockwise. To first order the rotation velocity in the disk is what we call keplerian where the local orbital velocity is ~sqrt(1/r) with r being the radial distance to the origin. So regions closer to the center move faster and regions farther out move slower.

Now, think of a simple picture of planet formation wherein some region becomes over dense at a particular radius. Now jump onto that little region of over density so you are co-orbiting with it and you are effectively sitting still.(This would be just like sitting in a car in the middle lane of a highway going 50 miles per hour and over taking a car in the right line going 40 miles per hour while a car in the left lane behind you going 60 miles per hour would overtake you.)

From your perspective material farther out is moving backwards because it is orbiting more slowly and regions closer in are moving forward compared to you because they are moving more quickly. This over dense region would begin to gravitationally attract material on either side of it which would cause it to grow. For the over dense region material farther out radially that is gravitationally attracted to you would strike you from the leading edge in the downward direction and material from closer in radially would strike your trailing edge in the upwards direction. The 'curve' in the hit is due to gravity radially bending incoming material. Both of these collisions hit in such a way as to increase the rotational spin of the growing clump in the counterclockwise direction. So, you've attracted material with a counterclockwise orbit with a corresponding angular momentum about the system's center. When the material hits your region it increases the spin of the over density in the counterclockwise direction as well thereby converting orbital angular momentum in one direction to spin angular momentum in the same direction and angular momentum is thereby locally conserved.

In reality this is one way this can happen. The details of how planets end up with the spins they have is not completely understood and it is possible to violate the above reasoning when interactions with larger bodies occur, when waves excited in the original disks transfer angular momentum in such a way as to violate this, collisions occur, etc. A lot of it depends on the precise details accompanying the planets in question. There is nothing that I know of that says planets HAVE to form this way and we certainly get counter-rotating examples. If you're interested I'll dig up some of the papers detailing this on Astro-ph and link them here.

[thoth]

Oh yes, I think the eddy formalism you are talking about is based in the fluid mechanical notion that a fluid element displaced from its location in a keplerian shear flow will exhibit retrograde epicyclic motion. This doesn't take into account local gravitational 'kicks' from material accreting onto overdense regions. You need some type of focusing effect to build up material into planets.

I'll email my old advisor and ask if I've gotten this more or less right and see if he can add to it.

[neufer]

I can visualize and reason that the planets orbit the Sun in the same direction as the Sun's rotation due to the protostar disk falling inward. Similarly, I can reason why the satellites orbit their parent planets in the same direction as the rotation of their parent planet. I have much difficulty reasoning why the planets and satellites rotate in the same direction as their orbits.

The nebula hypothesis uses accretion to create large bodies in the solar system. The accretion process is random and cannot create a consistent type of rotation for all bodies. If one considers fluid dynamics with velocity gradients increasing toward the center and viscosity of fluid increasing toward the hotter center, then eddy currents should be set-up that create cells of material rotating in an opposing direction. The planetary rotations would be like the planetary gears of a mechanical system that rotate opposite to the sun and ring gears.

How does the nebular hypothesis justify common directions for planetary orbits and planetary spins (rotation) ?

The answer probably lies in the solution to to fact that almost all of the angular momentum of the solar system lies in the planets themselves.

When the solar system was formed it was probably a plasma locked into a constant rotation rate by strong magnetic fields.

You cannot ignore the plasma/magnetic field aspects and expect to get the right answers.

[thoth]

Its true that most of the angular momentum in the solar system is in the planets (mostly Jupiter). Its also true that magnetic fields and plasma effects are a dominant driver of angular momentum transport and other dynamics in most accretion disks.

Its implausible that the magnetic fields played the dominant role in the angular momentum profiles in the planets themselves. You can determine ionization fractions in protoplanetary disks due to external irradiation, cosmic ray penetration, and radioactive decay of elements internal to the disk structure which in turn sets the coupling of the neutrals to the magnetic field (mediated by collisions with the ions and electrons, details in first source below). The coupling between the magnetic field and neutrals in protoplanetary disks is strong interior to 1 AU and past ~100 AU but intermediate to the regions (where planet formation occurs) you most likely get MHD quiet zones (where the B field is not a dominant driver).

On global scales the magnetic field probably moves the angular momentum in the gas of the disk around but in planet forming regions the dynamics are most likely mediated by the specific microphysics of planet formation i.e. streaming instabilities, gravitational instabilities, coagulation, oligarchic growth, transport of angular momentum via lindblad resonances, etc.

In short, the dominance of the magnetic field on the motion of the neutrals appears to be too weak in planet forming regions for magnetic fields to locally control the formative dynamics.

Protoplanetary disks remain a troubling area in astrophysics where the process by which angular momentum transport is not well determined because it lacks the coupling of the global field to the neutrals that probably does determine it in other disks such as cataclysmic variables, AGNs, SXTs, etc.

This is the present word on magnetic field interaction in protoplanetary disks http://xxx.lanl.gov/pdf/0906.0854

General background on angular momentum transport in disks http://www.lra.ens.fr/~balbus/araa.pdf

The other problem is with the idea of strong fields, those aren't good for redistributing angular momentum radially as they tend to enforce rigid rotation profiles which is bad because it generates a configuration wherein angular momentum transport is greatly inhibited. Strong fields are good for vertical angular momentum transport along filed lines out of the disk plane via winds, or azimuthal field lines near the central source via jets but these phenomenon are not generally in regions where planet formation is likely to be occurring actively and might be more disruptive than beneficial if they were.

You actually want (and have outside of regions near central sources permeated by strong dipole fields) weaker fields.

[thoth]

To the OP - you also have several scales/regimes during planet formation, some where the rocky bits are tiny and coupled to the gas flow, some where they are influenced by it, and some where they influence it. When you start talking about regimes where the dynamics are controlled by collisions of larger scale objects and gravity of over dense regions comes into play it is no longer in the classical fluid regime where you neglect particle particle collisions. Ultimately you can't rely on the same eddy generation formalism used in the classical navier-stokes equations although it does appear eddies may act as nurseries for smaller rocks which would otherwise be accreted to the central star via angular momentum loss to the background gas flow.

My advisor said it was subtle and situation dependent but when you get Hill Sphere accretion wide enough to see the shear you get sort of the picture I outlined. When the material build up is dominated by the last big impactor you get other configurations.

The truth is that planet formation is not clearly understood and the whole prototypical nebular hypothesis is being re-examined in light of the configurations we are seeing in exoplanetary systems.

Two very good books
http://www.amazon.com/Astrophysics-Plan ... 882&sr=8-1
http://www.amazon.com/Planetary-Science ... 904&sr=1-1

[dougettinger@verizon.net]

To the OP - you also have several scales/regimes during planet formation, some where the rocky bits are tiny and coupled to the gas flow, some where they are influenced by it, and some where they influence it. When you start talking about regimes where the dynamics are controlled by collisions of larger scale objects and gravity of over dense regions comes into play it is no longer in the classical fluid regime where you neglect particle particle collisions. Ultimately you can't rely on the same eddy generation formalism used in the classical navier-stokes equations although it does appear eddies may act as nurseries for smaller rocks which would otherwise be accreted to the central star via angular momentum loss to the background gas flow.

My advisor said it was subtle and situation dependent but when you get Hill Sphere accretion wide enough to see the shear you get sort of the picture I outlined. When the material build up is dominated by the last big impactor you get other configurations.

The truth is that planet formation is not clearly understood and the whole prototypical nebular hypothesis is being re-examined in light of the configurations we are seeing in exoplanetary systems. [/quote]

Thank you, Thoth, for very carefully and very gently answering my perplexing question. I have been away studying atolls for two weeks and have now returned to the forum. Thanks for informing me that the classical fluid regime do not necessarily apply for higher density clumping when a certain protostar disk region is still in the dusty phase. But you did admit that as the accreted bodies become larger it becomes harder to explain the spin of protoplanets. But some unknown mechanism does cause the planets to spin more or less consistently in a prograde direction.

The Grand Neufer ventured into forbidden territory trying to explain this phenomenon with magnetic fields. You explained that there are too many "neutrals" for magnetism to have any large affect. Are you referring to cold, uncharged particles, both dust and rock size?

Doug Ettinger, Pittsburgh, PA

[dougettinger]

Its true that most of the angular momentum in the solar system is in the planets (mostly Jupiter). Its also true that magnetic fields and plasma effects are a dominant driver of angular momentum transport and other dynamics in most accretion disks.

Its implausible that the magnetic fields played the dominant role in the angular momentum profiles in the planets themselves. You can determine ionization fractions in protoplanetary disks due to external irradiation, cosmic ray penetration, and radioactive decay of elements internal to the disk structure which in turn sets the coupling of the neutrals to the magnetic field (mediated by collisions with the ions and electrons, details in first source below). The coupling between the magnetic field and neutrals in protoplanetary disks is strong interior to 1 AU and past ~100 AU but intermediate to the regions (where planet formation occurs) you most likely get MHD quiet zones (where the B field is not a dominant driver).

On global scales the magnetic field probably moves the angular momentum in the gas of the disk around but in planet forming regions the dynamics are most likely mediated by the specific microphysics of planet formation i.e. streaming instabilities, gravitational instabilities, coagulation, oligarchic growth, transport of angular momentum via lindblad resonances, etc.

Thoth, why is the coupling between the magnetic field and neutrals in protoplanetary disks strong past 100 AU but not in the regions of planetary orbits ? For me there are no reasons to have any ionization or magnetic field in the most outer perimeters of a protostar (protoplanetary) disk ? Thoth, I still hope you are in the neighborhood to answer my question.

Doug Ettinger, Pittsburgh, PA 3/24/2011