A Question about Planet Formation

I have a problem with understanding the reason of why six of our eight
planets rotate in the same direction as does our solar system (and
Venus and Uranus initially seem to have done the same), but I haven´t
found this issue discussed on this forum. Laz Marhenke mentioned it in
1996 in his thread "Solar System questions" but the question basically
got sidestepped in the answer by Chris Carrier. Please inform me, if
I have overlooked any thread on this issue.

Neither have I been able to locate a webpage on which this issue is
explained. On http://www.bu.edu/core/cc105/lecture...formation.html
I found a mere mention of that this is due to the conservation of
angular momentum, and that supposedly the angular momentum (of
the ....?planet seed?) points in the same direction as it does in the
"initial rotating disk".

Whatever that means, it seems counterintuitive to (Laz and) me: If the
protoplanetary disk (proplyd) rotates in a keplerian manner, then, if
we put ourselves at any one point within such a disk, particles closer
to the center than us should overtake us on the side towards the
center, while particles further away from the center than us should
stay behind us on the opposite side. Thus, if we are fat enough to
capture particles from the disk by means of gravitation, the majority
of these particles should end up orbiting around us in a rotational
sense *opposite* to that of the proplyd, because otherwise they would
oppose the keplerian stream of particles around us, i.e. overtake us
on the far side from the center and stay behind us on the near side
from the center.

So where do I go wrong here? (or do I?) And of course, please give me
sources. I prefer webpages unless it´s a journal article.

And thank you in advance.

wrote:
I have a problem with understanding the reason of why six of our eight
planets rotate in the same direction as does our solar system (and
Venus and Uranus initially seem to have done the same), but I haven´t
found this issue discussed on this forum. Laz Marhenke mentioned it in
1996 in his thread "Solar System questions" but the question basically
got sidestepped in the answer by Chris Carrier. Please inform me, if
I have overlooked any thread on this issue.

Neither have I been able to locate a webpage on which this issue is
explained. On http://www.bu.edu/core/cc105/lecture...formation.html
I found a mere mention of that this is due to the conservation of
angular momentum, and that supposedly the angular momentum (of
the ....?planet seed?) points in the same direction as it does in the
"initial rotating disk".

Whatever that means, it seems counterintuitive to (Laz and) me: If the
protoplanetary disk (proplyd) rotates in a keplerian manner, then, if
we put ourselves at any one point within such a disk, particles closer
to the center than us should overtake us on the side towards the
center, while particles further away from the center than us should
stay behind us on the opposite side. Thus, if we are fat enough to
capture particles from the disk by means of gravitation, the majority
of these particles should end up orbiting around us in a rotational
sense *opposite* to that of the proplyd, because otherwise they would
oppose the keplerian stream of particles around us, i.e. overtake us
on the far side from the center and stay behind us on the near side
from the center.

So where do I go wrong here? (or do I?) And of course, please give me
sources. I prefer webpages unless it´s a journal article.

And thank you in advance.


It's an interesting question, but I'd have thought that as protoplanets
developed, they would gain more mass falling in from 'outside' their
orbit than from matter already interior to them.
hand waving argment
For collisions to occur, the infalling matter would be on highly
eccentric comet type' orbits with a semi major axis greater than that of
the protoplanet itself.
/hand waving argument

No sources, no references. Not claiming any credit, and quite happy to
see proof to the contrary

Dear p...:

On Apr 10, 9:58*am, wrote:
I have a problem with understanding the reason of
why six of our eight planets rotate in the same
direction as does our solar system (and Venus
and Uranus initially seem to have done the same),
but I haven´t found this issue discussed on this
forum.

Your intuition sees interior *circular* orbits passing you angularly
in the prograde direction. But the energy is lower, so if moved out
to your orbital radius, say via some event that makes the orbit an
ellipse with your radius as its aphelion, you will pass it. Now the
sunward side face of your forming body has objects that are going
slower (angularly and linearly), and likewise the anti-sunward face
has objects that are going faster.

Remember planets form by "traffic jam"... mostly. That darned seed
planet just wont get out of your way, or simply slams you in the butt
because you are not moving fast enough.

David A. Smith

On 10 Apr, 17:58, wrote:
I have a problem with understanding the reason of why six of our eight
planets rotate in the same direction as does our solar system (and
Venus and Uranus initially seem to have done the same), but I haven´t
found this issue discussed on this forum. Laz Marhenke mentioned it in
1996 in his thread "Solar System questions" but the question basically
got sidestepped in the answer by Chris Carrier. *Please inform me, if
I have overlooked any thread on this issue.

Neither have I been able to locate a webpage on which this issue is
explained. Onhttp://www.bu.edu/core/cc105/lectures/L20-SolarSystemFormation/L20-so...
I found a mere mention of that this is due to the conservation of
angular momentum, and that supposedly the angular momentum (of
the ....?planet seed?) points in the same direction as it does in the
"initial rotating disk".

Whatever that means, it seems counterintuitive to (Laz and) me: If the
protoplanetary disk (proplyd) rotates in a keplerian manner, then, if
we put ourselves at any one point within such a disk, particles closer
to the center than us should overtake us on the side towards the
center, while particles further away from the center than us should
stay behind us on the opposite side. Thus, if we are fat enough to
capture particles from the disk by means of gravitation, the majority
of these particles should end up orbiting around us in a rotational
sense *opposite* to that of the proplyd, because otherwise they would
oppose the keplerian stream of particles around us, i.e. overtake us
on the far side from the center and stay behind us on the near side
from the center.

So where do I go wrong here? (or do I?) And of course, please give me
sources. I prefer webpages unless it´s a journal article.

And thank you in advance.

Quite simple, planets rotate in the direction of the original clould
of gas that formed the Sun.


- Ian Parker