David Bowman has put forth an argument why a planet like Venus might
naturally come to have retrograde rotation. He asks if anyone can see
flaws in his arguments. I cannot be an authority on this simply
because I am not. But I can describe a couple things about the way I
view it that are different than how David views it. The only reason to
suspect I might be correct is because the standard assumption among
astronomers is that Venus is unusual and probably suffered a collision
after formation. That is clearly incongruent with David's assertion
that we might have expected Venus to be retrograde.
I don't want to put words in David's mouth, so he'll have to correct me
here if I am inferring the wrong thing. I think he views that the
angular momenta of gas and dust at smaller radii from the center of the
solar system are roughly the same as angular momenta of gas and dust at
larger radii. Hence, conservation of momentum would require that the
inner stuff revolve at higher angular velocity than the outer stuff.
This is in analogy to the figure skater pulling arms and legs in
tighter and thus spinning faster.
That might be true after the planets got formed and the sun became the
sun, but I don't think it was true when the planets formed. And I
don't think the planets formed around the sun. I think the planets
formed in the dark because the sun didn't exist yet when the planets
formed.
I think the early spinning gases and dust were revolving collectively
with similar angular velocities rather than similar angular momenta.
Hence the inner stuff had less angular momentum than the outer stuff
(L = mr^2w, w=omega=angular velocity). That is, for inner stuff, w is
about the same, m is about the same, r is less, hence L less. The
angular velocities of all stuff cannot be exactly the same or things
wouldn't bump into each other stick. But the angular velocities
couldn't differ by much or there would be too much energy in the
collisions and the forming planets would just self-destruct upon
collision. Growth of matter into planets assumes (I believe) that the
matter is coming together at fairly low speed. Hence my statement that
the angular velocities were somewhat lock-step together.
If the matter is condensing from a region of gas/dust moving with
similar angular velocity, then conservation of angular momentum will
force the condensed matter to rotate in the same direction as the
matter was revolving.
The picture David describes might be appropriate if the planets formed
from matter orbiting at different speeds around an already formed
central mass (the sun). But I think astronomers believe planet
formation took place at the same time as sun formation. That is, this
giant rotating blob of gas and dust was rotating somewhat as a unit and
forming the sun and planets at the same time. Once the sun "ignited,"
the solar wind blew away the "unplanetized" dust and gas, and planet
formation ceased.
So, unless I totally misread David, I think he has a different view
than I of what the gas and dust was doing at the time the planets
formed.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817