Expert: Tom Whiting - 9/1/2008

Question
QUESTION: Hi Tom,

I read about Uranus and Neptune forming closer to the sun and then migrating outwards past Jupiter and saturn. I am a little confused as to how a planet could choose to change its orbital path. Is there any scientific reason for how this could have come about.

Thanks Robert

ANSWER: Hi Robert,
Yes, the new evidence suggests that several major planets may have
originally formed in closer to the sun, and then migrated outward during the accretion phase of Solar System formation back 4 - 4.6 billion years ago.

Well, a planet doesn't 'choose' to do anything.  What causes the migration outward is the friction and resistance of all that dust and gas mass present, (and the frontal head-on collisions with smaller bodies during accretion phase) which slows the orbital velocity of the forming planet, causing it to slowly drift outward away from the sun.  [The longer the period...read that, slower moving....the more distant the orbit, per Kepler's third law where period squared is proportional to the distance cubed].  That's why Mercury only takes 88 days for one orbit, whereas Neptune takes nearly 165 years for one orbit of the sun.
In the case of Uranus and Neptune, there may have even been close
encounters with Jupiter early on, which would have flung the smaller gas giants out to a more distant orbit.  So there are at least two
viable scientific explanations to account for this to happen.
Or perhaps a combination of both...we just don't know yet.

There may have been even other planets present at that time which Jupiter completely ejected from the Solar System, or were sent flying right into the sun by Jupiter's immense gravitational field, depending on the encounter angle...we just don't know.
Clear Skies,
Tom Whiting
Erie, PA

---------- FOLLOW-UP ----------

QUESTION: Hi Tom,

Thanks for your reply but this doesn't make sense. Any satellite has to be boosted into a higher orbit, then it travels at a slower orbital speed in a higher orbit(as I understand it). I would like to know How the massive bodies of Uranus and Neptune could be accelerated into these higher orbits?

Thanks Robert

Answer
Ok, Yes, you are correct.  I got mixed up in the confusion of
higher/lower, bigger/smaller orbit, up/down/ inward/outward/ more/less energy/...etc. But Kepler's laws still hold.
You are forced to agree to with that.
....................  
So I was incorrect on my first of over 1000 questions.
Congratulations for confusing a 67 year old mind with your
question. So you can rate (or is it Rake)? me accordingly.
(I see you already have.)

In any case, as far as planetary migration goes, obviously something happened during the initial formation phase to cause it to happen. Perhaps, or obviously, you can come up with your own explanation and theories as to what must have happened 4 -4.6 billion years ago.
I might be an old fogey, but I wasn't around back then.
So all we are doing is theorizing anyway.
Clear Skies,
Tom