Planets

[wolram]

Can someone tell me which is the favoured theory for
planetary formation, core accretion or Gravitational
instabilities, and if these are the only valid theories?

[meteor]

The two theories that you mention are models of formation of gaseous giant planets. While the traditional point of view is that these planets formed through core accretion, a few years ago Alan Boss proposed the model of gravitational instability (aka disk instability model). the favoured theory is still the core accretion model. I don't know of any other model besides these two

[wolram]

Thanks Meteor.

[marcus]

while I'm ignorant of the details of these theories, I did happen to see
an interesting paper about this recently.

there is this problem that the solar system has its gas giants 5-10 or more AU out from Sol
and this seems to be atypical

maybe because of a bias in the data or the fact that we havent lookd for enough time to see slow wobbles, or maybe for some more basic reason

it turns out that the extrasolar planetary systems discovered so far tend to have the gas giant in close like at 1 AU.

So for whatever reason the Sol planetary system is an out-lying un-typical example.

this paper was trying to explain this by postulating two variations on the scenario for core-accretion formation of gas giants.
the other stars formed in more metal-rich clouds and rocky cores formed quickly and accretion began soon and gas giants appeared early while there was still a lot of gas that hadnt been blown out yet-----this gas slowed them down and made them spiral in close to the star.

but we on the other hand came from a metal-poorer cloud and rocky cores formed later and more slowly and our gas giants appeared later when most of the gas had already been expelled----so they had more stable orbits and didnt spiral in so much.

My take is: it is a hard problem why Sol system is exceptional (or even to be sure it really is!) and I am impressed by these guys making all that effort to explain it. I dont believe what they say, but I respect that it is a fascinating problem and I am glad they are trying to solve it.

If I come across that link again I will post it. It was a July paper, IIRC, in astro-ph arxiv.

[marcus]

http://arxiv.org/astro-ph/0407476

"How special is the Solar System?"
M.E. Beer, A.R. King, M. Livio, J.E. Pringle
6 pages

authors are variously from Cambridge, Univ. Leicester, and the Maryland
Space Telescope Science Institute.

[wolram]

I have just picked up the August edition of astronomy, and it has an article
on this subject, it points to some problems with the accretion model, ie that
it has problems producing gas giants in less than 100 billion yrs, a good read.
also it mentions the highly eliptical orbits of extrasolar planets and asks
why the planets in our solar system have such perfectly circular orbits.

[marcus]

I have just picked up the August edition of astronomy, and it has an article
on this subject, it points to some problems with the accretion model, ie that
it has problems producing gas giants in less than 100 billion yrs, a good read.
also it mentions the highly eliptical orbits of extrasolar planets and asks
why the planets in our solar system have such perfectly circular orbits.

some days I am proud to be a fish
it is after all just some evolved fish who are asking these questions.
As fish we understand very well what our job is
(find one of these places and impregnate it with life)
and we are being circumpect about it
(which is one reason evolution gave us eyes in the first place)

It's good to be circumspect and have a really good understanding
of the planet you want to fertilize.
So these fish are asking just the right questions like
"why are those orbits so ellipitical? why
cant they be nice circular ones like ours?"
Circular orbits is better for life because the climate is
more like California and doesnt get so hot and cold
and also you dont get bumped so much by crossing paths
with things.

this is just what people should be thinking about and
not be all the time talking about dark matter and that :smile:

[wolram]

A splash of cold water in the face, yes MARCUS as big as we are ,we have
little hope of understanding the larger issues until we have an understanding
of our immediate surroundings, to my mind the larger more fundamentals must
wait for the 22nd 23 rd centuries, that does not mean people should not
ponder them, it just means wait until some means of veryfiying them is possible.