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Imagine you're in a space ship in the very early Solar System, before planets form from the protoplanetary disk shown here.  At the frost line, about where the asteroid belt will be, it gets cold enough for ice grains.  When you pass this line, the density of solid particles in the disk abruptly increases by a factor of 3 or 4.   So, these particles will stick together to form larger bodies - and faster, too!  This means that gas giants are more likely to appear beyond the frost line, since the bodies that form beyond this line are bigger and have more time to accrete gas from the disk before it dissipates. 

The frost line is also called the snow line, and you can read more about it here:

http://ay201b.wordpress.com/the-snow-line-in-protoplanetary-disks/

This image is from:

http://lasp.colorado.edu/education/outerplanets/solsys_planets.php

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As they note, "Another open question is how to explain the presence of Earth’s large water content (both its oceans and the water locked in its mantle), since the Earth formed within the snow line where ice grains could not form. "

So the model is not complete.
 
I typically do imagine I am in a space ship in a very young system, however for other reasons though.
 
+Dave Pentecost - Right, there are a bunch of interesting open questions!  But ironically, the one that got me interested in this stuff is not why there's so much water on Earth, but so little:

http://www.universetoday.com/96341/why-doesnt-earth-have-more-water/

In terms of volume, the Earth is just 0.02% water, while gas giants formed well outside the frost line, like Uranus and Neptune, are something like 10-40% water.  But in some models of the early Solar System, the frost line was actually inside the Earth's orbit, at least for a while (it moved over time).  A new paper claims to explain why those models are wrong.

The fact that some scientists are wondering why there's so much water on Earth, while others are wondering why there's so little, suggests that there's a lot left to learn.
 
+John Baez , so little water or so much water. It all depends on the current model of the decade.
I respectfully suggest that THIS model needs additional work, as every other model does. Largely due to the fact that NONE have even closely or moderately managed to predict our CURRENT solar system.
Let's consider OTHER models, previous models. Where water could NOT exist in the frost line of the asteroid belt. Until it was observed.
I respectfully suggest that we lack sufficient understanding of physics to truthfully predict our or any other planetary system.
We're close, but not enough to even SEE that cigar.
Though, I DO long for that day!
Consider this: A generation ago (for humans), the Earth could not support water in formation, it was too hot. Some magma Earth thing going on at formation.
NOBODY considered what water would do in that environment. ALL assumed it would be blown off by Sol.
Today, we realize it would NOT be blown off. Indeed, I suspect our magnetic field was, erm, rather MORE interesting than it is today, due to magma oceans and no solids at that point, super fluids ARE super fluids and all...
Now, we're only BEGINNING to model such extreme environments. And we're seeing interesting, indeed, potentially bizarre results.
So, as usual, our models are imperfect. They describe things at best, problematically. But, they do better than some obscure bronze age story and gradually do a far better job of it.
No, I'm far from being atheist in nature. I'm more deist in nature, overall.
However, one does NOT explain to a small child PRECISELY HOW their baby sibling was conceived. One gives "baby steps" explanations.
In THAT concept, we're only entering the age of more advanced explanations.
But, some seek understanding through consensus of disparate views. Others seek understanding through current views.
I'll stick with the Geth consensus.  ;)
 
Isn't the whole point of a model is that we don't understand the whole story however we can do some fairly informed choices in parameter selection. A indication of a good model is simplicity. Thus, we make the best informed decisions we can under that model at that time.

As Richard Feynman once said:
”A great deal more is known than has been proved.”
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