Now we have the ingredients. 
We understand enough about the solar 
nebula to start following the process of 
how we got from a disk of gas and perhaps 
some dust to what we have today. 
And we'll start with the interior 
planets, the terrestrial planets. 
Remember that inside the snow line we 
have mostly hydrogen and helium class, 
but in addition, there are small amounts 
of solids that congeal. 
The solids in question are iron and 
nickel. 
Silicates, the kind of materials we find 
in meteorites and there's not a large 
quantity of those, because these elements 
are very rare in the nebula, but there's 
enough. 
And as the density increases, these dust 
grains that are in Keplerian orbits 
inside this massive cloud of dust collide 
with each other and they adhere, they 
adhere in the same way that grains of 
dust adhere to cleaning rag. They adhere 
electrostatically and then chemically 
bind. 
They're forming rocks. 
These rocks are held together by chemical 
forces. 
And slowly over time, the larger rocks 
have larger surface area. They collide 
with more of the dust. 
and they grow and eventually one forms 
objects of a size, order of magnitude of 
a kilometer. 
There will be about a billion of them in 
the inner solar system and these billion 
so-called planetesimals, 
microplanets, 
are reaching the size where as we'll see 
in the homework, when you reach a size of 
about one kilometer, 
an object is bound now by its 
gravitation. 
In other words, the gravitational force 
is enough to hold the object together. 
It's more gravitationally bound than 
chemically, and this is an important 
transition. 
Because now, these objects are heavy 
enough that they're moving in Keplerian 
orbits. 
They are no longer sustained by the 
pressure of gas, which is still enough to 
hold up, the dust is floating in the gas. 
But the larger, rocks are not floating in 
the gas, they're falling through it. 
So they're moving on Keplerian orbits, 
their velocities are higher than those of 
the gas and the dust. 
And so these large planetesimals are 
sweeping through the gla, the gas, 
and whatever grains of dust come close 
are immediately falling onto the 
planetesimals and being gravitationally 
attached. 
And so the gravitational attraction 
becomes important, 
and the larger objects grow faster. 
So there is this hierarchical growth, 
where the rate of, at which an object 
grows turns out to be proportional to its 
radius to the fourth. 
And this goes on for about a 100,000 
years in which these planetesimals, and 
certainly the larger among them, grow up 
to be rather large objects. And through 
these, at the end of this 100,000 years 
the largest of them have merged to form 
about a few hundreds of objects that are 
called protoplanets. 
These are objects with a radius on the 
order of 1,000 kilometers. 
so slightly smaller than the current 
moon. 
And here another important transition 
happens. When you reach radius or size of 
about 1,000 kilometers, the object is 
certainly gravitationally bound. 
What has been holding up the rock so far, 
the reason a rock doesn't. 
con, doesn't collapse under its own 
gravity, is basically that a rock is 
rigid material. 
Chemical bonds hold it up against its own 
gravity. 
When an object reaches 1,000 kilometers, 
two important things happen. 
One is, the heat of the collisions and 
the Kelvin-Helmholtz heating from 
converting gravitational potential energy 
as objects collide into heat, along with 
the radioactivity. 
The heat released by the radioactive 
elements that are trapped inside, melt a 
proto-planet. 
When you reach a size of about a 1,000 
kilometers, you melt. 
this is very important. 
Things that melt are going to be 
spherical. 
Why are they going to be spherical? 
Well, because gravity flattens them out 
to the form of a sphere. 
Imagine a molten liquid Earth well, we 
have parts of the Earth that are liquid, 
they are the oceans. 
There are no mountains in the ocean, 
because a mountain in the ocean would be 
leveled by gravity when the Earth was 
molten. 
And when a protoplanet is melted and 
liquid, it will naturally form into a 
symmetric spherical shape if it is 
twisting. 
It'll be, if it is spinning, it will be 
slightly oblate, slightly fatter at the 
equator than at the poles, 
because of, again, the centrifugal 
barrier that we talked about. 
But as long as it's not spinning too 
rapidly, it will be approximately 
spherical. 
This is one important process that takes 
place. 
All of these protoplanets are assuming 
spherical shape. 
And then, the other important thing that 
happens at 1,000 kilometers when things 
melt is something called chemical 
differentiation. 
these initially roughly uniform or 
objects where various elements were 
scattered haphazardly, depending on when 
they were accreted throughout the object, 
becomes segregated. 
Because once an object melts, then the 
heavier elements, iron, nickel, 
sink through the lighter silicates say, 
down towards the center. 
And one forms a chemically differentiated 
object where there be, there forms a core 
with, which is rich in the heavier 
elements, iron, nickel, etcetera. 
Along with an external envelope, which is 
the silicates. 
And. 
much cooler crust as the core as, as in 
falling matter falls in, we get 
additional heat at the crust. 
In addition, once an object is fluid, 
it's no longer held up by the chemical 
bonds, by rigidity. What holds the earth, 
or a proto planet up against collapse 
under it's own gravity is hydro-static 
pressure. 
The same kind of hydro-static pressure 
balance that held our slinky in balance, 
or the water in our cup. 
Pressure is largest in the middle, 
decreases as you move up through the 
Earth. 
Each layer is in hydro-static 
equilibrium, it's weight balanced by the 
extra pressure below it compared to the 
pressure above it, and so we obtain high 
pressure and high temperature in the core 
with decreasing pressure and temperature 
towards the outside. 
This is the situation on a planet. 
This happens when you create a 
protoplanet. 
So the transition from plan, well 
planitesimal, an object which is bound by 
gravity. 
But where gravity is not yet dominant, to 
a protoplanet, which melts, become 
spherical and chemically differentiates 
is the transition that we go through. 
After about a hundred thousand years, we 
have generated, protoplanets, a few 
hundreds of protoplanets orbiting the sun 
in this region that will become the inner 
solar system, within about 
two or three astronomical units from the 
Sun. 
how do we know about protoplanets? 
Well, we are fortunate enough to have 
some of them left. 
What we see in this beautiful video clip 
is an image of an actual proto-planet. 
This is the asteroid Vesta. 
As imaged, as it says, by the Dawn 
spacecraft. 
In 2011, there, the space craft was 
orbiting the asteroid. 
And so it got to take a 360 degree 
panoramic image. 
We see the roughly spherical shape. 
We see, the impact craters, we have to 
talk about what created those. 
This is an object that melted as it 
contracted gravitationally, so it's 
roughly spherical. 
The asteroid Vesta is an interesting 
object in itself. 
It, will be visible this winter. 
It will be in opposition. 
And if you get a chance to go out and 
take a look with a sufficiently large 
telescope, you're invited to view Vesta. 
You won't get a view like this, 
though. 
. 
Carrying on with our plan of producing 
planets, we now have these 
proto-planets orbiting. 
They rapidly, gravitationally accrete the 
remaining planetesimals. 
So now what goes on is that the 
gravitational force of these 
proto-planets begins to be important. 
And their interactions with the 
planetesimals are gravitational, they 
either accrete the planetesimals or, if 
they collide with them, they might, blow 
them up and eject them from the inner 
solar system. 
The result of this process is that you 
end up with about 100 objects the size of 
the moon, Mars, 
so smaller than the Earth. these are the 
protoplanets, and they have cleared gaps 
in the disc, so where a protoplanet is 
orbiting there will be no more 
planetesimals. 
Planetesimals will still be orbiting in 
the gaps between. 
These will be about 100 equally spaced 
orbits between the interior of the disc 
just outside the Sun and say two or three 
astronomical units out. 
And in this region will be a hundred 
proto planet orbiting in gap, in, in, in 
clear gap and in between them, there will 
still be planettissimals. 
Now. 
These, the gravitation, no interactions 
between proto planets themselves now 
become important. 
We'll talk about that in the next clip. 
And this distorts their orbit, so that 
they start moving through these gaps, 
ejecting or accreting the remaining 
planetesimals. 
These distorted orbits now lead to actual 
collisions between the protoplanets. 
And these collisions are massive 
collisions. 
These are moon sized objects crashing 
into each other with polarian velocities. 
this can lead to a complete destruction 
of some of the objects. 
The collisions are now violent. 
and there're suitable conditions. 
The two objects partially remelt, and 
merge, 
and this leads from these 100 or so 
moon-sized objects to a few large ones 
like Venus and Earth. 
This is where the larger planets come 
from, from these mergers of these 
protoplanets. 
one of such collision we think left 
Mercury stripped down to its core. 
We'll see that Mercury is a very dense 
planet. 
The explanation is that a collision 
evaporated and blew off into space its 
envelope, leaving just essentially the 
core, Mars never grows beyond a 
Mars-sized object. 
It's smaller, it's about the size of the 
moon, it's much lighter than earth, We'll 
see what it is that limits, or stunts the 
growth of Mars, and within ten to 100 
million years, remember ten million years 
is when the dust is gone, 
and the gas is gone. 
The T tauri winds clean out the system, 
and the orbits which have been perturbed, 
settle down by mutual friction, down into 
the near circular orbits that we see. 
So within ten to 100 million years, we 
have pretty much the inner solar system 
as we know it. 
We see this in this beautiful video. 
we start with, the orbiting, lift clouds 
of planetesimals. We see the gaps where 
larger proto-planets are forming. 
In a minute, we'll be able to see the 
actual proto-planets. 
Here's a proto-planet. 
And, the planets, as they orbit, clear 
away, lanes inside the planetesimals, and 
slowly their influence cleans out the 
leftover planetesimals, either accreting 
or ejecting them. 
So that, after about ten million years, 
we are left with roughly the solar system 
as we know it, and the sun blows away the 
remaining gas and dust, and we find the 
interior, interior solar system, 
the inner solar system looking pretty 
much the way we see it today. 
What happens beyond is going to be the 
subject of the next clip.