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.