Now that we have some idea of how star
systems form, we can turn our attention
to the formation of our own solar system.
When we look around our solar system, we
got to remember there's a few
characteristics that we want to keep in
mind. For starters, everything orbits the
Sun in the same direction. That is, if we
were looking at the solar system from,
let's just say, overhead, everything would
appear to be orbiting the Sun in a
counterclockwise rotation. But there are
some exceptions to this rule and that is
comets; sometimes the occasional comet
will seem to come in in a clockwise
direction as seen from overhead.
Otherwise, yeah, everything's orbiting in
the same direction. Another thing to
think about is that if you look at the
planets as they're arranged from the Sun
out to, say, the orbit of Neptune, it turns
out that these planets all seem to orbit
the Sun in roughly the same plane that
is essentially what we now think of is
the ecliptic.
But beyond Neptune, you see there's a an
abundance of highly inclined objects.
These are the dwarf planets, the Kuiper
belt objects, and so forth. They seem to
be a bit tilted above and below the
plane of the ecliptic, so if we're going
to put together a model of how the solar
system formed, it's important that
whatever we consider has to reproduce
what we currently see today. And
astronomers have a pretty good
hypothesis as to how our system formed.
We believe that our solar system formed
within a cloud like you see here. There
was once upon time a rotating disc, and
as this disc was coalescing around the
protosun which is depicted at the center.
There became little instabilities within
the disc. The disc fragmented and clumps
formed within the disc giving rise to
the ultimate formation of today's
planets. And these protoplanetary discs
are fairly common; sometimes we see them
edge-on or sometimes we see them nearly
face on, but when seen at microwave
wavelengths as we have here in the right
hand side of our screen, you can make out
the distinctive rings and
spoke like features of this disc. So this
is significant because it means that
there are proto planets inside this
protoplanetary disk that are beginning
to sweep out the concentrations of gas
and dust within their orbits. So as we
gaze around the solar system, we find
there's a variety of objects, namely
giant planets and terrestrial planets.
Giant planets are composed of mostly
lightweight materials, what we call
volatiles. For example, this is dry ice,
frozen carbon dioxide, and frozen ammonia.
So we're gonna have these very
lightweight materials such as hydrogen.
helium, methane, and chlorofluorocarbons
all cold enough to become an ice. And
it's for this reason that these planets
have a relatively low density. But
terrestrial planets, on the other hand,
have relatively few of these lightweight
elements, and they're mostly composed of
rocky or refractory materials. So
minerals, ollivines, and so forth. So why
then are these two types of planets so
very different from one another in terms
of their composition? The answer goes
back to the formation of the planets
themselves. Remember, it all formed inside
of a disk that you see here. So if we
think about this depiction of our proto-
solar system, we have an abundance of
refractory materials starting from just
around the protosun all the way to the
very edges. There's plenty of silicates,
plenty of carbonates and minerals and so
forth. However, it is not until you get
past, say, the orbit of Mars, and toward
the orbit of Jupiter that temperatures
drop low enough that volatiles can
condense. Water, molecular hydrogen and so
on. In other words, they freeze out. They
become solid enough or at least slushy
enough in order to fully condense and in
fact once you get out past the orbit of
Saturn, the temperatures are low enough
to even allow high volatile such as
hydrocarbons, methane, ammonia, and so
forth, they can even start to condense. So
the point where the temperatures drop
low enough is called the "frost line". It's
really just the distance where the
temperatures are
low enough for volatiles to condense.
So why then are the small rocky
terrestrial planets closer to the Sun,
and the large gas and ice giants farther
from the Sun? It's because of that
temperature difference. It is only in the
outer solar system where these large
planets can form because they have an
abundance of both refractory and
volatiles. However, inside the solar system
where the terrestrial planets live, there
are only refractory materials to build
with. So how does a spinning disk of gas
and dust go on to become planets? Well in
order for us to think about that we have
to change our perspective. We have to go
deep inside the circumstellar disk and
instead of thinking about the disk as a
whole, we need to change our scale and
get smaller and smaller until we are
finally at the scale of individual
grains of dust. These dust particles
will go on to become planets. Here's how
they do it. First of all, the dust
particles are not the same exact kinds
of dust particles that we think of in
our rooms and in our houses. Dust in
space is mostly composed of silicates
and chondrites,
whereas the dust in our rooms is
composed of dead skin cells, insect feces,
and so forth. Well, since there are no
people or insects in the space we are
left with a slightly different kind of
dust. Nevertheless, these fine dust
particles do carry an electric charge
and just as particles of dust and dirt
carry electric charge in our rooms and
cling together to become dust bunnies, so
do these particles as well; they become
cosmic dust bunnies. As a matter of fact,
they can grow quite large and they do so
by just undergoing very gentle
collisions which allows them to grow
into rocks, then into boulders. Anything
harder or faster would break these
things apart, but as these things grow
they can withstand harder and stronger
collisions until they grow into what are
now called "planetesimals".
At planetesimals' sizes - at about one
kilometer - they are massive enough to
exert a gravitational pull on one
another. This means that they can
withstand harder collisions, and in fact
we see leftover planetesimals around the
solar system today; they are modern-day
asteroids and comets and Kuiper belt
objects. These planetesimals collide
in a kind of proto-solar system
demolition derby. Most of these are
destroyed and are later accreted on to
other planetesimals. And the most massive
of these planetesimals survive and are
now proto-planets. They begin to clear
out their orbits. You can even do a
simple computer simulation like we have
here and you can easily see how as
objects collide into one another there
are fewer and fewer of these objects
remaining. So when we look in systems
like TW Hydrae, we can actually see those
rings. We can actually see those lanes
being carved out by protoplanets within
the disk. So returning our attention to
the gas giants, remember they too are
forming out of little disks within the
disk. Remember, they're gonna form well
beyond the frost line where there is a
high abundance of both refractory
elements and volatiles, so they have
everything. They gain mass by just
colliding with additional planetesimals
and because they have more mass. Because
they're now proper protoplanets in their
own right, they're able to accrete these
volatiles. Remember, they're farther
from the protostar, so there's less heat
there's less stellar wind and there's a
greater abundance of these volatiles to
begin with. So an accretion disk forms
around the protoplanet. It's like a disk
within a disk, and even moons can evolve
from inside the protoplanet's own
accretion disk, and this allows these
massive outer solar system protoplanets
to grow what are called their "primary
atmospheres". That means that the
atmospheres of Jupiter and Saturn, for
example, are the same atmospheres that
they pretty much formed with. However, in
the inner solar system where we have
lower mass protoplanets, it's a much
different situation.
They are unable to hold on to those
primary atmospheres. Remember the
temperatures are much warmer near the
protostar, or in this case the protosun.
Hot gases are going to always move
faster than cold gases, so if you have a
low mass planet with low mass gases, they
can easily achieve escape velocity from
those gases.
Remember, the hotter they are, the
faster they're going to be moving. They're also closer to the protosun. That
means that there's a stronger stellar
wind coming from the protosun which just
makes it that much harder for
lightweight volatiles to stick to the
protoplanets. So any protoplanetary
atmosphere of the inner solar system is
just going to be blown away by the Sun.
However, there's a lot of smaller objects
floating around in the solar system and
even from the outer solar system. And
when little tiny planetesimals from the
outer solar system fall toward
proto-Earth, they deliver these volatiles.
And there's already volcanism on the
planet, so now we get the formation of a
secondary atmosphere. So about four and a
half billion years ago the hot Earth was
really just one giant volcano; the entire
surface was covered in volcanism and
whatever trace volatile materials the
Earth did form with were quickly being
released through volcanism. Comets
and asteroids would deliver additional
volatiles from the outer solar system, so
our atmosphere that we live under today
is the second atmosphere, the 2.0 version
of Earth's atmosphere. To put the
whole thing into perspective, we can
imagine the rotating disc and we can
envision tiny clumps of material forming
inside that disk. Those clumps would
then go on to become planetesimals and
even protoplanets. Protoplanets would
then have the ability to start sweeping
out their orbits as the Sun was
undergoing its final fitful phases of
its own formation. Protoplanets began
to scoop up and sweep up their orbits,
thus clearing their paths, giving us the
solar system
we live in today.
