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.