An introduction to modern astronomy's most important questions. The four sections of the course are Planets and Life in The Universe; The Life of Stars; Galaxies and Their Environments; The History of The Universe.
Condensation Theory
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Confronting The Big Questions: Highlights of Modern Astronomy
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Are we alone in the Universe?
Planets and Life in The Universe - Exoplanets searches, exoplanet census, astrobiology
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Adam FrankProfessor
Physics and Astronomy
Welcome back, everyone.
So, now that we've studied our own solar system.
And we've studied what we've learned from exoplanet systems.
It's time to put it all together, and try and ask ourselves, how do planets form?
How do planetary systems form?
and the idea here is a very powerful theory called condensation theory.
The condensation theory for the formation of stars and planets.
And let's review sort of what we've learned from both
the exoplanets, and from our study of our solar system.
That any theory of star, of planet formation
is going to have to try to explain.
So one of the things that we know from the study of
our own solar system is, all the planets orbit in a disk.
And they are all orbiting in the same
direction, so that's a very important thing to remember.
all the planets orbit with a fairly low
inclination, meaning that, that they are in the disk.
Maybe there's a slight, you know inclination
in their orbit with respect to the, the
the disk, but it's really is a sense of all the orbits being in the
same plane.
the inner planets are all rocky and dense, and the outer
planets are made of gas and ice, that's something we want to understand.
there's a lot of debris left over.
Okay, so we have to understand that as well.
From the exoplanets, what we find is that a lot of planet, planets can migrate.
We see hot Jupiters, Jupiters, things that must have been born further out
in the solar system and then are right up against their own star.
So migration must be something that can happen in solar systems.
And we also see in exoplan, exoplanetary systems, highly eccentric orbits.
Which tell us that there must've been
interactions or scattering that must've gone on.
so these are the things we want to understand.
And the idea, the way we understand 'em, the,
the broad idea is what is called condensation theory.
And what we imagine is this.
So we have this cloud, and the cloud is in a
balance between it's own thermal pressure, it's own heat pushing outward and
gravity pushing inward.
But it's a very subtle balance and perhaps some
passing shock wave interstellar shock wave passes over the cloud.
And, and compresses it just a bit, enough
that now gravity can get hold of the cloud.
And the cloud begins to collapse under its own weight.
So this, what we now have to understand is a principle called conservation
of angular momentum which you're all
very familiar with from watching Olympic skating.
And we know from Olympic skating that if you have a skater
whose going into a spin with their arms out and they pull their arms in,
their rate of spin will increase and
that's basically something the conservation of angular momentum.
Which tells you the relationship between how your
spin rate has to change as your size changes.
Now these clouds, these interstellar clouds are all slowly rotating.
They all start off with some kind of rotation just from how the clouds form.
Now we know that when a skater changes their
size by just basically a factor of two by pulling
their arms in, you see how much faster they spin.
When a cloud collapses under it's own weight it changes
it's size by a factor of a million or more.
So what happens is as the material begins to fall inward and it's spinning,
it begins to increase it's rotation speed so much that eventually as it diving
downward in a big spiral the rotation speed is fast enough that it balances
the force of gravity of the thing, the star that's forming at the center.
so,
material. So imagine this is our cloud, okay?
And it's spinning this way.
Material that's along the poles of the cloud doesn't feel any angular momentum.
Or there's no conservation of angular momentum at all.
And that material can drop right on to form a protostar at the center.
But the material on the equator, as it spins
around, it goes, comes down through a giant spiral.
And eventually goes into orbit around the star.
Now what about material at mid-latitudes? Well that material oft, it falls in
as it spirals, but there's material on the other
side which is also spiraling in and it tends
to stay meet at the mid-plane and their inward
motion cancels out and you end up with a disk.
So, the, the net sum of this process of a collapsing rotating cloud is
the formation of a young star surrounded by a disk of gas and dust.
And that's where we begin to think about planet formation.
It's in this disk of gas and dust that we expect
the processes of planet assembly to begin.
Now, the thing we can calculate is the temperature through the disk.
And what we find is, if, of course, if you're closer to
the star, your material's going to be hotter than if it's farther away.
And this is what gives us the essential
clue to the formation of terrestrial and gas giants.
Because if you're very close to the star, the temperatures are
so high that all of what we would call the volatiles,
all of the things like water and methane, boil away.
That can't exist as a solid close to the star.
So the only thing which could exist as a solid
close to the star are things like metals or rocks.
So, the reason we have iron rich, metal rich,
or terrestrial type planets close in, or at least that's
where they form, is because that's the only region, in,
in those regions, all the water is essentially boiled away.
The reason we have gas giants and ice giants farther away.
Remember we talked about the snow line.
Those things are far enough away from the star, that in the
disk, the material in the disk is cold enough for ice to form.
So we expect to have lots of gas and lots of ice in those in those outer systems.
So, just from this this basic idea of a collapsing,
rotating cloud we've been able to explain a couple of things.
All the planets are going to be in
a rotating disk.
That explains why all the planetary orbits are in the same direction.
It also explains why they're all are
basically in a plane, with small inclinations.
We've also been able to understand from this
because of the temperature in the disk why it
is that there are terrestrial planets forming close
in and gas and ice giants forming further out.
Now, the thing that we've learned from exoplanets, however, is that
there are going to be interactions between the planets once they form.
So the
interaction for example between the Jupiter and the disk of gas
or the disk of debris early on in this, in planet's early solar system's
lifetime, is strong enough, that gravitational
interaction is strong enough to actually
exchange angular momentum between the newly
formed planet and the material around it.
Allowing the planet to get dragged in.
So as the, you know Jupiter will form out by
where Jupiter in our solar system is.
But it will slowly spiral in to perhaps end up, parked right
up against, or on an orbit that's very close to its star.
And that's where you can get hot
planets, hot Jupiters, hot Neptunes, even hot Earths.
if there are many planets that form in the
system, then what you're going to get is interactions.
Every now and then, one planet will
get close enough to another planet that they'll
gravitationally interact and perhaps even throw one of
the planets out of the entire solar system.
And will then, what you'll be left with, is a,
a, the other planet on a, a highly elliptical orbit.
So that explains scattering, planet
interactions, planet-planet interactions or planet-disc interactions
can explain both the crazy orbits that we see, the, the,
the, the very hot orbits that we see in extra solar
planets, extra solar systems as well as the the highly elliptical orbits.
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