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.
Planet Formation
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Confronting The Big Questions: Highlights of Modern Astronomy
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Adam FrankProfessor
Physics and Astronomy
Welcome back everyone.
What we want to talk about right now is the specifics of Planet Formation.
Just to briefly go over what we understand about exactly how planets formed.
Last time we talked about sort of the general formation of
planetary systems in the sense of a rotating cloud collapsing to
form a disk, and then the processes that happened to the
disk, but now we want to really zoom in on that.
So, we have these disks.
And the question is really, how do you go from a disk of
gas and dust to an Earth, or a Jupiter?
And the way that happens, is this, what we could
think of, can think of as a cascade of collisions.
So, we begin with, the gas, and then the dust.
And dust is absolutely essential to
the formation of planetary systems, or planets.
Excuse me. what we need are.
Tiny flex of minerals essentially. These are,
these are smoke may be a better word than actually dust.
There are about ten to the minus sixth of a
meter across little bits of silicon or so, or other elements.
And so what we what occurs is these dust grains are floating
around, they're moving with the rest of the disk orbiting the star.
And then through, their electric charges, they will collide and stick together.
So what occurs is the dust grains will collide to form larger dust grains.
And this begins the cascade.
And then what you get is these larger dust grains colliding
with other large dust grains and
eventually forming things like small pebbles.
And then the pebbles will collide with each other, and
they will form small rocks, and so on and so on.
So you go, you know, from rocks to boulders,
boulders to asteroids, asteroids to what we would think of
as planetesimals, and then planetesimals to planets.
Now, there's a lot of this process that we really don't understand.
for example, in some cases, what you'd
imagine if you get two rocks smashing together,
but they wouldn't necessarily stick together to
form a boulder, they would actually smash apart.
And so there's some competition between
processes of coagulation and processes of shattering.
and so that's one of the things that needs to be understood.
But you also have to under take,
keep in mind that these things are moving quite rapidly in their orbits.
And sometimes they're the speeds are high enough
that when they smack together, they will become molten.
And that is, sort of, you know, what will keep them together.
So, we have this cascade going from, grains, all the way up to planetesimals.
And it's the planetesimal part where you really had sort of asteroid sized bodies.
That we really begin to see the process where you
can, Get these things to collide.
And if they stick together basically on their own gravity, that gravity will get
hold of them when they get massive enough to begin to make them into spheres.
So that's where we begin to see
the beginning of dwarf planets, things like Pluto.
So now you have objects wherein you have
enough mass and enough gravity that they become spherical.
And you can also get differentiation in those objects
as well, where the metal, the, the, the temperatures and
pressures are high enough at the center
that things are melting, and you'll get rocky
cores sorry, you'll get the metal cores forming as the metals sink to the bottom.
So these are the processes, the general processes
we think that take us from a planet.
Or sorry, from, from dust grains to planets.
But there's more to it than that.
And so there are basically five stages
if we're thinking about terrestrial planets that we
need to think about.
The first is, this planetary differentiation.
And then as time goes on, the heat that came from the
planet forming was going to be dissipated and and that material will cool.
And then there, we expect that all the debris
that's left over in the solar system is going
to rain down on these planets because they're giant
vacuum cleaners in some sense because of their gravity.
So we're going to get impact creating.
And then we'll almost in almost all
cases, the center is, or, sorry, the inner parts
of the planet are hot enough that it's flowing.
It's molten rock.
So we expect sometimes magma to make to, make
it to the surface and to restructure the surface.
And if there's an atmosphere, we also expect weathering over time.
So that's for terrestrial planets.
And the process that we've described.
Really makes sense for understanding terrestrial planets.
But what about the gas giants right?
I mean you know this, this idea of dust grains forming boulders,
etc.
Or you know forming rocks going all the way up to boulders and planetesimals.
How does that explain gas giants?
Well gas giants are a different story because we think there we need
to start with a rocky core of some kind, and then once it gets
to a certain point if there is still a lot of gas left in
the disk by this time, then, the gas is going to start to accrete.
It's going to start gravitationally pulling in gas.
And the gas will pile onto the, the rocky core.
And eventually, you'll get it a gas or an ice giant.
Now, there's two ways to do this.
One is sort of what is called core accretion.
This idea I'm sort of thinking of is that, you know?
You start with a rocky core.
And then you get the gas falling onto it.
But one big problem with this is that it may take so long to get that rocky core.
Tens of millions, or even hundreds of millions of years.
That by that time the gas may have dissipated from the disk.
The gas eventually leaves the disk, or is blown away
from the disk by, via usually the winds from the young star.
So that may happen in only a few million years.
So if you wait too long to build your core there may be no gas around to accrete.
That's why there's a second model
that people like, called the gravitational instability.
Where basically the disk has enough mass in it that parts of the disk.
Just like the, the cloud it's that formed
the whole solar system collapsed under its weight.
Parts of the disk may collapse under their own weight.
To immediately form a
gas giant.
So you know, who you know, which, which of these
two models actually occurs depends on who you're talking to.
We don't really have enough evidence yet to know which of
the models, core accretion or hydrodynamics
instability, is really the appropriate one.
