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Lecture 3.08: The formation of the Oort cloud
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En provenance du cours de Caltech
The Science of the Solar System
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Unit 3: Big questions from small bodies (week 1)
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Mike BrownProfessor
Planetary Astronomy
Like the question of what comets are made out of, the question of
why there are comets at 100,000 AU from the sun was also solved in 1950,
in a very famous paper from Jan Oort.
So he, the papers of course say structure of a cloud of comets,
he's the one that used the word cloud.
Cloud of comets surrounding the solar system.
A hypothesis regarding it's origin.
This is the really important part of Oort paper was the hypothesis
concerning the origin.
This hypothesis is widely accepted which is why we call this
cloud of comets outside the Oort Cloud of comments.
Let's go through [INAUDIBLE] proposal of why there would be a cloud of
comments out there.
Which will tell us both, where they came from and why they come back.
To begin with, we have to ask ourselves what happens to a small body
when it encounters a large body?
A planet, okay.
Do the Sun again here now.
Let's have, we'll do Neptune.
Neptune is an orbit on the outside here of the solar system.
Here's the orbit of Neptune.
And close to the orbit of Neptune let's put some small bodies,
a bunch of small bodies in fact and they're in orbits like Neptune's orbit.
Every once in a while they come close to Neptune's orbit.
When they do, what happens?
Well they can get a gravitational slingshot, in fact if we come a little too
close we can get a gravitational slingshot in and you have a new orbit like this.
You can get a gravitational slingshot out, and you have a new orbit like this.
I can't draw the whole thing, but you get the picture.
The key thing about these slingshots is that you get a kick from Neptune at this
point, but the moment that you leave Neptune's influence, you're on a kick,
you're on an orbit, and orbits always come back to where they started.
And the orbit started right here so it comes back to right here.
You are always coming back to the location of Neptune,
as long as Neptune keeps kicking you.
So let me draw a process that happens after this.
I'm going to have to make it a little smaller now so you can see.
So let's imagine there was a small body that got a kick by
Neptune, and it went out.
Everything comes back to the orbit of where Neptune is,
now is Neptune right there?
No, because this is how to increase on a major axis,
it takes longer to go around the sun, so it doesn't mean Neptune is right there.
But eventually, as the two go around the sun, they will encounter again,
there's really no choice, so what's going to happen?
Well, they'll get scattered again.
You might scatter to an orbit that's not as wide, but
I'm just going to draw ones that are wider so it scatters like this.
Goes around a few more times.
What happens?
It can scatter again.
Keep doing it.
Eventually, something bad happens.
This is a slippery slope.
Every time you scatter, it gets easier and easier to scatter you because you're less
and less bound to the sun and eventually you could scatter so
much that you gone forever, you leave the solar system.
It's a very sad day for you.
But there's hope.
How is there hope?
There's hope that you can be saved.
And the easiest way to have you saved is if there is another star
that happens to be zipping along through here that also gives you a kick.
Imagine you get a little kick from the star right here and
your orbit changes again and now you caught the kick right here.
So you don't have to come back here anymore.
But you do have to come back here again.
And you keep going around the Sun.
That's a really terrible ellipse.
It looks a little bit more like an avocado, I think.
But the key point is, you are going around the Sun here.
And you are no longer coming back to where Neptune is.
You're no longer going to get kicks from the planets anymore.
What are you going to get kicks from,
well if other stars come screaming by [SOUND] You'll get kicks from them.
Maybe orbit will turn like this.
Many different things will happen, but you are now safe from the planets and
you are populating this region that we now call the Oort cloud.
It's nice to draw pictures.
It's nicer to do some cartoon models.
Of what really happens.
Cartoon models I think of it this way as these are computer models that I just
did to illustrate what's basically going on.
They're not very complicated.
They don't cover a lot of the important physics but
they just show what happens when you get kicks from planets and
you get kicks from stars, and where you end up going.
Okay, there are only two things happening in this cartoon model.
There are small bodies that are going in orbit around the Sun that
get too close to giant bodies.
And the same small bodies can sometimes get too close to a star.
I take this system and I throw stars into it at random locations.
So let's see what happens.
This is a plot of the semi major axis, which is the average distance
it is away from the sun, versus the other key parameter is the perihelion distance.
The perihelion distance is, if you have an orbit that's an ellipse like this, like
all these are, the perihelion distance is here, the closest approach to the sun.
That closest approach to the sun is a key parameter because it tells you whether or
not you're going to be encountering giant planets.
If your perihelion is above Neptune, no giant planets in your future you're safe.
If it's below the distance of Neptune you might be in trouble.
Okay, so that's why at the perihelion I actually put the location, so you can see,
of Jupiter, Saturn, Uranus, and Neptune.
Any object that has a perihelion up here above Neptune is safe.
If any object down here less safe.
The other interesting thing is the semimajor axis is s also something that
tells you whether you're safe because things that are closer to the sun are more
tightly bound, things that are further away are less tightly bound and
more affected by passing stars.
So, here's what I do I put an object here, it starts right here.
And they always start on a semimajor axis of a couple of hundred AU.
So, they're already elongated orbits affected by, maybe Neptune.
And I have them starting around the location of Uranus,
as it's perihelion distance.
And every orbit, it can be affected by one of the giant planets.
What happens when you're affected by a giant planet?
You change your semimajor axis.
But remember, if you're affected by a giant planet, you always come back
to that location where you affected, which is the perihelion.
So notice, you can't see all the little dots in here, but
there are a ton of little dots in here of the object moving around and
semimajor axis changing, but the perihelion staying very fixed.
At the location of Uranus, keeps on going.
The other thing that's going on, and if you see the red dots,
not the ones that I drew, but the ones that are there anyway,
every red dot is when a star comes screaming through the system.
You see red dots in through here, stars can come in all the time.
They don't always do anything.
If the semi major axis is too small, if the star came way over here,
and the object were here, the object wouldn't even care,
it's too tightly bound to the sun, it doesn't matter.
But once you get a large enough semi-major axis,
the star can give you a perturbation.
Now, the stars tend to not change your semi-major axis by very much.
What do they tend to change.
They tend to change your parihelion.
They tend to give you a little kick out here which might just change you a little
bit right in here.
So, sometimes you see red changes in parihelion distance.
A few more encounters over here with the same semi major access because of planets.
Red parihelion distance.
Semi major access because of planets.
And eventually this object, boring,
it gets ejected from the solar system by probably Saturn.
It's down here about the distance of Saturn.
Being close to Saturn, bad news for a small body.
Being ejected from the solar system, sad.
Let's look at the same plot now, just a different random object.
And they're different because I let them evolve differently each time as
a stochastic process, different stars, different timings.
Starts in this place.
It radles a little bit back and forth through here as you can see, I use blue so
you don't see my red marks.
Radles around it, must take a staller and canna raises its pair of healing but
just a little bit but it's still not too close enough to, so it's radling around.
Suddenly looks what happens right here,
a little stallion canner raises the pair just a little bit higher.
And that of Neptune and suddenly there are not any more encounters with Neptune.
This point the evolution is much slower because it only evolves with every
passing star.
And that's what every little point that you see through here is a passing star.
Each one raising the perihelion, raising the perihelion and
then it rattles around up here.
And this region stops here over 4.5 billlion years.
What do we have now?
Well we have an object With a very large semimajor axis.
Semimajor axis of nearly 10,000.
And a pretty high perihelion.
A perihelion of 1,000.
That means it never comes close to the inner part of the solar system,
where by inner part I mean, Neptune.
Okay, here's another one.
I hope you start to recognize what's going on.
You see the same semimajor axis.
Only evolution, little bit of evolution down, this way, that way,
you can't even quite tell what the path is.
And then, here's a nice one.
Here's a nice big jump in perihelion distance, that puts you above Neptune.
You're safe.
You rattle around forever.
I like this one it's fun.
Rattle, rattle, rattle, rattle, rattle.
What happens, this is, I can't tell exactly what happens in through here, but
an interesting thing happens here.
Clearly a semi major axis jump, puts you above Neptune.
But then, the evolution must come back down here,
another semi major axis, I said semi major axis, I mean peri Helion.
Peri Helion jump puts you up here,
another peri Helion jump puts you back down where Neptune is.
Bad news.
And that happens again, and that happens again, and eventually,
you get your final good paraheline jump puts you beyond the influence of Neptune.
You keep going on up, but now you're too far away and
the stars just cause you to get lost from the solar system.
Again, a sad thing.
sort of the same story here, semimajor axis, semimajor axis,
semimajor axis, one nice encounter.
You start to rattle around, rattle, rattle, rattle, rattle,
rattle just from stellar encounters.
Counters can take you down the same too and
accidentally you encounter Maybe that is Neptune.
And one bad encounter with Neptune and boom.
You are out of there.
Okay I am showing you all of these because what I want you to get the feeling for
is that there are just two processes that matter.
There are the planets that you are encountering over here.
There are stars that you are encountering over here.
And they both have different regions where they're most important.
The planets can only act if your perihelion is below that of the planets.
The stars can only act if you are far enough away that you can start to be
affected by the stars so here's what I want to do.
I want to show you an ensemble of 500 of these little simulations,
and see where the whole cluster of them ends up.
And I think this will help you really understand how the Oort cloud gets there.
And also, as I'll show you, how the comets get there.
Okay, here's how it looks.
You can see that.
Objects.
They all start out around in here.
They explore this whole region in through here.
They go up and down semi major access moving because of planets.
And eventually, if they get far enough away from the Sun.
Here it is, 10 to the third au.
1000 au, 10,000 au, 100,000 au.
If they get far enough away from the Sun.
They might get a good stellar encounter that pushes them up and
saves them from the giant planets.
And notice what you get.
You get sort of this wedge of material.
Now, I'm showing you perihelion and semimajor axis,
which is sort of showing you eccentricity also.
But, I want you to think of it as inclination also.
So, if I were looking out in space, these things would all be more or
less in the plane of the Solar System.
These would be all over the place and these would be in a wedge that's
increasing in size as you move outward in the solar system.
Why is it a wedge like this?
Well once you're out in here, stellar counters move you around all the time and
so you can explore this whole region in through here.
In through here, you steer a little too close to the sun.
So, it might be that you had one Fortuitous encounter that
popped you up in here, but
the other stellar encounters don't really do anything, and you stay like this.
This region, this wedge of stuff that goes all the way into a whole sphere once you
get to about here, this cloud of material is the Oort cloud.
The Oort cloud was populated by.
These objects that would have been escaping eventually.
They would have all escaped out this process here.
Had it not been for passing stars that came by.
There's another really key thing about this plot that I love.
Which is, where are the comets?
Well, let's think about what a comet is.
A comet is something that has a perihelion of more like.
One, Earth distance, that's down here.
Ten to the zero is one.
And ascending major axis of, well what were they?
They were like 30,000 to 100,000?
What is that?
Well that's right about here.
That's these objects here.
Here are your comets.
Where do the comets come from?
They come from the population here, the distant population that
has a stellar encounter where the perihelion goes through a big jump and
goes all the way down to here.
And suddenly we see the comet for the first time in the intersolar system.
This is why comets, although they're on these 5-ish million year orbits,
they have not been around the Sun that many times.
Or they've been around the sun that many times.
But because they have perihelia that are so high, they haven't been in the inner
solar system more than that one time when we first see them in through here.
Now, is this the whole story?
No, it's not actually the whole story.
This is what's hypothesis of how both you populate it and why you get comets.
We now know that the integrated effect of the entire galaxy is as important or
maybe a little bit more important than individual passing stars.
But, the story is still more or less the same, and
I think this is a more clear one for understanding what these things are doing.
People often ask, so is the Oort cloud theoretical, is there-
do we really now there's something out there or is it just some theory?
Theory, that you astronomers have?
And I have to tell you, we see these objects right here.
These objects have perihelia of one.
And they have semimajor axes in the Oort Cloud.
I think it's safe to call these members of the Oort Cloud.
It's just that we happen to see them when they come close, the perihelion.
Come close.
Their semimajor axis put them squarely in the earth board cloud.
There is no other place that these could have really come from and
the large numbers of comments that are coming into our
solar system means there must be a very big reservoir of stuff out there.
So this is great, these icy objects have been stuck out there in a very distant
Cloud for probably the entire history of the solar system.
And they have preserved whatever ices and
materials they originally have without having them being heated up by the sun.
The cool thing about comets is that we get a chance to
take these things that have been in deep freeze.
And just as they're evaporating we can see what they're made out of but
we're going to do fast, because then they are all gone.
We see all these ices that we would expect from things that formed
in these regions of the outer solar system.
In the drawings that I've had, I've started them at around the distance or
Uranus and Neptune, modern ideas kind of go back and forth between whether or
not they formed around Jupiter or Saturn or
around Uranus and Neptune, it could be sort of either could be both.
Could be just typical type of belt objects.
But, in general you are looking at these
things that were formed in the outer region of the solar system and
you get the pleasure of seeing them up close before they are gone forever.
Its one of the reasons again why comets are such cool things.
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