Okay. Welcome back, everybody.

So, today we're going to talk about the giant planets.

and so we're going to combine both the ice

giants and the gas giants together in this.

And again, these are the planets in our solar system

that are in the outer part of the solar system.

The outer part of where the planets live.

The interesting thing about our solar system is all the terrestrial planets are

close to the relatively close to the each other and to the sun.

And the gas giants and ice giants are relatively

far from the sun, and they also live

on Orbits where they're relatively far from each other.

That may not be the case, that we know that's not

the case, for other solar systems which makes things very interesting.

Of course, for a long time we only thought, we

thought that our solar system was the only solar system.

And we just always thought, of course all

of the terrestrial planets are going to be close

in and the gas giants are going to be

outside, and that's one of the beauties of science.

Is that you can think all you want until you get the data you don't know.

And of course nature

is far more creative, more fecund than we are.

And what we found when we started.

We were actually able to get the data, 20 beginning 20 years ago.

Is that that there is a, psychotic variety of planets and solar systems out there.

Things that we never would have imagined.

And so that's really one of the beauties of science.

Is the ability to be continually surprised.

So certainly, when we look at other solar systems,

we have, other solar systems, we have been continually surprised.

So let's look at the the

gas, the giant planets in our own solar system.

And let's start with this diagram here, or this table, which shows us the planets

and it gives us a good way of sort of seeing the differences between them.

So going outward from the sun we have Jupiter, Saturn, Uranus, and Neptune.

And let's look at their radiuses.

What you see is that Jupiter is a whopping ten tonings the size of the Earth.

It is an enormous planet.

it has 300 times the mass, and

in terms of the volume, you could fit over a 1,000 Earths inside of Jupiter.

That's how large it is.

Now, the important thing to notice here, is to

look at the density of the gas and ice giants.

1.3 grams per centimeter cubed.

Water has a density of one gram per centimeter cubed.

So you see, these planets are just about the density of water or even less.

The density of the terrestrial planets tends to be on the order

of two, or three, or four, grams per centimetre cubed.

So that's really one of the important differences

between the terrestrial planets and the gas giants.

and the ice giants.

The terrestrial planets are very dense because you've

got all that iron and rock in that.

The gas giants are not very dense.

Also, let's look at the Orbital period of Jupiter.

Ten hours, this planet is ten times the size of the

earth, and yet it's got a rotation period that is half that

of Earth.

These things are spinning very, very rapidly.

Now, there's other things we can look at like its tilt axis.

That's the, as we talked about for the earth, how important it was for

the earth to have a tilt axis that was stable, and it was 23 degrees.

we're going to see here a wide variety of tilt axies for these outer planets.

Now, let's look at Saturn.

Saturn is a bit smaller with, coming in at about 8.5 times the size of the Earth.

also quite large but nowhere near, or

massive, but nowhere near as massive as Jupiter.

So Jupiter is really the king of the solar system.

so now, the, we could, I'll let you look at this table in more detail

on your own, but notice that Uranus and

Neptune are clearly different from Jupiter and Saturn.

the radii here are about four times the size of the

earth, so they're, they're not enormous compared to the size of

the earth.

in terms at least radii and again also, one thing to notice is

they have densities that are similar to the to the gas giants as well.

Also, fairly rapid rotation periods.

Now one thing you might want to notice here, is that Uranus is

something bad happened to Uranus because it's axial tilt is not at 98%.

That basically means that if this is the way this is

the spin axis and here's the sun, Uranus is pointed like that.

So at one point in Uranus orbit, its north pole is pointing directly at the sun.

And then, six months of its, you know, of its

year later, its south pole is pointing at the sun.

So it's a very different kind of climate, then in terms

of its rotation axis, than anything else in the solar system.

And that's probably because of a collision.

One thing that really we've found is that

collisions between bodies really shapes the solar system.

A 100 years ago, people would have said, no, they're very rare.

It never happens. Well, not, it's not true.

It seems like there are collisions happening,

or happened all the time early on.

The collision that formed our moon, for example.

the collision which slowed Venus down.

Why does Venus, rotate so slowly?

Probably because of a collision.

and why does, Uranus have the strange inclination?

Probably because of collision.

So, so this gives us a good overview of the, g, of the giant planets.

Let's look at a little bit more detail.

So, one thing that's really important about the giant planets,

as we've seen, is they're all rotating very rapidly, and

that rapid rotation has a large impact on things that

happen in the planet, particularly, what happens in the atmosphere.

So this lovely animation here shows us

each of the different planets, including the earth.

it, comparing their rotation periods, and also shows you their axial

tilts based on the, you can see from the rotation periods.

Now because

all of these planets are rotating so rapidly, that has a large

effect, on the, what occurs in

the atmosphere because of, the coriolis effect.

So on the earth we know the coriolis

effect as something which creates the movements of hurricanes.

We all know that hurricanes.

Have this swirling pattern, that's because in any rotating system, if you try and

move this way, you also find a force moving perpendicular to you and so,

that is on the earth what curls around the wind systems on the earth.

Strong wind systems into things like the hurricane, hurricanes.

On these giant planets, the planet is rotating so rapidly that instead

of getting the coriolis force forming you know, these large circular patterns.

What you get is these clouds being pulled

apart into very stable bands, there's banded structure.

So Jupiter has this extraordinary

banded structure that we see on it.

now of course there are in between the bands, the bands are actually

counter rotating, one band is going this direction, one bands going that direction.

And we see all these beautiful structures forming these storms

that form where the two streams run up against each other.

And these can form, actually, large circulating

systems like the Great Red Spot in

Jupiter, which is an enormous hurricane that you could fit the entire Earth into.

And that hurricane seems to have been swirling or storming for around 300 years.

So it's really a remarkable system.

So these atmospheres, you may wonder about the atmospheres of these planets,

and think because they're gas giants the atmosphere goes all the way down.

But in fact, actually relative to the size

of the planets, the atmospheres are actually quite thin.

And what we mean by atmosphere is these the material in the gaseous state.

Because what happens as you dive into these gas giants

is that as you go down, just like if

you dive down into a swimming pool, the pressure goes

up, and as the pressure increases, the material that

composes the planet undergoes phase changes, it changes its state.

So, for example, as you were to dive down into

Jupiter's atmosphere, what you'd find is a very smooth transition.

From a gaseous state to a liquid hydrogen state.

so, in some sense you have this enormous hydrogen ocean,

that makes up most of Jupiter's volume.

So, the atmospheres are quite rich in terms of their phenomonology.

We see all this structure going on but it is

important to rec, repr, understand that they are actually very thin.

Now, we think of Saturn as being a planet with

rings, but remarkably, all of the giant planets have rings.

It's just that Saturn has rings that are the brightest and the largest and

the most spectacular.

But Jupiter has rings, Neptune has rings and Uranus has rings.

Now, why is it that giant planets all have ring systems?

Most likely it's because the giant planets have many moons.

Almost all of the giant planets have at least ten moons.

Many of these moons are captured asteroids.

If those asteroids get close enough, they will not

just be moons they will actually be torn apart.

There's something called the Roche. Distance.

And if you get closer than the Roche limit or the Roche

distance, then the gravity will actually, the gravity of the planet will actually

tear the object apart and all of the material that was in

that body, will then go into Orbit, and, it'll form a ring system.

And the important thing to understand is those rings don't last.

So, as spectacular as, Saturn's rings are, if you were to come

back billions of years from now, those rings would probably be gone.

Unless they were resupplied by perhaps another object being torn apart.

Or, the moon's just, material being blasted off the moons

just by, collisions with, meteorites can actually resupply the rings.

So, all the plan, the massive planets have rings, because they have a lot of moons.

And all because their, their, gravity's so large.

They tend to capture more things than the small planets do.

And the disruption of those bodies is actually what feeds the ring materials.

The interesting thing about the rings, too, is that, the rings, because they are.

The rings are not solid.

They're just lots of ice particles or rock particles in Orbit.

That the rings have their own dynamics.

And there's from the amazing space probes that we've sent,

we've seen all kinds of structure forming inside the rings.

and evolving with time.

And we have some beautiful videos for you to

look at to see the actual structure The gravitationally

induced structure inside the rings

of Saturn.

Giant Planets

Confronting The Big Questions: Highlights of Modern Astronomy

Meet the Instructors