Introduction to the Solar System

Can't see them at all without a telescope
They were not part of the wanderers of the ancients
OH: Right now, Neptune is behind Uranus, which is behind Jupiter
Almost in a straight line
Uranus to me looks like a green dot in a powerful telescope
Neptune looks like a smaller blue dot, but you can see the disk
There are so far out that you know that they have to be big to see a disk

William Herschel was a musician, who turned to astronomy
He was trying to map all the bright stars in the sky, and discovered Uranus
There are supposed to be only 2 kinds of true geniuses in the world -- mathematicians and music, but the aptitudes are very related
So Herschel discovered Uranus in 1781, well after the establishment of Newton's laws of gravitation and motion
Uranus is 2 billion km from the Sun, and goes around once every 84 years
So it has gone around the Sun only 2.5 times since its discovery
Uranus is the mythological grandfather of Jupiter (Zeus in Greek)
Saturn is the father of Jupiter
Uranus was watched for a few years until they could figure out its orbit
The problem is, it had some irregularities in its orbit that apparently couldn't be explained by Newton's Laws
It was a crisis in science -- some people suspected that Newton's laws were wrong
French mathematician Leverrier, in 1846 used Newton's laws to try and predict (with pencil and paper) where another large body might be to explain the discrepancies
It took some German astronomers only and hour of searching to find Neptune
This was cited as a great victory for Newtonian physics, and probably ushered in the great era of rationalistic thinking that characterized the Victorian era

In 1977, before Voyager got to Uranus, the planet passed in front of a star
This is called an occultation
Stellar occultations are important in planetary astronomy, because it allows you to figure out whether a planet has an atmosphere
If the star dims and flickers a little when it disappears behind a planet, you know the planet has an atmosphere
But with Uranus, the star dimmed before it got to the edge of the planet
We concluded that it had some rings around it

It took Voyager 6 years to cross the deep space between Saturn and Uranus, and it arrived there in 1986
What Voyager saw was a featureless bluish-green disk
The atmosphere of Uranus was made of H2 and He, just like Jupiter and Saturn
It has clouds of methane, at -320 F
Uranus and Neptune formed in the same way as Jupiter and Saturn
First a rocky core, several times the size of the Earth accreted from the planetesimals in the outer solar system
When the core got big enough, it began to gravitationally attract the gas in the solar nebula
It got lots of H2 and He, like Jupiter and Saturn, but it got lots of other frozen gases, too, like CH4
Because they are all almost the same, Jupiter, Saturn, Uranus and Neptune are called the Gas Giants
But on Jupiter and Saturn, we have yellow clouds and lots of turbulent weather
Why is Uranus blue-green, and why don't we see bands of clouds and weather?
Recall what drives the weather on Jupiter and Saturn
On the Earth, it is the Sun's energy
On Jupiter and Saturn, it is the energy coming up from the interior, the left over heat from the compression due to gravity
Because Uranus is smaller, it doesn't have as much compressional energy, and there is less heat from the interior to drive cloud motions
And why is Uranus blue-green?
On Uranus, the clouds are made of methane, rather than the sulfur and ammonia clouds of Jupiter and Saturn
Methane absorbs red and orange light, and Uranus looks blue-green
Voyager also discovered the shepherd moons that are necessary to keep the rings in place

Small rock core
mantle of liquid water and methane mixture -- an enormous ocean
Atmosphere of H2 and He
Clouds of methane, topped with a haze layer that may be a little like Titan's
Because Uranus it further out from the sun, the conversion of methane to heavier hydrocarbons takes place slower (it takes sunlight)
Uranus lacks the mantle of metallic and liquid hydrogen that Jupiter has, because it doesn't compress hydrogen enough

The most amazing thing about Uranus is that it is tilted onto its side
The north pole is pointing straight at the sun
The angle that the rotation axis makes with the axis of the solar system (same as the Sun's rotational axis) is called the planet's obliquity
Uranus' obliquity is about 90 degrees
As Uranus goes around the Sun, the equator points at the sun, and then it's south pole, and then the equator again, and then the north pole again
But the planet spins once every 17 hours
What it means is that the concept of years and days is really mixed up
The best explanation is that by a huge planetesimal, and that it wound up on its side
But why did its moons and rings get knocked over, too?
Tidal forces would have forced the moons to eventually orbit in the same plane as Uranus' rotation.

Voyager got to Neptune in 1989
Neptune actually violates Bodes' law -- it isn't as far out as you would expect
Uranus is 20 AU from the Sun, twice as far as Saturn. But Neptune is 30 AU from the Sun
What we saw there really surprised us
We expected another Uranus
Neptune is the same size as Uranus, so we don't expect much heating of the atmosphere due to compression
But what we saw were cloud features and storms, like on Jupiter
Neptune is a striking blue color, but it has noticeable clouds and storms
We know the sun is even dimmer there than at Uranus, so what drives the weather
We have no idea
Neptune may be bluer than Uranus because of N2 in the atmosphere
Maybe this has something to do with the weather and storms
Neptune has a blue storm, just like the Great Red Spot on Jupiter, rotating counter clockwise
It also has smaller features, like Jupiter, that rotate in the upper clouds
One feature seems to zip around the planet, a mini storm, called scooter
When we looked really closely at Neptune, we saw bands of cumulus clouds, like thunderstorms on the Earth
If Neptune was a basketball, Voyager flew 1/4" off the surface of the cloud tops
Before Voyager got to Neptune, in the 1980's, people tried to use occultations to show that Neptune had rings
The problem was that sometimes they saw the star dim before it passed behind Neptune, and sometimes they didn't
They concluded that Neptune didn't have rings, it had ring-arcs
Voyager saw that Neptune truly had rings all the way around the planet, but that they got thinner and thicker as you go around the planet
They are still called ring-arcs, and they are difficult to explain

VIDEO: NEPTUNE'S COLD FURY, NOVA (First half) 6/27/96