I hope you enjoyed last week's world wind 
tour of physics. 
But, I bet that you're ready to start 
thinking about astronomy again. 
We're going to do that now, we're 
going to take all tools we acquired and 
start applying them to understanding the 
rest of the universe. 
It seems fitting to start with our local 
neighborhood, the solar system. 
This is both, because our local 
neighborhood is very important. 
It's where we live. 
And also because in a way, we know a lot 
more about the solar system than about 
anywhere in this, rest of the universe, 
because over the past few decades we've 
been able to send spacecraft robotic 
spacecraft to visit many of the objects 
in the solar system. 
Allowing us to verify the predictions we 
had made based on ground base 
observations that are understanding of 
physics and compare them to actual 
measurements. 
So we will be able to verify a lot of our 
understanding of what goes on in the sky 
directly. 
For both of those reasons, we're going to 
spend the next week studying the solar 
system. 
I should point out at the beginning that 
there are at least two aspects of the 
science of the solar system, that I will 
not be able to give the appropriate time 
to. 
This includes A, the amazing and 
fascinating history of space travel, in 
our exploration of the solar system. 
that will be given short thrift because 
we don't have time to put into it. 
And the other aspect is the intriguing 
and exciting question of, the possibility 
for the existing of life elsewhere. 
in the solar system or beyond and this is 
not something about which I know very 
much and it's not the center of focus of 
this class. 
So we will be discussing the physics of 
the solar system leaving the biology 
aside. 
So with that said, what is the solar 
system? 
Well, primarily the solar system is all 
of the objects in bound orbits around the 
Sun and most of that is a star, it's the 
sun To a great extent, 99.9% of the mass 
of the solar system is in this 
average-sized, main sequence star we call 
our sun. 
And this star dominates the solar system 
in more ways than containing all of the 
mass. 
In addition to dominating the mass of the 
solar system, the sun is extends its 
influence throughout the solar system in 
two other ways. 
One, we talked about last week. 
Is the radiation. 
The black body radiation with which the 
sun glows. 
that is the source of our light and heat 
here on earth and elsewhere in the solar 
system. 
In addition, as we'll see as part of the 
processes that go in a star, the outer 
atmosphere of the sun is a very, active 
place, and it is constantly emitting a 
stream of charged particles called the 
solar wind that leave at high, the sun at 
high speed and propagate throughout the 
solar system. 
We will discuss this when we talk about 
the sun next week. 
But this is a nontrivial flux of 
high-energy particles away from the sun. 
And as we'll see it, it, it has lots of 
effects in the solar system. 
To give you a sense, every 150,000,000 
years the sun loses a full earth mass to 
this solar wind. 
So there is this constant stream of stuff 
coming out of the sun. 
Okay, we know about the sun. 
We'll discuss the sun next week when we 
talk about the stars. 
What else is out there? 
Well primarily what we think of is the 
solar system, is the eight planets. 
The sun is orbited by eight large, round 
objects, in slightly eccentric but almost 
exactly circular orbits of various sizes, 
ranging from point four astronomical 
units. Mercury is the closest plane to 
the sun, all the way out to 30 
astronomical units, or two orders of 
magnitude, of orbital radius all the way 
up to Neptune. 
And we see in the this nice image that 
there's a nice cluster of very small 
inner solar system orbits and then the 
more 
widely spaced outer solar system orbits 
and essentially the space between the 
planets is mostly empty. 
So, roughly, it's the sun with a planets 
orbiting it and the orbits are 
distinguished not only by being circular 
but note in this image, they are almost 
all in a plane. 
So there are no planets that orbit the 
sun this way or that orbit the sun that 
way. 
They all orbit in the same sense, in the 
same plane. 
Clearly crying for explanation. 
When one observes the sun as Galileo did 
when notices that the sun in fact spins 
about an axis. 
And the sun's axis, surprise is 
perpendicular to the ecliptic. 
So the sun, too, rotates about its axis 
in the same sense as do the planets. 
the planets themselves come in two very 
distinct families. 
There are the inner planets, Mercury, 
Venus, Earth and Mars, the outer planets, 
Jupiter, Saturn, Uranus and Neptune and 
they're different in as many ways as you 
can think of. 
the inner planets are dense, as we shall 
see, as dense as rock or denser. 
They are small, this image is not to 
scale, the left-hand side is to scale, so 
that you see that Mars and Mercury are 
smaller than Earth and Venus. 
the right hand side is the scale, which 
shows you that Jupiter is the largest. 
But, between the right and left hand side 
is about a factor of ten. 
In size, Jupiter in fact is eleven times 
as large in radius as Earth, 
if we put the two figures to scale, and 
either we wouldn't see the inner planets 
or the outer planets wouldn't fit. 
So, you have to draw a very sharp line 
between the right and the left. 
The outer planets are not just bigger, 
they are also far less dense. 
and their composition is different, 
as we will see, the inner plants are 
rocky. 
Their composition is silicates and iron 
and nickel. 
The outer planets are mostly hydrogen, 
helium and Isis and lighter elements, 
larger planets. 
Why this dichotomy? 
Certainly an interesting question. 
Okay, we have eight planets. 
The solar system is not just the sun and 
eight planets, there's other things 
floating out there. 
What else is there? 
Well, around many of the planets are 
objects that are captured in 
gravitational Keplerian orbits about the 
planets as they orbit the sun. 
They're called moons, we know of our 
moon. 
There are a total of about a 150 Moons in 
the solar system most of them orbiting 
Jupiter and Saturn, but all of the 
planets except Mercury and Venus have 
moons. 
the outer planets tend to have many more 
moons than the inner planets and some of 
these moons are planet sized. 
Our moon is about almost the size of 
Mercury, some of the moons of Jupiter are 
very large. 
Again notice that all of these large 
objects, like the planets. 
All have the same shape. 
So, they're all spherical. 
We want to understand why is everything 
round. 
and where do these moons come from and 
why do they orbit the planets and not the 
sun. 
in addition to moons orbiting the 
planets, there are these beautiful 
structures of rings. 
It turns out that, as we found in the 
twentieth century, all of the giant 
planets have ring systems surrounding 
them and we'll see that that's a 
ubiquitous fact. 
But Saturn's rings are certainly a 
spectacular exception. 
there's a reason why Galileo discovered 
the rings of Saturn and it was not until 
the twentieth century that we found the 
other giant planets had rings. 
and we'll want to understand what this 
structure is, where it came from, and 
what it tells us. 
Looking at the moons and the planets of 
the solar system we see another property 
that they all share, which tells us. 
Something about what goes on in the solar 
system. 
And that is, that they all seem to have 
suffered a little bit of damage. 
this image is an image of Mercury, and 
you see that it looks kind of like the 
moon. 
Because like the moon, it's pock-marked 
with craters, the results of collisions 
some time in its past. 
This image shows you that the Earth is 
not immune to such attacks. 
This famous crater in Arizona was formed 
by an impact only 50,000 years ago. 
And so, there have been and are impacts 
onto the Earth and this feature of 
cratering is one of the most ubiquitous 
things we see. 
Here's our familiar moon, with this is 
the nearside of the moon and the familiar 
cratered shape of Tycho and so on shows 
us that the moon too has taken the brunt 
of many hits. 
Here is Ganymede, Jupiter's moon and 
Ganymede too has had the impacts. 
What caused this, is it still happening, 
are we about to get hit? 
All questions we'll want to understand. 
So we have planets, moons, rings, 
something that crashes into things and 
creates craters. 
What else is out there? 
Well turns out that's all the big stuff 
but there are smaller objects out there 
The main collection of small objects is 
called asteroids. 
this is a picture of a particularly 
pretty one, this is the asteroid Eros 
close-up shot. 
And we see two things about Eros. 
One is that, like everything else in the 
solar system, it has suffered some 
collisions, it's cratered. 
But Eros is not round, it's shaped like a 
potato. 
And in general asteroids, 
do not necessarily follow the pattern 
we've seen where everything is spherical. 
We'll have to understand why it is that 
things can come in various shapes. 
There's this relatively densely populated 
region of the solar system. 
There's about a tenth of an Earth mass 
orbiting in the asteroid belt and in 
addition, for example in Jupiter's orbit, 
there are these so-called two groups of 
so-called Trojan asteroids that orbit in 
Jupiter's orbit but 60 degrees east or 
west of the planet and they orbit 
because, 
It's a Kepler orbit with the same period 
as Jupiter. 
Another thing we'll have to pay attention 
to. 
The solar system, we think, can not 
possible end at Neptune. 
There are objects that are a bit beyond 
Neptune's orbit, and our best hint about 
them comes first from comets. 
Here's a pretty view of Haley's comet 
from it's 1910 apparition. 
And it turns out that comets in their, 
cometary orbits, where they approach the 
sun, cannot survive billions of years. 
We'll talk about that later. 
And in order to explain the frequency 
with which comets are observed entering 
the inner solar system in the plane of 
the ecliptic. 
we conjecture the existence of a 
collection of objects. 
Orbiting beyond Neptune's orbit. 
It's called the Kuiper Belt, a 
disc-shaped distribution with it is 
assumed tens of thousands of objects, 
hundreds of which are the size of the 
moon or larger. 
Pluto would be an example of one of these 
Kuiper Belt object. 
And. 
these are objects which when perturbed 
from their orbits, form the source of the 
short and intermediate range comets that 
we see orbiting in the plane of the 
ecliptic and it turns out that in order 
to explain the frequency with which we 
see comets, we can predict the density of 
objects that must be out in the Kuiper 
belt. 
the Kuiper belt might extend out to as 
much as 100 astronomical units away from 
the sun. 
much farther away, it is conjectured that 
way past the orbits of the planets, is 
this spherically symmetric halo of 
object. 
the Oort cloud, it might have an inner 
Oort loud along the disk or the Hill 
cloud but these objects orbit the sun in 
a highly eccentric, 
Orbits outside the plane of the ecliptic, 
and they form the source for out long 
period comets. 
This is our collection. 
the first question you want to ask is 
What's it made of? And we can answer that 
rather directly. 
the answer of course to, "What is the 
solar system made of?" is mostly the 
answer of the question, "What is the Sun 
made of?" because 99.9% of the mass is 
the Sun. 
Of the.1 percent that is not the sun, 90% 
of the rest is Jupiter. 
and Saturn and we have here a plot of the 
abundances of various atomic species in 
the solar system, which roughly mirrors 
the abundance of the same atomic species 
throughout the universe. 
So when we see what the solar system's 
made of, we're really answering what the 
universe is made of. 
We'll come back and try to understand 
this later in the class but for now it's 
worth the observation and these are the 
relative numerical abundances of various 
species of atoms, 
organized by atomic number. 
And what you need, the, the scale is kind 
of random, but it's a logarithmic scale. 
So that the fact that helium registers 
one less than hydrogen means that there's 
about one helium atom for each ten 
hydrogen atoms. 
There's a factor of ten less helium than 
hydrogen and together, because everything 
is down by a factor of 10,000, they 
comprise almost the entire content of the 
solar system 
by mass, because a helium atom weighs 
four times as much as a hydrogen atom. 
that works out to about 70% hydrogen, 28% 
helium and 2% the rest. 
everything else is trace amounts, down by 
a factor, as I said, of at least 10,000, 
from the abundance of hydrogen and 
helium. 
And this is a sobering note when you 
remember that we and essentially, our 
entire planet is made up mostly of these 
trace left-over materials. 
Astronomers tend to call anything that 
is, heavier than lithium. 
So anything past the first three or four 
elements. 
Metals, carbon is not a metal in the 
chemical sense of the word. 
But in astronomy, heavier elements are 
all called metals. 
So we say that the mass of the solar 
system and the mass of the universe, 
essentially, has 2% metals. 
But mostly, it's a world of hydrogen and 
helium. 
That's our quick survey. 
this raises many questions. 
I've listed some of them here and we'll 
try to see how many of them we can answer 
and how many more we can raise as we get 
into more detail 
Why are all the planets orbits circular? 
Why are they all in a plane? 
Why are the comet orbits neither circular 
nor in a plane? 
Why are all the planets and the large 
moons spherical objects and if so, why 
are the asteroids not? 
Why are there two kinds of different 
planets? 
Why are there inner planets of one kind 
and outer planets of a completely 
different species of objects? 
why aren't the asteroid is a planet? 
There's clearly stuff there. 
there's room between Mars and Jupiter, 
why didn't a planet form there? 
Why didn't asteroid belts form elsewhere? 
Why didn't the rest of the solar system 
coalesced into planets? 
It's not a science question, but it's 
always asked. 
What happened to Pluto to get it demoted 
from planet to dwarf planet? 
What's a dwarf planet anyway? 
what are rings? 
Why does Saturn come equipped with these 
brilliant special rings? 
What made all the craters? 
Where did it go, is it coming back? 
Are we about to get 
it hit with some big impact. 
Remember we did our homework last week 
and we saw that this can be a significant 
event. 
Why do comets sometimes leave their 
comfortable orbits out in the Kuiper belt 
and the Oort cloud and come diving into 
the solar system? 
Why do some asteroids move from the 
asteroid belt into orbits that bring them 
very near earth? 
We keep hearing of near-earth asteroids 
and their potential for collisions. 
And, 
if orbits can change, if something can 
move from a comfortable orbit out of the 
Oort cloud and come crashing into the 
solar system. 
Will the planets orbits change? 
Will the solar system remain in its 
current configuration? 
Have planetary orbits changed? 
Was the solar system always the way it is 
now? 
And, 
underlying all of this, the question of 
where did all of this come from, and when 
did that happen? 
And we'll find that starting from the 
end, in fact, is the way we're going to 
go in the next clip, we're going to 
figure out when it started and from that, 
we'll try to go on to how it started. 
And that will try to generate as many 
answers as we can to all of these 
questions.