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3. Frontiers/Controversies in Astrophysics: Our Solar System and the Pluto Problem
Poziom:
Temat: Edukacja
Professor Charles
Bailyn: Okay now,
we didn't have sections this
week, in case you didn't notice,
and therefore you didn't have
an opportunity to discuss the
problem set during section.
So, I thought I would say a few
words about it now - here is the
problem set.
You probably can't read it at
this typeface but that's okay.
So, let's see,
problem zero is just a stupid
way of making you read the
policies.
Never mind that.
Problem set one,
problem one:
here are exercises in one-digit
scientific notation.
I don't have rules for this.
As I mentioned last time,
the only rule is common sense.
I think there might be some
difficulty with the last one.
This business of taking things
to the one third power is
important because you keep
ending up with a cubed equals
something,
and you have to figure out what
to do about that.
So, let me not do this
particular problem,
let me do a different one for
you.
Supposing you had (6 x
10^(4))^(1/3).
And you might be tempted to
say, well okay,
that's 6^(1/3) times 10^(4/3).
And that leads you to a bad
place, because 10^(4/3) is not
the notation we want.
We want this to be an integer
up there.
What does it mean to be a 1
with 4/3 of a zero after it?
And so, you don't like that.
So, the way to deal with this
is to regroup.
This is (60 x 10^(3))^(1/3).
That's 60^(1/3) x 10^(1).
What's 60^(1/3)?
Well, 4 times 4 times 4,
as it happens,
is 64, and that's close enough
for me.
So, this is 4 x 10^(1).
So, that's just an example of
how these kinds of things where
you take things to fractional
powers,
either the square root,
which is to the 1/2,
or the cube root to the 1/3.
All right, the next problem.
Let's see, Neptune's moon
Nereid has an orbital period of
almost exactly one Earth year.
If the mass of Neptune is
something or other,
what's its semi-major axis?
So, you have P and
M and you're asked for
A.
That's a completely
straightforward plug-and-chug
problem because there's an
equation that relates these
three things and the only tricky
thing about this is that you
have to make sure the units come
out right.
All right, the next one looks
similar in form,
but it isn't.
Consider a Sun-like star
orbited by a planet with a
period of eighty years.
So, we have P--the
separation of the planet and the
star appears to be 20 arc
seconds.
So we have an angle,
that's α.
How far away is the star?
And you want to know D.
Okay, there is no equation that
contains all three of those
things and so this,
although it looks similar in
form, is actually a
substantially more difficult
problem.
Going on, problem four.
Okay the important--in fact,
there is such a star,
blah, blah, does this fact make
any difference in the forgoing
calculation?
Explain.
The important thing to note
about this is that I'm not
asking you to do a calculation.
This is not a calculation
problem;
this is a comment.
You're supposed to say
something about how the
calculation would go if you were
to do it, but you don't have to
actually calculate anything
there.
And then, finally,
this last problem.
This is the sort of essay
question here.
The point here as I've written
down on the--there's about
Pluto.
The point here as I've written
down on the actual paper
itself--there's no right answer
to this.
This is a thought question.
There are some people I know
from the course evaluations who
get disturbed by this because
they have the feeling that
science ought to have right
answers;
that is, after all, the point.
And I keep asking these sort of
touchy-feely,
humanities kinds of questions.
Revel in it, go with it.
And what we're looking for here
is the same thing as your
English teacher would be looking
for, right?
Although perhaps less emphasis
on writing style,
but, just have some thoughts,
do some reading,
understand something and say
something intelligent and defend
it,
okay?
And a paragraph or two,
you know, two sentences
probably too little.
If it's more than two pages
double-spaced or one page
single-spaced I'm going to get
really irritated because I have
to read these things and there
are eighty of them,
and so don't go nuts.
Again, just be sane about it
and say something intelligent.
That's all we ask.
Questions about the problem set
or about procedural aspects of
the course at this stage?
If you do have some at some
point, send them to us on the
classes server.
Yeah, go ahead.
Student: What would you
say is too short for the answer
to that question?
Professor Charles
Bailyn: Oh the answer--well,
a sentence is too short,
two sentences probably too
short unless they're pretty
severe sentences.
I mean, it's got to be a
paragraph, you know;
otherwise, you haven't really
cleared your throat.
But I don't want to say
anything too precise about that.
If you've got a really keen
sentence, you know,
that might do the job.
Again, sanity ought to prevail.
Yes.
Student: Is the answer
to a question like that typical
of most of the problem sets?
Professor Charles
Bailyn: We'll probably have
one of that nature.
Not necessarily of this kind
about scientific controversies,
but about something.
Oh, and I should say,
the way the tests will work out
is it's going to be probably
slightly more than half
calculation and slightly less
than half other things.
There won't be essays because
there probably isn't time to do
that on a fifty-minute test but
there'll be short answer
questions and stuff like that.
Yeah, we'll have these kinds of
discussion things.
They won't be the dominant part
of the problem sets.
Just as in this case,
it's six points out of--this
adds up to twenty points;
this one's six [points at
problem set].
Other questions?
Okay, as I say,
if you've got some,
let us know.
Okay, so as I pointed out last
time, we're talking about
"exoplanets" – planets around
other stars.
And the problem with exoplanets
is you can't see them directly.
So, you can't see the
exoplanets directly as blobs of
light in the sky and so what do
you do?
So, you have to detect them
obviously indirectly and
here's--we've got to invoke
another one of Newton's laws.
So, the key law here is now
Newton's Third Law.
Newton's Second Law is F
= ma;
you may recall that explains
all the Physics 180.
Newton's Third Law is usually
phrased in textbooks and stuff
as "every action has an equal
and opposite reaction."
I hate this formulation because
it leads to really bad
philosophy.
You know, this is one of the
whole problems with physics in
general.
There are these words that seem
to mean something.
Words like "force," words like
"potential," and these have
technical meanings in physics
that are actually different from
the intuitive meaning that you
have in your head just from
using them in everyday life.
The big problem with
introductory physics is getting
people to use these words as if
they mean the physics definition
rather than as if they mean the
everyday life definition.
And this leads,
as I say, in extreme cases,
to all sorts of bad philosophy.
In fact, the two most misused
physics words that I know of
are, first of all,
"relativity"--"Everything is
relative," said Einstein.
Well, no, actually,
he said nothing of the kind.
And the other one is
"uncertainty," which has to do
with quantum mechanics and these
are technical terms in physics
and then they get sort of
promoted to use in philosophy as
if they meant what they mean in
English.
So, I don't like this "every
action has an equal and opposite
reaction."
You can get into all sorts of
bogus philosophy and I don't
want to go there.
So, I would rather phrase this
as "conservation of momentum."
And momentum,
as I say, is a technical term.
It means M,
mass, times velocity,
that's all it means.
And the kind of use that it's
put to in electoral politics or
other things has nothing to do
with what's going on in physics.
Oh, I should say:
velocity, one should be aware
of, consists of two things.
It's not just the speed that
something has,
but also its direction.
And so direction counts.
So, if you turn around and go
at the same speed you've
reversed your--you now have
negative velocity compared to
where you started with.
This, technically speaking,
makes this a vector quantity,
but again, we won't go there.
Okay, so how does this work in
the case of a planetary orbit?
So, here's a planet,
it's on one side of its star,
it's going this way [draws an
arrow up]
and it's got some momentum.
And then half an orbit later,
it's over here going the other
way.
Let's say it's orbiting some
star in the middle and it's got
more or less the same speed but
its direction is reversed.
So over half an orbit--so over
1/2 orbit, the momentum is
reversed.
So, it goes from being positive
to negative, or something like
that.
Now, Newton's Third Law says
that momentum is conserved.
The total momentum of the
system has to remain the same;
so something else has to change.
And here's the whole trick
about finding exoplanets,
and that is that the star moves
too.
So, when the planet's over here
at this blue arrow--the star is
here, and it's also moving in
the opposite direction.
And then half an orbit later,
the planet has come over to
this side.
The star has moved in the
opposite direction;
it's now going this way.
And so the star moves too,
that's key.
How much does the star move?
Well, the two momenta have to
be equal and opposite.
This is what is equal and
opposite about Newton's Third
Law is that these things have to
cancel out.
So what you find is that the
mass of the planet times the
velocity of the planet is equal
to the mass of the star times
the velocity of the star.
And because the mass of the
star is so much bigger,
so huge compared to the mass of
the planet,
its velocity must be much,
much smaller in order for these
two things to be equal.
Okay.
What are they orbiting around
then, if everything is moving
together?
They orbit around the "center
of mass," so-called.
This is just,
you know, where the balance
point of a seesaw would kind of
be.
And so here we have--here's the
center of mass,
here's the star moving this
way, way over here somewhere is
a planet moving much faster.
And let's define two
quantities: this is the distance
from the planet to the center of
mass.
Here's the distance of the star
to the planet of mass.
Star is moving at
V_star,
planet is moving at
V_planet.
And you can define a total
velocity, which is equal to
these two things added together.
And what does that mean?
That's the velocity where if
you're on one of these objects,
the other appears to be moving
relative to you.
So, if I'm going this way and
you're going that way,
we have a relative velocity
equal to the sum of our
individual velocities.
And there's also a total
distance, which can be defined,
which is, by analogy,
the sum of the distance from
the planet to the center of
mass, and the star to the center
of mass.
And that's just the distance
between these two objects.
The distance and the velocity
in an elliptical orbit can
change during the course of the
orbit.
But for circular orbits or
close to circular orbits they
remain about the same.
Orbits, you'll remember,
are generally elliptical.
And a way of saying this is
that the maximum of the total
distance between these two is
this quantity a,
the semi-major axis.
Remember, the semi-major axis
is the long slice through the
ellipse.
So, when this is at a maximum,
that equals the semi-major
axis.
For orbits that are nearly
circular you can sort of say in
an offhand way,
well this,
D_total,
doesn't change that much.
So, it's the distance between
the two objects.
All right, now as I mentioned,
V_p
M_p is equal to
V_star
M_star.
It is also true the way you
figure out where the center of
mass is, D_p
M_p is equal to
D_star
M_star.
And the whole point of this is
that this is a large quantity
compared to the mass of the
planet,
and therefore these are small
quantities compared to the
distances and velocities taken
by the planet.
Small, but as it turns out,
measurable.
In particular,
the velocity is the thing--the
velocity of the star turns out
to be the thing that you can
measure.
And so that's how you
find--determine that there's an
exoplanet there.
What you do is you look for the
reflex motion of the star.
Planets going around the star,
stars going around the center
of mass also.
And that is a motion that now,
these days, can be observed.
And you can see why this might
have happened only very
recently, because that motion is
really very small.
Let me give you some masses
just to give you a sense of
this.
I've already written down that
the Sun's mass is about 2 x
10^(30) kilograms.
Just for reference,
the Earth's mass is 6 x 10^(24)
kg, so down by almost a factor
of a million.
And so the Sun moves much
slower than the Earth does,
due to their mutual gravity.
Jupiter is the most massive of
the planets, and it's at about 2
x 10^(27) kg,
so 1,000 times smaller than the
Sun.
And so, of course,
the Sun moves 1,000 times less
because of Jupiter than it does
because of the Earth.
Now, of course,
the Sun actually responds to
all of these planets,
so it's actually executing some
complicated motion,
which is the sum of the motions
induced by all the planets.
But in fact,
Jupiter is significantly more
massive than the rest of the
planets,
so by far the dominant motion
that the Sun goes through has to
do with the orbit of Jupiter.
And so the consequence of this,
because the masses are so much
smaller, is that the velocity of
the star is much,
much less.
These two less-than signs
[<<]
means much, much less than the
velocity of the planet.
But it can nevertheless be
detected.
Okay, now, what do we expect to
see?
Supposing you can now go out
and through means that we'll
actually talk about on Thursday,
actually measure the velocities
of stars in response to planets.
What do you expect to see in
those--in other stars?
And basically,
the answer to that is,
what you expect to see depends
on what your expectations for
Solar Systems are.
We've got one example,
or at least ten years ago,
we had only one example.
And so, you have to take what
you know about our own Solar
System and infer what other
Solar Systems might want to look
like.
And so, at this point I want to
show you some things about our
own Solar System,
so a little slide show of the
Solar System here.
All right, don't take notes,
I'll tell you everything you
need to know after we finish the
pretty pictures.
Okay, so starting from the
innermost part of the Solar
System.
This is the innermost planet,
this is the planet Mercury.
Looks much like the Moon:
it's basically a rock with
craters on it.
There was a time when we
thought that its spin period was
exactly the same as its orbital
period, so it keeps one face to
the Sun.
There's all kinds of science
fiction based on that--that
turns out not to be true.
But basically,
it's kind of a hot rock,
that's all you need to know
about Mercury.
Let's see, oh,
here's a close up of a little
piece of Mercury surface,
and you would have a tough time
telling that this was Mercury
rather than the Moon or many
other objects--rocky objects in
the Solar System.
Next one out is Venus.
Venus looks quite different
because it's got a very thick
atmosphere, very thick carbon
dioxide atmosphere.
This is all clouds that you're
looking at here.
And, in fact,
the greenhouse effect,
which is supposed to be
responsible, perhaps,
for global warming,
was first studied and
identified on Venus,
because it appears to have run
amok on Venus.
The surface of Venus is
extremely hot and it's covered
with these really thick clouds.
So, it was quite hard,
for a long time,
to get a handle on what was
going on down on the surface.
This has now however been
accomplished.
They've put things into orbit
that have radar,
and can view the topography
through the clouds.
They've also dropped things
onto the surface of Venus.
The problem is at 700 degrees,
and it rains sulfur down there,
so it's an unpleasant
environment for machinery.
So things don't last very long.
But nevertheless,
they've gotten some
information.
Here's a little Venus landscape.
It's entirely artificially
colored, right?
But the topography comes from
these radar mapping missions.
Here is a map of the whole of
Venus made by these orbiting
missions.
And so Venus is important
primarily for its atmosphere,
and as a kind of warning for
what might potentially one day
happen here, if we're not
careful.
Okay, this is the third rock
from the Sun.
Ninety nine percent of all Yale
courses deal with what's going
on on this little piece of
cosmic debris.
I'm not going to say anymore
about it, therefore,
oh, except for one thing:
it comes with this companion
object.
This is the Moon.
The Moon is a very special
thing because,
relative to its planet,
it's huge.
This really shouldn't be
thought of as a planet and a
moon, but rather as a double
planet.
Here they are to scale,
and that's much closer in size
than any other moon-planet
system around the major planets.
Moving outwards,
we come to Mars.
This is about as good an image
of Mars as you can get from the
Earth, and you can see why
people got excited about it.
These blotchy things here turn
out to change with time.
And in fact,
they change with the Martian
seasons.
So people got very excited,
thought, "oh my goodness,
it's vegetation," you know,
the seasons come,
go.
And there's a polar ice cap up
there, obviously.
And 100 years ago,
people somehow convinced
themselves that there were
canals and maybe cities,
and maybe people all over this
planet.
This turns out to be wrong.
It isn't vegetation.
It's actually dust storms--that
changes what you see.
And by now we have some much
more close up views from things
like the Viking Missions and a
number of more recent missions.
And this is basically what the
surface of Mars looks like.
It has this slight reddish tint
overall, and it's a bunch of
rocks.
It has an atmosphere,
although it's less thick than
the Earth's atmosphere.
Now, one of the interesting
things about Mars is you can see
features that look like this.
And this looks very much like
river deltas.
You know, you see these little
tributaries coming into a big
river, this kind of looks like
Louisiana, or something like
that.
And so, people are pretty much
convinced that there was once
running water on Mars.
And that's important,
because it is thought that the
existence of life as we know it
is dependent on the existence of
liquid water.
For a long time,
people thought that there was
no liquid water.
Now, on Mars--it turns out that
the particular temperature and
atmospheric pressure that exists
on Mars means that water goes
from the solid state,
from ice and sublimes,
directly into the gaseous
state, much like carbon dioxide
does here.
That's why it's called dry ice:
because carbon dioxide,
when you freeze it,
and then warm it up again,
turns directly into gas.
Water is supposed to do the
same on Mars,
but there was,
just a month ago,
this interesting picture
published.
This is from a satellite
orbiting Mars that's been taking
a lot of pictures.
This is pictures of two
identical parts of the Martian
surface, one from 1999,
one from 2005.
And the claim is that there's
new stuff down here,
and that the way and the
pattern of that new stuff,
and the way it must have come
on, is from stuff flowing
downhill, down the side of this
crater.
And so now, people are
thinking, maybe there is
something flowing around on
Mars, although clearly not all
that much of it.
But that would be exciting if
it was confirmed.
Okay, out beyond Mars is the
asteroid belt,
filled with rocky chunks of
stuff that look vaguely like
this--many, many of them.
There are asteroids all over
the Solar System.
Most of them are between the
orbits of Mars and Jupiter,
but there are other families
that are elsewhere.
Some of these other families,
it has been suggested,
come from the asteroid belt,
but they've had collisions or
other catastrophes,
and have been bumped into
different orbits.
But most of the asteroids are
between Mars and Jupiter.
Now, out beyond the asteroid
belt are a number of other
planets, and much of what we
know about these other planets
come from a couple of satellites
that look kind of like this.
These are the Voyager
satellites that were launched in
the 1970s and have been
traveling through the Outer
Solar System ever since.
This is a clever thing that
they did.
It turned out that in the '70s
and '80s, the outer planets,
Jupiter, Saturn,
Uranus, and Neptune were
aligned in such a way that one
satellite could catch them all
as they went past.
And each time it goes past one
of these things,
it uses the gravitational
attraction of that planet to
swing itself to the next one,
and then to the next one,
and then to the next one.
And so, these wonderful
satellites, for many years,
gave us pictures of one planet
after another,
which we now know quite a lot
more about than we used to.
So, here's Jupiter,
this is by far the most massive
planet.
Here you've got the famous red
spot.
All that you see here is
atmosphere, and it's got very
elaborate weather.
And the red spot,
it turns out to be a hurricane
that has persisted for about 350
years.
To us, it seems like an almost
permanent feature,
although it's gotten fainter
recently.
But it's sort of as
if--supposing you were a race of
creatures whose lifetime was
about half a day,
and you were observing the
Earth.
And you observed it for many
lifetimes, and you saw the same
hurricane sitting down somewhere
in the Caribbean.
You would think that that
little spot was kind of a
permanent feature,
and that seems to be what this
is.
It's a sort of really
long-lasting hurricane.
If you have--they have
time-lapse movies of this.
You can find them on the
Internet, where you can see that
the wind is actually circulating
there.
Jupiter has moons,
many of them--these--the four
big ones you see here are the
so-called "Galilean" moons,
because they were discovered by
Galileo.
They've also included a little
one.
There are many dozens of moons
this size.
The moons--each of the moons
has its own peculiar
characteristics.
I'm quite fond of this one,
this is the innermost moon.
It's called Io,
sometimes referred to as the
pepperoni pizza moon.
And it's got the most elaborate
volcanoes anywhere in the Solar
System.
It spews up sulfur all the time.
And then, this sulfur sort of
melts and flows all over the
surface, and that's what gives
it its particular color.
Each of the other moons has
interesting characteristics of
its own.
Here's an interesting thing
that the Voyager satellites
discovered: they discovered that
Jupiter has rings.
It was not thought that Jupiter
had rings;
from the Earth,
you can't see them.
But from close up,
it became apparent that Jupiter
has rings the same way Saturn
does.
But, of course,
the Saturn rings are the most
spectacular.
Here's Saturn,
the next planet out.
You can see that it,
too, has weather-banded things
down here.
And then it has these very
spectacular rings,
seen here from various
different angles as Saturn goes
through its orbit.
And these rings,
we know now,
are made up of individual
little chunks of things.
The Voyager Mission,
this is obviously
artificially-colored so that you
could see all the different
rings.
And each one of those rings is
made up of many,
many, many little rocks.
Saturn too has moons.
This is Titan,
Saturn's big moon.
And you can see from here,
in this particular picture,
that Titan has an atmosphere.
That makes it very interesting.
People have the feeling that at
places where there are
atmospheres are potential
sources of life,
and so people find Titan an
interesting moon.
I actually like this one better.
This is Mimas.
It's just a rocky moon,
but you can see it's kind of
got the great grandmother of all
craters up there.
This thing got slammed into by
some asteroid that was just
barely not big enough to blow
the whole thing apart,
but it raised this big pucker
on the side of the moon.
Moving out to the next planet,
which is Uranus.
In ordinary light,
you can't actually see any
features.
Again, you're looking at the
atmosphere;
there are clouds.
But this picture was taken in a
particular kind of red light,
which brings out the cloud
features.
And what you can see is,
it's got a banded structure,
the same as Jupiter and Saturn,
but interestingly,
the bands are on its side.
One of the peculiar features of
Uranus is that it rotates
sideways, rather than kind of up
and down, uniquely among the
planets.
This planet, too, has rings.
It also has moons.
Here's another favorite moon of
mine, this is Miranda.
And this looks like what
happened was that it actually
did get blown apart by some
impact, but then fell back
together again.
And you can see that it looks
like it's been chopped into
pieces and then sort of thrown
back into--together again.
Moving out to the next planet,
here's Neptune.
Neptune, again, has weather.
Here is the big,
dark spot on Neptune,
similar to the big,
red spot on Jupiter.
This little cloud here is
called Scooter,
because it moves faster than
the rest of the weather on
Neptune.
It's actually kind of a mystery
how come Neptune has all this
weather, because it's very cold
out there.
There isn't a whole lot of
energy that should be in the
atmosphere, and nobody can quite
figure out how this is supposed
to work.
Neptune has moons.
Here's Neptune's biggest moon,
this is Triton.
You can see this sort of
frontier here between two types
of topography,
sort of moves around,
and that is thought to be due
to weather of various
kinds--methane snow,
stuff like that.
And then, by now,
this was the last planet
Voyager II examined,
and then it went past and it
took this--took a lovely shot
looking back at the Solar
System.
Here is Neptune and Triton as
the Voyager Mission moved out
into the Outer Solar System
beyond the large planets.
Now, Pluto wasn't aligned
properly to take part in this.
This is--these are pictures of
Pluto.
This is a picture of Pluto from
the ground.
Here it is from the space
telescope.
As you can see, it's got a moon.
This is kind of all we know
about Pluto at the moment.
There is a spacecraft that is
currently on route to Pluto,
and when it arrives there a
couple of decades from now--a
decade from now,
I guess, we'll know much more.
Now, you may be aware that
there was a little bit of fuss
about Pluto over the summer.
Let me show you why.
Here it is.
This is what all the fuss is
about.
This is three pictures,
sort of shown as a movie.
See this thing over here that's
moving?
That's moving because,
unlike all the other objects in
this picture,
that thing is not a distant
star.
That is a planet in the Outer
Solar System.
This is the one that for a
while was called Xena,
now it's called Eris.
And these are the discovery
photographs of this.
Then as they kept taking
pictures, they could plot the
orbit.
They determined the orbit of
the thing using an equation
you've already discovered.
They figure out how far away it
is.
From how far away it is and how
bright it is,
you can figure out how big it
is.
And the problem that Eris
presents is, it's bigger than
Pluto.
And there are a whole bunch of
other things out there that have
also been discovered,
also as big,
or bigger than Pluto.
Here are the eight currently
largest-known,
so-called "trans-Neptunian"
objects, sometimes also called
Kuiper Belt objects.
Here's Eris,
here's Pluto,
here are a bunch of other ones.
They find these things,
like, one or two of them a
year, now, and so we can
confidently expect that twenty
years from now,
there are going to 100 such
things.
So you have the problem--oh you
should--it's worth nothing,
these things also have moons,
and some of them have a
moon-to-planet ratio similar to
the Earth and the moon.
But here's the Earth for scale.
All of them,
including Pluto and Eris,
very much smaller than the
Earth.
And so you've got yourself a
problem.
If you're going to count Pluto,
how do you not count these
other things?
And so the first suggestion
was, well, you ought to count
Pluto, and you ought to count a
bunch of these other things too.
And so, there was going to be
twelve planets,
or something like that,
and presumably,
many more to come.
And then people decided,
you know, the heck with this
whole bunch of things.
We'll give them a different
name;
we won't call them planets at
all.
And that was what was finally
determined, leading to all sorts
of, you know,
"Save Pluto" campaigns from
disgruntled third-graders
who--[laughter]
yes, thank you very much--who
were unhappy about having to
re-memorize all the mnemonics
that tell you the planets in the
Solar System.
Actually, if you included
things like Quaoar,
you'll get yourself into
trouble trying to learn them
all.
And they're finding lots of
lots of these things.
Okay, so just a little about
the geography of the Solar
System.
Here's the Inner Solar System
out to the orbit of Jupiter,
with the asteroids and stuff.
Here's the Outer Solar System,
starting with Jupiter.
Here's Pluto,
and you'll notice that Pluto
really is an elliptical orbit,
much more elliptical than the
other planets.
It also turns out the orbit is
tipped.
All the other planets--the
orbits are in the same plane as
each other.
Pluto's orbit is tipped out of
the plane.
Here's one of the--these other
trans-Neptunian objects.
Here's its orbit,
and you can see that it goes
way, way out and is highly
elliptical, very much out of the
plane.
This is, in general,
true of these,
what are now called "dwarf
planets," or "trans-Neptunian
objects," or "Kuiper Belt
objects."
But it's also true that there
is a further component of the
Solar System even beyond that.
Here's Sedna's orbit--that
again is Sedna's orbit,
so we've now gone up a degree
in scale.
And then there's this whole
cloud of stuff out there called
the Oort cloud and this is--this
is where the comets live.
Comets are sort of ice balls.
We can't see them individually
in the Oort cloud,
but sometimes they collide with
each other, and one of them
falls into the Inner Solar
System.
And as it does, it heats up.
It's made out of ice,
the ice melts,
gets--streams backward,
and we get these very
spectacular things.
Comets cause all kinds of
mayhem, they kill dinosaurs,
for example,
and recently we've seen an
example of this.
This is a comet called
Shoemaker-Levy that's broken up
into a bunch of pieces.
And in 1994,
those pieces slammed into
Jupiter, and here you see the
various--the effect of the
various pieces of this comet
that hit Jupiter.
And so, comets can be dangerous
things when they land on your
planet.
Okay, this is the Outer Solar
System again.
Here's the tipped orbit of
Pluto.
That's the orbit of Neptune.
Here's Voyager I,
Voyager II traveling now
outward.
They are escaping from the
Solar System.
And the very edges of the Sun's
influence, where you might say
the Solar System ends and the
interstellar medium begins,
are indicated here.
Each of these things means a
slightly different boundary to
the Sun's influence.
But the Voyagers are going to
get there sometime in our
lifetime, so they will be the
first man-made things that
actually pass out of our Solar
System.
And that's as far as this goes.
Let me turn the lights back on,
here.
Okay, so much for the Solar
System.
Now.
Here's the important question:
I just threw a whole bunch of
facts at you.
Facts, information,
pretty pictures.
We have a lot of facts,
and information,
and pretty pictures.
The Voyagers took many
thousands of pictures and
other--acquired other forms of
information, as well.
There have been many other
spacecraft flying around our
Solar System for quite some
time.
We know a lot of things.
Here's the question:
now that you know all this
stuff, what do you do with that
information?
So, what do you do with all
this Solar System information?
One thing you could do is,
write it all down and memorize
it.
I strongly advise against that.
You know, this is what the
Internet is for.
There are a whole bunch of
websites out there that'll tell
you everything you could
possibly want to know about all
of the objects and all of the
planets,
moons, everything else.
So that isn't actually a
productive way,
I don't think,
of spending your time.
So, what would you rather do?
What would one--what would be a
more productive way of thinking
about this material?
And now, I want to step back
and remind you of how science
works.
Remember the scientific method?
They probably taught you
something about this when you
were, like, eleven or twelve
years old.
And you'll remember how this
works.
So, scientific method--so,
you have a hypothesis,
which is a fancy word for a
guess.
Maybe you have competing
hypotheses.
And on the basis of this
hypothesis you formulate some
kind of experiment.
Good experiments are sometimes
called "controlled" experiments.
And on the basis of the results
of the experiment,
you determine how much you
believe the hypothesis,
or which hypothesis you
believe.
And then, you know,
you may modify your hypothesis
or change it all together,
and then do more experiments.
And by iterating this
procedure, you develop an
understanding of whatever it is
you're trying to think about.
Okay.
No.
Not really, certainly not in
astronomy.
Astronomy doesn't work this way
at all.
Think about what an experiment
would be in astronomy.
Okay, so here's a ball of gas
the size of the Sun made out of
pure hydrogen.
Here's another ball of gas the
size of the Sun made out of pure
helium.
We stick them in the sky and
watch them evolve for ten
billion years,
okay.
No, you can't do experiments in
astronomy.
It doesn't work that way at all.
So, there has to be some other
way of approaching it.
And the reason for this is,
is that astronomy is not an
experimental science,
it's an observational science.
This is true of many other
sciences.
A lot of biology,
particularly environmental,
or ecological aspects of
biology, work this way.
All of the social sciences.
You can't do controlled
experiments in child
development, that's just ugly.
And so, these are observational
sciences, and this has a
different methodology.
And it starts,
as its name would suggest,
with observations.
And you go out and you find a
bunch of things,
whether they're butterflies,
or planets, or whatever they
are.
And what do you do when you've
found a whole bunch of things?
What's the next step?
The next step is classification.
You put them into categories.
And it's important to get this
right.
If you're dealing with,
for example,
animals, and you classify them
as things that live in the ocean
and things that live on land,
then you're going to have fish
and whales in the same category.
And you're going to have
lizards and frogs and snakes in
the same category with people
and bears, and things like that.
And this isn't going to lead
you to a deep understanding,
because you haven't got the
categories right.
And so the classification is
very important.
And what it leads to when you
get it right is a useful
interpretation of what is going
on.
This is--interpretation,
it's just a fancy word for a
good story.
These are the kinds of things
we write down in textbooks.
The astronomers sometimes
dignify this with the fancy word
"scenario."
But it's basically a story.
And on the basis of this story,
you say, well,
we better check out,
let's see whether this story
holds up.
Let's do more observations,
and--all of these connections
work in all directions.
And so, this is actually a
better description of how an
observational science is done
than this kind of thing up here.
So now, having gone through
that little piece of philosophy,
let's apply it in practice.
I've just described to you some
observations,
some observations of the Solar
System.
Now, what I'm going to do is,
I'm going to take those objects
that I just showed you and
classify them,
or attempt to.
So let us classify--these are
now going to be categories of
Solar System objects.
So, I claim,
or at least,
it's the start of a good story,
that there are six categories
of objects in the Solar System.
The first is the Sun,
which I didn't actually show
you, which is 99% of all of the
mass in the Solar System,
and almost all of the energy
and heat, and so forth.
Then there are the inner
planets, sometimes called the
terrestrial planets,
because the Earth is the prime
example.
And these are rocks,
basically that would be
silicon, iron,
elements like that,
with a very thin surface
coating of ice,
in some cases melted,
in some cases gaseous.
What I mean by ice is not
necessarily water-ice,
but also things like ammonia,
and methane,
and other compounds of that
kind.
This coating is very thin.
If you had a scale model of the
Earth that was six feet across
and you put your hand up against
it,
you wouldn't even feel that it
was wet from the oceans.
So, not very much.
And these have masses,
these things,
these inner planets of,
I don't know,
10^(-7) to 10^(-5) of the Sun.
And they're in basically
circular orbits.
Then you've got the asteroids,
which are irregular,
very small rocks in--mostly in
orbits between Mars and Jupiter.
And out beyond the asteroids,
you've got the outer planets,
sometimes called the "Jovian"
planets,
because Jupiter is the prime
example of these things.
And these are quite different
from the inner planets.
They've got a lot of gas and
ice, both gas,
which I'm defining here to be
hydrogen and helium,
and ice, which I'm defining to
be, as I said,
water, ammonia,
methane, and similar compounds.
And these are more massive,
10^(-4) to 10^(-3) of the Sun.
They have rings,
many moons, also circular
orbits.
And all these orbits,
both these planets and these
planets are basically in one
plane.
Out beyond the Jovian planets
are these trans-Neptunian
objects, or Kuiper Belt objects.
This is Pluto, etcetera.
We don't really know what they
are yet, but it seems like
they're probably going to be
rocky, just to judge from their
size and mass.
So we think they're rocky.
They're a low mass,
less than 10^(-7) of the mass
of the Sun.
And they are in elliptical and
inclined orbits,
way out there.
And then, finally,
in the outer region,
in this so-called Oort cloud,
you have the comets,
which are little snowballs.
Balls of ice,
again, ice in this generic
sense.
So, here are the six categories
that I would claim exist in the
Solar System.
And here's my problem with the
whole Pluto debate.
The Pluto debate was basically
about whether these guys are
going to count as planets.
But the thing is,
"planets" is already a bad
description, because it contains
two quite different categories;
namely, these inner terrestrial
planets, and the outer Jovian
planets.
So, it seems to me that arguing
whether category five should be
part of some category that
already contains two
fundamentally different kinds of
objects is kind of a strange
argument to be having.
Either we should split these
two things off from each other,
or, if we're going to join
these two kinds of the
categories,
fine, bring in anything you
like.
I don't care,
add the asteroids,
too.
And, in fact,
in the original proposal,
one of the asteroids qualified
as well.
And so, it doesn't seem to me
that this controversy was really
paying justice to an appropriate
classification of the things in
the Solar System.
Okay, so now.
Having classified it,
the next step is
interpretation.
And, I think I will leave
that--let's see,
how are we doing for time?
Yes, I'll leave that one for
next time.
So the question is going to be:
now that you have these six
kinds of objects,
what is the story you tell
about how the Solar System
evolved?
And what, in turn,
does that tell you about what
other kinds of planets you
should see around other stars?
