What we're going to start is going to be Comparative Planetology
because this section of the course is about planets and life.
So well the idea of comparative planetology is
to look at all the different kinds of planets
that we see, and we really see a
really remarkable variation even in our own solar system.
and to try and use that difference to compare
and contrast, to understand the forces which drive the evolution,
the formation and the evolution of planets.
Because it's interesting in its own right, but
also because it's going to effect things like habitability.
How habitable is a planet?
Is a planet remain habitable for a long time?
If it has moons, are those moons habitable?
So those are the kinds of questions we want to ask.
and so let's just remind ourselves of our guiding principles in thinking about this.
First, we have different planets and those
different planets are going to tell us different stories,
and we want to use those different stories.
And in each one of these, we want to understand how those planets might be
in terms of a home for life, either for long-term or even for short-term.
Okay.
So, let's focus in on one planetary system.
Our own.
To see what it tells us about its own evolution and perhaps
about more general principles for what we've learned about other planets as well.
Okay.
So, there's a couple of different things that we immediately
get by doing a survey of the bodies in our own solar system.
The first is there are essentially, in our solar system, three flavors of planets.
The first are what we would call terrestrial planets.
Rocky planets like the Earth or Mars.
The second kind of planet are what we call gas giants.
And these are enormous planets where there may or may not be a rocky core.
But essentially they're built of fairly
simple material, although in different states.
The third kind of planet are what we call
an ice giant, and these are planets that also
are quite large compared to the Earth, but, a
great deal of their material is locked up in ices.
So it's a mix of gases and ices in different states.
Okay so that's the three different kinds of planets, when
we look at individual planets we also find some similarities.
For example many of the objects in our solar system have moons, not all,
but many of them do have satellites orbiting around them and in some cases
there are many moons orbiting around a single planet.
Now, along with the planets, these three kinds of planets,
and their satellites, we also find a lot of debris.
material left over from the construction of
the solar system, which didn't form planets.
But was instrumental in forming the planets.
For example we have what, what are called Dwarf planets.
These are bodies that, some of them are you know spherical
or quite spherical, but are not as large as a planet.
in particular
they may not have done much in terms
of having their gravity vacuum up material around them.
That's one of the things we use to define a planet, the ability for
the gravity of the body to sort of clear out a gap around it.
