Lecture 16 4/10/2013 12:33:00 PM
The Terrestrial Planets
Review: Solar Nebula Model:
Solar system formed from a collapsing cloud.
Spin of the solar system (and all the objects in it) comes from the
spin of the original cloud.
Spin is also responsible for the flatness of most of the solar system
in the plane of the ecliptic. (Note: this is the same process that
gives the Milky Way galaxy its mostly flat shape.)
Chemical composition and size of planets changes with distance
from the sun.
This has to do with changes in temperature with distance from the
sun: farther away = colder, and therefore more solids available for
One important boundary was the ice line: the distance at which
temperatures were low enough to solidify water.
Process of planet formation would have stopped soon after the sun
“turned on” and drive away most of the remaining gas + dust.
Review: two main ways of detecting extrasolar planets:
o Doppler method:
Detect the slight wobble of the star as the planet's
gravity pulls on it.
o To detect the wobble, look for regularly recurring changes in
the star's radial velocity (motion toward or away from us)
using the Doppler effect.
o This allows us to figure out the orbital period (from the period
of the wobble) and the mass of the planet (from the size of
o Transit method:
Detect recurring changes in the star's brightness as the
planet moves in front of it.
o This allows us to find the planet's orbital period and its size
(from the amount of light it blocks.) o Transit only works if the planet's orbit happens to be almost
edge-on to our line of sight.
Revirew: Extrasolar Planets:
Detection of planets is easiest when the planets are massive and
close to the star (so that their gravitational pull is strong and their
orbital period is short).
Therefore the planets we have found so far are probably a very
biased sample: lots of “hot Jupiters.”
If our understanding of how solar systems form is correct, these
should be rare because large planets can normally only form farther
Learning about earth:
Eratosthenes figured out the size a couple of millennia ago.
We can also find the mass. How?
We know the orbital period and average distance of the Moon from
So Newton's version of Kepler's third law allows us to calculate the
Newtons Generalization of Keplers 3 rdlaw:
P = 4π a3
Where M is the mass of the sun (or other central object)
This can be used to figure out the mass of the Sun.
Similarly, the orbital periods and distance of Earth's or other
planets' moons can be used to figure out the planet's mass.
Fine print: Actually, the M in this equation should be the combined
mass of the two objects, but the planets' masses are much smaller
than the sun's.
Learning about earth:
Then we can divide mass by volume to get density. What is the earth made of?
We can get samples of rocks from near the surface...
We can also divide the mass by the volume to get the density,
which gives us at least some clues to what the planet as a whole is
made of. (This is true for any planet or other object.)
How else can we learn what's inside?
Some ways of learning about Earth’s interior:
Seismology: When an earthquake happens, vibrations travel
through the Earth in the form of s waves and p waves
Both types of waves get refracted as they travel through different
s waves cannot travel through liquids at all--- so the fact that s
waves do not travel through the middle of the Earth provides
evidence that part of the core is liquid.
Earth's magnetic field also provides indirect evidence that part of
the core is liquid metal:
combination of convection and the Earth's rotation creates the
Features of the surface due to plate tectonics also provide indirect
evidence of what is happening underneath.
a layer below the top crust, not quite liquid but soft enough to flow
Learning about Earths interior: other clues from the surface:
Height of mountains tells us something about how thick the crust is
and what is supporting it (mountains can only reach a certain
maximum height before their weight causes them to start sinking.)
Earth and its neighbours:
Some of our understanding of other planets comes from comparing
and contrasting their features with Earth.
What are some things we learn? Comparative Planetology: