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AS101 Lecture Notes - Doppler Spectroscopy, Formation And Evolution Of The Solar System, Methods Of Detecting Exoplanets

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Patrick Mc Graw

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AS101 Week 10 Lecture 1
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
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
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 forming planets.
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.
So, if this is typical for our solar system, it should be typical for other solar systems too
Review: Two main ways of detecting extrasolar planets I. Doppler method
Most successful methods are Doppler method and transit method
Doppler method: Detect the slight wobble of the star as the planet's gravity pulls on it.
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.
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 the wobble.)
Review: Two main ways of detecting extrasolar planets I. Transit method
Transit method: Detect recurring changes in the star's brightness as the planet moves in
front of it.
This allows us to find the planet's orbital period and its size (from the amount of light it

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Transit only works if the planet's orbit happens to be almost edge-on to our line of sight.
By measuring the amount of light blocked by the planet we can measure the diameter of
the planet
Review: 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 away.
What do we know about our planetary neighbours and how do we know it?
Our own planet is a good start
How do we know about the properties of Earth (composition, mass, etc)?
Learning about Earth
Eratosthenes figured out the size a couple of millennia ago.
o Eratosthenes found the diameter of the Earth by looking at the angle of the sun on
the same day from 2 different places on earth and using geometry to figure out the
circumference and diameter of the Earth
We can also find the mass. How?
We know the orbital period and average distance of the Moon from the Earth...
So Newton's version of Kepler's third law allows us to calculate the mass.
Then we can divide mass by volume to get density.
Newton's Generalization of Kepler's 3rd Law
p^2 = ((4pi^2)/GM) * a^3
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.
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