Exam Grades
Due to the rush to get exams back, I did not record
credit for those who wrote out their own answers.
If you did write out your own answer to a question,
please bring me your exam so I can update your
score.
Announcements
• Note: the exam questions were mostly taken from
the quiz and assignments. It is therefore important
to understand the questions, not just memorize the
correct answer, since they will be asked differently.
• Assignments will have TWO parts: (1) an
application or practice of what you have learned, (2)
questions about the reading for the next lecture.
• Planet Assignment 3, due Wednesday Feb 25,
– Part 1: Astronomy Place tutorial “Formation of the Solar
System”
– Part 2: Angel Planet Assignment 3
Confusions & Questions?
• Solar Nebula Theory - collapse & heating
– What triggers the collapse
– How does it collapse
– What heats it up
• Asteroids - why are they between Mars &
Jupiter
• Dating rocks, radioactive decay
• Doppler Effect
What is a Planet?
An object that has a mass between that of
Pluto and the nuclear reaction threshold of
13 MJ and that forms in orbit around a
star that generates energy by nuclear
reactions.
The Solar System
Solar System Properties
• Patterns of Motion
– All planets orbit the Sun in the same direction (as Sun
spins), in nearly the same plane, & in nearly circular orbit
• Two types of planets
– Terrestrial: small, rocky, inner
– Jovian: large, liquid H, He & ices, outer
• Astroids & Comets
– Comets dirty ice, Astroids rocks & iron
• Ubiquitous impact craters
– All old solid surfaces
• Exceptions
– Uranus rotates sideways, Venus rotates backwards
Patterns of Motion
Two Types of Planets
Terrestrial:
small
rocky, iron
inner
Jovian:
large
H, He, ices
outer
Asteroid & Comets
Dirty ice
Rocks & iron
Ubiquitous Craters
Mars
Moon
Callisto
Solar System - How did it Form?
What produced the organized motions?
• Planets formed as part of the
formation of the Sun.
• Begin with region of higher density
composed of H & He + traces of heavier
elements in the space between the stars
(called “interstellar cloud”)
• Gravity makes the “cloud” collapse
– Triggered possibly by explosion of a nearby
star
Collapse of Interstellar Cloud
Angular Momentum
Is conserved.
As “cloud” collapses
it spins faster.
Why are orbits in nearly same plane?
Collapses more along
poles than in equator.
Forms a disk.
Why are orbits nearly circular?
Solar System - How did it Form?
Why are there two types of planets?
• Planets formed as part of the
formation of the Sun.
• As “cloud” collapses gets hotter (falling
converts gravitational potential energy to
kinetic energy). Inner regions hotter than
outer (fell farther).
• Rocks & iron solid at high temperatures
• Ices solid only at low temperatures
Solidification Temperatures
Condensation Regions
Terrestrial & Jovian Planets
• Close to Sun - rocks and iron are solid
Make rocky terrestrial planets
• Far from Sun - Ices (water, carbon dioxide,
methane, ammonia) are solid
Make larger proto-planets with
Gravity large enough to attract H & He gas
Make Jovian planets
Terrestrial & Jovian Planets
Gravity captures H & He gas
Humans (& Earth) are made of the
flotsam and jetsam of the universe
Solar System - How did it Form?
What are Comets & Asteroids?
Leftover planetesimals.
Asteroids
• Left over
rocky & iron
planetesimals
from formation
of planets
Comets
• Left over
icy planetesimals
from formation of the
Jovian planets
Solar System - How did it Form?
Why ubiquitous
impact craters?
Remains of accretion
of planetesimals.
Accretion of Planetesimals
Review of
nebular theory
When did the Solar System Form?
Measurements of decaying elements in
meteorites indicate age of solar system:
4.6 billion years
Thought Question
Suppose you bought a carton of special eggs -each day half of the eggs in the carton
hatch. You open the carton and find 9
chicks and 3 eggs. How long ago was it
packed?
A.
B.
C.
D.
Yesterday
Two days ago
Three days ago
Last week
Example: Half of all potassium-40 nuclei decay into
argon-40 every 1.25 billion years
What about Moons?
Moons of Jovian Planets
form in miniature disks
Earth’s
moon was
probably
created
when a big
planetesimal
tore a chunk
out of the
newly
forming
Earth.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Collision
simulation
Other Evidence?
Other Planetary Systems
• Activity
– 1. Predict what you expect to be found about
the properties of other planetary systems
– 2. We will then see what was actually found,
and you will compare that with your predictions
You will not be graded on the accuracy of your
predictions, but on their relevance &
completeness.
How are Extrasolar Planets Detected?
By the Wobble
they produce in
their parent star.
Detection limit:
> 0.3 MJupiter
Doppler Effect
Analogy
Suppose photons are busses on Grand River Ave.
Busses come once every 10 min on average
If you are walking East towards Okemos, will the
busses heading west from Okemos pass you
every 10 min, or less or more?
If you are walking West towards Lansing, will the
busses heading west from Okemos pass you
every 10 min, or less or more?
What has been found?
• 120 Planets around
Main Sequence
(solar like stars)
• In 105 planetary
systems
• 15 multiple planet
systems
What has been found - Masses
What has been found - Eccentricity
Solar System
Eccentricities
< 0.1
Except
Mercury
& Pluto
What has been found - Composition
What we expected
• Nearly circular orbits
• Jovian planets far
from star
What we found
• Highly eccentric orbits
• Jovian planets close to
star
Implications for Life
• Eccentric Jovian planets will eject small
terrestrial planets from the system.
• Migrating Jovian planets will eject small
terrestrial planets as they pass.
• Young stars with more heavy elements are
more likely to form planets than older stars
with less heavy elements.
Questions?
• How are circular orbits produced? Are they
typical or atypical?
• How are hot, close Jovian planets
produced? Do they form as outer planets
and migrate inward? What makes them
migrate?