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Announcements
Mid-term Exam #2, this Wed, Oct 15th.
Extra-credit write-ups (Monsters, Dempsey)
due today.
Updated Grades online now (including
HW#2).
Last Time
Jovian planets: Giant, massive, gas-rich, far
from sun (5–30AU).
Formed “Ice” cores which gathered up the
abundance hydrogen and helium. Mostly
hydrogen.
Clouds of ammonia, methane, strong
weather.
Magnetic fields (strongest in jupiter):
aurora.
Last Time
Jovian Moons: many, including some larger
than the planet mercury!
Formed in orbit.
Tidal heating important.
IO has volcanoes, europa liquid water under
a thick sheet of ice, titan a dense
atmosphere, “lakes” of hydrocarbons, and
coastlines.
Last Time
All jovian planets have rings, formed and
shaped by small moons within them.
Asteroids: mostly in band between Mars and
Jupiter. A “failed planet” kept from
forming by Jupiter’s gravitational tugs.
Cratered, and rocky. Not large enough for
gravity to pull them into round shapes.
Last Time
Meteor (falling star) vs. meteorite (chunk
os asteroid, or even moon or another
planet, which has fallen to earth).
Comet: an ancient icy body. Near the sun,
has two tails of material pointing away
from the sun.
Come from the Kuiper Belt, and the much
larger “oort cloud” which stretches half
way to the next star.
Last Time
Pluto not like other jovian planets: small,
icy, eccentric orbit.
No longer called a planet, but a “dwarf
planet”, one of many objects in the kuiper
belt (including some larger than pluto).
How Solar Systems
Form
A cloud of gas and dust (the “solar
nebula”) collapsed under its own gravity.
The “nebular theory” for solar system
formation.
The Nebular Theory
Where did the
solar nebula come
from?
The cloud came
from the left over
material from a
previous
generation of
stars
The First Stages of the
Formation of the Solar System
Initially the cloud is
large and diffuse
As gravity causes the
cloud to collapse, it
heats up and starts to
spin faster
The heating and
spinning up are due to
the conservation of
energy and angular
momentum
The First Stages of the
Formation of the Solar System
The result of this
process is a flattened
disk of gas and dust
composed of 98%
Hydrogen/Helium, and 2%
heavier elements
The orderly motions of
the solar system are a
direct result of the
solar system’s birth in a
spinning flattened cloud
of gas and dust
We see evidence of
disks in other
regions of the
Milky Way galaxy
currently forming
stars
Temperature in the Solar
Nebula
The inner parts
of the disk are
hotter than the
outer parts
Rocks can
condense out at
higher
temperatures
than ice
The Frost Line
Inside the frost line, it is too hot for
hydrogen compounds to form ice
Outside the frost line, it is cold enough for
ices to form.
Orbit of Jupiter
Planetesimals
Tiny solid particles
stick together to
form planetesimals
This process of
assembly is called
accretion
Planetesimals
inside the frost line
are metal and rock
while planetesimals
outside the frost
line also contain
ices, and so are
larger.
Moons of Jovian Systems
Moons of Jovian systems form in
miniature disks of their own
Early
Jupiter
Early
Sun
What is the origin of the
asteroids and comets?
Asteroids and
comets are
leftover
planetesimals
Asteroids are rocky
because they
formed inside the
frost line
Comets are icy
because they
formed outside the
frost line
How do we explain the
exceptions to the rules?
Earth’s moon was
probably created
when a large
planetesimal (the
size of Mars)
slammed into the
young Earth
Similar events may
explain the tilt of
Uranus or the
backwards rotation
of Venus
Other solar
systems
Extra-solar planets
Extra-solar planets
Very (very) recent discoveries: direct
evidence for planets around stars aside
from our sun.
First found in 1995.
Very difficult to find.
Finding Extra-solar
planets
Stars are too bright, and planets are too
faint: seeing planets “directly” very difficult.
Main techniques are Indirect:
Infer a “wobble” of the star as one or more
planets orbit it.
Find solar systems aligned with our view in
which the planet “transits” in front of the
star. Look for star to change brightness.
Wobble Method
We detect an
unseen planet by
looking for the
periodic motion of
the star
We use the
“Doppler shift” to
detect this motion
in the star’s
spectrum
Shifts of ~10 m/s
(100m sprinter!)
Wobble Method
We detect an
unseen planet by
looking for the
periodic motion of
the star
We use the
“Doppler shift” to
detect this motion
in the star’s
spectrum
Shifts of ~10 m/s
(100m sprinter!)
Another Wobble
Method
Looking at the plane
of the solar system
from 30 light years
away.
Sun moves around
by tiny amounts
(millionths of an
arcsecond!).
Transit Method
Planet
Transits in
front of star,
partially
blocking it (an
eclipse!).
Transits
artist conception
The fraction of
the starlight
blocked tells us
how big the
planet is.
Only works for
planets with
orbits edge-on.
Workbook Time!
Groups, please.
If you forgot your workbook, find someone
who has one.
Do “Motions of Extra-solar planets”, page
117.
What have we found?
As of this week, 313
extra-solar planets
have been found.
Many very “unusual”:
jupiter sized or
larger, closer to their
star than earth is to
sun.
artist’s drawing
55 Cancri’s solar system
Have we seen any
directly?
Sort of:
in 2005, infrared
satellite was used to
see dimming of star
+planet as planet
went behind star.
Difference is direct
light from planet!
Atmospheres detected
Transit method
very powerful:
see change in
starlight as it
passes through
planets’
atmosphere.
Some “dry”
Other with
evidence of
water!
Mapping the weather!
What have we learned?
Nebular model
needs revision.
Planets must be
able to
“migrate”
inwards.
What about Earth?
No Earth-like
planets found
to date.
BUT, Doppler
technique is not
sensitive enough
to find Earthlike planets
We just do not
know much yet.
“Super-Earths”
In the last several years,
finding planets with masses
3–10× the mass of the earth.
Some very close to their
stars (2 day orbit!).
Smallest 3.3 earth masses,
orbiting a very cold “brown
dwarf” star.
Gliese 581c: In the zone
Discovered 2007.
5× earth’s mass.
13 day period.
Orbits close to a cool “red dwarf” star.
Is in the habitable zone, where water
would be liquid!
Mid-Term Exam #2
Wed., Oct 15, in class.
50 multiple choice problems.
Buy-back extra credit will be available
(keep your tests, and mark you answers!).
Covering 3.3 (Kepler’s laws), 4, 6–9
Use: end of chapter guides, online “Study
Area” quizzes.
Mid-Term Exam Topics
Kepler’s laws. Speed of planet during orbit
(equal areas in equal time).
P2=A3
Mass vs. Weight. Speed vs. Velocity.
Newton’s laws of motion: ❶ Body at rest
stays at rest (or constant motion). ❷ Force
= mass × acceleration. ❸ Equal and opposite
force. How they apply.
Mid-Term Exam Topics
Types of Energy (kinetic, radiative,
potential, mass-energy). Energy is
conserved.
Tides and their relation to phases of the
moon.
Newton’s universal law of gravitational
force: how does it depend on mass, distance?
How newton’s law underly kepler’s law (e.g.
angular momentum most be conserved:
Equal areas in equal time!).
Mid-Term Exam Topics
Terrestrial vs. Jovian planets.
Size of planets (e.g. compared to earth,
compared to Jupiter, compared to sun).
Ordering of planets.
Internal layers of terrestrial planets:
Core, mantle, crust.
Volcanism and plate tectonics: a result of
internal heat. Erosion.
Mid-Term Exam Topics
Cratering as an indication of the age of a
planet’s surface.
What can heat planets/moons: radioactivity,
differentiation, tidal heating (e.g. IO).
Meteor vs. meteorite
Aurora and magnetic fields.
Jovian Moons: what makes them special?
Mid-Term Exam Topics
Solar nebula: what’s it made of (mostly)?
Pluto demoted and dwarf planets. Kuiper
Belt, Oort cloud, objects of the K.B.
What’s a greenhouse gas?
Asteroids and the asteroid belt. Comets.
Formation of both. Sizes of asteroids.
Extra-solar planets: how we detect them?