Transcript Chapter 7

Chapter 7:
The Birth and Evolution
of Planetary Systems
Where did the solar system
come from? How was it made?
Up until the mid 1990’s the only planets known were those in our solar system.
As a result, the theories we developed to explain the formation of a solar
system fit our system. Since the 1990’s we have discovered hundreds of extrasolar planets. How does our theory match these newly discovered worlds?
“Facts” that must be accounted for in
any theory of solar system formation
•All the major planets orbit in almost the same plane
•All the planets orbit in the same direction
•Almost all the planets rotate in the same direction as
they orbit
•The inner planets are rocky bodies while the outer
planets are gaseous and/or icy bodies
•99% of the mass of the solar system is in the Sun
•Most of the angular momentum of the solar system is
in the planets, not the Sun
Look at ClassAction Solar System Properties Explorer
in the Solar System Characteristics module
We start with a cold cloud of
gas and dust
Since most of the mass is in the Sun and it is almost
entirely hydrogen and helium, we need to look for
places that have lots of hydrogen and helium
The cloud starts
to collapse due
to gravity
As it shrinks in size, angular
momentum causes the cloud’s
initial slow rotation to spin
faster and flatten out into a
thick disk.
Angular momentum is what
causes a skater to “spin-up”
It is what causes the pizza dough to flatten out when tossed
or an ice skater to spin-up as she pulls her arms and legs in
close to her body.
Watch Ice Skater Spin-up and Pizza Toss videos
The “Spin-up” causes the
cloud to flatten out
Angular momentum keeps stuff
from falling straight in. Instead,
it spirals down onto a disk. This
is the pizza toss effect
At this point we have something
that looks like a star surrounded
by a disk of gas and dust
The protostellar Sun is getting its energy from gravitational
collapse, not from fusion like “normal” stars.
The temperature in the
protoplanetary disk falls off as
you get farther from the protosun
Check out planet Formation Temperature Plot on ClassAction website
Solar System Characteristics module
The solar nebula is composed
mostly of hydrogen and helium
Since hydrogen is the most common element in the disk,
the most common things to condense will be hydrides of
carbon (CH4…methane), nitrogen (NH3…ammonia) and
oxygen (H2O…water). These condense at fairly low
temperatures. Elements like silicon and iron condense at
higher temperatures. Hydrogen and helium will never
condense, they always remain a gas.
What is found at different
distances from the protosun
depends on temperature and
abundance
Condensation begins to
form dust grains
The dust grains are tiny: about the size of particles in
smoke. They are also charged with static electricity
The dust grains
quickly start
sticking together
Close to the protosun the
grains are exclusively silicon,
iron and other heavy elements:
“rocky” materials. Farther out
there are more grains of “icy”
materials than rocky ones.
Static electricity plays an
important part in making the
grains stick together
Accretion is a snowball effect that
builds larger and larger objects
Eventually Planetesimals
are formed
Close to the Sun the
planetesimals look
like asteroids
Far from the Sun the
planetesimals are a mix of
ice and rock
Planetesimals
merge to form
protoplanets
The larger the planetesimal,
the stronger its gravity is.
The stronger its gravity, the
more it attracts stuff and the
more violent the collisions
become.
The gas giants form a large core of
ice and rock and then grow by
sweeping up large amounts of gas
There is still lots
of hydrogen and
helium around but
because they are
such light weight
gasses, only the
most massive
objects have
enough gravity to
hold on to them.
When the gasses get blown
away, the condensation
phase ends
Once the star at the center ignites, strong winds will blow
away any remaining gas and the condensation process ends.
The Solar Nebula Stage
Condensation starts and planetesimals begin growing. The
object at the center is still shrinking in size and gaining mass.
“Rocky” materials will begin to condense everywhere while
“icy” materials will only condense far from the new protostar.
The Accretion Stage
Planetesimals grow bigger by collisions. There may be
hundreds of moon sized protoplanets form in the inner
solar system. The outer planets have grabbed up the last of
the gas. The protostar at the center is beginning to start
fusion in its center. Violent winds from the new star blow
away any remaining gas.
The accretion stage was a
violent period with planet
smashing collisions
The final stages of accretion would have seen tremendous
collisions between planet sized objects.
Finally, we have a new star
and new planets
The new planets at this stage are nothing like the planets we
see today. They will evolve over time to become the eight
planets we see now. The new star, too, is not like the Sun of
today. It is larger and more violent with huge sunspots and
solar flares blasting the inner solar system with radiation.
Finding extra-solar planets
Our theory was designed to
explain the formation of our
solar system. How does it
match up with other planetary
systems around other stars?
We have seen lots of disks around
forming stars confirming some of
the nebular theory
Watch Orion Nebula Fly-through video
Actually seeing a planet has
only recently been done
Planets are tiny objects which shine by the reflected light
from their star. This makes them extremely difficult to see.
Newton’s 3rd Law applies to
the Sun and planets
If the Sun pulls on Jupiter, then Jupiter pulls on the Sun. The
two actually orbit a common point just outside the surface of
the Sun. The Doppler technique uses spectroscopy to detect
the tiny motion of a star caused by an orbiting planet.
Watch ClassAction Extrasolar Planet module Influence
of Planets on the Sun animation
The Doppler Effect technique
detects the motion of a star
caused by a planet
The planets orbit has to be aligned properly for the Doppler
technique to work. The orbit needs to be almost edge on for
the Doppler technique to work. Watch ClassAction
Extrasolar Planet module Radial Velocity Graph animation
The transit method measures a
planet directly if it passes in
front of its star
The planet will be a dark spot passing across the face of
the star. The dimming of the light from the star may be tiny
but it is measurable if the planet is large enough.
OGLE detects gravitational
microlensing caused by a planet
According to
Einstein’s theory
of gravity, if a
planet passes
directly between
us and some
distant star (not
its host star), the
light from the
distant star will
brighten and fade
in a very
particular way.
The Doppler method is the most
prolific but it finds large mass
planets close to their star
The transit method is starting
to catch up with the Doppler
technique in finding planets
because of the Kepler
mission but the Doppler
technique still has a slight
lead.
Visit http://exoplanet.eu
So what do we do about our
solar nebula model?
Our model
predicted small
rocky planets close
to the star. We are
finding large gas
giants close to their
star! Obviously, the
theory needs to be
modified.
The modification is to the
accretion phase. We call it
Migration theory: things
move, sometimes a lot.