Chapter 7:
The Birth and Evolution
of Planetary Systems
Where did the solar system
come from? How was it made?
“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
Because of the internal motions of the
gas and dust, the cloud almost always
has some slight overall rotation
The cloud starts
to collapse due
to gravity
Angular momentum causes
the cloud’s initial slow
rotation to spin faster and
flatten out
Angular momentum is what
causes a skater to “spin-up”
Angular momentum depends on both the velocity, V, and
the size, R. If R decreases, V must increase. It is also
what causes the pizza dough to flatten out when tossed
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
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.
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 also 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
When the gasses get blown
away, the condensation
phase ends
The Solar Nebula Stage
Condensation starts and planetesimals begin growing
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 accretion stage was a
violent period with planet
smashing collisions
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 gas giants were like mini
solar systems, forming a
system of moons
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
Actually seeing a planet has
only recently been done
Newton’s 3rd Law applies to
the Sun and planets
If the Sun tugs on Jupiter, keeping it in orbit,
then Jupiter tugs on the Sun, making it orbit. The
two actually orbit a common point just outside
the surface of the Sun
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
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
The Doppler method is the most
prolific but it finds large mass
planets close to their star
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!
The basic modification
is that things move,
sometimes they move
a lot: Migration theory