Download Chapter 6 - Formation of the Solar System

The Terrestrial (Rocky/Metal) Planets
Mercury
Earth
Venus
Mars
The Gas Giant (Jovian) Planets
Jupiter
Uranus
Saturn
Neptune
Pluto (and Other Dwarf Planets)
• Much smaller than major planets, and not like a Jovian or terrestrial
• Icy, comet-like composition (not as much rock and little/no gas)
• Also Ceres (rocky/metal asteroid), Haumea, Makemake, Eris
Swarms of Smaller Bodies
Many rocky/metal
asteroids and icy
comets populate
the Solar System.
Asteroid Belt – small rocky/metal objects between Mars/Jupiter
Kuiper Belt – icy comets outside Neptune’s orbit (including
Pluto) at 40-100 AU from Sun
Oort Cloud – icy comets at ~50,000 AU or about a light year
(not actually discovered yet)
Which of the following pairs of
objects are primarily rocky objects?
A) Terrestrial planets and asteroids
B) Gas giant (Jovian) planets and comets
C) Terrestrial planets and comets
D) Gas giant (Jovian) planets and asteroids
Which of the following pairs of
objects are primarily rocky objects?
A) Terrestrial planets and asteroids
B) Gas giant (Jovian) planets and comets
C) Terrestrial planets and comets
D) Gas giant (Jovian) planets and asteroids
asteroid = rock (and some metal)
comet = icy (and a little rock and metal)
Rules of the Solar System
Our Solar System follows a set of rules that give us
clues about how the Solar System formed.
This helps us to form a theory about how planetary
systems form around other stars.
Exceptions to these rules in our Solar System
challenge our theory.
Theory also challenged by discovery of other solar
systems around other stars within last 20 years.
Motion of Large Bodies
 All large bodies in the
Solar System orbit in
the same direction as
the Sun rotates and in
nearly the same plane.
 Most (but not all) also
rotate in that direction.
 Planets do not orbit in
random directions or
random inclinations,
but in nearly the same
plane (like a flat
pancake).
Two Major Planet Types
 Terrestrial planets made
of rock/metal (high
density), relatively
small, and close to the
Sun with few/no moons
and no rings.
 Gas giant (Jovian)
planets are mostly
gaseous + ice (low
density), larger, and
farther from the Sun
with many moons and
rings.
What properties of our Solar System
must a formation theory explain?
1. Patterns of motion of the large bodies
• Orbit in same direction and plane
2. Existence of two types of planets
• Terrestrial and Jovian
3. Existence of smaller bodies
• Rocky/metal asteroids and icy comets
4. Notable exceptions to usual patterns
• Rotations of Uranus and Venus, Earth’s Moon
What theory best explains the
features of our solar system?
•
The nebular theory states
that our Solar System
formed from the
gravitational collapse of a
giant interstellar gas
cloud—the solar nebula.
(Nebula is the Latin word
for cloud.)
•
A large amount of evidence
now supports this idea.
Galactic Recycling
 Heavy elements that
formed planets were
made in stars and
then recycled through
interstellar space.
 Sun is a second or
third (or later)
generation star to
explain 1% heavier
element (nonhydrogen/helium)
content of the Solar
System.
What caused the orderly patterns
of motion in our solar system?
Why do planets all go around Sun the same direction in
the same plane?
Conservation of
Angular Momentum
•
Rotation speed of the
large cloud from
which our Solar
System formed must
have increased as the
cloud contracted.
•
As size
speed
Rotation of a
contracting
cloud speeds
up for the same
reason a skater
speeds up as
she pulls in her
arms.
Cloud initially a
light year or so
in diameter.
Flattening
•
•
Collisions between
particles in the cloud
caused it to flatten
into a disk.
Collisions between
gas particles also
reduce up and down
motions.
 flat, pancake structure
forms, with everything
revolving in the same
direction
Collisions
between gas
particles in
cloud
gradually
reduce
random
motions.
Leads to
nearly circular
orbits of
contracting
gas.
Evidence from Other Gas Clouds
We can see
stars forming
in other
interstellar
gas clouds,
lending
support to
the nebular
theory.
We see protoplanetary disks elsewhere!
Protoplanetary disks in Orion star-forming region.
Disks around Other Stars
AU Microscopii
•
HD141569A
Observations of disks around other
stars support the nebular hypothesis.
Summary of Solar System Formation
1) Initial large (1 light year diameter) barely rotating gas
cloud begins to collapse gravitationally.
2) Cloud spins up (conserving angular momentum) and
flattens (due to collisions) as it collapses.
3) Left with gas in a rapidly-rotating, thin, mostly circular
pancake-shaped disk
Why are there two major types
of planets?
Conservation
of Energy
As gravity
causes cloud
to contract, it
heats up.
Gravitational
potential
energy 
kinetic energy
of gas 
thermal
energy (heat)
via collisions.
Inner parts of disk are hot; outer parts are cold.
Rock and metal can be solid at much greater
temperatures than ice.  rock/metal is present
everywhere in Solar System
Inner Solar System too hot for hydrogen compounds
(water ice, ammonia ice, methane ice) to be solids.
 ice, H compounds only survive as solids in outer
Solar System
Inner Solar System therefore contained only rock and
metal in solid form  terrestrial planets + asteroids!
Important
concept!
Inside the frost line: too hot for H/He/hydrogen compounds
to form ices (get terrestrial planets and rocky/metal asteroids)
Outside the frost line: cold enough for ices to form solids IN
ADDITION to rock/metal (get Jovian planets and icy comets)
Frost line between asteroid belt and Jupiter
Tiny solid particles
stick to form
planetesimals
Rock/me
tal only
Rock/metal/ice + H/He gas
Gravity draws
planetesimals
(tiny solid
particles) together
to form planets.
This process of
assembly is called
accretion.
Accretion of Planetesimals
•
Many smaller objects collected into just
a few large ones (dust grains to
planetesimals to planets).
Dust grain – 0.02mm across from interplanetary space
Inside frost line: small metal/rock planets form –
the terrestrial planets and asteroids
Gravity of terrestrials too weak to draw in
hydrogen/helium gas
Outside the frost line: large planets form – the
Jovians, plus comets
The gravity of ice and rock in large Jovian
planetesimals strong enough to draw in available
H and He gas  grow big (ice/rock/metal core
with large H/He gas envelope)
How would the Solar System be different if the
solar nebula had cooled with a temperature half
its actual value at every distance?
A) Jovian planets would have formed closer to
Sun.
B) There would be no asteroids.
C) There would be no comets.
D) Terrestrial planets would be larger.
How would the Solar System be different if the
solar nebula had cooled with a temperature half
its actual value at every distance?
A) Jovian planets would have formed closer
to Sun.
B) There would be no asteroids.
C) There would be no comets.
D) Terrestrial planets would be larger.
Frost line would have been closer to Sun, but
everything else would have proceeded as
before.
Which materials can be found in Jovian
planets?
A) hydrogen, helium
B) hydrogen, helium, hydrogen compounds
C) hydrogen, helium, hydrogen compounds, rock, metal
Which materials can be found in Jovian
planets?
A) hydrogen, helium
B) hydrogen, helium, hydrogen compounds
C) hydrogen, helium, hydrogen compounds, rock, metal
Nothing precludes rock and metal from being in the outer
part of the Solar System.
It’s just that there is a lot more H/He/hydrogen compounds in
the outer parts than rock and metal  Jovian planets
have small rocky/metal cores covered by a lot of
H/He/hydrogen compounds.
The gas giants are became “miniature solar
systems” with their own miniature accretion disks
which went to form moons.
Many moons form in miniature disks of dust/gas
around Jovians in a scaled-down version of how our
entire Solar System formed.
(form rocky moons)
(form icy moons)
Jovian systems formed like the Solar System in miniature.
What ended the era of planet formation?
Most of the nebula
material never gets
incorporated into a
(Jovian) planet.
H/He gas is blown
out of the young
solar system by a
strong solar wind —
outflowing protons
and electrons from
the Sun.
Planets stop growing
at this point.