 Two types of planets – Terrestrial & Jovian
 Orbits in nearly the same plane and in same direction
 Planet rotations in same direction
 Craters everywhere
 Two types of small objects – asteroids & comets
 Observations of exoplanetary systems are consistent
with this being a common pattern (sparse data though)
To understand why these
features of the solar system
exist, we need a little more
background material…
Faster (hotter) particles can
escape electrical bonds easier.
the phases
– solid
– liquid
– gas
– plasma
depend on how
tightly the atoms
and/or molecules are
bound to each other
• As temperature
increases, these
bonds are loosened:
●
Phases of Matter
Major Conservation Laws
Conservation of energy
Conservation of momentum
Conservation of angular momentum
Angular Momentum
• angular momentum – the momentum involved
in spinning /circling = mass x velocity x radius
● torque – anything that can cause a change in an
object’s angular momentum (twisting force)
Conservation of Angular Momentum
• In the absence of a net torque, the total angular
momentum of a system remains constant.
Angular Momentum & Orbits
The angular momentum of
an orbiting planet is
conserved, i.e., it is always
the same.
This provides yet another
reason why planets move
fastest at perihelion and
slowest at aphelion.
How do we account for what we see
in the solar system?
Group Activity
 Form into groups
 Select one member of group to serve as recorder
 Recorder:
1. Tabulate the group’s responses to the questions
asked in the activity
2. Print names ONLY of people that actively
participate in activity
 At end, individuals sign after printed name
Group activity
Consider a balloon that has been “blown up”:
 What is in the balloon?
 Why does the balloon inflate?
 What keeps it inflated?
 What happens to a balloon if you slowly cool it? Heat it?
 Why does this happen?
(hints: Think about the forces acting on the balloon. Think
about what is happening to the molecules of gas inside
the balloon)
The Nebular
Theory
The Solar system
was formed from
a giant, swirling
interstellar cloud
of gas and dust
(the solar nebula)
The Solar nebula may have
been part of a much larger
nebula
Protostellar
nebulae?
Spiral Galaxy:
Gas concentrated in
spiral arms
New stars being formed
Elliptical Galaxy:
No concentrated gas
Old stars
The struggle to form
stellar/planetary systems
Gravity:
Seeks to collapse
the cloud
Gas Pressure:
Seeks to expand
the cloud
Building the Planets. I
COLLAPSE OF PROTOSTELLAR CLOUD INTO A
ROTATING DISK
Composition of disk:
 98% hydrogen and helium
 2% heavier elements (carbon,
nitrogen, oxygen, silicon, iron,
etc.).
Most of this was in gaseous
form!
Collapse of the Solar Nebula
If cloud size and density exceeds a critical value, nebula collapses:
Temperature increased: Conservation of energy
Rotation rate increased: Conservation of angular momentum
Rotating cloud flattened into a disk: ”protoplanetary” disk
Motions of material in disk became circularized (from collisions)
Top view
Side view
Material in the newly formed
proto-planetary disk:
- similar orbital planes
- approximately circular orbits
Images of
protostellar
disks
Material in this disk will
form planets orbiting in
the same manner as the
material from which they
are formed.
According to our theory of solar system formation, what
three major changes occurred in the solar nebula as it
shrank in size?
(blue) It got hotter, its rate of rotation increased, and it
flattened into a disk.
(red) It gained energy, it gained angular momentum, and
it flattened into a disk.
(yellow) Its mass, temperature, and density all increased.
(green) I have no idea
According to our theory of solar system formation, what
three major changes occurred in the solar nebula as it
shrank in size?
(blue) It got hotter, its rate of rotation increased, and it
flattened into a disk.
(red) It gained energy, it gained angular momentum, and
it flattened into a disk.
(yellow) Its mass, temperature, and density all increased.
(green) I have no idea
Which law best explains why the solar
nebula spun faster as it shrank in size?
(blue) Law of universal gravitation.
(red) Einstein's law that E = mc2.
(yellow) Conservation of angular
momentum.
(green) Conservation of energy.
Which law best explains why the solar
nebula spun faster as it shrank in size?
(blue) Law of universal gravitation.
(red) Einstein's law that E = mc2.
(yellow) Conservation of angular
momentum.
(green) Conservation of energy.
Why did the solar nebula ended up with a disk shape
as it collapsed?
(blue) The force of gravity pulled the material
downward into a flat disk.
(red) It flattened as a natural consequence of
collisions between particles in the nebula, changing
random motions into more orderly ones.
(yellow) The law of conservation of energy.
(green) It was fairly flat to begin with, and retained
this flat shape as it collapsed.
Why did the solar nebula ended up with a disk shape
as it collapsed?
(blue) The force of gravity pulled the material
downward into a flat disk.
(red) It flattened as a natural consequence of
collisions between particles in the nebula, changing
random motions into more orderly ones.
(yellow) The law of conservation of energy.
(green) It was fairly flat to begin with, and retained
this flat shape as it collapsed.
Which law best explains why the central
regions of the solar nebula got hotter as
the nebula shrank in size?
(blue) Newton's third law.
(red) Law of conservation of energy.
(yellow) Law of conservation of angular
momentum
(green) The two laws of thermal radiation.
Which law best explains why the central
regions of the solar nebula got hotter as
the nebula shrank in size?
(blue) Newton's third law.
(red) Law of conservation of energy.
(yellow) Law of conservation of angular
momentum
(green) The two laws of thermal radiation.
Building the Planets. II
There was a range of
temperatures in the
proto-solar disk,
decreasing outwards
Condensation: the formation of solid or liquid particles
from a cloud of gas (from gas to solid or liquid phase)
Different kinds of planets and satellites were formed out of
different condensates
Ingredients of the Solar Nebula
Metals : Condense into solid form at 1000 – 1600 K
iron, nickel, aluminum, etc. ; 0.2% of the solar nebula’s
mass
Rocks : Condense at 500 – 1300 K
primarily silicon-based minerals; 0.4% of the mass
Hydrogen compounds : condense into ices below ~ 150 K
water (H2O), methane (CH4), ammonia (NH3), along with
carbon dioxide (CO2), 1.4% of the mass
Light gases (H & He): Never condense in solar nebula
hydrogen and helium.; 98% of the mass
Inner Solar System: Too hot for ices & carbon grains.
Outer Solar System: Carbon grains & ices form beyond frost line
Frost line inside
orbit of Jupiter
Building the Planets. III
Accretion
Accretion is growing by colliding and sticking
The growing objects formed by accretion –
planetesimals (“pieces of planets”)
Small planetesimals came in a
variety of shapes, reflected in many
small asteroids
Large planetesimals (>100 km
across) became spherical due to the
force of gravity
In the inner solar system (interior to
the frost line), planetesimals grew by
accretion into the Terrestrial planets.
In the outer solar system (exterior to
the frost line), accretion was not the
final mechanism for planet building –
nebular capture followed once
accretion of planetesimals built a
sufficiently massive protoplanet.
Building the Planets. IV. Nebular Capture
Nebular capture – growth of icy
planetesimals by capturing
larger amounts of hydrogen and
helium. Led to the formation of
the Jovian planets
Numerous moons were formed by the same processes
that formed the proto-planetary disk
Condensation and accretion created “mini-solar systems”
around each Jovian planet