The Layout of the Solar System
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Planets fall into two main categories
–  Terrestrial (i.e. Earth-like)
–  Jovian (i.e. Jupiter-like or gaseous)
[~5000 kg/m3]
[~1300 kg/m3]
What is density?
Average density = mass/volume
typical units: [kg/m3]
Density of water is 1000 kg/m3.
Useful formula: Volume of a sphere =
r = radius
Mars
Neptune
Terrestrial
Jovian
1 g/cm3 =
1000 kg/m3
Some facts about the Solar System
•  Planets orbit
the Sun under
the influence of
gravity.
•  The planets all
go around the
Sun in the
same direction.
•  Most of the
planets spin in
the same
direction as
their orbit.
• The orbits of all planets are aligned within a few degrees
of each other. (Pluto is no longer considered a planet)
The orbits of all the planets, as seen from directly above the Sun’s
equator, 100 AU away. Only Pluto’s orbit is more than 7 degrees
away from the rest.
Pluto’s
orbit
A Few Exceptions to the Rules…
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Uranus is tilted on its side.
Venus rotates “backwards” (i.e. clockwise).
Triton orbits Neptune “backwards.”
Earth is the only terrestrial planet with a
relatively large moon.
Origin of the Solar System
Theory must explain the data:
Large bodies in the Solar System have
orderly motions.
There are two types of planets.
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small, rocky terrestrial planets
large, hydrogen-rich Jovian planets
Asteroids & comets exist in certain regions
of the Solar System
The exceptions to the general patterns need
to be explained
Close-encounter theory
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Sun had a close encounter with another star
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Fails to explain why the orbits are almost
circular
Fails to explain the terrestrial/Jovian split
Close encounters are rare
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Gravity pulled matter off the surface of the Sun
Predicts that extra-solar planets should be very
rare
Doesn’t work
Nebular Theory
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Solar System formed from a giant, swirling
cloud of gas & dust
Depends on two principles of Physics:

Law of Gravity
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gravitational potential energy ⇒ heat
Conservation of angular momentum
And basic chemistry
Heating, spinning, flattening….
The Solar Nebula
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The nebular theory holds
that our Solar System
formed out of a nebula
which collapsed under its
own gravity.
observational evidence
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We observe stars in the
process of forming today.
The are always found within
interstellar clouds of gas.
10,000 AU
newly born stars in the Orion Nebula
solar nebula – name given to the cloud of gas
from which our own Solar System formed
Gravitational Collapse
•  The solar nebular was initially fairly
spherical and a few light years in
diameter.
–  very cold
–  rotating slightly
•  It was given a “push” by some event.
–  perhaps the shock wave from a
nearby supernova
•  As the nebula shrank, gravity increased,
causing collapse.
•  As the nebula “falls” inward, gravitational
potential energy is converted to heat.
–  Conservation of Energy
•  As the nebula’s radius decreases, it
rotates faster
–  Conservation of Angular Momentum
Collapse of the Solar Nebula
Orderly Motions in the Solar System
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The Sun formed in the very centre of the nebula.
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temperature & density were high enough for nuclear fusion
reactions to begin
The planets formed in the rest of the disk.
This would explain the following:
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all planets lie along one plane (in the disk)
all planets orbit in one direction (the spin direction of the disk)
the Sun rotates in the same direction
the planets would tend to rotate in this same direction
most moons orbit in this direction
most planetary orbits are near circular (collisions in the disk)
More Support for the Nebular Theory
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We have observed disks around other stars.
These could be new planetary systems in
formation.
β Pictoris
AB Aurigae
Ok the theory seems to pass the first test….
How about explaining the existence of planets?
And in particular 2 type of planets?
•  What key fact explains why there are two types
of planet?
•  Describe the basic steps by which the terrestrial
planets formed.
•  Describe the basic steps by which the Jovian
planets formed.
Making planets
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The gravity of the gas in the Solar Nebula
was too weak to form planets
Needs some ‘seeds’ to form another way
Metal/rocky seeds form close in; hydrogen
compounds form further out, by condensation
Condensation
Elements & compounds began to condense (i.e.
solidify) out of the nebula…. depending on
temperature
Building the Planets
So only rocks & metals condensed within 3.5 AU of the
Sun… the so-called frost line. Hydrogen compounds
(ices) condensed beyond the frost line.
Building the Planets
accretion -- small grains stick to one another via
electromagnetic force until they are massive enough to
attract via gravity to form...
…planetesimals which will:
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combine near the Sun to form rocky planets
combine beyond the frostline to form icy
planetesimals which…
capture H/He far from Sun to form gas planets
Evidence for condensation
Impacts happen even today (but rare)
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July, 1994: Comet Shoemaker-Levy/9 collided with
Jupiter
Why the outer planets are big
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Basic idea: the Hill Sphere:
Slightly stronger pull of
Sun on this side
Nebular
gas
Sun
Planet
Pull of gravity
We need the planet’s gravity to be stronger than the
extra Sun’s gravity, in order to pull in the nebular material
How big a region can the planet accrete
from?