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Formation of the
Solar System
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Formation of the Solar System
The Formation of the Solar System
What properties must a planetary formation
theory explain?
It must explain the patterns of motion of the
present solar system (last week).
2. It must explain why planets form into 2 groups.
3. It must explain the huge existence of asteroids
and comets.
4. It must allow for possible exceptions to the rules.
The theory may be able to be used on other solar
systems in the Galaxy
1.
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Formation of the Solar System
The Formation of the Solar System
Evolutionary Theories
All evolutionary theories have their start with
Descartes’s whirlpool or vortex theory
proposed in 1644.
Using Newtonian mechanics, Kant (in 1755)
and then Laplace (around 1795) modified
Descartes’s vortex to a rotating cloud of gas
contracting under gravity into a disk.
The Solar Nebular Hypothesis is an
example of an evolutionary theory.
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Formation of the Solar System
The Formation of the Solar System
Catastrophic Theories
Catastrophic theory is a theory of the
formation of the solar system that involves an
unusual incident such as the collision of the Sun
with another star.
The first catastrophic theory - that a comet
pulled material from the Sun to form the planets
- was proposed by Buffon in 1745.
Other close encounter hypotheses have been
proposed too.
Catastrophic origins for solar systems would be
quite rare (relative to evolutionary origins) due to
the unusual nature of the catastrophic incident.
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Formation of the Solar System
Solar Nebula Hypothesis
Origin of the Solar Nebula
Galactic recycling
– Most of the universe started as Hydrogen and Helium. All
other heavy elements (loosely called “metals” by
astronomers) were formed in stars
– When stars die they release much of the content into
space
– While this has been going on for 4.6 billion years, only
2% of all the have been converted to “metals”
Evidence from other gas clouds
– All new systems that we can observed formed within
interstellar clouds, such as the Orion Nebula
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Formation of the Solar System
Solar Nebula Hypothesis
Towards a Solar Nebula Hypothesis
A supernovae shock wave likely triggered the events
which led to the birth of our solar system
The nebular cloud collapsed due the force of gravity
on the cloud. But the cloud does not end up
spherical (like the sun) because there are other
processes going on:
– Heating – The cloud increases in temperature, converting
gravitational potential energy to kinetic energy. The sun would
form in the center where temperatures and densities were the
greatest
– Spinning – as the cloud shrunk in size, the rotation of the disk
increases (from the conservation of angular momentum).
– Flattening – as cloud starting to spin, collisions flattened the
shape of the disk in the plane perpendicular to the spin axis
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Formation of the Solar System
Testing the Model
If the theory is correct, then we should
see disks around young stars
Dust disks, such as discovered around
beta-Pictoris or AU Microscopii, provide
evidence that conditions for planet
formation exist around many Sun-like
stars.
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Formation of the Solar System
Solar Nebula Hypothesis
The Formation of Planets
As the solar nebula cooled and flattened into a disk some 200 AU
in diameter, materials began to “freeze” out in a process called
condensation (changing from a gas to a solid or liquid).
The ingredients of the solar system consist of 4 categories (with %
abundance):
–
–
–
–
Hydrogen and Helium gas (98%)
Hydrogen compounds, such as water, ammonia, and methane (1.4%)
Rock (0.4%)
Metals (0.2%)
Since it is too cool for H and He to condense, a vast majority of the
solar nebula did not condense
Hydrogen compounds could only condense into ices beyond the
frost line, which lay between the present-day orbits of Mars and
Jupiter
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Formation of the Solar System
Solar Nebula Hypothesis
Building the Terrestrial Planets
In the 1940s, Weizsächer showed that eddies would form in a
rotating gas cloud and that the eddies nearer the center would be
smaller.
Eddies condense to form particles that grow over time in a
process called accretion. Materials such and rock and metal
(categories #3 and #4).
These accreted materials became planetesimals which in turn
sweep up smaller particles through collision and gravitational
attraction.
These planetesimals suffered gravitational encounters which
altered their orbits caused them to both coalesce and fragment.
Only the largest planetesimals grew to be full-fledged planets.
Verification of this models is difficult and comes in the form of
theoretical evidence and computer simulations.
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Formation of the Solar System
Solar Nebula Hypothesis
Building the Jovian Planets
Planetesimals should have also grown in the
outer solar system, but would have been
made of ice as well as metal and rock.
But Jovian planets are made mostly of H and
He gas…
The gas presumably was captured by these
ice/rock/metal planetesimals and grew into
the Jovian planets of today.
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Formation of the Solar System
Solar Nebula Hypothesis
A Stellar wind is the flow of nuclear
particles from a star.
Some young stars exhibit strong stellar
winds. If the early Sun went through such a
period, the resulting intense solar wind
would have swept the inner solar system
clear of volatile (low density) elements,
molecules and compounds.
The giant planets of the outer solar system
would then have collected these outflowing
gases.
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Formation of the Solar System
Solar Nebula Hypothesis
An object shrinking under the force of gravity
heats up. High temperatures near the newly
formed Sun (protosun) will prevent the
condensation of more volatile (low density)
elements. Planets forming there will thus be
made of nonvolatile, dense material.
Farther out, the eddies are larger and the
temperatures cooler so large planets can form
that are composed of volatile elements (light
gases).
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Formation of the Solar System
Solar Nebula Hypothesis
Problem: The total angular momentum of
the planets is known to be greater than that
of the Sun, which should not occur according
to conservation laws (i.e. the present Sun is
spinning too slowly).
Solution: As the young Sun heated up, it
ionized the gas of the inner solar system.
– The Sun’s magnetic field then swept through the
ions in the inner solar system, causing ions to
speed up.
– As per Newton’s third law, this transfer of energy
to the ions caused the Sun to slow its rate of
rotation.
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Formation of the Solar System
Solar Nebula Hypothesis
Explaining Other Clues
Over millions of years the remaining
planetesimals fell onto the moons and
planets causing the cratering we see today.
This was the period of heavy bombardment.
Comets are thought to be material that
coalesced in the outer solar system from the
remnants of small eddies.
The Asteroid belt formed from debris that
could not coalesce into a planet due to the
gravitational influence of Jupiter
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Formation of the Solar System
Solar Nebula Hypothesis
The formation of Jovian planets and its
moons must have resembled the
formation of the solar system. Jupiter
specifically:
– Moons close to Jupiter are denser and
contain fewer light elements;
– Moons farther out decrease in density and
increase in heavier elements.
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Formation of the Solar System
The Exceptions to the Rule
Captured Moons – satellites which go the
opposite way were likely captured. Most of
these moons are small are lie far away from
the planet.
Giant impacts – may have helped form the
Moon and explain the high density of Mercury
and the Pluto-Charon system. Furthermore,
the unusual tilts of Uranus and Venus can
also be explained by giant impacts.
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Formation of the Solar System
Solar System Destiny
The nebular hypothesis accounts for all major
features in the solar system
It does not account for everything, however
It probably took about a few tens of million of years,
about 1% of the current age of the solar system
The solar system was probably not completely
predestined from the collapse of the solar nebula,
though the initial were orderly and inevitable
The final stage of accretion and giant impacts were
fairly random in nature and made our solar system
unique
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Formation of the Solar System
Radioactivity
Radioactivity
Certain isotopes (elements which contain
differing number of neutrons) are not stable and
will decay into two or more lighter elements
The time it takes for half of a given isotope to
decay is called the half-life
By noting what percentage a rock (or human
body) has left of a radioactive element can
enable us to estimate the age of that object. This
process is called radioactive dating. See
Mathematical Insight 8.1
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Formation of the Solar System
Radioactivity
Radioactivity - examples
Potassium-40 decays into Argon-40 with a half-life of
1.25 billion years
– Since Argon-40 is an inert gas, it is very unlikely to have formed
inside a rock as the solar nebula condensed, so it must have
formed via decay
Uranium-238, after a series of decays, turns into Lead206 with half-life of 4.5 billion years
– Lead and Uranium have very different chemical behaviors
– Some minerals have nearly no lead to begin with, so when
uranium is mixed with lead, we can assume that the lead formed
via decay
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Formation of the Solar System
Radioactivity
Radioactivity
The general formula for the age of a radioactive
material is (see Mathematical Insight 8.1):
 current amount 
log10 

original amount 

t  thalf 
1
log10  
2
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Formation of the Solar System
Radioactivity
Earth rocks, Moon rocks, and meteorites
The oldest Earth rocks date back to 4 billion
years and some small grains go back to 4.4
billion years. Moon rock brought back from the
Apollo mission date as far back as 4.4 billion
years.
– These tell us when the rock solidified, not when the
planet formed
The oldest meteorites, which likely come form
asteroids, are dated at 4.55 billion years,
marking the time of the accretion of the solar
system
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The End
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