Transcript Formation of the Solar System

Formation of the
Solar System
Courtesy: NASA
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 Nebula 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
Towards a Solar Nebula Hypothesis
• 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
increase (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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Courtesy: NASA
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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):
•
1.
2.
3.
4.
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
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
• 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 elements.
• 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
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.
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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:

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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 moon 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
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
Cosmic Calculations 6.1
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Formation of the Solar System
Radioactivity
Earth rocks, Moon rocks, and meteorites
• The oldest Earth rock 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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Formation of the Solar System – Chapter 10 in text
Planetary Systems Around Other Stars?
• Photographing planets around stars directly is very difficult
since planet merely reflect (visible) light from the nearby stars.
Using the infrared part of the spectrum, we can detect large
objects known as brown dwarfs which are neither stars or
planets
• Stars exhibiting a discernable wobble from gravitation tugs
can be evidence of an unseen companion - such as a large
planet or group of planets. One can try to look for positional
changes in the sky form this star – the astrometric
technique, but this is difficult.
• Since 1995, this Doppler Technique has found evidence of
over 200 planets orbiting stars in the near vicinity of the Sun.
• Some of the extrasolar planets can be detected when the
transit the star. The star’s brightness dims just a bit during
the transit. The Kepler mission has found over a 1000 planets
using the transit technique.
• Web link: http://exoplanets.org/
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Formation of the Solar System
Planetary Systems Around Other Stars?
• Comparisons to our Solar System
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Many of these planets are more massive than Jupiter
Many of these planets are closer to their star than
Mars is to the Sun
• These discoveries are in part due to a selection effect –
these are the easiest to detect

Jovian sized planets close to the star is not consistent
with the standard solar nebular model. So how does
one form a “hot Jupiter”?
• Planetary migration – the gas giant form in the cooler, outer
region of the nebular disk, but due to friction (and a loss of
angular momentum) from the nebular disk, the planet in
brought to a much closer distance.
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The End
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