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Formation of the Solar System
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)
•  Kant and Laplace proposed the nebular
hypothesis over two centuries ago
•  A large amount of evidence now supports this
idea
•  Predictions of this theory have been accurate
Galactic Recycling
•  The universe shortly after the Big Bang consisted of
hydrogen and helium
•  Heavier elements were made in stars and then recycled
through interstellar space when stars died
•  When the solar system formed 4.6 billion years ago,
about 2% of the original hydrogen and helium in the
universe had been converted to other elements.
•  The solar system was born from a cloud of gas (solar
nebula) consisting of 98% hydrogen and helium and
2% of other elements
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Evidence for the Nebular Theory
•  We can see
stars forming
in other
interstellar
gas clouds,
lending
support to the
nebular
theory
The Birth of the Solar System
•  The solar nebula was
initially a cold low
density cloud of gas of
diameter 100-200 AU
with 2-3 times the mass
of the sun
•  Collapse of the gas may
have been triggered by
a cosmic event such as
the shock wave of a
supernova
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Spinning cloud
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Initial slow rotation speed of the cloud increased as
the cloud contracted (similar to ice skater spinning
faster as she pulls her arms in)
The cloud heated up as it contracted
The sun formed at the centre where temperatures and
densities were highest.
Flattening
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Collisions between gas particles in cloud gradually
reduce random motions
Collisions between gas particles also reduce up
and down motions
Spinning cloud flattens as it shrinks
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Disks around other stars
• 
Observation
of disks
around
other stars
support the
nebular
hypothesis
Motion in the Solar System
•  The spinning disk explains the uniform
motions observed in the Solar System
today:
•  Planets all orbit in the same direction of
spin of the disk they were formed from
•  Planets orbit in the same plane because of
the flattening of the disk they were formed
from.
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Why are there two types of planets?
Why are there two types of planets?
Inner parts of disk are hotter than outer parts.
Rock can be solid at much higher temperatures than ice.
Inside the ice line (or frost line): Too hot for hydrogen
compounds to form ices. Only rocks and metals can be solid
Outside the ice line: Cold enough for ices to form.
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How did terrestrial planets form?
Small particles of rock and metal
(seeds) were present inside the ice
line
Planetesimals of rock and metal built
up as these particles collided. This
process is called accretion
Gravity eventually assembled these
planetesimals into terrestrial planets
over a few million years.
Heat caused melting and
differentiation inside the planets
Differentiation: separation by density
of material inside a planet
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Accretion of Planetesimals
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Many smaller objects collected into just a
few large ones
Computer simulations support this process
Meteorites provide evidence of early
condensation
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How did jovian planets form?
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Ice could also form planetisimals outside the
frost line.
Larger planets may have been able to form
from H and He gases by direct gravitational
collapse without an accretion phase.
Planets were large enough to draw in
surrounding gas to form moons and rings
What ended the era of planet
formation?
Outflowing
matter from
the Sun -- the
solar wind -blew away
the leftover
gases
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Where did asteroids and comets
come from?
Asteroids and Comets
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Leftovers from the accretion process
Rocky asteroids inside frost line
Icy comets outside frost line beyond
orbit of Pluto (Kuiper belt and Oort
Cloud)
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Asteroid Orbits
•  Most asteroids orbit in
a belt between Mars
and Jupiter
•  Trojan asteroids follow
Jupiter’s orbit
•  Rocky planetesimals
between Mars and
Jupiter did not accrete
into a planet.
•  Jupiter’s gravity stirred
up asteroid orbits and
prevented their
accretion into a planet.
Origin of Meteorites
•  primitive
•  about 4.6 billion years
old
•  accreted in the Solar
nebula
•  processed
•  younger than 4.6 billion
years
•  matter has differentiated
•  fragments of a larger
object which processed
the original Solar
nebula material
For a movie of a meteorite that hit the earth in 1992 check out
http://csep10.phys.utk.edu/astr161/lect/meteors/fireball.mpg
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Origin of Comets
•  We can tell where comets
originate by measuring their
orbits as they visit the Sun.
•  Most approach from random
directions and do not orbit in
the same sense as the
planets.
•  they come from the Oort
cloud
•  Others orbit along the
ecliptic plane in the same
sense as the planets.
•  they come from the
Kuiper belt
Pluto: Dwarf Planet or Kuiper Belt Comet?
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By far the smallest planet.
Not a gas giant like other outer planets.
Has an icy composition like a comet.
Has a very elliptical, inclined orbit.
Pluto has more in common with comets than with the eight
major planets
•  Pluto is currently classified as a dwarf planet.
•  Other dwarf planets: Ceres, Eris
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Other Icy Bodies
•  There are many icy
objects like Pluto on
elliptical, inclined
orbits beyond Neptune.
•  The largest of these,
Eris (previously called
Planet X) discovered in
summer 2005, is even
larger than Pluto
How do we explain “exceptions
to the rules”?
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Heavy Bombardment
•  Leftover
planetesimals
bombarded
other objects
in the late
stages of
solar system
formation
Origin of Earth’s Water
•  Water may
have come to
Earth by way
of icy
planetesimals
from outer
solar system
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Odd Rotation
•  Giant impacts
might also
explain the
different
rotation axes
of some
planets
Meteorite Impacts and Mass Extinctions
•  We know that larger objects have
impacted Earth
•  Meteor Crater in Arizona
caused by a 50 m asteroid
•  impact occurred 50,000 yrs ago
•  65 million years ago, many
species, including dinosaurs,
disappeared from earth
•  Sedimentary rock layer from
that time shows:
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Iridium, Osmium, Platinum
grains of “shocked quartz”
spherical rock droplets
soot from forest fires
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Impacts and Mass Extinctions on Earth
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Elements like Iridium, rare on Earth, are found in meteorites.
Shocked quartz, found at Meteor Crater, forms in impacts.
Rock droplets would form from molten rock “rain.”
Forest fires would ensue from this hot rain.
All this evidence would imply that Earth was struck by an
asteroid 65 million years ago.
•  In 1991, a 65 million year old
impact crater was found on the
coast of Mexico.
•  200 km in diameter
•  implies an asteroid size of
about 10 km across
•  called the Chicxulub crater
Iridium: Evidence of an Impact
•  Iridium is very rare in Earth surface rocks but
often found in meteorites.
•  Luis and Walter Alvarez found a worldwide layer
containing iridium, laid down 65 million years
ago, probably by a meteorite impact.
•  Dinosaur fossils all lie below this layer
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Impacts and Mass Extinctions on Earth
•  We have a plausible scenario of how the impact led to mass extinction.
•  debris in atmosphere blocks sunlight; plant die…animals starve
•  poisonous gases form in atmosphere
Could it happen again?
•  The odds of a large impact are small … but not zero!
•  Spaceguard programs and amateur astronomers around the
world watch the skies to detect any possible dangers early
enough
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First observation of two bodies colliding
Comet Shoemaker-Levy colliding with Jupiter
How can we tell the age of the
Solar System ?
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Radioactive Decay
•  Some (parent) isotopes
in a rock decay into
other (daughter) nuclei
•  Example: Potassium-40
decays into Argon-40
•  A half-life is the time
for half the nuclei in a
substance to decay
The amount of parent isotopes and daughter isotopes in a rock
allows us to calculate the age of a rock.
Radioactive Decay
•  Example: The halflife of Potassium-40
is 1.25 billion yrs.
•  Thus a rock with
equal amounts of
Potassium-40 and
Argon-40 is 1.25
billion years old.
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Radioactive Decay
•  After 2 half-lives (2.5
billion years) half of the
remaining Potassium will
be converted to Argon
•  Thus after 2.5 billion years
the amount of Argon will
be three times the amount
of Potassium
•  A rock with 3:1 ratio of
Argon-40 to Potassium-40
is 2.5 billion yrs old.
When did the planets form?
•  Radiometric dating tells us that oldest
moon rocks are 4.48 billion years old
•  Oldest meteorites are 4.56 billion years
old
•  Planets probably formed 4.6 billion years
ago
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Detecting planets around other
stars
•  A Sun-like star is about a billion times
brighter than the sunlight reflected from its
planets
•  Like being in Vancouver and trying to see
a pinhead 15 meters from a grapefruit in
Waterloo.
Planet Detection
•  Direct: Pictures or spectra of the planets
themselves
•  Indirect: Measurements of stellar
properties revealing the effects of orbiting
planets
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Indirect Detection: Astrometric Technique
•  We can detect planets
by measuring the
change in a star’s
position in sky due to
gravitational tug of
planets (wobble)
•  However, these tiny
motions are very
difficult to measure
(~0.001 arcsecond)
Indirect Detection: Doppler Technique
•  Measuring a star’s
Doppler shift can tell us
its motion toward and
away from us due to the
gravitational tug of a
planet
•  Current techniques can
measure motions as
small as 1 m/s (walking
speed!)
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First Extrasolar Planet
•  Doppler shifts of star
51 Pegasi indirectly
reveal a planet with 4day orbital period
•  Planet around 51 Pegasi
has a mass similar to
Jupiter’s, despite its
small orbital distance
•  Short period means
small orbital distance
•  First extrasolar planet
to be discovered (1995)
First Earth-Like Planet Found
•  In April 2007 scientists discovered the most Earth-like
planet since the first discovery of extrasolar planets
•  It is orbiting the star Gliese 581 about 20.5 ly away
•  Roughly 1.5 times size of earth
•  Lies in the habitable zone: may be a water planet!
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Transit Missions
•  NASA’s Kepler mission
began looking for
transiting planets in 2009
•  It can measure the 0.008%
decline in brightness when
an Earth-mass planet
eclipses a Sun-like star
https://www.youtube.com/watch?v=8v4SRfmoTuU
First Direct Picture of Extrasolar
Planets
Team led by Canadian
astronomer Christian Marois
~7 Jupiter-mass planet
orbiting at about 70 AU,
~10 Jupiter-mass planet
orbiting the star at about 40
AU.
In total almost 2000 planets
have been found including
numerous Earth-like planets.
http://phl.upr.edu/hec
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Summary
•  What does the solar system look like?
–  Planets orbit Sun in the same direction and in
nearly the same plane.
•  What can we learn by comparing the planets to
one another?
–  Comparative planetology looks for patterns
among the planets.
–  Those patterns give us insight into the general
processes that govern planets
–  Studying other worlds in this way tells us
about our own Earth
Summary
•  What are the major features of the Sun and planets?
–  Sun: Over 99.9% of the mass
–  Mercury: A hot rock
–  Venus: Same size as Earth but much hotter
–  Earth: Only planet with liquid water on surface
–  Mars: Could have had liquid water in past
–  Jupiter: A gaseous giant
–  Saturn: Gaseous with spectacular rings
–  Uranus: A gas giant with a highly tilted axis
–  Neptune: Similar to Uranus but with normal axis
–  Pluto: An icy “misfit” more like a comet than a
planet
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Summary
•  What are the three major groups of small bodies in
the Solar System?
•  Asteroids, comets of the Kuiper belt, comets of the
Oort cloud. The groups are distinguished by their
orbits and composition
•  What are asteroids like?
–  Most asteroids are small, potato-shaped, and orbit
in the asteroid belt.
Summary
•  Where do meteorites come from?
•  meteor: a trail of light caused by a small particle
entering our atmosphere. meteorite: a rock that
survives the plunge from space to reach the ground.
–  Primitive meteorites are remnants from solar nebula
–  Processed meteorites are fragments of larger bodies
than underwent differentiation
•  Did a meteorite impact kill the dinosaurs?
–  Iridium layer just above dinosaur fossils suggests that
an impact caused mass extinction 65 million years ago.
–  A large crater of that age has been found in Mexico
–  The probability of a major impact in our lifetimes is
very low, but not zero. The threat is still being
assessed.
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Summary
•  What are comets like?
–  Comets are like dirty snowballs
–  Most are far from Sun and do not have tails
–  Tails grow when comet nears Sun and nucleus heats
up
–  Comets in plane of solar system come from Kuiper
Belt
–  The Kuiper Belt contains objects as large as Pluto.
–  Comets on random orbits come from Oort cloud
Summary
•  What features of the solar system provide clues
to how it formed?
–  Motions of large bodies: All in same
direction and plane
–  Two main planet types: Terrestrial and jovian
–  Swarms of small bodies: Asteroids and
comets
–  Notable exceptions: Rotation of Uranus,
Earth’s large moon, etc.
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Summary
•  What properties of our solar system must a formation theory
explain?
–  Motions of large bodies
–  Two types of planets
–  Asteroids and comets
–  Notable exceptions like Earth’s moon
•  What theory best explains the features of our solar system?
–  Nebular theory states that solar system formed from a
large interstellar gas cloud.
Summary
What caused the orderly patterns of motion in our solar
system?
–  Solar nebula spun faster as it contracted because of
conservation of angular momentum
–  Collisions between gas particles then caused the
nebula to flatten into a disk
–  We have observed such disks around newly forming
stars
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Summary
•  Why are there two types of planets?
–  Only rock and metals condensed inside the frost line
–  Rock, metals, and ices condensed outside the frost
line
•  How did the terrestrial planets form?
–  Rock and metals collected into planetsimals
–  Planetesimals then accreted into planets
•  How did the jovian planets form?
–  Additional ice particles outside frost line made
planets there more massive
–  Gravity of these massive planets drew in H, He
gases
Summary
•  What ended the era of planet formation?
–  Solar wind blew away remaining gases
–  Magnetic fields in early solar wind helped reduce Sun’s rotation
rate
•  Where did asteroids and comets come from?
–  They are leftover planetesimals, according to the nebular theory
•  How do we explain “exceptions to the rules”?
–  Bombardment of newly formed planets by planetesimals may
explain the exceptions
•  How do we explain the existence of Earth’s moon?
–  Material torn from Earth’s crust by a giant impact formed the
Moon
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Summary
•  How does radioactivity reveal an object’s age?
–  Some isotopes decay with a well-known half-life
–  Comparing the proportions of those isotopes with
their decay products tells us age of object
•  When did the planets form?
–  Radiometric dating indicates that planets formed 4.5
billion years ago
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