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• Chapter 12
The Origin of the Solar System
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• The solar system is our home in the
universe.
• As intelligent species, we have the right and
the responsibility to wonder what we are and
where we live, to study the universe..
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Humans have inhabited the solar system for at least a
million years. However, only within the last
• 2400 years have we begun to realize that we live
in a ‘cosmos’ and a ‘planetary system’
• 400 years have we begun to understand the
details of the true heliocentric ‘architecture’ of our
solar system
• 300 years have understood its mechanics
• 200 years have we started to make detailed
scientific models of its origin
• 100 years have sensed that it’s chaotic yet stable,
and where it is located in a Galaxy
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Humans have inhabited the solar system for at least
a million years. However, only within the last
• 90 years have we completed the Copernican revolution:
Harlow Shapley removed the Sun from the center of the
Galaxy, and Edwin Hubble the Galaxy from the center of
the Universe
• 50 years have we figured out how the sun ‘works’ and
how the stars produced the atoms in our bodies
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Humans have inhabited the solar system for at least
a million years. However, only within the last
• 30 years have we begun to observe & understand
details of star formation such as protostellar disks and
their relevance to our origins
• 20 years have we begun to observe durect evidence for
planetary systems other than our own
• 10 years have we placed our system in a wider context
and compared it with hundreds of newly discovered
extrasolar systems
• …and started filling the gaps in our theories of the
origins
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The Great Chain of Origins
• You are linked through a great chain of
origins that leads backward through time
to the first instant when the universe
began 13.7 billion years ago.
• The gradual discovery of the links in that
chain is one of the most exciting adventures
of the human intellect.
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The Great Chain of Origins
• In the course ASTA02 (next semester) you
will have an opportunity to study some of
that story in some detail, including:
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The origin of the universe in the big bang,
The formation of galaxies,
The origin of stars, and
The production of the chemical elements.
Here, you will explore the origin
of planets.
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The History of the Atoms in Your Body
• By the time the universe was three minutes
old, the protons, neutrons, and electrons
now in your body had come into existence.
• You are made of very old matter (13.7 Gyr old)
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The History of the Atoms in Your Body
• Although those particles formed quickly,
they were not linked together to form the
atoms that are common today.
• Most of the matter was hydrogen (almost
75%)
• and about 25% was helium.
• Very few (< 1%) of the heavier atoms were
made in the big bang.
• The big bang also formed the so-called dark matter,
which permeates the planetary systems at very low
density, but has very little interaction with them.
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The History of the Atoms in Your Body
• Although your body does not contain
helium, it does contain many of those
ancient hydrogen atoms that have
remained unchanged since the universe
began.
• We are made mostly of water, and water is
made mostly of hydrogen (counting the
atoms), and of oxygen (which dominates
our mass)
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The History of the Atoms in Your Body
• During the first few hundred million years after
the big bang, matter collected to form galaxies
containing billions of stars. Our Galaxy was
built ~8.8 Gyr ago (so-called thin disk)
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recent galaxy formation also
occurs
• nuclear reactions inside stars combine low-mass
atoms, such as hydrogen, to make heavier atoms.
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The History of the Atoms in Your Body
• Generation of stars combined the original
particles, fusing them into atoms such as
carbon, nitrogen, and oxygen: CNO
• Those are common atoms in your body.
• Even the calcium atoms in your bones were
assembled inside stars.
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The History of the Atoms in Your Body
• Most of the iron in your body was
produced by:
• Carbon fusion in the explosions of stars called
supernovae, and
• Decay of radioactive atoms in the expanding
matter ejected by supernovae.
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The History of the Atoms in Your Body
• Atoms heavier than iron, such as iodine,
were created by rapid nuclear reactions
that can occur only during supernova
explosions.
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The History of the Atoms in Your Body
• Elements uncommon enough to be
expensive – gold, silver, and platinum in the
jewellery that humans wear – also were
produced during the violent deaths of rare,
massive stars.
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The History of the Atoms in Your Body
• Our galaxy contains at least 100 billion stars,
of which the Sun is one.
• The Sun formed from a cloud of gas and dust
about 4.56 billion years ago (4.56 Gyr ago).
• The atoms in your body were part of that cloud,
and previously part of another cloud that formed
an earlier star which no longer exists…
You are stardust, reprocessed many times over
the 13.7 Gyr, enriched in heavy elements
inside stars and in exploding stars.
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The History of the Atoms in Your Body
• How the Sun took shape, how the cloud
gave birth to the planets, and how the
atoms in your body found their way onto
Earth and into you is the story of this
chapter.
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The History of the Atoms in Your Body
• As you explore the origin of our solar
system, you should keep in mind the great
chain of origins that created the atoms.
• As the geologist Preston Cloud remarked,
“Stars have died that we might live.”
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The Origin of the Solar System
• Astronomers have a theory for the origin of
our solar system that is consistent both
with observations of the solar system and
with observations of star formation.
• Now, after the discovery of extrasolar planets
they are refining the details of the theories of
formation that would explain both our and
other planetary systems.
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The Origin of the Solar System
• The solar nebula theory supposes that
planets form in the rotating disks of gas
and dust around young stars.
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The Origin of the Solar System
• Our own planetary system formed in such
a disk-shaped cloud around the Sun.
• When the Sun became luminous enough, the
remaining gas and dust were blown away into space
by the solar wind (a stream of charged particles from
the Sun), and UV (ultraviolet) radiation, leaving the
planets orbiting the Sun.
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The Origin of the Solar System – early concepts
• According to the solar nebula hypothesis, Earth and
the other planets of the solar system formed 4.56
billions of years ago as the Sun condensed from the
interstellar medium.
Kant-Laplace nebula
Immanuel Kant (1724-1804)
Pierre-Simon de Laplace (1749-1824)
- dust & stones
- gas rings
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Kant
and Laplace
nebular models
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The Origin of the Solar System
• The theory predicts that most stars should
have planets because planet formation is
a natural part of star formation.
• Therefore, planets should be very common in
the universe – probably more common than
stars.
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The Origin of the Solar System
• These young stars form at the centre of a
rotating cloud of gas and dust that starts to
contract due to gravity.
• Collisions between particles in the rotating cloud
tend to flatten the cloud into a disk shape.
• In addition, as the cloud shrinks it spins faster due to
the conservation of angular momentum.
• Most of the material in the spinning disk forms a star
in the centre, while the remaining material forms the
planets and other bodies such as asteroids and
comets.
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The Origin of the Solar System
• There is clear evidence that disks of gas
and dust are common around young stars.
• The idea is so comprehensive and explains
so many observations that it can be
considered to have ‘graduated’ from being
just a hypothesis to being properly called a
theory.
• Bipolar flows from protostars were the first
evidence of such disks.
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The Origin of the Solar System
• Modern techniques, though, can image the
disks directly.
400 AU
Protoplanetary system
HH30 with jets
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The Origin of the Solar System
• The solar nebula theory supposes that
planets form in the rotating disks of gas
and dust around young stars.
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Protoplanetary
Disks in Orion
nebula
100 AU
HST (Hubble
Space Telescope)
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