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Structure & Formation of the
Solar System
• What is the Solar
System?
– The Sun and
everything
gravitationally
bound to it.
The planets to scale with a portion of the
Sun visible in the background.
• There is a certain
order to the Solar
System.
• This gives us
information on its
formation.
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Part 1: Structure of the Solar System
• All the planets orbit the
Sun in the same
direction.
• All the planets orbit
within nearly the same
plane. Like a disk.
• Two type of planets
– Solid, rocky, small
planets close to the Sun
(like Earth)
– Gaseous, large planets
far from the Sun (like
Jupiter)
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The Sun
• The Sun is a star.
• It is completely gaseous.
• It emits light and heat through
nuclear fusion in its core.
• It is by far the largest object in
the Solar System. 700 times
more massive than all of the
other objects in the Solar
System put together.
• It is composed mostly of
Hydrogen and Helium gas and
traces of many other elements.
• The Sun spins on its axis
counter-clockwise.
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The Planets
In order of
increasing
distance from the
Sun:
– Mercury
– Venus
– Earth
– Mars
– Jupiter
– Saturn
– Uranus
– Neptune
– Pluto
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The Planets
• All the planets
orbit in the
same direction
counterclockwise as
seen from
above Earth’s
North Pole.
• All the planets
spin counterclockwise too
except for
Venus, Uranus
and Pluto.
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The Inner or Terrestrial Planets
• Mercury, Venus,
Earth and Mars share
certain
characteristics:
– All are rocky bodies.
– All have solid
surfaces.
– Except for Mercury
all have at least a
thin atmosphere
• They are called
Terrestrial planets
because of their
resemblance to
Earth.
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The Outer or Jovian Planets
• Jupiter, Saturn,
Uranus and Neptune
share certain
characteristics:
– All are large, gaseous
bodies.
– All have very thick
atmospheres, with
possibly liquid
interiors and solid
cores
– All have rings
• They are called
Jovian planets
because of their
resemblance to
Jupiter.
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Pluto
• Pluto is unlike any of the
terrestrial or jovian
planets.
– much farther from the Sun
than the terrestrial planets.
– much smaller than any
jovian planet.
– composition is thought to
be a mixture of ice and
rock
• It is similar to some of the
satellites of the jovian
planets and similar to
some asteroids.
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Satellites (Moons)
• Most of the planets
have satellites.
• Most of the satellites
orbit in the equatorial
plane of the planet.
• Most satellites orbit
counter-clockwise.
Ganymede, Callisto, Io, and Europa.
• The jovian planets
Four of Jupiter’s largest satellites.
have more than a
These were discovered by Galileo
dozen satellites each.
Galilei and together are called the
Galilean satellites of Jupiter.
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Comets and Asteroids
• The Solar System is
filled with millions of
smaller bodies.
• Comets - composed of
ice and rock
• Asteroids - composed of
rock and/or metal
• There is also dust in
space which can be seen
in meteor showers
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Part 2: Solar System Formation
• Our Milky Way Galaxy is
filled with cold, dark clouds
of gas and dust.
• These clouds are mostly
hydrogen and helium with
dust containing mostly iron,
rock, and ice.
• The Solar System is thought
to have formed from a huge,
slowly rotating cloud about
4.5 billion years ago
• A nearby passing star or
stellar explosion may have
caused the cloud to collapse
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Collapsing Gas Clouds
• As the cloud collapsed the
original slow spin began
to speed up. This caused
the cloud to flatten into a
disk shape.
• The gravitational pull of
the cloud caused it to
shrink further and caused
most of the material to fall
towards the core forming a
large bulge.
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Collapsing Gas Clouds?
• In the Great Nebula of
the constellation Orion
are huge clouds of gas
and dust.
• Among these clouds
the Hubble Space
Telescope observed
lumps and knots that
appear to be new stars
and planets being
formed.
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Planets in Formation?
• Around the star Beta
Pictoris a large disk of
dust and gas has been
observed.
• The light from the star
is much brighter than
the disk so it had to be
blocked for the disk to
appear clearly.
• Disks have been seen
around other stars too
including Vega.
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Birth of the Sun
• As material falls into
towards the disk it
collides with other
material and heats up
and melts.
• The increasing mass of
the core also increases
the gravitational pull and
causes more material to
be pulled in.
• When the mass is large
enough and
temperatures high
enough nuclear fusion
reactions begin in the
core and a star is born!
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Heating and Condensation of the
Solar Nebula
• The heat from the Sun
prevents ices from reforming
on the dust grains in the
region near the Sun.
• Ices condensed only in the
outer parts of the Solar
nebula.
• In the inner portion of the
disk only materials like iron
and silicates (rock) can
condense into solids. Slowly
they form clumps of material.
• In the outer portion of the
disk much more material can
condense as solids including
ice. This extra material
allows clumps to grow larger
and faster.
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Gravity does the job
• Within the disk, material is
constantly colliding with one
another. If the collisions are
not too violent material may
stick together.
• In the outer parts of the Solar
Nebula the planets become
large enough to have a
significant gravitational pull
and collect gas around them.
• Planets in the inner nebula
can not grow enough to
collect much gas.
• Eventually most but not all
of the material was swept up
by the planets.
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The Last of the Planetesimals
The remaining material
exists today as
– comets which were flung
out to a region far beyond
Pluto called the Oort
cloud and
– asteroids mostly between
Mars and Jupiter (the
Asteroid Belt) and beyond
Pluto (the Kuiper Belt)
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