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Class 2: The Structure and
Formation of the Solar System
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Basic constituents of the solar system
The formation of the solar system
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What are the facts that we have to explain?
The collapsing gas cloud model.
The role of bombardment in the early solar
system.
I : Basic structure of the solar
system
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What makes up our solar system?
The Sun – our central star
Inner planets – small/rocky
Mercury  Venus  Earth  Mars
Outer planets – large/gaseous
Jupiter  Saturn  Uranus  Neptune  Pluto/Charon
What do small and large
mean?
Asteroids – rocky debris
Gaspra: 20 km
Mathilde: 59 km
Ida (58 km) & Dactyl (2 km)
Eros (33 km)
The Asteroid Belt
12566 Derichardson
Comets – dirty snowballs or
snowy dirtballs
Comet Hale-Bopp
(John Gleason)
Comet Ikeya-Zhang
II : Physical properties of the
Solar System
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All planets (apart from Pluto) orbit in
the same plane… solar system is flat
Properties, cont’d
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All planets orbit the Sun in same sense.
The Sun and all planets (apart from
Venus, Uranus, & Pluto) rotate in the
same sense as they orbit the Sun.
Inner planets are small and rocky, but
outer planets are large and gaseous.
There are also asteroids and comets
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All asteroids and short-period comets are
usually in planetary plane and orbit in same
sense
Long-period comets are random in direction
and sense.
III : Formation of the Solar
System
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How did all of this come to be?
A good theory needs to
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Explain the facts we just listed.
Be as simple as possible (but consistent
with the laws of nature).
Be testable.
Our best theory is the Collapsing Gas
Cloud theory.
Formation, cont’d
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Phase I (started 5 billion years ago)
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Phase II
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Gas cloud starts to collapse in on itself due
to its own gravitational pull.
As it collapses, it rotates faster and flattens.
The central part of the gas cloud forms the
Sun – nuclear reactions start in its core.
Phase III
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Dust grains and ice crystals start to
condense out of flattened gas disk.
Grains grow via collisions to form large
planetesimals (objects about Moon-size).
Formation, cont’d
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Stage IV – 100 million yrs have passed
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Planets grow by accreting planetesimals.
Eventually, there are hardly any large
planetesimals left – planets stop growing.
Outer massive planets grab the remains of
the gas from the cloud.
Stage V – 4.5 billion yrs ago to present
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Planets formed.
Still some debris around (asteroids and
comets) that occasionally hit the planets.
Questions…
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How would we ever test our theory for
the formation of the solar system?