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ASTR 200 : Lecture 6
Introduction to the Solar System
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Comparative Planetology
Studying the similarities among and differences
between the planets
this includes moons, asteroids, & comets
This approach is useful for learning about:
the physical processes which shape the planets
the origin and history of our Solar System
the nature of planetary systems around other stars
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Basic patterns that need explanation
• All planets orbit Sun in same sense (counterclockwise viewed from N)
• All planets orbit in almost same plane, with e~0
• Sun contains 99.9% of Solar System's mass
• Inner planets are rocky, while outer the
planets/satellites are icy or gas-rich
• Crater production (especially impact basins)
larger in past
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An obvious solar system division
Planets fall into two main categories
A) Terrestrial (i.e. Earth-like)
B) Jovian (i.e. Jupiter-like or gaseous)
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Danger: “Inner” and “Outer” are often used, but
they only work until Neptune
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Mars
Terrestrial
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Neptune
Jovian
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The Layout of the Solar System
Large bodies in the Solar System have orderly motions
- planets orbit counterclockwise in same plane
- orbits are almost circular
- the Sun and most planets rotate counterclockwise (seen from N)
- most moons orbit their planet counterclockwise
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A Few Exceptions to the Rules…
Both Uranus & Pluto are tilted on their sides.
Venus rotates “backwards” (i.e. clockwise).
Triton orbits Neptune “backwards.”
Earth is the only terrestrial planet with a
relatively large moon.
Is Pluto (and other large trans-neptunian
objects) to be considered a planet?
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The Sun – King
of the Solar
System
How does the Sun influence the planets?
• Its gravity regulates the orbits of the planets.
• Its heat is the primary factor which determines
the temperature of the planets.
• It provides practically all of the visible light in
the Solar System.
• High-energy particles streaming out from the
Sun influence planetary atmospheres and
magnetic fields.
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Mercury :
Battered
Remnant
• Recently
explored by
the
Messenger
spacecraft
• Impact
scarred like
our Moon
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Venus: An
extreme
climate
• Earth's twin
in size
• Massive
choking
atmosphere
• Clue to
planetary
climates
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Earth: The
cradle of life
• A complex
biosphere
• A massive
Moon
• Geologic
complexity
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Mars : The
most likely
second
home?
Earth-like in
some ways
Perhaps had
oceans?
Surface has
great variety
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Jupiter :
Lord of the
heavens
• More mass than
all other
planets put
together
• First of the
jovian planets
• Moons the size
of planets
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Ringed
Saturn
• Rings are
incredibly
flat
• Interesting
contrasts to
Jupiter
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Uranus and
Neptune :
the ice giants
• Much smaller
than Jupiter
and Saturn
• Dominantly
ices, not gases
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The Solar System has two 'small body belts'
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The main asteroid belt
• Swarms of asteroids and comets populate
the Solar System
• Rocky asteroids, mostly between Mars and
Jupiter
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Neptune's orbit
Comets
Pluto's orbit
- Visible comets (those
with tails) when they
make it to the inner
Solar System (rare)
Reservoirs are the :
- Kuiper Belt
- Oort cloud
Oort cloud
Main
asteroid
belt
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Comets and
Asteroids
Small bodies that hold
big clues to the birth
of planets
The two classes are
mainly composition
• Rocky asteroids
• Icy comets
This is linked to where
they formed.
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'Orbital elements' are the parameters that describe the
shape and orientation of the orbit
- In this course we are not worrying about the orientation,
so the only orbital elements we discussed were a and e.
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