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Chapter 6
Other Worlds: An Introduction to
the Solar System
What are the basic properties
of the solar system?
What are some of the general
patterns in the solar system?
What do these patterns tell us
about the how the solar
system formed?
Is (was) there life beyond
Earth?
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6.1 Overview of Our Planetary
System
• We have sent spacecraft to every
planet except Pluto.
– Also to moons, comets, and an asteroid.
• We’ve learned much about the
planets and the solar system.
• We want to get an overview of the
solar system, and then discuss in
more detail in chs. 7 to 13.
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Our Planetary System is the Solar
System
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6.1.1 An Inventory
Most of the mass of the solar system is in the Sun.
In this chapter we are discussing everything else.
Object
Percentage of Total Mass
Sun
99.80%
Jupiter
0.10%
Comets
0.05%
All other planets
0.04%
Satellites and rings
0.00005%
Asteroids
0.000002%
Cosmic dust
0.0000001%
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The Nine Planets
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The Nine Planets
• The masses of the Sun, Earth, Jupiter, and other
objects are measured by observing the effect of their
gravity.
• All the planets orbit the Sun in the same directions.
• All the orbits lie (almost) in the same plane.
• All the planets rotate (spin) about an axis.
– Most rotate in the same direction as they orbit.
– Venus rotates backwards
– Pluto and Uranus spin “on their sides”
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The Orbits of the Planets
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Terrestrial Planets
• The four innermost
planets:
–
–
–
–
Mars
Mercury
Venus
Earth
Mars
• Small, composed of
rock and metal, with
solid surfaces.
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Jovian Planets
• The next four planets are
called Jovian or giant
planets
–
–
–
–
Jupiter
Saturn
Uranus
Neptune
• Mostly liquids and gases
• No solid surface we can
stand on.
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6.1.2 Smaller Members of the Solar
System
• Satellites or moons
– All the planets except Mercury and Venus have moons
(small “m”)
• Rings
– All the giant planets have rings
• Asteroids
– Rocky metallic objects, mostly in the asteroid belt
• Comets
– Chunks of ice, mostly beyond the orbit of Pluto
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A Scale Model of the Solar System
Diameter
Distance from Sun
Sun
5cm
0m
Mercury
0.17mm
2.0m
Venus
0.43mm
3.9m
Earth
0.46mm
5.3m
Mars
0.24mm
8.1m
Jupiter
5.1mm
28m
Saturn
4.3mm
51m
Uranus
1.8mm
100m
Neptune
1.8mm
160m
Pluto
0.08mm
210m
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6.2 Composition and Structure of
Planets
• Three groups by composition
– Giant planets: Jupiter, Saturn, Uranus, Neptune
– Terrestrial planets: Mercury, Venus, Earth, Mars
– Pluto
• The different compositions and locations can
help us understand how they formed.
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6.2.1 The Giant Planets
• Core of metal, rock, and ice
• Surrounded by hydrogen
and helium
• Strong gravity holds and
compresses the hydrogen
and helium into liquid form
• The pressure at the core is
so great that any material
will be crushed  no solid
surface to land on.
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6.2.2 The Terrestrial Planets
• Composed primarily of
rocks and metals
– Silicon, oxygen, nickel,
iron
• Densest metals in the
central core
• Differentiated
– Interior of the planets
are or were molten long
enough to allow the
heavier elements to
settle to the core
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6.2.3 Moons, Asteroids, and Comets
• Earth’s Moon is like the
terrestrial planets
• Mars’ moons look like
captured asteroids
• Other moons more
similar to the cores of
the giant planets they
orbit -- differentiated
• Asteroids and comets
are probably unchanged
since they formed.
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6.2.4 Temperatures: Going to Extremes
• Generally, the farther something is from the
Sun, the colder it is.
– Mercury: 500K
– Pluto: 50K
• Venus breaks this rule, since at 700K it is
hotter than Mercury but further from the
Sun.
• Earth is the only planet at about the right
place to have liquid water
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6.2.5 Geological Activity
• Planetary crusts are
modified by both internal
and external forces.
• Good evidence that all the
planets have seen
considerable number of
impacts.
• Impacts more frequent in
the early history of solar
system
• Volcanos and seismic
quakes produced by internal
heat.
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6.3 Dating Planetary Surfaces
• We’d like to know the
age of a planet’s surface
• Two techniques:
– Counting the craters
– Radioactive rocks
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6.3.2 Radioactive Rocks
• Relies on the natural
abundances of certain
radioactive isotopes
• Used to determine the age
of the Earth and Moon
– 4.5 billion years
• Radioactive isotopes are
atoms that decay and
transform into a different
atom.
– In a half-life, half of the
radioactive isotopes decay.
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6.4 Origin of the Solar System
• Where did the solar system come from?
• How did it form?
• Look at the patterns in the solar system as a whole to
get a clue.
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6.4.1 Looking for Patterns
• All the planets revolve around the Sun in the same
direction, and in nearly the same plane
• The Sun rotates in the same direction
• Hypothesis: the Sun and planets formed from a
spinning system called the solar nebula. (discuss
angular momentum)
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6.4.2 The Evidence from Far Away
• In cool gas clouds, like the one pictured on the cover
of the book, we find evidence for new star
formation.
• In some instances we see evidence of a disk of
material around the star, a stellar nebula.
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Recent Photo from Spitzer Space Tele.
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The Search for Life Beyond Earth
• We are searching for evidence of life within our
solar system.
• Advantages: we can send probes or humans to the
locations to perform direct tests
• Disadvantages: limited possible locales
• The search is concentrated on Mars and Europa, one
of Jupiter’s moon’s.
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Dear Professor Robert Harr
Good morning, I like to present for you these
lines of our recently published book about Astronomy, that I have translated,
and which you can see at our website.
Your brother
E. A. Nassir The
Mistake of Astronomers about Mars Mars is bigger than Earth; it is about one
and a half times (less than! two times) that of Earth.Click here - [The Wrong
Calculations of Astronomers Concerning Mars]: If it were smaller than Earth, it
would be nearer than Earth to Sun.Click on: the cause of a planet being near
the sun and another being far , and see- at our website- the diagram 3 of a
simple experiment. Mars is inhabited: having rivers, seas, deserts, plains,
mountains, day and night, and the plant, the animal and intelligent human
beings to whom God has sent their apostles a! nd prophets, out of
themselves. Click here
The Planets Are Inhabited There will be an
anticipated meeting between the people of Earth and the people of Mars,
which may be a fruitful one. Click here
An Anticipated Meeting between the
Inhabitants of Earth and some other Planets The circumstances and
conditions on Earth will become bad in the future (when Earth will stop its
axial rotation; like what happened to Venus and Mercury before, on which life
was exterminated), then there will be a migration from Earth to Mars. Click
here
The Traveling to Mars Is Successful
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Discussion Question
• Here is planet Earth
• Which other object in
the solar system do you
think is most interesting
for the next NASA
mission, and why?
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