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Chapter 21
Our Solar System
Lesson 1
Planets orbit the Sun at different distances
 Planets have different sizes and distances
 Planets do NOT give off light-they reflect
sunlight like the moon
 The sun is the center of the universe and
all planets orbit the sun
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My Very Educated Mother Just
Served Us Nine Pizzas
Inner planets closest to the sun: Mercury,
Venus, Earth and Mars
 Asteroid belt
 Outer planets: Jupiter, Saturn, Uranus,
Neptune and Pluto
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Distances in Space
Astronomical unit (AU) is Earth’s average
distance from the Sun (about 150 million
kilometers or 93 million miles)
 1st four planets are somewhat close to the
Sun
 The other planets are very far away from
the Sun
 Distance in space is measured in AU’s
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Orbits
99% of all mass in solar system is in Sun
 Gravitational pull of Sun keeps planets
moving around the Sun
 This move is called an orbit
 An orbit is a flattened circle, or an oval
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Big Bang Theory
The solar system formed from a swirling
cloud of gas and dust according to this
theory.
 The cloud formed into a disk of whirling
material. Most the mass fell to the center
and became a star-the Sun.
 Tiny bits of dust froze and became largerplanets-they moved in the same direction
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Other Space Facts
Asteroids-rock or mountains in space
 Comets-like enormous snowballs or
icebergs
 Moons-objects orbit planets instead of
orbiting the Sun directly
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Lesson 2
The inner solar system planets are called
terrestrial planets
 Terrestrial planets are rocky and have
dense mantles and cores
 They all have similar patterns and design

Similarities
Tectonics-crusts change due to hot
material underneath
 Volcano-when molten rock moves from the
interior to the surface
 Weathering/Erosion-breaking down of
rocks through wind, water, ice and
temperature
 Craters-places where something has
impacted the Earth
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Atmospheres
Atmospheres on terrestrial planets formed
from gases that poured out of volcanoes
 Atmosphere can move energy from warm
to cool places
 Atmosphere can also make planet’s
surface warmer by slowly loosing energy

Earth
Mountains and valleys are formed from
tectonic plates colliding
 Volcanoes are formed on Earth
 Water, wind and gravity cause erosion
 Craters are usually covered by erosion
 Atmosphere keeps it warm enough for
water (Oceans cover 3/4 of Earth)

Mercury
Tectonics-Long high cliffs
 Volcanoes-Parts covered with cooled lava
 Erosion-Temperature and gravity
 Craters-Round craters have not changed
much for a long time
 Atmosphere-none but has extreme
temperature changes
 Longest cycle of day and night (3 months)

Venus
Tectonics-Cracks and cliffs
 Volcanoes-covered with lava
 Erosion-too hot for any form of liquid and
winds don’t move quickly
 Craters-Few
 Atmosphere-thick enough to protect it
from most craters

Mars
Tectonics-Valles Marineris as long as the
U.S. is a huge system of valleys
 Volcanoes-North has smooth cooled lavasome higher than mountains on Earth
 Erosion-Fast winds and gravity-appears to
have flash floods long ago
 Craters-some very old onesmostly in the
south

Mars
Atmosphere-Thin-Polar caps (mostly dry
ice), no liquid water, large difference in
temperatures causing fast winds
(sometimes wind storms)
 Two tiny moons (predicted to once have
been asteroids pulled into orbit)
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Lesson 3
The outer solar system consists of Jupiter,
Saturn, Uranus, Neptune
 Four giant gas planets with very deep
atmospheres
 Large planets that orbit slowly around the
sun
 All gas giants have rings

Jupiter
A world of storms and clouds
 Largest planet (10+ times Earth)
 Rotates in 10 hours and orbits in 12 years
 Great Red Spot-large storm twice the size
of Earth and at least 100 years old
 4 large moons-Io, Europa, Ganymede and
Callisto

Saturn
Ringed planet-ring is larger than the
distance between Earth and the moonmade of chunks of ice the size of our
school
 Weak gravitational pull
 Rotates in 11 hours and orbits in 29 years
 Largest moon-Titan
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Uranus
Rings and moons
 Seems to spin on its side
 40 years of sunlight and 40 of darkness
 Rotates in 17 hours and orbits in 84 years
 Has small moons

Neptune
A moon orbits in opposite direction of
rotation
 Appears very smooth
 Rotates in 16 hours and orbits in 164
years
 Largest moon-Triton
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Lesson 4
Pluto and most objects in the outer solar
system are made of ice and rock
 Asteroid vs Comet
 Meteor vs Meteorite
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Pluto
Smallest “dwarf” planet
 Smaller than Earth’s moon
 Very weak gravitational pull
 Thin atmosphere
 No spacecraft has passed close
 No clear images-mostly predictions
 Small moon-Charon
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Asteroids
Asteroids-small solid rocky bodies that
orbit close to the sun
 Irregular shapes
 Asteroid belt between Mars and Jupiter10,000+
 Collisions form craters
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Comets
An object that produces a coma (trailing
cloud)
 Appears as a fuzzy spot in the night sky
 Sun turns ice into gas so tail points away
from sun always
 Most are not visible on Earth to the naked
eye

Meteor-Meteorite
Brief streaks of light from very hot objects
 Meteorite when it reaches Earth’s surface
 Fireballs that last only seconds in the sky
 Meteor showers are when several are
visible per hour
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Test Time
Complete review on page 749
 Study vocabulary and review for test
 Study pictures in chapter for test
 Do your best! 
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