LESSON 3
THE SOLAR SYSTEM
Chapter 8, Astronomy
OBJECTIVES
Identify planets by observing their movement
against background stars.
Explain that the solar system consists of many
bodies held together by gravity.
MAIN IDEA
The solar system consists of the planets, their
moons, and many other bodies orbiting the
Sun.
VOCABULARY
planet
moon
solar system
inertia
asteroid
comet
meteoroid
meteor
meteorite
WHAT IS THE
SOLAR SYSTEM?
• Before telescopes were invented people
studied stars by looking up at the night sky.
• People noticed the bright lights in the sky
changed.
• A solar system has stars like the sun as well
as moons and planets.
• All but 2 planets have one or more than one
moon.
PLANETS AND ORBITS
• Gravity is the force of attraction among all
objects.
• The amount of gravitational force between
the sun and a planet depends on mass and
distance.
• The greater the mass, the greater the force.
• The greater the distance, the less force.
• The second property that keeps planets in
orbit is inertia.
• Without it we would be pulled into the Sun
because of its mass.
• With only inertia the planets would move in
a straight line.
• Gravity and inertia are balanced so the
planets’ paths are curved.
IDEAS ABOUT
PLANETARY MOTION
• Ancient astronomers saw that the planets
moved across the field of stars but didn’t
know why.
• Over time two explanations emerged.
• The first explanation was that the Earth was
the center of the universe.
• The other explanation was that the Earth,
the Moon, the stars, and other planets
revolve around the Sun.
QUICK CHECK
Classify
List the planets in order of increasing distance
from the Sun.
Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, Neptune
Critical Thinking
Would the pull of the Sun’s gravity on a space
probe be greater near Mercury or near Saturn?
Explain.
The Sun’s gravity on a space probe would be
greater near Mercury because it is closer to the
Sun.
WHAT IS IN THE INNER
SOLAR SYSTEM?
• Mercury, Venus, Earth, and Mars are closest
to the Sun and called the inner planets.
• They have similar sizes and mostly rocky
structures.
• They also have closely spaced orbits and few,
if any, moons.
• All the inner planets rotate relatively slow and
none of them have rings.
• Earth is the largest of the inner planets.
ASTEROIDS
• Between the orbits of Mars and Jupiter are
asteroids, rocky or metallic objects, that orbit
the Sun.
• Most asteroids are located in the asteroid belt.
• The largest object is about one fourth the
diameter of the Moon.
• Asteroids orbit the Sun just like planets.
• Some asteroids travel as far from the Sun as
Saturn’s orbit, other asteroids have orbits that
cross Earth’s path.
• Scientists have a lot of knowledge about
asteroids by using space probes.
• Galileo, a space probe, passed by Gaspra and
Ida, two asteroids.
• NEAR Shoemaker, another space probe,
encountered the asteroid Mathilde and landed
on Eros.
QUICK CHECK
Classify
List the inner planets in order from smallest to
largest.
Mercury, Mars, Venus, Earth
Critical Thinking
In what ways are asteroids similar to planets?
Asteroids are similar to planets because they
are also made of rock and metal. They orbit
the sun, as do the planets.
WHAT ARE THE
OUTER PLANETS?
• Beyond the asteroid belt is another group of
planets that includes Jupiter, Saturn, Uranus, and
Neptune, known as the outer planets.
• They are gas giants which are huge planets with a
small, metallic core, and a thick atmosphere.
• The gas giants all have rings and many moons.
• They spin very rapidly, so a day on a gas giant is
very short.
• Pluto was once known as the ninth planet.
• In 2003, astronomers discovered a similar,
slightly larger world beyond the orbit of
Pluto.
• In 2006, the International Astronomical
Union reclassified Pluto as a dwarf planet.
QUICK CHECK
Classify
List the outer planets in order from smallest to
largest.
Neptune, Uranus, Saturn, and Jupiter are the
outer planets listed from smallest to largest.
Critical Thinking
How is Pluto different from the outer planets?
Differences between Pluto and the outer
planets include size and rings.
The outer planets are huge, while Pluto is very
small.
The outer planets have rings and Pluto does
not.
WHAT ARE OTHER
OBJECTS IN OUR SOLAR
SYSTEM?
• A comet is a ball of ice and rock that orbits
the Sun.
• They come from the outer fringes of the
solar system.
• Some comets come from the Kuiper belt
located just beyond Pluto’s orbit.
• Other comets originate in an area called the
Oort cloud, a region surrounding the solar
system at a distance of about 30 trillion
kilometers from the Sun.
• Meteoroids are small, rocky, or metallic
objects that orbit the Sun in both the inner
and outer regions of the solar system.
• A meteor is a meteoroid that enters Earth’s
atmosphere.
• A meteoroid that strikes Earth’s surface is a
meteorite.
QUICK CHECK
Classify
How are space objects classified as meteoroids,
meteors, and meteorites?
In space, they are meteoroids;
In Earth’s atmosphere, meteors;
And on Earth, they are meteorites.
Critical Thinking
Is the tail of a comet in front of or behind the
comet?
Explain your answer.
Sunlight and solar radiation push the cloud
(tail) away, forming a tail behind it that always
points away from the Sun.