UNIT NINE: Matter and Motion
in the Universe
 Chapter 26 The Solar System
 Chapter 27 Stars
 Chapter 28 Exploring the Universe
Chapter Twenty-Six:
The Solar System
 26.1 Motion and the Solar System
 26.2 Motion and Astronomical Cycles
 26.3 Objects in the Solar System
Section 26.3 Learning Goals
 Explore theories about how the Moon was
formed.
 Compare and contrast properties of planets.
 Identify features of objects— other than the
Sun, the Moon, and planets, in the solar
system.
26.3 Objects in the solar system

A planet in the solar system is a
celestial body that:
1. is in orbit around the Sun;
2. is nearly round in shape; and
3. has cleared its orbit of other objects.
26.3 The planets
 The planets are
commonly classified in
two groups.
 The terrestrial planets
include Mercury, Venus,
Earth, and Mars.
 The gas giants include
Jupiter, Saturn, Uranus,
and Neptune.
26.3 Moons
 A moon is a natural
satellite that orbits
a planet or other
body, such as a
dwarf planet.
 The planet the
moon orbits is
called the primary.
26.3 Earth and moon
 If you have ever observed
the Moon, you may have
noticed that the same
side of it faces Earth at
all times.
 This does not mean that
the Moon does not rotate.
 Over millions of years,
Earth’s gravity has
locked the Moon’s
The time it takes the Moon to
rotation to its orbit
complete a rotation is the same time
around Earth.
it takes it to revolve around Earth.
26.3 How the moon was formed

What evidence did Apollo
moon rocks supply?
Before the Apollo
landings that began in
1969, there were three
main theories.
1. The Moon split off Earth.
2. The Moon formed
somewhere else.
3. The Moon and Earth
were formed at the same
time.

Apollo discoveries gave rise to the giant
impact theory that is widely accepted
today.
26.3 Mercury

Mercury, the
closest planet
to the sun, is
the second
smallest (after
Pluto) in both
size and mass.
26.3 Venus
 Venus appears as
the brightest planet
in the evening sky
and is the third
brightest
observable object
(after the sun and
moon).
26.3 Earth
 Earth is a small,
rocky planet with
an atmosphere
that is made of
mostly nitrogen
(78 percent N2)
and oxygen (21
percent O2).
26.3 Mars

The fourth
planet out from
the sun, Mars
appears as a
reddish point of
light in the night
sky.
26.3 Jupiter

The fifth planet out
from the sun, Jupiter
is by far the largest.

Jupiter’s mass is
greater than the
combined mass of all
of the other planets.

With 63 known
moons, Jupiter is like
a mini solar system.
26.3 Saturn

Saturn, at almost
10 times the size of
Earth, is the
second largest
planet.

The most striking
feature of Saturn is
its system of rings
and like Jupiter,
has many natural
satellites.
26.3 Uranus
 The seventh planet
from the sun,
Uranus can barely
be seen without a
good telescope
and was not
discovered until
1781.
26.3 Neptune

Neptune, the eighth
planet from the sun, is
the outermost of the
gas planets.

It was discovered in
1846 and its discovery
almost doubled the
diameter of the known
solar system because
of its great distance
from the sun.
Comparing properties of the planets
26.3 Triton, Pluto and the Kuiper belt
 Triton is Neptune’s largest
moon.
 Triton and Pluto are similar
objects in both composition
and size.
 Some astronomers believe
Pluto may actually be an
“escaped” moon of Neptune.
26.3 Pluto
 Pluto is a dwarf planet.
 Most of the time Pluto
is the farthest from the
sun.
 Discovered in 1930,
Pluto was named for
the Roman god of the
underworld.
26.3 Pluto and the Kuiper Belt
 Pluto is grouped along
with Sedna, Xena, and
similar distant bodies
in the Kuiper Belt
Objects (or KBOs).
 It contains at least
three dwarf planets:
Pluto, Haumea, and
Makemake.
26.3 Asteroids and comets
 An asteroid is an
object that orbits the
sun but is too small to
be considered a
planet.
 The largest asteroid,
named Ceres, is 933
kilometers (580 miles)
across.
26.3 Asteroids and comets
 We believe comets
are made mostly of
ice and dust.
 Comets revolve
around the Sun in
highly elliptical orbits.
26.3 Asteroids and comets
 The inner core of the
comet is the nucleus.
 As a comet gets
closer to the Sun, it
forms a tail.
26.3 Meteors and meteorites
 Occasionally, chunks of
rock or dust break off
from a comet or asteroid
and form a meteor.
 As Earth orbits the sun, it
passes through this
debris, creating a meteor
shower as the small bits
of dust burn up in the
atmosphere.
26.3 Meteors and meteorites
 If a meteor is large
enough to survive the
passage through
Earth’s atmosphere
and strike the ground,
it becomes a
meteorite.
Optional* Investigation 26C
Solar System
 Key Question:
How big is the solar system?
What happened to Pluto?
 The 2006 meeting, held in
Prague, Czech Republic,
distinguished astronomers
from all over the world
came together to exchange
ideas at a conference held
by the International
Astronomical Union (IAU).