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Chapter Introduction
Lesson 1 The Structure of
the Solar System
Lesson 2 The Inner Planets
Lesson 3 The Outer Planets
Lesson 4 Dwarf Planets and Other
Chapter Wrap-Up
Dwarf Planets and Other Objects
• What is a dwarf planet?
• What are the characteristics of comets
and asteroids?
• How does an impact crater form?
Dwarf Planets
According to the International Astronomical
Union (IAU), a dwarf planet is an object that
orbits a star and has enough mass and
gravity to pull itself into a spherical shape.
Dwarf Planets (cont.)
• Unlike a planet, a dwarf planet has
objects similar in mass orbiting nearby
or crossing its orbital path.
• Ceres is the smallest dwarf planet with
a diameter of about 950 km.
• Pluto is so far from the Sun that it takes
about 248 years to complete one orbit.
• Eris is the largest dwarf planet.
All of the dwarf planets are smaller than
Earth’s moon.
Pluto: Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA
Ceres: NASA, ESA, and J. Parker (Southwest Research Institute)
Eris: NASA, ESA, and M. Brown (California Institute of Technology)
Most asteroids orbit the Sun in the
asteroid belt between the orbits of Mars
and Jupiter.
Vesta: Ben Zellner (Georgia Southern University), Peter Thomas (Cornell University), NASA/ESA
Eros: NASA/Goddard Space Flight Center Scientific Visualization Studio
• Asteroids are chunks of rock and ice that
never clumped together to form a planet.
• Some astronomers think the strength of
Jupiter’s gravitational field might have
caused the chunks to collide so violently
that they broke apart instead of sticking
Comets are mixtures of rock, ice, and dust.
Comet: Roger Ressmeyer/Getty Images
Wild 2: NASA/JPL-Caltech
• The particles in a comet are loosely held
together by the gravitational attractions
among the particles.
• Comets orbit the Sun in stretched out
elliptical orbits.
• The solid, inner part of a comet is its
• As a comet moves closer to the Sun, it
heats up and can develop a bright tail.
• A meteoroid is a small rocky particle
that moves through space.
• A meteor is a streak of light in Earth’s
atmosphere made by a glowing
Meteoroids (cont.)
• A meteorite is a meteoroid that strikes a
planet or a moon.
• An impact crater is a round depression
formed on the surface of a planet, moon,
or other space object by the impact of a
• An asteroid, such as Ida, is a chunk
of rock and ice that orbits the Sun.
• Comets, which are
mixture of rock, ice,
and dust, orbit the Sun.
A comet’s tail is
caused by its
interaction with
the Sun.
Roger Ressmeyer/Getty Images
• When a large meteorite strikes a
planet or moon, it often makes an
impact crater.
Which term refers to chunks of
rock and ice that never clumped
together to form a planet?
A. meteoroid
B. meteor
C. comet
D. asteroid
Which is a round depression
formed on the surface of a planet,
moon, or other space object by the
impact of a meteorite?
A. coma
B. impact crater
C. meteor
D. meteoroid
Which objects orbit the Sun in
stretched out elliptical orbits?
A. asteroids
B. comets
C. meteoroids
D. meteors
Do you agree or disagree?
7. Asteroids and comets are mainly rock
and ice.
8. A meteoroid is a meteor that strikes