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SUMMARY

The planets can be classified into two major groups: (1) the terrestrial is called a meteorite. Most meteors are fragments and pieces of dust
planets of Mercury, Venus, Mars, and Earth and (2) the giant planets of from comets. Most meteorites are fragments that resulted from colli-
Jupiter, Saturn, Uranus, and Neptune. Table 15.1 summarizes the char- sions between asteroids.
acteristics of the various planets. The protoplanet nebular model is the most widely accepted
Comets are porous aggregates of water ice, frozen methane, fro- theory of the origin of the solar system, and this theory can be con-
zen ammonia, dry ice, and dust. The solar system is surrounded by the sidered as a series of events, or stages. Stage A is the creation of
Kuiper Belt and the Oort cloud of these objects. Something nudges one all the elements heavier than hydrogen in previously existing stars.
of the icy bodies, and it falls into a long, elliptical orbit around the Sun. As Stage B is the formation of a nebula from the raw materials cre-
it approaches the Sun, increased radiation evaporates ices and pushes ated in stage A. The nebula contracts from gravitational attraction,
ions and dust into a long, visible tail. Asteroids are rocky or metallic forming the protosun in the center with a fat, bulging accretion disk
bodies that are mostly located in a belt between Mars and Jupiter. The around it. The Sun will form from the protosun, and the planets will
remnants of comets, fragments of asteroids, and dust are called mete- form in the accretion disk. Stage C begins as the protosun becomes
oroids. A meteoroid that falls through Earth’s atmosphere and melts to established as a star. The icy remains of the original nebula are the
a visible trail of light and smoke is called a meteor. A meteoroid that birthplace of comets. Asteroids are other remains that underwent
survives the trip through the atmosphere to strike the surface of Earth some melting.

KEY TERMS 2. The distance from Earth to the Sun is called a (an)
a. light-year.
b. solar year.
achondrites (p. 392)
c. astronomical unit.
asteroids (p. 391) d. astronomical year.
astronomical unit (p. 378)
3. What type of planets are Mercury, Venus, Earth, and Mars?
chondrites (p. 392) a. Early planets
chondrules (p. 393) b. Terrestrial planets
comet (p. 389) c. Small planets
Copernican system (p. 397) d. Hot planets
dwarf planet (p. 378) 4. Which of the following is most likely found on Jupiter?
giant planets (p. 378) a. Hydrogen
iron meteorites (p. 392) b. Argon
Kepler’s first law (p. 397) c. Nickel
Kepler’s laws of planetary motion (p. 397) d. Carbon dioxide
Kepler’s second law (p. 397) 5. What is the outermost planet?
Kepler’s third law (p. 398) a. Mercury
Kuiper Belt (p. 389) b. Mars
c. Neptune
meteor (p. 392)
d. Jupiter
meteorite (p. 392)
6. The planet that was named after the mythical Roman messenger
meteoroids (p. 392)
of speed is
meteor shower (p. 392)
a. Jupiter.
Oort cloud (p. 389)
b. Mercury.
planet (p. 378) c. Saturn.
protoplanet nebular model (p. 393) d. Mars.
Ptolemaic system (p. 396) 7. A day on which planet is longer than a year on that planet?
small solar system bodies (p. 378) a. Mercury
stony-iron meteorites (p. 392) b. Venus
stony meteorites (p. 392) c. Neptune
terrestrial planets (p. 378) d. Jupiter
8. The day on which planet is about the same time period as a day
on Earth?
APPLYING THE CONCEPTS a. Mercury
b. Venus
1. The mass of the Sun is how much larger than all of the other c. Mars
planets, moons, asteroids, and other bodies in the solar system? d. Jupiter
a. 100 times larger
b. 200 times larger
c. 500 times larger
d. 700 times larger

400 CHAPTER 15 The Solar System 15-24