CHAPTER 19 | THE ORIGIN OF THE SOLAR SYSTEM 401
*Recall from Chapter 2 that the words revolve and rotate refer to diff erent types
of motion. A planet revolves around the sun but rotates on its axis. Cowboys
in the Old West didn’t carry revolvers. Th ey actually carried rotators. And you
don’t rotate your tires every 6 months, you actually revolve them.
A Survey of the Solar
SystemTo test their hypotheses about how the solar system was
born, astronomers searched the present solar system for evidence
of its past. In this section, you will survey the solar system and
compile a list of its most signifi cant characteristics that are poten-
tial clues to how it formed.
You can begin with the most general view of the solar sys-
tem. It is, in fact, almost entirely empty space (look back to
Figure 1-7). Imagine reducing the scale of the solar system until
Earth is the size of a grain of table salt, about 0.3 mm (0.01 in.)
in diameter. Th e moon is a speck of pepper about 1 cm (0.4 in.)
away, and the sun is the size of a small plum located 4 m (13 ft)
from Earth. Jupiter is an apple seed 20 m (66 ft) from the sun.
Neptune, at the edge of the planetary zone, is a large grain of
sand over 120 m (400 ft) from the central plum. Although your
model solar system would be larger than a football fi eld, you
would need a powerful microscope to detect the asteroids orbit-
ing between Mars and Jupiter. Th e planets are tiny specks of
matter scattered around the sun—the last remains of the solar
nebula.
Revolution and Rotation
Th e planets revolve* around the sun in orbits that lie close to a
common plane. Th e orbit of Mercury, the closest planet to the
sun, is tipped 7.0° to Earth’s orbit Th e rest of the planets’ orbital
planes are inclined by no more than 3.4°. As you can see, the
solar system is basically fl at and disk shaped.
Th e rotation of the sun and planets on their axes also seems
related to the rotation of the disk. Th e sun rotates with its equa-
tor inclined only 7.2° to Earth’s orbit, and most of the other
planets’ equators are tipped less than 30°. Th e rotations of Venus
and Uranus are peculiar, however. Venus rotates backward com-
pared with the other planets, whereas Uranus rotates on its side
with its equator almost perpendicular to its orbit. You will
explore these planets in detail in Chapters 21 and 23, but later in
this chapter you will be able to understand how they might have
acquired their peculiar rotations.
Th ere is a preferred direction of motion in the solar system—
counterclockwise as seen from the north, like the curl of the
fi ngers of your right hand if you point your thumb toward your
eyes. All the planets revolve counterclockwise around the sun;
and with the exception of Venus and Uranus they rotate counter-
clockwise on their axes. Furthermore, nearly all of the moons in
the solar system, including Earth’s moon, orbit around their
respective planets counterclockwise. With only a few exceptions,
19-2 most of which are understood, revolution and rotation in the
solar system follow a single theme. Apparently, these motions
today are related to the original rotation of a disk of solar system
construction material.Two Kinds of Planets
Perhaps the most striking clue to the origin of the solar system
comes from the obvious division of the planets into two groups,
the small Earth-like worlds and the giant Jupiter-like worlds. Th e
diff erence is so dramatic that you are led to say, “Aha, this must
mean something!” Study Terrestrial and Jovian Planets
on pages 402–403, notice three important points, and learn two
new terms:
Th e two kinds of planets are distinguished by their location.
Th e four inner Terrestrial planets are quite diff erent from the
four outer Jovian planets.
Craters are common. Almost every solid surface in the solar
system is covered with craters.
Th e two groups of planets are also distinguished by proper-
ties such as presence or absence of rings and numbers of
moons. A theory of the origin of the planets needs to explain
these properties.
Th e division of the planets into two groups is a clue to how our
solar system formed. Th e present properties of individual planets,
however, don’t tell everything you need to know about their ori-
gins. Th e planets have all evolved since they formed. For further
clues about the origin of the planets, you can look at smaller
objects that have remained largely unchanged since soon after the
birth of the solar system.Space Debris
Th e sun and planets are not the only products of the solar nebula.
Th e solar system is littered with three kinds of space debris: aster-
oids, comets, and meteoroids. Although these objects represent a
tiny fraction of the mass of the system, they are a rich source of
information about the origin of the planets.
Th e asteroids, sometimes called minor planets, are small
rocky worlds, most of which orbit the sun in a belt between the
orbits of Mars and Jupiter. More than 100,000 asteroids have
orbits that are charted, of which about 2000 follow paths that
bring them into the inner solar system where they can potentially
collide with a planet. Earth has been struck many times in its
history. It is a Common Misconception that the asteroids
are the remains of a planet that broke apart. In fact, planets are
held together very tightly by their gravity and do not “break
apart.” Astronomers recognize the asteroids as debris left over
from the failure of a planet to form at a distance of about 3 AU
from the sun.
About 200 asteroids are more than 100 km (60 mi) in diam-
eter, and tens of thousands are estimated to be more than 10 km