The Solar System

494 PART 4^ |^ THE SOLAR SYSTEM

The Outer Planets

Th e outermost planets in our solar system are Jupiter, Saturn,

Uranus, and Neptune—the Jovian planets, meaning they resem-

ble Jupiter. ■ Figure 23-1 compares the four outer worlds, and

one striking feature, of course, is their sizes. Figure 23-1 shows

Earth in scale to the Jovian planets, and it seems tiny in compari-

son. You can also see that the four Jovian planets can be divided

into two pairs, with Jupiter and Saturn being large and nearly the

same size, whereas Uranus and Neptune are smaller but very

similar in size to each other.

Th e other feature you will notice immediately when you

look at Figure 23-1 is Saturn’s rings. Th ey are bright and beauti-

ful and composed of billions of ice particles, each particle follow-

ing its own orbit around the planet. Astronomers have discovered

that Jupiter, Uranus, and Neptune also have rings, but they are

not easily detected from Earth and are not visible in this fi gure.

As you visit these worlds in this chapter and the next, you will be

able to compare and contrast four giant planets, four moon sys-

tems, and four diff erent sets of planetary rings.

Th ere is something fascinating about

science. One gets such wholesale returns of

conjecture out of such a trifl ing investment

of fact.

— MARK TWAIN, LIFE ON THE MISSISSIPPI

W

hen Mark Twain wrote the sentences that open
this chapter, he was poking gentle fun at science, but
he was right. Th e exciting thing about science isn’t
the so-called facts, the observations in which scientists have great-
est confi dence. Rather, the excitement lies in the understanding
that scientists get by rubbing a few facts together. Science can
take you to strange new worlds such as Jupiter and Saturn, and
you can get to know them by combining the available observa-
tions with known principles of comparative planetology.

A Travel Guide to the

Outer Planets

If you travel much, you know that some cities make you feel
at home, and some do not. In this and the next chapter, you will
visit worlds that are truly un-Earthly. Th is travel guide will warn
you what to expect.

23-1

■ Figure 23-1

The principal worlds of the outer solar system are the four massive but low-

density Jovian planets, each much larger than Earth. (NASA/JPL/Space Science

Institute/University of Arizona)

Uranus and Neptune are green

and blue colored because of

small amounts of methane in

their hydrogen-rich atmospheres.

Earth is the largest of

the Terrestrial worlds,

but it is small compared

with the Jovian planets.

Shadow of one

of Jupiter’s

many moons

Uranus and Neptune

are both both about four

times Earth's diameter.

The cloud belts and zones on

Saturn are less distinct than

those on Jupiter.

Jupiter, more than 11 times Earth’s diameter,

is the largest planet in our solar system.