The Solar System

400 PART 4^ |^ THE SOLAR SYSTEM

Gaseous cloud

evaporated from

dust disk

Visual-wavelength image

Light from

central star

scattered by dust

Gaseous cloud

evaporated from

dust disk

Dust disk

Dust disk

Visual-wavelength image

Dust disk seen

edge-on

The So-Called Scientifi c Method

How big a role have sudden, catastrophic

events played in the history of the solar

system? Many theories in science can be

classifi ed as either evolutionary, in that they

involve gradual processes, or catastrophic, in

that they depend on sudden, unlikely events.

Scientists have generally preferred evolution-

ary theories. Nevertheless, catastrophic events

do occur.

Some people prefer catastrophic theories,

perhaps because they like to see spectacular

violence from a safe distance, which may

explain the success of movies that include

lots of car crashes and explosions. Also,

catastrophic theories resonate with scriptural

accounts of cataclysmic events and special

acts of creation. Thus, many people have an

interest in catastrophic theories.

Most scientifi c theories are evolution-

ary. Such theories do not depend on unlikely

events or special acts. For example, geologists

study theories of mountain building that are

evolutionary and describe mountains being

pushed up slowly as millions of years pass. The

evidence of erosion and the folded rock layers

show that the process is gradual. Because

most such natural processes are evolutionary,

scientists sometimes fi nd it diffi cult to accept

any theory that depends on catastrophic

events.

You will see in this and later chapters

that catastrophes do occur. For example,

the planets are bombarded by debris from

space, and some of those impacts are very

large. As you study astronomy or any other

natural science, notice that most theories are

evolutionary but that you need to allow for

the possibility of unpredictable catastrophic

events. Mountains evolve to great heights by rising slowly,

not catastrophically. (Janet Seeds)

19-1

Two Kinds of Theories: Catastrophic

and Evolutionary

■ Figure 19-3

Many of the young stars in the Orion Nebula are

surrounded by disks of gas and dust, but intense

light from the brightest star in the neighbor-

hood is evaporating the disks to form expanding

clouds of gas. These particular disks may evapo-

rate before they can form planets, but the large

number of such disks shows that planet construc-

tion material around young stars is common.

(C. R. O’Dell, Rice, NASA; Dark Disk: M. McCaughrean,

Max-Planck Inst. für Astronomie, C. R. O’Dell, NASA;

Lower left inset: J. Bally, H. Throop, C. R. O’Dell, NASA)