398 PART 4^ |^ THE SOLAR SYSTEM
encounter with another star. More important, the gas pulled
from the sun and the star would be much too hot to condense to
make planets, and it would have dispersed instead. Furthermore,
even if planets did form, they would not go into stable orbits
around the sun.
Th e hypotheses of Descartes and Buff on fall into two broad
categories. Descartes proposed an evolutionary hypothesis
involving common, gradual processes to produce the sun and
planets. If it were correct, stars with planets would be very com-
mon. Buff on’s idea, on the other hand, is a catastrophic hypoth-
esis. It involves unlikely, sudden events to produce the solar
system, and thus implies that planetary systems are very rare.
While your imagination may enjoy picturing the spectacle of
colliding stars, modern scientists have observed that nature usu-
ally changes gradually in small steps rather than in sudden, dra-
matic events. Th e modern theory for the origin of the planets,
based on abundant evidence, is evolutionary rather than cata-
strophic (How Do We Know? 19-1).
Th e modern theory of the origin of the solar system had its
true beginning with Pierre-Simon de Laplace (1749–1827), a
brilliant French astronomer and mathema-
tician. In 1796, he combined Descartes’s
vortex idea with Newton’s laws of gravity
and motion to produce a model of a rotat-
ing cloud of matter that contracted under
its own gravitation and fl attened into a
disk—the nebular hypothesis. As the disk
grew smaller, it had to conserve angular
momentum and spin faster and faster.
Laplace reasoned that, when it was spin-
ning as fast as it could, the disk would shed
its outer edge to leave behind a ring of mat-
ter. Th en the disk could contract further,
speed up again, and leave another ring. In
this way, he imagined, the contracting disk
would leave behind a series of rings, each ofuniverse was fi lled with vortices of whirling invisible particles. In
1644, he proposed that the sun and planets formed when a large
vortex contracted and condensed. His hypothesis explained the
general properties of the solar system known at the time.
A century later, in 1745, the French naturalist Georges-
Louis de Buff on (1707–1788) proposed an alternative hypothesis
that the planets were formed when a passing comet collided with
or passed close to the sun and pulled matter out of the sun. He
did not know that the solid parts of comets are small, insubstan-
tial bodies, but later astronomers modifi ed his hypothesis to
propose that a star, rather than a comet, interacted with the sun.
According to the modifi ed hypothesis, matter ripped from the
sun and the other star condensed to form the planets, which were
driven into orbit around the sun by the motion of the two stars’
collision. (■ Figure 19-1a). Th is passing star hypothesis was
popular off and on for two centuries, but it contains serious
fl aws. First, stars are very small compared to the distances
between them, so they collide very infrequently. Only a tiny frac-
tion of stars in our galaxy have ever suff ered a collision or close
■ Figure 19-1
(a) The passing star hypothesis proposed that
the sun was hit by, or had a very close encoun-
ter with, another star. Matter torn from the sun
and the other star formed planets orbiting the sun
and, perhaps, the other star. This is an example
of a catastrophic hypothesis. (b) Originally pro-
posed in the 18th century by Laplace, the nebular
hypothesis proposed that a contracting disk of
matter around the sun conserved angular momen-
tum, spun faster, and shed rings of matter that
then formed planets. This is an example of an evo-
lutionary hypothesis.ab