CHAPTER 19 | THE ORIGIN OF THE SOLAR SYSTEM 399
which could become a planet circling the newborn sun at the
center of the disk (Figure 19-1b).
According to the nebular hypothesis, the sun should be
spinning very rapidly, or, to put it another way, the sun should
have most of the angular momentum of the solar system.
(Recall from Chapter 5 that angular momentum is the ten-
dency of a rotating object to continue rotating.) As astrono-
mers studied the planets and the sun, however, they found that
the sun rotates relatively slowly and that the planets moving in
their orbits actually have most of the angular momentum in
the solar system. In fact, the rotation of the sun contains only
about 0.3 percent of the angular momentum of the solar sys-
tem. Because the nebular hypothesis could not explain this
angular momentum problem, it was never fully successful,
and astronomers instead considered various versions of the
passing star hypothesis for over a century. Today astronomers
have a consistent theory for the origin of our solar system that
is a modifi ed version of the nebular hypothesis.
The Solar Nebula Theory
By 1940, astronomers were beginning to understand how stars
form and how they generate their energy, and it became clear
that the origin of the solar system was linked to that story.
Th e modern explanation of the origin of the planets is the
solar nebula theory, which proposes that the planets, including
Earth, formed in a rotating disk of gas and dust that surrounded
the sun as it formed (■ Figure 19-2). Laplace’s nebular hypoth-
esis included a disk, but his process depended on rings of matter
left behind as the disk contracted. Also, it was not based on a
clear understanding of how gas and dust behave in such a disk.
In the solar nebula theory, the planets grew within the disk by
carefully described physical processes. Th is evolutionary hypoth-
esis is so comprehensive and explains so many of the observa-
tions, both of our solar system and of other systems, that it can
be considered to have “graduated” from being just a hypothesis
to being properly called a theory. Astronomers are continuing
to refi ne the details of that theory.
You have seen clear evidence that disks of gas and dust are
common around young stars. Bipolar fl ows from protostars (see
Chapter 11) were some of the fi rst clues to the existence of such
disks, but modern techniques can image the disks directly
(■ Figure 19-3). Th e evidence is strong that our own planetary
system formed in such a disk-shaped cloud around the sun.
When the sun became luminous enough, the remaining gas and
dust were blown away into space, leaving the planets orbiting the
sun.
According to the solar nebula theory, our Earth and the
other planets of the solar system formed billions of years ago as
the sun condensed from a cloud of gas and dust. If planet forma-
tion is a natural part of star formation, most stars should have
planets.
SCIENTIFIC ARGUMENT
Why does the solar nebula theory imply planets are common?
Often, the implications of a theory are more important in build-
ing a scientifi c argument than the theory itself. The solar nebula
theory is an evolutionary theory; and, if it is correct, the planets
of our solar system formed from the disk of gas and dust that sur-
rounded the sun as it condensed from the interstellar medium. That
suggests it is a common process. Most stars form with disks of
gas and dust around them, and planets should form in such disks.
Planets should therefore be very common in the universe.
Now build a new scientifi c argument. Why would a catastrophic
hypothesis for the formation of the solar system suggest that
planets are not common?■ Figure 19-2
The solar nebula theory implies that the planets formed along with
the sun. The scale of the top panel is much larger than the lower two
panels.The Solar Nebula TheoryPlanets grow from gas
and dust in the disk
and are left behind
when the disk clears.to form a thin disk of
gas and dust around
the forming sun at the
center.A rotating cloud of
gas contracts and
flattens...