CHAPTER 19 | THE ORIGIN OF THE SOLAR SYSTEM 413
in detail in Chapter 25. But for now you can consider how the
sun blew away the last of the gas and dust in the solar nebula.Clearing the Nebula
Th e sun probably formed along with many other stars in a swirl-
ing nebula. Observations of young stars in Orion (Figure 19-3)
suggest that radiation from nearby hot stars would have evapo-
rated the disk of gas and dust around the sun and that the gravi-
tational infl uence of nearby stars would have pulled gas away.
Even without the external eff ects, four internal processes would
have gradually destroyed the solar nebula.
Th e most important of these internal processes was radiation
pressure (see Chapter 11). When the sun became a luminous
object, light streaming from its photosphere pushed against the
particles of the solar nebula. Large bits of matter like planetesimals
and planets were not aff ected, but low-mass specks of dust and
individual atoms and molecules were pushed outward and eventu-
ally driven from the system.
Th e second eff ect that helped clear the nebula was the solar
wind, the fl ow of ionized hydrogen and other atoms away from
the sun’s upper atmosphere. Th is fl ow is a steady breeze that
rushes past Earth at about 400 km/s (250 mi/s). Young stars have
even stronger winds than stars of the sun’s age and irregular fl uc-
tuations in luminosity, like those observed in young stars such as
T Tauri stars, which can accelerate the wind. Th e strong surging
wind from the young sun may have helped push dust and gas out
of the nebula.
Th e third eff ect that helped clear the nebula was the sweeping
up of space debris by the planets. All of the old, solid surfaces in
the solar system are heavily cratered by meteorite impacts
(■ Figure 19-11). Earth’s moon, Mercury, Venus, Mars, and most of
the moons in the solar system are covered with craters. A few of
these craters have been formed recently by the steady rain of mete-
orites that falls on all the planets in the solar system, but most of
the craters appear to have been formed before roughly 4 billion
years ago in what is called the heavy bombardment, as the last of
the debris in the solar nebula were swept up by the planets.
Th e fourth eff ect was the ejection of material from the solar
system by close encounters with planets. If a small object such as
a planetesimal passes close to a planet, the small object’s path will
be aff ected by the planet’s gravity fi eld. In some cases, the small
object can gain energy from the planet’s motion and be thrown
out of the solar system. Ejection is most probable in encounters
with massive planets, so the Jovian planets were probably very
effi cient at ejecting the icy planetesimals that formed in their
region of the nebula.
Attacked by the radiation and gravity of nearby stars and
racked by internal processes, the solar nebula could not survive
very long. Once the gas and dust were gone and most of the
planetesimals were swept up, the planets could no longer gain
signifi cant mass, and the era of planet building ended.Th e icy Kuiper belt objects appear to be ancient planetesi-
mals that formed in the outer solar system but were never incor-
porated into a planet. Th ey orbit slowly far from the light and
warmth of the sun and, except for occasional collisions, have not
changed much since the solar system was young. Th e gravita-
tional infl uence of the planets defl ects Kuiper belt objects into
the inner solar system where they also are seen as comets.
Th e large satellite systems of the Jovian worlds may contain
two kinds of moons. Some moons may have formed in orbit
around forming planets in a miniature version of the solar neb-
ula. Some of the smaller moons, in contrast, may be captured
planetesimals, asteroids, and comets. Th e large masses of the
Jovian planets would have made it easier for them to capture
satellites.
In Table 19-1, you noted that all four Jovian worlds have
ring systems, and you can understand this by considering the
large mass of these worlds and their remote location in the solar
system. A large mass makes it easier for a planet to hold onto
orbiting ring particles; and, being farther from the sun, the ring
particles are not as quickly swept away by the pressure of sunlight
and the solar wind. It is hardly surprising, then, that the
Terrestrial planets, low-mass worlds located near the sun, have no
planetary rings.
Th e last entry in Table 19-1 is the common ages of solar
system bodies, and the solar nebula theory has no diffi culty
explaining that characteristic. If the theory is correct, then the
planets formed at the same time as the sun and should have
roughly the same age.
Th e solar nebula theory can account for all of the distin-
guishing characteristics of the solar system, but there is yet
another test you should apply to the theory. What about the
problem that troubled Laplace and his nebular hypothesis—the
angular momentum problem? If the sun and planets formed
from a contracting nebula, the sun should have been left spin-
ning very rapidly. Th at is, the sun should have most of the
angular momentum in the solar system, and instead it has very
little.
To study this problem, astronomers used the Spitzer Space
Telescope to examine 500 young stars in the Orion Nebula.
Th ose that rotate slowly are fi ve times more likely to have a sur-
rounding disk of gas and dust than the faster rotators. Th is con-
fi rms astronomer’s expectations that the strong magnetic fi elds of
young stars extend out into their disks. Th is allows the transfer
of angular momentum, speeding the disk’s rotation and slowing
the star’s rotation. Th us, the angular momentum problem is no
longer an objection to the solar nebula theory.
Your general understanding of the origin of the solar system
gives you a new way of thinking about asteroids, meteors, and
comets. Th ey are the last of the debris left behind by the solar
nebula. Th ese objects are such important sources of information
about the history of our solar system that they will be discussed