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C H A P T E R
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THE ORIGIN OF THE SOLAR SYSTEM
SUMMARY - GUIDEPOSTS
19-1 THEORIES OF EARTH’S ORIGIN
What theories account for the origin of the solar system?
René Descartes proposed that the solar system formed from a contracting vortex of matter—an evolutionary
hypothesis. Buffon later suggested that a passing comet pulled matter out of the sun to form the planets—a
catastrophic hypothesis. Later astronomers replaced the comet with a star to produce the passing star hypothesis.
Laplace’s nebular hypothesis required a contracting nebula to leave behind rings that formed each planet, but it
could not explain the sun’s low angular momentum, which was known as the angular momentum problem.
The solar nebula theory proposes that the planets formed in a disk of gas and dust around the protostar that became
the sun. Observations show that these disks are common.
19-2 A SURVEY OF THE SOLAR SYSTEM
What properties must a successful theory explain?
The solar system is disk shaped in the orbital revolution of the planets and their moons and in the rotation of the
planets on their axes.
The planets are divided into two types. The inner four are terrestrial planets—small, rocky, dense Earthlike worlds.
The next four outward are Jovian planets that are large and low density.
All four of the Jovian worlds have ring systems and large families of moons. Jupiter’s Galilean satellites were
discovered by Galileo. The terrestrial planets have no visible rings and few moons.
Studies of craters on the moon show that the planets suffered a heavy bombardment of meteorite impact when they
were young, but that the rate of cratering fell rapidly. A sudden burst of cratering impacts from 4.1 to 3.8 billion
years ago is called the late heavy bombardment.
Most of the asteroids, small, irregular rocky bodies, are located between the orbits of Mars and Jupiter.
The Kuiper belt is composed of small, icy bodies that orbit the sun beyond the orbit of Neptune.
Comets are icy bodies that fall into the inner solar system along long elliptical orbits. As the ices vaporize and
release dust, the comet develops a tail that points approximately away from the sun.
Meteoroids that fall into Earth’s atmosphere are vaporized by friction and are visible as meteors. Larger and stronger
meteoroids may survive to reach the ground, where they are called meteorites.
The age of an object can be found by analyzing the abundance of radioactive elements with long half-lives. The
oldest rocks from Earth, the moon, and Mars have ages approaching 4.6 billion years. The oldest objects in our solar
system are the meteorites, which have ages of 4.6 billion years. This is taken to be the age of the solar system.
19-3 THE STORY OF PLANET BUILDING
How do planets form?
Modern astronomy reveals that the matter in our solar system was formed in the big bang, and the atoms heavier
than helium were cooked up in a few generations of stars. The sun and planets evidently formed from a cloud of gas
in the interstellar medium.
In general, planets that are farther from the sun have lower densities. Even among the terrestrial planets, their
uncompressed densities decrease with distance from the sun.
The outer solar system beyond the ice line could form large amounts of ice particles made of water, methane, and
ammonia. Ices could not form in the inner solar system; only metal and rock particles could form there.
The condensation sequence describes the kind of material that can form solids in the solar nebula with increasing
distance from the sun.
Condensation in the solar nebula converted some of the gas into solid bits of matter. Accretion combined particles to
build planetesimals that combined to form protoplanets.
Planets begin growing by accreting solid material. But once a planet approaches 10 to 15 Earth masses, it can begin
growing by gravitational collapse as it pulls in gas from the solar nebula.
According to the condensation sequence, the inner part of the solar nebula was so hot that only metals and rocky
minerals could form solid grains. The dense terrestrial planets grew from those solid particles and did not include
many ices or low-density gases.
The outer solar nebula was cold enough for metals, rocky minerals, and large amounts of ices to form solid particles.
The Jovian planets grew rapidly and incorporated large amounts of low-density ices and gases.
The terrestrial planets may have formed slowly from the accretion of planetesimals of similar composition. Dense
cores and low density crusts could have formed later by differentiation when radioactive decay heated the planet’s
interiors.
It is also possible that the planets formed so rapidly that the heat of formation melted the planets and they
differentiated as they formed. In that case, Earth’s first atmosphere was not captured from the nebula but was
outgassed from Earth’s interior.
Gas in planet forming disks can be blown away quickly by nearby stars, so Jovian planets, which grow primarily
from gas, must form quickly. Models suggest that under some circumstances they can form by direct gravitational
collapse without first accreting a core of solids.
Newer models suggest that forming planets may gradually migrate to new orbits, and that helps them sweep up
material and form quickly. It may not be necessary to conclude that the Jovian planets formed by direct gravitational
collapse.
Models of planet formation suggest that Jupiter may have migrated inward slightly as it formed and Saturn was
pushed outward. That pushed Uranus and Neptune further outward and scattered smaller bodies into the inner solar
system where they produced the late heavy bombardment.
In addition to ultraviolet radiation from hot nearby stars and the gravitational influence of passing stars, the solar
nebula was eventually cleared away by radiation pressure, the solar wind, the sweeping up of debris by the planets,
and ejection.
19-4 PLANETS ORBITING OTHER STARS
Is our solar system unique?
Hot disks of gas and dust have been detected in early stages of star formation and are believed to be the kind of disk
in which planets could form.
Cold dust disks, also known as debris disks, appear to be produced by dust released by collisions among comets,
asteroids, and Kuiper belt objects. Such disks may be home to planets that have already formed.
Planets orbiting other stars, extrasolar planets, have been detected by the way they tug their stars about, creating
small Doppler shifts in the stars’ spectra. Planets have also been detected as they cross in front of their star and dim
the star’s light.
Some extrasolar planets are big enough to be Jovian worlds but are quite close to their star and are called hot
Jupiters. A few of these planets have been detected when they crossed behind their star and their infrared radiation
was cut off.
Nearly all extrasolar planets found so far are massive, Jovian worlds. Lower-mass terrestrial planets are harder to
detect but are presumably common.
Because planets can migrate as they form and can interact to distort orbits, Jovian planets orbiting very close to their
suns or moving in elliptical orbits are not objections to the solar nebula theory.